Misbehaving encoders

[Jared Russell]

Originally Posted by nathanww

Hey guys,
We've been trying to retrofit our Lunacy bot with quadrature encoders so that we can use it for some off-season projects. Unfortunately, the encoders are not cooperating with our plans. Disconnected from the robot, we can apparently read the shift in values from the encoder--but the robot's program doesn't seem capable of reading them(we checked to make sure we were using the right ports, actually starting the encoders, etc)

So, basically what we have is a situation where the robot's digital inputs are not reading the shifts. Our theory is that both the encoder HIGH and the encoder LOW signals are falling on the same side of the HIGH/LOW division when the robot reads it, so it can't pick up the pulse modulation. Does this make sense? Or is there something else that could cause this?

The theory you are describing (that you aren't reading the encoders fast enough and that they are going HIGH->LOW->HIGH so fast you don't see the "LOW" part) is known as aliasing. The Shannon-Nyquist Theorem basically states that you need to sample at least twice as fast as the highest frequency in your input in order not to miss anything. However, I don't think that's the case here, as the cRIO reads the encoder signal at many tens of KHz; you'd have to be spinning your encoder at a blazing speed to hit that.

It's more likely that it's a wiring or programming problem.

What kind of encoder are you using?

How is it currently wired?

Are you using LabView or C++, and what encoder code are you using?