Encoder drift on elevator

Hello, we are using the US digital S4T encoder with single ended output, 500 CPR, and 1/4 inch output shaft. It is attached to a shaft that rotates the drum on our continuous 2 stage elevator. It is plugged into a Talon SRX breakout board, and we use motion magic to control the elevator. There are two 775 pro’s controlling our elevator.
We find that over time, the elevator accrues a lot of encoder drift. When we move the elevator up at Max speed, it either skips or repeats clicks, and it messes up our sequences. We have a limit switch on the bottom and middle of our elevator to reset it but it still has such high amounts of drift.
Has anyone had any experience with a similar scenario or have some ideas on how to stop this issue?

Check the datasheet on your encoder. It’s more than possible that you’re exceeding the RPM limit.

I’m unfamiliar with that encoder, does it have dip switches to select different pulse counts per rotation? If it does, decreasing your pulse count will increase the RPM limit, at the cost of slightly lower resolution on position.

Where along on your reduction is the encoder, and what speed does it spin at? Again, check the datasheet.

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That could totally be the issue. The encoder is only rated for 100 rpm, and the encoder velocity according to the talon is around 100 rotations per second. We will look into fixing that. Thanks!

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Glad I could help! Good luck and feel free to reach out if you have any other issues.

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It’s also always a good idea to have some way to dynamically calibrate an encoder for an actuator which controls an intrinsic position [arm or piston vs drive or input wheel]. Whether it’s a hard stop or a limit switch or something else, having the ability to intentionally re-calibrate midmatch is a great safety net.

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This isn’t the only problem. The bushing version of the S4T is only rated for 100 RPM because the bushing heats up. It is far more likely that the shaft is slipping due to the drag, meaning that your connection between encoder shaft and drum is bad.
Electrically, you can totally go above 100 RPM. The optical encoder bits of it shouldn’t outright fail above that speed.

We have two reset points to dynamically recalibrate the encoder, one at the bottom of the elevator and one halfway up

We epoxy’d the encoder in and it looks solidly connected, is there a better way to create the connection between the encoder shaft and drum?

How long are the wires between your single-ended output encoder and what is ultimately reading the encoder? Is the encoder wiring bundled with nasty edgy signals like motor controller outputs?

The wires are pretty long and travel underneath two neos on the drivebase, although we noticed that the elevator still drifts when the drive base isn’t moving

Here’s why I ask:

Our wrist position was drifting intermittently during matches. The wiring from our single-ended output wrist encoder ran 7 feet through drag chain along with the wrist motor wiring. It turns out that the very edgy PWM motor currents were coupling additional edges into our encoder signal. Separating the encoder wiring from the motor wiring solved that problem for us.

I didn’t believe that 7 feet was long enough to allow corruption of the encoder signal. Live and learn. In the future we will be using differential signalling for encoders with more than a couple feet of wiring.

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What sort of wire or cable did you use for the encoder? Ribbon cable? Shielded twisted pair should give you the immunity to EMI needed. Twisting the motor wires (about 2 twists per inch) will also help.

We used the standard 4 pin molex wire that US digital sells. It has shielding but we haven’t grounded it

Unconnected shields cause problems all their own. Simply grounding one end may resolve your issue.

Do we just solder a small line from the foil to a ground pin somewhere? Or does it go to the frame?

Hopefully the foil is accompanied by a drain wire you can work with. Connect it to whatever is supplying the negative power pin to your encoder. Don’t connect to the robot frame. Keep the connection short.

Yes, you should find a bare stranded wire that is in contact with the foil shield along the whole length of the cable.

You should connect the shield to the Ground or 0V wire at the RoboRio. Second best would be to extend the shield drain wire and twist it with the 0V wire that goes into the power connector for the Roborio.

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