A typical motion system can be thought of as a motion controller, amplifier, and servomotor/encoder. Though there are a wide variety of servoloop implementations, the most common tend to be either a PID position loop or a cascaded position/velocity loop. In the former, the motion controller generates the P, I, and D terms which operate on the position error, i.e., the difference between the commanded and measured position. The resulting signal goes to the amplifier, which closes a velocity loop. In the case of a cascaded loop, the motion controller gets or synthesizes both position and velocity feedback, applies coefficients to each, and feeds the resulting signal to the servoamp.

For many engineers, the process of tuning a servomotor is a mysterious black art. But the procedure needn't be painful. Just a modest amount of knowledge can bring familiarity with some of the standard techniques and add consistency to what can otherwise be a hit-or-miss process. It can be helpful to look at servo tuning by first examining the usual procedure for PID-type loops, then progressing to more advanced techniques such as feedforward. Fluency in the different types of servoloops helps lead to a common, reproducible method to tune servos.

The most common positioning servomotors in use today are dc brush motors and brushless dc motors, also called brushless PM (permanent-magnet) motors. They are driven by an amplifier which may also commutate the motor coils and, in turn, is driven by the motion controller.

Unlike stepmotors, which move in discrete steps determined by the motor construction itself, servomotors require a position-feedback device to sense where they are. The most common type of position-feedback device in use today is the optical-incremental encoder. Other types of encoders include resolvers, magnetic encoders, and sine/cosine encoders. Tachometers, found on some motors, are another common feedback device that sense velocity.

The servoloop, or compensator, is the portion of the motion controller that tries to match the desired position specified by the profile generation with the actual position provided by the motor-feedback device. There are a wide variety of servoloop implementations. But most tend to be one of two schemes, the PID position loop and the cascaded position/velocity loop.

The PID position loop uses a single feedback loop which combines P (proportional), I (integral), and D (derivative) terms along with position sensor feedback to generate a motor command. The cascaded position/velocity loop controls the motor using a more complicated scheme that has an inner velocity loop and an outer position loop.

Which scheme is better? For most systems there is no clear advantage to either. Most consider PID tuning to be simpler and it is certainly much better known. To tune a cascaded position/velocity loop you must adjust the inner velocity loop first, then the outer position loop. In theory this is easier than a PID, but for most engineers the less to adjust the better.

If you have purchased a velocity-mode amplifier, the only loop that makes sense is the PID loop because the amplifier already contains a velocity loop. The output of the PID will become the input to the velocity loop within the amplifier. Generally speaking, you must make sure the type of amplifier you use is compatible with the servoloop your controller provides.

Zone-based tuning starts by driving the motor with a velocity square wave, then setting both the integral and proportional gains to zero or small values. The value of the D term then gets gradually increased until the actual axis velocity overshoots the profile velocity. The next step is to change the velocity profile to that of the actual application and begin tuning the P and I terms. Intuitive tuning starts by driving the system with a step-function position command. With D and I terms at zero, the P term gets increased until just before the motion axis oscillates, then backed off to half that value. The D term then gets increased until the resulting motion is critically damped. Then the I term is set to satisfy long-term motion needs.

Focus on PID

There are two widely used methods for tuning a PID loop. The first, which may be called zone-based tuning, adjusts the D terms first, then the P terms, and finally the I term. The second, which may be called tuning by intuition, adjusts P first, then D, then I.

Both of these tuning methods will work best with the servosystem operating under conditions that approximate those of the actual application. In other words, the motor should be exercised using inertias and load conditions similar to those used by the system in the field. If you have purchased an off-the-shelf motion chip, motion card, or a motion box, it will also probably include a PC-based diagnosis tool that can help exercise the motor and visualize the results.