IMPROVED SAMPLED SERVO SEEK AND TRACK FOLLOW DISC DRIVE

Abstract: Method and apparatus for performing seek and track follow operations in a disc drive provides time varying acceleration and position reference signals which have the same time duration and which are developed as a function of seek length. These signal are successively simultaneously sampled at the same points in time. The sampled acceleration and position reference signals from the same points in time are simultaneously applied as a feed forward signal and a position reference signal, respective­ ly, in the disc drive to position a magnetic head at a selected track. During a seek operation, each actual track position signal and subtrack position signal from the trans­ ducer is compared with an existing sampled position reference signal. At the end of the seek movement, track following continues under the control of the subtrack signals.

Description: This invention relates to a sampled servo magnetic head positioning system for disc memory drives. Magnetic disc memory drives comprise a plurality of magnetic memory discs axially spaced on a common shaft and driven at constant rotational speed. In one type of disc memory drive a single surface of one disc is provided with concentric tracks of servo code. The surfaces of the remaining discs have concentric tracks of recorded data. Corresponding tracks on the disc surfaces define track cylinders. An armstack supports magnetic heads in corresponding positions adjacent the surfaces of the discs. The head adjacent the servo code may be called the servo head and the other heads the data heads. A motor driven carriage coupled to the armstacks moves the magnetic heads across the tracks or supports the heads at a selected track cylinder. Control systems for such a disc memory drive have included dedicated servo arrangements for track cylinder seeking and track following using only a dedicated servo head. This assumes that track centering of the dedicated servo head results in track centering of the data heads which is unlikely when the track width is very small. The dedicated servo approach limits track recording density because of mechanical tolerances in track recording, in magnetic head mounting on the armstack and in the changing physical relationship among the differing parts, for example, armstack tilt, or disc axis tilt, or both. In sampled servo disc drives, instead of employing a disc surface and a magnetic head dedicated to servo code, the servo code is recorded on each disc usually in equally angularly spaced sectors in each track, interspersed with the data code. The head on each disc is then used in the control system loop for both track seeking and track following over the adjacent disc surfaces as well as for reading and/or writing. In some of these systems, as referenced in GB-A-20 11 654 using the same drive system for both track accessing, or track seeking, and track following modes, after the addressed track has been reached, switching from the track seeking mode to the track following mode takes place. Switching is undesirable since it frequently results in switching transients or drive force discontinuities producing magnetic head oscillation, interfering with the speed and accuracy of positioning of the magnetic head. Improvement was achieved, according to the teachings of GB-A-20 11 654 reflected in the prior art portion of claim 1, in the provision of a servo system wich couples position error signals and position signals to a summing junction at the input to a compensator circuit in the control loop of the servo. With this arrangement there is no switching between track seeking and track following modes of operation which eliminates the switching transients causing the detrimental oscillations of the magnetic heads but control loop saturation is a problem. Systems of the type referred to above and others, as described in US-A-3 936 876, entitled "Rotable Data Storage Apparatus with Track Selection Actuator Having Multiple Velocities", filed January 21, 1974, US-A-4 439 800, entitled "Servo Control Seek Operations in Magnetic Disc Drive", filed April 24, 1981, and US-A-4 134 053, entitled "Method and Means for Capturing Magnetic Tracks", filed October 26, 1976, during track accessing or track seeking, sometimes provide signals to the carriage motor or actuator control system, defining a velocity profile, to cause the magnetic head to be accelerated over a first predetermined distance to a selected velocity, to be moved at the selected velocity over a second predetermined distance, and to be retarded over the remaining distance so that the velocity of the head as it approaches the addressed or selected track is reduced to the value called the capture velocity that it may be captured and controlled by a track follower control for track centering. The track follower remains connected throughout the seek operation, however, it is not functional since its effect in the seek operation is swamped or overpowered by the seek power until the head velocity is reduced to track follower capture velocity. Microprocessors or computers are used to provide velocity profiles for the control systems. Speed regulation in some has been provided using tachometer