How are teams mounting drivetrain encoders on WCD?
 

My team is experimenting with WCD this summer and we've begun discussing the various ways to go about mounting encoders. The three options we've thought of are mounting to the ouput shaft of the gearbox, mounting to one of the wheel axles, or utilizing a CIMencoder.

What have your teams used and what have you learned from it? Pros? Cons? Would you do anything differently?

Re: How are teams mounting drivetrain encoders on WCD?
 

While my team has not built a WCD, I just want to say that I cannot recommend the CIMCoder. 2/4 of ours stopped functioning by competition season and I have heard of their shells cracking in high vibration applications.

YMMV

The method I have heard most commonly is mounting something to the output shaft of the gearbox, using something like an SRX Mag Encoder.

Re: How are teams mounting drivetrain encoders on WCD?
 

The one thing I have against mounting to the output shaft is that it removes our ability to convert to a dogshifter or ballshifter

Re: How are teams mounting drivetrain encoders on WCD?
 

I assume that you are speaking of the andymark CIMcoder. Am I correct?

Re: How are teams mounting drivetrain encoders on WCD?
 

Correct. I should clarify. The one you posted seems to be fine, though I must admit I haven't personally tested it. I hope to do so next year.

Re: How are teams mounting drivetrain encoders on WCD?
 

For FRC, it probably doesn't matter, just get and encoder in there... :)

So for discussion purposes, what are you trying to measure? Seeing this is a drivetrain, probably distance. So if you put the encoder on the wheel axis, you are limiting your electrical accuracy, as the final distance per count, will be the largest... Say you have 100 count per rotation encoder, and 7.75" pneumatic wheel, roughly 24" circumference, 100 counts per rotation, each count equals a 1/4". So you can't position the wheel any finer than a quarter of an inch. But hey, in FRC if you can put a bot within a quarter of an inch you are typically fine.

Mechanically, what is happening if you place the encoder on the wheel axis, you have eliminated any gear mesh slop, errors, backlash, so mechanically, putting the encoder here, will be the most accurate mechanically.

If you move the encoder to the motor, like the cimcoder, now your distance calculation is going through a gearbox, there may be multiple stages, backlash, etc. that is generating an error between what the motor shaft is doing and what the wheel axle is doing. Say your gearbox 15:1, so the encoder distance at the motor, is now 0.24" divided by 15 or 0.016" per encoder count. Sounds more accurate, electrically, but mechanically you probably have more than 1/4" of backlash in the geartrain, (hold the motor shaft, and wiggle the wheel back and forth.) now when the programmer tells the motor to stop at 1000 encoder clicks, all of that backlash is an error in stopping. Do you have a two speed transmission? Now you programmer needs to know what are the different gear ratios, and what gear the bot is in to keep track of the distance traveled. Did you want to switch gears, during auto, now there are errors when the controls tell the gear to switch, and when the pneumatics actually switch the gears.

Can you do it with a CIMcoder, yes. Will it be better than nothing, Yes... You want to make it easier on your programmer, so they only need to deal with one distance per count calculation, put the encoder on the wheel axle shaft.

There is another drawback of putting the encoder on the motor, is you need to be aware of how many counts per second, your controller can handle. If you are counts are being generated quicker than you can process them, you miss counts, and all bets are off. In FRC with the roborio, this is pretty much a mute point now, but it is a design consideration in industry, and it is dependent on the controller used.

So if you are using a roborio, we used a cimcoder on our shooting wheels last year, and was fine. If you are using the Talon SRX to handle the counts, we also used the 775pro and vexpro versaencoder at 1:1 and was find too.... 14,000 rpm and 4096 counts per rev... no missed counts. (engineer close enough to control as expected)

Re: How are teams mounting drivetrain encoders on WCD?
 

We mount it to a wheel shaft. Gearbox output shaft usually doesn't have enough clearance for Grayhill encoders we use.

CIMcoder worked great for our shooter motor this year. We had no issues at all once we got the system mounted correctly. Don't think I would put one on a drive train gearbox. Those CIM's are usually not easy to remove if the encoder required maintenance.

David

Re: How are teams mounting drivetrain encoders on WCD?
 

We haven't actually run the drivetrain yet so I can't vouch for how well it holds up, but the Apex offseason robot is running encoders on the front wheel axles and made a small polycarb bracket to hold it in place.




