Abstract:
A system and method for damping the motion of the head of a tape drive is disclosed. The method includes measuring the position of the head over time with a position sensor; damping the motion of the head based on the measured position of the head over time; and tuning the damping by varying a parameter of the position sensor.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to tape drives. More particularly, the present invention relates to damping the movement of the read-write head of a tape drive.  
       BACKGROUND OF THE INVENTION  
       [0002]     High-density recording on multiple tracks of a magnetic tape is known. In certain arrangements, parallel tracks extend along a longitudinal direction of the magnetic tape. The magnetic tape is moved transversely across a read/write head and data is recorded or read. During recording or playback, the head needs to stay in a fixed lateral position relative to the tape as the tape moves in a longitudinal direction.  
         [0003]     Some existing tape storage systems contain a mechanism that allows the read/write head to be located at the track&#39;s centerline at the beginning of the read/write process. Once the read/write process begins, there is no correction if an offset should arise between the head and the centerline of the track.  
         [0004]     To increase storage capacities to meet increased demands, track density, which is the number of tracks per distance (e.g., inches), should be increased. As track density increases, the track pitch and width decrease. For proper read/write operation, the magnetic head must stay at, or very near, the centerline of the track, and the need for precision increases as track density increases.  
         [0005]     A conventional tape drive system is shown in  FIG. 1 . This system comprises a tape drive  12  connected to a host computer  10  by a cable  16 , and an associated tape cartridge  14 . The tape drive  12  includes a receiving slot  22  into which the tape cartridge  14  is inserted. The tape cartridge  14  comprises a housing  18  containing a length of magnetic tape  20 . The tape drive  12  is preferably compatible with the associated host computer, and can assume any one of a variety of cartridge or cassette linear formats.  
         [0006]     A typical configuration of the tape drive  12  is shown in  FIG. 2 . The tape drive  12  in  FIG. 2  comprises a deck  24  including movable parts, and a control card  26  including various circuits and buses. The deck  24  includes a head assembly  28  which contacts the tape  20  of the tape cartridge inserted into the tape drive  12  to read and write data and read a servo pattern, and motors  34  and  36  for respectively rotating a supply reel  30  and a take-up reel  32 . For a tape cartridge  14  of a dual reel type, both of the reels  30  and  32  are included in the tape cartridge  14 . For a tape cartridge  14  of a single reel type, however, only the supply reel  30  is included in the tape cartridge  14  while the take-up reel  32  is provided in the tape drive  12 . Although not shown in  FIG. 2 , the deck  24  additionally includes a mechanism for moving the head assembly  28  across the width of the tape  20 , a mechanism for holding the inserted tape cartridge, and a mechanism for ejecting the inserted tape cartridge.  
         [0007]     The control card  26  includes a microprocessor (MPU)  38  for the overall control of the tape drive  12 ; a memory  42 , a servo control unit  44 , a data flow unit  46  and an interface control unit  48  all of which are connected to the MPU  38  via an internal bus  40 ; a motor control unit  50  and a head control unit  52  which are connected to the servo control unit  44 ; and a data channel unit  54  which is connected to the data flow unit  46 . While the memory  42  is shown as a single hardware component in  FIG. 2 , it is actually preferably constituted by a read only memory (ROM) storing a program to be executed by the MPU  38 , and a working random access memory (RAM). The servo control unit  44  manages speed control for the motors  34  and  36  and position control for the head assembly  28  by transmitting the respective control signals to the motor control unit  50  and the head control unit  52 . The motor and head control units  50  and  52  respond to these control signals by physically driving the motors  34 ,  36  and positioning the head assembly  28 , respectively.  
         [0008]     The head assembly  28  includes servo heads which read data from servo tracks or bands on the tape  20 . Control card  26  utilizes data from the servo tracks to generate a position error signal (PES), and the PES is used by the servo control unit  44  to cause the head control unit  52  to position the head assembly  28 . In some conventional designs the head assembly  28  includes a voice coil motor (VCM)  56  which receives electrical signals from the head control unit  52  and positions the head assembly  28  according to the received signals.  
         [0009]     The data flow unit  46  compresses data to be written on the tape  20 , decompresses data read from the tape  20  and corrects errors, and is connected not only to the data channel unit  54  but also to the interface control unit  48 . The interface control unit  48  is provided to communicate data to/from the host computer  10  via the cable  16 . The data channel unit  54  is essentially a data modulating and demodulating circuit. That is, when data is written to the tape  20 , it performs digital-analog conversion and modulation for data received from the data flow unit  46 , and when data is read from the tape  20 , it performs analog-digital conversion and demodulation for data read by the head assembly  28 .  
         [0010]     In some types of tape drives the head control unit  52  and head assembly can be considered to be essentially a second order type spring-mass actuator system. Actuator systems of this type can have a fundamental resonance that is normally within the loop bandwidth. One of the common features of a second order type spring-mass actuator system is the lack of damping unless damping is designed in as part of the mechanical assembly. Damping provides a controlled step response to the actuator. Too much damping will make the system sluggish while the lack of damping will cause it to ring at the fundamental frequency.  
