Reducing the Cogging Torque Effects in Hybrid Stepper Machines by Means of Resonant Controllers

Permanent magnet machines are not free from the interaction between magnets and the stator and rotor slots, which causes an undesired disturbing torque. Such cogging or detent torque is especially larger with salient pole machines, as it is the case of the permanent magnet hybrid stepper machines (PMHSM). Depending on the application requirements, these torque perturbations can be unacceptable and the application of solutions that minimizes the cogging torque effects are mandatory. This paper analyzes the minimization of the cogging torque using resonant controllers. More specifically, this paper details the analysis and design of a speed-adaptive resonant controller, which is not only directly designed in the Z-domain but also considers the current (or torque) inner-loop delay. Pole-zero placement and the disturbance-rejection-frequency response are attained in the design of the speed and position speed-adaptive controllers. Experimental results with two off-the-shelf PMHSMs demonstrate the superior performance of the proposal in both speed and position closed-loop applications for tracking, as well as in disturbance (load impact) rejection tests and against inertia variations. A comparison with a conventional PI is carried out from the design stage to experimental results and the improvement of the proposal is numerically quantified.