Trebuchet Design - College of Engineering and Computer

Transcription

The WSU Trebuchet Contest and The Engineering Design Process
A Sample Lesson Plan For Teachers
By: Jeff Bohrer
Introduction
As high school students begin to think about career choices, many are curious about engineering.
Providing these students with the experiences and knowledge needed in the engineering
profession is necessary to help solidify their college major decision. Physics, Algebra, and
Chemistry are good examples of opportunities offered by high schools to meet these needs.
However, these courses usually require analysis methods of problem solving, i.e. one solution is
expected, and the solution procedure is sequential. This is not usually the way engineers
problem solve. The Engineering Design Process is more typical. The Engineering Design
Process is a set of steps used by engineers to solve design problems. It is different than ordinary
problem solving in that engineering problems are often open-ended and can have many different
solutions. It is the job of the engineer to try to find the most appropriate solution. Because the
design problems are open ended, the Engineering Design Process is often iterative, i.e.
completed steps in the process may need revisited and the design may require refining
(Khandani, 2005). Giving students an opportunity to experience this process is highly beneficial
in helping them understand what engineering is all about. The WSU trebuchet contest provides
students with an excellent opportunity to experience the Engineering Design Process first hand.
What follows is a more detailed explanation of the Engineering Design Process and a sample
lesson plan for incorporating this process into the trebuchet competition preparations.
The Engineering Design Process
Although the Engineering Design Process is different than traditional analysis-type problem
solving, the steps used in solving a design problem are fairly similar. The main difference is that
the design process is often iterative and can require a lot of back tracking and re-working of
steps. In addition, multiple solutions to the problem usually result, and so the engineer must
decide on the most appropriate solution for the problem (Khandani, 2005).
One way to visualize the steps involved in the Engineering Design Process is illustrated in Figure
1.
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Step Eight: Redesign
Step One: Identify the
problem/need
Step Seven:
Communicate the
solution
Step Six: Test and
evaluate the solution
Step Two: Research
the problem/need
Step Three: Develop
possible solutions
(computer models are
often used)
Step Five: Construct a
prototype
Step Four: Select the
most appropriate
solution
(JETS, 2007)
Figure 1. Visualization of the Engineering Design Process. Note that step order may not be
sequential and may be iterative.
These steps might be applied to the trebuchet project as follows:
• Step 1: Identify the problem. Four trebuchets must be designed and built such that
they meet the checksheet requirements for the WSU trebuchet competition.
• Step 2: Research the problem. Learn about the history of trebuchets, the rules and
requirements of the trebuchet competition, the physics behind trebuchets, important
trebuchet design elements, computer models for trebuchet design, etc.
• Step 3: Develop possible solutions. Using results of research, sketch several possible
designs. Use a computer model to simulate designs performances.
• Step 4: Select the most appropriate solution. Through class and group discussions,
computer model simulation results, and personal judgment, decide on best design.
• Step 5: Construct a prototype. Build the design for one trebuchet.
• Step 6: Test and evaluate the solution. Check that the trebuchet meets checksheet
requirements. Check that the trebuchet throws distances as modeled in the computer
simulation. If the trebuchet fails any of these checks, hypothesize possible reasons for
the failures. If no reasons are determined, go to Step 2.
• Step 7: Communicate the solution. Continually document this process
photographically and with copious notes. Upon final completion of the project, a
technical report of the process should be written.
• Step 8: Redesign. Decide on design changes to make and modify the prototype. Once
the prototype meets design requirements, the remaining three trebuchets should be
constructed and tested.
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It is probably obvious that these eight steps cannot be done without going back to previous steps
and/or jumping steps non-sequentially. For example, perhaps the results of testing the prototype
trebuchet lead students to believe their originally chosen design solution is not the best. They
may jump back to Step 4 to choose one of their other designs, or they may jump back to Step 2 to
look more in depth at literature suggestions for designs. This is the essence of the Engineering
Design Process. For an even more detailed look at the Engineering Design Process, see the wellwritten paper Engineering Design Process by Seyyed Khandani, Ph.D. which is included in
Appendix A.
Works Referenced
JETS (2007). “The Engineering Design Process”,
<http://www.jets.org/ewb/cambodia_engineering_design_process.pdf> (Aug. 5, 2009).
Khandani, Seyyed Ph.D. (2005). “Engineering Design Process”,
<http://www.iisme.org/etp/HS%20Engineering-%20Engineering.pdf> (Aug. 5, 2009).
Contact Information
Please direct comments or questions about this document and lesson plans to Jeff Bohrer at
[email protected] or at 937-253-8188 x716.
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Sample Lesson Plan For Implementing The Engineering Design Process Into
The Wright State University Trebuchet Contest
Objective: The student(s) will be able to successfully prepare four trebuchets for the Wright
State University Trebuchet Contest through the implementation of the Engineering Design
Process.
Materials/Space Needed: Students will provide their own construction materials and tools.
Handouts in Appendices B, C, F, and G. School gymnasium will need to be reserved 1 to 2
weeks prior to the competition.
Implementation Notes: This lesson plan might be integrated into an existing unit (such as a
physics chapter on rotational motion or energy) to help apply material in a hands on way, it
might stand alone as a self contained enrichment project on engineering and the design process,
or it might be implemented as an after school project for students. How much of the project is to
be done in class vs. at home can be adjusted to individual needs.
Procedure:
1. EDP, Step One—Identify The Problem
a. Briefly explain that students will soon be competing in the WSU Trebuchet
Contest. The purpose of the contest is to successfully design and construct four
trebuchets that fulfill the constraints set forth in the contest rules.
2. The Engineering Design Process
a. Use the diagram in Appendix B to run a class discussion on the Engineering
Design Process.
b. Explain how the process will be applied to the design and construction of the
trebuchets. The document in Appendix A is a good reference for this discussion.
3. EDP, Step Two—Research The Problem
a. Use on hand material (pictures, videos, websites) to adequately inform students
about the competition. The NOVA special, Secrets of Lost Empires II—Medieval
Siege, is a good video to introduce trebuchets and to motivate students.
b. Pass out Research Assignment Handout (Appendix C).
i. Strongly encourage students to begin researching design elements of the
trebuchet as well.
ii. The next day run a discussion of any questions they may have of the
competition rules.
c. Classroom education of physics principles involved in the operation of trebuchets
(i.e. rotational motion concepts) may also be completed here.
4. EDP, Step Three—Develop Possible Solutions
a. Direct student teams either in the classroom or on their own to go through their
researched material and begin sketching out some possible design solutions. By
the end of this step, teams should have sketches of their designs or construction
drawings (if they have drafting skills).
i. To make sure teams complete this, require students to begin turning in
progress reports (Step Seven).
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1. A progress report could include a discussion (what and why) of
designs under consideration, sketches of the designs, progress or
changes made since the last progress report and/or a discussion of
the next steps anticipated in the design process.
2. Have students turn in progress reports as often as deemed
necessary.
b. Computer model
i. Direct students to visit http://www.algobeautytreb.com/ and download
WinTreb.
1. Explain to students that engineers often begin their problem
solving by and/or narrowing of proposed solutions by running
computer models of designs. Computer models can give a general
idea of what to expect in the performance of a physically
constructed trebuchet.
2. Have students explore the software program and read the Help
menus.
3. Once students are familiar with WinTreb, they should use it to help
determine the dimensions for their possible design solutions.
c. Cost Estimates (optional)
i. One way engineers can help decide on the most feasible design solution is
to complete cost estimates for their solution options. For the trebuchet
contest, a cost estimate of all or some of the leading possible design
solutions may help students narrow their choice for the best design.
ii. Direct students to choose a few of their best design options, and create
cost estimates.
1. Appendix D contains reference information on creating cost
estimates.
5. EDP Step Four—Select The Most Appropriate Solution
a. Direct teams to look at the possible design solutions they came up with and
discuss among themselves which solution they would like to construct as a
prototype.
i. They should use their judgment and the results of their computer
modeling, cost estimates, and research to help them decide.
ii. Remind students that in the EDP, the chosen design may not be the final
design. They may be coming back to this step at a later time to decide on
a different design if this one doesn’t work.
b. Students should turn in a copy of the sketch or construction drawing of the chosen
design, and they should provide an explanation of their reasons for their selection
(Step Seven).
6. EDP Step Five—Construct a Prototype
a. Teams use their sketches to construct a prototype trebuchet.
i. This should most likely be done at home.
ii. Tell students they should be taking photographic evidence of the
construction process and of other important events for the remainder of the
design process. These photos can be used in the technical report at the end
of the project.
