Locking Motors?

We are kind of new to vex and still learning. We need to have motors in an arm stay rigid when not commanded to move. This will have to be with varying loads also as the robot must pick up several objects (the heaviest of which is a d battery) to meet our requirements? We are wondering if this is a programming or mechanical problem. We would also like to know how to fix it.

I moved this to the General forum where a lot of VEX users can respond. This problem has been solved in several ways and the community can share them in this thread.

There’s a couple of ways to solve this… a programming solution could use a P control loop and potentiometer. See this article for some ideas: http://www.inpharmix.com/jps/PID_Controller_For_Lego_Mindstorms_Robots.html

//Andrew

I think its a mechanical problem, im guessing your saying that the motor dose not have enough torque to pick up the objects. you must add a gear ratio to the motor to improve this torque

here is a great video on gear ratios

A quick and dirty solution (which can be used temporarily until you learn one of the other methods) is to apply an arm reverse using programming. Basically, in your code, put in a bit of code that says that while it is not receiving any other input, it should drive upwards at some nominally small value (such as 20 on a 0-127 scale). Experiment with that number with the weights until it holds it steady (I usually end up using something between 10 and 20).

Again, this is only a very quick solution that you can use until you have time to use a better solution such as a different gear ratio or using a PID loop.

or use elastics to help balance out the weight of the arm itself
if you have the old remote controllers, you can just adjust the trim buttons on the joystick itself

If you use a worm gear on the motor it will lock the movement when not powered.

You must be careful with worm gears. They cannot take that much force. Last year I tried to take advantage of the inability to back-drive worm gears, to climb the latter. The result was the worm wheel would snap and fly across the room. I had to string five or six together not counting the countless rubber-bands I used to counterbalance it.