Control of Precise Positioning System with Cascaded Colinear Actuators

With the current emphasis on precision in science and engineering, there are increasing requirements for precise manipulation systems that operate over a range of several centimeters or greater with an accuracy in the sub-micrometer range. A control concept is presented which addresses the problem of using two controllers operating in a parallel fashion to form the inputs to a pair of cascaded colinear actuators. This is the problem encountered in precision positioning systems when one actuator acts as a long range course positioner and the other a shorter range fine positioner. Typically the short range actuator has a much higher bandwidth than the long range actuator and this poses a difficult control problem when fast accurate positioning is required. A linear mechanical positioning system is described and techniques for controlling position to 80 nm over a 25 cm range using cascaded colinear actuators are presented. The computer control system included a high speed Digital Signal Processing (DSP) chip to implement the digital control algorithms. A unique method of handling known decaying periodic disturbances based on the Internal Model Principle was also developed. This method resulted in a 70% reduction of positioning error while in the presence of this external disturbance.