Reliable track crossing detection in data storage apparatus

Abstract: Data Storage apparatus is provided which employs a track-crossing sensor to produce track-crossing signals during track seek. An oscillator is slaved to the sensor for supplying substitute track-crossing pulses in the absence of the sensor providing such pulses or when the radial velocity exceeds a threshold velocity. A velocity profile means alters the oscillator frequency so that the oscillator produces track-crossing pulses in accordance with the profile.

Description: Field of the Invention The present invention relates to data storage apparatus having reliable track counting apparatus. Background of the Invention Disk recorders of either the magnetic or optical type employ transducer positioning systems. Such positioning systems often use a so-called velocity servo loop for long transducer motions, termed seeks. Such long transducer motions cause a sensing transducer to traverse a large number of concentric circular record tracks on the disk record member. The velocity positioning servo loop is optimally switched to a track-following position servo mode at a one-quarter track pitch distance from a target track. Such a track following control may be favorably compared to "Stop-Lock" positioning control in other servo positioning applications. Upon reaching the target track, the track-following position servo positions the transducer to faithfully scan or follow the target track. In a subsequent seek operation, the track following position loop is interrupted to return to either a velocity loop, a second positioning loop or other form of seek positioning servo control. It has been observed that when inter-track spacing is reduced for obtaining higher track densities, then reliable track counting during a velocity seek becomes more acute. In particular, the so- called runout or eccentricity of rotation of the optical disk can cause false direction indications and false counting of tracks such that a target track is not faithfully reached. Accordingly, it is desirable to provide for more reliable counting and control of the velocity servo during a seek operation in which a transducer transverses a plurality of tracks. In a seek operation, because of the high track densities, any asperity in the record medium surface can cause a track not to be counted. Accordingly, it is desirable to provide a system which obviates medium surface asperities from detracting from a successful track-seeking operation. Many optical recorders have the goal of high performance at low cost. In some of these high performance optical recorders, a so-called fine servo or fine actuator (also termed a secondary head-arm) is carried on a primary transducer carrying-arm which is radially moved by a so-called coarse actuator. Typically, the fine actuator has high-frequency response characteristics and provides for rapid and short distance positioning of the transducer with respect to a track being followed or for moving from one track to a second or target track. The coarse servo, which positions the relatively large mass primary or head-carrying head-arm, typically has low-frequency characteristics for handling the longer moves. For optimizing the relationship for top performance between the fine and coarse actuators, servo systems also provide for relative positioning of the fine actuator with respect to the coarse actuator to a central or reference position. Such arrangements have been colloquially called "piggy-back" carriage servo systems. The application of such a piggy-back carriage system is not limited to disk recorders. Actually, the concept was established many years ago (see for example Gardiner USP 2,717, 979) for a pattern-following or template- controlled, coarse-fine positioning servo mechanism. Such an arrangement enabled higher production rates of a pattern-controlled machine, such as a welding or cutting machine. The carried fine or secondary actuator rapidly responds to sharp changes in the pattern, such that the welding or cutting operation faithfully follows the guiding pattern template while overcoming only minimal inertia of the pattern-control machine mechanisms. Gardiner teaches that the fine actuator, which Gardiner terms a topping servo, is to be controlled by the absolute positioning of the pattern template, while the coarse servo (Gardiner's main servo) is slaved or always follows positioning motions of the topping servo. Such an arrangement means that the rapidly responding topping servo controls the pattern-control machine, while the main servo follows the motions of the topping servo for maintaining the topping servo in an optimal position with respect to the main servo-controlled carriage; this arrangement can maximise the range of operation of the topping servo. This type of servo arrangement is also shown in Meyer USP 4,627,029. McIntosh et al., USP 3,924,268 and Merrit et al. USP 4,513,332 show magnetic disk recorders having piggy-back, head-positioning arrangements which are servo-position controlled for optimising the relative position of the fine actuator with respect to a coarse actuator. Simmons USP 3,924,063 shows yet another coarse-fine control, wherein the fine actuator is permitted to move over a predetermined minimal distance before a coarse actuator operation is invoked. Van Winkle USP 4,191,981 shows fast and slow servo-positioning mechanism in a multiple magnetic disk recorder in which the slow servo mechanism is slaved to a fast servo mechanism. The latter arrangement is not a piggy-back arrangement in a true sense. EP 103493 against which claim 1 is delimited discloses a disk information storage including a data storage disk, a transducing head for sensing track indicating information on the surface of the disk and a positioning system including motive means for moving the transducer over multiple tracks on the disk surface and tracking circuit means responsive to the transducer produced information signals to generate sensed track crossing signals indicative of the transducer position for application to the motive means. Another problem in track seeking is the reliable track counting during the seek to ensure access to a target track upon the first try. Defect accommodation or avoidance has been practised in the magnetic disk recorders for a long time. For example Shoji et al. in USPs 4,406,000 and 4,414,655 show defect accommodation during a track following operation. Both of these references show employing the current value of the track-following control signal for maintaining the track following relationship while traversing a defect in the track. In fact the time out timer is used to determine the maximal length of time corresponding to the size of the defect. This accommodation does not suggest how to accommodate loss of track counts during a high-speed, track-seek operation. Fujiie in USP 4,587,644 shows reducing track-following servo loop gain during occurrence of defects of the track being scanned. Another problem in track seeking is the eccentricity or run out problem. When the track-seek operation is relatively slow with respect to the rotational or lineal speed of the track being crossed, then the runout may occur while the radial motion is less than one track width. To assist in accommodating such runout, the eccentricity is measured and stored in a table, then the stored values are fed into the positioning servo such that the positioning servo tends to follow the eccentricity. Such a runout compensator is shown by Jacques et al., in USP 4,135,217. Another technique for accommodating the runout condition is shown by Sordello in USP 3,458,785. Sordello teaches that generating a quadrature signal within a pattern recorded on the disk provides direction-indicating information. For example, a first signal is generated which is alternating in accordance with the rapidity of the seek. A second signal generated from a second transducer scanning a second signal portion on the disk surface