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7. EDP Step Six—Test and evaluate the solution.
a. Teams should begin testing their trebuchet to see if it complies with the minimum
throwing distance requirements of the competition rules.
i. Results may be observation based (i.e. do there appear to be any problem
areas during the throws, are throws approximately the minimum throwing
distance), or more advanced students may wish to take data (i.e. throwing
distance changes while changing one variable at a time)
b. Comparisons to the WinTreb model results can also be made.
i. Classroom discussion could ensue regarding model results vs. reality
1. Discussion could include topics like assumptions made by the
model or aspects that could affect trebuchet performance that are
not included in the model (such as angle of the sling hook, type of
material used for hook, etc.)
c. At this point it is very likely that the trebuchet is not performing as planned, and it
may not be throwing forward at all. This is the point where the Engineering
Design Process really comes into play.
i. Based on their observations of or data from the tests, teams should begin
to decide how their trebuchet can be improved.
ii. Appendix E is a brief discussion of several factors/physics principles that
students may wish to consider in coming up with design changes.
d. Students may need to go to Step Eight, Redesign, and/or return to Steps Two,
Three, or Four depending on their situation.
i. This process may need repeated several times.
e. Once teams are satisfied with the performance of their prototype, they should
return to Steps Five and Six and build their remaining three trebuchets.
f. During the last two weeks before competition, teams are required to bring in all
four trebuchets.
i. Class time will be used to work with trebuchets in the gym.
ii. Use check sheets and check trebuchets in for each team. For trebuchets
that do not completely qualify, students need to go to Step Eight to make
design adjustments
iii. While checking in trebuchets, teams should be working to make last
minute adjustments.
iv. Once trebuchets are all qualified, students can use remaining time to begin
to practice for the competition
1. It is at this time that teams can begin to perfect aiming and
strategy.
v. If there is remaining time before the competition and all teams are checked
in, a practice competition may be conducted.
8. EDP Step Seven—Communicate the Solution
a. Progress reports, preliminary construction drawings, and other assignments all fit
under Step Seven. Students should often be returning to this step throughout the
design process.
b. Assign an actual cost analysis at the end of the project. This looks just like a cost
estimate, except the items, quantities, and prices are the actual ones used in the
project.
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i. Discuss the results of the actual cost analysis vs. the cost estimate.
1. Discuss possible reasons why actual costs are/are not the same as
estimated.
ii. This cost analysis will be used in the technical report assignment
c. At the end of the design process, an optional report may be assigned.
i. This is technical writing, and some hints as well as a template for the
report are presented in Appendix F.
9. EDP Step Eight—Redesign
a. This step is actually referenced in the earlier steps of this lesson plan. Students
will be doing a fair amount of redesign throughout the Engineering Design
Process.
10. Competition
a. Competition day at Wright State University is a chance for students to implement
their designs that they’ve worked so hard to construct.
b. At this point, the goals of the EDP lesson have been fulfilled, and it is time to
enjoy a day of competition and fun.
11. Assessment
a. Assessing the project can be difficult, since most of the work will most likely take
place at home. A suggestion is to break the grade up into two main parts:
i. Technical Report
ii. Peer Review
b. Appendix G contains information on assessing the project.
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Appendix A
Engineering Design Process by Seyyed Khandani, Ph.D.
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ENGINEERING DESIGN PROCESS
Education Transfer Plan
Prepared by
Seyyed Khandani, Ph.D.
[email protected]
August 2005
IISME/Solectron/2005
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Seyyed Khandani
TABLE OF CONTENTS
PREFACE
3
BACKGROUND
4
ENGINEERING DESIGN
4
THE DESIGN PROCESS
5
1. DEFINE THE PROBLEM
6
2. GATHER PERTINENT INFORMATION
9
3. GENERATE MULIPLE SOLUTIONS
11
4. ANALYZE AND SELECT A SOLUTION
12
5. TEST AND IMPLEMENT THE SOLUTION
19
REFERENCE
23
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PREFACE
This document is designed as an introduction to how engineering products are designed.
It is intended for use in an introductory design course in engineering with the objective of
providing some hands-on experience for people interested in exploring engineering
disciplines.
This document is prepared based on the experience of the author* while completing a
summer fellowship at Solectron Corporation in Milpitas, California. This fellowship was
coordinated by the Industry Initiatives for Science and Math Education (IISME) in 2005.
I would like to specially thank Mr. Hoshang Vaid, as my principal mentor, at Solectron
Corporation whose continuous support and guidance has made my fellowship experience
very productive and educational. Furthermore, I would like to extend my appreciation to
the other members of the Design and Engineering department at Solectron Corporation
for making my experience pleasant.
__________________________________________________________________ _____
Seyyed Khandani has a Ph.D. in Mechanical Engineering from MIT and is currently a
professor of engineering at Diablo Valley College in Pleasant Hill, California.
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BACKGROUND
If you take a moment to observe your surroundings, you will see examples of
technological creativity. The physical objects you see, whether they are telephones,
automobiles, bicycles, or electric appliances, all came into being through the creative
application of technology. These everyday inventions did not miraculously appear but
originated in the minds of human beings and took time to develop. Engineering is the
creative process of turning abstract ideas into physical representations (products or
systems). What distinguishes engineers from painters, poets, or sculptors is that engineers
apply their creative energies to producing products or systems that meet human needs.
This creative act is called design.
ENGINEERING DESIGN
Most engineering designs can be classified as inventions-devices or systems that are
created by human effort and did not exist before or are improvements over existing
devices or systems. Inventions, or designs, do not suddenly appear from nowhere. They
are the result of bringing together technologies to meet human needs or to solve problems.
Sometimes a design is the result of someone trying to do a task more quickly or
efficiently. Design activity occurs over a period of time and requires a step-by-step
methodology.
We described engineers primarily as problem solvers. What distinguishes design from
other types of problem solving is the nature of both the problem and the solution. Design
problems are open ended in nature, which means they have more than one correct
solution. The result or solution to a design problem is a system that possesses specified
properties.
Design problems are usually more vaguely defined than analysis problems. Suppose that
you are asked to determine the maximum height of a snowball given an initial velocity
and release height. This is an analysis problem because it has only one answer. If you
change the problem statement to read, "Design a device to launch a 1-pound snowball to
a height of at least 160 feet," this analysis problem becomes a design problem. The
solution to the design problem is a system having specified properties (able to launch a
snowball 160 feet), whereas the solution to the analysis problem consisted of the
properties of a given system (the height of the snowball). The solution to a design
problem is therefore open ended, since there are many possible devices that can launch a
snowball to a given height. The original problem had a single solution: the maximum
height of the snowball, determined from the specified initial conditions.
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Solving design problems is often an iterative process: As the solution to a design problem
evolves, you find yourself continually refining the design. While implementing the
solution to a design problem, you may discover that the solution you've developed is
unsafe, too expensive, or will not work. You then "go back to the drawing board" and
modify the solution until it meets your requirements. For example, the Wright brothers'
airplane did not fly perfectly the first time. They began a program for building an airplane
by first conducting tests with kites and then gliders. Before attempting powered flight,
they solved the essential problems of controlling a plane's motion in rising, descending,
and turning. They didn't construct a powered plane until after making more than 700
successful glider flights. Design activity is therefore cyclic or iterative in nature, whereas
analysis problem solving is primarily sequential.
The solution to a design problem does not suddenly appear in a vacuum. A good solution
requires a methodology or process. There are probably as many processes of design as
there are engineers. Therefore, this lesson does not present a rigid "cookbook" approach
to design but presents a general application of the five-step problem-solving methodology
associated with the design process. The process described here is general, and you can
adapt it to the particular problem you are trying to solve.
THE DESIGN PROCESS
The basic five-step process usually used in a problem-solving works for design problems
as well. Since design problems are usually defined more vaguely and have a multitude of
correct answers, the process may require backtracking and iteration. Solving a design
problem is a contingent process and the solution is subject to unforeseen complications
and changes as it develops. Until the Wright brothers actually built and tested their early
gliders, they did not know the problems and difficulties they would face controlling a
powered plane.
The five steps used for solving design problems are:
1. Define the problem
2. Gather pertinent information
3. Generate multiple solutions
4. Analyze and select a solution
5. Test and implement the solution
The first step in the design process is the problem definition. This definition usually
contains a listing of the product or customer requirements and specially information
about product functions and features among other things. In the next step, relevant
information for the design of the product and its functional specifications is obtained. A
survey regarding the availability of similar products in the market should be performed at
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this stage. Once the details of the design are clearly identified, the design team with
inputs from test, manufacturing, and marketing teams generates multiple alternatives to
achieve the goals and the requirements of the design. Considering cost, safety, and other
criteria for selection, the more promising alternatives are selected for further analysis.
