Model and Control of a Compact Long-Travel Accurate-Manipulation Platform

Stick-slip accurate manipulator (composed of piezo and slider) is widely applied in scanning electron microscope (SEM). However, for its positioning control in SEM, conventional displacement sensors are hard to be used because of their big physical dimensions or magnetic working principles. To solve these problems, this paper proposes to use strain gauge as the accurate manipulator's positioning sensor and develops a displacement-prediction method to achieve a long-travel accurate positioning (positioning error no more than 2% of the traveling range) in SEM. The slider's feedback displacement is obtained indirectly, which is calculated and predicted by the LS friction model and the piezo's motion statements obtained from the strain gauge. Meanwhile, the application of strain gauge can minimize the manipulator's geometric dimension guarantee a nonmagnetic working environment and avoid the thermal radiation caused by sensor in SEM. An LS friction model with the main friction characteristics is developed to provide the friction calculation between the slider and the piezo. Feedforward PID control method is employed to improve the system's dynamic characteristics. Experiments are carried out to validate the effectiveness of displacement-prediction control applying on the positioning platform.