Automatic optimization of load angles for a linear hybrid stepper motor
The objective of this contribution is a linear direct drive based on the working principle of hybrid stepper technology. Herein, reluctant forces enable the thrust of this type of drive. In order to improve the dynamic performance a method adjusting the optimal load angle with respect to the driven velocity is presented. Commonly, the phases of the linear hybrid stepper motor (LHSM) are commutated sinusoidal with a constant load angle of 90 degrees. Due to delay times of sensors, actuators and hardware, the coils of the phases are not energized optimally in terms of maximum force application. Thus, variable load angles subject to velocity are introduced. This contribution comprises the optimization of the load angles. To solve this one-dimensional optimization task, bracketing methods can be used. These algorithms work without derivatives and find the minimum through iterative decreasing of the interval until a desired tolerance is achieved. Regarding the implementation, signal processing has to be done beside the optimization algorithm to ensure feasible solutions. The entire optimization process can be carried out automatically on the test rig. As a result, a characteristic curve is obtained describing the optimal load angle to velocity relation. Including the directionality, the characteristic curves are distinguished between forward and backward drive. Further properties of the optimization algorithm such as convergence and reproducibility are examined and discussed. The curves are implemented on a real-time system facilitating a comparison with constant load angle commutation. Velocity control measurements exhibit an improved performance, especially at high motion dynamics.
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