Table of contents
3.Introduction
4.Specification requirements
5.Solutions suggestions
6.The magnetic fields sensors
7.The inclination measurement system
8.The gyroscope
9.The data acquisition system
10.Communication system
11.The power supply
12.Realisation of the PCB
13.The embedded system
14.Static Library Util.a
15.ViewPort
16.Xcompass
17.Sensors controller commands
18.Test
19.Future improvements
20.Conclusion
21.References
The gyro is an angular rate sensor. Its
main purpose on our system is to validate or invalidate the magneto data. The
different categories of gyro are:
Ø
Mechanical
Ø
Piezoelectric
Ø
Optical
Ø
Resonator
We chose to implement a gyro that
operates on the principle of a resonator first because this type has already
been use in IAU. It has no inner moving part, it is integrated, and it is
cheap.
The different gyros on the market operate
on the principle of the resonator gyro. Two polysilicon-sensing structures each
contain a dither frame, which is electrostatically driven to resonance. This
produces the necessary velocity element to produce a Coriolis force during
angular rate. At two of the outer extremes of each frame, orthogonal to the
dither motion, are movable fingers to form a capacitive pickoff structuring
that sense Coriolis motion.
The resulting signal is fed to a series
of gain and demodulation stages that produce the electrical rate signal output.
The dual-sensor design rejects external g-forces and vibration.
An important specification to choose the
gyro is the maximum angular rate that it can measure. We read on the report of
the MMR[5] that the
maximum speed approximately is 10km/h (approximately 2,78m/s). From this value,
we can calculate the maximum angular rate:
? = V / (R * p)
V: MMR’s speed
R the
distance between the two wheel divided by 2 (= 22,5cm on the MMR).
? » 3.93 rad / s ? ?
»
225.17 º / s
Equation 8: Maximum angular rate
Nevertheless
this angular rate is obtained from the maximum speed possible by the robot. It
seems realistic to estimate that this value will be 3 times inferior to the
previous results when the MMR turns. In this case a gyroscope that is able to
measure a maximum angular rate of 75 º / s could be sufficient to achieve our
application.
But to prevent
this risk, we will implement this component on a support in the aim to replace
easily this sensor by another one with a wider operation range if necessary.
We found several gyroscopes from
different companies:
Name
|
Range [°/s]
|
Sensitivity
[mv/°/s]
|
Noise
[mv rms]
|
Noise Density
[°/s/ÖHz]
|
Supply voltage [v]
|
Price [$]
|
ADXRS150
|
± 150
|
12.5
|
5
|
0.05
|
5 ± 0.25
|
30
|
ADXRS300
|
± 300
|
5
|
|
0.05
|
5 ± 0.25
|
30
|
ADXRS401
|
± 75
|
15
|
3
|
0.025
|
5 ± 0.25
|
22.5
|
Table 6: Gyroscopes from Analog Devices
Name
|
Range [°/s]
|
Sensitivity
[mv/°/s]
|
Noise
[mv rms]
|
Noise Density
[°/s/ÖHz]
|
Supply voltage [v]
|
Price [$]
|
CRS03-02
|
± 100
|
20
|
1
|
|
5 ± 0.25
|
285 **
£290
|
CRS03-04
|
± 200
|
10
|
1
|
|
5 ± 0.25
|
£290
|
CRS03-011
|
± 573
|
3.49
|
1
|
|
5 ± 0.25
|
£290
|
CRS04
|
± 150
|
12.75
|
1
|
|
5 ± 0.15
|
363 **
|
Table 7: Gyroscopes from Silicon Sensing Systems
Name
|
Range [°/s]
|
Sensitivity
[mv/°/s]
|
Noise
[°/s]
|
Noise Density
[°/s/ÖHz]
|
Supply voltage [v]
|
Price [£]
|
KGF01-1001
|
± 75
|
26.7
|
0.35
|
0.05
|
5 ± 0.25
|
160
|
KGF01-1002
|
± 250
|
8
|
0.35
|
0.05
|
5 ± 0.25
|
160
|
Table 8: Gyroscopes from Kionix
Name
|
Range [°/s]
|
Sensitivity
[mv/°/s]
|
Noise [mv rms]
|
Noise Density
[°/s/ÖHz]
|
Supply voltage [v]
|
Price [$]
|
MRG
|
± 60
|
25
|
4
|
|
5 ± 0.25
|
|
Table 9: Gyroscopes from Microsensors
** Found on supplier elfa.se (comparison ® ADXRS150 cost
138 $)
The components provided by the company
Silicon Sensing Systems and Kionix have been excluded of our selection due to
their cost. From the previous requirements, it seems the gyroscope ADXRS401
will be sufficient and it is low cost compare to the other ones.
The signal of the component is centred to
2,5V.
The z-axis rate-sensing device is also
called a yaw-rate-sensing device. It produces a positive-going output voltage
for clockwise rotation about the axis normal to the package top (clockwise when
looking down at the package lid).
We choose also to use the evaluation
board ADXRS401EB because it was impossible for us to solder the ADXRS401.
A great part of the gyro is implemented
on the evaluation board. We have just to proceed the output of the gyro in the
same way as the accelerometer (see section 7.3). The -3db breakdown frequency of the
low pass filter is the same and the amplification is equal to 1,3. The output
of the INA2126 is then sent to a channel of the ADC MAX186 and the temperature
reference on the internal ADC of the microcontroller in the first card or also
on the ADC MAX186 on the second card. This parameter will allow compensating
the output’s derivation due to the temperature’s variation.
|