Dual track servo system.

0001This invention relates to control systems and methods for translating a transducer relative to a disc, and more particularly to methods and apparatus for moving a trans­ducers relative to a rotating disc and achieving more accurate registration between the transducer and the disc in a structure wherein a plurality of heads are being aligned with tracks on a stack of discs.

0002Owing to the capacity of modern electronic processors for processing large masses of information, large capacity stores for data and instructions must be provided. In other words, the need is clearly established for external stores, the capacity of which can be increased at will and in which searching and reading are rapid. These re­quirements are met by magnetic disc storage units, which typically in large mass storage situations include a cylindrical package of vertically stacked discs.

0003In magnetic discs, the information is recorded in serial digital form on each face of a disc along a plurality of concentric circular tracks laid down in the circular band between the outer edge of the disc and a central circular area. The tracks may be subdivided into sectors, so that it is possible to select a single segment of track as a storage area for recording or reading. To accomplish this reading and recording, transducer elements are positioned over vertically aligned tracks on the stack of discs.

0004Rapid reading of the information sought is limited by the availability of means for rapid, accurate positioning of the magnetic transducer relative to the vertically aligned tracks on the stacks of discs that contain the information.

0005Normally, read/write transducers are mounted facing one another on a pair of arms movable in a radial direction with respect to the disc and disposed in the form of a fork so as to operate on both sides of the disc. In high capacity disc drives, a plurality of such pairs of recording heads are provided, operating on and facing opposite sides of a stack of discs. Each transducer must be accurately positioned, both in a normal direction with respect to the surface of the disc and in a radial direction with respect to the axis of the disc.

0006The positioning of the transducer is generally carried out by electromechanical systems. It is obvious that the closer the tracks are brought to one another and the smaller the tolerance in the radial distance between two tracks (with a consequent greater amount of information recordable on the surface of the disc), the greater the problems that arise with the positioning mechanism. The mechanism must not only achieve a low time of access to the information sought, but must also effect precise positioning of the transducer over the desired track. FIG 1 schematically depicts a group of discs and a cor­responding group of data heads such as are typically used in a high capacity disc drive. If the gaps of the record­ing heads are arranged on a line designated as HB-HT, they would record information or data on the discs which would ideally form another line DB-DT. The two lines should be coincident with each other.

0007To position the transducers over the desired tracks, the disc drive unit receives from the computer an indication of the track in which the information sought is located. The disc drive unit must be able to execute the order of the computer by positioning the transducers over the track addressed, and thereafter be able to perform the function of reading or writing (or possibly erasure) which the processor transmits to it.

0008However, due to dynamic and thermal disturbances which occur in disc drives, the recorded data can become displaced from the data heads. Several typical examples are diagrammed in FIG 2. The error is emphasized the higher the stack of discs, and the closer the tracks are aligned radially on the disc. The resulting displacement of the recorded data and the head which is trying to read the data represents an error. The maximum error is known as ΔE in the subject figure.

0009Disc drives often use a servo head to measure the error of the type exemplified in FIG 2 and then correcting for it by moving the heads a corresponding amount. In known systems, a single servo head is used located at the mid­dle or at one extreme of the disc stack. This solution works well if the error is uniform from one end to the other of the disc stack as shown in FIG 2A. When a tilting, or non-parallel condition exists, as shown in FIGs 2B and 2C, errors still occur, even after the servo head applies its correction. For example, assume the servo head was at the bottom of the disc stack and the condition shown in FIG 2B occurred. No error would be detected by the servo head and no correction would be applied. However, a significant error would exist at the top of the disc stack, resulting in reading or writing data from a track that differed dramatically from that actually addressed.

0010One objective of this invention is to provide a novel apparatus for positioning a data transducer relative to a rotating data storage medium.

0011Another objective is to provide an improved servo control system for accurately aligning a stack of transducer heads with vertically aligned tracks on a stack of discs.

0012Yet another objective is to provide an accurate servo control system especially useful in aligning a stack of transducer heads with discs having data recorded at high data track densities.

