The Book

Transcription

The Book

The
Book
The CanSat Team
Project Manager; Torstein Wang; [email protected]
Issued: Friday, 29 January 2010
Page 1 of 32
Programmer; Dag Martin Nilsen; [email protected]
Contents
Chapter 1: The primary mission, experimenting with sensors ................................................... 3
Introduction ............................................................................................................................ 3
Analog to Digital .................................................................................................................... 3
Sensors ................................................................................................................................... 5
Calibrating the sensors ........................................................................................................... 7
Altitude Calculations .............................................................................................................. 8
Example Assignments .......................................................................................................... 10
Chapter 2: Ordering guide ........................................................................................................ 12
Secondary mission ideas ...................................................................................................... 12
Order Components ............................................................................................................... 12
Chapter 3: The secondary mission ........................................................................................... 14
Adjusting the structure ......................................................................................................... 14
Additional Connections ........................................................................................................ 14
Communication .................................................................................................................... 16
Power Supply ....................................................................................................................... 16
Chapter 4: Parachute design ..................................................................................................... 17
Introduction .......................................................................................................................... 17
Descend Physics ................................................................................................................... 17
Requirements Descent Parameters ....................................................................................... 20
Parachute production ............................................................................................................ 20
Chapter 5: Telemetry; sending data, setting up the ground station and processing the data.... 22
Introduction .......................................................................................................................... 22
Transmitting data.................................................................................................................. 22
Setting up the Ground Station .............................................................................................. 25
Processing the data ............................................................................................................... 27
Appendix A: Example transmission code ................................................................................ 29
Appendix B: Transmitter Frequencies ..................................................................................... 29
Appendix C: Scanner Guide ..................................................................................................... 30
Appendix D: The CanSat kit MicroController Board .............................................................. 31
The CanSat Team
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Chapter 1: The primary mission, experimenting with
sensors
Subject:
This lesson will explain the operation of the temperature and pressure sensor
provided in the CanSat kit. After the necessary theory some experiments are
described which can be conducted.
Introduction
The CanSat kit used for this competition comes equipped with a sensor board.
Connected to this board are two sensors, a pressure sensor and a temperature sensor. Figure 1
shows a picture of this sensor board. These sensors produce an analogue signal, varying from
0 to 5V, and can be used to measure the temperature and pressure in the CanSat. Utilizing
these sensors is part of the primary mission for the CanSat Competition. From these sensor
readings you can calculate the altitude of the CanSat. This lesson will help you in completing
the primary mission:
Use the CanSat kit to measure temperature and pressure, and then make graphs
displaying altitude profiles of the mission flight.
Figure 1: The Sensor board of the CanSat kit
Analog to Digital
The sensors used produce a voltage, this voltage depends on the value of the parameter
the sensor measures. It can take on any value in a certain range; such a signal is called analog.
The temperature sensor produces an analog output signal between 0 and 5 volts.
On the contrary a signal that can only take on some discrete values is called a digital
signal. In Figure 2 these two different signals are depicted. The top graph shows an analog
(continuous), signal. The bottom graph shows a digital (discrete), signal.
A computer and also the small computer in the CanSat, called microcontroller, can
only process digital signals. To convert the analog signal from the sensor into a digital one we
use an Analog to Digital Converter (ADC), which as the name implies converts an analog
signal into a digital signal.
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Figure 2: An example of an analog signal, a digital signal
and a bitstream
The ADC converter is incorporated in the microprocessor and has 8 input channels. It
is a 10 bit’s ADC; it will convert a signal into a digital signal with 10 0‟s and 1‟s. So it can
generate a 10 bit binary number. Bit stands for binary digit, and is a value of 0 or 1, high or
low, on or off.
Each digit in the binary number can have 2 values, 0 or 1, so a 10 bit binary number
can have, 210 = 1024, different values. This results in an integer, meaning a solid number,
ranging from 0 to 1023. The microcontroller can understand this value and use it for
computations. These computations can be programmed into the processor by writing a
program code. An example of such a code is shown in the lessons included with the CanSat
kit.
Each sensor in the CanSat is sampled by the ADC, making each analog value into a 10
bit number. 0 voltage will be converted into the binary number 0000000000 = 0, and 5
voltages will be converted into the binary number 1111111111 = 1023 as a decimal number.
These numbers will be sent to the transmitter at a bit stream with high and low values. Such a
bit stream is shown in the leftmost image in figure 2.
The sequence of events is thus as follows:
1) The temperature sensor converts the measured temperature into a voltage, this
is an analog signal.
2) The ADC converts this analog signal in a digital signal, which the processor
can understand.
3) Inside the microcontroller the signal is a 10 bit binary number, and can be used
for computations.
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Sensors
Two sensors are used in the CanSat kit, a pressure sensor and a temperature sensor.
Pressure sensor
The pressure sensor used is the
MPX4115A from Motorola. It uses a silicon
piezoresistive sensor element. Figure 3
presents the internal built up of the sensor.
