In some prior art apparatus, servo systems are used to control the speed and position of magnetic head assemblies, which are driven by a motor, across the surface of a rotating storage disk. The servo system provides an initial acceleration, a subsequent uniform speed, and a deceleration of the magnetic head towards the end of the positioning path before the magnetic head reaches the selected track. The speed regulation of the magnetic head is ensured by a tachometer associated with the servo circuit, which is disconnected from the drive motor for the magnetic head when the selected track is reached. Simultaneously, the motor that drives the head assembly is switched on in a servo regulating circuit which causes the magnetic head to be guided over and to follow the center of the recorded track. The servo pattern is recorded on a different surface of a magnetic disk than the data being stored. The servo signals are sampled by a servo head and the data signal by a separate magnetic head. Both servo and data heads are associated with different, but jointly driven access arms, which serve to position the magnetic heads to a selected track.
The disadvantage of such prior art apparatus is that the arrangement of the servo and data signals on different disk surfaces lead to positional differences between the servo head and the data head. The tolerance values of these positional differences are so high that they limit the density of the data tracks of a disk drive.
It is also known, as described in U.S. Pat. No. 3,185,972, to regulate the track following of a magnetic head of a disk drive by means of servo signals that are sampled from a servo pattern that is recorded in servo sectors of the data tracks on a disk surface. The servo sectors are arranged at uniform angular distances between the data sectors of the magnetic disk surface. The sampling of the data and servo sectors is achieved by a magnetic head positioned to a selected data track. For this purpose, the servo circuit is controlled by a switching device that is synchronized with disk rotation and which, as the servo sectors are sampled by the magnetic head, applies sampled servo signals at predetermined time intervals to the magnetic head.
This type of device makes it possible to associate the servo and data magnetic head in a single head arm structure so that, as a result, position differences between the data and servo magnetic transducing elements of the head are eliminated, thereby enabling the increase of density of data tracks. However, the device has the disadvantage that for track selection and track following of the magnetic head, different drive systems are required, which leads to an increase in the mass moment of inertia of the elements being moved, and to a reduction in the positioning speed of the magnetic head.
It has also been proposed, as in copending patent application, Ser. No. 815,394, filed July 13, 1977, now U.S. Pat. No. 4,120,505, and assigned to the same assignee, to regulate magnetic head position during the track accessing and track following modes in such a manner that the same drive system can be used during both modes. In this case, the servo signals for regulating the track following of the magnetic head are recorded in servo sectors of a disk surface. At the beginning of an access motion of the magnetic head, the drive motor of the magnetic head which effects the radial advancing motion of the head is connected to a constant drive voltage during an acceleration phase. The duration of the acceleration phase is controlled by clock signals which are sampled by a fixed magnetic head. The clock signals are utilized for controlling the switching periods of the data and servo sectors. After sampling of a particular number of servo sectors, the drive motor of the magnetic head is connected to a servo regulating circuit for a particular period of time during which the speed of the magnetic head is constant.
After completion of the access motion of the magnetic head, the servo regulating circuit is used to regulate track following. For the duration of a particular number of servo sectors, the servo signals, by means of which the radial position of the magnetic head is indicated during each sector time, are sampled by the servo and data magnetic head for the duration of a sector time. By means of the regulating circuit, an acceleration or deceleration component of the drive signal is produced depending upon whether, at the time of sampling, a leading or lagging track deviation of the magnetic head from the center of a data track is detected. At the end of the period in which the access speed is constant, the drive motor is disconnected from the regulating circuit and connected to a constant voltage for decelerating the magnetic head. The polarity of this voltage is opposite to the polarity of the constant drive voltage to which the drive motor was connected during the acceleration phase. After a particular number of servo sectors has been passed, the magnetic head is stopped on a selected recording track. After the address track position has been reached, the drive for the radial advance of the magnetic head is connected to the regulating circuit for track following.
The apparatus described above offers the possibility of utilizing the drive of a magnetic head both for the duration of an access motion and, after the address position has been reached, for the regulation of track following. There is, however, the disadvantage that the switching processes of the drive voltage lead to discontinuities of the drive force acting on the drive motor of the magnetic head, so that oscillations of the moving head assembly reduce the positioning accuracy and the positioning speed of the magnetic head.