Design and analysis of a high bandwidth disk drive servo system using an instrumented suspension

The magnetic disk drive industry exhibits a continuing technological trend of ever increasing storage capacity leading to track densities of 25000 tracks per inch (984 tracks per millimeter) by early in the next century. Successful operation at these track densities will require an increase in the closed-loop bandwidth of the head-positioning servo system. Resonance modes in the suspension of hard disk drives limit the closed-loop bandwidth. The bandwidth of the servo can be increased by active vibration control of the resonance modes. This paper considers the optimal placement of strain gauge sensors on a suspension to observe the vibration states of the suspension. Using a finite-element simulation of an actual suspension, a state-space model is identified for the two normal strains and the shear strain at each finite element. The state-space model includes the dynamics of the three primary resonance modes. A numerical search algorithm is used to determine the sensor location and orientation which maximizes the minimum singular value of the observability grammian, With the strain gauge output signal, a multirate inner loop controller is designed to be used with the existing head-positioning system. Simulations and analysis show that increased servo bandwidth is achieved without loss of system robustness to resonance frequency changes and high frequency spillover, Results suggest that use of an instrumented suspension is a viable candidate method for improved disk drive servo performance.