Servo - July 2008

by Robert Doerr

One of the joys of the robotics hobby is mastering the art of interfacing. A lot of the parts are already in front of us and we just need to make them work together. Recently, I ran into a problem with a pair of quadrature encoders for the drive train on one of my robots. The encoders themselves worked fine and were generating perfect quadrature outputs. However, they were sending out data faster than the controller could handle at higher speeds. As a result, the encoder readings were worthless and could not be trusted.

Getting a valid lower resolution reading of the encoder is certainly better than high resolution unreliable data. I needed to know how far the motors really moved but didn’t necessarily need to know with as high of a resolution as the encoders were providing. So, the task at hand was a way to lower the resolution enough so that I could always count on the readings and help keep track of the movements. A bit of encoder resolution may be lost but it should still be close enough for this particular project.

Symptoms of the Problem

the counts started going backwards! As it sped up some more, it counted forward again. This cycle continued back and forth a bit and then the counts were completely erratic. It appears that the encoder was exceeding the polling time used by the controller and would start missing pulses at higher speeds. Obviously, the encoders were not matched up well with the controller. I’ve seen this happen with my robot drive base and also when I was using some salvaged encoders from HP scanners and DeskJet printers. Whenever you see symptoms like this, it should throw up a red flag and make you take a look to see if this may be the problem.

I was watching the encoder counts while running some robot drive motors. During the initial bench testing, the motors weren’t running at full speed and the results I saw were just as expected. However, things got interesting when I started ramping up the speed. A very peculiar thing started happening. The encoder counts started to rise as expected but as the motors sped up,