Control of linear motors for machine tool feed drives: Design and implementation of H-infinity optimal feedback control

Direct drive linear motors have good 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. To exploit the high speed and high response direct drives for machining the servo control must achieve as high as possible tracking performance while at the same time establishing as much as possible the dynamic stiffness in order to maintain machining stability and reduce the effect of machining disturbance forces on the tool position. This paper investigates the use of optimal H-infinity control to design for large stiffness and closed-loop tracking performance. Position feedback alone is first considered, with cutting force feedback later added to augment closed loop stiffness. Optimal position feedback is experimentally seen to achieve up to a 46 percent stiffness improvement over that achievable with proportional-derivative control. The addition of force feedback to the servo-loop resulted in a further 70 to 100 percent stiffness improvement over the position feedback alone values.