Disk drive servo system

Claim 1

An electronic tachometer for a magnetic disk subsystem of the type including: magnetic disks which are mounted for common rotation; an access mechanism carrying magnetic heads into read-write relationship with the tracks of said magnetic disks; a motor for driving said access mechanism; a servo disk mounted for concurrent rotation with said other disks, said servo disk having recorded thereon a magnetic pattern which produces from a head in reading relationship therewith an oscillating position signal representing the position of said access mechanism with respect to the tracks of said disks during a seek for a particular track; a variable bandwidth filter for receiving and filtering said position signal; differentiating means coupled to said variable bandwidth filter to receive a filtered signal therefrom and to produce a velocity signal which represents the velocity of said access mechanism across the tracks of said magnetic disks except during those intervals when said oscillating position signal undergoes a reversal in amplitude change; means for generating a current signal representing the current supplied to said motor driving said access mechanism; integrating means for integrating said motor current signal; combining means for combining the differentiated position signal and the integrated current signal to produce an output signal representing access mechanism velocity with respect to said tracks; a velocity detector coupled to said combining means for producing an output signal representing the velocity being applied to said velocity detector, the output of said velocity detector being applied to said variable bandwidth filter to change the bandwidth of said filter so that said filter has a broad bandwidth for high velocity signals and a lower band width for low velocity signals; said motor current signal having a polarity in the reverse direction which is the opposite of its polarity when said rotary access mechanism is being driven in the forward direction, said integrated motor current signal being applied to add to the voltage on said combining means during the acceleration portion of a seek and being applied to subtract from said voltage during a deceleration portion of a seek; and a variable gain circuit, said motor current signal being applied through said variable gain circuit to said combining means, said variable gain circit including means for switching the gain of said motor current signal between a gain less than one and a gain greater than one during the accelerate and decelerate portions of each seek.