         [0011]     In some applications, damping is achieved by mechanical means, such as the use of grease, ferro fluids, etc. A system of this type is taught in U.S. Pat. No. 5,739,984. In other applications, either the PES or back EMF signal is used to estimate the velocity state variable for the head assembly  28  to be damped, and this information is fed back to the servo control unit  44 . U.S. Pat. 6,359,748 teaches such use of a back EMF signal. However, these two methods have certain disadvantages. For example, PES is continuous only in defined zones and also it has the relative position information between the head and the tape making the estimation process rather difficult and limited to the availability of PES. In some situations the servo heads are very near the edges of a servo band. Shock and vibration disturbances as well as large lateral tape motion due to staggered wrap tape layers may cause the servo head to move outside the servo band thereby causing loss of the PES. The back EMF signal is difficult to use since its value is normally dependent on the electrical and physical parameters of the actuator, inductance, coil resistance, sense resistance, and magnetic characteristics, making the task to estimate the actuator velocity as a function of back EMF a very difficult and potentially inaccurate. Moreover the back EMF signal is not tunable.  
         [0012]     For systems which have predefined sections of tape where the feedback signal is located, such as the Linear Tape Open (LTO) servo feedback method, discontinuous type feedback signals can present a problem. Discontinuous type servo signals occur for e.g. the following reasons: If the tape suddenly moves up or down the head can leave the servo band. While the head is tracking very near the edges of the servo band external shock or vibration can push the head off the servo band.  
         [0013]     The use of a VCM with little or no damping in an environment that exhibits discontinuous type feedback can cause the head to oscillate at its natural frequency. With the head oscillating, the reacquisition of tracking normally takes time which results in reduction of system performance.  
         [0014]     Accordingly, it is an object of the present invention to provide a system and method to reduce head oscillation. It is another object of the invention to reduce head oscillation by providing a tunable damping system including an optical sensor to sense the position of the head.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present invention and, together with the detailed description, serve to explain the principles and implementations of the invention.  
         [0016]     In the drawings:  
         [0017]      FIG. 1  is a view of a conventional tape drive system.  
         [0018]      FIG. 2  is a block diagram showing an exemplary arrangement of a control card and tape drive according to the prior art.  
         [0019]      FIG. 3  is an exploded view of a portion of an optical position sensor according to a preferred embodiment of the present invention.  
         [0020]      FIG. 4  is a view of the components shown in  FIG. 3  with the components assembled.  
         [0021]      FIG. 5  is a view of the optical position sensor according to a preferred embodiment of the present invention as connected to the read-write head and the VCM.  
         [0022]      FIG. 6  is a cross sectional view of the optical position sensor, the read-write head and the VCM.  
         [0023]      FIG. 6A  is a partially exploded, isometric view of the optical position sensor, the read-write head and the VCM.  
         [0024]      FIG. 7  is a block diagram of a control system according to a preferred embodiment of the present invention.  
         [0025]      FIG. 8  is another block diagram of a control system according to a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0026]     Embodiments of the present invention are described herein in the context of a system and method for tape drive control. Those of ordinary skill in the art will realize that the following detailed description of the present invention is illustrative only and is not intended to be in any way limiting. Other embodiments of the present invention will readily suggest themselves to such skilled persons having the benefit of this disclosure. Reference will now be made in detail to implementations of the present invention as illustrated in the accompanying drawings. The same reference indicators will be used throughout the drawings and the following detailed description to refer to the same or like parts.  
         [0027]     In the interest of clarity, not all of the routine features of the implementations described herein are shown and described. It will, of course, be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made in order to achieve the developer&#39;s specific goals, such as compliance with application- and business-related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art having the benefit of this disclosure.  
         [0028]     In accordance with the present invention, the components, process steps, and/or data structures may be implemented using various types of operating systems, computing platforms, computer programs, and/or general purpose machines. In addition, those of ordinary skill in the art will recognize that devices of a less general purpose nature, such as hardwired devices, field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), or the like, may also be used without departing from the scope and spirit of the inventive concepts disclosed herein.  
         [0029]     Turning now to  FIGS. 3-6A  there is shown an optical position sensor  59  which includes optical emitter assembly  60 , which includes a light emitting diode, LED  62 , and light sensor assembly  64 . The emitter assembly  60  and light sensor assembly  64  are mounted in housing body  66  and housing cap  68 . The optical position sensor  59  is assembled with read-write head assembly  72  and VCM  102 . An adjustable screw  74  is attached in threaded engagement with a center spring clamp  78 .  FIG. 6  shows the adjustable screw  74  located adjacent to the LED  62  so that the head of the screw  74  partially interrupts the light beam from the LED  62  to the light sensor assembly  64 . The adjustable screw can be rotated to move up and down to a selected position as indicated by arrow  76  for the purpose of calibration.  
         [0030]     As best shown in  FIG. 6A , the VCM  102  comprises a voice coil assembly  80  which is substantially cylindrical, and the voice coil assembly  80  is located in a cylindrical bore formed in a voice coil housing  82 . A top spring element  84  is connected to the top of the voice coil assembly  80  and the voice coil housing  82  by center spring clamp  78 , and the top spring element  84  provides a flexible connection between assembly  80  and housing  84 . A similar bottom spring element, not shown, is connected to the bottom of the voice coil assembly  80  and the voice coil housing  82 . The top and bottom spring elements provide a flexible connection and permit limited vertical motion of the voice coil assembly  80  relative to the voice coil housing  82 . The read-write head assembly  72  is rigidly connected to the center spring clamp  78 .  
