Hybrid force-position control of clamping with a piezo-stepper

The demands concerning shape accuracy, surface roughness, and subsurface damage on machined parts are continuously increasing. Traditionally, the production process of accurate and smooth surfaces consists of some roughing steps, like turning, and subsequent finishing steps, like polishing or precision grinding. Two new grinding processes, however, electrolytic in-process dressing (ELID), and high-speed chipping (HSC), can combine high removal rates with high surface quality through a mechanism of submicron chip removal. For the successful application of these processes, very stiff, ultra-precision positioning systems are needed. Classical solutions, based on traditional guide way technology, cannot meet the requirements of these new machining processes. The piezo-stepper solution presented in this paper can provide accurate motion, combined with high stiffness, through the use of piezo-electric actuators with high resolution. High passive stiffness is provided thanks to the integration of the driving, bearing and transmission functions. The same actuators can provide an extremely high active stiffness by referring the position of the positioning stage with respect to an absolute reference frame. In contrast with existing (position-controlled) piezo-steppers, this paper presents a way to provide smooth stepping by avoiding hammering through hybrid force-position controlled clamping and by rolling traction.