Practical control of precision positioning mechanism with friction

This paper describes the practical control of precision positioning mechanisms with friction. In a controller design, the microdynamic characteristic of these mechanisms is taken into account. First, the behavior of an experimental mechanism immediately before stopping is examined experimentally and its characteristics, namely, the micro- and macrodynamic ones are modeled. Next, a practical control method is proposed for the mechanism. The controller is designed based on the nominal characteristic trajectory following (NCTF) controller design procedure. It consists of a nominal characteristic trajectory (NCT) and a PI compensator. The NCT is determined using an open-loop time response of the mechanism and is represented on a phase plane. The compensator is used to make the mechanism motion follow the NCT and to stop the motion at the origin of the phase plane. Although the compensator is designed based on the microdynamic characteristic, it is useful in a wide working range. The positioning performance of the system with the proposed controller is examined in comparison with those of the system with conventional PID controllers. These results prove that the system with the proposed controller has a better positioning performance than those with conventional PID controllers and its positioning resolution is higher than 50 nm.