Protip: Don't be like us, put your set screw hole somewhere where it can be accessed, not underneath the pulley/sprocket. Definitely overlooked that one.

Re: How are teams mounting drivetrain encoders on WCD?
 

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.

Re: How are teams mounting drivetrain encoders on WCD?
 

We use the encoder attachment on the 3 cim ball shifters by vex. (http://www.vexrobotics.com/3cimballshifter.html) It makes it super easy to attach and use encoders though the gearboxs in general are quite heavy and expensive. In addition we have had some trouble with the retaining clip for the shifter shearing off. We use the entire line of WCD for our robot and have found that this is the easiest wasy to mount encoders.

Re: How are teams mounting drivetrain encoders on WCD?
 

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.

Re: How are teams mounting drivetrain encoders on WCD?
 

You could try this hack: https://www.chiefdelphi.com/forums/s....php?p=1569705. The only challenge is to clear the hex collar. A 3D printed housing /standoff (or just a bit of bent lexan) could work really nicely.

Re: How are teams mounting drivetrain encoders on WCD?
 

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.

Re: How are teams mounting drivetrain encoders on WCD?
 

2014 we used Ball shifters and used the encoder setup that comes with it.
2015 we used the AM CIMple Box and 3D printed an encoder gearbox that mounted to the 1x2 frame. It had a 3D printed gear on the hex drive axle and we pressed a 3D printed gear onto the Greyhill encoder. Worked well.
2016 we used the WCP SS gearbox (single speed with 3 motor mounts). We only used 2 CIMs so we 3D printed a mounting plate and gear so that the encoder could mount using the CIM bolt holes. Worked great. Much easier to get to and work on if needed compared to 2015.

Re: How are teams mounting drivetrain encoders on WCD?
 

We had 2 different ways of including an encoder on our drive this year. Originally with our 775pro and VP gearbox we just included the encoder stage in our VP. When we moved to a CIM/miniCIM gearbox in an effort to keep it as low profile as possible, in this case it was ~2.625in tall, we had to move the encoder away from the gearbox. We moved it to the center wheel axle.

Re: How are teams mounting drivetrain encoders on WCD?
 

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?

Re: How are teams mounting drivetrain encoders on WCD?
 

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.

Re: How are teams mounting drivetrain encoders on WCD?
 

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

Re: How are teams mounting drivetrain encoders on WCD?
 

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.

Re: How are teams mounting drivetrain encoders on WCD?
 

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.

Re: How are teams mounting drivetrain encoders on WCD?
 

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.)

Re: How are teams mounting drivetrain encoders on WCD?
 

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).

Re: How are teams mounting drivetrain encoders on WCD?
 

This is clever. Thank you!

Re: How are teams mounting drivetrain encoders on WCD?
 

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.

Re: How are teams mounting drivetrain encoders on WCD?
 

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).

Re: How are teams mounting drivetrain encoders on WCD?
 

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).

Re: How are teams mounting drivetrain encoders on WCD?
 

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.

Re: How are teams mounting drivetrain encoders on WCD?
 

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.

Re: How are teams mounting drivetrain encoders on WCD?
 

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.

Re: How are teams mounting drivetrain encoders on WCD?
 

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

Re: How are teams mounting drivetrain encoders on WCD?
 

I don't have any pictures unfortunately, but a couple of 1712s students designed a novel mount for the Versa block system. They 3D printed a piece that interfaced with the contour of the Versa block above the 2x1 channel (including the existing mounting holes) and stick out just beyond the end of our drive axle. There was an adjustable encoder mount that was then slid into that overhung lever, allowing a structural element to hold the encoder that could also slide with the Versa block as the block moved to tension chain.

Re: How are teams mounting drivetrain encoders on WCD?
 

Would be cool to get a picture of this. Sounds like a cool add-on.

Re: How are teams mounting drivetrain encoders on WCD?
 

FWIW, 254 generally (or always?) mounts their encoder on a wheel shaft and has some of the fanciest autos out there.

Re: How are teams mounting drivetrain encoders on WCD?
 

Just to make sure people understand here: Adam's example is valid proof that putting an encoder on an outer wheel axle will not inherently add too much variability to produce reliable autos.

I feel that it's worth adding that this is possible because 254 properly tensions their chains, which means the backlash/slop between the outer wheels and directly driven wheels is minimal. If you're using WCP sliding versablocks, you should be able to tension your chain/belt well enough to make this a viable solution as well.

