More specific types of controllers, such as servo controllers, are used to control servomotors. When dealing with servo systems, it’s important to note that definitions for servomotor, servo system, and servo controllers can differ widely throughout industry. When selecting a servo system for an application, it’s best to ask suppliers what exactly their offerings entail.
A servo controller is the heart of a servo system. A typical servo system consists of a motor, feedback device, and the controller. The control circuitry typically involves a motion controller, which generates the motion profile for the motor, and a motor drive which supplies power to the motor based on the commands from the motion controller. Servo systems are closed-loop systems which have some benefits over open-loop systems including the fact that they improve transient response times, reduce steady state errors and reduce system sensitivity to load parameters.
Servo controllers perform two types of tasks; tracking some commanded input and improving a system’s disturbance rejection. One of the most powerful methods of control is PID control, which stands for proportional-integral-derivative control. PID control is a combination of proportional control, integral control and derivative control. A PID control method works on the error signal which is the difference between a commanded value and the actual value of an output variable, and driving the error to zero. The proportional value can be thought of as a simple gain value. The integral value integrates the error over a period of time and helps to drive the error to zero. The derivative value helps to stabilize a system that uses an integral and proportional term only.
There are a few important factors to consider when selecting a servo controller for an application. The first thing is knowing which type of motor is to be controlled. Is the servomotor an ac or dc motor? If dc, is it brushless or brushed? This will help in determining the kind of commutation the motor needs and if the controller can accommodate it.
How many axes of motion does the application have? Is it a single axis of control or are there multiple axes? Servo controllers are available to control simple single axis applications as well as more complex motion such as robotics applications involving multiple axes.
Next, how many channels of I/O are needed? Are special input types needed beyond inputs for feedback signals such as speed and position? Be sure that the controller can accommodate the appropriate feedback device, such as incremental encoder signals, resolver signals, SSI, Hall sensor signals, or tachometer inputs.
Another important factor that is sometimes overlooked is the controller set up. Is the controller easy to set up and program? Is programming done via keypad or can it be programmed from a computer screen? Also consider the available communication links. Are there basic RS232 or RS485 links? Does the controller include bus interfaces for common networks such as CAN, DeviceNet, Sercos or Ethernet?