provides a second alternating signal with 90 degrees phase change or quadrature, which unambiguously provides for direction indication. While the Sordello technique provides a satisfactory system, it is expensive to implement; in fact, the Sordello system is implemented in those disk recorders having a dedicated servo surface. Having the quadrature signal recorded thereon is used to guide a so-called comb-head supporting transducer for data recording and reading on a plurality of co-rotating disks. It is desirable to avoid separate quadrature patterns on a record surface. Disclosure of the Invention Thus the prior art does not show an efficient means of accommodating for track defects during track seeking operations. Accordingly the invention provides data storage apparatus comprising at least one data storage disk, a transducer for sensing track indicating information on the surface of the disk and a positioning system including motive means for moving the transducer across multiple tracks on the disk surface and tracking circuit means responsive to the transducer produced information signals to generate sensed track crossing signals indicative of the transducer position for application to the motive means, characterised in that the tracking circuit means includes means for producing substitute track crossing signals, the tracking circuit means further including switch means for selectively applying either the sensed or substitute signals to the motive means during motion of the transducer in accordance with predetermined conditions. It is preferred that the tracking circuit means further includes velocity circuit means which generates a signal indicating the transducer velocity during track to track motion, the substitute track crossing signal means being responsive to this velocity signal to generate the substitute track crossing signals. It is further preferred that the velocity circuit means further includes comparison means for comparing the transducer velocity with a reference velocity and supplying a signal to the switch means to select the substitute track crossing signals when the transducer velocity exceeds the reference velocity. Thus when the velocity of the transducer exceeds a predetermined reference value, the switch means is actuated to select the substitute track crossing signals. The substitute signals are then used until the transducer velocity drops below the reference velocity at which point the sensed track crossing signals are used during the remainder of the seek. The invention will now be described by way of example only with reference to the accompanying drawings. Description of the Drawings Fig. 1 shows simplified and abbreviated position and velocity detection circuits as employed in data storage apparatus according to the present invention; Fig. 2 is a simplified block diagram of an optical disk recorder/player according to the present invention; Fig. 3 is a diagram showing the track-positioning portion of the tracking and focusing circuits of Fig. 2; Fig. 4 shows a portion of the velocity circuits shown in Fig. 3; Fig. 5 shows a decode circuit used in the generation of tracking error signals; Fig. 6 shows a portion of the disk surface with associated tracking error signals and error signals as generated by the circuit of Fig 5; Fig. 7 is a detection window generation circuit usable in the circuits of Fig. 4 as a noise rejection circuit; Fig. 8 is a simplified and idealized set of signal waveforms used for describing the operation of the circuit of Fig. 7; Fig. 9 shows a forward-reverse counting control for counting tracks in the system shown in Fig. 1; Fig. 10 shows a set of idealized signal waveforms used in connection with the description of the operation of Fig. 9; Fig. 11 shows a velocity signal smoothing circuit which enhances operation of the illustrated system. Detailed Description of the Invention A preferred embodiment of the invention in the form of an optical recorder is shown in Fig. 2. A magnetooptic record disk 30 is mounted on spindle 31 for rotation by motor 32. Optical head-carrying arm 33 on head-arm carriage generally denoted by numeral 34, moves radially of disk 30. The reciprocal radial motions of carriage 34 enable access to any one of a plurality of concentric tracks or circumvolutions of a spiral track for recording and recovering data on and from the disk. Linear actuator 36 which is mounted on frame 35, radially moves carriage 34 for enabling track accessing. The recorder is suitably attached to one or more host processors 37, such as control units, personal computers, large system computers, communication systems, image processors, and the like. Attaching circuits 38 provide the logical and electrical connections between the optical recorder and host processors 37. Microprocessor 40 controls the recorder including the attachment to the host processor 37. Control data, status data, commands and the like are exchanged between attaching circuits 38 and microprocessor 40 via bidirectional bus 43. Included in microprocessor 40 is a program or microcode- storing, read-only memory (ROM) 41 and a data and control signal storing random access memory (RAM) 42. The optics of the recorder include an objective or focusing lens 45 mounted for focusing and tracking motions on head-arm 33 by fine actuator 46. This actuator includes mechanisms for moving lens 45 toward and away from disk 30 for focusing and also for radial movements parallel to the motion of carriage 34; for example, for changing tracks within a range of 100 tracks so that carriage 34 need not be actuated each time a track adjacent to a track currently being accessed is to be accessed. Reference numeral 47 denotes a two-way light path between lens 45 and disk 30. In magnetooptic recording, magnet 48 (eg an electromagnet) provides a weak magnetic steering field for directing the remnant magnetisation direction of a small spot on disk 30 illuminated by laser light from lens 45. The laser light heats the illuminated spot on the record disk to a temperature above the Curie point of the magnetooptic layer (not shown, but can be an alloy of rare earth and transitional metals as is described by Chaudhari et al., US Patent 3,949,387). This heating enables magnet 48 to direct the remnant magnetisation to a desired direction of magnetisation as the spot cools below the Curie point temperature. Magnet 48 is shown as oriented in the "write" direction, i.e., binary ones recorded on disk 30 normally are "north pole remnant magnetisation". To erase information from disk 30, magnet 48 rotates so that the south pole is adjacent disk 30. Magnet control 49, which is mechanically coupled to rotatable magnet 48 as indicated by dashed line 50, controls the write and erase directions. Microprocessor 40 supplies control signals over line 51 to control 49 for effecting reversal of the recording direction. The radial position of the lens 45 is required to be carefully controlled, such that a track or circumvolution is faithfully followed and that a desired track or circumvolution is quickly and precisely accessed. To this end, focus and tracking circuits 54 control both the coarse actuator 36 and fine actuator 46. The positioning of carriage 34 by actuator 36 is precisely controlled by control signals supplied by circuits 54 over line 55. Additionally, control signals from circuits 54 travel over lines 57 and 58, respectively, for focus and fine tracking and switching actions of fine actuator 46. Lines 57, 58 respectively carry a position error signal to circuits 54 and a position control signal from circuits 54 to the focus and tracking mechanisms of actuator 46. Sensor 56 senses the relative position of fine actuator 46 to head-arm carriage 33. The focus and tracking position sensing is achieved by analysing laser light reflected from disk 30 over path 47, thence through lens 45, through one-half mirror 60 and to be reflected by half-mirror 61 to a so-called "quad detector" 62. Quad detector 62 has four