feed back in a systems closed control loop. In other systems as described in US-A-4 333 117, entitled "Disc Head Arm position Controller Using Digital Velocity Sensing", filed June 30, 1980, head velocity is measured using the transit time between adjacent tracks of the servo code. In still other systems such as described in US-A-3 936 876, referred to below, the integral of the coil current and the derivative of the position error signal are used to provide a measure of head velocity. Such loop stabilizing feedback techniques add to system complexity and cost. While such prior art disc drives have individually provided improvements in performance, it is the object of the invention to achieve further improvements in performance while simplifying the disc memory drive. The above object is solved by the features characterised in claim 1. The dependent claims 2 to 14 characterise advantageous developments thereof. Thereby, the acceleration drive, the track position drive and a track following control function simultaneously during the seek mode of operation to minimize tracking errors. At the end of a seek movement the track following control function continues. Adaptive gain adjustment of the acceleration profile signal at the end of a seek movement is achieved by an off-track detector, the output of which adjusts the acceleration profile signal to compensate for overshoot or undershoot of a selected track in a succeeding seek move. By this expedient overshoot or undershoot of the target track is virtually eliminated. This seek adaption process corrects for manufacturing variations that exist in the actuator motor. Consequently all drives will tend to have the same seek characteristics. Means are provided for tailoring the time rate of change of the acceleration profile to the time constants of the actuator to minimize magnetic head overshoot at the selected or target track. According to a further aspect, a track follower loop is provided which is always closed. A time varying track position trajectory signal is provided during a seek operation. The terminal end of the trajectory represents a desired track and subtrack position. The actual track position of the magnetic head as the seek movement progresses is subtracted from the sample point track position values along the trajectory up to and including the desired track at' the terminal end of the trajectory to produce a partial position error. The desired track position may define a desired track and a subtrack. The partial position error is then the difference between the actual track and the desired track p!us the desired subtrack position. The actual subtrack position is then combined with the partial position error to obtain a final or net position error signal which represents the position error in tracks with subtrack resolution. By leaving the track follower loop closed at all times, the loop functions to follow the trajectory of the track position trajectory reference. By applying the acceleration drive as a feed forward input to the track follow loop, ie., to the output section of the loop, the loop need only correct for errors between actual position and desired position. Loop saturation problems are also avoided. A track following compensator is used in the input section of the track follow loop to stabilize the loop. By this expedient, the loop velocity state does not have to be measured or generated. According to still another aspect odd/even track signals are combined with a servo code signal to produce a single error signal having a value that is valid anywhere over the memory disc surface. The character of this error signal is such that the track follower control loop always has negative feedback as distinguished from prior art drives, for example, of the type of the above mentioned US-A-3 936 876, which alternate negative and positive feedback (or the reverse) on odd and even tracks. Further, head to head misalignment when switching from one selected head to another can now be tolerated. The invention will be better understood by reference to the following specification when considered in conjunction with the accompanying drawings, in which: Figure 1 is a plan view of a sector of a memory disc fragmentarily illustrating magnetically recorded tracks. Figure 2 is an enlarged plan view of a fragment of the magnetic zones or transitions of the sampled servo code defining a track and typical track centered track following signals which are shown therebelow; Figure 3 illustrates a typical servo code derived track crossing signal; Figure 4 illustrates the corrected track crossing signal of this invention; Figure 5 is a plot of a track position error signal based upon the corrected track crossing signal of Figure 4; Figure 6 plots the timing relationship of an acceleration profile signal and a reference or desired position trajectory signal as employed in this invention; Figure 7 is a block diagram illustrating the improved sampled servo seek and track follow control loop of this invention; Figure 8 compares rectangular and trapezoidal wave forms for applied acceleration (current); Figure 9 depicts the function of the system in which the seek closely approximates the profile of the acceleration current