Controlling RPM of two wheels with one encoder?

[Cal578]

Originally Posted by thecakeisalie

So this year we will be using a shooter with two wheels. ...

Originally Posted by asteb27

We are using two cim motors hooked up to each wheel. ...

Two wheels x 2 motors/wheel = 4 CIM motors for the shooter? That's a lot of power. Maybe I misunderstood, or maybe one of the above statements isn't quite right. But I'll go with that, for now...

Two motors running on one wheel must be running at the same speed, so only one encoder is needed per wheel.

If the wheels are coupled, such as a chain running around both, then they also must be at the same speed, so you only need one encoder total.

If the two wheels are independent, then it would be ideal to have one encoder on each wheel. The assumption that the two wheels will run close enough to the same speed falls apart if the loading on the two wheels differs. This can happen if one wheel is doing more work to accelerate the disc than the other (in our two-wheeled shooter, the first wheel brings the disc from standstill to near full speed, and the second wheel just boosts it to final speed). It can also happen if one wheel has more friction than the other, which can change as parts wear.

If you have determined that your shooter can tolerate a difference between the two wheels, even if that difference changes over time, then it does simplify things a little to have only one encoder. But, what's so bad about having two encoders? Once you've learned how to make one work correctly, it doesn't cost much (in money, time, weight, power) to add another. Why not use two encoders, stop debating, and rest assured that your shooter will work more consistently for the whole competition season?