photo elements which respectively supply signals on four lines collectively denominated by numeral 63 to focus and tracking circuits 54. Aligning one axis of the detector 62 with a track centre line, track following operations are enabled. Focusing operations are achieved by comparing the light intensities detected by the four photo elements in the quad detector 62. Focus and tracking circuits 54 analyse the signals on lines 63 to control both focus and tracking. Recording or writing data onto disk 30 is next described. It is assumed that magnet 48 is rotated to the desired position for recording data. Microprocessor 40 supplies a control signal over line 65 to laser control 66 for indicating that a recording operation is to follow. This means that laser 67 is energised by control 66 to emit a high-intensity, laser light beam for recording; in contrast, for reading, the laser light beam of reduced intensity in order that the laser illuminated spot on disk 30 is not heated above the Curie point. Control 66 supplies its control signal over line 68 to laser 67 and receives a feedback signal over line 69 indicating the emitted light intensity. Control 68 adjusts the light intensity to the desired value. Laser 67, a semiconductor laser such as a gallium arsenide diode laser, can be modulated by data signals so the emitted light beam represents the data to be recorded by intensity modulation. In this regard, data circuits 75 supply data signals over line 78 to laser 67 for effecting such modulation. This modulated light beam passes through linear polarizer 70 thence through collimating lens 71 toward half mirror 60 and is then reflected toward disk 30 through lens 45. Data circuits 75 are prepared for recording by the microprocessor 40 supplied control signals over line 76. Microprocessor 40 in preparing circuits 75 is responding to commands for recording received from a host processor 37 via attaching circuits 38. Once data circuits 75 are prepared, data is transferred directly between host processor 37 to data circuits 75 through attaching circuits 38. Data circuits 75 also includes ancillary circuits (not shown) relating to disk 30 format signals, error detection and correction and the like. Circuits 75, during a read or recovery operation action, strip the ancillary signals from the readback signals before supplying corrected data signals over bus 77 to host processor 37 via attaching circuits 38. Reading data from disk 30 for transmission to a host processor requires optical and electrical processing of the laser light beam from the disk 30. That portion of the reflected light which has its linear polarization from polarizer 70 rotated by the disk magnetisation travels along the two-way light path 47, through lens 45 and half-mirrors 60 and 61 to the data detection portion 79 of the head-arm 33 optics. Half-mirror or beam splitter 80 divides the reflected beam into two equal intensity beams both having the same reflected rotated linear polarization. The half-mirror 80 reflected light travels through a first polarizer 81, which is set to pass only that reflected light which was rotated when the remnant magnetization on disk 30 spot being accessed has a "north" or binary one indication. This passed light impinges on photo cell 82 for supplying a suitable indicating signal to differential amplifier 85. When the reflected light was rotated by a "south" or erased pole direction remnant magnetization, then polarizer 81 passes no or very little light resulting in no active signal being supplied by photocell 82. In contrast, polarizer 83 passes only "south" rotated laser light beam to photo cell 84. Photocell 84 supplies its signal indicating its received laser light to the second input of differential amplifier 85. The amplifier 85 supplies the resulting difference signal to data circuits 75 for detection. The detected signals include not only data that is recorded but also all of the so-called ancillary signals as well. The term "data" as used herein is intended to include any and all information-bearing signals, preferably of the digital or discrete value type. The rotational position and speed of spindle 31 is sensed by a suitable tachometer or emitter sensor 90. Sensor 90, preferably of the optical sensing type that senses dark and light spots on a tachometer wheel (not shown) of spindle 31, supplies the "tach" signals (digital signals) to RPS circuit 91 which detects the rotational position of spindle 31 and supplies rotational information-bearing signals to microprocessor 40. Microprocessor 40 employs such rotational signals for controlling access to data storing segments on disk 30 as is widely practiced in magnetic data storage disks. Additionally, the sensor 90 signals also travel to spindle speed control circuits 93 for controlling motor 32 to rotate spindle 31 at a constant rotational speed. Control 93 may include a crystal controlled oscillator for controlling motor 32 speed, as is well known. Microprocessor 40 supplies control signals over line 94 to control 93 in the usual manner. A simplified description of the position and velocity detection circuits employed in the optical recorder will now be given with reference to fig 1. Tracking Error Signal (TES) generator circuits 10 respond jointly to a sensed Tracking Error Signal (TES) supplied over line 63 (line 63 corresponds to a part of and is included in later described line 57 of Fig. 2), as later described, and to a track count of distance to go supplied by track position circuit 11 received over bus 12 to supply a corrected or valid TES signal over line 13. The valid TES signal more faithfully indicates track crossings than the indicia signal. The valid TES signal on line 13 travels to track position circuits 11 for effecting reliable track counting. A set of velocity circuits 14 respond to the valid TES signal on line 13 to generate a true velocity signal on line 15 for servo circuits for controlling the motive means, as later described. The line 15 velocity signal also travels to TES generator circuits 10 for more validly producing a valid TES signal. The actual track count to go is supplied not only over bus 12 but over bus 12A for use in other parts of the servo circuits. At the beginning of each seek, the value received over bus 59A is stored in the track position circuits 11 to represent a predetermined number of tracks to go (this value can be the number of tracks times two). TES generator circuits 10 include a timed electrical circuit for generating a substitute TES signal which is used during predetermined portions of the relative motion controlled by the positioning systems. At other times the sensed TES signal supplied over line 63 is passed as an indicia signal through TES generator circuits 10 to become the valid TES signal on line 13. Describing the various components of Fig 1 and Fig 2 in detail, Fig. 3 illustrates the seeking and track following portions of focus and tracking circuits 54. The tracking and seeking portions include fine positioner circuits 110 and coarse positioner circuits 111. The coarse positioner circuits 111 operate such that the coarse actuator 36 always moves the head-arm to follow the motions of fine actuator 46. Fine positioner circuits 110 actuate actuator 46 to move objective lens 45 such that a laser beam travelling along light path 47 follows a single track during track following operations and moves radially across the disk 30 during a track seek operation. For track following operations track follow circuit 112 receives a sensed tracking error signal over line 63 from decoder 113 to provide track-following control signals over line 114. The line 114 control signal then traverses electronic switch 115, entering at track follow terminal 116, for controlling power output amplifier 117. Power amplifier 117 supplies an actuating signal over line 57D to actuator coil 46A of fine actuator 46. The drive current on line 57D causes the actuator 46 to move radially along the head-arm 33 to maintain