How are teams mounting drivetrain encoders on WCD?

[AdamHeard]

Originally Posted by gc_coxen

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

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

[Nuttyman54]

Originally Posted by AdamHeard

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

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.

[AdamHeard]

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.

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

[Nuttyman54]

Originally Posted by AdamHeard

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

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.

[Thad House]

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.

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.

[Chris is me]

Originally Posted by Thad House

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.

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.

[Thad House]

Originally Posted by Chris is me

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.

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.

[AdamHeard]

Originally Posted by Chris is me

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.

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

[Chris is me]

Originally Posted by AdamHeard

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

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

[ollien]

Originally Posted by Thad House

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.

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!

[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 Hitchhiker 42

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.

[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?