Design and Control of a Six Degree-of-Freedom Magnetic / Fluidic Motion Control Stage

A six degree–of–freedom electromagnetically suspended motion control stage (the Angstrom Stage) has been designed and constructed for use in short-travel, high-resolution motion control applications. The stage consists of a platen suspended in oil and acted upon by twelve electromagnets in opposing pairs. Position is sensed using six capacitance probes within a working volume of 100 microns cubed. A digital controller implements a feedback linearization algorithm in real-time to account for nonlinearities in both the magnetic actuators and the squeeze film dampers. This effectively yields a linear plant to be controlled by an outer loop. Damping provided by the oil dominates the stage dynamics, and so an outer loop proportional controller is sufficient for normal operation. Under controlled conditions and with limited range, the stage has achieved angstrom resolution. In general operation over the full 100 micron range, the peak-to-peak error motion is about one nanometer. We envision applications of this stage in scanned probe microscopy and for integrated circuit measurement. Other methods of precision motion control such as voice coils and piezoelectrics offer viable alternatives in these applications. However, the ability to independently control and decouple the six degrees of freedom gives our stage an advantage when vibration rejection, mechanical simplicity, accurate trajectory following, or precise metrological data are required.