Programming Motors for Arm

[DonRotolo]

Originally Posted by dmitch

Ok...what on earth does the PID control actually do?

It's a way for software to 'gracefully' bring some quantity to a specific value.

OK, some examples: You are riding a bicycle. You want to stop right at a fire hydrant by the curb. You wouldn't go at full speed, and then slam on the brakes at the right moment - too much chance for missing the spot. Instead you slow down gradually, stopping exactly where you want. This is PID control, the quantity being the exact spot where you will stop.

Another example: A Cruise control in a car. You set the system to maintain a certain speed, and it does. A good system does not apply full throttle when the speed drops one or 2 MPH because of a hill, it applies a little throttle, only adding more if the hill is too steep to maintain the 'set' speed. This is PID control, the quantity being the set speed.

PID stands for Proportional, Integral, Derivative. Proportional means that you apply a corrective signal (like a car's throttle) in proportion to the difference ("error") between the actual value and the desired value. A car going 2 MPH slower than the set speed needs only a little throttle, but one going 30 MPH too slow needs a lot of throttle.

Integral helps keep a steady value when the P "error" is very small. It adds up the error over time, so that even a small error gets compensated for after several seconds.

Derivative keeps you from making changes too rapidly, but allows you to make big changes when necessary. You don't want the cruise control at full throttle until 1 MPH below the setpoint - you'll "overshoot" the set speed and have to slow down. Instead this works with P do gradually hit the set speed.

In robotics, PID can be used to move an arm to a set position. Depending on the mechanism, P may be enough, or possibly PI. Full PID is often overkill and unnecessary in a typical FRC design.