Simple and Practical Control Method for Ultra- Precision Positioning - Application to a Ballscrew Mechanism

INTRODUCTION Precision positioning systems are essential in precision industry. These positioning systems often have one or more elements that introduce friction. Friction is well known to cause steadystate errors and limit cycles, and to slow the motion of the mechanism. Thus, it is important to consider friction compensation in controller design. Simple controllers like PID controllers do not offer the best possible performance for the mechanisms. Thus the use of advanced controllers such as controllers including model based compensators and variable structure controllers is increasing. However the advanced controller design usually requires the identification of mechanism characteristics and sufficient knowledge of the control theory. Ballscrew mechanisms have friction characteristics and are widely used in precision positioning systems. In this research, a conventional ballscrew mechanism is used as a positioning mechanism. Although some studies have achieved nanometric accuracy with a ballscrew mechanism, this research differs significantly in the ease of design and controller structure. The NCTF (nominal characteristic trajectory following) controller, has a simple structure and its design procedure does not require exact parameter identification, which makes it easy to design, understand, and adjust [1-3]. In this paper, first, the concept and the design procedure of the NCTF control method are introduced. Next, the NCTF control method is implemented to a conventional ballscrew mechanism and the positioning performance of the controller is examined. The controller is designed without any exact identification of parameters or modeling.