Abstract:
A voice coil motor control circuit provides control signals to a voice coil motor circuit drivel that is coupled to a voice coil motor. A current sensing resistor is coupled in series with the voice coil motor. The control circuit includes a sense amplifier having inputs that couple to the current sensing resistor and includes a feedback circuit that includes an input and also includes an output that couples to the voice coil motor driver. In a first mode of operation, the feedback circuit input is coupled to an output of the sense amplifier. The control circuit also includes an inverting operational amplifier. In the first mode of operation, the inverting operational amplifier is bypassed. In a second mode of operation corresponding to deployment of a read/write head from a parked position onto the disc, the inverting operational amplifier is coupled in series between the sense amplifier output and the feedback circuit input. As a result, a velocity of the voice coil motor is more precisely and accurately controlled.

Description:
TECHNICAL FIELD 
     This invention relates to improvements in electronic circuitry used in moving read/write heads in memory disc drives for use with computers, and, more particularly, to improvements in such circuitry for providing drive signals to voice coil motors for such disc drives. 
     BACKGROUND OF THE INVENTION 
     Voice coil motors are linear actuators that are widely used for moving read/write heads and their support assemblies across discs in computer system disc drives in order to read data from or write data to the disc. Voice coil motors also remove the head from the areas of the disc that store data when the disc drive is turned off and deploy the head onto the disc when the disc drive is turned on. The head floats across the disc surface on a cushion of air resulting from rotation of the disc. In a conventional disc drive, the disc is roughened on at least portions of the disc surface to obviate sticking of the head to the disc surface as the disc is spun from a stop to an operating speed. 
     As data densities on the discs have increased, the need for greater precision and accuracy in head positioning has also increased. Additionally, spacings between the heads and the discs have decreased to a point where the roughening of the disc surface is impractical. As a result of these changes, a prior art practice of “parking” the head in the innermost data track no longer provides adequate safeguarding of the head or of the disc when the computer system is not in use. 
     In increasing numbers of disc drives, the head is parked by causing the head and support assembly to traverse a ramp to remove the head from proximity to the disc when the disc drive is deactivated as the system is shut down. When the support assembly for the head reaches the end of the ramp, the head is latched into a storage position. The head then cannot collide with the disc if the disc drive is jarred or bumped, avoiding one potential source of damage to the head or to the disc. 
     As the system is reactivated, the head is unparked by releasing the latch in response to a UNPARK HEAD command. The head and support assembly then traverse the ramp towards the disc in response to signals delivered to the voice coil motor from a controller. The head must be moving with the correct speed when the support assembly arrives at the end of the ramp in order to be maintained in proximity to the disc without collision between the head and the disc. As a result, the controller must provide drive signals to the voice coil motor resulting in the correct speed for the head when the support assembly for the head exits the ramp. 
     One method for driving a voice coil motor includes applying a constant voltage to a voice coil in the voice coil motor. However, the voice coil motor generates a back electromotive force (BEMF) because the voice coil is moving in a magnetic field. The actual voltage driving the voice coil motor thus is the sum of the applied voltage and the BEMF, which varies with voice coil motor velocity v M . As a result, the applied voltage is not the actual voltage driving the voice coil motor. 
     FIG. 1 is a simplified schematic diagram of a driving circuit  10  and voice coil  11  of a voice coil motor in a disc drive, in accordance with the prior art. The voice coil has a first terminal  11 ′ and a second terminal  11 ″. The driving circuit  10  includes a current sensing resistor  12  having a resistance R SENSE  and coupled to the voice coil  11 . A voltage across terminals of the current sensing resistor  12  is proportional to the current through the voice coil  11  which is driven by power amplifiers  13 ,  14  and  15  having a gain A P , as is explained below in more detail. The voltage across the current sensing resistor  12  is sensed by a sense amplifier  16  having a gain A S  to provide a feedback signal. The feedback signal is added to an analog control signal V IN  and the resulting voltage is then compared to a reference voltage V REF  by an input amplifier  17  having a gain A E  that is determined by resistor  18  and RC network  19 . An output signal from the input amplifier  17  is then applied to the voice coil  11  by the drive amplifiers  13 - 15 . 
     Conventional voice coil motor controller circuits employ a digital to analog converter circuit (not shown) for providing the analog control signal V IN  to the driving circuit  10  in response to digitally preprogrammed profiles. However, these voice coil motor controller circuits have limited ability to compensate for effects due to wearing of the ramp and of those portions of the head supporting assembly that are in contact with the ramp. Additionally, the feedback provided by the driving circuit  10  does not compensate for voltage errors in the voltage actually present in the voice coil  11  that result from a dc resistance R MOTOR  of the voice coil  11 . 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention includes a voice coil motor driver circuit providing feedback from a voice coil motor coupled to the driver circuit in a disc drive. The voice coil motor driver circuit provides positive feedback having a first loop gain in a head parking or unparking mode of operation and provides negative feedback together with a second loop gain in other modes of operation. The voice coil motor driver circuit is thus able to compensate for errors in a voltage driving the voice coil motor resulting from a dc resistance of a voice coil in the voice coil motor. The voice coil motor driving circuit is also able to compensate for changes in the voice coil motor properties with time or with temperature changes and for effects caused by motion of the disc drive. As a result, head velocity is more precisely controlled during a head unparking operation, reducing probability of damage to the head or disc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic diagram of a driving circuit and voice coil for a voice coil motor in a disc drive, in accordance with the prior art. 