applied as a feed forward signal in the control loop; Figure 10 depicts the function of the system in causing the actual position trajectory signal to follow the reference or the desired position trajectory signal. Figure 11 is a plot of the table of position values for full power position trajectory; and Figure 12 plots the timing relationship of an acceleration profile signal and a desired position trajectory signal in which a constant velocity control interval is provided. Best Mode for Carrying Out the Invention A disc drive comprising a sampled servo seek and track follower control loop for a high performance disc memory drive is disclosed herein. This control loop is a single servo loop that is used in an identical manner for both track following and track seeking modes of operation. The character or format of track recordings employed in this disc drive is depicted in Figure 1 on a sector of a magnetic recording disc 1. Only a part of two data tracks 3 and 5 are shown to demonstrate the general pattern of the magnetic recordings. Each track of the memory disc comprises a plurality of equally angularly spaced sectors 7 interspersed with recorded data. Each sector comprises a section of servo code recordings 9 and a section of track number recordings 11. The servo code comprises magnetic zones or transitions A and B in circumferentially spaced positions on opposite sides of the track centerline. The magnetic zones B are in circumferential positions intermediate the magnetic zones A. The track numbers 11 are recorded in Gray code. The system requires knowledge of whether a track is even or odd. The track number in Gray code provides this informa-. tion. A single bit also may be recorded with the track number code to indicate whether the track is even or odd if desired. The remainder of each track between the sectors contains recordings of data 13. Track following is achieved as a magnetic head traverses the servo code in each sector, by determining the radial position of the magnetic head in relation to the magnetic zones A and B. Figure 2 is an enlarged view of a fragment of the servo code 9 of one of the sectors 7 of Figure 1. A magnetic head 15 is shown on the left in a track centered position. The surface of the disc, represented in the plane of the drawing, is of one magnetic polarity. The magnetic zones are of the opposite magnetic polarity. Disc motion from right-to-left, as viewed, beneath the magnetic head 15 generates servo code voltages in the magnetic head which, after amplification and processing, are characterized by the time varying servo code wave forms A and B shown below and aligned with the magnetic zones 9. The difference between the A and B voltages (A-B) indicates the radial position of the magnetic head with respect to track center and is used in the track following mode of operation for keeping the magnetic head 15 track centered. During the seek mode of operation the magnetic head 15 is moved across the tracks from a present track position to a selected or target track. The servo code signal relationship (A-B) is now processed to produce a track crossing signal 17 of the type of Figure 3, which arbitrarily spans the centers of tracks 98 to 102. At track center the servo code signal (A-B) is zero, becoming maximum (positive or negative) at half track positions when the magnetic head is aligned, respectively, with either an A or a B magnetic zone. When this track crossing signal is used as feed back of magnetic head position in relation to a track center, the feed back voltage is alternately positive and negative with even and odd track center crossings. By combining the track address (odd/even) or the odd/even bit recorded in the track addresses, with the track crossing signal (Figure 3) (known also as the fine servo position signal) a track crossing signal 19 is produced, as illustrated in Figure 4, in which the negative going voltage excursions are inverted. The resulting demodulated position error voltage P6 is shown in Figure 5. This position error signal is valid over all of the tracks on the disc. As a result, the control loop always has negative feedback. Seek movement is initiated and performed by coupling acceleration and desired position signals to the track following loop. These are shown in the upper and lower portions, respectively, of Figure 6. The acceleration signal is defined as an acceleration current having a profile, the time rate of change of current or acceleration, for moving the actuator to position a selected or target head at a designated track without overshoot or undershoot. The desired position signal is defined as a position trajectory. signal, which is the time rate of change of track position. These signals begin and terminate simultaneously. They are based upon known characteristics of the system. The acceleration values for different length seeks are tabulated and stored and are accessed as a function of seek length. There is a single position displacement table that corresponds to full power acceleration. The acceleration signal profile and the position trajectory signal have designated sample points which are equal time divisions on the common time axis of the signal plots. In practice, signal values on