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Correction of Data Track Misregistration in Servo Controlled Disk Files

IP.com Disclosure Number: IPCOM000082368D
Original Publication Date: 1974-Nov-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 3 page(s) / 36K

Publishing Venue

IBM

Related People

Paton, A: AUTHOR

Abstract

The figure shows schematically a magnetic disk store having a number of magnetic disks 1 rotatably mounted on a spindle 2. Magnetic read/write heads 3 are moved backwards and forwards from one transducing position to another over the surfaces of the disks, by an actuator mechanism 4 common to all the heads. The movement of the actuator and the heads is depicted simply by the double-headed arrow 5.

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Correction of Data Track Misregistration in Servo Controlled Disk Files

The figure shows schematically a magnetic disk store having a number of magnetic disks 1 rotatably mounted on a spindle 2.

Magnetic read/write heads 3 are moved backwards and forwards from one transducing position to another over the surfaces of the disks, by an actuator mechanism 4 common to all the heads. The movement of the actuator and the heads is depicted simply by the double-headed arrow 5.

The actuator mechanism 4 forms part of a closed-loop servo system and receives controlling signals from a file control unit 6 via an arithmetic unit 7, the function of which will be described later. The controlling signals to the actuator mechanism 4 supplied on control line 8 energizes the mechanism 4 to cause it to move in the appropriate direction during an access operation, and to bring the heads 3 to rest over the destination track at the end of the access operation.

A servo head 9 derives track crossing and on-track information from servo tracks recorded over the entire surface of one of the disks 1 in the stack, and supplies this information to the file control unit 6 to close the servo loop via an adder 10, the function of which will also be described later. When the destination track has been located, data from a data channel 11 may be written into and subsequently read from the corresponding data tracks by one or more of the data heads 3. The transfer of data between the data channel 11 and the data heads 3 is under control of a switch 12.

In an arrangement where the positions of all data heads are controlled from servo information derived from a single servo disk, a data head associated with a remote disk may be off-track even though the controlling servo head 9 is on- track. The situation occurs in a file with a high-track density and is caused, for example, by differential expansion of the actuator arm and stack of disks, due to temperature variations occurring between the time when data is written on a particular track and the time when it is subsequently read.

This problem is overcome in this apparatus by a calibration procedure which is performed during initial start up of the file, and again at predetermined intervals during the operation of the file as found to be necessary. Although there is only one disk surface dedicated entirely to servo information, each data disk 1 is supplied with a few prewritten servo tracks spaced across the surface of the disk. These "sparse" servo tracks appear at the same location on each data disk 1.

The calibration process consists first of all of moving the data heads 3 to each of the ""sparse'' servo tracks in turn, under control of the servo head 9. When the servo head 8 is in the on track position corresponding to the first cylinder of sparse servo tracks on the data disks 1, the data heads 3 are used to read any servo error derived from the sparse servo track or the associated disk. These errors are indicative of...