6.2 Dual-Servo Stage
6.2.1 Introduction
Owing to the rapid growth and great demand in up-to-date technologies such as semiconductor manufacturing, ultraprecision machining and micro-electro-mechan- ical-systems (MEMS), the development of a precision-positioning system is an urgent need in these days. For example, the required precision-positioning accuracy
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FIGURE 6.5 Power spectrum density of 8× CD-ROM drive. (From Choi, S.B. et al., Mechatronics, 11, 691, 2001. With permission.)
is 0.13 μm for 300 mm silicon wafer, which is the mainstream in the semiconductor manufacturing process, and in the near future, the required accuracy will be grow- ing up to 65 nm. So far, there are many researchers who devote their every effort to realize a positioning system having long working distance with ultraprecision level.
Among those candidates, a dual-servo control system now attracts great attention since it has a salient advantage that can overcome current limits without changing existing facilities by just integrating the fi ne positioning system.
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FIGURE 6.6 Position tracking responses for single sinusoidal trajectory. (a) yd = 100 sin(2π × 8 × time): fi rst track and (b) yd = 100 sin(2π × 4.3 × time): last track. (From Choi, S.B. et al., Mechatronics, 11, 691, 2001. With permission.)
To realize high precision and fast manipulation, Omari et al. [15] proposed a fi ne positioning system consisting of a piezostack actuator and a displacement amplifi er attached on the end effector of an industrial SCARA robot arm. They designed the disturbance estimator and the robust feedback control system to eliminate exter- nal disturbances such as unwanted vibration on the fi ne positioning system due to coarse motion of the robot arm. Lee and Kim [16] developed an ultraprecision wafer
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FIGURE 6.7 Position tracking responses for combined sinusoidal trajectory. (a) yd = 50 sin(2π × 2 × time) + 50 sin(2π × 5 × time) and (b) yd = 50 sin(2π × 7 × time) + 50 sin(2π × 8 × time). (From Choi, S.B. et al., Mechatronics, 11, 691, 2001. With permission.)
stepper for the microlithography process. In their research, a linear servo motor was adopted as a coarse positioning actuator, and the error of the coarse positioning has been successfully compensated up to 20 nm by controlling the fi ne positioning stage consisting of multi-fl exure hinges and piezostack actuators. Moriyama et al. [17]
made a dual-servo X–Y moving stage for the step and repeat lithography system.
As a result of their work, the coarse positioning stage had the accuracy of 5 μm for the feeding speed of 100 mm/s by using conventional DC servo motors. On the X–Y moving stage, the fi ne positioning stage that has an accuracy of ±50 nm for a 10 mm step movement was achieved by employing piezostack actuators. Instead of conventional DC servo motors and ball screws, Sakuta et al. [18] proposed a dual-servo positioning system that used air-bearing slides for the coarse motion of 20 nm resolution and piezoelectric elements for the fi ne motion of 2.5 nm resolution.
Most of those researches on dual-servo mechanism have been mainly focused on
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FIGURE 6.8 Tracking control durability of the optical pickup. (From Choi, S.B. et al., Mechatronics, 11, 691, 2001. With permission.)
the design and control of the fi ne servo mechanism composed of the fl exure hinge system with piezoelectric actuators. A few of them have made an effort to develop a new type of coarse motion actuator that can substitute the conventional DC or AC servo motor system. The ER clutch actuator is one of the most potential candidates for the coarse motion control in dual-servo system. Sakaguchi et al. [19] proposed a multi-cylindrical-type ER clutch for force display system. They analyzed its perfor- mance by comparing the proposed ER clutch with conventional powder clutch. Saito and Sugimoto [20] made a cylindrical-type ER clutch and applied it as a positioning actuator for a single-link rigid robot arm. Han et al. [21] presented position control of an X–Y stage mechanism driven by a pair of ER clutch actuators. From these works on the ER clutch as a positioning actuator, it has been proved that the ER clutch can be successfully adopted in various industrial fi elds to substitute conventional DC or AC servo motors.
This section presents a bidirectional ER clutch as a coarse motion actuator and a piezostack actuator associated with the displacement amplifi er as a fi ne motion actuator to construct a “smart” dual-servo system that wholly consists of smart materials [22]. After deriving the dynamic model for the coarse positioning stage, a sliding mode controller with the friction compensator is designed to achieve robust control performance. On the other hand, the Preisach model–based feed-forward compensator with PID feedback controller was designed to compensate the hyster- esis nonlinearity of the fi ne positioning system. These controllers are experimentally realized in a decentralized strategy, and the position control responses are evaluated in terms of accuracy in order to demonstrate the effectiveness of the “smart” dual- servo system.