Let’s Talk Accuracy
Gurley Encoders explain motor encoder accuracy. In the world of motion control encoders, the terms resolution, accuracy and repeatability are often confused. Definitions: This document refers to rotary encoders because they are more common than linear encoders, however information can be extend to linear encoders also.
The resolution is a measure of how many counts per unit distance the encoder generates, or, the size of the measuring step. With rotary encoders, resolution is expressed in either units of angle (degrees-minutes-seconds, decimal degrees, grads, or radians) or in number of measuring steps per revolution (e.g., 10,000 counts/rev). [PPR, or pulses per rev, is quite common, but we avoid this term on purpose. Often, when a binary number of measuring steps per revolution is required, the term “bit” is used to indicate the resolution’s highest power of 2; for example, a 16-bit encoder generates 216 = 65,536 counts/rev. Resolution is a basic encoder parameter.
Accuracy is a measure of where the encoder says it is as apposed to where it actually is. They are expressed as units of an angle, although sometimes also expressed as electrical degrees. Accuracy and error are expressions of the difference between actual and reported position, However with a slightly different meaning. The accuracy is the difference between the indicated reading and the true position. Error is how far the indicated position is off true.The higher theaccuracy is better.
The resolution of an encoder is fixed on the electronics or the optic wheel when the encoder is made however, encoder errors can crop up later with influences from temperature, shaft loading, service life and dust. The number of counts per revolution does not change unless in the case of severe damage. Accuracy effects all encoders but not all manufacturers specify it. The reported position over time is expressed as repeatability.
Instrument Error + Quadrature Error + Interpolation Error + Quantization Error = Total Optical Encoder Error.
Phasing and duty-cycle tolerances is Quadrature Error It is usually expressed as electrical degrees or an angular measure.
Interpolation Error is the error in an electronically increased value.
Quantization Error = the characteristic of a digital instrument. To measure encoder accuracy it is possible rotate the encoder at a controlled speed and measure the time interval between transitions of the output.
The classic way to measure position errors is with an autocollimator a multi-faceted mirror that attached to the encoder shaft.