# the gyroscope

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.

## 8.1.                     The gyro’s operation mode

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.

## 8.2.                     The requirements of the gyro

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.

## 8.3.                     The selection of the gyro

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.

## 8.4.                     The design of the system

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.