The care and feeding of your servo...forward

In a perfect world, every force that your motor experiences could be predicted in advance. In the real world however, some forces are predictable, while others, such as loads that are larger or smaller than expected, or motor characteristics that change over time, are not. In fact, one of the most valuable aspects of servo control is that even if we know nothing at all about the motor or the load, by using techniques such as those described above, we can still develop passable PID parameters.

But if we do know something about the motor or the load, it is possible to improve system performance further, even for a welltuned system, by “feeding-forward” offsets directly into the output of the servo loop. In the context of motion control, this technique is generally used with the profile generator to provide velocity feed-forward and acceleration feed-forward controls.

Velocity feed-forward is useful to compensate for any viscous friction or velocity-proportional lagging force. This includes some types of friction forces on the motor or load. It is also common if a voltage-mode amplifier (one without a torque loop) is used, because in these types of amplifiers back-EMF introduces a velocity-proportional lag.

Acceleration feed-forward is useful to compensate for any acceleration- proportional lagging force. This includes, in theory, all hardware with non-zero inertia, because basic physics dictates that if we change velocity, the object will resist this change, and this resistance will show up as an acceleration-proportional lag.

Figure 5 shows a plot of position error versus time for a system that exhibits velocity and acceleration-dependant characteristics, along with the position error after application of velocity and acceleration feed-forward gain values.

Practically speaking, feed-forward only works if inertia, friction, and other system forces can be predicted. Many motion systems have a variable load, or friction forces that change dramatically over the expected operating temperature range or product lifetime. Be sure that feed-forward gains that are helping you under one set of conditions are not hurting you for a different set of conditions.