Piezoresistive effect means that the
resistance of a material will change when a
mechanical stress is applied. In this case
silicon is used. The changes of resistance for
silicon are magnitudes of times larger than
for example for metals, making this material
very useful to use in a pressure sensor.
Figure 3: Cross Sectional Diagram SOP (not to scale)
Figure 4 shows a more detailed look into the sensor. It shows the dimensions of the
sensor and the layout of the connections. To relate the measured voltages back again to values
for the pressure the transfer function of the sensor is needed. Such a function describes the
relation between the voltage output of the sensor and the equivalent pressure. This function
can be found in the datasheets of the sensor. Figure 5 presents a graph from these datasheets
with the accompanying transfer function.
Figure 4: Unibody Package dimensions
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Figure 5: The transfer function
For more information on the pressure sensor look into the datasheet.
Temperature sensor
The temperature sensor used in the CanSat is the
NTCLE203E3103GB0 manufactured by Vishay/BC components. It is
a so called NTC, or Negative Temperature Coefficient thermosistor.
An increase of temperature the thermal conductivity rises.
Most ceramic materials exhibit such an behaviour. Other
materials however will have an opposite behaviour, with rising
temperature the conductivity decreases.Most NTC thermosistors are
therefore made out of semi conductive materials, something in
between an insulator and a conductor, with some special qualities.
Simply put, when the material is heated the electrons in the material are energized.
More electrons are able to move around, thus the material can conduct electricity more easily.
When a material can conduct electricity more easily its resistance will obviously decrease. So
with an increase of temperature the resistance is decreased. Therefore it is called a Negative
Temperature Coefficient, NTC.
On the sensor board the temperature sensor is connected in series with a resistor, R1
with a constant resistance of 10 kΩ. The NTC In the figure you can find a simplified
schematic of this. When resisitors are connected in series the current in the circuit will be the
same everywhere. The total resistance can be calculated by:
RT = R1 + RNTC
To find the current, I, trough the circuit we can use ohms law U = R I, where U = 5 V
I = U/R =5V/( R1 + RNTC)
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The same current, I, flows trough the fixed resitstor R1 giving a voltage Vmessure across the
resistor. This can be put into the following equation:
I = U/R = Vmessure/R1
Since the current is the same everyhvere in the circuit, we can sett upp the following equation:
I=I
5V/( R1 + RNTC) = Vmessure/R1
From this we can get the relations between the messured voltage (Vmessure) and the resistanse
of the NTC temperature sensor (RNTC).
Vmessure =5V* R1/( R1 + RNTC), or
RNTC = (Vmessure /( R1 + RNTC)) - R1
To get a complete transefer function you will also need the relation between the temperature
and (RNTC). You can find this in the sensor datasheet.
Calibrating the sensors
In some cases the transfer function, to compute the corresponding values to the measured
voltages, is not precisely known. We will have to come up with a function ourselves. A
simple method to compute the function is by using measurements and a graph, assuming
linearity and interpolating the results. In reality the behaviour of the sensor will probably not
be linear. However on a certain range it can be very well described as being a linear relation.
Linearity means we assume the relation between voltage and the parameter is directly
proportional. Describing this relation by means of the standard linear formula:
Parameter = A * Voltage + B
100
Tabel 1: Measured test results
80
Voltage [V]
0,5
1,5
2,5
Temperature [C]
60
40
Temperature [C]
0
20
40
20
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
-20
-40
Sensor Output voltage [V]
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To estimate the values of A and B use a graph. In this graph plot the measured voltages
on the x-axis and the parameter, in this example temperature, on the y-axis. In the table next
to the graph you will find some example measured values for this sensor. These points are
also plotted in the graph. Step two is to draw a straight line through the points. The more
points are used the more accurate our result will be. However it will be more difficult to fit
the line exactly through all points. Try to fit the line as good as possible. This line can now be
used to determine the values of A and B from the standard linear formula.
A:
B:
is the slope of the line,
is the intersection with the y-axis.
Altitude Calculations
The atmosphere is all around us; it is a thin
gaseous layer surrounding our planet. The atmosphere
is composed of most importantly nitrogen (78%) and
oxygen (21%). Furthermore it contains some water
vapour, CO2 and other trace gasses.
The Earth’s atmosphere consists out of different
layers with different properties (temperatures,
pressure, composition, etc.
In Figure 6 the different layers are
represented, along with the different
human and weather activities in these
layers.
In contrary to our CanSat
most satellites are operated in the
exosphere. Here the density of the
atmosphere is very low. The CanSat
however operates in the troposphere,
the bottom layer. This layer contains
about 80% of the total mass of the
atmosphere, and stretches to about 10
kilometres altitude. In this layer all
sort of meteorological phenomenons
occur, wind and clouds for instance.
As seen in the graph there is a
relation between two properties of the
atmosphere, temperature and
pressure, and the altitude.
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There is a linear relation between the temperature and the height in the atmosphere.
Ascending one kilometre in the air will result in a 6,5 degrees Celsius decrease in
temperature.
