Stalling an RS775

[Oblarg]

Does anyone know if it's possible to keep an RS775 stalled indefinitely without damaging the motor by giving it a sufficiently low voltage, and, if so, how low that voltage is?

Thanks.

[Joe Ross]

We have not done it with an RS775, however, we have indefinetly stalled fisher price and RS550 motors with outputs of between 10 and 20%. Note that depending on the speed controller you use, that value might have to change.

[JamesCH95]

Originally Posted by Oblarg

Does anyone know if it's possible to keep an RS775 stalled indefinitely without damaging the motor by giving it a sufficiently low voltage, and, if so, how low that voltage is?

Thanks.

Yes. Our 2011 robot stalled two RS775 motors to hold our arm in position.

Doing math on the torque required showed each motor was only getting 6-7W of power, which is pretty darn safe.

Can't find the internal resistance spec for a 775 off-hand... You could take a multimeter to a 775 and measure the resistance, figure out a voltage that applies less than, say, 10-12W to the motor, and you're off to a good start.

[CRITICALThinker]

There is also the choice of shorting the motor, which is the same procedure the Victor 888's use for braking the drive motors.

[Ether]

Originally Posted by JamesCH95

Can't find the internal resistance spec for a 775 off-hand...

The FRC 2014 motor spec says:

RS-775-18 stall current = 87 amps -> R = 12/87 = 0.138 ohms

RS-775-12 stall current = 30 amps -> R = 12/30 = 0.400 ohms.


So, for 10 watts input power, the RMS voltage at stall would be given by:

V = sqrt(P*R) = sqrt(10*0.138) = 1.17 volts for the 775-18 for 10 watts

V = sqrt(P*R) = sqrt(10*0.400) = 2.0 volts for the 775-12 for 10 watts.

[Ether]

Fun fact:

the average of 1 volt at 20% duty cycle is 0.2 volt;
the RMS is 0.44; RMS is ~2.23 times the average.

same as above, only 40% duty instead of 20%:
average=0.4; RMS=0.63; RMS is ~1.58 times the average.

same as above, only 60% duty:
average=0.6; RMS=0.77; RMS is ~1.29 times the average.

same as above, only 80% duty:
average=0.8; RMS=0.89; RMS is ~1.12 times the average.

same as above, only 100% duty:
average=1; RMS=1; RMS is equal to the average.

[Oblarg]

Thanks very much for all the helpful replies; we'll be running our arm against the hard stop at ~5-10% to keep it in place.

[Dad1279]

Originally Posted by Oblarg

Thanks very much for all the helpful replies; we'll be running our arm against the hard stop at ~5-10% to keep it in place.

Hard to visualize without a picture, but if it's an arm against a hard stop, can't you balance it with springs (surgical tubing) so that stalling the motor(s) won't be necessary? Or use a latch to hold it in place?

[Oblarg]

Originally Posted by Dad1279

Hard to visualize without a picture, but if it's an arm against a hard stop, can't you balance it with springs (surgical tubing) so that stalling the motor(s) won't be necessary? Or use a latch to hold it in place?

Can't do the surgical tubing, as we'd like it to apply pressure in both of its possible locations, and would like not to do a latch as that requires additional actuators and complexity.

[cgmv123]

Originally Posted by CRITICALThinker

There is also the choice of shorting the motor, which is the same procedure the Victor 888's use for braking the drive motors.

All speed controllers, as well as the Spike, use this method. It doesn't hold position and requires the motor to have some speed before having any substantial effect.