Controlling RPM of two wheels with one encoder?

So this year we will be using a shooter with two wheels. I believe that in order to correctly control the RPM of each wheel, we would need 2 encoders - one for each wheel. My friend on the other hand believes that only one encoder is needed since the wheels are both running at the same speed.
Which one of us is right and why?

In an ideal world, you would need one.

However, in reality, two would be better.

The performance of motors is, in our experience, quite volatile and unexpected. Minute differences between the states and conditions of the two motors you’re using will produce different results. As motors heat up they will also perform differently, spin at difference speeds, etc.

An encoder on both wheels would be ideal; they will almost definitely run at slightly different speeds.

We are using two cim motors hooked up to each wheel. In our prototype testing we found the RPMs to be within 5% of each other. Because we all we’re doing with the encoder is ensuring that both wheels are within a certain tolerance of RPM, I don’t think we need two. When wheel 1 is traveling at the ideal RPM (as determined by the encoder), we can offset the PWM value sent to wheel 2 to ensure it will also be at (or very close to) that ideal RPM. We are not changing the speed of the wheel based on distance, or anything fancy like that, so a second encoder is extraneous.

–The Friend

I am reading this literally to mean that each wheel has 2 CIM motors. If that is what you meant (probably not? but that’s the way it reads), then yes you need only one encoder. The effect of the differences mentioned in the earlier post by ksafin will be that the motors don’t share the load exactly equally (i.e. their torques will not be exactly equal).

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?

I agree with everything you posted:) …up until this point.

Use two encoders if need be, but keep “debating” (asking questions) if you are puzzled or think you still have a useful point to make.

Ok, point taken, and generally agreed. But at this point in the build season, I’m usually trying to reduce (though not eliminate) debates. It’s time to make decisions and execute. The debates should be on smaller and smaller issues for continuous improvement.

I agree that at this point in the build season, in the shop it’s time to make decisions and execute. The context for my post was discussion on the CD forum, which can continue to take place “after hours”, and may result in the occasional epiphany.

An epiphany (from the ancient Greek ἐπιφάνεια, epiphaneia, “manifestation, striking appearance”) is an experience of sudden and striking realization. Generally the term is used to describe breakthrough scientific, religious or philosophical discoveries, but it can apply in any situation in which an enlightening realization allows a problem or situation to be understood from a new and deeper perspective.

I meant to say “two cims, one hooked up to each wheel.” Sorry for the confusion, and thank you for all the replies.

If you’re quite certain that you’re only interested in shooting with a particular ratio (I would be, in your shoes), then I would advise coupling the wheels anyway… It may require a little extra work and about .5-2 pounds, but it would give you a mechanically defined ratio. I expect this could provide you some of the following benefits:

  • Increased shot accuracy
  • Need for only one encoder to precisely control shot speed/rotation
  • Both motors are used to power both wheels (if one wheel takes more load, the second wheel and the two motors can all instantaneously apply energy to the more heavily loaded wheel)
  • May allow you to “downgrade” one or both motors to Mini CIMS (which actually have a higher free speed, which many have touted as making them better for shooters, as it seems that higher torque (and certainly power) are less necessary for shooters)
  • May allow you to remove one motor entirely

So, in my mind, the ideal solution is to chain/belt the two wheels together… seems that may actually improve your shooter, enable the PID tuning to be simpler, and to remove the need for the second encoder.

If of course you really can’t add a mechanical coupling, the debate for 1 or 2 encoders is opened back up…

Really, it seems like the debate is there only because of a difference in goals… one side says 1 encoder is good enough, while the other says the improvement 2 brings is actually quite important! This all depends on your requirements… if you’re aiming to be one of those “field-length snipers” I’m personally skeptical that 1 encoder is “good enough.” If you’re instead shooting from the front or side of the pyramid, it does seem like 2 would be overkill…