The equation below provides the relation:
T T1
a
is Temperature in Kelvin;
is the start temperature at h1 altitude
is the altitude in meters;
is the starting altitude
is the temperature gradient: -0,0065 K/m.
h
T
T1
h
h1
a
h1
The relation between the pressure and the altitude is somewhat more complicated. The
pressure is not only dependent on the altitude but also on the temperature. Let’s start with the
relation of pressure to temperature:
p
p1
p
p1
g0
R
T
T1
g0
aR
is pressure in Pa;
is the start pressure in Pa
is gravitational acceleration 9,81 m/s2
is specific gas constant 287,06 J/kg*K
Inserting this formula in the formula for the temperature we arrive at the following relation:
h
T1
a
p
p1
aR
g0
1
h1
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Example Assignments
The following assignments can be performed when experimenting with the sensors.
Temperature Sensor assignment:
The aim of the assignment is to calibrate and test the temperature sensor.
In addition to the CanSat kit you will need the following for this assignment:
Thermometer,
Hair dryer
1) Performing reference temperature measurements
a. Measure the temperature on several places with a thermometer. Try to get as
much difference as possible between the points. More points and a larger
difference will produce a better fitting curve. You can try to use a hot dryer to
heat up the temperature, be careful not to heat it up to much!
b. Use the CanSat to measure the voltage from the sensor at these places too.
2) Plot the voltages, on the x-axis, against the temperature, on the y-axis. You could use
MS Excel.
3) Draw a straight line through the measured points. Calculate the describing formula, in
the form: Temperature = A * Voltage + B
NOTE: This formula can be used to measure the temperature with the CanSat.
4) Implement the formula into a program for the CanSat. Use the CanSat to measure the
temperature at different places, use a thermometer to check your results.
Pressure Sensor assignment
The aim of the assignment is to test the pressure sensor.
A plastic tube or straw, between 15 and 30cm long and with a maximum diameter of 6
mm
1) Write a program for the CanSat to read out the voltage from the pressure sensor.
Implement the scale formula to convert the voltage in a pressure.
2) Measure different pressure levels
a. Place one end of the tube very close to the pressure port of the pressure sensor
b. Place the other end in your mouth and suck away some air.
3) If you have done this correctly the pressure should be decreasing. If you are having
troubles ensure the tube is connected rightly to the sensor port.
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Altitude estimation assignment
Use both the pressure and the temperature sensor to estimate the altitude.
A height difference, you could use a tall building however a hill would be preferable
1) Use the programs written to measure the temperature and the pressure. Modify them to
provide you with a value for the altitude as well.
2) Perform measurements on different locations on different heights. If possible try and
check the measured altitude with a known altitude.
NOTE: The equation to calculate the altitude uses a reference altitude, denoted by T1 and
p1.
Extra assignment: Incorporating errors
Take a look at the figures below. They are found in the datasheet for the pressure sensor. The
indicated the errors which are associated with the sensor.
1) Try to incorporate the temperature error in your program. Use it to estimate a upper
and lower band of the measured pressure.
2) Keep the pressure the same and try to change the temperature, for example with the
hair dryer. See if you can catch the temperature error band.
3) Now also include the error band for the pressure to your graph.
The CanSat Team
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Chapter 2: Ordering guide
Secondary mission ideas
Satellites can be used to accomplice a lot of different missions. Some are used by scientist to
measure certain parameters of the atmosphere of our planet, increasing our knowledge of for
example Global Warming. Other satellites are used to capture images of the planet or of the
stars. The pictures are sent down and analysed by researchers. A well known result is the
pictures in Google Maps. Some satellites, like the International Space Station, are used to
perform experiments. These experiments can vary from testing new equipment, physical
experiments of astronauts or biological experiments. Furthermore satellites can be used to
determine your position, like Galileo or GPS. Some satellites are even used to go to other
planets and explore. Some of these even carried small rovers to Mars.
Our CanSat is a small satellite, so in the secondary mission we are going to try and
mimic one of the functions of a satellite. You are entirely free to choose this secondary
mission, limited only by your imagination. A lot of information on satellites missions can be
found on the internet. Try and see what ESA is doing at the moment, or NASA. Search on the
website of Arduino to see what would be possible. Under Library you find some extra
functions of the processor. Maybe this will provide additional inspiration.
Some ideas for a secondary mission could be:
1. Try to determine the position of the CanSat more precisely
2. Steer the CanSat when airborne
3. Deploy an experiment outside the CanSat
4. Take videos when descending
5. Generate power when descending
There are off course more than a thousand different possibilities. Just try to pick a mission
that will best fit with your team’s wishes. To further assist you we have listed a few useful
components on the next page. We hope this will help in starting the search for your own
components.
When you are not sure about the compatibility of a component you would like to use you can
always contact the CanSat Team. We will try to help you out.
Order Components
It is possible to connect a whole range of components to your standard CanSat kit. The
following items were found on www.elfa.se, they have a wide variety of useful components.
