Testing Results

I have been able to verify most of Godfrey's hardware components are working correctly using the test software described in the last post. The one exception is the motor fault indicator which I didn't test because I was uncomfortable shorting the pins to see if it worked.

The IMU was already working and tested previously. This data is passed straight through to the status report.

To test the odometer I manually spun the rear shaft and saw the odometer counts increased and decreased appropriately. This count was seen in the status output. I tested each motor independently and they each worked.

Next I tested the motors themselves using the 'm' command. I was able to move the motors forward and backwards on command. Each motor worked independently and changed speed and direction depending on the value specified. However it was immediately obvious that the motors were not running at the same speed for the same input value. To determine the exact behavior I ran a series of tests in the forward and backwards direction and recorded the results. Below is a graph of the actual speed as measured by the odometer based on the input value. The right motor is in green and the left motor is in red and the odometer was sampled over 1 second periods.

It is obvious from the graph that the right motor is slower than the left motor. For example, at the maximum forward speed of 400 the left motor's odometer is 10392 over one second and the right one is 9563 (around an 8% difference in speed). To determine whether the speed difference is the motors or the driver chips I flipped which driver drove which motor and reran the tests. To two motors showed almost exactly the same odometer readings for each input value. So it seems that the issue is with the motors and not the drivers.

This graph also shows that the motor response is not linear to the input values requested. As we approach the maximum value the velocity tapers off. The right/left differences along with the non-linearity will have to be considered when creating the balancing algorithm. It is very important to know the maximum speed the motors can go so Godfrey doesn't attempt to exceed this speed and ultimately fall over.

I monitored the current used by each motor and plotted it in a graph similar to the odometer graph. Here it is:

I am not exactly sure what to make of this graph. I noticed that the current for both motors was not very steady from one reading to the other. But it is pretty interesting that the right motor, which is the slower one, had a much poorer behavior. Perhaps the right motor just had a bad choice of samples. Maybe if I averaged the values over several readings the graph will make more sense. That is my next step and I will report the results in another post.