As an aside, 254 and 971 gave a great talk at Champs 2015 which is recorded on Youtube about Motion Profiling and Control which is super helpful to understand how they get such reliable auto control. Worth looking at.

TLDR; If you have a reasonable tensioning method for your chains (esp sliding bearing blocks) you should be fine putting an encoder on one of the outer wheel axles.

Re: How are teams mounting drivetrain encoders on WCD?
 

I don't believe 254 ran tensioners in 2016 or 2015.

Re: How are teams mounting drivetrain encoders on WCD?
 

I think think another trick is making sure you put the encoder on the weighted wheels. That way you don't have to worry about chain tension, and the encoder should always be touching the ground, at least in most cases. Another thing some teams do is preload their chains. Instead of pushing your robot forward into its starting position, push it too far forward, and then pull back. This will preload the chains to they are tensioned in the right direction for moving forward, which helps reduce shock loads.

Re: How are teams mounting drivetrain encoders on WCD?
 

This is an exceedingly smart idea that I think we will have to steal next time we do a DS drive.

Re: How are teams mounting drivetrain encoders on WCD?
 

Correct. I actually had a much longer post drafted up and then deleted most of it to keep it shorter and more straightforward. Tension your chains and you shouldn't have a problem.

I would consider a well-designed exact C-C setup to be "properly tensioned", such as 254 ran in 2015 and 2016 and 118 for the past few years as well.

Some quick math also shows that a few degrees of "slop" of the wheels relative to eachother will only result in a positional error of fractions of an inch (1 degree of slop on a 4" diameter wheel is less than 0.040" linear). While you may get some "noise" in your encoder values, you should be able to tune a PID loop to be stable with that amount of error and still give extremely precise auton navigation.

Re: How are teams mounting drivetrain encoders on WCD?
 

Isn't this backwards? If the robot last moved backwards, when forward voltage is applied it will have to drive through all of the slop in the system before moving forward. If the robot last moved forward, the slop is "behind" the applied power to the system, and it won't jerk as much. So you want the last "push" of a robot being set up for auton to be forward.

It's a good idea to push the robot back and forth a few times as you set it up, in either case, to make sure everything is running smooth and your drivetrain isn't stuck on a burr or anything like that. I've just always made sure the last push was in the direction the robot intends to travel.

Re: How are teams mounting drivetrain encoders on WCD?
 

Its definitely pull if you want to go forward. If your motor is to the left of the wheel, when the motor attempts to drive the wheel, it will pull the bottom length of the chain tight. Pulling the wheel backwards also causes the bottom part of the chain to go tight, which is what you need.

Re: How are teams mounting drivetrain encoders on WCD?
 

You're thinking in the wrong reference frame, forward in the motor frame is reverse when externally pushed.

Re: How are teams mounting drivetrain encoders on WCD?
 

Well, I've been very meticulously setting up robots for auton wrong for like three seasons now. :o Thanks for the clarification.

Re: How are teams mounting drivetrain encoders on WCD?
 

Hate to ask such a stupid question (my team has never used chain. Only belts and even then only for the DT), but here goes.

A) What do you mean by the "weighted" wheel? Do you mean the dropped wheels?

B) What do you mean by preloading chain? How would this help? You said something about moving the robot forward and then back, but I don't understand how this would help.

Thanks so much!

Re: How are teams mounting drivetrain encoders on WCD?
 

The weighted wheel would be the wheel with the most weight on it that is not the dropped wheel. For the setup being discussed, you couldn't easily put an encoder on the center dropped shaft. So the encoder would be mounted to one of the outer wheels. It the encoder is mounted to the wheel that is on the heavier side of the robot, that wheel will usually be in contact with the ground, and therefore the encoder is directly driven instead of through chain or belt. Its not a big difference, but sometimes the little differences help.

As for preloading, if your chains are not preloaded, as you start to drive the motors, there will be a split second where the motors will accelerate, but the wheels will stay stationary. This is because there is some slop in the gears and the chain or belt. This will then jolt the wheels when everything in the geartrain gets loaded, which can cause control issues if you want to be precise. If you preload the geartrain, which you do by moving the robot manually in the opposite direction you want to move, this jolt goes away, which allows much more controlled starts and more controlled acceleration.