The CanSats are equipped with 0.64 mm. pins for connecting external sensors/instruments for
the secondary mission. The easiest way to connect something to the pins is to buy 400mm
cables with connectors (Item 16 and 17). These can be fitted inside of plastic connectors (item
18). Item 1 and 2 is a simple card which can be used to make your own electronic schematics.
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Item
Component:
Description
Price (Nkr)
Comment
Copper board
Elfa.se part
number
48-326-63
1
With tracks and
holes
46
Option 1
2
Copper board
48-324-81
46
Option 2
3
4
TS912
Opamp Socket
73-459-45
48-135-49
With tracks and
holes
Dual Opamp.
Socket for
mounting opamp
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Sensors:
SFH300
LM35D
SFH300
GPS EM 411
HiH 4000
Resistors
Resistor 1 kΩ
Resistor 10 kΩ
Resistor trimmer
Connectors:
Cable
Cable
Connector
Solder pins
Connector 2 pin
Connector 3 pin
Connector 4 pin
Photo Transistor
0 to 100 C temp
Photo Transistor
GPS module
Hygometer
10
25
10
350
167
60-722-84
60-734-23
64-360-91
0 to 10 kΩ
2
2
5
43-566-40
43-566-42
43-566-08
43-716-13
48-354-01
48-354-19
48-354-27
400mm read
400mm black
3 pin
3 pin
Screw terminal
Screw terminal
Screw terminal
7
7
3
2
7
10
14
75-221-39
73-090-57
75-221-39
78-400-02
73-056-83
12,20
5
SFH300
Other links:
Electronics: http://farnell.com/
Electronics and gadgets: http://www.sparkfun.com/commerce/categories.php
The CanSat Team
Page 13 of 32
Chapter 3: The secondary mission
Subject:
This manual provides some tips and tricks which can prove to be useful for the
secondary mission
Adjusting the structure
In the CanSat kit an aluminium structure was provided. It has been designed to incorporate
the primary mission and has standard no possibilities to attach additional components for a
secondary mission. However there is still some space left on the structure, especially at the
battery side, to utilize for extra components. The structure may freely be adapted to the needs
for the secondary mission. It is also allowed to design and built a new structure, if the
secondary mission requires so. Off course this is easier said than done. Some important
aspects when designing a new structure:
Figure out where to put the different components,
Make sure the wires can still be connected. They have a limited length.
Ensure the different components are still easily accessible.
Also the skin, so the soda can, is allowed to be replaced. However it is not allowed to go
beyond the boundary of a normal 33cl soda can. It has to be the same size. When adjusting the
structure or the skin, make sure the antenna wire is still entirely outside the can. Otherwise the
reception of the signals during the flight will fail.
Additional Connections
The microcontroller used in the CanSat is the ATMEGA 168 20UA. You can study the
datasheet to get detailed information on inputs and connections. Only some of the
connections of the microcontroller are used by the primary mission. In appendix D the ports
of the primary mission are defined. It is thus possible to use the controller for the secondary
mission as well, connecting new components to the remaining connections. In Appendix D, a
mapping of the additional ports of the microcontroller can be found.
Analog sensors
Analog sensors are easily connected to the microcontroller. These sensors produce a
voltage between 0 and X Volt, depending on the measured value. Hooking it up to a ADC of
the microcontroller will turn it into a binary number which can be used in calculations. It is
the same principal as the temperature and pressure sensor of the primary mission. These
sensors can be easily connected to the still remaining other ADC ports. On the sensor board
one additional connection is still free and would be ideal to use.
Sensors that have a 5 volt maximum for their output are ideal to use. If the output is
smaller than 5 volts you will get less accuracy because not the entire range of the ADC is
used. When the output is larger, every value above 5 volts will not be measured. These issues
can be solved by using an amplifier to increase or decrease the signal to the desired 5 volts
range. More information on amplification can be found on internet.
Digital sensors
Digital sensors can be found more and more often these days. These sensors doesn’t use a
analog signal for the information, instead the make use of digital protocol. Two important
protocols are supported by the Microcontroller, Serial and I2C, they are discussed here.
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Serial: An often implemented communications protocol. The bits are sent in a serie,
one after the other. It has a separate transmission and receiving connection. The
microcontroller can support a software serial solution on all the digital pins.
Software Serial can be used if data is transmitted or received once in a while. The
information is only received when the function is called. A disadvantage would be the
loss of data in between listening cycles. More information can be found on the
Arduino website, under the SoftwareSerial library.
Hardware Serial can be used when sensors transmit information irregularly or very
frequently. The data is now stored inside the microcontroller and no data is lost. The
hardware serial are the same pins as the ones to which the USB programming cable is
connected.
I2C is a different protocol which can be used. It is very useful when multiple processors are
connected to each other. It uses a master/slave architecture to enable communication.
Although it is supported by the microcontroller it is a little bit harder to implement. For I2C
make sure no clock stretching is used, because it is badly supported.
Electric motors
It is also possible to control a small electric motor with the microcontroller. Two main types
exist: DC motors and servos.
