Victor Braking Feature

[brennerator]

We want our arm to freeze when the joysticks are in neutral. I changed the jumper on the victors but it still does not work. Can anyone shed some insight on this?

[tdlrali]

it is hard to have the motors "freeze". That is not what the jumper setting accomplishes. The following is from memory, don't take my word on it...
one setting feeds the power generated by the motor spinning back into the battery
the other one, "dynamic braking", feeds the power back into the motor with reversed polarity, causing it to slow down faster

So dynamic braking will not stop your arm from falling down, but it might help with preventing overshooting. To prevent falling, you have to power the motor slightly "upwards". You can either do this with programming (recommended) or by "calibrating" the joystick until the arm stops.

[Alan Anderson]

The Victor "brake" feature uses what's known as dynamic braking. It connects the motor leads together. The braking force increases as the motor speed increases, but when the motor is not moving there is no braking force. It will slow down and perhaps even stop a moving arm, but as soon as the arm comes to rest it will move again easily. It'll stop again, or at least move more slowly than without the brake jumper in place, but it won't stay stopped. Basically, you can't "freeze" a motor with dynamic braking.

[Alan Anderson]

Originally Posted by tdlrali

...To prevent falling, you have to power the motor slightly "upwards"...

Be very careful when choosing this option. The motor will be stationary, and a stalled motor draws a lot of current. Some motors will be okay with that for a while (I think the window motor is one). Others will heat up quickly but should survive because they're massive, like the CIM minibike motor. But others will fry if they're powered for very long without spinning -- the scent of a smoking Globe motor is familiar to many.

The other options are to provide a mechanical brake or to use a gearing system that won't backdrive.

[KTorak]

I think the issue you are having is with motors backdriving. The window motors and the denso are the only motors I know of that do NOT backdrive. Other wise, you'll need constant current as described above.

[tdlrali]

The window motors and the denso are the only motors I know of that do NOT backdrive.

Basically, any worm drive will not be backdriveable

Another option, although this is almost never possible on robots, is a counterweight

[KTorak]

Originally Posted by tdlrali

Basically, any worm drive will not be backdriveable

I almost made that generalization, but I wasn't 100% sure. Thanks!

[Tristan Lall]

Originally Posted by KTorak

I almost made that generalization, but I wasn't 100% sure. Thanks!

In reality, you can backdrive some worm gears. It's a function of the lead angle and the efficiency (which is itself dependent on material, finish, lubrication, wear, etc.). If you want to really be sure, test it with an equivalent torque, and see what happens. Also consider that the gear may wear out over time, and can begin to behave differently.

You're on the right track, though; locking the mechanism with a worm gear is better than stalling your motor to hold it in place.