each signal are sampled in corresponding sequential time intervals. Their midway and terminal sample points represent the midpoints and terminal points, respectively, of a seek movement. The acceleration signal is zero at the midpoint of the seek movement at which point it reverses to initiate actuator retardation, and, ideally, is zero again at the termination of the seek movement. The position trajectory signal is a parabolic function (voltage/current) plotted, in Figure 6, as track position per unit or interval of time. The acceleration profile is trapezoidal to match the slew rate capability of the actuator. A system which implements the principles described above is illustrated in Figure 7 which comprises a track follower loop 21 having an input section 23 and an output section 25. The input section conventionally includes a compensator circuit 27 and a notch filter circuit 29 coupled to the input of a power amplifier 31 in the output section 25 by means of a summing junction 32. The power amplifier 31 drives an acutator 33 which carries and moves an armstack 35 on which the magnetic heads H1, H2 and H3 are conventionally flexibly mounted. Either a linear or rotary actuator for armstack 35 is contemplated for the purpose of moving the magnetic heads as a unit across the associated discs. This is a sampled servo system and the magnetic heads are individually employed for both track seeking movement and track following operation. To this end a host computer 37 or other central control unit makes requests which include selecting an individual magnetic head and a track at which that head is to be positioned for reading or writing purposes. These requests are transmitted to a signal processor 39, forming part of a microcomputer or microprocessor 41, which addresses a target head select circuit 43 to couple the selected one of magnetic heads H1, H2 or H3 to a position demodulator 45 in the feed back path 47 of the track follower loop and to a Gray code detector circuit 49 in a track position feed back path 51 wrapped around the track follower feed back path 47. The input signals to the position signal demodulator 45 are the servo code signals from the selected magnetic head resulting from transducing the servo code 9 (Figure 2). These servo code signals are amplified, filtered and subtracted by the position signal demodulator to produce the track crossing and subtrack or off-track servo code signal (A-B) used for track seeking and track following purposes. These servo code signals (A-B) are coupled to the input section 23, at the compensator circuit 27, of the track follower loop via an off-track or subtrack summing circuit 53. The input signals to the Gray code detector 49 from the selected magnetic head are transduced from the Gray code recordings of the track number in the selected magnetic track. These are converted to binary code in a Gray code to binary converter 55. The actual track number indicates whether the track is odd or even; however, as discussed in connection with Figure 1, an odd/even bit may be recorded with the Gray code as an additional indication of whether a track is odd or even. The output of the Gray code to binary converter 55 is the actual track position, digitally expressed, of the selected magnetic head, either in the seek mode or the track following mode of operation. This digital actual track position signal is coupled as a feed back signal to one input of a desired position summing circuit 57. The output of the position summing circuit is coupled as input to a position digital-to-analog converter 59 the output of which is coupled to the second input of the off-track summing circuit 53. The digital desired track position signal, developed as a time varying position trajectory signal P1 (Figure 6) in the micro-computer 41, is coupled to the second input of the position summing circuit 57. This signal which is a position reference signal, includes a desired track number and a subtrack position at which it is desired to position the selected magnetic head. Thus the reference position at which it is desired to position the selected head is defined, and both components of the reference position are represented in the actual track position and servo code feed back signals, respectively, in the actual track position and subtrack or off-track position feedback paths 51 and 47, to achieve precise positioning. An acceleration signal developed in the microcomputer 41 is coupled via an acceleration digital-to-analog converter 61 and a slew rate circuit 63 to a second input of the summing circuit or junction 32. This is a feed forward signal which is the primary power to the actuator during a seek move. The closed track follower loop, now responding to the position reference signal, provides the small correction signals necessary to maintain alignment with the trajectory of the reference signal. As noted, requests from the host computer 37 are coupled to the signal processor 39 which is also coupled to the feedback path 51 and receives the actual track number signal at which the selected magnetic head is presently positioned. These binary signals are subtracted. The difference, signal L, represents the direction of seek movement and the seek length, the