A DC motor is an electric motor, it will start turning once connected to a DC current,
for instance from a battery. It is not possible to use the
microcontroller to power such a motor. The digital port is not
capable to deliver enough current. However you can use the
controller to open an electric switch that will power the motor.
This could be a transistor. The figure shows an example on how
to use a transistor to control a DC motor with the CanSat. The
diode is necessary to safeguard the transistor.
Using a Servo is the other option. A servo is an
electric motor of which the position can be controlled. It
can be useful for precision control. However it draws a lot
of current, making additional power supply a necessity. The
position is controlled by using a third signal wire,
commonly the white or yellow one. The other two wires are
used for powering the servo. Mostly black will be ground
and red will be the positive voltage.
Make sure that the ground of the servo is connected
to the ground of the microcontroller or it will not work.
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Communication
To transmit data on the secondary mission it is possible to use the same transmitter.
However as was mentioned before the capacity of this transmitter is limited. It could be that
you would want to transmit more information than is possible. The easiest solution is to
transmit the information as efficiently as possible. Trying to make it small enough for the
transmitter will be able to handle it. Another possibility would be to adjust the transmitter.
This is possible however rather complicated and not recommended.
Thirdly a separate second downlink could be used. Using an additional transmitter to
sent down the information. Make sure to use a frequency band which is legal. If the other
transmitter uses the same band as the primary one, make sure the bandwidth is not too wide. It
is not allowed to interfere with the transmission of the other CanSat’s.
Power Supply
The secondary mission will need enough energy for it to function. Two power sources are
available: the 9V battery and the 5V from the microcontroller. The 5V of the microcontroller
is only able to supply 100mA, of which 70mA is used by the primary mission. For most
additional sensors this should suffice. However in many cases the GPS and/or motors require
more power. Also some sensors may need a different input voltage to function. A solution is
to add an extra power supply.
There is a lot of information available on the internet on how to construct a simple power
supply yourself. For instance search for the L7805 Voltage regulator. Just make sure to
connect the different grounds with each other. Otherwise it won’t work.
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Chapter 4: Parachute design
Subject:
This lesson will address the Parachute design for the CanSat. The lesson
explains the calculations involved in the design and provides some practical
tips for fabricating the parachute.
Introduction
Satellites normally do not return to Earth on a
parachute. At the end of the life of a satellite it will be put
in a different orbit. For satellites orbiting at a low altitude
this could mean they will burn up in the atmosphere.
Satellites further away will end up in a parking orbit and
circle our planet forever. Sometimes however the
spacecraft has to return to earth with samples or
astronauts. One of the solutions is then to descent on a
parachute.
When the CanSat is deployed it has to have a
device to slow it down, otherwise it would crash in the
ground. Furthermore we would like the CanSat to be
oriented in an upright position. Specifically for the antenna this is important. This will enable
the best chance of receiving the telemetry. These functions are performed by the parachute.
This lesson will guide you through the different steps needed to design and built your
parachute.
Descend Physics
Before we can start on producing the parachute we will have to figure out how big it
should be. More specifically: How big should the area of the parachute be to fulfil the
requirements?
Logic will say that the bigger the parachute the slower the object will drop down.
Later on this principal is demonstrated with some basic equations. Although it would be very
beneficial for the mission and the CanSat to have a very low descent speed there is a limit.
For safety reasons a minimal descent speed is set. This limit is set to ensure that the CanSat
will land in an area near the launch area. When the descent speed is too slow the satellite
could drift kilometres away on the wind, this is not allowed nor desired.
To design your parachute we’ll use some simple physics. We use a simplified model
to estimate the area of our parachute. After which we can start on the production.
During the descent two forces will be acting on the CanSat. Gravity will pull on the can and
accelerate it towards the ground. The parachute will pull the CanSat in the opposite direction
and slow down the descent. In the picture you can clearly see the two forces.
When the CanSat is deployed it will be accelerated by the gravity force. After a few
second the drag force of the parachute will be equal to the gravity force. From now on the
acceleration will be zero and the CanSat will descent at a constant velocity. This constant
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velocity has to be larger than the minimum descent velocity which is required. During the
following calculations we will use this value as our constant velocity of the CanSat.
The gravity force is equal to:
Fg
=
m*g
[1]
In this equation:
m:
is the mass of the CanSat.
g:
is the acceleration of gravity, equal to 9,81
m/s2.
The drag force of the parachute is equal to:
FD
=
0,5 * CD * ρ * A * V2 [2]
In this equation:
A:
is the total area of the parachute (not just
the frontal area)
CD:
is the drag coefficient of the parachute. This value
depends on the shape of the parachute.
ρ:
is the local density of the air, assumed to
be constant at 1,225 kg/m3.
V:
is the descent velocity of the CanSat
You can easily rewrite these equations to calculate
the needed area of the parachute.
The CanSat Team
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Semi-spherical Parachute Design
A semi-spherical parachute is the most common shape of a parachute. Although it is
not hard to make one it can be quit tedious to get the right shape. The figure below should
help out.
n
r
stands for the number of needed parts
stands for the radius of the parachute.
