Abstract:
A circuit ( 90 ) and method are presented to accurately determine a BEMF voltage of a VCM coil ( 20 ) after termination of a driving current in a first current direction in the coil ( 20 ). The circuit includes a circuit for activating selected VCM coil driver transistors ( 44–47 ) to apply a current to the coil ( 20 ) in a direction opposite the first current direction to generate a magnetic field to oppose eddy currents established in structures adjacent the coil ( 20 ) by the driving current. The time that the eddy current opposing current may be applied may be determined, for example, by determining a magnitude of the original current command, a time that the coil spends in flyback, or a magnitude of the original driving current, and adjusting the time of application of the eddy current opposing current accordingly.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of application Ser. No. 09/451,697, filed Nov. 30, 1999, now U.S. Pat. No. 6,768,977, which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to improvements in methods and circuits for operating voice coil actuator/motors (VCMs) of the type used in mass data storage devices, or the like, and more particularly to improvements in such methods and circuits that may be used to move the head mechanism of such VCM to a parked position from an operating position. 
   2. Relevant Background 
   A well-known hard disk drive assembly (HDA) is a typical mass data storage device of the type to which the invention pertains. Generally the HDA includes one or more rotating disks that carry a magnetic media to which data may be written, and from which previously written data may be read. The data is written to and read from the disk by one or more magnetic heads or transducers that are a part of a voice coil motor (VCM) assembly, which moves the heads to the desired locations at which data is to be written or read. 
   An exploded view of a portion of a typical HDA  5  is shown in  FIG. 1 . The HDA  5  includes a VCM apparatus  10  in conjunction with a plurality of rotating disks  12 . The VCM assembly  10  includes one or more arms  14  that are pivoted about a bearing point  16  to carry and move the heads or data transducers  18  radially inwardly and outwardly within the stack of data disks  12 . 
   The outboard end of the arm  14  carries a coil  20  that is selectively energized by currents from VCM positioning circuitry  22 . The outwardly extending end  24  of the arm  14  is located between two horizontal magnets  26  and  28 , which are mounted to base plates  30  and  32 . The base plates  30  and  32  and magnets  26  and  28  are spaced apart by spacers (not shown) to allow the arm and coil portions  24  and  20  to freely swing between the magnets  26  and  28 . The plates  30  and  32 , spacers, and magnets  26  and  28  are securely fastened to the base plate  34 . A top cover plate  35  encloses the top side of the base plate  32 . The two plates  32  and  35  may physically touch or barely touch each other. Thus, as the currents from the VCM positioning circuitry  22  are applied to the coil  20 , magnetic fields are established by the current induced field of coil  20  that can precisely position the heads  18  at a desired location under control of the VCM positioning circuitry  22 . 
   When the apparatus  5  is powered down, typically the head mechanism is moved to a position (not shown) at which the heads  18  are “parked” or “landed”, often at the inner radius of the disk. In other cases, such as when the head is parked on a ramp, they may be parked along the outer radius of the disk. In order to properly move the heads to the park position, generally a driving current is applied to the coil  20  that is of sufficient magnitude to bring the head assembly just to the park position. However, it will be appreciated that if the head mechanism is overdriven, the delicate head mechanism and other parts of the disk assembly may sustain damage. On other hand, if the head is underdriven, the head mechanism may not reach the park position, which may result in loss of the air bearing between the head and disk surface, which may also cause damage both to the head mechanism and to the underlying magnetic media of the disk assembly  12  above which the heads  18  fly. 
   The heads are positioned by the positioning circuitry  22 , also referred to herein as a servo circuit, of the type shown in  FIG. 2 , which also operates in the retraction or parking of the heads to their landing zone or landing ramp. The servo circuit  22  may incorporate a floating-terminal BEMF detection scheme (FLBD)  23  in its design to control the retract of the heads to their parked position. The purpose of FLBD is to extract the BEMF signal from the VCM terminal voltage difference, Vpn=Vp−Vn, at nodes  62  and  58 . This is done normally by turning off all four FET&#39;s  44 – 47  to let Vp and Vn on nodes  62  and  58  float for a short time. After the flyback current in the VCM coil decays to a predetermined level, which is defined to be at or near zero and the rate of change of the current is also at or near zero, Vpn theoretically will approximate the BEMF voltage, since with no current, there should be no voltage drop across resistor R 0    60 , resistor R SEN    55 , and the motor inductor L 0    49 . 
   One technique controlling a VCM is shown in U.S. patent application Ser. No. 09/388,508 now U.S. Pat. No. 6,184,645, filed Sep. 1, 1999, incorporated herein by reference. One technique measuring the BEMF of the coil of the actuator used in said application Ser. No. 09/388,508 now U.S. Pat. No. 6,204,629 is shown in U.S. patent application Ser. No. 09/193,803, filed Nov. 17, 1998, incorporated herein by reference. 
   With reference again to  FIG. 2 , the circuit  22  includes a VCM predriver circuit  42  that provides signals to drive transistors  44 – 47  in a selective manner by which current flows through the coil  20  of the VCM in one direction or the other to move the head of the VCM in the desired direction. Thus, for example, to move the head in one direction, transistors  44  and  45  are turned on to establish a current flow path between the voltage terminal  51  and a ground terminal  53  to move the head in a first direction. To move the head in the opposite direction, transistors  46  and  47  are turned on to establish a current flow path through the motor coil from the motor driving potential  51  to ground  53 . In the circuit embodiment shown, a sense resistor, R SEN ,  55  is shown in series with the motor inductance, L 0 ,  49  and the node Vn  58 . The resistance of the coil  20  is shown as resistor  60 , in series between the motor inductor  49  and the node, denoted Vp. For clarity, the remainder of the circuit elements of the VCM model  50 , described in detail below with reference to  FIG. 3 , are lumped into element  61 , except for the capacitance C 0    56  and the resistor R h    66 , which can be disregarded. 
