Servo controller design for high bandwidth HDDs

Summary form only given For high TPI Hard Disk Drives (HDDs), wide servo bandwidth and fast seek settling time are needed. The servo bandwidth is primarily limited by the actuator´s mechanical resonance modes. Including the actuator resonance mode in plant modeling gives an additional degree of freedom in controller design. Controller performance improvement is confirmed using second phase margin stabilization. Also, the LTR approach, starting from a double integrator plus resonance plant, allows one to achieve a better sensitivity shaping. Since the resonance mode reduces achievable phase margin, fast seek settling time with allowable percentage overshoot cannot be achieved. In addition to this, the frequency peak of sensitivity and complementary sensitivity function are increased. Including the resonance mode in plant modeling is proposed to increase the phase margin. However, including such a resonance mode does not necessarily bring better performance. Furthermore, the controller may be sensitive to parameter variations. Including such a resonance mode, we propose a controller design method to obtain the desired stability margin, servo bandwidth and frequency peak of sensitivity function using a Sbode plot.