Abstract:
An embodiment of the present invention includes a method for driving a voice coil motor in response to signals from a feedback network that senses voice coil motor velocity. The method includes steps of providing a drive signal to an H-bridge for a first interval. At the end of the first interval, the H-bridge is placed in a high impedance state. Following a pause for a second interval during which transient voltages extinguish, a sample and hold circuit is coupled to the voice coil motor. The sample and hold circuit measures a voltage from the voice coil motor that is directly proportional to voice coil motor velocity and thus is directly related to head velocity. After the sample and hold circuit measures the voice coil motor voltage, the input to the sample and hold circuit is disabled. An output signal from the sample and hold circuit is coupled to the feedback network and thus to the H-bridge. As a result, voice coil motor and head velocity is more accurately controlled, reducing probability of collision between heads and discs in a disc drive and thereby increasing reliability of the disc drive.

Description:
TECHNICAL FIELD 
     This invention relates to improvements in electronic circuitry used in moving read/write heads in a memory disc system for use with computers, and, more particularly, to improvements in such circuitry for providing drive signals to a voice coil motor for such a system. 
     BACKGROUND OF THE INVENTION 
     Voice-coil motors are linear actuators that are widely used for moving heads and their support assemblies across discs in computer system disc drives in order to read data from or write data to the disc and in activating or deactivating disc drives. The heads float across the disc surface on a cushion of air resulting from rotation of the discs. 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, 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 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 in an area removed from areas of the disc that store data no longer provides adequate safeguarding of the head or of the disc when the computer system is not in use and particularly when the head is deployed from the parked position. 
     In increasing numbers of disc drives, the head is parked by causing the head 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 head support assembly reaches the end of the ramp, the head support assembly 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 head support assembly from the latch. The head support assembly then traverses 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 head support assembly reaches the end of the ramp in order to maintain the head in proximity to the disc without collision between the head and the disc. As a result, the controller must provide the proper drive signals to the voice coil motor resulting in the correct speed for the head when the head support assembly exits the ramp. 
     One method for driving the voice coil motor is to apply 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 resistive voltage (I.R.) and the BEMF, which varies with voice coil motor velocity V M . As a result, the applied voltage is not the actual BEMF of the voice coil motor. 
     Some conventional voice coil motor controller circuits employ a digital to analog converter circuit for providing analog control signals to the voice coil motor controller in response to digitally preprogrammed profiles. However, these conventional controller circuits have limited ability to compensate for wearing of the ramp and of the portions of the head supporting assembly that are in contact with the ramp. Additionally, conventional controller circuits have limited capability for providing control signals responsive to head velocity variations originating from other sources, such as motion of the disc drive. 
     In prior art approaches to driving voice coil motors and compensating for the BEMF, as described in U.S. Pat. Nos. 5,566,369 and 5,297,024, both issued to F. Carobolante, a current sensing resistor is coupled in series with the voice coil motor. A differential buffer amplifier has inputs coupled to the terminals of the current sensing resistor and provides an output signal that is proportional to a current through the voice coil motor. A comparison circuit then allows the current through the voice coil motor to be corrected to a desired value. However, the effective resistance of the voice coil motor causes some of the energy from the current through the voice coil motor to be lost as heat. As a result, this form of feedback, while providing improved performance for the voice coil motor, does not result in optimal performance, especially as voice coil motor characteristics change with age, temperature and the like. 
     SUMMARY OF THE INVENTION 
     In several aspects, the present invention includes circuits and methods for providing feedback from the motion of a head to a voice coil motor controller circuit to correct head velocity during ramp loading of the head from a disc into a storage position and particularly during ramp unloading from the storage position into proximity to the disc. As a result, voice coil motor velocity may be monitored and corrected to compensate for temperature-induced mechanical changes and also for wear of moving components that are in contact with other components. 
     In one aspect, the present invention includes a power supply circuit coupled to the voice coil motor that in turn is coupled to the head. A controller provides signals to the voice coil motor to correct voice coil motor velocity in response to signals from a feedback network. The feedback network includes a sample and hold circuit that is coupled to the voice coil motor during intervals when the power supply circuit is not providing drive signals to the voice coil motor. 
     In another aspect, the present invention includes a method for driving a voice coil motor in response to signals from a feedback network that senses voice coil motor velocity. The method includes steps of providing a drive signal to an H-bridge for a first interval. At the end of the first interval, the H-bridge is placed in a high impedance state. Following a pause during a second interval while transient voltages extinguish, a sample and hold circuit is coupled to the voice coil motor. The sample and hold circuit measures a voltage from the voice coil motor that is directly proportional to voice coil motor velocity and thus is directly related to head velocity. After the sample and hold circuit measures the voice coil motor voltage, the input to the sample and hold circuit is disabled. An output signal from the sample and hold circuit is coupled to the feedback network and thus to the H-bridge. As a result, head velocity is more accurately controlled, reducing probability of collision between the heads and the discs and thereby increasing reliability of the disc drive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified block diagram of a voice coil motor driving circuit, in accordance with embodiments of the present invention. 