     FIG. 2 is a simplified block diagram of an embodiment of a disc drive, in accordance with embodiments of the present invention. 
     FIG. 3 is a simplified schematic diagram of an embodiment of a first control circuit for a voice coil motor useful in the embodiment of the disc drive of FIG. 2, in accordance with embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 is a simplified block diagram of an embodiment of a disc drive  20 , in accordance with the present invention. The disc drive  20  is coupled to a host computer  22  through a controller  24  that provides instructions to a disc drive microprocessor  26 . The disc drive microprocessor  26 , in turn, provides commands to control logic  28 , which decodes the commands into control signals, some of which are coupled to a voice coil motor driver circuit  30 . A voice coil motor  32  that includes the voice coil  11  of FIG. 1 moves in response to these control signals, causing a head support system  34  to move heads  36  across discs  38 , or to park or unpark the heads  36 . A spindle motor and spindle motor drive circuit  40  cause the discs  38  to rotate in response to other control signals from the control logic  28 . A read/write (R/W) head circuit  41  is coupled between the heads  36  and the control logic  28 . In response to control signals from the control logic  28 , the R/W head circuit  41  delivers data read from the discs  38  by the heads  36  to the control logic  28  and delivers write data from the control logic  28  to the heads  36 , which write the data to the discs  38 . 
     FIG. 3 is a simplified schematic diagram of an embodiment of the voice coil motor driver circuit  30  for the voice coil motor  32  of FIGS. 1 and 2 that is useful in the embodiment of the disc drive  20  of FIG. 2, in accordance with the present invention. The voice coil motor driver circuit  30  includes first and second sense lines  42 ,  43  that are connected to first and second terminals  44 ,  45  at opposite sides of the sense resistor  12  (see also FIG.  1 ). The first and second sense lines  42 ,  43  are coupled to a sense amplifier  46 . The sense amplifier  46  corresponds to the sense amplifier  16  of FIG.  1  and includes a first operational amplifier  47  having non-inverting input  48  coupled to the first sense line  42  through a first input resistor  49 . The first operational amplifier  47  also has an inverting input  50  coupled to the second sense line  43  through a second input resistor  51  and has an output  52  that is also an output of the sense amplifier  46 . The sense amplifier  46  also includes a feedback resistor  53  that, in conjunction with the second input resistor  51 , sets the gain A S  for the sense amplifier  46 . 
     In one embodiment, the voice coil motor driver circuit  30  also includes a ramping amplifier  55  that includes a second operational amplifier  57  having a gain −K determined by a ratio of resistances of a resistor  58  and a resistor  59 . The resistor  58  is coupled to an inverting input  60  to the operational amplifier  57  and to an output  61  of the operational amplifier  57  that is also an output to the ramping amplifier  55 . The resistor  59  is coupled between the input  60  of the operational amplifier  57  and the output  52  of the sense amplifier  46 . A noninverting input  62  to the second operational amplifier  57  is coupled through a resistor  63  to the noninverting input  48  to the first operational amplifier  47 . The noninverting input  62  is also coupled to a reference voltage V REF . In one embodiment, the ramping amplifier  55  is included in a feedback path for the voice coil motor driver circuit  30  in head parking and unparking modes of operation and is bypassed in other modes of operation through operation of one or more switches  65 ,  70 , as is explained below in more detail. 
     The switch  65  may be realized as a pass gate or as a N-channel MOSFET switch. The switch  65  includes a first terminal  67  coupled to the output  52  of the sense amplifier  46  and a second terminal  68  coupled to the output  61  of the second operational amplifier  57  through the optional switch  70 . 
     The switch  65  also includes a control terminal  69  that is coupled to an output (not shown) of the control logic  28  (FIG.  2 ). 
     The switch  70  includes a first terminal  72  that is coupled to the output  61  of the second operational amplifier  57  and a second terminal that is coupled to the second terminal  68  of the first switch  65 . The switch  70  also includes a control terminal  74  that is coupled to an output (not shown) of the control logic  28  (FIG.  2 ). The switch  70  may be realized as a solid state switch such as a pass gate or as a N-channel MOSFET switch. 
     In the head parking and unparking modes of operation of the voice coil motor driver circuit  30 , the switch  65  provides an open circuit between the first and second terminals  67 ,  68  of the switch  65  in response to control signals from the control logic  28  that are coupled to the control terminal  69 . The switch  65  thus allows the ramping amplifier  55  to modify signals from the sense amplifier  46 . The switch  70  provides a short circuit between terminals  72 ,  74  in these modes of operation in response to control signals coupled to the control terminal  74  from the control logic  28 . The switch  70  thus couples the output  61  of the ramping amplifier  55  to an input  79  to a feedback amplifier  80  in these modes of operation. 
