For a given servo, varying P and D terms produces damped responses. The goal is to match the actual position to the desired position as closely as possible (as the critically damped response shows), minimizing error between them.

Chuck Lewin
President and CEO
Performance Motion Devices Inc.
Lincoln, Mass.

There are two types of motion control engineers: Those who are comfortable tuning a servo loop, and those who aren't. The latter basically, have two options. The first is to use a nonservo device such as a stepmotor; the alternative, a better idea, is to get comfortable.

Whether you're a novice or an experienced hand with servo tuning, this article can help you become more proficient when applying PID (proportional, integral, derivative) based servo loops. For starters, it explains two standard manual tuning methods that work well for many systems. It also covers an ever more common remedy to manual tuning, autotuning, which despite its name, isn't necessarily as automatic as one would hope. Beyond that, in the realm of advanced servo techniques, it explores feedforward and frequency domain biquad filtering, which, in certain instances, produce smoother motion profiles, resulting in better system operation.

PID-based servo loops

No discussion of servo tuning can begin without addressing how servomotors behave. There are two types of servomotors that engineers commonly use for positioning applications. One is a dc servo, which uses mechanical brushes to commutate the motor. The other is a brushless dc servo, also known as a permanent magnet (PM) brushless motor, commutated electronically by external circuitry. Unlike stepmotors, which move in discrete position steps, servomotors have no built-in sense of angular position, and thus require a feedback device, such as a quadrature encoder. The servo loop, or "compensator," keeps the motor at a desired position by comparing the actual position from the feedback device to the desired position and applying corrective motor commands. The better a servo loop performs, the more accurately the motor tracks desired position under various loads and motion profiles. While a number of servo compensation schemes are available, the most common is the PID loop. Several implementations exist, however, and different vendors often use different approaches. Generally, PID controllers fall into two groups: the PID position loop and the cascaded position/velocity loop.

The more common PID position loop requires users to determine three values — the position loop gain Kp, the integral gain Ki, and the derivative gain Kd. Even for such a basic servo system, motion vendors provide additional options, namely integrator limits, feedforward gains, motor bias, and frequency-domain filtering using notch and bandpass filters.

Cascaded position/velocity loops, on the other hand, are tuned from the inside out with users setting either four or five parameters. The inner velocity loop (usually a PI controller) is tuned first, then the outer position loop (generally a PI or PID controller). External amplifiers that provide velocity control are one example of a cascaded position/velocity loop. Although this control approach is not discussed further, other techniques presented here may be adapted to it.