A higher bandwidth servo design with strain feedback control for magnetic disk drives

To overcome the mechanical resonances limiting the servo bandwidth for head-positioning control, a control method using a strain-feedback controller added to a conventional head-position feedback loop, was developed. The strain feedback controller was designed so that the gain and phase delay of the sensitivity function for the strain feedback control system are both reduced in the frequency region below the primary mechanical resonance. Experiments showed that the controller suppresses the gain by about 10 dB at the primary resonance, and it removes the phase delay of the controlled plant in the frequency region below the primary resonance. It was found that the strain-feedback control (i.e., strain feedback plus a conventional head-position feedback loop) increased the open-loop bandwidth of the head-positioning servo system by 17% and improved the positioning accuracy by 18%. It was also found that the sensitivity function of the head-positioning servo system with the strain feedback did not change when the temperature increased from 5°C to 60°C.