PID How to hold an arm in a fixed position without a locking gear box

[Biff]

We used the two motor solution a few years back with an extendable ladder like structure. The trick is to make the window motors holding direction so that you are driving the armature toward the back of the motor. I.e. hold the motor in your hand and if the output shaft is facing you and the motor part is to the right and up, the stopped load you would want turning the output shaft clockwise. We have had very bad luck with the holding load the other direction. The other thing is to match the free speeds of the two motors you are using. The published info is good. Hooking them up and counting revolutions is even better. In 2005 A Globe and the Keyang window motor was a good match. We had a coupler with 16 teeth for #25 chain machined the linked the two shafts together. It was the best working part of the arms on that robot. And it stayed where you left it with out driving power. This year you get two Globes and two Keyangs. I will attach the inventor file. We did have some issues with the machining and had to do some of it by hand.
Biff