Talon SRX Magnetic Encoder Cable Issues

[rafi]

We are making a flywheel shooter. The flywheel is powered by a mini-CIM, and has an integrated Talon SRX Magnetic Encoder in a gearbox attached to it. Due to the fact that we have this shooter mounted on a turret, we need to have a long enough encoder cable for it to be able to reach the encoder, no matter where it is in the rotation of the turret. Whenever we test the encoder using a 12" 10-wire ribbon cable that was bought from VEX, the motor runs perfectly. When we use a longer 128" cable that we crimped, which is necessary for the turret, the motor runs at about half the speed we expect and makes a lot of loud grinding-y noises, which we figured are not good. We tested all the crimps and the crimp seems to work. The resistance for the shorter cable read as -0.3 ohms, which I assume means it's about 0. The resistance for the longer cable read as 0.8 ohms.

Our current hypothesis is that the encoder is diverting power from the motor, since they are both connected to the same Talon. We are not really sure why this might be, could anyone give us any ideas on what could be wrong, and how to fix it?

[saikiranra]

We had a similar issue during build season, but our problem was actually the encoder values. The noise from the flywheel motors + the noise from the turret motor + the length of the cable made the signal very noisy, which gave us lots of problems. We were able to find this issue by connecting the signal to an oscilloscope and seeing the waveforms as we ramped the flywheel power. To fix this, we shielded the cable and made a very simple RC filter to take out the noise.

Do you have any pictures of the setup?

[GeeTwo]

If an encoder where drawing enough current from the motor controller to cause the motor a problem, it would almost certainly be releasing the magic smoke, melting insulation, or engaged in some other catastrophic failure.

I concur that noise in the encoder line is a much more likely cause of error. If you have a long wire harness with the encoder lines and the two power lines from the Talon to the motor all close to each other, there are several forms of cross talk than can get you, not to mention external sources. An alternative (or additional) solution to shielding the long cable would be to use a shorter encoder cable in the first place, by relocating the SRX to be closer to the encoder (probably on the turret). This would mean a longer CAN bus, but CAN is engineered to reduce noise, and to be more tolerant than most encoder counters. If you do this, don't forget to use twisted pairs for CAN!