Re: How are teams mounting drivetrain encoders on WCD?
 

I'd assume this would mean stretching out the chain before actually putting it on finally, so that it stretches less during actual use.

Re: How are teams mounting drivetrain encoders on WCD?
 

I'll hop in and answer, but I'm not the most qualified in the subjects, so I welcome any correction if I say something incorrect.

Weighted wheels would be the wheels actually contacting the ground when your robot drives in the direction it's about to go. For the vast majority of 6-wheel drop centers, this will be your back wheels due to the torquing of the wheels. For 8-wheelers, it kinda depends on your specific wheel placement whether the robot is on its center sets or back sets of wheels.

As for preloading this chain, they're mostly talking about removing any slack from the side of the chain that is about to be driving the robot. This prevents much shock loading from being put on your chain runs right as you start auto or teleop. And if you consider pushing the robot to be the directional opposite of the robot driving itself, pulling the robot "pre-loads" the chains so they're ready to go when you start accelerating.

Re: How are teams mounting drivetrain encoders on WCD?
 

I believe what they're really talking about is temporarily pre-tensioning, or removing the slack before each match. You could call that pre-loading, but I consider what you mentioned to be a better description for pre-loading.

Re: How are teams mounting drivetrain encoders on WCD?
 

The plan has shifted to using 9mm belts on a direct c-c with the wcp side bearing blocks. My thought was utilizing feed forward as suggested by a teammate to minimize any slop. Is there a noticeable difference between mounting to the output shaft or rear wheel axle, specifically in regards to necessary programming to counter backlash/slop?

Re: How are teams mounting drivetrain encoders on WCD?
 

Properly tensioned timing belt runs have minimal backlash and will not require any code to compensate for play.

If you're going to use direct c-c, why bother using the WCP side bearing blocks? Teams have been very successful using flanged bearings directly in 0.125" and 0.100" wall tubes. The benefits of dropping the bearing blocks are lower weight, lower cost, slightly reduced tolerance-stackup and easier/more precise machining (circular holes vs rectangular cutouts).

Also, what size pulleys and wheels do you plan to use?

Re: How are teams mounting drivetrain encoders on WCD?
 

Size constraints in the gearbox allow for a 30T pulley running 2x 9mm HTD belts. Will be running 4" colsons.

http://imgur.com/a/jkSMH <-- My thought behind using this bearing block was additional strength around the bearing to help mitigate the effect of load on the wheel axle. I vaguely remember seeing a thread on here awhile back showing a broken frame due to placing the bearing directly in the tube

Re: How are teams mounting drivetrain encoders on WCD?
 

Link? If I'm thinking of the same picture, that was in 1/16" wall tube.

Re: How are teams mounting drivetrain encoders on WCD?
 

Hm, quite a bit of searching and I came up blank. I think you're right that it was 1/16' tube

https://www.chiefdelphi.com/media/photos/43555

This?

Re: How are teams mounting drivetrain encoders on WCD?
 

That's exactly what I was looking for.

Cool, happy I could help. I just remembered it had the words "hulk" and "thumb" in the description so that's how I found it :D.

Re: How are teams mounting drivetrain encoders on WCD?
 

I've only ever seen this happen in .063''-walled tubing, which I would not advise for use in a drive chassis anyway.

Re: How are teams mounting drivetrain encoders on WCD?
 

Here we go

Re: How are teams mounting drivetrain encoders on WCD?
 

A couple quick notes on those pictures.


The one prominanetly displayed there was actually used with a VersaBlock we had on a manipulator joint, rather than our drivetrain. We didn't intend to have that VersaBlock slide, thus why we fastened the encoder mount down to the 2x1 aluminum with the bolt you can see. One of the drivetrain encoder mounts can be seen in the background of a couple of the pictures. For the drivetrain mount, you can see the bolts that are used for the VersaBlock are also used to secure the encoder mount.



The electrical tape was a pit repair since a crack developed in the piece that holds the encoder itself. Next year we'll likely beef up that piece. Unfortunately the tape obscures the design somewhat, but the idea is that the piece that holds the encoder can slide on the main mount structure in order to find your ideal fit. If you know your shaft lengths beforehand, you could make this all one print custom fitted to work with your shaft overhangs.

Re: How are teams mounting drivetrain encoders on WCD?
 

I absolutely love that versablock encoder mount idea, and will probably urge 449 to steal it :P