Backdriving PG71?

[scuba1000]

We are planning on using a PG71 gearmotor to rotate a pneumatic cylinder on our bot for climbing. Currently the design calls for the motor to be backdriven (I think that's the right term) to follow whatever angle the cylinder ends up at. It is not connected directly, but uses a lead screw.

Sorry if the setup explanation doesn't make sense, but here are my questions:

1) Considering the high-torque gearmotor and the leadscrew and nut (kinda similar to a worm gear i guess), will the motor even reverse when a large force is applied to the nut. We can't really test this right now because it involves connecting the leadscrew to the shaft and we don't have any coupling to do that, and I don't want to turn down the shaft of the leadscrew just to test it.

2) If the motor is backdriven at relatively slow speeds, will it hurt a Talon or Jaguar? I've pushed the whole robot when it was on and nothing seemed to happen, but my mentors are weary of this. I was thinking of adding a spike relay upstream from the talon and switching it off when the motor is finished rotating. That would solve the problem right?

Thanks

[PAR_WIG1350]

1) the possibility of back driving that setup is doubtful.

2) The motor controller should be fine. The spike and talon combo is illegal and won't do anything to prevent the motor from back-feeding the talon anyway. When a motor stops driving and is driven instead it becomes a generator.

[scuba1000]

cool, thanks for the info

[Spirator]

(1) What you may discover is that yes, PG71's will backdrive on their own, but connected to a lead screw they shouldn't. Of course, this all comes down to the pitch of the screw. I don't know the math, but it's unlikely that a vertical load will cause the screw to undo.

(2) You can set the Talons to brake mode and they'll effectively 'short' the motor terminals when no input is given to the Talon. This will result in back EMF which will resist the rotation of the shaft, but I doubt it'll completely stop it from happening.

Best of luck!

[MichaelBick]

Depending on the lead screw and the wear on it you might be able to back drive it. 192 ran 4 start leadscrews in 2012 on their drive and initially it didn't backdrive. Later the lead screws wore in and were backdriveable after that.