Testing Software

It is now time to build the software necessary to drive and test the hardware. Each piece of hardware in the robot base must be initialized and then either read or written in a specific manner. Ideally, the software should be simple to use and understand.

The complete test program can be found here on GitHub.

Test Interface

The test software is designed to read commands from Ardunio's serial port and write status back to this same port. The port is set up at 115200 baud.

Unless otherwise specified, the software periodically reports the status from the IMU as well as the motor current and odometer counts. The format of this status message is:

 46 -151 -291 43 -31 17 418 304 14348 12947

The numbers are the the accelerators x, y, z, the gyros x, y, z, the left and right motor currents, and the left and right odometer readings from the rotary encoders.

This port accepts several input commands:

m left-motor right-motor    Set left and right motor speed. The values are decimal numbers between -400 and 400. Positive values move the motor in the forward direction, negative in reverse, and zero stops the motor. Missing arguments default as zero. An 'm' without any arguments stops both motors.

f report-divider    Set the frequency of the status reports as a multiple of 50Hz. It defaults to a report every five seconds. Setting this value to 50 will report once a second.

r    Toggles the status reports on and off.

How the Hardware is Programmed

The program in the IMU hardware was discussed in a previous post. The data outputted by the IMU is passed straight through this system into the status message.

The motor controller outputs a voltage proportional to the current being consumed by each motor. Godfrey uses two analog ports to read these voltages and then multiplies them by a constant to convert them into values that represent the milliamps used.

The motor controller outputs a signal when the H-bridge is overheating either due to driving the system too hard or because of a direct short. This error condition is detected by digital pins and reported as an error.

The odometer measures how far each wheel rotates over a specific period of time. The actual value is calculated by decoding the 2-bit gray code signal supplied by rotary encoders attached to the shafts of the motors. Each motor's odometer count is updated by adding a delta value generated through a lookup table indexed by the rotary encoder current 2 bits along with the last 2 bits.

Finally, there is the actual motor control itself. The Arduino can generate the PWM signals necessary to drive the motors. So the test program initializes the necessary timers and uses the Arduino to create a 20KHz PWM signals sent to the driver chips. The Arduino also specifies the motor direction. The speed value can ranges between -400 to 400.

The next post will outline the results of testing.