latter in terms of the number of tracks between the present track and the selected or desired track. This number, or other number developed therefrom, is used as an address for accessing memories for storing seek reference signals. The micro-computer 41, of which the signal processor is a part, contains the memories for storing the seek reference signals which include the acceleration and the position reference signals. Since the actuator characteristics are well known it is possible to develop a table of feed forward acceleration commands, one acceleration value for each seek length and a single table of full power position commands. The acceleration values, which are individually addressed as a function of seek length are used to scale the full power position values to generate position values for an individual seek length. The selected acceleration value and the scaled full power position values are then used to simultaneously produce an acceleration profile signal and a position trajectory signal of the same time duration which are applied simultaneously in the single control loop for precisely performing a seek movement. To this end the micro-computer 41 comprises an acceleration signal memory 67 and a position signal memory 65. The position signal memory 65 comprises a table of digital track position values that would result from full power acceleration. Digital track position trajectory values under the control of clock pulses C and requests R are coupled via a multiplying operation 66 to a position trajectory generator 68, controlled by clock pulses C and requests R, which couples the digitally expressed track position values to the summing junction 57, one at a time, in sequence, in said equal sample time intervals determined by clock pulses C, beginning with the present track and continuing to the target track of the seek movement, for either direction of seek movement. The digital track position values for each sample interval lie along, and, in fact define, the track position trajectory and correspond to a track position along the Y axis of the trajectory plot of Figure 6. The trajectory, which is parabolic, represents the time rate of change of position of a magnetic head and defines the actuator response to the acceleration profile signal for that seek length. The specific shape of the trajectory is a function of the seek length L. Since the sample time intervals are the same for all seek lengths, the number of sample time intervals in a seek movement is therefore a function of seek length. The acceleration memory 67 contains a table of acceleration values, discussed at a later point, the individual magnitudes of which are related to seek length. An individual acceleration value is accessed by each output address signal L of the signal processor 39 depending upon the seek length. The acceleration value is coupled via a multiplying operation 69 as an input to an acceleration profile generator 71, controlled by clock pulses C and requests R, which digitally defines the acceleration profile. The analog equivalent of the digital acceleration profile from the digital-to-analog converter 61 is coupled to the slew rate circuit 63 which establishes the slope of the acceleration current transitions, as seen in Figure 6, to match the dynamic response characteristics of the actuator. The advantages of the slew rate correction are evident from Figure 8, which compares a rectangular acceleration signal profile with the preferred trapezoidal acceleration signal profile. The dotted lines defining the boundaries of the shaded areas in the rectangular profile approximate the actuator response and depict lost acceleration as the accelerating current is applied, lost deceleration at the midpoint current swing from positive to negative and lost deceleration at the end of the profile cycle as the negative current goes to zero. The area within the acceleration current profile defined between the dotted lines and the internal boundaries of the profile are unequal. Since the positive and negative magnitudes of acceleration current are the same, and since the accelerating and decelerating characteristics of the actuator are the same, it is evident that this lack of symmetry in the application of energy to the actuator during acceleration and deceleration will result in a net magnetic head velocity at the end of the seek movement, substantially as shown. The feed forward acceleration current is applied so that the track follower control loop, which is always closed, corrects only for errors between actual position and desired position. That is, it follows the reference position trajectory (Figure 6) during seek movement. Since the compensator 27 stabilizes the control loop there is no need for introducing velocity feedback. Acceleration signal values at individual sample points in time on the acceleration profile signal CC2 translate into magnetic head positions with respect to individual tracks on the disc surface and each corresponds closely to a track position signal value on the track position trajectory P1 at the same point in time. Ideally the actual track position of the magnetic head, represented by the value of the signal P2, is equal to the value of the track