Tests of incremental rotary encoders

Inexpensive optical rotary incremental encoders now available can provide resolution approaching one arcsecond. However, several factors limit the accuracy of measurement that can be obtained. We report on test results of rotary incremental encoders obtained with a test setup that compared the output of two such encoders driven by the same shaft. Although intrinsic non-linear response of the encoders tested is specified to be less than plus or minus 15 arcseconds, additional errors are often caused by the coupling of the encoder to a rotating device. Bearing runout and shaft misalignment typically require use of a flexible coupler, but tests with several types of small inexpensive flexible couplers have shown that these can contribute additional errors including windup and non-uniform rotation that is affected by small changes in alignment. An additional minor source of error is due to a reproducible periodic error of several arcseconds generated in the interpolation electronics used to provide high resolution by subdividing the analog signal from the encoder. The driving torque required by a typical Gurley encoder is larger than might be expected, and has been measured at various speeds by determining the amount of windup with a solid aluminum coupling shaft.