   As mentioned, when the head is to be moved to the park position, one method that may be employed is to tristate the transistors  44 – 47 , wait a period of time to allow the flyback current to occur and dissipate down to a predetermined magnitude. Thus, after the flyback current has dissipated to the predetermined level, the voltage appearing between nodes  62  and  58  is measured, which, at least in theory, should represent the BEMF developed across the coil  20 . Since the BEMF has a value almost directly proportional to the speed of the coil of the VCM, knowing the velocity of the coil  20  enables the precise required drive current to be determined that will properly move the heads to the parked position at a controlled velocity. 
   However, in practice, it has been found that the BEMF that is measured using the prior art techniques does not always accurately represent the correct velocity of the coil  20 , and, consequently, the head assembly controlled thereby. As discussed below, we have determined that this is due at least in part to the influence of eddy currents induced in the structures adjacent the coil  20  of the VCM on the voltage induced into the coil during its movement at the same time that the BEMF is measured. 
   What is needed, therefore, is a method and circuit for more accurately determining the BEMF when the VCM drivers are tristated to enable the current needed to be applied to the coil to properly park the heads at a controlled velocity to be determined. 
   SUMMARY OF THE INVENTION 
   In light of the above, therefore, method and circuit are presented for more accurately determining the BEMF when the VCM drivers are tristated to enable the voltage needed to be applied to the coil to properly park the heads to be determined. 
   Thus, according to a broad aspect of the invention, a circuit to determine a velocity of a coil to which a driving current is applied in a magnetic field is presented. The circuit includes a circuit to terminate the driving current in the coil and a circuit to apply a current to the coil to create a magnetic field to oppose eddy currents established in structures adjacent the coil by the driving current. A circuit is provided for measuring the BEMF in the coil after the current has been applied to oppose the eddy currents. 
   The time that the eddy current opposing current may be applied may be determined, for example, by determining a magnitude of the original current command, a time that the coil spends in flyback by measuring, for example, the time that coil voltage exists above a predetermined magnitude, or a magnitude of the original driving current, and adjusting the time of application of the eddy current opposing current in accordance with this value. 
   According to yet another broad aspect of the invention, a circuit is presented to determine a BEMF voltage of a VCM coil after termination of a driving current in a first current direction in the coil. The circuit includes a circuit for activating selected VCM coil driver transistors to apply a current to the coil in a direction opposite the first current direction to generate a magnetic field to oppose eddy currents established in structures adjacent the coil by the driving current. The time duration of the current may be determined as described immediately above. 
   According to still another broad aspect of the invention, a circuit is presented for use in determining a velocity of a head assembly of a VCM after termination of a driving current in a coil of the VCM. The circuit includes a circuit for activating selected VCM coil driver transistors to apply a current to the coil of the VCM to create a magnetic field that opposes eddy currents established in structures adjacent the coil by the driving current. 
   According to yet another broad aspect of the invention, a method is presented for determining a velocity of a coil to which a driving current is applied in a magnetic field. The method includes terminating the driving current and allowing a flyback current in the coil to reduce to below a predetermined magnitude. After the flyback current has been reduced, the method includes applying a current to the coil of magnitude and direction to cancel eddy currents in structures adjacent the coil, and measuring a BEMF in the coil, wherein a magnitude of the BEMF is directly related to the velocity of the coil. 
   According to still yet another broad aspect of the invention, a method is presented for determining a BEMF voltage of a coil of a VCM after termination of a driving current in the coil. The method includes determining when the driving current has been terminated, and activating selected VCM coil driver transistors to apply a current to the coil to create a magnetic field to oppose eddy currents established in structures adjacent the coil by the driving current. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is illustrated in the accompanying drawings, in which: 
       FIG. 1  is a perspective view of a portion of a mass data storage device and associated VCM assembly with which the circuit and method in accordance with a preferred embodiment of the invention may be employed. 
       FIG. 2  is an electrical schematic diagram of a driver circuit used for positioning the head mechanism of the VCM of  FIG. 1 , in accordance with a preferred embodiment of the invention may be employed. 
       FIG. 3  is an electrical schematic diagram of a model of the VCM assembly of  FIG. 1 , in accordance with a preferred embodiment of the invention. 
       FIG. 4  is an electrical schematic diagram of a portion of one circuit that may be used to determine the time at which the flyback current has dissipated to a predetermined level to enable the measurement of the BEMF, in accordance with a preferred embodiment of the invention. 
       FIG. 5  is an electrical schematic diagram of a counter circuit that may be used in  FIG. 4 , in accordance with a preferred embodiment of the invention. 
   

