| As the density of data on magnetic disk drives increases, so does the need for more precise position control of the read/write head, especially in the presence of internal and external disturbances. This is achieved by two separate algorithms, acceleration feedforward control and a disturbance observer.; In the first algorithm, the acceleration of the drive is measured and fed forward to the actuator. By matching the electromechanical impedance between the disturbance and the position error, the feedforward controller can cancel the effects of the disturbance. Two techniques are presented for designing the feedforward controller. The first technique is an infinite impulse response filter that is designed off-line, and the second is a finite impulse response filter that is adapted on-line using the filtered-x LMS algorithm. Both feedforward techniques were tested through shake table experiments, resulting in reductions of the position error signal between 50% and 95% for vibration disturbances in the frequency range from 10 Hz to 500 Hz. Simulation of the system response to a shock disturbance resulted in a 89% reduction of the peak error.; The second algorithm does not require an external sensor. This is particularly relevant in products such as disk drives where cost is a major concern. The disturbance observer uses the position error signal and a nominal model of the plant to create an estimate of the disturbance. This estimate is then used to compensate for the disturbance effects. Vibration simulation indicated that the performance of the disturbance observer was comparable to the feedforward control in the 50 Hz to 500 Hz range, but offered significantly better results below 50 Hz. Experimental results verified that the disturbance observer was effective below 200 kHz, but noise in the system degraded performance between 200 Hz and 1 kHz. Simulation of shock testing with the disturbance observer was similar to the feedforward control results. |
