Laptop Computer Shock and Vibration Characterization

Laptop Computer Shock and Vibration Characterization in
Commuter Bike Packs
16.622 Final Report
Fall 2010
Authors:
Ryne Barry
Cory Kays
Advisor: Dr. Kim Blair
December 8, 2010
1
Abstract
Bicycle commuting has seen an increase in popularity over the past decade; with this influx of bicycle
commuters, there has developed a need to understand the equipment associated with bicycle commuting.
Specifically, the protection that carrying devices provide their contents – notably expensive electronic
devices such as laptop computers – has not been well understood. It was hypothesized that a backpack
would reduce the forces due to shock and vibration on a laptop computer during a typical bicycle commute
by 10% compared to other commercial bicycle commuting packs. An experiment was performed which
investigated the relative protection that four common carrying devices - a backpack, a courier bag, and
two rear-wheel panniers – provided a laptop computer. A 3-axis accelerometer was mounted to a laptop
computer and placed inside each carrying device. A second 3-axis accelerometer was mounted to the
bicycle frame seat post. Data was collected for three bicycle paths of varying terrain: a smooth, paved
road, a rough sidewalk, and several curb drops. Five human subjects were used to test each carrying
device on each of the three courses. Results show that the laptop in the backpack experienced the lowest
root mean square force over all courses while the laptop in the courier bag experienced the lowest peak
force over each course; however, the respective root mean square forces experienced by the laptop in each
of the four tested carrying devices over all tests were all within 20% and are well below the force threshold
of a laptop computer. Furthermore, transfer function estimations were found for each of the carrying
devices and provide further insight into the protection that each carrying device provides. The results
of this study indicate that common commuter bike packs provide adequate protection for commuters’
laptops.
2
Contents
1 Introduction
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Historical Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Project Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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2 HOS
2.1 Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Success Criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3 Literature Review
3.1 Bicycle Frame Shock and Vibration Experimental Design Methodology . . . . . . . . . . . . .
3.2 Accelerometer Data Analysis Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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4 Experimental Description
4.1 Experimental Design . . . . . . . . . . . . . . . . . .
4.1.1 Experimental Overview . . . . . . . . . . . .
4.1.2 Description of Apparatus . . . . . . . . . . .
4.1.3 Description of Manufacturing Procedures . .
4.1.4 Variables and Measurements . . . . . . . . .
4.2 Testing . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1 Description of Test Courses . . . . . . . . . .
4.2.2 Description of Human Subjects and Protocol
4.2.3 Test Methods . . . . . . . . . . . . . . . . . .
4.2.4 Error Mitigation and Calibration . . . . . . .
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5 Results
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6 Discussion
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6.1 Analysis of results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.2 Error in results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.3 Relation of results to hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7 Summary and Conclusion
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7.1 Summary of findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.2 Assessment of hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.3 Suggestions for future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8 Acknowledgements
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A Full Experimental Test Matrix
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B Technical Drawings of Testing Apparatus
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List of Figures
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Example of a backpack (left), pannier (middle), and courier bag (right)
experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Experimental setup of accelerometers . . . . . . . . . . . . . . . . . . .
Dimensioned drawing of laptop design . . . . . . . . . . . . . . . . . .
Dimensioned drawing of seat post mount . . . . . . . . . . . . . . . . .
Carrying devices tested . . . . . . . . . . . . . . . . . . . . . . . . . .
Test courses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Peak force experienced by laptop computer . . . . . . . . . . . .
RMS force experienced by laptop computer . . . . . . . . . . . .
Input and Output PSD of Backpack on Rough Course . . . . . .
Input and Output PSD of Courier Bag on Rough Course . . . . .
Input and Output PSD of Deuter Pannier on Rough Course . . .
Input and Output PSD of Vaude Pannier on Rough Course . . .
Transfer Function Estimate of Backpack on Rough Course . . . .
Transfer Function Estimate of Courier Bag on Rough Course . .
Transfer Function Estimate of Deuter Pannier on Rough Course
Transfer Function Estimate of Vaude Pannier on Rough Course .
Experimental test matrix . . . . . . . . . . . . . . . . . . . . . .
Laptop mount drawing . . . . . . . . . . . . . . . . . . . . . . . .
Bicycle seat post clamp drawing . . . . . . . . . . . . . . . . . .
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Values calculated from t-tests on peak forces . . . . . . . . . . . . . . .
Values calculated from t-tests on RMS forces . . . . . . . . . . . . . . .
95% confidence intervals of peak force and RMS force for each test case
Quantitative comparison between each carrying device . . . . . . . . . .
