Research And Realization Of Velocity Measurement Algorithm For Servo Motor Based On FPGA

In modern high accuracy servo control system, such as CNC machine tool and wire-bonding machine, incremental photoelectric encoders are widely used as position sensors and combined with corresponding time information to get accurate velocity measurement of servo motor. The results will be provided to the closed-loop servo control system as velocity feedback. However, in practical applications, only low resolution encoders could be used for the consideration of cost. Besides, due to manufacturing error and disturb introduced by signal trasmmission and measurement, the calculated velocity always contains inconvenient error. Thus, a proper algorithm is needed to reduce the error and improve the overall performance of control system.At first, this paper analyzes the main problem in velocity measurement from encoder signal which is how to both improve the accuracy and keep the real time effect at the same time, based on theoretical research and off-line experiments of serval basic methods. After comparing some advanced methods introduced by scholars, the Least Square Estimation method is adopted. Considering the practical situation in velocity measurement, the Dynamic Fitting Points Selection method is proposed for Least Square Estimation. Its good performance under various situations is shown by off-line experiments. In order to reduce the difficulty and hardware consumption of realization in FPGA, an Approximate Least Square Estimation method is designed. The off-line experiments show the error introdued is negligible compared with standard method. Thus, it could be used in practical applications.Then, the proposed algorithm for velocity measurement is realized in FPGA by using Verilog HDL. The program contains a set of self-designed float point calculator. Simulation results show that the algorithm runs very fast and the hardware consumption is also acceptable in FPGA.At last, on-line experiments have been done in the velocity measuring platform. The results show velocity measurement accuracy from low resolution encoder of 10,000 pulses per round by using proposed algorithm is very close to that from high resolution encoder of 160,000 pulses per round. Thus the expected goal has been reached, which also means the whole project is successful, from theoretical research to realization.