Lowering injection amplitude in sensorless control by means of current oversampling

Sensorless control approaches for low and zero speed determine the rotor position by injecting high frequency voltage signals. Current measurement noise and quantization necessitate a minimum amount of injection amplitude, which implies an acoustic noise emission that for many applications is unacceptable. This paper proposes to sample the current with the maximum AD conversion frequency while processing the data with an FPGA. By means of a recursive least squares method and a theoretical extension, high precision current slopes are calculated from low injection voltage. Experimental results show that the injection magnitude could be reduced by factor 7 to 10 compared to standard PWM centered current measurement, but also that such weak excitation shifts the HF injection approach to a scale where normally negligible effects have to be taken into consideration.