Oscillating Motor - PID Subsystem

[GeeTwo]

I haven't looked at your details yet, but in general a larger magnitude for D (that is, derivative of the error, or velocity for a mechanical system going for a position) will act like a dashpot/shock absorber and dampen your system.

[Asymons]

Originally Posted by GeeTwo

I haven't looked at your details yet, but in general a larger magnitude for D (that is, derivative of the error, or velocity for a mechanical system going for a position) will act like a dashpot/shock absorber and dampen your system.

Thanks for the advice. Once you read the details, I have a couple of questions: Are you suggesting I should add in the derivative of error to fix the oscillation of the motor?
How would the tuning work if the system has an issue with just a P-closed loop essentially? Would that not continue having error in the sense of a continued oscillation?

[GeeTwo]

Originally Posted by Asymons

Thanks for the advice. Once you read the details, I have a couple of questions: Are you suggesting I should add in the derivative of error to fix the oscillation of the motor?

No, just to use a non-zero, probably constant value of D. It is convenient to think of D as standing for dampening as well as derivative.

Originally Posted by Asymons

How would the tuning work if the system has an issue with just a P-closed loop essentially? Would that not continue having error in the sense of a continued oscillation?

A P-only PID feedback system is (assuming that the mechanical system is linear) is mathematically identical to an undamped spring-and-mass; you would expect it to oscillate for a long time. When the system is far from the target, it is accelerated towards the target. By the time it reaches the target, it has significant speed, but there is no frictional term to slow it down, so it overshoots. You have essentially made a system in which F=kx (Hooke's Law). In terms of a differential equation, it becomes the simple x''=(k/m)x, where k/m is proportional to your P term. If you have done calculus 1 it is easy to verify that one solution to this equation is x=sin(sqrt(k/m)t), that is displacement is described as an undamped sine wave. Some sort of friction term is required to dissipate the initial "potential energy"; neither P nor I do this. Both D and mechanical friction will dampen the oscillations.

[Asymons]

Originally Posted by GeeTwo

No, just to use a non-zero, probably constant value of D. It is convenient to think of D as standing for dampening as well as derivative.

A P-only PID feedback system is (assuming that the mechanical system is linear) is mathematically identical to an undamped spring-and-mass; you would expect it to oscillate for a long time. When the system is far from the target, it is accelerated towards the target. By the time it reaches the target, it has significant speed, but there is no frictional term to slow it down, so it overshoots. You have essentially made a system in which F=kx (Hooke's Law). In terms of a differential equation, it becomes the simple x''=(k/m)x, where k/m is proportional to your P term. If you have done calculus 1 it is easy to verify that one solution to this equation is x=sin(sqrt(k/m)t), that is displacement is described as an undamped sine wave. Some sort of friction term is required to dissipate the initial "potential energy"; neither P nor I do this. Both D and mechanical friction will dampen the oscillations.

Thank you for this excellent response! As soon as I get access to the robot again, I'll try adjusting the variables and add a D constant. Hopefully this is the only issue, otherwise I'll be back here. Would it be acceptable to private message you if another issue arises as it seems you have some experience dealing with PID loops? I'll try not to be too bothersome

[GeeTwo]

Originally Posted by Asymons

Thank you for this excellent response! As soon as I get access to the robot again, I'll try adjusting the variables and add a D constant. Hopefully this is the only issue, otherwise I'll be back here. Would it be acceptable to private message you if another issue arises as it seems you have some experience dealing with PID loops? I'll try not to be too bothersome

Certainly, though my PID knowledge is far more theoretical than practical. If you don't mind, though, it probably makes more sense to put this on the open forum where someone else may benefit. I know there are a lot of "lurkers" who never post answers and rarely ask questions themselves, but find answers on CD regularly. CD is here for them, too!

--Gus

[otherguy]

Regarding the command continuing to run between the robot being enabled/disabled:

Your command will only finish if the onTarget method returns true.

protected boolean isFinished() {
    	return Robot.pidelevator.onTarget();
    }

When the robot is disabled commands aren't automatically disabled, the command is still trying to move the motors, the outputs to the motor controllers are disabled. So when you re-enable, the command continues to try to get the elevator to the setpoint.

Add something like:

Scheduler.getInstance().removeAll();
Scheduler.getInstance().disable();

in your disabled periodic() method inside your main robot class. This will disable any command that's active when the robot is disabled.

[Asymons]

In that case, Gus' suggestion, as well as James worked!

So my solution was as followed:
-Added the D value, set it to 1.0 and eventually lowered this value to 0.1 due to the lift "skipping".
-Absolute Tolerance was adjusted from 200 to 20, which it finally remained at 50 to keep the lift moving smoothly.
-P value was adjusted to 0.05
-Added Robot.elevator.disable(); at end of command

Doing all this, I was able to get the lift to stop at a set point, or have it skip once and get to the set point.

I'm most likely going to learn common tuning practices;however, I've mostly seen people use the "guess and check" method.