Detail design and analysis step enables a complete study of the solutions and result in
identification of the final design that best fits the product requirements. Following this
step, a prototype of the design is constructed and functional tests are performed to verify
and possibly modify the design.
When solving a design problem, you may find at any point in the process that you need to
go back to a previous step. The solution you chose may prove unworkable for any
number of reasons and may require redefining the problem, collecting more information,
or generating different solutions. This continuous iterative process is represented in the
following Figure.
Define the
Problem
Test and
Implement
solution
Gather
Information
Engineering
Design
Process
Generate
multiple
solutions
Analyze
and select
a solution
This document intends to clarify some of the details involved in implementing the design
process. Therefore a description of the details involved in each step of the design process
is listed below. Although the descriptions of the activities within each step may give the
impression that the steps are sequential and independent from each other, the iterative
nature of the application of the process should be kept in mind throughout the document.
1. DEFINE THE PROBLEM
You need to begin the solution to a design problem with a clear, unambiguous definition
of the problem. Unlike an analysis problem, a design problem often begins as a vague,
abstract idea in the mind of the designer. Creating a clear definition of a design problem
is more difficult than, defining an analysis problem. The definition of a design problem
may evolve through a series of steps or processes as you develop a more complete
understanding of the problem.
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Identify and Establish the Need
Engineering design activity always occurs in response to a human need. Before you can
develop a problem definition statement for a design problem, you need to recognize the
need for a new product, system, or machine. Thomas Newcomen saw the need for a
machine to pump the water from the bottom of coal mines in England. Recognizing this
human need provided him the stimulus for designing the first steam engine in 1712.
Before engineers can clearly define a design problem, they must see and understand this
need.
Although engineers are generally involved in defining the problem, they may not be the
ones who initially recognize the need. In private industry, market forces generally
establish the need for a new design. A company's survival depends on producing a
product that people will buy and can be manufactured and sold at a profit. Ultimately,
consumers establish a need, because they will purchase and use a product that they
perceive as meeting a need for comfort, health, recreation, transportation, shelter, and so
on. Likewise, the citizens of a government decide whether they need safe drinking water,
roads and highways, libraries, schools, fire protection, and so on.
The perceived need, however, may not be the real need. Before you delve into the details
of producing a solution, you need to make sure you have enough information to generate
a clear, unambiguous problem definition that addresses the real need. The following
example illustrates the importance of understanding the need before attempting a solution.
Example: Automobile Airbag Inflation - How Not to Solve a Problem
A company that manufactures automobile airbags has a problem with an unacceptably
high rate of failure in the inflation of the bag. During testing, 10 percent of the bags do
not fully inflate. An engineer is assigned the job of solving the problem. At first the
engineer defines the problem as a failure in the materials and construction of the inflation
device. The engineer begins to solve this problem by producing a more robust inflation
device. After considerable effort, the engineer discovers that improving the inflation
device does not change the failure rate in the bags. Eventually, this engineer re-examines
the initial definition of the problem. The company investigates the airbag inflation
problem further and discovers that a high degree of variability in the tightness of folds is
responsible for the failure of some bags to inflate. At the time the bags were folded and
packed by people on an assembly line. With a more complete understanding of the need,
the engineer redefined the problem as one of increasing the consistency in tightness of the
folds in the bags. The final solution to this problem is a machine that automatically folds
the bags.
Often the apparent need is not the real need. A common tendency is to begin generating a
solution to an apparent problem without understanding the problem. This approach is
exactly the wrong way to begin solving a problem such as this. You would be generating
solutions to a problem that has never been defined.
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People have a natural tendency to attack the current solution to a problem rather than the
problem itself. Attacking a current solution may eliminate inadequacies but will not
produce a creative and innovative solution. For example, the engineer at the airbag
company could have only looked at the current method for folding airbags-using humans
on an assembly line. The engineer might have solved the problem with inconsistent
tightness by modifying the assembly line procedure. However, the final solution to the
problem proved to be more cost effective and reliable, in addition to producing a superior
consistency in the tightness of the folds.
Develop a Problem Statement
The first step in the problem-solving process, therefore, is to formulate the problem in
clear and unambiguous terms. Defining the problem is not the same as recognizing a need.
The problem definition statement results from first identifying a need. The engineer at the
airbag company responded to a need to reduce the number of airbag inflation failures. He
made a mistake, however, in not formulating a clear definition of the problem before
generating a solution. Once a need has been established, engineers define that need in
terms of an engineering design problem statement. To reach a clear definition, they
collect data, run experiments, and perform computations that allow that need to be
expressed as part of an engineering problem-solving process.
Consider for example the statement "Design a better mousetrap." This statement is not an
adequate problem definition for an engineering design problem. It expresses a vague
dissatisfaction with existing mousetraps and therefore establishes a need. An engineer
would take this statement of need and conduct further research to identify what was
lacking in existing mousetrap designs. After further investigation the engineer may
discover that existing mousetraps are inadequate because they don't provide protection
from the deadly Hantavirus carried by mice. Therefore, a better mousetrap may be one
that is sanitary and does not expose human beings to the Hantavirus. From this need, the
problem definition is modified to read, "Design a mousetrap that allows for the sanitary
disposal of the trapped mouse, minimizing human exposure to the Hantavirus."
The problem statement should specifically address the real need yet be broad enough not
to preclude certain solutions. A broad definition of the problem allows you to look at a
wide range of alternative solutions before you focus on a specific solution. The
temptation at this point in the design process is to develop a preconceived mental
"picture" of the problem solution. For example, you could define the better mousetrap
problem as "Design a mousetrap that sprays the trapped mouse with disinfectant." This
statement is clear and specific, but it is also too narrow. It excludes many potentially
innovative solutions. If you focus on a specific picture or idea for solving the problem at
this stage of the design process, you may never discover the truly innovative solutions to
the problem. A problem statement should be concise and flexible enough to allow for
creative solutions.
Here is one possible problem definition statement for our better mousetrap problem:
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A Better Mousetrap: Certain rodents such as the common mouse are carriers and
transmitters of an often fatal virus, the Hantavirus. Conventional mousetraps
expose people to this virus as they handle the trap and dispose of the mouse.
Design a mousetrap that allows a person to trap and dispose of a mouse without
being exposed to any bacterial or viral agents being carried on the mouse.
Establish Criteria for Success
Criteria for success are the specifications a design solution must meet or the attributes it
must possess to be considered successful. You should include criteria in the problem
statement to provide direction toward the solution. At this point in the design process, the
criteria are preliminary. As the design solution develops, you will most likely find that
the initial criteria need to be redefined or modified. Preliminary criteria must not be too
specific so they allow flexibility through the design process.
The criteria that apply to a particular design problem are based on your background
knowledge and the research that you've conducted. Since each problem or project is
unique, the desirable attributes, or criteria, of the solution are also unique. Some criteria
are unimportant to the success of the design. The list of criteria is developed by the
design team. The design team is made up of people from various engineering
backgrounds that have expertise pertinent to the problem. This team may also include
people from backgrounds other than engineering, such as managers, scientists, and
technicians. The design team must evaluate each criterion and decide if it is applicable to
the design effort. Later in the design process, value judgments must be applied to the list
of criteria. Therefore, it makes little sense to include those criteria that will be of
relatively low priority in the evaluation of design solutions. For example, if you were
designing a critical life support system, you would not include the criterion of "must be
minimum cost," because cost is not an important factor in evaluating this design.
The following is a list of preliminary criteria for a better mousetrap design. This list
would be included in the problem definition statement.
•
•
•
•
•
•
•
The design must be low cost.
The design should be safe, particularly with small children.
The design should not be detrimental to the environment.
The design should be aesthetically pleasing.
The design should be simple to operate, with minimum human effort.
The design must be disposable (you don't reuse the trap).
The design should not cause undue pain and suffering for the mouse.
2. GATHER PERTINENT INFORMATION
Before you can go further in the design process, you need to collect all the information
available that relates to the problem. Novice designers will quickly skip over this step and
proceed to the generation of alternative solutions. You will find, however, that effort
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spent searching for information about your problem will pay big dividends later in the
design process. Gathering pertinent information can reveal facts about the problem that
result in a redefinition of the problem. You may discover mistakes and false starts made
by other designers. Information gathering for most design problems begins with asking
the following questions. If the problem addresses a need that is new, then there are no
existing solutions to the problems, so obviously some of the questions would not be
asked.
•
•
•
•
•
•
•
•
•
Is the problem real and its statement accurate?
Is there really a need for a new solution or has the problem already been solved?
What are the existing solutions to the problem?
What is wrong with the way the problem is currently being solved?