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List of Tables
1
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4
1
1.1
Introduction
Motivation
Every day, thousands of bicycle commuters ride their bicycles to and from work or school, and many of
them bring their laptop computer with them. Recently, there has been an influx of bicycle commuters
due to rising fuel costs, eco-friendly crusades, and healthy lifestyle campaigning. These commuters use a
number of carrying devices to transport their laptops, with the most common being a backpack, pannier,
and courier bag (Figure 1). During these commutes, the laptop computer experiences constant forces due to
shock and vibration that can possibly lead to damage of the computer. While backpacks seems to provide
greater protection for their contents from road transmission of forces of shock and vibration, many bicycle
commuters complain of the resultant discomfort brought on by a backpack. Therefore, many commuters
look to alternative carrying devices to transport their computers. However, the protection these devices
provide for their contents is not well understood. The protection that these carrying devices provide for
laptops during a bicycle commute is the focus of this experiment.
Figure 1: Example of a backpack (left), pannier (middle), and courier bag (right) to be used during the
experiment
1.2
Historical Perspective
Bicycles have always been a popular method of transportation due to the convenience, low cost, and userfriendliness. However, only quite recently have bicycle riders started to use bicycles as means of getting to
and from work and school – where they need to carry their laptop computer with them. Many different
carrying devices are available to transport laptops such as the ones examined in this experiment. While
these devices have been designed with padding and special pockets for computers, the exact forces due to
shock and vibration that a computer experiences during a bicycle commute in these carrying devices is not
well understood.
1.3
Project Overview
To obtain these measurements and analyze the data, an accelerometer was fixed to a laptop computer that
could be loaded into all of the carrying devices. Another accelerometer was attached to the bicycle frame
for a transfer function analysis. Four different carrying devices were tested on three different courses. Data
was continuously taken at 40Hz during each test case and from this data peak force, RMS, PSD plots,
and transfer functions were computed. The data from our experiment shows that for most test cases, the
backpack provides the most protection for the computer; that is, the laptop within the backpack experienced
the lowest peak and RMS forces throughout most test cases. Furthermore, while the panniers provide the
least amount of protection, they still protect the computer enough to keep it from being harmed.
5
2
2.1
HOS
Hypothesis
A backpack will reduce the forces due to shock and vibration on a laptop computer during a bicycle commute
on a smooth course, rough course, and curb drop by 10% compared to other commuter bike packs.
2.2
Objective
Design, build, and operate instrumentation to measure the forces due to shock and vibration on a laptop
computer during an urban bicycle commute and compare the performance of the carrying devices.
2.3
Success Criterion
Measure the forces due to shock and vibration on a laptop to assess the hypothesis.
6
3
3.1
Literature Review
Bicycle Frame Shock and Vibration Experimental Design Methodology
A prior study was conducted to measure the forces due to shock and vibration on bicycle frames. A.Z. Hastings [1] led this study in 2001, where the effect of road vibrations on bicycle frames of different material was
measured. The measurement and analysis techniques used in the study provide a methodological framework
on which to build our project. Hastings and several colleagues fitted the different bicycle frames directly
under the seat on the seat post with a 3-axis accelerometer and data logger to measure the transmission of
forces due to shock and vibration on a bicycle frame, which was then compared to data that was measured
concerning the riders power output.
The experimental design and subsequent data analysis for this project will use similar techniques to those employed by Hastings et al. [1]s studys method of employing bicycle frame-mounted accelerometers to measure
the forces due to shock and vibration from road transmission on the bicycle frame will be almost identically
conducted in our experiment. Much like [1]s methodology, our study will mount the accelerometer to the
seat post of the bicycle frame and log continuous data throughout each test trial.
Moreover, the accelerometer output data analysis techniques employed by [1] serve as a useful guide in the
data analysis. In their study, [1] took the raw voltage output data from the accelerometer and, through the
various data reduction techniques discussed below, reduced this data to acceleration and, thus, force. This
allowed a maximum acceleration to be found in the time domain and a maximum amplitude to be found in
the frequency domain.
Our study looks to expand upon [1]s experiment by broadening the scope of measuring the forces of shock
and vibration due to road transmissions on simply the bicycle frame to measuring these forces on a laptop
computer situated in different carrying devices. This new area of measurement will be achieved by mounting
accelerometers onto the laptop computer and taking continuous data for each of the test cases.
3.2
Accelerometer Data Analysis Techniques
To successfully assess the hypothesis, we must conduct an appropriate analysis of the data output by the
accelerometers. Rogers et al [2] and Shust [3] provide an in-depth discussion of the analysis and presentation
techniques used when dealing with raw voltage output of accelerometers. The data analysis and presentation
of this data will primarily be based on the discussions of Rogers et al. and Shust.
