Magnetic disc drives use magnetic heads and recorded servo code in a track following servo mode for keeping the magnetic heads track centered during reading operations. The magnetic heads comprise a magnetic core having an air gap therein and having a coil wound thereon. These magnetic cores vary in effective magnetic widths due to their design and due to the manufacturing process. These physical variations among the magnetic heads result in variations in servo gain when they are individually connected in the servo loop. Efforts to compensate for these variations in servo gain have included manual adjustments of servo gain of the individual heads in the servo loops and arrangements for automatically compensating for such variation which latter are made in the circuits coupling the individual heads to the servo system.
Such arrangements usually involve, individually calibrating the heads, determining the correction to bring all of the heads within a predetermined servo gain band width and externally storing the individual corrections which are to be made to compensate the servo gain variation with different heads. These approaches however, do not suggest the use, on the memory discs, of servo code patterns which are designed to compensate head-to-head servo gain variations, to compensate servo gain variations resulting from the radial position of the individual heads on the discs and to compensate servo gain variations resulting from the aerodynamic behaviour of the individual heads in flight.
Along this line, U.S. Pat. No. 4,412,165 describes a sampled servo position control system. Here servo codes provided at intervals on a record medium are used in a sampled servo system to define data tracks. The patent notes that problems arising from the magnetic medium or from the servo writing process can in some instances lead to misalignment of a servo path or track defined by the servo code with a corresponding desired data track. The problem is solved by measuring the position error signals with a head constrained in the correct on-track position. These position error signals are stored in the disc data section immediately preceding the associated faulty sample and used to keep the head on track with the data.
While recording on the disc is described to store information to correct this off-track problem, there is no consideration with regard to specific configurations of magnetic zones or patterns of such configurations of magnetic zones for achieving uniform servo gain.
There is one patent known to the applicants, however, that does refer to the use of the magnetic media for compensating servo gain variations. This is U.S. Pat. No. 4,530,019. Here a code pattern, useful in providing data to a processing scheme to control the fine positioning of a transducer head of a disc drive unit, comprises an erased gap followed by an automatic gain control information code burst followed by a first burst of servo control information code followed by a second burst of servo control information code. Mechanical indexing is used in writing this code. Rotation of the disc is used as a primary time reference with all other time references being based on a transition between the erased gap and the automatic gain control burst. The servo control information bursts are written alternately off track by one half of the track separation between the track of interest and the next adjacent track to each side thereof. The pattern is decoded using a comparator circuit and an integrator. The decoded information being used to center the transducer over the track.
Although the AGC information is stored on the disc it is only an automatic gain correction for that particular head and is not a servo code pattern which together with other servo code patterns in different tracks across the disc result in relatively uniform servo gain for that particular head at any location on the disc.
U.S. Pat. No. 4,588,636 is directed to a magnetic memory disc which has controlled magnetic characteristics in the radial direction achieved by controlling the thickness and composition of the magnetic layer which is formed on the disc. The procedure for forming such a disc coating is described in the patent and involves an electroplating operation.
Specific servo codes for achieving relatively uniform servo gain with a particular head at any location radially of the disc are not discussed.