Compensation of nonlinear distortion effects for signal injection based sensorless control

An accuracy and robustness comparison is made between three high-frequency carrier-based sensorless control techniques. The high-frequency carrier signal may be injected on an arbitrary stationary three-phase axis or the estimated, synchronously rotating d or q-axis. Nonlinear distortion effects introduced by the pulse-width modulation switching process lead to distortion of the high-frequency carrier signal which causes a degradation of position detection accuracy. It is shown that all three signal injection techniques suffer loss of accuracy but the distortion is least when the carrier is injected on the torque producing q-axis. This leads to a tradeoff between the undesirable production of high-frequency torque ripple and the radiation of audible noise, and robustness to nonlinear distortion effects. To overcome this tradeoff a new compensation model is proposed. An experimental implementation of the new compensator is applied to the three signal injection techniques used to estimate the rotor angle of a permanent magnet ac machine. Experimental results confirm that the new compensator minimises the distortion of the high-frequency injection signal irrespective of its axis of injection which leads to an improvement in position estimation accuracy and robustness.