What is right about the way the problem is currently being solved?
What companies manufacture the existing solution to the problem?
What are the economic factors governing the solution?
How much will people pay for a solution to the problem?
What other factors are important to the problem solution (such as safety,
aesthetics and environmental issues)?
Search for Information Resources
As an engineering student in the 2000s you have many more sources of information
available to you than engineers did only 20 years ago. This section discusses some of the
most current resources available, but because our world is witnessing an information
explosion, by the time you read this many more resources will be available that are not
mentioned here.
Traditional publications are still an essential source of information to engineers and
scientists. However, electronic information transfer and retrieval are quickly becoming a
standard source for engineers and scientists. When you begin a search for information
relating to a design problem, you must be prepared to go to many different sources. The
library is still the primary source of information for an engineering student. Your success
as an engineer and student will be enhanced if you are able to use the library effectively.
For specific help on using our library, you should consult the library staff at the college;
they probably offer courses or seminars on library usage.
Some of the common resources available at a university library are discussed below:
Scientific encyclopedias and technical handbooks. These sources are a good place
to start when you are investigating an area or problem that is new to you. An
encyclopedia or handbook provides a brief general overview by an authority in a
particular field and includes references for more detailed information. The
McGraw-Hill Encyclopedia of Science and Technology covers all scientific fields.
Technical handbooks, such as the Electrical Engineers Handbook or Mark's
Handbook of Mechanical Engineering, cover various fields such as chemical, civil,
electrical, or mechanical engineering. The information contained in these
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Seyyed Khandani
handbooks is presented in a very concise form and can be good starting points for
an in depth search.
Electronic catalog. Electronic catalogs give a listing of all the sources available at
your library. The catalogs allow search by subject matter, author, or title. They
give a brief summary of the book's content, including the title, author, publisher,
copyright date, and total number of pages. The call number tells you where to
locate the book in your library.
Indexes. Indexes categorize current works in various disciplines. They list the
subject, title, and author of recent articles in technical and trade journals under
various subject headings. Some indexes include brief abstracts of articles. Most
indexes are updated monthly, so a complete search through an index may be
tedious. A familiar index to scientists and engineers is the Index of Applied
Science and Technology. It lists articles from 335 journals and is updated monthly.
The Engineering Index is another popular index for engineers. It selects articles
from approximately 2700 journals and periodic publications and includes an
abstract of each article.
The Internet. There is a wealth of information on the Internet from a variety of
sources. Manufacturers, professional and trade organizations, suppliers of
products, and many government agencies have valuable resources on their
websites. Search engines such as Google(www.google.com) and
Teoma(www.teoma.com) offer tools to locate relevant information quickly and
efficiently.
3. GENERATE MULTIPLE SOLUTIONS
The next step in the design process begins with creativity in generating new ideas that
may solve the problem. Creativity is much more than just a systematic application of
rules and theory to solve a technical problem.
You start with existing solutions to the problem and then tear them apart-find out what's
wrong with those solutions and focus on how to improve their weaknesses. Consciously
combine new ideas, tools, and methods to produce a totally unique solution to the
problem. This process is called synthesis. Casey Golden, age 13, did this when he
invented the BIOtee. Casey noticed that discarded and broken wooden golf tees littered
golf courses, damaging the blades and tires of lawn mowers. He decided to design a new
biodegradable tee. After experimenting with different mixtures, he devised a recipe made
of recycled paper fiber and food byproducts coated with a water-soluble film. When the
film is broken, moisture in the ground breaks down the tee within 24 hours. As a result of
his creative efforts, Casey's family started a company to manufacture BIOtees producing
several million tees per year.
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Seyyed Khandani
Psychological research has found no correlation between intelligence and creativity.
People are creative because they make a conscious effort to think and act creatively.
Everybody has the potential to be creative. Creativity begins with a decision to take risks.
Listed below are a few characteristics of creative people. These are not rigid rules to be
followed to experience creativity. You can improve your creative ability by choosing to
develop these characteristics in yourself.
•
•
•
•
•
•
Curiosity and tolerance of the unknown. Creative people have a positive curiosity
of the unknown. They are not afraid of what they don't understand.
Openness to new experiences. Creative people have a healthy and positive attitude
toward new experiences.
Willingness to take risks. Creative people are not afraid to take risks and try new
experiences or ideas, knowing that they may be misunderstood and criticized by
others. They are self-confident and not afraid to fail.
Ability to observe details and see the "whole picture." Creative people notice and
observe details relating to the problem, but they also can step back and see the
bigger picture.
No fear of problems. Creative people are not afraid to tackle complex problems,
and they even search for problems to solve. They seek solutions to problems with
their own abilities and experience if possible. They have the attitude of "if you
want something done, you'd better do it yourself."
Ability to concentrate and focus on the problem until it's solved. Creative people
can set goals and stick to them until they're reached. They focus on a problem and
do not give up until the problem is solved. They have persistence and tenacity.
Solutions to engineering design problems do not magically appear. Ideas are generated
when people are free to take risks and make mistakes. Brainstorming at this stage is often
a team effort in which people from different disciplines are involved in generating
multiple solutions to the problem.
4. ANALYZE AND SELECT A SOLUTION
Once you've conceived alternative solutions to your design problem, you need to analyze
those solutions and then decide which solution is best suited for implementation.
Analysis is the evaluation of the proposed designs. You apply your technical knowledge
to the proposed solutions and use the results to decide which solution to carry out. You
will cover design analysis in more depth when you get into upper-level engineering
courses.
At this step in the design process, you must consider the results of your design analysis.
This is a highly subjective step and should be made by a group of experienced people.
This section introduces a systematic methodology you can use to evaluate alternative
designs and assist in making a decision.
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Seyyed Khandani
Analysis of Design Solutions
Before deciding which design solution to implement, you need to analyze each
alternative solution against the selection criteria defined in step l. You should perform
several types of analysis on each design. Every design problem is unique and requires
different types of analysis. The following is a list of analysis that may need to be
considered; bear in mind that the importance of each varies depending on the nature of
the problem and the solution.
•
•
•
•
•
•
•
•
Functional analysis
Industrial design/Ergonomics
Mechanical/Strength analysis
Electrical/Electromagnetic
Manufacturability/Testability
Product safety and liability
Economic and market analysis
Regulatory and Compliance
The following paragraphs provide details of some of these analysis types.
Functional analysis. This part determines whether the given design solution will
function the way it should. Functional analysis is fundamental to the evaluation and
success of all designs. A design solution that does not function properly is a failure even
if it meets all other criteria. Consider for example the invention of the ballpoint pen. This
common instrument was first invented and manufactured during World War II. The
ballpoint pen was supposed to solve the problems of refilling and messiness inherent to
the fountain pen. Unfortunately, this new design had never been evaluated for
functionality. The early pens depended on gravity for the ink to flow to the roller ball.
This meant that the pens only worked in a vertical upright position, and the ink flow was
inconsistent: Sometimes it flowed too heavily, leaving smudgy blotches on the paper;
other times the flow was too light and the markings were unreadable. The first ballpoint
pens tended to leak around the ball, ruining people's clothes. An elastic ink developed in
1949, allowed the ink to flow over the ball through smooth capillary action. Not until the
1950s did the ballpoint pen finally become a practical writing instrument, thanks to
proper ink and engineering. Economy, appearance, durability, and marketability of a
design are unimportant if the product does not function properly.
Ergonomics. Ergonomics is the human factor in engineering. It is the study of how
people interact with machines. Most products have to work with people in some manner.
People occupy a space in or around the design, and they may provide a source of power
or control or act as a sensor for the design. For example, people sense if an automobile
air-conditioning system is maintaining a comfortable temperature inside the car. These
factors form the basis for human factors, or ergonomics, of a design.
A design solution can be considered successful if the design fits the people using it. The
handle of a power tool must fit the hand of everybody using it. The tool must not be too
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Seyyed Khandani
heavy or cumbersome to be manipulated by all sizes of people using the tool. The
geometric properties of people-their weight, height, reach, circumference, and so on-are
called anthropometric data. The difficulty in designing for ergonomics is the abundance
of anthropometric data. The military has collected and evaluated the distribution of
human beings and published this information in military standard tables. A successful
design needs to be evaluated and analyzed against the distribution of geometry of the
people using it. The following Figure shows the geometry of typical adult males and
females for the general population in millimeters. Since people come in different sizes
and shapes, such data are used by design engineers to assure that their design fits the user.
A good design will be adjustable enough to fit 95 percent of the people who will use it.
