Gearbox

[Andrew]

This is just my opinion but, I believe that using two different types of motors is bad news waiting to happen.

I predict that four motors in the drive system is going to become a "must" as time goes on.

I'm still reserving my opinion on shifting.

[dddriveman]

I believe that Andrew is correct. I don't want that to happen but, as the games get tougher and as FIRST wants to see what we can make out of what we are given it could happen in the near future.

[Joe Johnson]

I used to be a multi motor skeptic.

This year, I became a believer (at least in some instances).

With this year's rules allowing pneumatic tires, it was easy to get enough traction to make multiple motors worth the bother.

We initially had a 2 motor system, after 2 regionals and a ton of hours on our practice robot, it became clear that more power to the wheels was a big factor (and this with a shifting transmission).

The MAIN improvement (imho) came from having multiple current sources.

When you have a lot of traction and a reasonable top speed (10 ft per second, for example), turning requires that the motors run too close to their stall torque for too long, resulting in tripping breakers.

With multiple motors you share the torque somewhat -- this helps -- but more importantly you have multiple paths to draw current from. Rather than being limited to 40 amps (nominal) you may have twice that. If you are geared to be on the fast side and you have a lot of traction, the 40 amp breakers become your power limiting factor rather than overheating the motors or breaking the tires free.

From our experience this year, it can be worth the bother of designing multi-motor drives.

As to matching free speeds or matching stall torques or matching some other speed-torque point, again, I restate that there is no real magic here: The two motors act like a single motor with different characteristics.

You do not have to match free speeds -- Really.

The story is made a bit more complex by the fact that you can give different voltages to the different motors but it is still a fairly simple arrangement.

I feel a white paper coming on... ... stay tuned.

Joe J.

[Andrew]

The MAIN improvement (imho) came from having multiple current sources.

Hear! Hear! This is the thing which drove us to multi-motors.

Another advantage (especially if you direct drive separate wheels with your multi-motors) is that you have additional failure modes. Even if you trip your breakers on one set of motors, you can still limp around. It takes a double failure on one side of your robot to put you out of commission.

[Veselin Kolev]

Originally posted by JVN
You should try to match the free speeds as close as possible. I think ours are within 5% error.

Since the characteristics of the motors vary so much (in my experience) it is not critical to be overly accurate.

Anyone wanna back me up on this?
How much error does everyone else have in their speed matching?

John

I'll back you up. We have about 1.2% error at max power output and about 3.5% on free spinning. This drive train is planned for next year though, so I can't say too much. Last year was about 4% error on average, using the drills and cims.

It is easier just to match the max power speeds of the motors, since that is the time when you really need the error percentage to be less.

[Veselin Kolev]

Originally posted by Veselin Kolev


It is easier just to match the max power speeds of the motors, since that is the time when you really need the error percentage to be less.

Wow. What was I saying? Sorry, but I seem to have written "match the max power speeds".
This is completely wrong. It's no load speed.

There doesn't seem to be much a difference in the gear ratios, but if you calculate it, the percent error almost doubles. Bad.