Encoder: Can't Find Consistent Values for One Rotation

[Alan Anderson]

I agree, the documentation says a Tetrix motor encoder provides 1440 counts per revolution. How are you measuring it?

Perhaps more importantly, how are you using it? It's possible that your motor code is letting the wheel spin a teeny bit longer after the encoder reaches the desired value, instead of bringing it to an immediate halt.

[PAR_WIG1350]

Base on anecdotal evidence, the tetrix encoders are (expensive) junk that are nearly impossible to use for anything other than determining if the motor is moving, an if so, the direction in which it is moving. They are often used for detecting motor stall and preventing motor damage.

Welcome to Chief Delphi! I have no actual answer to your question, but since this is your first post, I'm going to assume you're at least somewhat new. Therefore, the Chief Delphi community gives a huge WELCOME to you!

[Alan Anderson]

Originally Posted by PAR_WIG1350

Base on anecdotal evidence,...

Based on more extensive documentation, the Tetrix Motor Encoders appear to be essentially the same thing as the encoders supplied in the FRC Kit of Parts for the past few years. When installed correctly, using the proper spacing of the plastic encoder disk from the circuit board, they work consistently well.

[rudun]

Is it possible to turn the shaft by hand and read the ticks output for one revolution. That is how we do it with the encoders provided with the FRC kit. Then based on wheel size you can figure out the distance per 1 revolution. The formula is floating around CD.

[Ether]

Originally Posted by rudun

Then based on wheel size you can figure out the distance per 1 revolution. The formula is floating around CD.

The distance the wheel travels per revolution is equal to the circumference of the wheel. One circumference is equal to pi times the wheel diameter.

So you can turn the wheel one revolution and count the encoder ticks... or you can count the encoder ticks for one encoder rev, then reduce that by the gearbox (and chain, if present) ratio to get the fractional wheel revs.

[feverittm]

There are also a couple of other possibilities (at least):

1 - The wheel shaft could be slipping in the encoder causing an changing number of ticks per revolution.

2 - The encoder wheel could be damaged (scratched). This can cause ticks to be missed or miscounted. Not at all deterministic.

We had these issues last year with our arm and needed to switch to a pot to measure arm angle. Not that a pot is that much more reliable.