0013Another objective herein is to provide a unique method an apparatus for driving a transducer carriage to rapidly move a stack of transducer heads radially across data tracks during track seeking operations and aligning the stack of heads with the tracks for reading and writing operations.

0014Another objective is to provide a unique servo control system which adjusts for relative tilt between a stack of transducer read/write heads and the vertically aligned tracks with which they are to be aligned for reading and writing operations, so that the error between any single head and the track with which it is to cooperate can be effectively eliminated.

0015Yet another objective of this invention is to adjust the positioning of a stack of read/write heads relative to a vertical set of data tracks so that the error is rela­tively proportioned from the top to the bottom of the stack, whereby the error is at a minimum at the center of the stack, and varies to a maximum at the top and bottom of the stack so that in every case, the error at the extreme ends of the transducer stack is minimized.

0016These and other objectives of this invention are achieved in a system wherein a plurality of discs in a stack are accessed using a vertical head array including two servo heads, one located at each end of the disc stack. In order to eliminate error due to relative tilting of the head stack relative to the discs, two servo heads are provided, one located at each end of the disc stack. In order to exactly center any given head over a desired track location in the disc stack, the error distance of the given head relative to the track can be determined by combining proportions of the two servo head signals from the top and bottom servo heads, their proportions being based on the vertical distance of each of the servo heads from the given head which is to be exactly centered over a track. If the servo distance is represented by S1 and S2, and the vertical distances from the head to the center of the servo heads is represented by Y1, Y2, then the general formula for calculating the error correction for centering the typical data head is E_c=1/Y1+Y2 × (S2Y1+S1Y2).

0017The objectives and advantages of this invention can be best understood by reference to the following detailed description of the drawings wherein <ul id="ul0001" list-style="none"><li>FIG 1 is a representation of the physical elements of the disc drive which the present system is meant to control;</li><li>FIG 2 is a block diagram of a simple form of track seek­ing electronic circuitry;</li><li>FIG 3 is an expanded schematic view of the head-to-data track relationship;</li><li>FIG 4 illustrates several typical head-to-data track error condition situations; and</li><li>FIG 5 illustrates a general case and two specific exam­ples of the determination of data head error correction using two servo head schemes of this invention.</li></ul>

0018FIG 1 is a perspective view schematically illustrating a mechanical portion of the typical magnetic disc storage apparatus accessing multiple discs with a plurality of magnetic heads. In FIG 1, reference numbers 11 indicate a plurality of magnetic discs. In this figure, the top and bottom discs may be servo discs. The stack of discs is fixed on a rotating shaft 12 rotated in the direction of arrow A. The shaft is rotated by an electric motor, not shown, at a constant high speed. The numbers 13 and 13ʹ indicate transducers, the number 13ʹ especially indi­cating a servo transducer which cooperates electromagnet­ically with the servo discs. The transducers 13 and 13ʹ are supported by a carriage 14 by means of corresponding arms 15 and are moved by the carriage 14 forward and backward in the directions of arrows B and Bʹ. According­ly, the transducers 13 and 13ʹ can travel in a radial direction relative to and slightly above or below the corresponding magnetic discs 11 and 11ʹ.

0019On each of the discs 11, a great amount of data stored is in a plurality of circular tracks (not shown) which are arranged concentrically on the disc. When a control unit commands the readout of data from a particular track of a particular disc 11, or the writing of some data onto a particular track of a particular disc 11, the correspond­ing transducer 13 is moved to the desired track. There­after, transducer 13 can read out data from the desired track or write data onto the desired track. The above-­mentioned movement of the transducers is accomplished by a linear motor 16 via the carriage 14 and arms 15.