the position of the light beam 47 on a single track. The quad detector 62 having independent photoresponsive elements A, B, C and D, arranged in a rectangular array, provides tracking error photo element signals to decode circuit 113. The axis of quad detector 62 lying between photoelements A, D and B, C is aligned with the axis of the track being followed. Decode circuit 113 responds to the four photoelement signals to provide a tracking error signal, as is well known and as later described herein. In track follow circuit 112, the tracking error signal actuates servo circuits in a known manner. Operation of track follow circuit 112 is modified by a radial runout input received over line 118. A disk profile is generated for the disk 30 which indicates that the expected radial runout. Track follow circuits 112 respond to the radial runout signal for modifying the tracking error signal to anticipate the radial run out thereby providing more faithful track following. Additionally, relative position error sensor 56 mounted on head-arm 33 of carriage 34 senses relative displacement of actuator 46 and head-arm 33. It supplies a relative displacement error signal RPE over line 58E, thence line 119, to track follow circuits 112 to compensate for the TES offset. Such offset is caused by relative motion between carriage 33 and fine actuator 46. Velocity seek loop circuits, generally denoted by numeral 123, constitute all of the electronic circuits providing a signal to the seek input terminal 124 of seek-follow switch 115. Whenever a seek is instituted by microprocessor 40, electronic switch 115 is moved from terminal 116 to terminal 124 for disconnecting the track follow circuit 112 from amplifier 117 and connecting the velocity servo loop circuits 123 to amplifier 117. Circuits 123 respond to several input signals for effecting the velocity-controlled seek. Track crossing circuit 125 receives the sensed TES signal over line 63 for detecting when the beam 47 is crossing a track on disk 30. Each time a track crossing is detected by circuit 125, an output decrementing pulse is supplied to track counter 126 for decrementing one from the number of tracks to go. It may be noted that in some embodiments a single track crossing is represented by two zero crossings of the tracking error signal on line 63 and thus two pulses provided to the track counter indicate a single track crossing. At the same time the microprocessor actuates circuits 123 to do a seek, it also supplies the number of tracks to be crossed over bus 59A (part of line 59 of Fig. 2) presetting track counter 126 for the upcoming seek operation. Track counter 126 continuously outputs the number of tracks-to-go over bus 127 to velocity circuits 130 as well as to track crossing circuits 125. The track crossing circuits 125 use the track count to verify the accuracy of the number of tracks counted. Velocity circuits 130 respond to a velocity profile designed for the seek operation for generating a speed reference signal and a measured speed signal. The speed reference signal supplied over line 132 is based upon the velocity profile desired for the seek operation and the instant distance to go. The measured speed signal is supplied over line 133 to be subtracted from reference signal on line 132 by sum circuit 131. The resultant speed error signal supplied by sum circuit 131 alters the operation of fine actuator 46 to closely follow the velocity profile. Operation of velocity circuits 130 are described later. Circuits 123 also include a gain control circuit which includes gain control switch 135 having its output terminal connected to the seek terminal 124 of switch 115. Switch 135 is actuated to an acceleration position 139 whenever microprocessor 40 supplies a SEEK signal over line 136 (line 136 is shown as two different line portions in Fig. 3 for purposes of simplifying the drawing). The SEEK signal sets flip-flop FF 137 to the inactive state causing FF 137 to supply a switch actuating signal over line 138 to move switch 135 to connect acceleration terminal 139 to seek terminal 124 of switch 115. Acceleration circuit ACCEL 140 provides high gain to the sum circuit 131 error signal; that is, the error signal is accentuated by ACCEL circuit 140 for initially maximizing the drive power to actuator 46. This acceleration high gain portion minimizes the time it takes the fine actuator 46 to move beam 47 to a speed or velocity corresponding to the desired velocity profile. Once the velocity profile and the actual velocity are the same, then detector circuit 141 detects a small error signal supplied by sum circuit 131. At this time, detector circuit 141 resets FF 137 to the active state to supply an activating signal over line 138 switching switch 135 from terminal 139 to receive signals from seek compensator circuit COMP 142. Compensator circuit 142 is designed to maximize velocity servo operation whenever the reference velocity profile and the measured velocity have a small error condition. Compensator 142 continues to couple sum circuit 131 through switch 135 to fine actuator 46 until the end of the seek, which occurs at one-quarter track pitch from the target track. At this point, track follow circuit 112 is again re-energized and the velocity circuits 123 are disconnected from actuator 46. Track capture, i.e., switching from track seeking to track following on the target track, is indicated by a one-quarter track pulse from pulse track counter 126 pulse supplied over line 145, resetting seek priming flip-flop 146 to the reset state. Initially the SEEK signal from microprocessor 40 on line 136 set FF 146 to the active state causing switch 115 to move from follow terminal 116 to seek terminal 124. Resetting FF 146 at one-quarter track pitch-to-go from the target track causes a deactivating signal to be supplied over line 147 to move the switch 115 from seek terminal 124 back to the follow terminal 116. To assist in track capture, the dynamic range of power amplifier 117 is momentarily increased to supply a maximal control signal over line 57D to actuator coil 46A. This additional control current ensures fast capture but is not desired for faithful track following operations. Accordingly, the one-quarter track pitch signal on line 145 actuates a monostable multivibrator, or other time delay circuit MONO 148, to supply an actuating signal over line 149 to amplifier 117. This actuating signal causes the amplifier 117 dynamic range to be increased, electronically changing the dynamic range of amplifiers as is well known and not further described for that reason. It may be desired to verify that track counter 126 has faithfully counted the tracks, particularly when two pulses per track crossing are employed. To this end, decode circuit 113 supplies a signal which is the sum of all photocurrents from the elements of detector 62 over line 192 to track counter 126. This sum signal is in quadrature to the TES signal on line 63. Whenever the sum signal on line 192 has a maximum positive amplitude, then the beam 47 is in the centre of a track being crossed. When two pulses per track crossing are employed, this means that the numerical content of track counter 126 should be even. If the track counter is odd, then the track count is changed by unity for synchronizing the track count to actual track crossings. This paragraph completes the description of circuits 123. The coarse positioner circuits 111 receive the relative position error signal RPE from detector 56 over line 58E. Compensator and integrator COMP/INT 155 responds to the error signal to supply a smoothed and integrated error signal to sum circuit 156. Sum circuit 156 compares the error signal from compensator integrator 155 with a reference signal (zero or ground reference potential) to supply a control signal through amplifier 157 causing actuator 36 to move the head-arm to follow the motions of actuator 46. Such following operations reduce the error signal sensed by detector 56 in the relative