The primary analysis conducted in the our analysis will be acceleration versus time data. This type of
analysis yields the most precise accounting of the variation of acceleration magnitude as a function of time
[among the time domain plots] [2] and is the most intuitive [of all the analysis techniques] [3]. Since our
experiment seeks to measure the average force experienced by the laptop over the duration of each test trial,
this type of analysis seems to be the most effective. Of particular interest in acceleration versus time data
analysis is the sampling rate (measured as the number of samples per second) of the data, from which the
length of the time plot is extracted. In our experiment, we must adjust the sampling rate of the accelerometer
so that it provides useful data at our test case duration (which will likely be around one minute). Rogers et
al. calculated an interval average acceleration versus time for the x-axis component as,
xavg =
M
N
1 X
x(k−1)M +i , k = 1, 2, ..., [ ]
M i=1
M
(1)
Corresponding expressions for the y- and z-axis data can be combined to form the interval average acceleration
vector magnitude as,
q
2
2
accelavgk = x2avgk + yavg
+ zavg
(2)
k
k
7
This averaging tends to smooth the appearance of the data and allow for longer periods of time to be plotted
on a single page [2]. This type of analysis will be performed in our experiment.
A second type of insightful analysis is a frequency domain analysis which provides insight as to the underlying
behavior [of the physical system] [3]. To perform a frequency domain analysis, the time domain must first
be transformed to the frequency domain. Both [2] and [3] show that the most efficient way to make this
transformation is through the use of the Fast Fourier Transform (FFT) method. Once an FFT is taken, one
can compute directly the Power Spectral Density (PSD) versus frequency plot, which provides a useful way
to view the distribution and magnitude of energy with respect to frequency.
8
4
4.1
4.1.1
Experimental Description
Experimental Design
Experimental Overview
To compare the forces due to shock and vibration on a laptop computer during a bicycle commute in a
backpack versus a pannier and a courier bag, a laptop computer was fitted with an accelerometer and was
placed in the backpack, pannier, or courier bag. Another accelerometer was mounted to the bicycle seat
post to measure the forces due to shock and vibration that the bicycle frame was experiencing. The bicycle
frame-mounted accelerometer allowed for a transfer function analysis between the forces on the bicycle frame
and the forces on the laptop computer to be done. Several test cases were performed for the carrying devices
using different bicycle riders and different courses.
4.1.2
Description of Apparatus
The accelerometers used in this experiment were Gulf Coast Data Concepts X6-2 three-axis ±6g accelerometers. One accelerometer was fixed to the laptop computer. The axes of the accelerometer were oriented
with the geometrical axes of the computer. This laptop was then placed in the carrying device which was
being tested. The other accelerometer was attached to the bicycle frame directly underneath the seat on the
seat post. The experimental setup, with the location of the accelerometers on the laptop and bicycle frame,
is shown in Figure 2.
Figure 2: Experimental setup of accelerometers
Both of the accelerometers were synced with a computer so that the time stamp was the same for each test
case. This allowed a transfer function estimation for each test case. At the start and the end of each test
case, the button on the back of the accelerometer was pressed to initiate and cease data collection. The data
was stored on the 1GB memory card on the accelerometer and later transferred via USB to the computer
for data analysis.
9
4.1.3
Description of Manufacturing Procedures
The accelerometers used in this experiment were unobtrusive to both the carrying device and the rider during
testing. The laptop that used in this experiment was a DELL Inspiron 5100. It was powered off during the
testing. The accelerometer mounted on the laptop was attached to the center of the laptop by double-sided
tape as shown in Figure 3.
Figure 3: Dimensioned drawing of laptop design
The second accelerometer was attached to the bicycle frame directly below the seat on the seat post as shown
in Figure 2. It was fixed to the seat post via the seat post mount shown in Figure 4. The seat post mount
was constructed on the water jet out of 12 -inch thick aluminum.
4.1.4
Variables and Measurements
The primary goal of this experiment was to measure the RMS and peak forces due to shock and vibration
on the laptop during a bicycle commute. To obtain these measurements, the acceleration of the laptop was
recorded during the aforementioned test cases. The acceleration on the bicycle frame was also recorded in
order to obtain transfer functions for the different cases. For these transfer functions, the input was taken
to be the disturbances on the bicycle frame-mounted accelerometer, while the output was taken to be the
disturbances on the laptop-mounted accelerometer.
The parameters which were varied in this experiment were the carrying device and the course. Four different
carrying devices were tested: a backpack, a courier bag, a Deuter pannier (not well padded), and a Vaude
pannier (more padding). The different carrying devices tested are shown below in Figure 5. Three different
courses and multiple riders were used. By having different riders complete different courses, enough data
would be obtained to make a generalization about which carrying device provides better protection for the
laptop. Even though only one backpack, two panniers, and one courier bag were used, this generalization
is valid because there is not enough variation in backpacks and panniers to change the forces that a laptop
computer experiences while being transported in them.
