Abstract:
A capacitor powered electronic emergency device retracts and parks an actuator into a resistive parking position with a crash stop barrier. Due to variations of the kinetic status of the actuator at the begin of the retraction, the actuator reaches the resistive parking position with different velocities, which can be at a level that causes the actuator to bounce off the crash stop barrier. A modulating pulsing of the energy flow extends the discharge time of the capacitor long enough to provide sufficient energy to compensate an eventual bounce-off. The use of transistors in the device keeps the internal energy consumption low. An increase of the pulsing frequency in dependence on the capacitor voltage results in a high average current flow.

Description:
FIELD OF INVENTION 
     The present invention relates to the field of power-off actuator retract devices in hard disk drives. 
     BACKGROUND OF THE INVENTION 
     Hard disk drives are the most commonly employed data storage devices in personal computers, which are being continuously improved to meet the demand for higher performance. The amount of data to be stored on a hard disk drive is rapidly increasing due to more sophisticated software applications and larger data files. At the same time, personal computers are becoming smaller in size. For example, new types of portable computers known as notebooks and miniaturized desktop computers have been introduced to meet the demands for less space consumption in an office environment. 
     The reasons described above together with an additional demand for faster access to the data stored on the hard disk drive are forcing the manufacturers to reduce the size of hard disk drives. 
     A typical hard disk drive includes the following: a number of spinning disks stacked above each other on a spindle, a disk controller, a rotary actuator and an actuator retract circuit. All these elements are mounted in a chassis or housing and supplied with external cable connectors. 
     The spinning disks have a magnetic recording layer for data storage. The rotary actuator consists of a number of arms equipped with heads for reading and writing data in generally radial and concentric tracks in the recording layers of the individual disks. The actuator is usually driven by an attached voice coil motor(VCM). Flexible cables are connected with the actuator and the controller to transmit signals to and from the heads and to power the VCM. The disk controller is typically an electronic circuit that controls all functions of the hard disk drive. 
     During regular operation of the drive, the controllers control the actuator motions including the movements to and from a parking position, at which the actuator is placed when the drive is not under operation. However, if the power supply to the drive is shut off unexpectedly, the actuator may not be in the proper parking position. Since the controller requires external power to operate, it cannot park the actuator after unexpected power supply shut-off. An independent retract circuit parks the actuator in such cases. 
     Such a retract circuit has to be able to power and control the retraction or withdrawal of the actuator from the disk surface into a parking position within a critical time period during which the spinning disks slow down to a minimal rotational speed. The minimal rotational speed guarantees sufficient supporting airflow between the disk surface and an air bearing surface of the read and write heads to keep them at a minimum flying height. In case the supporting airflow should fall beneath a critical value, the heads are likely to crash and damage the disk surface. Moreover, if the heads were to come to rest on the smooth disk surface, they would adhere to the disks through a process known as stiction. 
     There are two main methods for parking heads during power-off of the disk drive. The first is known as contact start-stop (CSS). In this method, the heads are moved to a special central location of the disks, the so-called landing zone. During the down spinning period of the disks, the supporting airflow decays and the heads land on the landing zone. To prevent stiction between the heads and disks, this landing zone is roughened or textured. 
     One of the main shortcomings of CSS is wear between the heads and the disks, which is caused by a sliding contact during the landing process. Another problem is that the flying height of heads in modern disk drives is becoming lower than the minimum required surface roughness, such that the heads tend to contact the textured disk surface, while the disks are still rotating at full speed. As a result, the interface is subject to excessive wear. 
     External imposed mechanical shock causes the heads, parked on the central landing zone to vibrate with the danger of vertically impacting the disk surfaces, an effect commonly known as head slap. Head slap can damage the heads and disks, and can generate particles that could cause the heads to crash. 
     Another method for parking heads during power-off is to move them to a parking ramp, located at either the inner or outer diameter of the disks. Each head of an actuator is mounted on a suspension. A tab or sliding element is provided at the end of the suspension. The tab is pushed onto the wedge shaped ramp. When the tabs reach the predetermined parking position on the ramp, the actuator is held in place with either a mechanical or an inertial latch. A mechanical latch locks the actuator in place, while an inertial latch engages a mechanism against the actuator when the drive is exposed to mechanical shock or acceleration. This ramp-parking process overcomes some of the shortcomings of CSS parking described above. Since the heads are not sitting on the disk during power-off, they are less prone to damage from external shocks. This is especially important in portable disk drives. 
     The electrical energy necessary for the retract movement to a parking position and the internal energy consumed by the actuator retract circuit is typically generated by a back electromotive energy generated from the kinetic energy stored in the rotating disk stack. The kinetic energy is thereby converted into a back electromotive voltage (BEMV) by utilizing the disk motor as a generator. The rectified BEMV is electrically connected across the voice coil motor VCM, which generates a torque on the actuator in the desired direction into the parking position. An example of such a retract circuit is described in U.S. Pat. No. 5,486,957. 
