Active damping in HDD actuator

One of the limitations inherent in high-track-density hard disk drive design is the effect of actuator mechanical resonant modes on the head-positioning servo. The first servo bandwidth limiting mode is the so-called “butterfly mode,” which is the major mode in the head mechanical transfer functions and can not be avoided through mechanical design. A special active damping technique has been developed for rotary actuators of 3-1/2" drives. With an additional sensor on the actuator and an external feedback loop, the butterfly mode can be damped and stiffened while the system stability is maintained. A theoretical system model, based on FEM, modal superposition and feedback control, is developed for analytical prediction of system stability and dynamics. Drive level experiments are conducted on actuators of two different designs, and show that the butterfly mode can be totally damped such that the gain of the head mechanical transfer function is close to 1/s ². The end arm mode is also damped by about 8 dB for one actuator design. Servo improvement is also studied. It shows that the butterfly mode notch filter can be removed, and the phase margin can be improved by 7 degrees