Overview

The precise control of speed, position or acceleration requires appropriate measuring systems to be applied to the controlled variable. Although considerable progress has been made in the development of sensorless drive systems, these schemes tend to result in improvements in dynamic and shaft performance as opposed to precise positional or speed accuracy.

This chapter deals with sensors external to the drive module itself.

Before considering the various forms of sensor, it is necessary to clarify the, often misunderstood, difference between resolution and accuracy of a feedback device:

Resolution

The resolution of a feedback device is most simply described as the number of measuring steps in one revolution of the motor shaft. For an incremental encoder it is the number of pulses per revolution.

Resolution is often described in terms of number of bits. This is related to the twos complement: for example, a 12-bit feedback device has the equivalent of 2 ¹² = 94096 measuring steps per revolution.

For analogue feedback devices such as resolvers, where a resolution is quoted it usually refers to an associated resolver-to-digital converter.

Accuracy

The accuracy of a sensor is best described as the position deviation within one signal period or measuring step.

In an encoder the accuracy is influenced primarily by the eccentricity of the graduation to the bearing, the elasticity of the encoder shaft and its coupling to the motor shaft, the graduation, the optics and the electronic signal processing.

For analogue feedback devices, the accuracy is influenced primarily by the...