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This paper presents a design method of robust controller using the modified dual feedback controller to improve transient response in track seeking while attaining robust performance and stability in the presence of plant uncertainties. Coupled dynamics model is identified from the Newton-Euler formulation. We use discrete-time LQG/LTR controller design method. Robustness analysis is performed using the l-analysis method. With the proposed method we are able to obtain an improved transient response. Furthermore, we can achieve the desired sensitivity functions while assuring robustness to the parameter variations to known structured uncertainties. It is demonstrated that the proposed method can be extented to short track-seek control without change of the controller structure. From simulation and experiment results, we confirm the effectiveness of the proposed method. IntroductionIn the design of dual-stage actuator control systems, the relative contribution of VCM and microactuator in the both time and frequency domains is important because destructive interference may occur (Schroeck et al., 2001). The VCM is used as a first stage actuator to provide coarse and slow motion, while the microactutor is used as a secondary actuator to provide fine and fast positioning. In the frequency domain, microactuator controller should be designed to reject high frequency disturbances while VCM controller should be designed to reject low frequency disturbances. Thus, the sensitivity function of each actuator as well as that of the dual-stage actuator is important design specification. Conventionally, the master controller controls the microactuator in order to compensate for position error and the slave controller controls the VCM to compensate for relative position error (Koganezawa et al., 1999). But the interactions between the two actuators are neglected in controller design. Thus control performance degradation is unavoidable. Furthermore, it is difficult to design sensitivity function for disturbance rejection although this method has an advantage that two loops (master and slave) can be designed separately. In Schroeck et al., 2001, DISO system is changed to a SISO system so that the relative actuator contribution in the frequency domain can be considered. Furthermore, recently, a new controller design methodology for DISO system was proposed by using Sbode plot and PQ method (Rapley et al., 2001). It provides insight into the effect of the open loop frequency on the closed-loop sensitivity function. A number of control design methods for track following have been addressed. However, track-seeking performance of dual-stage actuator has not been addressed much and it has been only evaluated based on step response. A new decoupled track-seek controller for dual-stage servo was developed to prevent overshoot and obtain improved seek-setting performance (Kobayashi et al., 2001). Three steps approach was used in the design of reference trajectory and feed forward compensator to improve the transient response in the paper....

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