10
Figure 4: Dimensioned drawing of seat post mount
4.2
4.2.1
Testing
Description of Test Courses
Three different courses were tested for each carrying device. All of the courses were on the block between
Marlborough Street and Commonwealth Avenue and Hereford Street and Gloucester Street in Boston. The
rough course was a very bumpy brick sidewalk on Marlborough Street, the smooth course was a paved
asphalt alley in between Gloucester Street and Hereford Street, and the curb drops were done on Hereford
Street. The different courses are shown below in Figure 6.
11
Figure 5: Carrying devices tested
4.2.2
Description of Human Subjects and Protocol
Five human test subjects were used to operate the bicycle during the test runs. The human subjects were
recruited from MIT, and were all undergrads. They were all required to give their consent before riding the
bicycle in a test case.
Before testing began, the subjects underwent a briefing session where the investigators fully explained the
experiment and testing procedure to them. After the briefing session, the subjects were taken to the testing
area and shown the test courses. They were provided time to test the bicycle and get comfortable riding it.
After this, data collection began. Each subject did the rough course, smooth course, and curb drop with all
four carrying devices. Each subject was walked through the specific course before they began and one of the
investigators was at both the start and end points to signal the beginning and end of the course.
4.2.3
Test Methods
Once the test subjects were ready to collect data, the laptop would be loaded into the carrying device to be
tested and the button on both accelerometers would be pushed to turn them on and start collecting data.
Once the subject reached the end of the first course, the button was pressed again to stop data collection
and create a data file on the accelerometer to later be transferred to the computer. Then, the accelerometers
were started again and data was collected this way until the subject had completed all three courses with
the carrying device. After this, the next carrying device was tested and the subject did all three courses
with this carrying device. This process was repeated until each subject completed all three courses with
each of the four carrying devices. Once data collection was completed, the accelerometers were connected
to a computer and the data was downloaded for analysis on the computer.
Ten test cases were performed with the backpack and five test cases were performed with the courier bag
and each type of pannier.
12
Figure 6: Test courses
4.2.4
Error Mitigation and Calibration
There were several sources of error in our data. These errors came from the fact that the exact same path
was not ridden each time so the input acceleration was not the exact same and the +/- 0.004g accuracy of
the accelerometers [6]. Additional error in the data comes from the fact that the peak force on the laptop for
the curb drop in the panniers is unknown due to the range of the accelerometers. In each of the test cases
with the panniers on the curb drop, the acceleration maxed out at 6g so the true value of these data points
is unknown. Lastly, in the transfer function analysis the time stamps of some of the cases do not exactly
line up due to the drift of the accelerometers. This adds error to our transfer functions because for some
data points the input to the bicycle frame does not exactly line up with the output of the acceleration on
the computer.
While these sources of error could not be mitigated because they are the cause of uncontrollable factors such
as the exactness of the path ridden and the uncertainty inherent in the accelerometers, error analysis was
performed to quantify the effects of these uncertainties present in the experiment. First, the uncertainties
in the measuring instruments were propagated throughout each data set and the subsequent analysis. Next,
a normality test was performed on each test case to determine the proper statistical methods and to further
validate the data. For those data sets accepted as normal, 95% confidence intervals were calculated and are
presented in the following sections.
13
5
Results
The full test matrix for this experiment is shown in Figure 17 in Appendix A. Included in this test matrix
are the the peak force in the 3 axes of the laptop computer, the root mean square RMS force in the 3 axes
of the laptop computer, and the average force in all 3 axes of the laptop computer, as well as the magnitude
of the peak force, the RMS force, and the average force on the laptop computer.
To visualize the data concerning the protection that each carrying device provides the laptop, boxplots of
the peak force in the longitudinal axis of the laptop over each test case are presented in Figure 7. Only those
forces experienced in the longitudinal axis of the laptop are shown since this is the axis which was oriented
to measure the largest forces on the laptop in each carrying device.
Figure 7: Peak force experienced by laptop computer
Furthermore, boxplots of the RMS force in the longitudinal axis of the laptop over each test case are presented
in Figure 8.
14
Figure 8: RMS force experienced by laptop computer
Additionally, power spectral density (PSD) plots for a single test of each carrying device on the rough course
are shown in Figures 9 - 12. Each plot shows the PSD of the input – the disturbances on the bicycle-mounted
accelerometer – overlaid with the PSD of the output –the disturbances on the laptop-mounted accelerometer.
Each PSD plot effectively shows the envelope of frequencies which were most prevalent on the laptop within
each carrying device.
