PID crossing 0

[Ether]

Originally Posted by ratdude747

Is there an alternative to a PID that will let me cross 0 or a way to make the PID cross 0? or do i need to write my own algorithm to render that?

If you want want to use the WPI library PIDs, you can create "setpoint" and "process_variable" inputs for them as follows (assuming angles in degrees):

angle_error = target_angle - measured_angle;
angle_error -= 360*floor(0.5+angle_error/360);

then:

setpoint = angle_error;
process_variable = 0;

OR

setpoint = 0;
process_variable = -angle_error;


The disadvantage of the first approach is that the Derivative term will be disabled since the WPI Lib PIDs only look at the derivative of the process_variable.

The disadvantage of the second approach is that, although the Derivative term will be active, it will respond to changes in BOTH the measured_angle as well as the target_angle, so be careful how you adjust it.

Notice that the first two lines of code above (calculating the angle_error) calculate the shortest angle distance to the target. You need extra conditional logic only if you want to change the sign of wheel speed. See the discussion here.