A magnetic disk memory is distinguished by its large storage capacity and by the relatively short time taken by the magnetic heads, which read and write the data contained on the disks, to access any information contained at any point on the disks, beginning at the moment when they have received the command to access this information. This time as defined above is known as the access time.
The data on the magnetic disks are recorded in binary encoded form on concentric circular recording tracks, the width of which does not exceed a few tens of microns and which are disposed on both sides of the disks.
Associated with each side of a magnetic disk is at least one magnetic reading/writing head disposed a few tenths of a micron away from this side.
In current practice, and more particularly in the case of memories that include a limited number of disks (generally less than four or five), the data contained on each side of a disk are preferably distributed over equal and adjacent sectors of a circle. Conventionally, one side of a disk is divided into several tens of sectors (most often, 40 to 50). This kind of distribution of data on magnetic disks is described in French Pat. No. 2.466,078, for example, filed by the Compagnie Internationale, pour l'Informatique Cii Honeywell Bull, predecessor in interest to Bull, S. A., assignee of the present invention, on Sept. 21, 1979, corresponding to U.S. Pat. No. 4,383,207.
Each sector is divided into two unequal areas. The larger area includes the data that are intended to be processed by the data processing system of which the disk memory is a part (these are called data to be processed), while the smaller area includes the data required for automatic control of the position above the tracks of the magnetic head associated with the side of the disk containing these data. The track position finding data include in particular the addresses of the tracks. It will be recalled that the address of a track is the encoded expression of the serial number j of a track affected thereby, j being an integer varying from 0 to NP-1, NP being the total number of recording tracks. When the magnetic reading head associated with one side of a disk is located facing a predetermined sector thereof and reads a given address, this means that it is located facing the track having the serial number j corresponding to this address. The address accordingly represents the position occupied by the head facing the side of the disk.
Since the disk has a constant speed of rotation, the time taken by the disk for two successive sectors of the disk to travel past the reading head is identical from one sector to the other. This time T separating the passage of two successive sectors before the reading head is called the "sampling period".
The position occupied by the head (that is, the address read by the head) is thus known every T seconds, and the corresponding instants are called "sampling instants".
The present trend in the development of disk memories is to attempt to shorten the access time. To do this, the head must be capable of displacement from a departure track to the arrival track within the shortest possible time and must be capable of being positioned precisely with respect to the arrival track.
Apparatuses which enable displacing and positioning the head in response to these requirements are also known. These apparatuses utilize an electrodynamic motor of the "voice coil" type, which includes a coil that is displaced linearly on the inside of a permanent magnet. This coil is mechanically connected to a carriage, which via a suspension arm carries the magnetic heads associated with each of the sides of the disks in the disk memory.
A movement comprising two phases, one of acceleration and the other of deceleration, is imposed on a displacement and positioning apparatus of this kind. In the course of the first phase, a direct current having a predetermined direction is applied to the coil. Generally, this first phase is done in an open loop (that is, the head positioning system is not slaved) and with a constant acceleration.
In the course of the second phase, the movement of the head is generally under automaic control, that is, slaved, and the current direction is opposite that of the first phase.
Various means for slaving the movement of the head during this second phase are known. Such means are described, respectively, in French Pat. No. 2,336,768, corresponding to U.S. Pat. No. 4,166,970, filed on Dec. 24, 1975, and No. 2,258,661, corresponding to U.S. Pat. No. 4,031,443, filed on Jan. 18, 1974, as well as the aforementioned No. 2,466,078, all these patents having been filed by the Compagnie Internationale pour l'Informatique Cii Honeywell Bull.
French Pat. No. 2.336.768 describes an apparatus for displacing a system that is movable with respect to a magnetic disk in which the current in the coil is a function of the comparison between the position occupied by the head and the position of the arrival track. The movement of the head is thus slaved to the position occupied by the head.
French Pat. No. 2.258.661 describes a system for displacing a system that is movable with respect to a magnetic disk in which the current in the coil during the second phase is a function of the comparison between a theoretical reference speed furnished by a memory and an actual measured speed of the head positioning system.
French Pat. No. 2.466.078 describes a positioning system with slaved acceleration. The current in the motor, at the time of the deceleration phase, is a function of the comparison between a set-point acceleration (which can also be called a reference acceleration) and the measured acceleration of the head positioning system.
If the positioning system described in principle do enable obtaining correct access time and precision, still they no longer meet current requirements in terms of material.
Furthermore, for a given energy supplied to the positioning system, the access time of the systems described above is considered to be overly long. Similarly, for a given access time, the energy expended by these positioning systems is considered to be excessive.