movement of the head-arm 33 (carriage 34) and actuator 46 away from a reference position on head-arm 33 (not shown). During seek operations, it is desirable to actuate coarse actuator 36 to move the head-arm 33 more quickly in order to limit the relative displacement of the fine and coarse actuators. To this end, the drive signal supplied to amplifier 117 for driving fine actuator 46 is also supplied over line 159 to feedforward circuit 158. Feedforward circuit 158 is basically a gain control and signal smoothing circuit of any design. Feedforward circuit circuit 158 supplies its output signal to sum circuit 156. The feedforward circuit 158 output signal is added by sum circuit 156 to the signal from element 155 for actuating actuator 36 to maximal actuation, particularly during acceleration mode. Thus, during acceleration phase the fine servo loop passes the effects of circuit 140 to the coarse servo loop. Accordingly, both the fine actuator 46 and the coarse actuator 36 receive enhanced drive signals during the acceleration phase for ensuring both actuators 36 and 46 accelerate in a minimal time according to the desired velocity profile. Fig. 4 is a block diagram showing the track crossing circuits 125, which accommodate defects or other error conditions during the track seeking operations. Electronic switch 160 selects either processed TES zero axis crossing signal pulses received over line 161 or substitute zero axis crossing pulses received over line 162. Under certain operating conditions, the line 162 pulses provide a more reliable indication of track crossings than the processed TES signal pulses. The substitute zero axis crossing pulses are supplied by a phase-lock loop (PLL) 171, which is phase synchronized to the processed TES pulses and frequency controlled in accordance with the current radial velocity of the beam 47 crossing the disk 30. The processing of the received TES signal on line 63 into zero crossing pulses is first described. Plural band filter 166 band pass filters the line TES signal 207 (Fig. 6) in accordance with current velocity of the beam crossing disk 30. The current velocity, is the velocity profile of the seek which is digitally stored in velocity profile EPROM 164 as a table of values. Track counter 126 supplies the number of tracks-to- go indication over bus 128 to EPROM 164 and to velocity circuits 130. The distance-to-go value actuates EPROM 164 to emit control signals over bus 165 to select a pass band of plural band filter 166; for example, filter 166 may have eight pass bands, one of which is selected at any given instant. The higher the velocity, the higher frequency and width of the pass band selected to process the line 63 TES signal. The actual values and number of bands are best empirically determined. The pass band filtered TES signal travels over line 168 to zero axis crossing detector 167. Detector 167 emits a train of pulses 231 (Fig. 8), corresponding to zero axis crossings of TES signal 207 (Fig. 6), to line 161, thence switch 160. At low pulse frequencies on line 169, switch 160 passes the pulses as valid TES zero axis crossing pulses over line 172 for decrementing track counter 126 and as an input to velocity measuring circuits 130. The substitute zero axis crossing pulses on line 162 are PLL 171 output pulses. PLL 171 has the usual construction of a digital phase compare portion 175 which receives the line 161 pulses via noise rejection circuit 230 for phase comparison with the line 162 pulses. The phase error signal generated by phase compare portion 175 passes through the usual transfer function 176 to phase control voltage controlled oscillator (VCO) 177. The frequency of VCO 177 is controlled to represent the current velocity of the beam 47 crossing disk 30. In a test embodiment, the current velocity was derived from the measured velocity signal on line 133 or from the velocity signal from EPROM 164; both velocity-indicating signals, the measured or the reference, were fed forward to frequency control VCO 177. The line 133 measured velocity signal has positive and negative polarities, hence absolute value circuit 178 converts the bipolar velocity signal to a unipolar (absolute value) signal traveling over line 179 to frequency control VCO 177 to the current velocity. In the test embodiment, the bus 165 carried reference velocity signal was alternately supplied for frequency controlling VCO 177. In a commercial embodiment, either of the two velocity-indicating signals may be employed or the two signals may be averaged together for joint frequency control of VCO 177. Switch control 163 actuates switch 160 to couple the PLL 171 to line 172 whenever the radial velocity of the beam 47 exceeds a predetermined threshold indicated by a reference signal on line 180. In the frequency domain, the threshold is the track crossing frequency. Comp circuit 181 compares the line 179 measured velocity signal with the line 180 reference signal. When the measured velocity signal amplitude exceeds the reference signal amplitude, then compare circuit supplies a high-speed indicating signal over line 182 to switch control circuit 163. Switch control circuit 163 responds to the high-speed indicating signal to actuate switch 160 to decouple line 161 from line 172 and to couple line 162 to line 172. When the line 179 measured velocity signal decreases to be less than the line 180 reference signal, then compare circuit 181 switch control 163 responds to the compare circuit to actuate switch 160 to recouple line 161 to line 172 for passing the processed sensed TES pulses as valid TES pulses. A phase error signal produced by the digital phase compare portion 175 indicates a loss of phase synchronization between the sensed TES pulses and the PLL 171 output pulses. Switch control 163 includes a phase error detector (not shown) operative during the high-speed portion of the seek for indicating a loss of synchronization; then switch control 163 actuates switch 160 to decouple line 162 from line 172 and recouple line 161 to line 172 for again passing the processed sensed TES pulses as valid TES pulses. Once a loss of phase synchronization is detected, then the PLL 171 output pulses are not used during a current seek. It has been found that PLL 171 enables the seeking operation to continue in an accurate manner over medium defects extending over a relatively large number of tracks. While no TES pulses are being sensed, digital phase compare portion 175 does not indicate a loss of phase synchronization. The fed forward current velocity signal causes PLL 171 to track the velocity profile such that when sensed TES pulses are again generated, the PLL 171 operation quickly regains phase synchronization for continuing the seek operation under its control. If phase synchronization is not reestablished, then the sensed TES pulses on line 161 are used to complete the seek operation. It has been found that PLL 171 enables seeking directly to a target track even when large defects are encountered during the seek. It is well known that each track crossing can be indicated by two zero crossings of the tracking error signal. Track counter 126 counts two such zero crossings for each track crossing. The centre of a track is indicated by the numerical contents of track counter 126 having an even count. This even or odd count (the last or least significant digit of the counter) is supplied over line 190 to verify count circuit 191. Verify count circuit 191 receives the sum (quadrature) signal over line 192 for comparison with the odd-even count of track counter 126. If the count is odd, unity is subtracted from track counter 126 by a signal supplied from verify count circuit 191 over line 192. This minor change synchronises the numerical contents of track counter 126 to the actual position of beam 47. Fig. 5 is a simplified diagram of the circuits employed for generating the TES and quad signals respectively over lines 63 and 192. The light beam reflected from disk 30, when in