PID control loops - closed loop feedback

[velocipenguin]

Have you made sure that you're calling the PID function at a consistent time interval? If there's code that occurs before that in the main loop that might vary in execution time, you might want to consider using an interrupt-driven timing scheme to make sure you're calling the function at precisely the right time interval. Are the excessive output values appearing as soon as the control loop starts, or are they occurring gradually? If they're occurring after you move the arm, particularly if you move it rapidly, try reducing Ki to make sure you're not saturating the integral term of the PID equation.