   In the various figures of the drawing, like reference numerals are used to denote like or similar parts. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A model  50  of the voice control actuator/motor (VCM), according to a preferred embodiment of the invention, is shown in the electrical schematic diagram of  FIG. 3 . The model is more fully described in said copending application Ser. No. 09/451,697 now U.S. Pat. No. 6,768,977. The model  50  takes known VCM effects into account, including eddy current effects in all the structures in the neighborhood of the actuator coil, which have been unrecognized heretofore, and which, therefore, have not been modeled. The model  50 , therefore, is believed to be a more accurate representation of a physical VCM assembly and its associated electrical components than models used heretofore. 
   The model  50  includes a number of ideal model parts between the input terminals  52  and  54 , which model or represent the actual VCM terminals of a physical VCM, such as that shown in  FIG. 1 . A capacitor  56  is connected between the input terminals  52  and  54 , to represent the input capacitance of the system. The inductance of the motor coil  20  is modeled by serial inductor  64  and inductor  58  connected at point  67 . The inductor  64  represents a winding leakage inductance of the VCM coil  20 . The inductor  58  represents the mutual inductance between VCM coil and its metallic neighborhood including parts  26 ,  30 ,  28  and  32 . The low end of inductor  58  is connected to input terminal  54  through a voltage source  60 , which represents the back EMF (BEMF) of the coil  20 . The BEMF, of course, is a time varying quantity; therefore, the voltage source  60  is likewise a varying voltage source that follows the BEMF waveform generated in the physical device. 
   A resistor  62  is connected in series between the top input terminal  52  and the left end of the inductor  64 . The resistor  62  represents the resistance of the physical VCM coil  20 . A resistor  66  is connected from a node  67  between the inductor  64  and inductor  58  to the bottom end of the inductor  58 . The resistor  66  representing the magnetic hysteresis loss is in parallel with the VCM inductor  58 , and would typically be of very high value. Consequently, in many applications, the resistor  66  may be ignored. 
   As mentioned, one of the reasons that the physical VCM does not behave as predicted by prior art models is that the coil  20  of the VCM creates eddy currents in the adjacent magnets and other structures of the VCM assembly. The eddy currents do not self-extinguish as rapidly as the flyback current, and consequently result in the creation of a voltage across the coil when the excitation voltage has been removed. Thus, top and bottom current loops  68  and  70  are included in the model  50  to consider the eddy current effects. 
   The loop  68  includes a mutual inductor  72 , having an inductance equal to the value of the VCM mutual inductor  58 , an inductor  74 , and a resistor  76 , connected in series. The inductor  72  represents the mutual inductance between VCM coil and the top VCM magnetic plate. The magnet plate includes the top VCM magnet  28  and the surrounding structures, including the mounting plate  32  and top cover plate  35 , into which eddy currents are induced. The inductor  74  represents the leakage inductance of the top VCM magnet plate, and the resistor  76  represents the resistance of the top VCM magnet plate. 
   Likewise, the bottom loop  70  includes a mutual inductor  78  having a value equal to the mutual inductance of the VCM inductor  58 , and inductor  80  and a resistor  72 , all connected in series. The inductor  78  represents the mutual inductance between VCM coil and the bottom VCM magnetic plate, which includes the bottom VCM magnet  26 , and the surrounding structures, including the mounting plate  30  and base plate  34 , into which eddy currents are induced. The inductor  80  represents the leakage inductance of the bottom VCM magnet plate, and the resistor  72  represents the resistance of the bottom VCM magnet plate. The first and second loops  68  and  70  are interconnected, as shown, at one side of the inductors  72  and  78 . 
   First and second parasitic capacitors  86  and  88  are connected between the top and bottom ends of coil  58  and the interconnection nodes of inductors  72  and  74  and inductors  78  and  80 , respectively. The values of capacitors  86  and  88  may be very small. Consequently, they may be ignored in many applications. 
   With the recognition that the effects of the induced eddy currents affects the accuracy of the measurement of the BEMF, according to the above described model, their effects can now be taken into account in measuring the BEMF. More particularly, in accordance with a preferred embodiment of the invention, prior to measuring the BEMF, a current may be injected into the coil  20  that is of magnitude and polarity such that the eddy currents existing in the structures surrounding the coil  20  may be substantially cancelled. 
   One circuit  90  by which eddy currents can be canceled or significantly reduced is shown in  FIG. 4 . The circuit  90  includes a number of logic gates that are enabled by a float signal on line  92  that becomes high when the VCM predriver  42  ( FIG. 2 ) floats or tristates the driver transistors  44 – 47 . The signals at the driver circuit nodes  62  and  58  are applied to differential amplifiers  96  and  98 , respectively, which are referenced to ground or other reference potential  100  to produce outputs on respective output lines  102  and  104  when the input signals to the differential amplifiers  96  and  98  exceed the potential on the reference line  100 . 
