How are teams mounting drivetrain encoders on WCD?

[Thad House]

Originally Posted by ollien

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!

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.

[Hitchhiker 42]

Originally Posted by ollien

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!

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

[Cothron Theiss]

Originally Posted by ollien

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.

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.

[gc_coxen]

Originally Posted by Nuttyman54

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.

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?

[Greg Woelki]

Originally Posted by gc_coxen

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?

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?

[gc_coxen]

Originally Posted by Greg Woelki

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?

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

[frcguy]

Originally Posted by gc_coxen

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?

[gc_coxen]

Originally Posted by frcguy

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

This?

That's exactly what I was looking for.

[frcguy]

Originally Posted by gc_coxen

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 .