0020The fine positioning of the corresponding transducer to the desired track on the desired disc is made under con­trol of servo information stored in tracks on the servo discs 11ʹ wherein servo information is read out by the servo transducer 13ʹ. The storage of servo data on sepa­rate discs or on sectors on data discs is well known, and will not be described in detail herein. It is sufficient to note for the purposes of this application wherein it is desired to compensate for potential tilt of a data transducer 11 located intermediate the top and bottom of vertical array of transducers 13, both the top and bottom discs contain servo information, and servo transducers are provided adjacent both these top and bottom surfaces of the disc stack. The servo information thus read out can be supplied to a controlling circuit and the control circuit controls the linear motor 16 so as to locate and hold the corresponding transducer on the desired track. The number 17 indicates a tachometer which detects the moving speed of the transducer, the detected moving speed being supplied to the control circuit to move the trans­ducers with predetermined controlled speed.

0021FIG 2 shows the essential elements of a control circuit that can be used to move a transducer to a desired disc track, in accordance with technology which is also well known in this art. Basically, a desired track address 30 is provided to a new track address register 32. This new track address is compared at comparator 34 with the pre­sent track address as stored in register 36. The present track address register 36 can be continually updated by a position signal 37 which comprises a signal received back from a detector coupled to the actuator motor 16 to pro­vide a track crossing signal via track crossing detector 38. The resulting difference between the present track address from register 36 and the new track address from register 32 provides a signal to difference register 40, which can be converted 41 to a signal 42 to drive the actuator motor.

0022The objective of this invention is to provide fine positioning to any data head in the vertical array of data heads relative to a track on any disc surface. Thus, referring to FIG 3, it can be seen that in the ideal situation the line HB-HT passing through the recording head gap should to inside with a line DB-DT which passes through vertically aligned data tracks on all of the discs.

0023It can be seen from FIG 4 that a number of data error conditions can occur, which would result in differing error distances depending on which head 13 was selected for reading. Thus, if a head 13R near the approximate middle of the stack were selected for reading, and an error condition such a shown in FIG 2A occurred where the error is uniform from the top to the bottom of the disc stack, regardless of the location of the reading head 13R, relative to the servo head 13ʹ, the error would be corrected. However, if the error on head 13R were less than the error detected at the head 13ʹ which might be part way up from the stack or even at the extreme end of the stack, as illustrated in FIG 4B, then either an over correction or no correction at all would occur. In the example of FIG 4C, the head 13R is in fact exactly cor­rectly located. However, the servo head would be located closer to one end or the other of the disc stack, with the result that some error would be perceived to be pre­sent, and a correction movement would occur, misaligning head 13R.

0024In order to eliminate this problem and provide means for making corrections in all cases, a circuit 50 has been added to the position control electronics of FIG 2 to perform the calculations shown in FIG 5. Specifically, the data error for any head can be calculated by provid­ing two servo heads S1 and S2 preferably at the top and bottom of the stack of transducer heads, although for engineering reasons, they might be moved in slightly towards the center of the stack. If the selected data head 13R which is to be centered exactly over the track suffers from an error E_c, this error can be broken down into two components. A first component E1 can be calcu­lated from the bottom servo error S1 according to the formula E1 = S1Y2/(Y1+Y2) where Y1 and Y2 are the verti­cal distances from the selected head 13R to the servo heads S1 and S2. The other component can be calculated from the servo distance error E2 using the formula E2 = S2Y1/Y1+Y2. Therefore, the accumulated error that must be calculated and corrected for is the sum of E1+E2 or S2Y1/(Y1+Y2)+S1Y2/(Y1+Y2).

0025The result of this calculation in exactly locating a head 13R over the desired track without overcorrection as would otherwise occur is shown in examples 1 and 2. For example, if the tip of the head array is such that the center head 13R is in fact centered over the track to be read, but servo errors S1 and S2 are detected, then the accumulated error would be zero and no compensation would be made, although in cases using a single head, compensa­tion would occur. Similarly, where the tilt is of the type shown in Example 2 and previously illustrated in FIG 2B, the correction to be applied to a head 13R is 1/2 of the servo error S2.

0026Thus, the general formula known as Equation 1 in FIG 5A provides for a calculation of the error at a given read­ing heads 13 so that it may be exactly centered over a desired reading track.

0027Alterations and modifications to this invention may occur to a person of skill in the art who studies the claimed invention disclosure. Therefore, the scope of this invention is to be limited only the following claims.