Future Servo Technologies for Hard Disk Drives

This paper focuses on some of the newer servo technologies that have been added to the actuator servo control system in order to meet the higher track density requirements for future HDD. After a brief description of the basic digital actuator servo control system, the paper focuses on the use of elliptic filters to reduce the effects of mechanical resonance disturbances. Then a short discussion follows on the position sensor errors. A development of the governing equation for the total position error consisting of repeatable and non-repeatable error components is made. Feedforward control can be used to increase track density by reducing the repeatable position error. A brief discussion is given on the use of accelerometers to protect the HDD from unrecoverable data errors caused by external shocks and vibrations. A predictive write/read inhibit decision using fuzzy logic shows a way to relax the tracking requirements. Increasing the HDD track density to 25,000 tracks/inch requires a five fold increase in todays servo bandwidth. This can be accomplished by placing a rotary micromechanical actuator piggyback on the slider. Some thoughts around the use of piggyback actuators in HDDs are given.