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Unread 21-03-2004, 23:06
Jay Lundy Jay Lundy is offline
Programmer/Driver 2001-2004
FRC #0254 (The Cheesy Poofs)
Team Role: Alumni
 
Join Date: Jun 2001
Rookie Year: 2001
Location: Berkeley, CA
Posts: 320
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Re: PID control loops - closed loop feedback

Yeah the elbow loop works really well. It took us a lot of testing to get the right constants but 1/8 and 1/8 works best for us.

The wrist loop was a little more difficult to get working because of chain slack. Our wrist chain runs from the base of the robot up 5 ft to the joint so we can maintain a low CG by keeping all the motors in the base. What happened was when we moved the arm down it would overshoot a lot and be really bouncy, even though it worked perfectly on the way up. To fix that we reduced the constants on the way down.

We did target velocity because that's basically what was coming out of the joysticks -- a target velocity. With target position, if something is holding back your arm, and your joystick is full forward, your target position is changing, but the actual position never changes. Then if you bring the joystick back to neutral and allow the arm to move again, it will shoot forward to its target position, even though that isn't necessarily what you wanted.

Our arms sag a little bit when a ball is placed in them, but not much and the integral adjusts accordingly pretty quickly. In fact, we tested once with the robot at 90 deg (we were hanging from the bar) and the arm fully extended to 10 feet, and we placed a ball in it and it sagged about 3 inches with a huge lever arm before the integral picked up the change.