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Product Safety and Liability. The primary consideration for safety in product design is
to assure that the use of the design does not cause injury to humans. Safety and product
liability issues, however, can also extend beyond human injury to include property
damage and environmental damage from the use of your design. Engineers must also
consider the issues of safety in design because of liability arising from the use of an
unsafe product. Liability refers to the manufacturer of a machine or product being liable,
or financially responsible, for any injury or damage resulting from the use of an unsafe
product.
The only way to assure that your design will not cause injury or loss is to design safety
into the product. You can design a safe product in three ways. The first method is to
design safety directly into the product. Ask yourself, "Is there any probability of injury
during the normal use and during failure of your design?" For example, modern downhill
ski bindings use a spring-loaded brake that brakes the ski automatically when the ski
disengages from the skier's boot. Older ski bindings used an elastic cable attached to the
skier's ankle, but this had a tendency to disconnect during a severe fall.
Inherent safety is impossible to design into some products, such as rotating machinery
and vehicles. In such cases you use the second method of designing for safety: You
include adequate protection for users of the product. Protection devices include safety
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Seyyed Khandani
shields placed around moving and rotating parts, crash protective structures used in
vehicles, and "kill" switches that automatically turn a machine off (or on) if there is
potential for human injury. For example, new lawnmowers generally include a protective
shield covering the grass outlet and include a kill switch that turns the motor off when the
operator releases the handle.
The third method used in considering safety is the use of warning labels describing
inherent dangers in the product. Although this method does not implement safety in
design, it is primarily used as a way to shift the responsibility to the consumer for having
ignored the safety guidelines in using the product. In most cases, however, a warning
label will not protect you from liability. Protective shields or other devices must be
included in the design.
A product liability suit may be the result of a personal injury due to the operation of a
particular product. The manufacturer and designer of a device can be found liable to
compensate a worker for losses incurred during the operation or use of their product.
During a product liability trial, the plaintiff attempts to show that the designer and
manufacturer of a product are negligent in allowing the product to be put on the
market. The plaintiff's attorney may bring charges of negligence against the designer.
To protect themselves in a product liability trial, engineers must use state-of-the-art
design procedures during the design process. They must keep records of all calculations
and methods used during the design process. Safety considerations must be included in
the criteria for all design solutions. The designer must also foresee other ways people
could use the product. If a person uses a shop vacuum to remove a gasoline spill, is the
designer responsible when the vacuum catches fire? The courts can decide that a design is
poor if the engineer did not foresee improper use of the product. It is imperative that you
evaluate all of your alternative solutions against safety considerations. Reject or modify
any unsafe elements of your design at this stage in the design process.
Economic and Market Analysis. The net result or purpose of most engineering designs
is to produce a product that generates a profit for the company. Obviously, each
alternative design has to be evaluated against criteria such as sales features, potential
market, cost of manufacturing, advertising, and so on. Large companies often conduct
marketing surveys to obtain a measure of what the public will buy. These surveys may be
conducted by telephone interviews with randomly selected people, or they may be
personal interviews conducted with potential users of a product. Our society is based on
economics and competition. Many good ideas never get into production because the
manufacturing costs exceed what people will pay for the product. Market analysis
involves applying principles of probability and statistics to determine if the response of a
selected group of people represents the opinion of society as a whole. Even with a good
marketing survey, manufacturers never know for certain if a new product will sell.
Mechanical/Strength Analysis. Engineering analysis of a preliminary design often
include the analysis of its mechanical features. The engineer conducts mechanical
analysis to answer questions such as, "Will the device or structure support the maximum
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Seyyed Khandani
loads that it will be subjected to?" You must also determine the effect of shocks and
repetitive or dynamic loading over the life of the product. Many systems generate heat, so
you need to determine if the design can dissipate all of the heat being generated during
normal operation. Thermal analysis is an area important to the design of electronic
equipment. Many pieces of electronic equipment fail prematurely due to inadequate heat
transfer. For example, the early releases of Intel's Pentium microprocessor could not
operate at their rated speed due to overheating. The production of this microcircuit was
delayed while engineers figured out ways to dissipate the excess heat.
You need to perform strength calculations to determine whether the design alternative
will be able to support the specified mechanical loads. As a mechanical system is
subjected to applied loads, it will deform or deflect.
Many products contain several subsystems and, quite often, the evaluation is done on
each of the subsystems rather than the complete product itself.
The Decision Process
After analyzing your alternative solutions, you need to decide and document which
design solution is the best. You will refine and develop the best solution in more detail
during the later stages of the design process. At this stage, to evaluate each solution
objectively against the stated design criteria or requirements, you need a quantitative
basis for judging and evaluating each design alternative. One widely used method to
formalize the decision-making process is the decision matrix. The decision matrix is a
mathematical tool you can use to derive a number that specifies and justifies the best
decision.
The first step in creating a decision matrix is for the design team to rank, in order of
importance, the desirable attributes or criteria for the design solution. These attributes can
include factors such as safety, manufacturing considerations, the ease of fabrication and
assembly, cost, portability, compliance with government regulations, etc. You then assign
to each attribute or criteria a value factor related to the relative importance of that
attribute. For example, suppose you decide that safety is twice as important to the success
of your design as cost. You would assign a value factor of 20 for safety and a value factor
of 10 for cost. You assign value factors on a basis of 0 to 100, representing relative
importance of each criterion to the decision.
Next you evaluate each design alternative against the stated criteria. A rating factor is
assigned to each solution, based on how well that solution satisfies the given criterion.
The rating factor is on a scale of 0 to 10, with 10 representing a solution that satisfies the
given criterion the best. To make an accurate evaluation, you need as much information
as possible. Unfortunately, engineers seldom have enough information to make a
"perfect" evaluation. If you have done the analysis phase of the design process properly,
those results can provide a basis for evaluation. Computer models and prototypes can also
yield valuable information to assist in the decision phase. In most cases you must use
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engineering judgment, and the decision is subjective. The following example illustrates
the use of a decision matrix in deciding the best alternative design for a can crusher.
Example: Aluminum Can Crusher
Students are being asked to design a simple device to crush aluminum cans. A student
design team proposes four solutions to the problem. They develop six criteria that are
important to a successful design. The student team agrees that the most important criteria
(or desirable attributes) of the design and assigned weights are
•
•
•
•
•
•
Safety: 30 percent (30 points)
Ease of use: 20 percent (20 points)
Portability: 20 percent (20 points)
Durability and strength:10 percent (10 points)
Use of standard parts:10 percent (10 points)
Cost:10 percent (10 points)
This team also proposes four alternative solutions to this problem, which are illustrated in
the following Figure. They are
1.
2.
3.
4.
A spring-loaded crusher
A foot-operated device
A gravity-powered dead weight crusher
An arm-powered lever arm crusher
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Seyyed Khandani
After analyzing each solution against the six criteria, the team evaluates each design
alternative. After assigning a rating factor to each design alternative for each of the
specified criteria, the team multiplies the rating factor by the value factor. The product of
the value and rating factors is then summed down the column for each design alternative.
The total sum at the bottom of each column determines the best design alternative. The
results of this decision matrix are illustrated in the following Table.
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Although rating each design against the six stated criteria is subjective, the rating factor
for each design alternative is assigned according to the consensus of the design team. The
results of an analysis are used to evaluate and rate each design. The rating factor R is
assigned according to the following scale:
•
•
•
•
•
Excellent 9-10
Good 7-8
Fair 5-6
Poor 3-4
Unsatisfactory 0-2
Design 4 was chosen the best design largely due to the rating assigned for safety,
criterion l. The team felt that the chances of human injury were negligible for this design.
Since safety is the most important factor (30% of the total weight), the high safety rating
for design 4 gives it the highest overall score (9 x 30, or 270).
5. TEST AND IMPLEMENT THE SOLUTION
The final phase of the design process is implementation, which refers to the testing,
construction, and manufacturing of the solution to the design problem. You must consider
several methods of implementation, such as prototyping and concurrent engineering, as
well as distinct activities that occur during implementation, such as documenting the
design solution and applying for patents.
Prototyping. The first stage of testing and implementation of a new product, called
prototyping, consists of building a prototype of the product-the first fully operational
production of the complete design solution. A prototype is not fully tested and may not
work or operate as intended. The purpose of the prototype is to test the design solution
under real conditions. For example, a new aircraft design would first be tested as a scale
model in a wind tunnel. Wind tunnel tests would generate information to be used in
constructing a full-size prototype of the aircraft. Test pilots then fly the prototype
extensively under real conditions. Only after testing under all expected and unusual
operating conditions are the prototypes brought into full production.
