Motion control is all about control. But you cannot control what you cannot measure. So there is an important measurement component to the control of moving systems. The difficulty lies in knowing what to measure, how to measure and what to do about things you can’t measure.
The obvious thing to measure is motor speed. That part is easy. Servo motors have built in feedback devices. In the old days, the preferred feedback device was a small generator that produced a voltage proportional to the speed. In the digital age feedback is by quadrature encoder that outputs a digital pulse that is primarily used for position control. Most control systems are able to easily integrate the pulse train to derive the speed of the motor.
Unfortunately, most applications require relatively low speed. Most motors are engineered for high speed. This is in an effort to package more work related power in a smaller physical package. Often, the motor is connected by pulleys or gear reducers to get the speed of the motor to more closely match the desired speed of the load.
Some of the important attributes of motion cannot be easily measured. In addition to speed, torque is extremely important to controlling motion. Torque can be measured directly from the drive electronics, but this is rarely used for control.
Torque and current are direct equivalents with a slight variation due to the temperature of the motor winding. As the temperature of the motor goes up, the resistance goes up and the current required goes up at the same time. Since high performance motors have fairly high internal temperatures, this swing can be in excess of 100 degrees centigrade, and should be considered in the control scheme.
Most of the emphasis on current control is in terms of protecting the motor and drive electronics. The first derivative of current over time is the limiting parameter of the power electronic devices and is an important boundary condition in safe operation of the electronics.
More important information can be derived by considering the region of the motion profile and the current or torque requirements that are presented. In order to accelerate a load, a lot of current is needed to overcome the mass of the load. But once the load is moving the torque requirement drops off. This creates an opportunity to profile the current requirement while using the conventional error detection scheme of the traditional control.
Other variable that are part of the mechanical system are things like momentum and center of mass. In multi-axis mechanisms, there is usually a dependency of one axis upon another. The idea that the mass of one axis is changing it’s center of mass and momentum with respect to the other axis is generally ignored. This too is an opportunity to gain increased stability in the control and possibly improve throughput by having a better model of the application from which to create the ideal control.
Looks to me like there is a lot of room for improvement. Let me know if you agree or disagree.