Determine Motor's Ability to Hold Load

Hello Everyone,

We hooked up two FP motors to AM planetary gearboxes, single stage I believe, and then we hooked them both up to a toughbox. I know that 2 FPs are illegal, but this was our attempt to simulate some banebot stuffz.

The problem that the FP motors faced was keeping the load up. We’re using a 2.5" spool with the setup above. It takes about 40 pounds of force to lift our mechanism, but then when we get to the top, the FP motors don’t hold the position.

So I was wondering what property of a motor helped to determine whether a motor will hold a load or not?

Also something else about our setup, we’re running the motors with alligator clips from the battery, not through a motor controller. Is setting the motor to 0 on a motor controller different from simply removing power from the motor?



That is the same as coast mode on the speed controllers. Try shorting the alligator clips together (not connected to the battery, of course) when you want it to hold position. That will be the same as break mode on the speed controllers, and should hold more of a load. As a side note, it might be worth it to alligator clip a fuse into the middle if your circuit in case something goes wrong.

Austin Schuh

As a word of caution, the FP motors have a thermal cutout inside the motor. This might be the reason the FP’s are not holding.

In 2008, we had similar issues. If I had it to do aver again, I would use a brake to hold the load, and turn off the motor. DC motors do not like to be stalled, they heat up very fast. If you are using pneumatics, try a small cylinder pushing against something in the gearbox. Maybe a UHMW pad against the pinion gear? Every time you set motor to 0, turn on the valve to extend the cylinder.

An excellent solution would be a bicycle disk brake on the arm, actuated by a pneumatic cylinder.

In 2008, the problem came up at competition, and with no time to implement a mechanical solution, the programmers just put in a routine to pulse the motor when were at the extreme. Not an ideal solution (we still had issues) but it helped.

We use an FP on our elevator from 2008, and had a similar problem, even with the Victors in brake mode - you get to the top and set it to 0, and it would slowly drift down. There are really two ways to fix a problem like this - implement a mechanical brake (which we’ll probably be doing this year), or stall the motor intentionally. In 2008, we ended up stalling the motor. By telling it to go forward very slow, the elevator would perfectly hold position. While we didn’t have any problems with the motor that year, it always worried us that it would burn out. Maybe we were lucky… we only had the elevator stalled for a few seconds at a time.

Another solution may be to mount a window motor in parallel to the rest of the gear/sprocket set up. Make sure the free speeds of the motor matches up with the free speed shaft you attach it to (As per the free speed of the other motors). The worm drive in the window motors should prevent or significantly slow down any back drive, and it will add a little power to the system.

The following VEX FAQ covers several ways to make it less-backdriveable.

To engineer this properly you need to determine the torque needed to hold the arm in position. Then factor in the gear box and determine the torque required by the motor. Assuming the motor is stalled use this current and the resistance to determine the total watts to hold this in place. Compare this to a conservative continous rating of the motor. The motor is not turning so it doesnt have the fan running to help cool it. So a motor that can deliver 300 watts peak might have a 100 watt continous rating and a conservative rating might be as low as 20 watts.
For a FP motor you might be limited to under 2.0 amps or less as a holding current. The answer is more gear ratio lower holding current.

All the stuff about the electronic brake and regen just helps with controlability when the arm is being lowered and keeps it from running too fast when the speed is set to zero.

Much like martin417’s team, we had a simple brake system designed for the winch that ran our forklift for our 2008 season. We used the FP motor, but had a terrible problem with it coming back down with the huge ball weighing it down.

We ended up taking some spare tire tread for our AM performance wheels and wrapping it around the drum that winched that was actuated by rotating a servo. Very simple, yet very effective.

2008 Drum.png

2008 Drum.png

2008 Drum.png

Determining the current using the motor’s Kt from the motor curves will give you average current.

Attached is a simulation plot of the current waveform for a CIM driven by a Victor at 5% PWM*

The instantaneous current is shown in red. The average current is in aqua. The rms current is in green.

Power consumption should be computed using the rms current, which can be higher than the average current when using Victor controllers at lower PWM in stalled condition. In this case, the power consumption is 60% greater than the value computed using average current.

I suspect this condition is even more exaggerated in the smaller motors, but I don’t have inductance specs for them. Has anyone measured the locked-rotor inductance of the Banebots or FP motors?

*note: this is based on CIM inductance of 200uH, which I was told but have not independently confirmed.