position trajectory signal at each corresponding point in time, neglecting the subtrack position, if any. Differences between the track position trajectory signal and the actual track position signal result in a digital desired position error signal P3, which is actually a partial position error signal, indicative of the error. The polarity of this error signal indicates whether overshoot or undershoot exists. The analog equivalent P4 of this error signal P3 at the output of the position digital-to-analog converter 59, is coupled to the summing junction or circuit 53 where it is algebraically summed with the analog actual subtrack position signal P5. The result is an analog final or net position error signal P6 coupled at the input section 23 of the track follower 21 to the compensator circuit 27. This error signal produces a current command signal CC1 at the output of the compensator circuit to correct for the position error P6 and which is algebraically summed at the summing circuit 32 with the acceleration profile current command CC2 to drive the actuator. The sample intervals of the acceleration profile signal CC2 and the desired track position trajectory signal P1 controlled by clock pulses C are shown in Figure 6. The time intervals are equal, are equal in number and are coincidental in time. The sample points are numbered and corresponding sample points are sampled at the same time in generating the respective signals. Two tables are all that are needed to define the reference signals. One is the table of values for the full power position trajectory, and the other is a table of acceleration values A, one for each seek length, or for individual groups of seek lengths, so that the midpoint and the target track end of the seek movement m and 2m, respectively, are reached on exact sample point boundaries of the acceleration profile CC2 of Figure 6. In one embodiment of this invention the sample time intervals are about 280 micro sec. long. Calculation of a track position at each sample point along the track position trajectory usually results in a decimal number, rather than a whole number, for example, track and subtrack position number 492.2, in which the whole number 492 denotes the desired track and .2 denotes the subtrack position, that is, the fractional track position off track center or beyond the center of track number 492. Referring to Figure 7, the track reference number 492, which, as seen in Figure 6, may be any sample point, 1, 2, m-1, etc., but not including sample point 2m (a whole number), is the present value of the track position reference signal sample of signal P1 coupled to the summing junction 57. At this instant, assume the selected magnetic head is at an actual track and subtrack position 491.4. The actual track number feed back signal, P2, in the feed back path 51, is 491 and the subtrack position, P5, in the feed back path 47, is 0.4. The digital signal P3, subtracting 492.4 - 491 = 1.2, which becomes the analog signal P4 at the summing junction. 53. The net position error signal P6 = 1.2 -.4 = 0.8 at the output of the summing circuit 53. The use of a single control loop 21 for both track following and track seeking reduces the hardware parts count. It also avoids the problem of switching from a seek control system to a separate track follower which requires settling out the residual seek velocity and the seek position error at the end of a seek movement. More importantly signal tracking errors and seek time are reduced. The seek adaption process for correcting the acceleration profile as a function of off-track position (overshoot or undershoot) does not interfere with normal drive operation and causes succeeding seek movement to more exactly follow the desired references, acceleration and position. This adaptive process corrects for manufacturing variations that exist in the actuator motor. Consequently, by the seek adaption process, all drives have the same seek movement characteristics and the same seek times. The seek adaptation process is implemented by developing an adaptive gain adjustment signal as a function of an off-track position (track overshoot or undershoot), evidenced by the servo code signals, which may exist at the end of a seek movement. The adaptive gain adjustment signal is used to correct acceleration signal values selected from the acceleration memory 67. The adaptive gain adjustment signal may be either a fixed increment used on successive overshoots or undershoots to correct the acceleration signal, or may be proportional to the overshoot or undershoot to make the entire correction at once. Off track detection is provided by an off-track detector circuit 73 in the feedback path 47 and which responds to the servo code signal P5 to produce output signals at circuits 75 and 77 indicative, respectively, of track overshoot or undershoot. POT is Positive Off Track and indicates the head is displaced in the positive direction from track center. NOT is Negative Off Track and indicates negative direction displacement. The circuits 75 and 77 are connected to an adaptive gain adjustment value circuit 79 which is coupled by a line 81 to a multiplier 69. The adaptive gain adjustment value which is less than 1 for overshoot