Servo Motion Control
In servo motion, a feedback sensor – usually encoder – is used to continuously measure position and speed of motor to correct motor speed. By using feedback sensor, we can get very accurate results. Most servo motors now have on-board ‘Motor Encoder’. However, additional encoders could be placed on the actual mechanical axis to compare against actual line speed at different locations on a manufacturing line. Feedback encoder sensor could be linear as well as rotary and both types could further provide relative position measurement or absolute position measurement. It is often worth at stage of design and procurement to establish if to use relative or absolution measurement system.
Relative and Absolute Position Measurement For Motion Control
In relative measurement system, only relative position difference between mechanical axis and motion controller is measured. This often requires definition of travel range for the movement and establish reference point ‘Home’ every time power is established to motion controller.
Absolute position measurement system knows actual position of mechanical system and does not require homing. However, it might still require ‘absolute position offset’ – difference between encoder and axis zero point – during initial commissioning of the motion axis. It is only to compensate for encoder location which might as well be installed away from motion controller.
Choice between Relative and Absolute Encoders
Design requirements and procurement budget limits encoder choice. Relative encoders are cheap but when used as ‘Motor Encoders’ fitted on synchronous body, these have to be homed every time motor is powered up. Homing would also result in setting commutation offset for synchronous machines. Such limitation does not make relative encoders suitable for Servo Motors which have on-built encoder arrangement. On the other hand, Absolute encoders are ready for operation when power is established.
In applications where an encoder is required away from motor somewhere on manufacturing line, absolute encoders are not often the best choice. For absolute encoders, full length of travel bed has to be defined in terms of absolute actual position values over encoder shaft revolutions. Relative encoders could measure large travel range and are cheaper. Accordingly, when used in applications where a line speed is required somewhere along the production line, it is worth to opt for relative encoders. Irrespective of encoder choice, I have always opted to use Heidenhain Endat encoders. Heidenhain encoders both rotary and liner types are available for relative or absolute measurement applications.
Motion control engineering is exciting field and it is great fun to be able to control motion of gigantic machine. In this article I touched fundamental hardware often used in motion controllers. I hope information has been useful and please feel free to drop a line should you require assistance with your motion control application.