Locking Winch Motor

[ShaneWong]

Hi, my team recently did a test of our winch system on our robot and we found that when it wasn't being moved by any code we could move it freely by hand. Is there any way I could lock the winch in place using code? We're using Talon motor controllers if it makes any difference.

[Ozuru]

This year we've chosen to take a mechanical approach rather than a software approach because it's a lot easier than programming it to hold the motors. If you want to lock them programmatically you need to constantly supply a voltage of just enough to keep tension.

[Arhowk]

One possible solution would be to put your talon from "Coast" mode to "Brake" mode (http://www.crosstheroadelectronics.com/Talon_User_Manual_1_1.pdf). Another would be to increase the gear ratio on the winch motor. If neither work and it is truly a problem, than the only solution would be to attach an encoder and use a PID loop.

[matthewdenny]

As I understand (someone can elaborate if they know more) the braking force is proportional to the speed, so at slow speeds their isn't much of a braking force.

[GeeTwo]

As I understand (someone can elaborate if they know more) the braking force is proportional to the speed, so at slow speeds their isn't much of a braking force.

That is essentially correct for the electric braking triggered by setting the motor controller in "brake" mode. It quite simple to add "always on" braking force to a system, just by applying pressure on an shaft or wheel, or even just silghtly mis-aligning a sprocket. Active braking is most commonly provided by pneumtics, because it is great at going right up to the stop and applying constent pressure without experiencing undue stress. It is also possible to do an electrically actuated brake, for example by driving a screw with a motor to clamp a bicycle brake onto a wheel, disk, or even just a hub.

The active feedback (PID) approach requires that you put a sensor on the winch to determine how fast it is moving, and if you want to stop it at defined locations, where it is at any given time. Encoders and potentiometers are the two favorite solutions for this, though at certain ranges an optical rangefinder could be used. This method also requires that you apply a steady voltage to hold a load at a given altitude. The specific amount is dependent on the motor and gearbox; calculate the torque placed on the motor by the load (working down through the gear reductions) and find out how much current your motor needs to apply the appropriate counter-torque. Note that since it isn't doing any work, ALL of the energy used to drive the motor turns into heat. Do not do this with any small motor that has a built-in fan; these almost certainly will build up heat quickly when run in a stall condition. CIMs are pretty good for this sort of work; they have a high heat capacity and are designed for (relatively) high torque. This year, we added a second CIM to our lift not for lifting, but to better disperse the heat for holding.