     FIG. 2 is a simplified schematic diagram of the feedback network of FIG. 1, in accordance with embodiments of the present invention. 
     FIG. 3 is a simplified block diagram of a disc drive, in accordance with embodiments of the present invention. 
     FIG. 4 is a simplified flow chart of a process for inactivating and activating a head for a disc drive, in accordance with embodiments of the present invention. 
     FIG. 5 is a graph showing voice coil motor current (top trace) and voltage (bottom trace) during the process of FIG. 4, in accordance with embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a simplified block diagram of a voice coil motor driving circuit  10 , in accordance with embodiments of the present invention. The driving circuit  10  includes a controller  12  having a first input  14  coupled to a computer system and a second input  16  coupled to an output of a feedback network  18 . 
     The feedback network  18  has an input  20  coupled to the output of a sample and hold circuit  21 . 
     The sample and hold circuit  21  includes a high input impedance amplifier  22  having a first input  24  and a second input  26 . In one embodiment, the high input impedance amplifier may be a FET input operational amplifier  22 . A capacitor  27  is coupled across the first  24  and second  26  inputs. Switches  28  and  30 , which may be solid state switches such as pass gates or FET switches, or other devices that act to couple or decouple a voice coil  31  from the capacitor  27  in response to sampling signals from an output  34  of the controller  12 . In one embodiment, the switches  28  and  30  are formed from a pair of isolation FETs in the sample and hold circuit  21 . 
     Outputs  34 ,  36 ,  38 ,  40  and  42  of the controller  12  are coupled to a power supply circuit  95 . A preferred power supply circuit  95  includes FETs  44 ,  46 ,  48  and  50  having their respective outputs coupled via lines  33 ,  35  to the voice coil  31  and that are coupled in a conventional “H-bridge” configuration. The transistors are all N-channel type in one design or, if desired, transistors  48  and  50  are P-channel in an alternative design. In one embodiment, the FETs  44 ,  46 ,  48  and  50  are constructed such that they could be modeled as an FET having an integral diode with an anode coupled so a source of the FET and having a cathode coupled to a drain of the FET. As a result, signals on the lines  33  and  35  cannot have voltage excursions greater than one forward-biased diode voltage above the power supply voltage or below ground. In one embodiment, the controller  12  provides analog control signals to pairs  44 ,  48  or  46 ,  50  of the FETs to provide current to the voice coil  31  to drive the head (shown in FIG. 3) in a first or a second direction, or turn OFF all of the FETs  40 ,  44 ,  48  and  50  to decouple external power sources from the terminals of the voice coil  31 . 
     It will be appreciated that other arrangements may be used to implement the connection to sample and hold circuit  21 . For example, the controller  12  could cause one side of the other of the voice coil  31  to be grounded through the transistor  44  or  46 , with another side of the voice coil  31  being coupled to one side of the capacitor  27  and the other side of the capacitor  27  being coupled to ground. In this embodiment, the amplifier  22  may be implemented as a one-sided voltage follower, e.g., an operational amplifier  22  configured to provide, for example, unity gain. 
     FIG. 2 is a simplified schematic diagram of the feedback network  18  of FIG. 1, in accordance with embodiments of the present invention. A feedback signal at the input  20  is added to an analog control signal V IN  and the resulting voltage is then compared to a reference voltage V REF  by an amplifier  55  having a gain A E  that is set by a ratio of resistors  57  and  59 . As a result, when the comparison between the voltage V REF  and the sum of V IN  and the output voltage from the sample and hold circuit  21  indicates that the heads are moving too slowly, a larger drive signal is generated by the controller  12  in response to the output signal from the feedback network  18  in order to speed the voice coil motor up. Conversely, when the comparison between the voltage V REF  and the sum of V IN  and the output voltage from the sample and hold circuit  21  indicates that the head is moving too fast, a reduced drive signal is generated by the controller  12  in response to the output signal from the feedback network  18  in order to slow the voice coil motor down. An output signal from the amplifier  55  is then applied to the input  16  to the controller  12 . 
     Conventional voice coil motor controller circuits employ a digital to analog converter circuit (not shown) that outputs an analog control signal V IN  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  31  that result from a dc resistance R MOTOR  of the voice coil  31 . 
     In one embodiment, the sample and hold circuit  21 , the feedback network  18  and the controller  12  are integrated into a single integrated circuit. The capacitor  27  may be external to the integrated circuit. In one embodiment, the H-bridge is also external to the integrated circuit. while in another embodiment, the FETs  44 - 50  in the H-bridge are included in the integrated circuit. The integrated circuit may be formed using known processes, such as full CMOS or BiCMOS that combines complementary metal-oxide-semiconductor transistors with bipolar transistors. 
