Let's see if you can help me... (Motors)

Ok, out robot uses two pivot points in the arm. One is at the top of the supports… the arm goes down and another near the ground that goes back up. Understand?.. Kinda like a shoulder/elbow arm.

We have two window motors mounted halfway up the supports that have a smaller sprocket that run up to the arm with a bigger sprocket. I think its a 4 to 1 ratio.

The arm raises fine and stays up fine with no problems. The problem comes in when we want to drop the arm down. Right when you start dropping the arm it will not stop. It just keeps dropping all the way until it hits the floor. Then we have to raise it up and repeat the process when we want to go down again. This gives us very little control.

Any help on how we can fix this problem?


Given the motors you’ve chosen, I think the most appropriate fix would be a mechanical one. Provide a counterweight or spring to balance the weight of the arm, so you don’t have to worry about gravity when moving it.

A counter balance would be best, and you still might want to couple that with a position sensor, such as a potentiometer, and a closed loop position control system. Given that your arm wants to sink to the ground, the control loop would cause apply enough energy to counteract that force and essentially keep the motor stalled to hold the arm in place. This may result in excessive heating of the motor, and subsequent failure.

If you could balance the arm AND provide a closed loop control of the arms position, then it would easily maintain position.

A “quick & dirty” fix would be to add some surgical tubing on the back of the arm. If you can imagine something sort of like a tendon on your elbow. It takes a bit of tension adjustment but it should help with the tension on the arm.

on our arm, we decreased voltage input to the motors when the arm moves down because gravity is doing the work. We also use pots to determine arm position to adjust the voltage depending upon arm angle.

that sounds like a coding problem to me. Since the window motors have worm gears, they should have anti-backdrive, i.e. they should never just fall when you are going down. You should be able to program the arm to go to a certain height and stay there without stalling the motors or anything.

You would think that the motors you have chosen would not back drive but in the configuration you are using them, once they are moving the arm weight will continue the motion. Even if you set the “brake” jumper on your victors, it might not help. Some of the suggestions above will help the situation, I am a big fan of surgical tubing but it might not be suitable for you design. Changing the ratio of the output shaft to arm might also help. I think a combination of a few ideas will solve your problem.