Concurrent Engineering. Traditional design practices are primarily serial or sequential:
Each step in the process is completed in order or sequence only after the previous steps
have been completed. The implementation of the design occurs after a prototype or model
is created from engineering drawings. A machinist working from the engineering
drawings generated by a drafter, or an engineer, makes the prototype. Only after creating
a prototype of the design would the engineer discover that a hole was too small, parts
didn't mate properly, or a handgrip was misplaced. The part would have to be redesigned
and the process completed until a satisfactory solution was reached.
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In the competitive manufacturing climate of the 1990s, the serial practice of design
proved inadequate. In a matter of months, a manufacturer may find that factors such as
markets, material prices and technology, and government regulations and tax laws may
have changed. This competitive environment required a company to design high-quality
products faster, better, and less expensively than their competitors. One solution to the
traditional design paradigm was concurrent engineering.
Concurrent engineering is the ability to implement parallel design and analysis in which
safety, manufacturability, serviceability, marketability, and compliance issues are
considered early on and during the process. Concurrent engineering is however possible
through the application of modern computer-aided design (CAD), analysis, and
manufacturing software. A designer starts with an idea of a new product in which the
above factors are considered and uses the CAD software to create a preliminary design.
With the appropriate software, the preliminary design can also be analyzed for
functionality as the design is being created. Using the results of this analysis, the designer
then makes any necessary modifications and reanalyzes the computer model. An engineer
designing a bicycle frame, for example, would use concurrent engineering to minimize
the weight and maximize the supported loads in a new frame design. The engineer would
first create a design and model the physical behavior of the frame on the computer before
actually manufacturing the frame.
The next stage in concurrent engineering is called rapid prototyping or sometimes called
"art to part." Here the three-dimensional computer model of the finished design is used
with computer-aided manufacturing (CAM) software to drive appropriate machinery to
physically create the part. The entire design cycle therefore becomes nearly paperless.
Engineers can go from design to prototype in a matter of days, instead of weeks or
months as with earlier serial design practices. Since design is an iterative process,
concurrent engineering significantly shortens the time between iterations. A product can
therefore get to market much quicker, at a lower cost, and with a higher quality.
Documentation. One of the most important activities in design is documenting your work,
clearly communicating the solution to your design problem so someone else can
understand what you have created. Usually this consists of a design or technical report.
Communicating the solution to a design problem through language, both written and oral,
is a vital part of the implementation phase. Many people you will be communicating with
do not have technical training and competence. They may be the general public,
government officials, or business leaders. Successful engineers must possess more than
just technical skills. The ability to communicate and sell a design solution to others is
also a critical skill.
You can use graphs, charts, and other visual materials to summarize the solution process
and present your work to others. Multimedia techniques, including Power Point
presentations, slides, sounds, videos, and computer-generated animations, are often used
to clearly communicate the solution to a design problem.
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Applying for Patents. If you develop an original and novel solution to a design problem,
part of the implementation phase may include applying for a patent on your solution. A
patent will not protect you from someone else copying your solution, but it does give you
specific rights to make and sell your design for a specified period. A patent is an
agreement made between you-the designer or inventor-and the U.S. government.
Through a patent document you agree to make public all the details and technology of
your invention. You agree to provide an invention disclosure, which provides enough
details to allow anyone to build a working model of your invention. Most large libraries
now have files of issued patents, which are available for anyone to see. These can be a
good source of ideas for engineering design solutions. In return for making your
invention or design solution public, the U.S. Patent Office grants you the exclusive right
to your invention for a specified period of time.
Pursuing a patent is not a trivial process and may take a long time, costing hundreds or
even thousands of dollars. Before considering a patent you should have a general
understanding of patent requirements and what can be patented. Ideas by themselves
cannot be patented. To obtain a patent, you must prove that your idea can be applied to
produce a "new and useful process, machine, manufacture, or composition of matter, or
any new and useful improvement thereof." These categories include just about everything
made by people and the processes for making them.
Most engineering design problems fall into the patentable categories of utility patents or
design patents. All mechanical and electrical devices fall into the category of utility
patent, which is granted for 20 years. At the end of the patent period, your protection
expires, and anyone can copy, manufacture, and sell your invention without giving you
credit or payment. A design patent is granted to protect the styling or ornamental features
of a design. A design patent is only granted for the appearance of an item, not for how it
works or is made. For example, if you invent a telephone that looks like a shoe, you
might apply for a design patent. The design patent would be granted on the appearance of
the phone, not on the electronic and mechanical workings of the phone. Design patents
are granted for 3-1/2, 7, or 14 years, depending on the patent fee paid. The fees range
from $200 to $600.
Patents are only granted to the inventor of a device. However, the inventor can assign the
rights to the patent to another party. If you develop an invention while working as an
engineer for a company, you will probably be required to assign the patent rights to that
invention to your employer.
Once a patent is granted, there is no guarantee that someone else will not try to copy the
invention. The U.S. Patent Office does not enforce patent rights. It is the responsibility of
the patent holder or a patent attorney to police the patent and make sure no one else
copies it while it is in effect. Since a patent makes all information about your design
public, some people choose not to pursue a patent, but rather keep the details of the
invention secret. If no one else learns how the invention works, you will have protection
until another inventor figures it out. For example, the formulas for Coca-Cola and Silly
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Putty have never been patented, and the secrets are only known to selected company
officials.
To apply for a patent, you need to prepare and include the following items:
A written document clearly describing your invention and stating that you are the
original inventor. Enough information must be provided so that someone else can
make your invention from the information you provide. You must also make
claims about your invention which describe the features which distinguish it from
already patentable material.
Engineering drawings that follow the format documented in Guide for Patent
Draftsmen, which is available from the U.S. Patent Office.
The filing fee. This is a basic fee of at least $ 150 that must accompany the patent
application. If the patent is granted, you will be charged an additional patent issue
fee. The total charges for obtaining a patent can be hundreds of dollars.
A patent is granted only after an extensive review process of the U.S. Patent Office. The
office will first search the nearly 5 million existing patents to determine whether your
design has been previously patented or infringes on an existing patent. This process can
take several years and be very expensive. Many inventors employ patent attorneys or
agents to conduct a preliminary patent search. Most large libraries have records of all the
patents filed with the U.S. Patent Office. This information is also available on a CDROM database at many libraries. You can look at this database and read the patent
applications filed under the same product category as yours. This will give you a good
idea on how an application is written and might help you improve your own design.
Before spending more time and money pursuing a patent, it is a good idea to find out if
someone else has already patented your invention.
Testing and Verification. Testing and verification are important parts of the design
process. At all steps in the process, you may find that your potential solution is flawed
and have to back up to a previous step to get a workable solution. Without proper testing
at all stages in the process, you may find yourself making costly mistakes later.
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REFERENCE
1. Ertas, A., Jones, J. C., The Engineering Design Process, John Wiley and Sons, New
York, 1996.
2. Lumsdaine, E., Lumsdaine, M., Shelnutt, J. W., Creative Problem Solving and
Engineering Design, McGraw-Hill, Inc., New York, 1999.
3. Sanders, M. S., McCormick, E. J., Human Factors in Engineering and Design,
McGraw-Hill, Inc., New York, 1993.
4. Dym, C. L., Little, P., Engineering Design: A Project-Based Introduction, John
Wiley, New York, 1999.
5. Hyman, B., Fundamental of Engineering Design, Prentice Hall, New Jersey, 1998.
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Appendix B
The Engineering Design Process Handout
9
The Engineering Design Process
Step Eight: Redesign
Step One: Identify the
problem/need
Step Seven:
Communicate the
solution
Step Six: Test and
evaluate the solution
Step Two: Research
the problem/need
Step Three: Develop
possible solutions
(computer models are
often used)
Step Five: Construct a
prototype
10
Step Four: Select the
most appropriate
solution
Appendix C
Research Assignment Handout
11
Research Assignment for Trebuchet Contest
1. Determine a team of six. Also come up with a team name. Email member names and team
names to me.
2. Visit http://www.engineering.wright.edu/cecs/trebuchet/ and download/print off the
competition and design rules. Read through the rules and begin to think about the project
and the role each person will play on the team. Come to class prepared with questions.
3. Conduct research on the history of trebuchets. Write a 250 word minimum summary on what
you found. You will want to use your research results in your final report. Be sure to cite
your sources at the end of your summary. Print off your summary for class.
4. You may also wish to research design hints and tips for the trebuchet; however, this is not
required for this assignment. This research may help you in your initial trebuchet design.
12
Appendix D
Cost Estimation
13
Cost Estimating
In engineering, cost is always a large factor in deciding the feasibility of a design option. The
basic idea of a cost estimate is to determine the materials, quantities, labor, and unit costs
required to construct a design option. This information is combined in a tabular fashion and is
used to help determine the total cost for a design option. An example of a cost estimate table is
shown in Table 1.