         [0031]     It should now be understood that the read-write head  72 , the voice coil assembly  80 , the center spring clamp  78  and adjustable screw  74  are all rigidly connected to each other so that they all move up and down together as a unit. On the other hand, optical position sensor  59  is isolated from those components so that optical position sensor  59  does not move with those components. Rather, the optical position sensor  59  is rigidly connected to the voilce coil housing  82 , which, in turn is rigidly connected to the housing of the tape drive. Accordingly, as the read-write head assembly  72  moves up and down as indicated by arrow  86 , the head of the adjustable screw  74  partially interrupts the beam of light received by the light sensor assembly  64  from the LED  62 , and the extent of light interruption is directly proportional to the relative vertical position of the optical position sensor  59  with respect to the read-write head assembly  72 . The light sensor assembly  64  produces electrical signals corresponding to the light it receives, and the electrical signals are used by the control system discussed hereinafter.  
         [0032]     Turning now to  FIG. 7  there is shown a block diagram schematically illustrating a control system  98  according to a preferred embodiment of the present invention. The control system includes a summer  101 , an amplifier  100 , and a VCM  102 , which is affixed to a head assembly  28 . The gain of the amplifier  100  is Ka, and the transfer function of the VCM and head assembly is Gp. The control system  98  further includes feedback loop  104  including the optical sensor  60  and a filter  106 , and the block  108  indicates that the gain C of the feedback loop can be controlled. The filter  106  is AC coupled and has the transfer function:  
       F   =     ks       τ   ⁢           ⁢   s     +   1           
 
         [0033]     The variable k is the gain and τ is the time constant. The filter  106  can be a lead-lag filter which differentiates the input signal and applies low pass filtering.  
         [0034]     The control system  98  is connected so that the summer  101  receives a signal from the tracking loop and subtracts from the tracking loop signal a signal from the feedback loop  104 . The signal from the tracking loop represents the target position of the VCM and is generated by the servo control unit  44  and head control unit  54 . However, the signal from the tracking loop can be generated by other conventional means.  
         [0035]     In practice, variability in the gain of optical sensor  60  due to aging, temperature, humidity and lot to lot variations could result in variability of damping for the system. Furthermore, excessive damping could cause a sluggish and slow system while insufficient damping may result in too much overshoot during transients. Therefore it is desirable to have the ability to control the extent of damping to account for the particular characteristics of a system. In other words, the damping is tunable.  
         [0036]     Moreover, the ability to control damping is desirable in order to obtain a predictable response of the system in both the gain and phase of the loop. For example, for systems which we have modeled we have found that higher damping provides more phase at the critical region, which is desirable for phase margin and stability. However, lower damping provides more suppression in the critical region. Therefore there is an optimum damping value that will satisfy both the phase and suppression requirements. Consequently we provide a calibration algorithm that adaptively tunes the damping to maintain the optimized value.  
         [0037]     We have found that controllable damping can be accomplished by calibrating the feedback loop  104  using the LED current. This is true because LED current is proportional to the damping applied by the feedback loop  104 .  
         [0038]     To calibrate the feedback loop  104  a calibration system  120  shown in  FIG. 8  is used. The calibration system  120  includes a damping tuning algorithm  122  which can execute in a microprocessor such as microprocessor  38 . The system also includes two digital to analog converters (DAC&#39;s)  124  and  126  to receive digital signals computed by the damping tuning algorithm  122 . The first DAC  124  transmits analog signals to the summer  101  and the second DAC  126  establishes setting for the LED current to the optical sensor  60 .  
         [0039]     In operation, the damped VCM position signal controls the position of the head assembly  28 . As represented by summer  130 , lateral motion of the tape is added to lateral motion of the head assembly, and the resulting total motion is measured by conventional servo control system  44  to generate PES  132 .  
         [0040]     The damping tuning algorithm  122  sends a set of digitized sine wave signals to the VCM amplifier reference DAC  124  that controls the line labeled “From Tracking Loop” in  FIG. 7 . The damping tuning algorithm  122  collects PES data as the tape is moved across the head at a constant speed. The damping tuning algorithm  122  implements this as a background task as the tape is moving and the drive is not doing read/write data transfer but rewinding or searching for a location. Therefore the normal operation of the drive is not interrupted. This background calibration continuously maintains optimum damping.  
         [0041]     The damping tuning algorithm  122  uses a curve fit algorithm to calculate the transfer function of PES to the sensor current LED setting using a second order function fit. It does this for three different LED currents, high, medium and low. Once the damping tuning algorithm  122  has computed the damping coefficients for all  3  different LED currents, the algorithm  122  fits a third order function to calculate the LED current for the targeted damping coefficient. It can now load this value in the LED current control DAC  126 .  
         [0042]     While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art having the benefit of this disclosure that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.