Optimal feedforward tracking control of linear motors for machine tool drives

Direct drive linear motors have potential for use as next generation machine tool feed drives since they can increase machining rates and improve servo accuracy by eliminating gear related mechanical problems. This paper investigates the use of optimal l₁ and H _∞ model reference optimal feedforward control to enhance the tracking performance of a linear motor servo. Experimental work is presented which studies the effects of signal preview, tracking constraint, and reference model choice on tracking performance. Sub-optimal l₁ control in the case where the closed loop system has a zero on the unit circle due to feedback integral action is seen to give near optimal performance for the system under study here. For the specific reference trajectory employed, the best performing feedforward controllers were experimentally seen to reduce the maximum and rms tracking errors of the (feedback only) closed loop system by more than half