Cost for each unit of the item.
Visit a local store to find prices.
Item
1 !” Dia. PVC Pipe, 10 ft lengths
2”x4”x8’ Interior Pine Lumber
Twine
8d Hot Dipped Galvanized Nails
Labor
Quantity
3
4
5
1
5
These are estimated based
on construction drawings
and/or judgment
Unit
EA
EA
LF
LB
HR
Unit Price x Quantity
Unit Price
$ 10.50
$ 2.10
$ 0.10
$ 8.00
$ 20.00
Grand Total =>
Abbreviations for common units are:
EA = Each
CY = Cubic Yard
LF = Linear Foot SF = Square Foot
LB = Pound
HR = Hour
Table 1. An example of a cost estimate table.
14
Total Cost
$ 31.50
$ 8.40
$ 0.50
$ 8.00
$ 100.00
$ 148.40
Add up column
Appendix E
Trebuchet Physics Hints
15
Trebuchet Physics Hints
The trebuchet is a great learning and teaching tool on many levels. From a physics standpoint,
there are many applications of physics principles at play in a trebuchet throw. What follows is a
brief look at several of the applicable principles, how they affect the trebuchet throw, and what
considerations students may want to make in considering each principle in their design.
Moment of Inertia
• The Principle
The moment of inertia can be thought of as the resistance an object has to rotating about a
point. For example, if you hold a stick in the middle and rotate it and then hold that same
stick at its end and rotate it, you will notice there is much more resistance to rotation
when held at the end. The mass of an object as well as its shape (i.e. distance of the edge
from the rotating point) affect the moment of inertia.
• How The Principle Applies To The Trebuchet
The throwing arm of the trebuchet rotates about a fixed axis. Just like in the example
above, where the rotation point is located affects the resistance the throwing arm has to
rotation. The goal is to keep this resistance to a minimum, so that the squash ball can be
rotationally accelerated as quickly as possible.
• Design Implications
Watch how students’ trebuchets are firing. If the throwing arms are not speeding up
quickly when released, part of the problem may be due to a high moment of inertia.
Some ways to lower the moment of inertia include:
• Reduce the mass of the arm by making the arm thinner, by drilling holes along the
arm, or by using lighter materials like PVC. If, for example the throwing arm is
one inch thick, reducing the thickness to ! inch reduces the resistance to rotation
by !.
• Place the rotation point as close to the balance point (i.e. the point where you
would place your finger to balance the arm on your finger) as possible. This is
the position of least resistance to rotation. Be careful in choosing the rotation
point, however, in that the pencil test rules for the contest are not violated.
Angle of Release (Throwing Angle)
• The Principle
When an object is launched at an angle, how far it travels horizontally (this is known as
the range) is affected by not only the velocity of the object when released but also the
angle at which it is released. If an object is launched at a very steep angle, then its range
will be small. Most of its motion is vertically directed. If an object is launched at a very
low angle, then its range will also be small. This is because while its motion is mostly
directed horizontally, it stays so close to the ground that gravity makes sure it has almost
no time in the air. The result is a short range. It may seem like somewhere in the middle
of very steep and very flat launch angles will give the best range, and this is true. A
launch angle of 45 degrees is optimal for throwing an object at a certain velocity.
The trebuchet launches a squash ball, and this ball must throw a minimum distance. If
the trebuchet is not launching squash balls very far, it may be that the release angle is too
16
•
high or too low. While 45 degrees is the optimal launch angle for an object thrown at a
certain velocity, for the trebuchet, the optimal angle may be somewhat greater than 45
degrees. This is because the speed of the squash ball will continue to increase for a bit if
released past the 45 degree mark.
Design Implications
Visually check the angle of release of a throw. If the angle does not appear to be 45
degrees or slightly higher, then the following adjustments can help to change the release
angle:
• Adjust the angle of the nail holding the free end of the trebuchet sling. The
slightest changes of the nail angle can have dramatic effects on the launched
object’s release angle. Generally, bending the nail further forward will allow the
sling to slip off faster, resulting in a higher launch angle.
• Adjust the sling length. Shorter or longer lengths will affect the squash ball’s
angle of release.
Torque
• The Principle
Torque is a word that is used to describe the ability of a force to cause a rotation around
an axis. Torque is required to get any sort of rotational motion going. It is affected by
the size of the force, the distance of the force from the point of rotation (this is called the
lever or moment arm), and the angle of the force with the lever arm. The more torque
that is applied to an object, the quicker the object will speed up rotationally (this is called
angular acceleration). To get high torque there needs to be a high force and/or a large
lever arm. Additionally, torque is greatest when the angle between the force and lever
arm is 90 degrees.
Torque is what causes the trebuchet arm to angularly accelerate. The torque comes from
the pop can weight (the force) pulling down on the trebuchet throwing arm (the lever
arm). The pop can weight is fixed for the competition, and so the only way to increase
torque is to increase the throwing arm distance.
• Design Implications
Increasing the distance between the pop cans and the axis of rotation on the trebuchet will
increase the torque. This is good, but there can be a cost to increasing that length. Recall
that moment of inertia is dependent on length. If the arm is longer, the torque will be
higher, but the moment of inertia (the resistance to rotation) will also be higher. There is
an optimal length that will have to be experimentally determined.
Tangent Velocity
• The Principle
Imagine spinning a ball attached to a string that you hold. If you let go of the string, the
ball will no longer rotate in a circle and will instead take off in the direction it was
traveling at the moment the string was let go. The direction and speed with which the
ball takes off is known as the tangent velocity. The tangent velocity is affected by two
things: how fast the ball is spinning at the time of release (this is called angular velocity)
and the string length (i.e. the radius). The higher the angular velocity and/or the longer
the radius, the higher the tangent velocity will be.
17
•
•
How The Principle Applies To The Trebuchet
The tangent velocity is the key principal affecting the range of a launched squash ball.
As previously mentioned, there is an optimal launch angle (45 degrees). Additionally,
the faster the tangent velocity is, the further the squash ball range will be. So, it is
imperative to launch the squash ball with as much speed as possible and at an angle of 45
degrees (or slightly greater as previously discussed).
Design Implications
To get a high tangent velocity, the distance between the squash ball and the trebuchet’s
axis of rotation at launch must be large. The trebuchet must also be spinning quickly. A
large radius can be obtained by making the trebuchet arm between the axis of rotation
and the sling long and/or making the sling length longer. However, while these
adjustments could help the by making the radius longer, they could also reduce the
angular velocity at release. This is because lengthening the trebuchet arm and/or sling
will increase moment of inertia. Also, if the radius is to be lengthened solely by moving
the axis of rotation further towards the counterweight end of the throwing arm, then the
torque will be reduced. This would lower the angular velocity at release. Thus, there is a
point where lengthening the radius will be counterproductive in increasing the tangent
velocity.
Frictional Force
• The Principle
Frictional force is a force that opposes motion. It exists in almost all circumstances. If
the frictional force is applied to a rotating object and it is not directed through the rotation
point, then there exists a frictional torque. The frictional torque will oppose the rotational
motion of the object.
Most of the frictional force that comes into play for the trebuchet occurs at the axis of
rotation—i.e. where the rotation axis and the throwing arm turn inside one another. The
frictional force acts over a short lever arm (Figure 2) producing a torque. This torque
acts oppositely to the torque applied by the pop cans, thereby reducing the overall torque
(the overall, or net, torque is what accelerates the throwing arm) applied to the trebuchet.
Reducing friction by making smoother interface surfaces and/or reducing the lever arm
length will help reduce frictional torque.
Interface between trebuchet
arm and axis of rotation—
friction force applied here
Lever arm distance Axis of Rotation
Trebuchet
throwing arm
Figure 2. An illustration of the interface between the trebuchet arm and the axis of
rotation.
•
Design Implications
The key is to try and make the interface between the throwing arm and the axis of
rotation as smooth as possible. Ball bearings are an excellent way to do this. However,
18
they can be expensive to purchase. Experimentation with different interface materials is
necessary here. Also, the smaller the diameter of the axis of rotation means the smaller
the lever arm will be for friction to act on. This can help lower frictional torque
Observations should also be made to see that the axis of rotation does not have much play
at the interface. Too much play can also reduce the amount of net torque available for a
throw.
Putting It All Together
This is how all of the principles fit together. The overall goal is to have the trebuchet throw a
squash ball with a large range. To get a large range, the ball must be launched with as high a
tangent velocity as possible at an angle close to 45 degrees. To get a high tangent velocity, the
throwing arm must be long and the rotational speed must be high. To get a high rotational speed,
the trebuchet arm must be rotationally accelerated from rest very quickly. To get a high
rotational acceleration, the moment of inertia must be kept small and the net torque must be as
large as possible. To get a large net torque, the lever arm must be as long as possible and the
frictional torque must be as small as possible. To get a low frictional torque, the axis of rotation
diameter must be small and the interface surfaces must be smooth.