   The outputs from the amplifiers  96  and  98  are connected to one input of each respective AND gate  106  and  108 , which are enabled by the float signal on line  92  that is applied to the other inputs thereof. The output signals on output lines  110  and  112 , therefore, indicate the direction that the flyback current is flowing in the motor coil  20 . 
   In addition, an exclusive OR (XOR) gate  114  receives the signals on lines  110  and  112  to produce an input to an AND gate  116 , which also is enabled by the signal by the float signal  92 . The output from the gate  116  on line  118 , therefore, represents a logic state that exists only during the time that the flyback current in the motor coil  20  is above the reference voltage on line  100 . The signal on line  118  thus represents an indication that the flyback is in existence, and issues a signal to enable AND gates  120  and  122  to which the output lines  110  and  112  from gates  106  and  108  are applied. 
   The output signals from gates  120  and  122  are applied on lines  121  and  123  to input terminals of a counter or timer  124 , which also is enabled by the float signal on float line  92 . The counter  124 , below described in detail, is configured to determine a time that the flyback voltage exists above a predetermined magnitude, then to determine a time that either of the signals produced by AND gates  120  or  122  is high. 
   Thus, after the termination of the flyback current, or more particularly after its dissipation to a predetermined level, the output signals from gates  120  and  122  are both low. In this state, the counter  124  counts down, or in an opposite direction from the count that was produced during the existence of a high state from either AND gate  120  or AND gate  122 . During the count down time, the counter  124  applies drive signals on lines  126 – 129  to selected pairs of AND gates  130 – 133  to produce control signals to respective pairs of drive transistors  44  and  45  or  46  and  47  in an opposite direction to the most recent drive current direction that existed in the motor coil  20 . The output gates  130 – 133  are enabled by a user supplied enable signal on line  134 , so that the user may, if desired, exclude the eddy current canceling feature provided by the invention by removing the enabling signal. 
   It should be noted that although reference is made to the counter counting up and down, it should be understood that the count may be performed by analog counting or timing devices, such as the time of charge and discharge of a capacitor. On the other hand, the timing may be performed by a physical digital counter that is clocked by appropriate clock pulses (not shown). 
   Details of the counter circuit  124  are shown in  FIG. 5 , to which reference is now additionally made. The counter  124  receives inputs from the detector circuit  90  on lines  121  and  123 , as well as the float signal on line  92 . As above described, the signals on lines  121  and  123  represent the existence of a flyback current in the coil  20 . The circuit  124  has a first portion  140  that conditions the signals on lines  121  and  123  for application to the output AND gates  142 – 145  to deliver the output signals on lines  126 – 129  to the AND gates  130 – 133 , described above. The circuit  140  insures that the flyback current indicating signals are sharp, and that they do not exist contemporaneously so that only two of the four output AND gates  142 – 145  will be on at the same time. 
   In addition, the circuit  124  includes a timing circuit  150 , which receives an input on line  152  that indicates that the flyback current exists on either input line  121  or  123 . The signal on line  152  is applied to a switch controller  154  that controls a switch  156 . When the flyback current exists, the switch controller  154  operates to charge a capacitor  160  by a current source  162 . On the other hand, when the flyback current does not exist, the switch controller  154  operates to discharge the capacitor  160  by current source  164 . 
   The charge on the capacitor is detected by first and second differential amplifiers  170  and  172 . The differential amplifier  170  is referenced to a voltage source  174  so that it can detect a minimum flyback voltage threshold, if desired. Thus, if the flyback current is not sustained for a predetermined minimum time, the eddy current canceling current is not applied. The output from the differential amplifier  170  is used to enable the switch controller  154  on line  153  to charge the capacitor  160  when the float signal exists on line  92 , if the capacitor charge has exceeded the predetermined threshold. 
   The second differential amplifier  172  operates to enable the output AND gates  142 – 145  so long as the charge on the capacitor exceeds the reference voltage determined by voltage source  176 . The output from the differential amplifier  172  is applied through an AND gate  178 , which is enabled by the output from the threshold detecting differential amplifier  170 . It should be noted that the AND gate  178  may include a predetermined time delay, if desired, depending upon the particular circuit requirements. When the output from the second differential amplifier  172  falls back to zero after the capacitor  160  has been discharged, the enable signal on line  173  is extinguished, disabling the AND gates  142 – 145 , and also resetting the output to the switch controller  154  on line  153 . 
   With reference again briefly to the operation of the switch controller  154 , the switch positions are determined by the states of the signals on input lines  152  and  153 , in accordance with the following table: 
   
