Basics of Feedback

In any closed loop control system, a method for measuring the characteristic being controlled (i.e. position) needs to be implemented. This is known as feedback. Some of the different kinds of feedback used in motion control are relative position, absolute position, velocity, and analog voltage feedback. For relative position feedback, the most common device used is the incremental encoder. This device increments as it turns and can be used to keep track of the total distance traveled from some point of refrence. The most common absolute position feedback devices are absolute encoders and resolvers. An absolute encoder is similar to an incremental encoder except that it does not need to have a reference position and can know what position the shaft is in on start up. A resolver is also another absolute position device that is commonly used for both position and velocity feedback. For velocity feedback, a tachometer is commonly used. Some applications require feedback other than the type that has been mentioned above. In these cases, analog feedback describing some characteristic of the system such as temperature or pressure can be useful in controlling motors. For more information about the different kinds of feedback devices, see the following links:

Absolute Encoders and NI products
Linear and Rotary Encoders
Resolvers
Tachometers

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