Construction and control of an ultra-stiff nanopositioning system

This paper describes the design and manufacturing of a rigid positioning system with high resolution based on piezostepper technology. The planar piezostepper is able to move along the three in-plane directions and is able to make small correction movements in the out-of-plane directions. The piezostepper integrates the drive system, the transmission, and the bearing system into a single stepping unit. This leads to a unique combination that combines high positioning accuracy with high bearing stiffness, obtained by active control. The position deviation of the stage under influence of the disturbance forces of an ELID process proves to lie within 20 nm. Introduction Recently, very promising new grinding methods became available. Ductile or shearmode instead of brittle removal can now be realised for all kinds of materials, such as metals, glass and ceramics. An example of these new grinding methods is ”Electrolytic In-process Dressing” (ELID) [1] grinding. This process requires submicron track control of the relative position between tool and workpiece in all directions. Existing grinding machines are not accurate and stiff enough to perform this task so that new positioning methods have to be developed. This paper presents the development of a piezo-driven positioning method for accurate and stiff positioning. At the same time it eliminates traditional problems of linear guides like backlash, stick-slip and rolling resistance. Existing piezosteppers, like the inchworm drive [2] or others [3,4], do not provide the necessary stiffness or do not provide a continuous motion. Zhang et al. [5] and Shamoto et al. [6] were the first to recognise the necessity of high stiffness next to high accuracy. Concept of the piezostepper Contrary to existing piezosteppers, suffering from hammering phenomena and discontinuous motion velocity, this stage [7] is able to generate a smooth, continuous motion within tight tolerances. Sub-micron resolution and high passive stiffness are obtained by the use of piezoelectric actuators. Moreover, piezoelectric actuators have a short response time and can generate large forces. Their limited stroke, a few micrometers, can be overcome by using a stepping algorithm. The stage consists of a series of drive units held in a rigid frame. Each unit has a spherical hard contact element held in place by a steel membrane. The stepping motion is obtained by rolling the preloaded contact element over the base surface using a push-pull set of piezo-actuators. The force transmission between unit and base is by rolling friction. The use of rolling friction, in stead of sliding friction reduces the wear of the contacting surfaces. Spreading the drive function over a