Cross Parachute Design
Instead of using a semi spherical shaped
parachute you can also choose a cross shaped. The
advantage is in the ease of production. If you want
to know more of cross shaped parachutes you can
look at the following link:
http://www.nakka-rocketry.net/xchute1.html
Parapent
A parapent shaped parachute acts a little bit
like a wing. Because of its shape you can use it to
steer. The design of a parapent is more complex
than of the other shapes. You will have to perform
some more research.
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Requirements Descent Parameters
The values are still preliminary; they could change in the future. They can be used as a
first guideline. Uncertainties in the Launch Campaign could lead to a different value for the
eventual descent velocities.
Minimal descent Velocity: 8 m/s
Maximal descent Velocity: 11 m/s
Maximum allowed mass:
350 grams
Drag coefficients:
Semi Spherical:
Cross Shaped:
Parapent:
1,5
0,8
depends on the design, can be determined by tests
Parachute production
When the design of the parachute is finished you can start on producing it. There are a
few important issues when starting the production. The deployment of the parachute will be
relatively violent, so the fabric and fibres you use need to be strong. Most often you can get
nylon wires and rib stop fabric at a kiting shop. These materials are ideally suited for the
parachute.
When cutting the fabric, you should take into account the fact that some of the fabric
needs to be double to be able to sew it.
Some more handy tips on the production of the parachute can be found here:
http://www.nakka-rocketry.net/paracon.html
After the parachute is produced the best way off course to see if it works is to test it.
The CanSat Team
Page 20 of 32
Example Assignments
The following assignments can be performed when working on the parachute.
1. Calculate the impact speed of the can without a parachute, when released from 1
kilometre altitude?
2. Calculate the needed minimal area for your parachute when you use a cross parachute?
What would be the sizes of the squares of the chute?
3. The same but with a spherical parachute? What will be the radius?
4. Test the descent velocity of your parachute with a soda can?
5. Try out different solutions for the parachute. A parachute with some holes or multiple
small parachutes? Both will enhance the stability of the CanSat.
The CanSat Team
Page 21 of 32
Chapter 5: Telemetry; sending data, setting up the ground
station and processing the data.
Subject:
This lesson will provide a background to the telemetry part of the CanSat
system. It will explain the operation of the transmitter board, help in setting up
a ground station and provides a guide to processing the received data.
Introduction
Telemetry is a technology that allows performing remote measurements. It is derived
of the Greek words “tele”, meaning remote, and “metron”, meaning measure. Telemetry is an
essential part of rocketry and satellite technology. Information is transmitted wirelessly
mostly using radio waves. On the ground these signals are collected by receiving stations.
Large space agencies have networks of these ground stations stretching all over the globe,
tracking, monitoring and receiving telemetry from their satellites.
Telemetry data can be divided into two groups, from internal and external sources.
Rockets and satellites are equipped with countless sensors that measure internal parameters.
Parameters can be temperature, pressure, attitude, power usage and much more. The
information from these sensors is called “housekeeping data”. It is used to monitor a
satellites health, and necessary for the operation of the system.
Information from the external sources is mostly what interest’s scientists. It is the data
collected from sensors or equipment measuring parameters from our planet, the space
environment or somewhat else depending on the mission. This information is called the
“mission data” or “scientific data”. In your CanSat this would be the information from the
sensor board. This data is often collected at the ground station and then processed or used by
scientist for research.
The CanSat telemetry has three distinct components, transmitting, receiving and
processing the information. The transmitter board inside the CanSat will collect the
information and sends out a radio signal. This signal is picked up by the ground station and
received on a laptop, where it is stored.
Transmitting data
Radio enables the transmission of information over
electromagnetic waves. The information can be sent in different
kind of signals. The earliest and most simple method is Morse
code. A radio signal is switched on and off on specific intervals. A
pattern emerges of short and long radio pulses. Remember the sign
for SOS: three times short, followed by three longs concluded with
three short pulses, rendering help when in distress.
Frequency Modulation
More complicated forms of radio communication have
emerged since, using a modulation of the signal to transmit
information, for example the familiar AM and FM.
The CanSat Team
Page 22 of 32
AM stands for Amplitude Modulation. The information is contained in the amplitude
of the signal.
FM stands for Frequency Modulation. The information is contained in the frequencies
of the signal.
The CanSat kit uses FM modulation to send its signals to the ground station. The frequency of
the radio signal is combined with two audio signals and then transmitted. It is the same
principal as with your FM music radio.
AX.25 Protocol
For communication to go smoothly a protocol is needed. A protocol is simply the method
used to format the information. The information is structured to enable the receiver to
understand the sent data. It contains information of the header, a call sign, identifying the
information and detecting and correcting errors. The CanSats use part of the AX.25 protocol.
This is a protocol for digital communication used by radio amateurs.
The CanSat uses a part of the protocol called the UI Frame.
It has been modified from the X.25 protocol to support call signs.