and greater than 1 for undershoot, corrects the acceleration values selected from the acceleration memory 65. These corrected acceleration values are coupled to the digital acceleration profile generator 71. This circuit now produces a digital representation of the corrected acceleration profile. A plot of track position values Xn against sample number n, for equal intervals of time, at full power, constant acceleration, is seen in Figure 11. The curve is nonlinear, the discrete position points defining a parabola. The values Xn for each sample number are quantized and stored as a table for a full power position trajectory stored in memory 65. From these values in sample intervals T a position trajectory P1, such as characterized in Figure 6, is developed. If it is desired in long seeks to limit head velocity to some value below that resulting from full power acceleration to the seek midpoint, the acceleration current may be reduced to zero in advance of the seek midpoint and held at zero to a corresponding point beyond midpoint, at which point the acceleration current is reversed to define a reversed acceleration current profile for deceleration of the head. The area under the reversed acceleration current profile is equal to that under the head acceleration current profile for the head acceleration phase. The head moves with constant velocity when the acceleration current is zero. The time rate of change of position is linear resulting in a straight line plot of position change on the position trajectory of Figure 12. The position values for constant velocity are also plotted in Figure 11 over a linear section of that curve. Scaling of position values for other than full acceleration power is achieved by multiplying the output of the position trajectory memory 65 by the value A/A FULLPOWER, as noted above, at the multiplying junction 66. The manner in which the incremental values of the table for the full power position trajectory of Figure 11 and acceleration commands are determined is explained below. In the equations which follow, the position values are determined: Acceleration level = A c (Tracks/Sec.²) Midpoint = sample #m; m = [samples]; and T = sample interval = [seconds] At any sample point n, the track position is given by Xn. The equations above show that each point for a position Xn during a seek is proportional to A/2. Therefore a look-up table of track position values based on a standard position trajectory or profile at, say, full power acceleration can be used to get all other position profiles by scaling table values by A/A FULLPOWER. Also only the acceleration phase of the position values needs to be in the table since deceleration values are obtained by indexing backwards through the table. Two tables are needed. 1) A table of values for track position Xn given a full power seek (Figure 11). 2) A table of values of acceleration "A" to use for each seek length so that the target track and seek midpoint are reached on exact sample boundaries. In the equations below, acceleration values are determined: From the equation for n = m; Let L = Total Seek Length; Xm = L / 2 i.e., midpoint This gives acceleration value A necessary to do a seek movement of length L in exactly 2m sample times using a profile of the type of Figure 6. An example of a calculation of the number of samples in a given seek length at maximum acceleration A is given below. Example: L = 1600 Tracks; T = 280 µs A - 6G's = 3.7 × 10⁶ Tracks,/Sec² @ 1600 Tracks/Inch M = 74.1; This seek length would use m=75 and A is slightly less than full power. The position trajectory table (see Figure 11) which can be read backwards, for deceleration, only needs to be 75 entries long for 1600 tracks. For deceleration the negative value of a selected acceleration value is used. High performance seek movement is achieved whether few or many tracks are crossed. The micro-computer which generates the acceleration profile signal CC2 and the track position trajectory signal P1 is a precision reference signal generator. These signals, as employed in the improved track follower loop, move a magnetic head to a target track in the shortest time possible. In operation: a) The seek movement is synchronized with the position samples 1, 2, m-1, m, etc. b) The exact midpoint is on a sample point, m. This guarantees that the turn around from acceleration to deceleration is made at the proper instant and at the midpoint of the seek movement. c) The endpoint or target track point of a seek movement is reached coincidentally with the end point 2m of the acceleration profile signal CC2. d) The control of the time rate of change of the acceleration current (slew rate) removes dependence on the rise time of the coil current. e) The reference generator is an inexpensive, yet precise, 8 bit microcomputer. An example being an 8052 microcomputer or processor made by the Intel Corporation of California. The final or net position error signal P6 is simply formed from the odd/even track addresses and the corrected track crossing signal 19, and provides negative feedback to the track follower. This implies that any amount of head missalignment can be tolerated when switching from one head to another in the head selection process.