15
Figure 9: Input and Output PSD of Backpack
on Rough Course
Figure 10: Input and Output PSD of Courier
Bag on Rough Course
Figure 11: Input and Output PSD of Deuter
Pannier on Rough Course
Figure 12: Input and Output PSD of Vaude
Pannier on Rough Course
16
Figure 13: Transfer Function Estimate of
Backpack on Rough Course
Figure 14: Transfer Function Estimate of
Courier Bag on Rough Course
Figure 15: Transfer Function Estimate of
Deuter Pannier on Rough Course
Figure 16: Transfer Function Estimate of
Vaude Pannier on Rough Course
Finally, empirical transfer function estimates for each carrying device over a single test case on the rough
course are presented in Figures 13 through 16. It is assumed that each system was linear time-invariant (LTI)
and thus a transfer function does exist for each carrying device. Welch’s averaged periodogram method for
transfer function estimation [4] was used for each of these transfer function estimations. Each transfer
function shows the estimated magnitude of the disturbance for a given normalized frequency.
17
6
Discussion
In this section there will be a discussion of the results of the experimental project, including error sources
and error analysis, as well as a relation of the results to the hypothesis presented in section 2.
6.1
Analysis of results
The boxplots given in Figures 7 and 8 show qualitatively the relative protection that each carrying device
provided the laptop for each of the courses. From these plots, it is clear that the both the backpack and the
courier bag – commuter packs which directly interact with the cyclist throughout a ride and, thus, use the
rider to further damp the input disturbances – provide better protection in nearly every test case. Further, it
is observed that the laptop within the backpack experienced the lowest RMS force over nearly all test courses.
To quantitatively analyze the results, a Jarque-Bera test for normality [7] was performed for both the peak
force and RMS force data sets from each test case. Because the underlying distribution of the data is not
known, this test allows us to accept or reject each data set for normality and thus perform well known
statistical analysis on those sets deemed normal. Only the peak force and RMS force data sets for the
courier bag test case over the rough course were rejected for normality. For all data sets accepted as normal
from the JB-test, a one-tailed, two-sample t-test with independent variances was performed between each
data set of equivalent courses. The t-value is computed as
X1 − X2
t= q 2
s1
s22
n1 + n2
(3)
where X i is the sample mean of the ith data set, s2i is the unbiased sample variance of the ith data set, and
ni is the number of data points in the ith data set. Furthermore, the degrees of freedom for each t-test were
computed as,
d.f. =
s21 /n1 + s22 /n2
(s21 /n1 )2 /(n1 − 1) + (s22 /n2 )2 /(n2 − 1)
(4)
.
The results of the t-tests between each carrying device comparing the peak forces are shown in Table 1.
For each test case, the t-value and degree of freedom (rounded to the nearest integer) are reported. For
the calculated degrees of freedom of each test case, the associated critical t-value (at a significance level of
α = 0.05) was looked up in a t-test table [6]. If the calculated t-value lies outside of the critical t-value range,
then it can be concluded that the two compared values of interest are statistically significantly different with
95% confidence. Therefore, in the tables below, an ’Accept’ under the ’Hypothesis Testing’ column indicates
that the values of interest calculated for the two carrying devices are statistically significantly different. In
both Tables 1 and 2 the carrying device which is listed first is taken to be X 1 in (3) above; thus, a negative
t-value indicates that the first carrying device listed has a smaller value of interest compared to the other
carrying device and, therefore, provides more protection to the laptop computer. Then, from the results
shown in Table 1, it is clear that the backpack statistically significantly reduces the peak forces on the laptop
ovewr the courier bag and both panniers on the rough course, while the courier bag reduces the peak forces
over the backpack on the curb drop to statistical significance.
The results of the t-tests between each carrying device comparing the RMS forces are shown in Table 2.
From this table, it is clear that the laptop within the backpack experienced to lower RMS forces on each
course compared to the other carrying device to 95% statistical significance. This suggests that over the
full duration of a bicycle commute the backpack provides the most protection to the laptop. Furthermore,
the Deuter pannier – the less padded of the two panniers – reduced the forces on the laptop compared to
the courier bag and Vaude pannier to statistical significance, which seems to suggest that the protection
the panniers provides the laptop over a full bicycle commute is independent of the respective padding in
either pannier. However, it should be noted that the Vaude pannier did provide statistically significant
protection for the peak forces over the Deuter pannier on the smooth course, and nearly so on the rough
18
Table 1: Values calculated from t-tests on peak forces
Test Case Comparison
t-value d.f. Hypothesis Assessment
Backpack vs. Courier Bag
-2.880
7
Accept
Rough Course
Backpack vs. Courier Bag
Curb Drop
2.965
4
Accept
Backpack vs. Deuter Pannier
Rough Course
-2.827
5
Accept
Backpack vs. Deuter Pannier
Smooth Course
-1.5603
5
Reject
Backpack vs. Vaude Pannier
Rough Course
-3.507
8
Accept
Backpack vs. Vaude Pannier
Smooth Course
2.059
8
Accept
Courier Bag vs. Deuter Pannier
Smooth Course
-0.5724
4
Reject
Courier Bag vs. Vaude Pannier
Smooth Course
4.867
6
Accept
Deuter Pannier vs. Vaude Pannier
Rough Course
1.157
5
Reject
Deuter Pannier vs. Vaude Pannier
Smooth Course
2.462
8
Accept
course; therefore, while the less-padded pannier seems to provide better protection over the full course, the
better padded pannier provides superior protection against large impulses.
