Speed Measurement for Incremental Position Encoder Using Period-Based Method With Sinc3 Filtering

Precise measurement of the rotor speed is mandatory for machine tool spindle drives and other drives having the same demands for the accuracy of speed stabilization. These drives commonly use incremental position encoders for speed measurement as a derivative of the position in time. Currently, the best practice is the implementation of constant elapse time methods, the accuracy of which is limited by the accuracy of the position encoder itself and the discreteness of time. However, it was noted that the constant elapse time method ignores all the data in-between the measurements. An analysis of the periods between pulses from the encoder led to the discovery that these periods have the properties of delta–sigma modulated signals. Thus, it was suggested to process the results of the period-based method with a sinc filter of a high order to obtain extra information from the incoming data. The proposed period-based method with sinc filtering (PBSF) uses third-order filter as the most commonly used type in the measuring equipment. The simulation model was developed to show the error distribution for competing methods, and the experimental results were obtained in the steady state and during acceleration of the drive. The comparison of the methods was performed with identical data at the input and the same measurement time. Both the simulation and experimental results showed that at high speeds the proposed period-based method with sinc3 filtering (PBS3F) provides a much smaller deviation from the actual value than the conventional constant elapsed time (CET) methods. The computational complexity was analyzed as well showing the possibility of method implementation using modern microcontrollers.