Force-Driving motors from M+/- End of Victors

[Danny Diaz]

Apparently several members on our team like to bypass using the RC/OI in order to programmatically move motors when they're doing work on the robot that involve our robot arm; since they cannot get direct access to the motor to power it directly with a 12v battery, they touch 12v battery leads to the M+ and M- terminals on the victors directly. When they do this the RC powers up really quick, runs while they have the leads connected, then powers down when they stop powering the motors (we don't have a backup battery connected right now). Several members have asked me if this is safe - they're afraid that may be possibly damaging components.

I told them that the Victor is naturally designed to withstand some serious currents that are generated by the motors (when you drive motors and then immediately throw them into reverse, you get a SERIOUS current spike between the Victor and the motor) so it should be relatively safe to do what they're doing, it just weirds you out because the RC and hardware connected to it are being powered up when they do that. However I didn't know the ramifications if a competition battery was also hooked up to the robot when they power the motors with a completely different battery, so I told them to make sure the robot battery was unplugged if they do that.

Is there anything else I should be aware of when students are doing this? Can this damage electrical components?

-Danny

[EricVanWyk]

I can't guarentee that you are or are not damaging things. However, it sounds like a patently bad idea. The breakers are there for a reason. I would recomend creating a simple PWM generating box to do these sorts of things.

We made one from a 555 and a potentiometer, it was rediculously useful.

As for why things get power:
The Victor is a simple H-Bridge. When you apply voltage to the middle terminals of the bridge, the current flows backwards through the MOSFET's body diodes to the rest of the robot. These body diodes are rated for 64amps a piece, so you _probably_ won't destroy them that way. However, they do have a 1.2V drop, so you will generate a good portion of heat if you try to power anything else.

The two 1.2V drops add up to a fifth of the voltage coming from the battery. Anything powered by the battery through the victor the wrong way therefore dumps 1/5th of its power through those FETs.

Long story short, tell the students not to do so. If the RC decides to tell another motor to turn on during this proceedure, it could go pop.

[Mike Betts]

Danny,

I agree with Eric. There are a lot of reasons not to do this and very few defensible reasons why you should.

If you must, then have them pull the CB for the motor they are powering. This will limit any possible damage to the single Victor and eliminate the RC from the equation.

Regards,

Mike

[Al Skierkiewicz]

Danny,
From what you describe, they are back feeding the Victors with 12 volts and I would agree that is a very bad idea. Although the Victors are back driven all the time when the robot is moving, the danger of slipping with battery terminals or getting across the input, backwards, presents the possibility of some really costly errors.
I know that this isn't a non cost alternative, but we use Anderson Power Locks on most of our motors. That way we can easily disconnect a motor from all electrical systems on the robot. We have modified a battery operated drill with Power Lock connectors so that we can provide variable voltage and reversing through the drill switches. This little setup also allows us to trouble shoot wiring. With an adapter we can power anything on the robot with confidance.