19
Table 2: Values calculated from t-tests on RMS forces
Test Case Comparison
t-value d.f. Hypothesis Assessment
Backpack vs. Courier Bag
-5.648
4
Accept
Rough Course
Backpack vs. Deuter Pannier
Rough Course
-3.796
8
Accept
Backpack vs. Deuter Pannier
Smooth Course
-3.712
7
Accept
Backpack vs. Vaude Pannier
Rough Course
-4.671
4
Accept
Backpack vs. Vaude Pannier
Smooth Course
-5.839
4
Accept
Courier Bag vs. Deuter Pannier
Smooth Course
2.121
5
Accept
Courier Bag vs. Vaude Pannier
Smooth Course
-0.2421
5
Reject
Deuter Pannier vs. Vaude Pannier
Rough Course
0.7051
4
Reject
Deuter Pannier vs. Vaude Pannier
Smooth Course
-2.339
4
Accept
The PSD plots show further differences in the carrying devices. Particularly, the output frequency ranges
in Figures 9 and 10 – the frequency ranges present in the backpack and courier bag – are similar, with
the majority of this frequency distribution lying between 2-5 Hz. The output frequency ranges evident in
Figures 11 and 12 – the frequency ranges present in the panniers – are also similar; however, these frequency distributions lie over a much larger range (2-12 Hz) than those ranges present in the backpack or
courier bag. It seems, then, that the frequency filtering from the panniers is not dependent upon the relative
padding in the panniers, since the frequency envelopes of the panniers (which are differently padded) are
nearly identical. Therefore, the backpack and courier bag serve as much better low-pass filters than either of
the panniers; because laptop harddrives are more sensitive to high frequencies, it is clear the backpack and
courier bag, in addition to better protecting the laptop from the magnitude of the disturbance, also serve to
filter the high-frequencies which can damage a laptop. Furthermore, it is worth noting that the frequency
ranges present in both the backpack and courier bag are nearly identically the force bandwidths of both the
human knee and hip [5]. This result shows that those carrying devices which are carried by the cyclist have
the added effect of damping from the body of the rider, specifically in the shock absorbent regions of the rider.
The transfer function estimation plots in Figures 13 to 16 provide further insight into the comparison
of each carrying device. While each transfer function is quite noisy, the general trend of each transfer
function provides some comparative insight into each carrying device. Comparing the transfer function of
the backpack given in Figure 13 to that of the courier bag in Figure 14, we see that both carrying devices
provide similar protection at low frequencies; that is, both transfer functions have similar magnitude at
normalized frequencies less than 0.3. However, at normalized frequencies greater than this, the corresponding
magnitudes in the backpack transfer function are approximately 30% less than the magnitudes in the courier
bag transfer function. This finding suggests that the backpack provides superior protection to the courier
bag across all frequencies. Comparing the transfer functions of the panniers, we note the similarity in the
two transfer functions. Both pannier transfer functions show nearly identical magnitudes for the given range
of normazlied frequencies and thus we further see the ineffectiveness of the padding within each pannier.
20
6.2
Error in results
From the boxplots given in Figures 7 and 8, we can clearly see the relative spread in each data set, which
provides a qualitative glimpse into the general error present in each set. To quantify this error, 95% confidence
intervals for all data sets which were accepted as normal from the JB-test were found. The results are
presented in Table 3. These confidence intervals provide some quantitative verification of the quality of
each data set. Specifically, those test cases which have a small confidence interval (i.e. the courier bag on
the smooth course) can be validated as having small error in the measurements, whereas those test cases
with large confidence intervals (i.e. the backpack on the curb drop) are validated as having more relative
uncertainty in the measurements.