     In other modes of operation, the switch  65  provides a short circuit between the first and second terminals  67 ,  68 . The switch  65  thus couples signals from the output  52  of the sense amplifier  46  to the input  79  of the feedback amplifier  80 , bypassing the ramping amplifier  55 . The switch  70  decouples the output  61  of the ramping amplifier  55  from the input  79  to the feedback amplifier  80 , disabling the ramping amplifier  55 . 
     The feedback amplifier  80  includes a third operational amplifier  82  that corresponds to the amplifier  17  of FIG.  1  and acts as a summing circuit, combining signals from the first or second operational amplifiers  50 ,  57  with the signal V IN  (FIG. 1) from the control logic  28 . The feedback amplifier  80  includes the RC network  19  of FIG. 1 coupled between an output  84  of the third operational amplifier  82  and an inverting input  85  of the third operational amplifier  82 . The output  84  of the third operational amplifier  82  is also the output of the feedback amplifier  80 . The feedback amplifier  80  includes a resistor  86  that is coupled between the inverting input  85  and the input  79  to the feedback amplifier  80 . The resistor  86  and the RC network  19  set the gain A E  for the third operational amplifier  80 . A resistor  87  couples the inverting input  85  to the third operational amplifier  80  to signals V IN  from the control logic  28  (FIG.  2 ). In one embodiment, the signals V IN  from the control logic  28  are analog signals from a digital to analog converter (not illustrated), The digital to analog converter is provided with predetermined digital values by the disc drive microprocessor  26  (FIG. 2) to influence the velocity v M  of the voice coil motor  32 . 
     As a result, in one embodiment, the voice coil motor driver circuit  30  uses the ramping amplifier  55  to provide positive feedback during the park and unpark operations and also to approximately compensate for voltages resulting from the resistance R SENSE  of the current sensing resistor  12  or from the dc resistance R MOTOR  of the voice coil  11 . The positive feedback provided by the ramping amplifier  55  in the voice coil motor drivel circuit  30  does not result in oscillation when a magnitude of an open loop gain of the voice coil motor driver circuit  30  is less than unity. In turn, this requires that (R MOTOR +R SENSE )&gt;A S ·A P ·A E ·R SENSE ·2·K. 
     Additionally, the gain −K for the ramping amplifier  55  allows the magnitude of the loop gain for the voice coil motor driver circuit  30  in the bead parking and unparking modes of operation to be optimized for the resistance R MOTOR  and also for the resistance R SENSE . The loop gain for the voice coil motor driver circuit  30  in the head parking and unparking modes of operation can be different from the loop gain when the ramping amplifier  55  is bypassed. 
     The second operational amplifier  57  in the ramping amplifier  55  and ancillary circuitry associated with the ramping amplifier  55  require relatively little additional silicon area when the voice coil motor driver circuit  30  is fabricated as an integrated circuit. In one embodiment, the integrated circuit forming the voice coil motor driver circuit  30  may optionally include the power amplifiers  13 - 15 . 
     Also, the BEMF of the voice coil motor  32  is related to the actual motor velocity v M  as given by BEMF=K e v M , where K e  is a motor design constant whose value can be calculated with greater accuracy than is required for the motor velocity v M . The second operational amplifier  57  allows the actual BEMF in the voice coil  11  (FIG. 1) to be forced to a desired value by compensating for effects due to the resistance R MOTOR . As a result, the BEMF and K e  are known and thus the motor velocity v M  can be controlled more precisely during head parking or unparking than is possible without the ramping amplifier  55 . 
     In one embodiment, the present invention includes two switches  90  and  92 . The switch  90  has a control input  74  that is coupled to the control input  74  of the switch  70 . As a result, when the ramping amplifier  55  is included in the feedback path, the open loop gain reflects feedback via the resistor  83 , rather than the RC network  19 . 
     The switch  92  has a control input  69  that is coupled to the control input  69  of the switch  65 . As a result, when the ramping amplifier  55  is bypassed by the switch  65 , the open loop gain reflects feedback via the RC network  19 , rather than the resistor  83 . The switches  90  and  92  provide an additional technique whereby the open loop gain may be different in the two modes of operation. 
     Disc drives  20  including the head unparking and control circuitry for such applications may provide significant advantages over other types of disc drives, including reduced head and disc wear and increased data storage density leading to increased storage capacity. The present invention also allows increased overall disc drive reliability due to reduced probability of collision between the heads and the disc. Disc drives find application in most computers where, for example, operating systems as well as programs and data are stored and may be modified. 
     Improved disc head parking and unparking control circuits and methods have been described. Although the present invention has been described with reference to specific embodiments, the invention is not limited to these embodiments. Rather, the invention is limited only by the appended claims, which include within their scope all equivalent devices or methods which operate according to the principles of the invention as described.