Thread: Gearbox
View Single Post
  #18   Spotlight this post!  
Unread 20-06-2003, 13:53
Joe Johnson's Avatar Unsung FIRST Hero
Joe Johnson Joe Johnson is online now
Engineer at Medrobotics
AKA: Dr. Joe
FRC #0088 (TJ2)
Team Role: Engineer
 
Join Date: May 2001
Rookie Year: 1996
Location: Raynham, MA
Posts: 2,648
Joe Johnson has a reputation beyond reputeJoe Johnson has a reputation beyond reputeJoe Johnson has a reputation beyond reputeJoe Johnson has a reputation beyond reputeJoe Johnson has a reputation beyond reputeJoe Johnson has a reputation beyond reputeJoe Johnson has a reputation beyond reputeJoe Johnson has a reputation beyond reputeJoe Johnson has a reputation beyond reputeJoe Johnson has a reputation beyond reputeJoe Johnson has a reputation beyond repute
multi-motor drives have applications

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.