Table 3: 95% confidence intervals of peak force and RMS force for each test case
95% Confidence Interval 95% Confidence Interval
Carrying Device
Test Course
for peak forces
for RMS forces
Backpack
Rough Course
3.267 ± 0.251
0.996 ± 0.045
Backpack
Smooth Course
2.282 ± 0.236
0.931 ± 0.037
Backpack
Curb Drop
3.626 ± 1.195
–
Courier Bag
Smooth Course
2.693 ± 0.150
1.039 ± 0.009
Courier Bag
Curb Drop
1.803 ± 0.153
–
Deuter Pannier
Rough Course
4.504 ± 0.820
1.120 ± 0.046
Deuter Pannier Smooth Course
2.921 ± 0.767
1.013 ± 0.023
Vaude Pannier
Rough Course
3.986 ± 0.314
1.103 ± 0.006
Vaude Pannier
Smooth Course
1.894 ± 0.285
1.040 ± 0.004
6.3
Relation of results to hypothesis
The hypothesis of this experiment stated that the backpack would reduce the forces due to shock and
vibration over all test courses by 10% compared to any other commuter bike pack. This reduction in forces
has been specified to include the peak force on the laptop over each test course and the RMS force on the
laptop over each test course, respectively. The backpack did not reduce the forces on the laptop by at least
10% over the other carrying devices in all test cases and thus the original hypothesis cannot be accepted
for every test case; regardless of the validation of the original hypothesis, though, the experiment allows a
quantitative comparison of protection between the carrying devices, which is the intent of the project. Table
4 shows this quantitative comparison between the average peak forces over each test case and the average
RMS forces over each test case for all carrying devices.
21
Table 4: Quantitative comparison between each carrying device
% difference in
Carrying Devices
Test Course
average peak force
Backpack vs. Courier Bag
Backpack vs. Deuter pannier
Backpack vs. Vaude pannier
Courier Bag vs. Deuter pannier
Courier Bag vs. Vaude pannier
Deuter pannier vs. Vaude pannier
Rough Course
Smooth Course
Curb Drop
Rough Course
Smooth Course
Curb Drop
Rough Course
Smooth Course
Curb Drop
Rough Course
Smooth Course
Curb Drop
Rough Course
Smooth Course
Curb Drop
Rough Course
Smooth Course
Curb Drop
22
Courier bag by 33%
Backpack by 18%
Courier Bag by 50%
Backpack by 38%
Backpack by 28%
Backpack by at least 65%
Backpack by 22%
Vaude pannier by 17%
Backpack by at least 64%
Courier bag by 107%
Courier bag by 8%
Courier bag by at least 230%
Courier bag by 83%
Vaude pannier by 30%
Courier bag by at least 230%
Vaude pannier by 12%
Vaude pannier by 35%
–
% difference in
average RMS force
Courier bag by 5%
Backpack by 12%
–
Backpack by 12%
Backpack by 9%
–
Backpack by 11%
Backpack by 12%
–
Courier bag by 18%
Deuter pannier by 3%
–
Courier bag by 16%
Vaude pannier by 0.1%
–
Vaude pannier by 1%
Deuter pannier by 3%
–
7
7.1
Summary and Conclusion
Summary of findings
From this study, it was found that the backpack reduced the average peak force on the laptop on the rough
course by 38% over deuter pannier and by 22% over vaude pannier. The courier bag reduced the average
peak force on the rough course by 33% over the backpack, 107% over the deuter pannier, and 83% over
the vaude pannier. The backpack reduced the average peak force on the smooth course by 18% over the
courier bag and 28% over the deuter pannier, while the vaude pannier reduced the average peak force on the
smooth course by 21% over the backpack. Furthermore, the backpack reduced the RMS force to statistical
significance on the laptop over nearly every test course compared to the other carrying devices. Between the
two panniers, we can see the effectiveness of the padding on reducing the forces on the laptop. Particularly,
the more padded Vaude pannier reduced the peak forces on the laptop on the rough course by 12% and
on the smooth course by 33%. However, both panniers provided nearly identical protection over the entire
course in each test case, evidenced by the similarity in the average RMS force over each test course for both
panniers. Therefore, the padding seems to provide more protection for large impulses, but has little effect
over the duration of a commute.
Additionally, the frequency envelope of both the backpack and courier bag is significantly smaller than those
of the panniers, as the backpack and courier bag have the added damping effect from the body of the rider.
While none of the frequenies in any of the test cases are beyond the bandwidth of a laptop, both the backpack and the courier bag served as low-pass filters and restricted the frequency range on the laptop to a
significantly smaller envelope than the panniers. Because higher frequencies are more detrimental to a laptop
than lower frequencies, the courier bag and backpack serve as more protective devices from forces due to the
transmitted vibrations to the bicycle.
