On-off robust design of hard disk ramp loading

As the hard disk media data density is increasing, it is beneficial to park the read/write head of hard disk at an external parking area, which will save the data space area and further will improve the reliability performance of the hard disk against the external shock and vibration during non-operation time. Before read/write data from the disc media, the head have to be moved out of the parking area, usually through a ramp. This procedure is called ramp-loading procedure. To prevent the read/write head from colliding with the disc media at the end of the ramp, which can damage; both the head and the media, it is crucial to limit the velocity of the head moving out of the ramp. To control the velocity of the head during ramp loading, the velocity has to be measured. Usually the velocity information is obtained by measuring the back-EMF of the voice coil motor (VCM) that is proportional to the value of the head velocity. There are a lot of methods to measure the back-EMF. But the most cost-effective method is to set tri-state the VCM driver, wait till the current attenuate to zero and then measure the voltage between the two ends of the VCM that equals to the value of back-EMF. However, this method needs to turn on and off the VCM driver that presents a discontinuous control problem for velocity control. This paper deals with this on-off control problem for ramp loading procedure. After analysis of the problem, a new design method is presented and the velocity at the end of ramp when the head is leaving out of the ramp and moving above the media area is limited through a provided switching controller.