The protocol is used for transmitting short sets of data. Radio amateurs use it to
transmit weather reports, position coordinates and such. Very similar to the data the
CanSat likes to transmit.
An example of a protocol is shown in the table below. This protocol is nothing more than
a format.
The format means:
1. The first 8 bits are the header, used by the receiver to recognize the start of the
message.
2. The next 32 bytes are reserved for the information to be transmitted.
3. The last 16 bits are a checksum; the receiver can use it to determine if the message
was received correctly, without errors.
Header
Data bytes CheckSum
8 bits
32 bytes
The CanSat Team
16 bits
Page 23 of 32
Transmitter Hardware
The transmitter board has two important
components, a processor and a transmitter. The processor
gathers the information from the processor board and
generates a data stream. The data stream is sent to the
transmitter as two tones. One of 1200 Hz and one of
2400Hz, representing a logic 1 or 0. The transmitter
employs frequency modulation to produce a 433 MHz
carrier signal. This modulated signal is transmitted by
using a wire antenna.
The information is transmitted at 1200 bits per second, not very fast. Your local
wireless network can easily go up to 11 Megabits per second. The low data rate is chosen to
keep the electronics simple and easy. Additionally the amount of information to be
transmitted is small, making the data rate sufficient. Higher data rates would demand more
complex electronics and a more advanced ground station, making it very costly.
Adding telemetry to the programming code
Luckily communication with the CanSat is very straight forward and can be setup by
adding some lines to your code.
1. Insert the frequency you want to transmit on. Add it in the void setup, by adding:
a. Serial.println(‘Freq_string‟);
b. See Appendix B for more information on the Freq_string.
c. Use a delay of 1000 before setting the frequency, use a delay of 500 after the
frequency is set. This enables the transmitter to adjust to the new frequency.
2. Add a call sign:
a. Serial.println(‘Call_sign‟);
b. The call sign has to be unique; it could be your team name.
c. However it is limited to a maximum of 6 characters and has to be preceded by
a C.
3. Now the transmitter is setup. You can use it to sent telemetry in the void loop. The
transmitter is activated when you add the following command to the code:
a. Serial.print(`S`);(the same as used for communicating with the computer).
b. All Serial.print(``); commands afterwards will be read by the transmitter and
made ready for sending.
c. Use Serial.println(``); to end the data string which will be sent down.
4. Now your program is ready to transmit telemetry. See Appendix A for an example
program.
The CanSat Team
Page 24 of 32
Setting up the Ground Station
The ground station is all the equipment used to receive
the telemetry signal. It will always consist out of three
basic components:
An antenna to collect the signal,
a receiver which is able to process this signal,
and a storing device.
To the left you see the ground station for ANSAT. It is
used to receive signals from student satellites orbiting
in Low Earth Orbit.
The ground station of ANSAT in Andenes.
The Basic ground station for the CanSat consists out of the following equipment:
Omni directional or Pointing Antenna
Radio Receiver, Uniden UBC69 XLT-2
Laptop with installed software, Packet Engine and AGW Monitor
Stereo cable (3,5mm jack)
Follow the following steps to setup the ground station:
1. Install the ground station software on the laptop.
2. When using a pointing antenna connect it to the scanner.
3. Adjust the scanner to the correct frequency. Use the same frequency as used in the
code of the transmitting CanSat.
4. Connect the headphone output on the scanner with the microphone input of the laptop.
Use the stereo cable.
5. Open the AGW packet Engine and AGW monitor to start receiving data.
Ground station software
Your computer or laptop will be used as a modem to receive the information from the
scanner and use it. To prepare the computer for this function software needs to be installed.
Installing the software is very easy. Just double click on the gssetup.exe file. This will install
all the necessary software on your computer. Just follow the instruction on the screen, just
keep clicking next. (The program was originally made for the Dutch CanSat competition so
the instructions are in Dutch.)
When installed, open the program AGW Packet Engine, it is added to the startmenu.
This will enable your laptop to start receiving data packets from the microphone input. To see
the actual data the program AGW Monitor is used. When the ground station is setup correctly
you will see the data coming in here. The File >Save as text function can be used to store the
received data in a txt file.
Note: Remember to store the data before closing the program, otherwise the
information is lost.
The CanSat Team
Page 25 of 32
Radio Receiver
The radio receiver used is the Uniden
UBC69XLT-2 Handheld Scanner. It can contain up to 80
frequencies to be scanned ranging in three bands in both
AM and FM: 25-87 MHz, 138-174 MHz and 406-512
MHz. It has a BNC connector to easily switch between
different types of external antennae. It comes equipped
with a 3.5mm mini-jack to connect it to a headphone or
external device.
The scanner is very easy to use; the next steps explain the basic setup for the scanner,
appendix C shows a picture of the radio scanner.