focus, impinges on the centre of detector 62 as indicated by dash-lined circle 204. Such an indication indicates true track following and true focus. As the beam 47 goes off centre from the track being followed, then the impingement of the beam 47 moves the circle 204 along an axis 205 of detector 62; axis 205 is aligned with the track centre lines 206 (Fig. 6). All operations of decoder 113 are in analog signal form. TES signal 207 on line 63 is generated by two sum circuits 210, 211 and differential amplifier 212. Sum circuit 210 sums the photocurrents from the photodetector elements A and D received over lines 63A and 63D respectively. The sum represents the total light received on one side of the axis 205 which corresponds to the physical position of beam 47 radially of the track being followed. Similarly, sum circuit 211 sums photocurrents from photoelements B and C, received over lines 63B and 63C respectively, corresponding to a second signal representative of the light received relative to the track being followed. Differential amplifier 212 compares the sum signals from circuits 210 and 211 to generate the sensed TES signal 207 (Fig 6) on line 63. Line 63 may be of the differential type wherein two signal lines carry the TES signal in differential form (A1 and C1). Single-ended circuits may be employed as well. The quad signal on line 192, which is in quadrature to the sensed TES signal as will be explained with respect to Fig. 6, is generated by sum circuit 213. Sum circuit 213 receives photocurrents from all of the four elements A, B, C and D of detector 62, providing a signal indicative of the total light received by the detector 62. Referring next to Fig. 6, a portion of the information- bearing surface of disk 30 is illustrated. The disk is formed with two sets of concentric rings, one set being mesas 215 and the second set being grooves 216. Beam 47 impinges on the outer surface consisting of the interleaved grooves 216 and mesas 215. The track centre lines 206 of the three tracks illustrated in Fig. 6, lie along the centre of the grooves. At the point of light beam impingement on any track of disk 30, axis 205 (Fig 5) is parallel to track centre lines 206. When the beam 47 is focused into a groove, the tracking error signal is at a zero crossing position 220, i.e., when exactly centred, the sensed TES signal should be at zero. As the beam moves radially from the track centre line (crosswise), then the amplitude of the TES signal 207 changes as a sinusoid with the direction of change indicating the direction of tracking error, wherein zero axis crossings in a first direction signify crossing track centre lines 206 as indicated by dimension line 220. Similarly, when the beam is crossing the mesas 215, then the zero access crossing of TES signal 207 is in the opposite direction as at points 221. The quad signal 224 on line 192 is displaced 90 degrees from the TES signal 207 (Fig 6). At the centre of each track, quad signal 224 has a maximum positive amplitude (the polarity of the signal may be inverted in other embodiments), whereas at the centre of the mesas, a maximum negative amplitude is reached. Because of varying focus conditions, maximum positive and negative magnitudes may be different. Referring again to Fig. 4, a noise rejection circuit 230 is interposed between line 161 and digital phase compare portion 175. Figs. 7 and 8 show such a noise rejection circuit and its operation. During a seek operation, sensed TES zero crossing pulses 231 travelling over line 161 (Fig. 4) actuate ramp generator 241. The seek operation is indicated by a seek signal received on line 136. Generator 241 outputs the ramp signal 243 over line 245 to a pair of comparators 247 and 248. The desired velocity profile digital signals provided over cable 165 are converted to analog form by DAC 250. The desired profile is supplied to generator 241 for indicating the slope of the ramp 243. Comparator 247 compares the amplitude of ramp signal 243 with a reference signal supplied over line 255. When the ramp amplitude equals the reference signal amplitude on line 255, a set or priming pulse 257 (fig 8) is supplied by comparator 247 to set check latch 260 to the active condition; simultaneously, the set pulse S from comparator 247 triggers generator 241 to begin discharging, thereby identifying apex point 252. A next succeeding sensed TES pulse 240 resets ramp generator 241 at point 274 returning ramp signal 243 to a reference amplitude. A new ramp cycle then begins. This action is the usual operation when sensed TES pulses 231 are being reliably generated. Latch 260 supplies the B and B signals detection defining windows respectively over lines 262 and 263. When latch 260 is in the reset condition, AND circuit 265 is enabled and it passes a sensed TES pulse on line 161 as noise over line 266. On the other hand, when latch 260 is set to the active condition, AND circuit 265 is disabled, while AND circuit 268 is enabled to indicate a drop-out signal over line 269. The other input to AND gate 268 is generated by comparator 248. Comparator 248 determines when the discharging of ramp signal amplitude 243 falls below a certain reference signal value received on line 275, i.e., no sensed TES pulse 231 had reactivated ramp generator 241 to again begin a ramp signal. For example, in Fig 8, a drop out of the sensed TES pulses occurs at point 272. As a result, the ramp signal 243 is not reset at point 273 on the ramp signal 243. Instead ramp generator 241 continues to discharge until a threshold value indicated by the signal on line 275 is reached as indicated at 276. This point corresponds to comparator 248 supplying a drop-out pulse 280 over line 281 which resets latch 260. Drop-out pulse 280 also passes through AND circuit 268 to line 269. Lines 266 and 269 go to phase compare 175 for inhibiting its operation. Fig. 7 also shows comparator 181 and a part of switch control 163. Comparator 286 compares the velocity profile with a reference signal supplied from DAC 250 for indicating when the velocity of the beam is less than the predetermined value. The output signal of comparator 286 travels over line 288 to AND circuit 290. The valid TES pulses supplied over line 172 time the setting of switch 160 control latch 291. As long as latch 291 is set, no enabling signal is supplied over line 169 to switch 160 keeping line 161 coupled to line 172. On the other hand, when the velocity profile indicates a velocity greater than the predetermined value, comparator 285 determines that the profile value signal from DAC 250 is greater than the reference value signal on line 293. Then, comparator 285 enables AND circuit 295 to pass the valid TES pulses on line 172 to reset latch 291 thereby providing a continuous select PLL line 162 signal on line 169 keeping switch 160 in the setting shown in Fig 4. As soon as the profile is reduced to below the predetermined value, then latch 291 is again reset to the inactive condition by comparator 286. If a continuous phase sync loss signal from PLL 171 portion 175 is received over line 295, latch 291 stays in the reset state for the remainder of the seek. Figs. 9 and 10 show a track-crossing indicating circuit which performs the function of verify count circuits 176 and supplies count input to a counter in track counter 175. The optical signals are retrieved from "quad" detector 62 as shown in Fig. 5. The differential tracking error signal (TES) on line 63 has a +TES signal 207 (Fig. 6) and a -TES signal which is the complement of signal 207. The output signals of all four photo diodes A-D of detector 62 are scanned to create a sum or "quad" signal 224 (Fig. 10) having phase quadrature to both and TES and -TES signals. The quad and TES signals are combined to reliably detect track crossings with an indication of relative direction of radial motion of the beam 47 to disk 30. The block diagram of Fig. 9 is described with respect to the signals shown in Fig. 10. The inputs A1, C1 are