Gyroscope Code

[Kevin Watson]

For those who might be interested, I've just posted updated gyroscope interface code for IFI's robot controllers. The code has been significantly reworked to use the new ADC interface code, which allows the programmer to use other analog sensors alongside a gyroscope without affecting the performance or operation of the gyroscope. In addition, significant performance improvements can be realized using the new oversampling and deadband features. The code can be found here: http://kevin.org/frc. As always, if you find a bug in the code or a problem with the documentation, please let me know.

-Kevin

Here's the readme file:

The source code in gyro.c/.h contains a driver and supporting 
software to interface a variety of inexpensive gyros to your
robot controller. This software was tested with Analog Devices'
ADXRS401EB, ADXRS150EB and ADXRS300EB gyros. Data sheets for 
these devices are included. By default, this software is 
configured to work with a ADXRS150EB gyro, sampling at 800Hz, 
downconverting to an update rate of 50Hz by averaging sixteen 
samples per update. These parameters can be changed by editing 
gyro.h and/or adc.h.
 
Version 0.5 of the gyro interface software has been altered
to use the ADC interface provided by adc.c/.h. This change 
allows the programmer to use additional ADC channels for other 
analog sensors without affecting the performance or operation 
of your gyro. See the documentation included with adc.c/.h for 
information on how to use this new functionality.
Another new feature is the ability to specify a measurement 
deadband centered about the gyro bias. ADC measurements within 
this deadband will not be used to calculate the gyro angle or 
angular rate. This feature can have a significant impact at 
minimizing short-term drift just after a bias calculation has
taken place. To use this feature, follow the instructions 
embedded within gyro.h.
 
This source code will work with the Robovation (A/K/A EDU-RC) 
robot controller and the FIRST Robotics robot controller.
Wiring-up the ADXRS401EB, ADXRS150EB and ADXRS300EB gyro
evaluation boards is straightforward: Grab a PWM cable and cut
off the male end and strip the three wires back a centimeter 
or so. With a low wattage soldering iron, solder the white wire 
to the RATEOUT pin, solder the black wire to the AGND pin, the 
red wire to the AVCC pin, and finally connect a jumper wire 
between the AVCC and PDD pins. Plug the female end into one of 
the robot controller's analog inputs. These gyro evaluation
boards can be purchased from analog devices (www.analog.com),
and Digi-Key (www.digikey.com). Another great source for
Analog Devices gyros is SparkFun Electronics (www.sparkfun.com).
 
For optimum performance, you'll need to calibrate the scaling
factor to match that of your gyro. One way to calibrate your 
gyro is to mount it very securely to a hefty, square or 
rectangular object. Mounting the gyro to a hefty object will 
help dampen higher frequency vibrations that can adversely 
effect your measurements. Place the mounted gyro against 
another square object and start the included demonstration 
software. To get good results, the mount must be absolutely 
still when the "Calibrating Gyro Bias..." message appears on 
the terminal screen. After a few seconds, the gyro angular rate 
and angle will be sent to the terminal screen. If the angle 
drifts rapidly while the mounted gyro is motionless, you need 
to restart the software to acquire a new gyro bias measurement. 
Again, gyros are very sensitive and must be still while the bias 
is calculated. Once the gyro is running with little drift, 
rotate the mount 180 degrees and note the reported angle. If the 
angular units are set to tenths of a degree, the ideal reported 
angle is 1800. If set to milliradians, the ideal angle 1s 3142 
(Pi times a thousand). For every tenth of a percent that the 
angle is high, decrease the GYRO_CAL_FACTOR numerator by one.
Conversly, for every tenth of a percent low, increase the 
numerator by one. Repeat until you're satisfied with the 
accuracy.
 
The included project files were built with MPLAB version 7.20.
If your version of MPLAB complains about the project version, 
the best thing to do is just create a new project with MPLAB's 
project wizard. Include every file except: FRC_alltimers.lib 
and ifi_alltimers.lib and you should be able to build the code.
 
****************************************************************
Eight things must be done before this software will work 
correctly on your robot controller:
 
1) The gyro's rate output is wired to one of the analog inputs
of your robot controller and gyro.h/#define GYRO_CHANNEL is
updated with the analog channel your gyro is attached to.
 
2) A #include statement for the gyro.h header file must be 
included at the beginning of each source file that calls the 
functions in gyro.c. The statement should look like this: 
#include "gyro.h".
 
3) Initialize_Gyro() must be called from user_routines.c/
User_Initialization().
 
4) You must select the gyro you're using from a list in gyro.h
and if needed, remove the // in front of its #define.
 
5) The default angular unit is milliradians. If desired, this
can be changed to tenths of a degree by editing gyro.h
 
6) A gyro bias calculation must take place using the functions 
Start_Gyro_Bias_Calc() & Stop_Gyro_Bias_Calc() described below.
This must be done several hundred milliseconds after the gyro 
powers-up and is allowed to stabilize.
 
7) For optimal performance, you'll need to calibrate the gyro 
scaling factor using the instructions above or those included
in gyro.h.
 
8) Follow the instructions found in adc_readme.txt for
installation instructions.	
 
****************************************************************
Here's a description of the functions in gyro.c:
 
Initialize_Gyro()
This function initializes the gyro software. It should be called 
from user_routines.c/User_Initialization().
 
Get_Gyro_Rate()
This function returns the current angular rate of change in 
units of milliradians per second. If desired, the angular unit 
can be changed to tenths of a degree per second by modifying
the angular units #define entry in gyro.h
 
Get_Gyro_Angle()
This function returns the change in heading angle, in 
milliradians, since the software was initialized or 
Reset_Gyro_Angle() was called. If desired, the angular unit
can be changed to tenths of a degree by modifying the angular
units #define entry in gyro
 
Start_Gyro_Bias_Calc()
Stop_Gyro_Bias_Calc()
These functions start/stop a new gyro bias calculation. For best 
results, Start_Gyro_Bias_Calc() should be called about a second 
after the robot powers-up and while the robot is perfectly still 
with all vibration sources turned off (e.g., compressor). After
at least a second (longer is better), call Stop_Gyro_Bias_Calc()
to terminate the calibration. While a calibration is taking
place, gyro rate and angle are not updated. Once calibrated, a
call to Reset_Gyro_Angle() should take place.
 
Get_Gyro_Bias()
This function returns the current calculated gyro bias. For
extra precision, this software internally maintains a gyro bias
value that is the sum of GYRO_SAMPLES_PER_UPDATE samples of the 
gyro's analog output. By default, GYRO_SAMPLES_PER_UPDATE is
set to a value of eight in gyro.h
 
Set_Gyro_Bias()
This function can be called to manually set the gyro bias. For
extra precision, this software internally maintains a gyro bias
value that is the sum of GYRO_SAMPLES_PER_UPDATE samples of the 
gyro's analog output. By default, GYRO_SAMPLES_PER_UPDATE is
set to a value of eight in gyro.h
 
Reset_Gyro_Angle()
This function resets the integrated gyro heading angle to zero.
 
Process_Gyro_Data()
This function should be called when the ADC software reports
that new gyro data is available. Ideally this should be done
within the user_routines_fast.c/Process_Data_From_Local_IO()
function. See the enclosed copy of user_routines_fast.c for
an example of how to do this.