1. Turn the scanner on
2. Press HOLD, you will see HOLD appear in the screen
3. Type in a number to store the frequency under, for example „1‟
4. Press Func and then E(Pgm)
5. Type in the frequency you are using, for example „433,650‟
6. Press E again to save the frequency
7. To switch between saved frequencies:
a. Press a number, for example ‘2‟
b. Press SCAN
c. If a frequency was stored under this number it will appear in the screen
Ground station problem solving
Hopefully you will have received your data by now. If you do not receive anything, you
can try the following:
1. Do you hear the signal when the audio cable is disconnected from the Scanner? Adjust
the scanner, frequency or volume, until you hear a beeping signal coming from the
scanner. The CanSats transmit between 433,050 and 434,800 MHz.
2. Check if the CanSat is working correctly. Is the battery still full? Trying powering the
CanSat off and on again.
3. Check the volume setup for your microphone input of the laptop. You might have to
look into the advanced settings. Try switching off all but the basic settings.
4. If the laptop has a line-in, try using it.
The CanSat Team
Page 26 of 32
Processing the data
Now the data is received it will have to be processed to get some more meaningful
results.
The steps below can guide you through
the process:
1. Make sure you save the received
telemetry. Use File>Save as Text
file.
2. Before beginning with the
processing make a back up of the
data file. This way if something
goes wrong you will still have the
original file.
3. Import the file into excel,
Data>Import external data. A
wizard will guide you through the
process of importing. Get the data
into different columns. You can
add some formatting in your code
to make this process easier.
4. Excel has a sorting option,
Data>sort. Use it to filter out the
not wanted data. If the telemetry
has been structured correctly this
should work easily. TIP: Use a
counter in the telemetry.
5. Now the data can be used for
analysis. Perhaps some
calculations have to be performed.
Excel is really good in doing these
for you.
6. Present the results in tables or
graphs.
To make sorting out of the necessary data easier you could add some aids to your code.
Add a counter, counting the runs through the loop of the code. It will make sorting the
data on a time scale a lot easier.
Adding the time, just enter time = millis(); to the code. It can be used for different
purposes. Recording the exact time the data was acquired or to calculate how much
time one loop takes. It can be used instead of a counter to sort the data.
Format the information on the CanSat, before it is send out. Some small adjustments
here can make life a lot easier when processing. Pay attention to “,” or “.” and choose
a handy separator between measurements.
The CanSat Team
Page 27 of 32
Example Assignments
The following assignments can be performed to test the ground station.
1. Write a test code for the CanSat to test the transmission of telemetry.
a. Try different call signs.
b. Try out different frequencies. Do not forget to adjust your scanner as well!
c. Add some housekeeping data to the code, for easier processing afterwards.
2. Setup the ground station:
a. Install the software.
b. Setup the radio receiver and connect it together.
c. Use the CanSat to test if the setup is working. Some fiddling might be required
to get it operational.
3. Test the CanSat and sent down information to your ground station. See if everything is
working as anticipated; if necessary make adjustments or improvements.
4. Adjust the sensor read program so it will send out information on the primary mission.
5. Prepare an Excel sheet to process data received from the CanSat.
6. Write a checklist of the steps that need to be taken to acquire the telemetry. For
example the steps to correctly setup the ground station, receive the data from the
CanSat and store the information for the data processing.
The CanSat Team
Page 28 of 32
Appendix A: Example transmission code
Appendix B: Transmitter Frequencies
Frequentie
(MHz)
433,050
433,100
433,150
433,200
433,250
433,300
433,350
433,400
433,450
433,500
433,550
433,600
433,650
433,700
433,750
433,800
433,850
433,900
Code
F8CF79
F8CFBD
F8CFFD
F8D03D
F8D07D
F8D0CD
F8D10D
F8D14D
F8D18D
F8D1D1
F8D215
F8D255
F8D299
F8D2D7
F8D31D
F8D361
F8D3A5
F8D3E5
Frequentie
(MHz)
433,950
434,000
434,050
434,100
434,150
434,200
434,250
434,300
434,350
434,400
434,450
434,500
434,550
434,600
434,650
434,700
434,750
434,800
The CanSat Team
Code
F8D429
F8D46D
F8D4AD
F8D4F1
F8D535
F8D575
F8D5B9
F8D5FD
F8D63D
F8D681
F8D6C5
F8D705
F8D749
F8D78D
F8D7CD
F8D811
F8D855
F8D895
Page 29 of 32
Appendix C: Scanner Guide
The CanSat Team
Page 30 of 32
Appendix D: The CanSat kit MicroController Board
The figure on the following page shows the MicroController Board of the pratt hobbies
CanSat Kit. To improve the usability some information has been added. This would enable a
better understanding of the board. It is helpful when connecting additional components to the
board. In the figure below you will find a more schematic view of the board.
Table with the explanation of the acronyms:
Acronym
Explanation
ADC
This a pin which leads to the Analog Digital converter on the processor
VThis is ground
V+
This is the 5 volts line (except for the entrance voltage which is 9 volts
PD
…?
PWM
TMX
Transmitting
RCV
Receiving
The CanSat Team
Page 31 of 32
The CanSat Team
Page 32 of 32

The Book

Transcription

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