differential TES signal 207 from differential amplifier 212 (Fig. 5). These inputs go to Schmitt triggers ST1 300 and ST2 301, respectively. The Schmitt triggers have switchable hysteresis levels which eliminate "pick up" errors when track seeking at low radial velocities (hysteresis increased) and drop outs when seeking at high velocities (hysteresis decreased). The baseline tracking circuit (303) varies the reference level of the first Schmitt triggers 300 and 301. Circuit 303 is only operative at high seek velocities. A velocity detector 304 compares the desired velocity profile signal (a current velocity) on line 219 (DAC 250 of Fig. 7) with a reference value signal on line 305 and determines when to switch between different hysteresis levels of the Schmitt triggers and a variable reference level. The quad signal 224 is used to detect retrograde motion during a seek operation. Retrograde motion may occur whenever disk runout is high or vibrations occur. The quad signal has a 90-degree phase lag with respect to the TES signal 207 when seeking radially inward versus a 90-degree phase lead when seeking radially outward. Output signals 308 and 309 respectively from AND 311 and AND 312 indicate track crossings in respective opposite radial directions, such as caused by a change in relative direction of motion of the beam and track during a seek. A change in direction causes pulses which appeared at one output to switch to the other output. The quad signal 224 used to detect the occurrence of retrograde motion is only used when seeking at low velocities. Once the track crossing frequency is high, this circuit path is gated off. Baseline tracking circuits 303 and 313, respectively, restore the baseline references for both the TES and quad signals. The positive peak amplitude level of the TES signal 207 activates one of the Schmitt triggers, ST1 or ST2, 300 or 301. Triggers ST1 and ST2 each have asymmetry which is accommodated by the complementary connections. The time duration that each peak level is stored varies as a function of velocity (track-crossing frequency). The baseline reference voltages are connected to one leg of a comparator 315 (Schmitt trigger in the TES path). The other leg of the comparator receives the quad signal 224. The outputs of the comparator 315 are square waves B2 316 and B2 317. Fig. 10 shows two different runout conditions having retrograde motions, the solid line being one example and the dotted line the other example. The signal designated as "A1" is the TES 207 and the one designated as "B1" is the quad signal 224. "AP" is the signal made up of pulses occurring each positive-sloped zero crossing of the TES and "AN" is the signal made up of pulses occurring each time a negative zero crossing occurs. B2 and B2 are the in-phase and out-of-phase digital track summing signals. The AND of AP+ B2 , ORed with the AND of AN+B2 produces track-crossing pulses 308 indicating a radial outward motion of the beam (RADIAL OUT in Fig 9). The AND of AP+B2 ORed with AND of AN+ B2 produces track-crossing pulses 309 indicating a radial-inward motion of the beam (RADIAL IN in Fig 9). In Fig. 9, gating logic 320 receives a direction signal from microprocessor 40 over line 321 which indicates the direction of commanded seek direction radially inward or radially outward. When a direction signal on line 321 indicates a radially-inward motion, then the radial impulses from AND circuit 312 are subtracted from the numerical contents of track counter 126 which indicates distance to go, while the radial-outward pulses from AND circuit 311 are added to the counter 126 contents. In a similar manner, when the direction signal on line 321 indicates a radial-outward motion, the pulses from AND circuit 311 are subtracted from the contents of counter 126, while the output pulses from AND circuit 312 are added to the counter 126. The output of AND circuits 311 and 312 represent the output signal of track-crossing circuit 125 which includes the illustrated NAND circuits and the illustrated Schmitt triggers and baseline circuits. The operation of NAND circuits is well known and the logic of operation can be determined from inspection of Fig. 9. Referring next to Fig. 11, the illustrated velocity-state estimator circuit is a preferred construction of velocity circuits 130 which smoothes the measured velocity signal and causes the measured velocity signal to change when a defect is being crossed (and thus no TES signal 207 is being produced) using servo system parameters as will become apparent. This smoothing provides a continuous analog-type velocity signal throughout a seek operation even with significant defects. Such smoothing includes modelling the mechanical dynamic characteristics of the fine actuator and the relative-position error sensor 56. When the track-crossing frequency during a high speed SEEK exceeds a predetermined threshold, digital velocity circuits 330 are employed for generating the measured velocity signal on line 133. Such circuits may be the circuits shown in the co-pending application, supra; while at speeds below the track-crossing frequency threshold, the TES signal on line 63 is employed for measuring the radial velocity. The Fig. 11-illustrated circuits selectively modify the measured velocity signal or the TES signal on line 63 to supply a continuous analog-type velocity-indicating signal on line 133. In Fig. 11, summation circuit 400 receives three inputs to be summed for providing the measured velocity signal on line 133. A first input signal is received from the fine actuator drive signal on line 57D (the drive signal without the radial runout compensation effect provided over line 118 (Fig. 3)). The fine actuator 46 drive current on line 57D is modified in circuit 401 by a model of the force constant of fine actuator 46. A second input is the RPE signal on line 58E that is modified by circuits 402. In circuits 402, the RPE signal is modified by a model of the spring constant and viscous damping constant of the fine actuator 46. The RPE effect on summation circuit 400 is to subtract or counteract the other two inputs. In a practical embodiment, the portions in circuits 402, respectively modelling the spring constant and the viscous damping constant, may result in separate summation inputs to the summation circuit 400. The third input to summation circuit 400 on line 403 provides a timed velocity recalibration input from either velocity circuits 330 or from the line 63 TES signal 207, depending upon the track-crossing frequency, as will become apparent. The RPE error signal from circuit 402 is subtracted from the sum of the first and third inputs, from circuits 401 and 403 respectively. Summation circuit 400 supplies its sum signal through compensation circuit 404 which modifies the summation circuit by an inverse of a model of the fine actuator 46 mass value. Compensation circuit 404 supplies its compensated signal through low-pass filter LP 405 to line 133. The actual measured velocity calibration change of the smoothed measured velocity signal on line 133 is timed to coincide with the ends of respective velocity-measuring periods. Switch 410 times the modification of the estimated measured velocity throughout the seek operation. The timing is controlled by circuit 411 which holds electronic switch 410 closed for a predetermined period, such as by a monostable multivibrator indicated in circuit 411 by the word "HOLD". Circuit 411 has four inputs into two AND function circuit portions A1 and A2. Circuit A1 times the switch 410 whenever the digital velocity-measuring circuit 330 is being implemented; i.e., above the track-crossing frequency threshold. Below the track-crossing frequency threshold, when the TES signal on line 63 is used for generating the velocity signal, AND circuit A2 controls the timing of switch 410. An OR circuit "OR" passes the outputs of AND circuits A1 and A2 in a logic OR manner for actuating the HOLD portion of circuit 411. Circuit 411 is controlled