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Original Publication Date: 1999-Dec-01
Included in the Prior Art Database: 2003-Jun-19

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Disk drive servo systems are designed to work over a range of operating environments and manufacturing variations; accomplishing this requires a sacrifice to performance in the nominal environment with the nominal plant. Specifically, as the temperature of the VCM increases, the coil resistance increases and consequently the maximum acceleration of the actuator decreases. And the coil resistance itself may vary from drive to drive due to manufacturing variations; this, too, will alter the maximum acceleration. Further, the power supply is allowed to fluctuate within a few percent of nominal and when the voltage supply is lower than nominal, the maximum acceleration is also lower than nominal. These factors may individually or in sum result in a limit to the control that is significantly lower than nominal. To prevent seek failures, the velocity profile must be designed with this control limitation in mind; this in turn reduces the maximum performance when these factors and thus the control limit are nearly nominal. The novel advance is that this invention degrades performance only in proportion to the limitation on the maximum control. The solution takes three steps. First, measure the maximum acceleration of the drive by performing a set of seeks using maximum available control. The outcome of these seeks is a predicted distance traveled and an actual distance traveled. Second, determine from the results of these calibrating seeks the ratio of the actual maximum acceleration to the nominal maximum acceleration: this ratio is equal to the ratio of the actual distance traveled to the predicted distance traveled, and it provides a great deal of information on the state of the disk drive. Specifically, if the ratio is low, then the maximum acceleration is low, and hence the control is more constrained than nominal. This means that either the coil resistance is high, or the power supply voltage is low, or both. The third step is to take corrective action when the control is too constrained. This action consists of derating the velocity profile so that the maximum control required to follow the profile is still less than the maximum available control. (This derating can take the form of multiplying the desired velocity by a constant smaller than one, and/or adjusting the desired velocity by a constant, or even just using a different velocity profile.)