We’re thinking of making a PID loop to hold our elevator in particular positions. Current plan is to drive the elevator with 2 775Pro motors. Will the 775s stall and overheat while holding position?
Check out the stall test for the 775pro here: http://motors.vex.com/vexpro-motors/775pro
My analysis is that if you stall at 6V or less, the motor will be okay. How do you ensure that it only takes that much voltage to hold your elevator steady? Reduce the force it takes to hold the elevator steady. Some possible ways to do that are keeping the weight you’re carrying small, counterbalancing the load, using a mechanical brake, using a low-efficiency gearbox (like a worm drive), or using a high gear ratio on the 775pros.
If you run a PID loop to control your elevator and tune it properly, the PID output will converge on a voltage that stalls the motor to hold the elevator at your setpoint. If your elevator is well balanced, this will be effectively 0. If your elevator is not balanced, you can figure out how much torque you need at the motor to hold it and refer to the chart Basel linked to find out how the motor with take it over time.
Yes it will stall.
Overheating and burning out depends on how much voltage is applied. This depends on the motor’s gear ratio, load on the arm, and any counter-balancing you’ve done.
It should be straight-forward to determine any dangerous conditions using the JVN design calculator (search for it here) and the motor data supplied by Vex (as linked above).