by comparator circuit 412. Comparator circuit 412 receives the reference velocity signal on line 132 (Fig. 3) for comparison with a track-crossing frequency threshold value received over line 413. Since the threshold signal on line 413 is a constant, it can be provided by a voltage divider or other suitable voltage or current source. Comparator circuit 412 is of the switching type such that when the reference velocity on line 132 is less than the threshold signal on line 413, an actuating signal is supplied by comparator circuit 412 only over line 414; when the reference velocity signal on line 132 exceeds the threshold signal on 413, then an actuating signal is supplied over line 415. The reference velocity signal on line 132 is representative of the track-crossing frequency. The actuating signal on line 414 is supplied to AND circuit A2 for passing the track-crossing signals received from line 63 to actuate the "hold" portion of circuit 411 for closing switch 410. AND circuit A1 is enabled to pass the end of measuring period signal on line 331 by the line 415 signal. The line 331 signal is delayed by circuit 416 to compensate for circuit delays. When the digital velocity circuit 330 is providing the measured velocity signal on line 332, AND circuit A1 is actuated at the end of each of the measuring periods for timing the closure of calibrating switch 410. Analog summation circuit 420 supplies the calibrated signal through switch 410 as the third summation input on line 403 for analog summation circuit 400. Line 421 couples the output of compensation circuit 404 as a subtractive input into summation circuit 420. The actual measured signals are supplied to summation circuit 420 respectively through either switch 422 or 425. Switch 422 is closed by the line 415 actuating signal for passing the digital velocity measured signal on line 332 as a positive summation signal to summation circuit 420. Therefore, when the seek speed or velocity is greater than the threshold on line 413, the correction signal to summation circuit 400 is the difference between the measured signal on line 332 from the digital measuring circuit and the feedback signal on line 421. When the seek speed is below the threshold value on line 413, the velocity measurement is as follows: The TES signal 207 on line 63 supplied though gain control block circuit 426 goes to switch 427, thence to sample and hold circuit S&H 428. Sample and hold circuit 428 receives the input TES pulses on line 161 at each TES signal 207 zero crossing (two zero crossings per track crossing). Electronic switch 425 remains closed when comparator 412 supplies the actuating signal on line 414. Switch 410 supplies the timing control for the calibration of the measured velocity signal on line 133. Polarity corrections have to be provided for the velocity signal calculated from the line 63 TES signal 207. Referring next to Fig. 6, a portion of the grooved medium 30 is shown with track centre lines 206 being in the grooves with no tracks being on the mesas 215 intermediate the grooves. Tracking error signal 207 (TES signal on line 63) is shown in spatial relationship to the grooved media 30. An actual track-crossing signal is represented by the vertical line 220 and the zero crossings of TES signal 207 on the mesas are represented by numeral 221. Depending on the relative direction of motion with respect to the groove or track crossings 206, the polarity of the signal supplied to switch 427 varies. Accordingly, the actual relative direction of the beam 47 as it crosses the tracks 206 have to be polarity reversed. To achieve this, the output signal of gain-adjusting circuit 426 is supplied over line 435 to direction circuit 436 which receives a direction indicating signal over line 437. The direction indicating signal on line 437 is the actual direction as determined by the comparison of the quadrature signal and the TES signal (Fig 9). Circuit 436 basically is a phase compare such that the control of switch 427 will always provide a properly-phased signal to circuit S&H 428. Line 439 causes a polarity inverted signal from circuit 426 for supplying an inverted signal to terminal 440 of switch 427. The operation of the circuit is such that when the beam is relatively moving across the tracks as seen in Fig. 6 from left to right, all signal crossings will be positive. That is, the signal crossings at points 221 will be polarity inverted rather to have the same polarity velocity-indicating signal in S&H 428. On the other hand, when the beam movement is from right to left, the indicating signals are reversed.

Claims: Data storage apparatus comprising at least one data storage disk (30), a transducer (45) for sensing track indicating information on the surface of the disk and a positioning system including motive means (36) for moving the transducer over multiple tracks on the disk surface and tracking circuit means (54) responsive to the transducer produced information signals to generate sensed track crossing signals indicative of the transducer position for application to the motive means, characterised in that: the tracking circuit means includes means (171) for producing substitute track crossing signals, the tracking circuit means further including switch means (160) for selectively applying either the sensed or substitute signals to the motive means during motion of the transducer in accordance with predetermined conditions. Data storage apparatus as claimed in claim 1, the tracking circuit means further including velocity circuit means (130,164) which generates a signal indicating the transducer velocity during track to track motion, the substitute track crossing signal means being responsive to this velocity signal to generate the substitute track crossing signals. Data storage apparatus as claimed in claim 2, the velocity circuit means including means (400, 401, 402) for measuring the transducer velocity during track to track motion and means (164) for storing a track to track velocity profile of the desired transducer velocity, the substitute track crossing signal means (171) being responsive to either the measured transducer velocity or the desired transducer velocity or a combination of the two to generate the substitute track crossing signals. Data storage apparatus as claimed in claim 2 or claim 3, the velocity circuit means further including comparison means for comparing the transducer velocity with a reference velocity and supplying a signal to the switch means to select the substitute track crossing signals when the transducer velocity exceeds the reference velocity. Data storage apparatus as claimed in any of claims 2 to 4 including detection window generating means (260) responsive to signals from the velocity profile means to produce detection window signals and detection means (265,268) jointly responsive to the detection window signals and the sensed track crossing signals to indicated whether or not a sensed track crossing signal occurred within a detection window, the sensed signals being selected for application to the motive means when there is such an occurrence and the substitute signals being selected when there is no such occurrence. Data storage apparatus as claimed in claim 5, the detection window detection window signals having periods in inverse proportion to the value of the velocity signals from the velocity profile means. Data storage apparatus as claimed in claim 5 or claim 6, the tracking circuit means including comparison means responsive to signals from the detection means to synchronise the phases of sensed and substitute signals.

Inventors: CHOW WILLIAM WAI-CHUNG, FENNEMA ALAN AUGUST, FIORINO BENJAIN CARMEL, HENDERSON IAN EDWARD, KADLEC RONALD JAMES, ROBERTS SPENCER DONALD, CHOW, WILLIAM WAI-CHUNG, FENNEMA, ALAN AUGUST, FIORINO, BENJAIN CARMEL, HENDERSON, IAN EDWARD, KADLEC, RONALD JAMES, ROBERTS, SPENCER DONALD
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: G11B7
IPC codes: G11B21, G11B11, G11B7, G05D3