19
Appendix F
Technical Writing Guidelines and Template
20
Trebuchet Technical Writing—Guidelines and Template
In the format example below, all highlighted words are to be replaced with your own words. Do
not include highlighting in your report. The highlighting is there to indicate what must be
changed to your own words.
Include a title page and a table of contents.
I. Problem Statement
A logically organized, cohesive paragraph that provides the who, what, where, when, how, and
why for the project. Key to this paragraph is making sure the purpose of the project/question
being asked/hypothesis being tested is clearly stated. The paragraph should not be written as a
list of answered questions. Make it flow like a story. (i.e. not: Golder Associates requested the
project. They wanted the project completed so they could satisfy their curiosity about crawdads.
The question being asked is: Can crawdads be microwaved successfully? Testing was
completed at Carroll High School, Room 109. It was completed on August 23, 2008. etc.)
Rather, it should be more “story like” in form (i.e. Golder Associates approached Bohrer and
Associates about completing a study on crawdads. For many years the company was curious as
to whether they could be microwaved successfully. Bohrer and Associates tested the suitability
of microwaving crawdads in Carroll High School, Room 109 on August 23, 2008. etc.)—btw,
you are designing and constructing trebuchets, subject to tournament rules, that, when combined
with your team’s agreed upon strategy, will help your team win a tournament.
II. Background
2.1 Background Topic Number 1
A logically organized paragraph(s) thoroughly discussing background topic number 1. Note that
your background needs at least a history of trebuchet development and design, a discussion of
the tournament with a rules summary, AND a discussion of the physics of a trebuchet’s
operation! You also will want to briefly discuss any research you did in coming up with your
design. All references to outside works must be cited correctly! See:
http://pubs.asce.org/authors/book/generalresources/references.htm If you are going to have
more than one major idea for this background topic number 1 (or any topic, for that matter), then
you need to do as follows:
2.1.1 Subtopic 1
A logically organized paragraph(s) thoroughly discussing subtopic 1.
2.1.2 Subtopic 2
2.1.3 Subtopic 3 (etc.—use as many as you need)
2.2 Background Topic Number 2
21
A logically organized paragraph(s) thoroughly discussing background topic number 2
2.3 Background Topic Number 3 (etc.—use as many as you need)
A logically organized paragraph(s) thoroughly discussing background topic number 3
III. Design and Construction
Begin with a paragraph here briefly explaining that the trebuchets were designed and constructed
based on your knowledge of physics coupled with the results of your research. This provides the
reader with a background as they begin to read the section.
3.1 Trebuchet Design Development
Begin with a paragraph that explains in general how you approached your trebuchet design
process. You will need to reference the rules of the competition either here or in the subsequent
paragraphs as you explain your design. Then in the following subsections, you can either detail
the design process chronologically or by revision number, or by some other method. Be sure to
include drawings and/or photos of the design!
3.1.1 Initial Design (or whatever method you choose to illustrate your design)
A logically organized paragraph(s) thoroughly describing what your initial ideas for your
trebuchet were. You must also describe why you thought this way. You should include a
diagram or a picture of the design. Always refer to the diagram or picture in the text first,
and then place the diagram or picture after you discuss it. All diagrams, pictures, figures,
etc. MUST be mentioned in the text, and there must be a label and caption under the
figure.
Figure 3.1 The microscope used for observing test results. (for example)
3.1.2 First Design Revision (i.e.—use as many as you need)
A logically organized paragraph(s) thoroughly describing the “what” and “why” of your
first design revision. You should include a diagram or a picture of the design.
22
Figure 3.2 The stopwatch used for recording test results.
3.2 Trebuchet Construction
Logically organized paragraphs detailing your team’s trebuchet construction process. You
should include many photos (referenced within the text, of course) to help tell the story. I
recommend a chronological method of explanation. This should include everything up to the
point of bringing in the trebuchets to school to test them out. You may or may not need to use
subsections here.
IV. Field Modifications and Tournament Preparations
4.1 Field Modifications
Logically organized paragraphs detailing your team’s trebuchet testing and subsequent
modifications during the two weeks in the gym. You should include many photos (referenced
within the text, of course) to help tell the story. I recommend a chronological method of
explanation. This should include everything from bringing in the trebuchets to school to the day
of the competition. Do not include the final results of how far your trebuchet threw the ball.
You may or may not need to use subsections here.
4.2 Tournament Preparations
Logically organized paragraph(s) detailing what your team did besides testing of your trebuchets
to prepare for the tournament. This includes any strategy plans your team created for the
competition, team uniforms or other intimidation factors planned for the competition, and any
other bits of information that should be included to illustrate how you prepared for the
tournament. You may or may not need to use subsections here; however, if you don’t use
subsections, you must be very clear and logical on how you lay out your information (i.e. don’t
randomly skip around from idea to idea.)
V. Results
5.1 Trebuchet
Logically organized paragraph briefly explaining if your trebuchets ended up qualifying for the
competition (refer to checksheet).
5.2 Team Performance at Competition
Logically organized paragraph(s) detailing your team’s performance at the competition. Include
any notable “situations” from check-in (if any), the effectiveness of your strategy, the
performance of the actual trebuchet’s, and anything else relevant to illustrate the results of your
preparations. Refrain from including feelings and other non-tangibles in the text. This should be
a straightforward reporting on the performance of your trebuchet and team strategy.
5.3 Tournament Results
23
Logically organized paragraph indicating your overall placement in the tournament. Discuss
how many teams you beat before being eliminated and, if applicable, what place you came in for
the tournament.
VI. Cost Estimate and Analysis
A brief paragraph that explains what is included in the cost estimate and analysis (i.e. labor,
materials, etc.). If you did not keep track of hours put into the project, then you need to explain
that this analysis is materials only, and labor costs are not presented. Then make sure you refer
to the cost estimate and analysis tables in your text. Discuss any differences and/or similarities
between the two, and give some explanations for why you believe these differences and/or
similarities exist.
Cost Estimate
Table here
Table 6.2 Cost Estimate
Cost Analysis
Table here
Table 6.2 Cost Analysis
VII. Summary and Conclusion
7.1 Summary
A logically organized paragraph(s) recapping the entire report. This should be more extensive
and detailed than Section 1. It should briefly summarize each section of the report.
7.2 Conclusion
A brief paragraph that makes a concluding statement about your project. Your conclusion
should describe how well your group fulfilled the purpose of the project that you stated in
Section 1. (i.e.—did you create qualified trebuchets? Did you win? Etc.)
VIII. Recommendations
24
Write this section as if you are making recommendations for next year’s design group. Write a
well organized paragraph(s) explaining what should be done differently/same to make this
project work well in the future. Do NOT make recommendations about things not associated
directly with the project (i.e. it would be good to see if catapults or trebuchets are better siege
weapons). To help you with this, think about what things you would like to share with next
year’s group that might save them stress!
IX. Works Cited
Per ASCE site: http://pubs.asce.org/authors/book/generalresources/references.htm
X. Addendum
A list of who did what for the report.
25
Appendix G
Assessment Handouts
26
Trebuchet Assignment
PART I:
As a team of six, complete a report following the technical report guidelines/template handout.
This is worth 50 points, so please be thorough and detailed.
Email your completed report to me by _____________.
Reports will be edited and sent back for final revisions.
PART II:
You will be asked to rate your peers on their team performance over the trebuchet project. I will
be taking your peer evaluation scores and combining it with my observations of each of you
during the project to come up with a fair score.
Please fill out the peer/teacher evaluation form and turn in on _____________. The results of
the peer/teacher evaluation will also be worth 50 points.
These forms will be given to you in the next few days.
27
Trebuchet Peer Evaluation Form
Please evaluate your teammates based on their effort and whether or not
they did their fair share of work.
Circle a grade and write a comment(s) for each person, including yourself.
You do not need to place your name on this.
Group 1
Name:
Comment(s):
Grade:
A
A-
B+
B
B-
C+
C
C-
D+
D
D-
F
A
A-
B+
B
B-
C+
C
C-
D+
D
D-
F
A
A-
B+
B
B-
C+
C
C-
D+
D
D-
F
A
A-
B+
B
B-
C+
C
C-
D+
D
D-
F
A
A-
B+
B
B-
C+
C
C-
D+
D
D-
F
28

Trebuchet Design - College of Engineering and Computer

Transcription

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