Claims: A sampled servo magnetic head positioning system for a magnetic disc memory drive having magnetic memory discs, means for rotating said discs, sampled servo code magnetically record d in each track on said discs, a magnetic head (H1 to H3) for each disc, means (43, 45, 49, 55) including a movable member (35) supporting each magnetic head in a position to scan an adjacent disc surface and to select a magnetic head to sense said servo code for producing servo code signals and for producing present track position signals (P2) for the selected magnetic head, and a control loop (21) having an output section (25) and an input section (23), said output section including an actuator (33) which is coupled to said movable member (35) to move said member and thereby move said magnetic heads over the respective adjacent surfaces of said discs, characterized by comprising: means (65, 66, 67, 68) for producing a time varying track position trajectory signal (P1) of given time duration; means (67, 69, 71) for producing a time varying acceleration signal (CC2) having the same time duration as said time varying track position trajectory signal (P1); and means (57) for simultaneously coupling said time varying track position trajectory signal (P1) and said present track position signals (P2) to said input section (23) of said control loop (21) for driving said actuator and coupling said acceleration signal to said output section (25) of said control loop (21) to provide the primary power for driving said actuator (33). The system according to claim 1, wherein said acceleration signal producing means comprises: means (63, 71) responsive to said acceleration signal for controlling the time rate of change of said acceleration signal. The system according to claim 1, further comprising means (49) responsive to said present track position signals for producing signals indicative of odd and even numbered tracks for modifying said servo code signal resulting from track crossings during a seek movement so that the servo code signal has similar characteristics for both odd and even track crossings. The system according to claim 1, wherein said track position signals comprise odd and even track indication signals and said odd and even track indication signals and said servo code signal are combined into a single position error signal (P3) so that the control loop has always negative feedback whether a magnetic head is crossing an odd or an even track. The system according to claim 1, in which said means for producing said time varying position trajectory signal (P1), comprises: means (37) for producing a selected magnetic head signal identifying a selected magnetic head, and a requested track position signal identifying a track at which said selected magnetic head is to be positioned; means (39) responsive to said requested track position signal and said present track position signal for producing a seek signal (L) indicative of the seek length of a seek movement from said present track to said desired track; wherein said means for producing said time varying track position trajectory signal are responsive to said seek signal (L). The system according to claim 5, in which said last named means comprises: storage means (65) for storing a plurality of different desired track position values. The system according to claim 1, comprising: means (63) responsive to the servo code signals of said selected one of said magnetic heads at the end of a seek movement for adjusting the gain of said acceleration signal in a sense to reduce the overshoot or undershoot of a selected magnetic head in a subsequent seek movement. The system, according to claim 7, wherein said means responsive to the servo code signals of said selected one of said magnetic heads, comprises: an off-track detector (73) responsive to said servo code signals (P5) for producing correction signals (POT, NOT) indicative of the magnitude of track center overshoot or undershoot; and an adaptive gain adjustment operation responsive to said correction signals for adjusting the gain of said acceleration signal. The system according to claim 6, comprising: means responsive to said acceleration signal for modifying signals from said storage means. The system according to claim 1, in which said means for producing a time varying track position trajectory signal comprises: means (65) for storing a single full power position trajectory table; and means (66, 68) responsive to an addressed full power position trajectory signal for producing said time varying position trajectory signal. The system according to claim 10, in which said last named means comprises: a position trajectory generator. The system according to claim 10, comprising: means (67) for storing different individual acceleration values, differing seek signals (L) addressing corresponding individual acceleration value; and means (32, 61, 63, 69, 71) for coupling an addressed acceleration value to said output section (25) of said track following control loop (21) to provide the primary power for driving said actuator. The system according to claim 12, in which said last named means comprises: an acceleration profile generator (71) responsive to said addressed acceleration value for producing an acceleration command signal having a predetermined profile and having a time duration corresponding to that of said time varying track position trajectory signal. The system according to claim 13, comprising: means providing an equal number of equal sampling intervals (C) for said addressed desired track position signal and said addressed acceleration signal; and means for sampling each addressed signal in the same interval of time.

Inventors: KNOWLES, VERNON L.
Applicants: HEWLETT-PACKARD COMPANY
Application year: 1987
Main classification: G11B5
IPC codes: G11B5, G11B21