More importantly, no peak force – nor output frequency range – measured was beyond the threshold of a
laptop computer; thus, all carrying devices adequately protect the laptop throughout each of the courses
which were tested. However, due to the accelerometers maxing out on the curb drop for both panniers, it is
possible that one or both of the tested panniers does not adequately protect the laptop in situations of very
high impulses, such as the curb drop test case. Bicycle riders can therefore be confident that their contents
are adequately protected in at least the four carrying devices tested in this experiment over the duration of
a typical bicycle commute. Furthermore, this study has shown that the backpack and courier bag provide
statistically significant protection over either of the panniers in nearly all paths which could be encountered
during a commute in both shock absorption and vibration filtering.
7.2
Assessment of hypothesis
The hypothesis was assessed with 95% confidence for all test cases deemed normal by the Jarque-Bera test.
From this assessment, it was found to statistical significance that the backpack reduces the peak forces due
to shock and vibration on a laptop computer on the rough course by over 20% compared to both panniers;
additionally, the courier bag reduces the peak forces on the laptop on the rough course by at least 30% over
all other carrying devices. The entire comparative analysis is summarized in Table 4; it is clear from this data
that the backpack did not reduce the forces on the laptop by 10% over the other carrying devices. Therefore,
the hypothesis cannot be accepted for every test case; however, the forces on the laptop were measured with
sufficient confidence for each carrying device and thus the hypothesis was successfully assessed.
7.3
Suggestions for future work
To further validate the generalization of the results of this project, future work could be performed to
characterize a wider range of carrying devices. Additionally, more test courses could be used to better
model a typical bicycle commute. Furthermore, the maximum peak forces experienced by the laptop in each
of the panniers could be quantified using an accelerometer with larger measuring range capabilities; these
results would allow a quantification in the relative protection that each carrying device provides across all test
courses of this experiment. Additionally, the transfer functions which were obtained contain significant noise.
23
The data was filtered using several techniques, but the results were never adequately improved. Therefore,
additional filtering of the input-output data could be performed to further resolve each transfer function.
Moreover, because the transfer functions rely on the data from two time-synced accelerometers, a more
thorough analysis could be performed using an apparatus which starts both accelerometers simultaneously
and accounts for the respective electronic drift inherent in each accelerometer.
24
References
[1] Hastings, A.Z., Blair, K.B., Culligan, K.F., & Pober, D.M., ”Measuring the effect of transmitted road
vibration on cycling performance.” ISEA Conference, (pp. 619-625).
[2] Rogers, M.J.B., Hrovat, K., McPherson, K., Moskowitz, M.E., & Reckart, T., ”Accerelerometer Data
Analysis and Presentation Techniques.” Paper by the Tal-Cut Company at the NASA Lewis Research
Center in Cleveland, OH, 1997, September.
[3] Shust, W.C., ”Dynamic Data in the Time, Frequency and Amplitude Domains – As Viewed by the
Mathematically Challenged,” Sound & Vibration, Vol. 36, No. 1, 2002, pp. 64-68.
[4] Ilvedson, C.R. (1998). Transfer Function Estimation Using Time-Frequency Analysis. M.S. Thesis. Massachusetts Institute of Technology.
[5] Ferris, D.P. & Lewis, C.L., ”Robotic Lower Limb Exoskeletons Using Proportional Myoelectric Control.”
Conference Proceedings IEEE Eng Med Biol Soc. 2010, March.
[6] Rice, J.A., ”Mathematical Statistics and Data Analysis,” 3rd ed., Brooks/Cole Publishing, 2007.
[7] Jarque, C.M., & Bera, A.K., A Test for Normality of Observations and Regression Residuals. International
Statistical Review, 1987.
25
8
Acknowledgements
We would like to thank REI for their generous donations to this project. Hopefully the results of this
project have provided insight into the effectiveness of those carrying devices donated by REI (and hopefully
these results help sell a few extra courier bags, too). We are also greatly indebted to Todd Billings, Dave
Robertson, and Dick Perdichizzi for their technical assistance in the early stages of our experimental design.
Furthermore, we would like to thank the faculty and staff of 16.62x for their guidance throughout the data
collection and analysis portions of the work and for the great experience which 16.62x provides. Finally,
we would like to thank Dr. Kim Blair for his continual insight and support throughout the duration of the
project.
26
A
Full Experimental Test Matrix
The full test matrix for this experiment is shown in Figure 17. Included in this test matrix are the the peak
force in the longitudonal axis of the laptop, the RMS force in the longitudonal axis of the laptop, and the
average force in the longitudonal axis of the laptop.
Figure 17: Experimental test matrix
27
B
Technical Drawings of Testing Apparatus
Figure 18: Laptop mount drawing
28
Figure 19: Bicycle seat post clamp drawing
29