You're in the zone

This is exactly what “zone-based tuning” does, the second manual method that we will introduce. “Zone-based” refers to the frequency zones of the P, I, and D terms, and is adapted from George Ellis’ excellent book, Control System Design Guide.

In this method we will plot velocity versus time and the desired profile will be a step function of the velocity (not the position). Set the profile so that it accelerates instantaneously between a velocity of zero and a fixed velocity, and back to zero. Leaving the P and I terms at zero, increase D until the actual velocity profile closely matches the desired velocity profile. Do not worry about whether the destination positions match, you are only examining differences in velocity (velocity error) at this stage. Figure 3 shows a well-tuned D term using this approach.

Hardware trace key to high performance

Driving waveform for intuitive tuning

Driving waveform - zone-base tuning

Now set up your profiler so that you are using moves with accelerations and velocities typical for your application, and change the capture facility so that it plots the desired position, actual position, and position error. Increase P until the servo error is minimized. At some point as you increase P the motion may have high overshoot, or become unstable, at which point you should back off of this value by at least 20% for the final value.

Zone-based tuning has a number of advantages over step-response tuning. For one, it is less iterative, because it tunes the PID terms in order of the frequency response domain. Secondly, it allows you to utilize real motion profiles with ramps, rather than unrealistic position jumps. In all cases, whether using stepresponse or zone-based manual tuning, check the motion in both the positive and negative direction to make sure the gain parameters work well in both directions.