Chief Delphi - How are teams mounting drivetrain encoders on WCD?

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- - How are teams mounting drivetrain encoders on WCD? (http://www.chiefdelphi.com/forums/showthread.php?t=150254)

Cothron Theiss

26-08-2016 18:33

Re: How are teams mounting drivetrain encoders on WCD?

Quote:

Originally Posted by ollien (Post 1603109)

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.

Cothron Theiss

26-08-2016 18:35

Re: How are teams mounting drivetrain encoders on WCD?

Quote:

Originally Posted by Hitchhiker 42 (Post 1603131)

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

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.

gc_coxen

29-08-2016 15:50

Re: How are teams mounting drivetrain encoders on WCD?

Quote:

Originally Posted by Nuttyman54 (Post 1602691)

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

29-08-2016 16:41

Re: How are teams mounting drivetrain encoders on WCD?

Quote:

Originally Posted by gc_coxen (Post 1603674)

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

29-08-2016 23:52

Re: How are teams mounting drivetrain encoders on WCD?

Quote:

Originally Posted by Greg Woelki (Post 1603685)

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

Knufire

30-08-2016 00:22

Re: How are teams mounting drivetrain encoders on WCD?

Quote:

Originally Posted by gc_coxen (Post 1603751)

I vaguely remember seeing a thread on here awhile back showing a broken frame due to placing the bearing directly in the tube

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

gc_coxen

30-08-2016 00:35

Re: How are teams mounting drivetrain encoders on WCD?

Quote:

Originally Posted by Knufire (Post 1603757)

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

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

frcguy

30-08-2016 00:50

Quote:

Originally Posted by gc_coxen (Post 1603759)

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

30-08-2016 00:58

Re: How are teams mounting drivetrain encoders on WCD?

Quote:

Originally Posted by frcguy (Post 1603761)

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

This?

That's exactly what I was looking for.

frcguy

30-08-2016 01:01

Quote:

Originally Posted by gc_coxen (Post 1603762)

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.

Oblarg

30-08-2016 13:17

Re: How are teams mounting drivetrain encoders on WCD?

Quote:

Originally Posted by gc_coxen (Post 1603751)

I vaguely remember seeing a thread on here awhile back showing a broken frame due to placing the bearing directly in the tube

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

GKrotkov

27-09-2016 17:54

Re: How are teams mounting drivetrain encoders on WCD?

5 Attachment(s)

Quote:

Originally Posted by Lil' Lavery (Post 1602516)

I don't have any pictures unfortunately

Here we go

Lil' Lavery

27-09-2016 18:59

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.

Oblarg

27-09-2016 22:35

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

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