     
       
             
             
             
             
           
         
             
                 
                 
             
             
                 
               152 
               153 
               Switch Position 
             
             
                 
                 
             
           
           
             
                 
               0 
               0 
               Ground 
             
             
                 
               1 
               0 
               Charge 
             
             
                 
               1 
               1 
               Charge 
             
             
                 
               0 
               1 
               Discharge 
             
             
                 
                 
             
           
        
       
     
   
   One of the advantages realized by the timing circuit  150  is that by appropriate selection of the values of currents sourced by current sources  162  and  164 , the timing of charge and discharge of the capacitor  160  can be independently controlled. Thus, during the existence of the flyback current in the winding  20 , the capacitor may be charged at a first charge rate, but during the subsequent discharge of the capacitor during which the eddy current canceling current is enabled, the capacitor may be discharged at a different rate. 
   It can be seen that by virtue of the action of the counter/timer  124 , the driving signals produced by the AND gates  142 – 145  exist for the necessary time to produce a current in coil  20  to eliminate or substantially reduce the eddy currents that exist in the structures surrounding the coil  20 . This time would need to be established for each different model of drive, since each drive model may be differently constructed from the other, but can be determined by characterizing the drive using the VCM model described above with reference to  FIG. 3 , by trial and error, or by other technique. 
   Thus, it can be seen that the time of application of the eddy current reversing current in the embodiment presently described is based upon the time duration of the flyback current. The magnitude of the eddy currents that are established in the structures surrounding the coil  20  which are to be cancelled is a function of the duration of the flyback current which is also related to the magnitude of the driving current established in the motor coil  20 . 
   It will be appreciated, of course, that other techniques may be employed for determining the time that the eddy current canceling current is applied. For example, a circuit may be employed to directly measure the magnitude of the driving current applied to the coil  20 . Alternatively, a circuit may be employed to apply an eddy current canceling current for a time duration directly related to the magnitude of the original current command. Those skilled in the art will recognize still other techniques that may be suitable for determining the time and magnitude of the eddy current canceling current it may be suitable in each particular application. 
   Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention, as hereinafter claimed.