How are teams mounting drivetrain encoders on WCD?

[ollien]

Originally Posted by tr6scott

Did you wire your cimcoders to the srx? If you did, did you solder in the pull up resistors? If not that is probably the issue with missing counts. We ran them on Minicims for our shooter wheels, to srx running speed control loop on the srx, and worked beautifully.

From Omar, at CTR when I asked question about cimcoder and pullups.

Well that's interesting... That might solve some of our problems. Surprising. We used the Andymark Universal Breakout board, which seem to have spaces for pull up resistors. Might have been good to read in on that.

Do other encoders generally need pull up resistors?

[tr6scott]

Originally Posted by ollien

Do other encoders generally need pull up resistors?

It's less about the encoders, as to what is reading the encoders. The roborio has pullup resistors built in. In the cRio days the digital side card had pullup resistors built in too. Read an input on either with nothing wired up, the all will return TRUE, or that the signal is high.

[ollien]

Originally Posted by tr6scott

It's less about the encoders, as to what is reading the encoders. The roborio has pullup resistors built in. In the cRio days the digital side card had pullup resistors built in too. Read an input on either with nothing wired up, the all will return TRUE, or that the signal is high.

Ah ok. I'm assuming the srx doesn't have one, then. The more you know.

[Nuttyman54]

Originally Posted by pwnageNick

Any reason you guys didn't bring that polycarb bracket across to mount the tube on the other side of the pulley? I know it's just an encoder but it would drastically lower any worry about that encoder moving too much for basically no extra weight.

I don't want to hijack the thread here, but mainly because the cross-brace attaches on the other side of the pulley, so there's no room. It's also intended to be flexible by design. Realistically, this mount is only there to keep the encoder body from rotating. It's pretty well retained inside the shaft, and because it's a rigid attachment there and a very snug fit, we want to avoid over-constraining the encoder and ending up putting a side load from any kind of misalignment.

The last benefit is this mounting lets us remove and replace a belt without taking the encoder off.

[Oblarg]

449 tends to use WCP gearboxes.

If we're using a SS gearbox, we place the encoder on the gearbox output shaft, mounted under the motors.

If we're using a DS gearbox, we place the encoder on a wheel shaft.

The latter can be a pain if you're using chain and need adjustability (such as w/ versablocks) to maintain tension, but there are workarounds (such as slotting the mounting holes for the encoder bracket).

We used to use thick surgical tubing for the coupling, but found that this actually causes a fair amount of noise in the encoder reading (my best guess is that the torsional flex in such a coupling leads to something like a stick-slip behavior, but I have no way of actually testing this). We now use helical beam couplers instead.

[tr6scott]

Originally Posted by Oblarg

We used to use thick surgical tubing for the coupling, but found that this actually causes a fair amount of noise in the encoder reading (my best guess is that the torsional flex in such a coupling leads to something like a stick-slip behavior, but I have no way of actually testing this). We now use helical beam couplers instead.

We found the same on our elevator in 2015, switched to 1000psi 1/4" fuel line and the problem was resolved. (available in all auto stores for a couple bucks a foot, max.)

[thinker&planner]

Originally Posted by tr6scott

We found the same on our elevator in 2015, switched to 1000psi 1/4" fuel line and the problem was resolved. (available in all auto stores for a couple bucks a foot, max.)

I discovered this summer that adhesive-lined heat shrink tubing works well too. It is designed to be used for underwater applications, but works well for attaching encoders too. It's easy to install, but a little more difficult to remove (you can just cut it off).

The McMaster part number is 7861K44, but don't buy the 4' length unless you want it to be shipped in a 4' long cardboard tube (not coiled up in a bag).

[marshall]

Originally Posted by thinker&planner

I discovered this summer that adhesive-lined heat shrink tubing works well too. It is designed to be used for underwater applications, but works well for attaching encoders too. It's easy to install, but a little more difficult to remove (you can just cut it off).

The McMaster part number is 7861K44, but don't buy the 4' length unless you want it to be shipped in a 4' long cardboard tube (not coiled up in a bag).

This is clever. Thank you!

[MichaelBick]

Originally Posted by Oblarg

We used to use thick surgical tubing for the coupling, but found that this actually causes a fair amount of noise in the encoder reading (my best guess is that the torsional flex in such a coupling leads to something like a stick-slip behavior, but I have no way of actually testing this). We now use helical beam couplers instead.

If your encoder mount has some free play or reasonably tight tolerances, I also really like slotting the given shaft and then clamping down with a hex or round shaft collar.

[Oblarg]

Originally Posted by MichaelBick

If your encoder mount has some free play or reasonably tight tolerances, I also really like slotting the given shaft and then clamping down with a hex or round shaft collar.

This is clever, and seems to me a somewhat nicer solution than press-fitting - I will have to add this to our team's list of things to try.

It'd also be a good opportunity to get the students to practice drilling into the end of a shaft accurately with our hobby lathe, which is something we seldom do (c.f. turning down a shaft).

[MichaelBick]

Originally Posted by Oblarg

It'd also be a good opportunity to get the students to practice drilling into the end of a shaft accurately with our hobby lathe, which is something we seldom do (c.f. turning down a shaft).

I really like thunderhex for this, just because it has the pre-bored hole. This keeps the drill relatively concentric to the axle and helps with chip clearing (the two biggest issues you have to face with small diameter drills).

[AdamHeard]

Originally Posted by MichaelBick

I really like thunderhex for this, just because it has the pre-bored hole. This keeps the drill relatively concentric to the axle and helps with chip clearing (the two biggest issues you have to face with small diameter drills).

The bummer is this hole isn't always concentric.

We generally bore encoder holes for this reason.

If your tailstock is reasonable concentric just boring with a 1/4" end mill is adequate.

[gc_coxen]

Originally Posted by AlexanderLuke

In the past with our fully custom WCD systems, Team 696 has placed encoders on the drive shafts of wheels other than the output shaft of the gearbox. It was perfectly reliable for us, with no slippage of the encoders or wandering of the encoders out of the shafts during practice, competition matches, or offseason events. I do not personally see any reason for us to change this practice in the future for such a setup.

We have often relied on US Digital S4 encoders for drivetrains and other systems, which are now replaced with the S4t as far as I understand. We have not had any experience with the S4t.

One tip for encoders in shafts though: feel free to use some hot glue to aid in the interface between the encoder shaft and the shaft it is connected to. This will certainly help in keeping that encoder from slipping inside the shaft, or the encoder from becoming removed from the shaft unintentionally.

Did your team experience any backlash with mounting to the drive axle? One of my teams mentors is worried that we will experience backlash if using PID on the drivetrain if mounting to the axles.

[AdamHeard]

Originally Posted by gc_coxen

Did your team experience any backlash with mounting to the drive axle? One of my teams mentors is worried that we will experience backlash if using PID on the drivetrain if mounting to the axles.

The drive axle is closer in gearing to the floor than somewhere near the motor, it will have a much more accurate measurement of floor position.

[gc_coxen]

Originally Posted by AdamHeard

The drive axle is closer in gearing to the floor than somewhere near the motor, it will have a much more accurate measurement of floor position.

My mistake, I meant to say between mounting to the drive axle or a separate wheel axle.