     FIG. 3 is a simplified block diagram of a disc drive  66 , in accordance with embodiments of the present invention. The disc drive  66  is coupled to a host computer  68  through a controller  70  that provides instructions to a disc drive microprocessor  72 . The disc drive microprocessor  72 , in turn, provides commands to control logic  74 , which decodes the commands into control signals. Some of these control signals are coupled to the voice coil motor drivers  10 . A voice coil motor  75  that includes the voice coil  31  of FIG. 1 moves in response to the control signals, causing a head support system  76  to move heads  78  across discs  80 , or to park or unpark the heads  78 . A spindle motor and spindle motor drive circuit  82  cause the discs  80  to rotate in response to control signals from the control logic  74 . Read/write head electronics  84  are also responsive to control signals from the control logic  74 . The read/write head electronics  84  deliver read data from the discs  80  to the control logic  74  to read data from the discs  80  and write data from the control logic  74  to the heads  78  to write data to the discs  80 . 
     FIG. 4 is a simplified flow chart of a process  100  for inactivating and parking, or activating and unparking, the heads  78  of FIG. 3 for the disc drive  66 , and FIG. 5 is a graph showing voice coil  31  (FIG. 1) current  120  (top trace) and voltage  128  or  130  (bottom trace) during the process  100  of FIG. 4, in accordance with embodiments of the present invention. In a step  102 , the voice coil motor driving circuit  10  of FIGS. 1 and 3 supplies drive signals to one of the pairs of FETs  44 ,  48  or  46 ,  50  to move and park the heads  78  of FIG. 3 when the disc drive  66  is to be deactivated as part of a normal system shutdown, or to move and unpark the heads  78  when the system is to be reactivated as part of a normal system boot operation. In a step  104 , the drive signals from the voice coil motor driving circuit  10  are maintained during a first interval having a first predetermined length. In one embodiment, the first predetermined length is about one millisecond, although longer or shorter intervals may be used. The top trace  120  of FIG. 5 has a first segment  122  corresponding to a portion of the drive signal of the step  102  during the inferral of the step  104 . 
     In a step  106 , the voice coil motor driving circuit  10  supplies a control signal to set all of the FETs  44 ,  46 ,  48  and  50  of FIG. 1 to a high impedance condition, i.e., turns OFF all of the FETs  44 ,  46 ,  48  and  50 , at a time corresponding to the end of the first segment  122  and the beginning of a second segment  124  of FIG.  5 . This creates an open circuit on both ends of the voice coil  31 . In one embodiment, the current formerly passing through the inductive voice coil  31  is shunted through the integral diodes in the FETs  44 ,  46 ,  48  and  50 , causing the voltage to be clamped to the power supply or ground, as shown in FIG.  5 . During the second segment  124 , the voice coil  31  of FIG. 1 exhibits a voltage (lower trace, FIG. 5)  128  or  130  given by Ldi/dt, where L represents an inductance of the voice coil  31  and di/dt represents the change in current through the voice coil  31  per unit time. 
     In a step  108 , the process  100  pauses for a second interval lasting for a second predetermined length that is longer than the length of the second segment  124  of FIG. 5 in order to allow the Ldi/dt voltage  128  or  130  during the second segment  124  to extinguish. In a step  110 , during a time represented in part by a segment  126  of the top trace of FIG. 5, the process  100  triggers the sample and hold circuit  21  of FIG. 1 to measure the BEMF across the voice coil  31  of the voice coil motor  75  of FIG.  3 . The BEMF is directly related to the velocity of the voice coil motor  75  because it is due to relative motion of the voice coil  31  and a magnet (not shown) in the voice coil motor  75 . The BEMF is equal to K e V M , where K e  is readily calculated. The segments  124  and  126  together represent a pause of between 50 and 200 microseconds, although longer or shorter intervals could be used, depending on the inductance L of the voice coil  31  in the voice coil motor  75 , parasitic resistance R MOTOR  in the voice coil  31 , friction and other factors. During the segments  124  and  126 , the head  78  continues to move. Therefore, the BEMF generated by the motion of the head  78  can be used to calculate the velocity V M  of the voice coil motor  75 . In one embodiment, the voice coil motor driving circuit  10  includes nonvolatile memory (not shown) coupled to the disc drive microprocessor  72  for storing delay parameters for different voice coils  31  employed in different disc drives  66 . 
     In a step  112 , an output signal from the sample and hold circuit  21  is supplied to the feedback network  18  of FIGS. 1 and 2. In a query task  114 , the process  100  determines if the heads  78  (FIG. 3) have reached a terminal position, either latched and parked, or unparked and deployed on the disc  80 . When the query task  114  determines that the heads  78  have not yet reached a terminal position, control passes back to the step  102  and a revised drive signal incorporating feedback from the feedback network  18  is sent to the FETs  44 ,  48  or  46 ,  50 . The steps  102 - 114  iterate until the query task  114  determines that the heads  78  have reached a terminal position, i.e., are either parked or unparked. Typically, this iteration has a periodicity of between 800 microseconds and two milliseconds. When the query task  114  determines that the heads  78  have reached a terminal position, the process  100  ends. 
     Disc drives  66  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. The circuits of the present invention may be implemented in an integrated circuit, with the improvements of the present invention resulting in very little additional silicon area being needed. The methods and apparatus of the present invention compensate for effects of wear in head deployment apparatus. Programmable delays may allow a variety of different types of disc drives to be improved with a single integrated circuit. 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.