     As disk drives are made increasingly smaller for portable applications, several design parameters become limiting to the use of spindle motor BEMV for the retract event. The primary demand to save battery power reduces the size and mass of all moving or accelerated parts with the following results: First, the disk drives employ fewer disks of smaller diameter and the disk motors have lower torque constants and a relatively high internal resistance. These parameters reduce the magnitude of the back electromotive voltage that can be obtained from the spinning motor, as well as the total energy that can be extracted to park the actuator. Secondly, the VCM also has a low torque constant and an increased internal resistance due to a reduced wire diameter of the windings. Since the bias force from the actuator cables is not reduced proportionally with the torque constant of the VCM, the minimal required retract energy is higher relative to that in conventional disk drives. 
     Some parameters become more advantageous, such as the lower moment of inertia of the shorter actuator, and a reduced number of heads with their tabs that have to be moved onto a parking ramp. 
     Still, the combination of all parameters impose an increase of the demanded retract energy relative to the electric energy that can be extracted from the kinetic energy of the down spinning disk stack. 
     To overcome these problems in small drives, an alternate approach is to power the retract circuit with energy stored in a capacitor. A combination of both types of energy sources may also be used. 
     U.S. Pat. No. 4,786,995 to Stupek teaches a retract system for use with a stepper-motor actuator. Energy for the stepper motor is derived from the back electromotive energy generated from the down-spinning disk stack assisted by a charged capacitor. Highly complicated electronics are used to supply the energy in a number of pulses with a predetermined rate that defines the velocity and the rotational angle of the stepper motor. The stepper motor imposes its controlled rotation over a geared transmission system onto the actuator. The retract system is sophisticated and relies heavily on resistive components, which increase internal energy consumption. Even in hard disk drives equipped with large diameter disks and a relatively large amount of available kinetic energy additional capacitors have to be used to power the electronics and the stepper motor for the retraction and parking. The retract system is not feasible for small hard disk drives that use voice coils. 
     U.S. Pat. No. 5,325,030 to Yamamura et al. describes the general use of capacitors as possible energy source for a voice coil actuator retract. To initiate the retraction the capacitor is simply connected to the voice coil actuator to provide a single energy pulse. The actuator is accelerated and moves away from the disk surface. A shortcoming of single-pulse type capacitor retract systems is that the actuator may not properly enter and stay in the parking position under all circumstances. At the moment of power-off, the velocity and position of the actuator can vary over the operational range of positions and velocities during the regular usage of the drive. The torque needed to reach the parking position varies widely depending on these initial conditions. With a single pulse retract, without any adjustment for initial conditions, a predetermined torque pulse is applied to the actuator regardless of initial conditions. In some situations the torque pulse may be too large, such that the actuator bounces out of the parking position, which results in drive failure. 
     U.S. Pat. No. 5,495,156 to Wilson et al. discloses an actuator retraction circuit that uses the back electromotive energy generated from a down spinning disk stack to move the actuator into a parking position. The velocity of the actuator in or against retraction direction at the beginning of the retraction event generates an electromotive voltage, which is utilized to adjust the amount of energy imposed in an initial energy pulse onto an actuator motor. The invention sufficiently compensates the velocity differences of the actuator at the beginning of the retraction event to park the actuator with a predetermined end velocity. Because of the sophistication of the circuit with its reliance on resistors its internal energy consumption is too high to utilize it in very small disk drives. The disclosed invention is also only designed to impose most of its energy in an initial impulse leaving no energy reserves for the moment when the actuator reaches the ramp to ensure the proper retention in the predetermined parking position. 
     U.S. Pat. No. 5,615,064 to Blank et al. and U.S. Pat. No. 5,663,846 to Masuoka and Toru describe actuator control systems that use a back electromotive voltage from the actuator motor as a control signal during regular operation of the hard disk drive. The hard disk drive has an external energy supply. The control signal is processed together with other derived parameters within the hard disk controller for a regular operational retraction. The back electromotive voltage is changed into a control signal with a relative high energetic expenditure. The disclosed inventions are thus not useful for a power-off retract circuit with a very limited energy supply derived from capacitors. 
     A further shortcoming of capacitive retract systems is that they require physically large and expensive capacitors to store energy to power the retract. In a very small drive, such as a 1-inch drive in a CompactFlash form factor, it is challenging to budget enough space for a large capacitor. Since single-pulse capacitive retract circuits are inefficient in their conversion of stored energy into mechanical energy of the rotary actuator, the capacitors have to be larger than necessary. 
     Therefore, there exists a need for an actuator retract circuit with a low internal energy consumption and a highly efficient distribution of the available energy to retract an actuator from its operational position over a disk into a fixed parking position. The current invention addresses this need. 
     OBJECTS AND ADVANTAGES 
     Accordingly, it is a primary object of the present invention to provide an energetically self-supportive electronic device to power an initial retraction and a final positioning onto a resistive parking ramp of a voice coil actuator. 
     It is a further object of the present invention to provide an energetically self-supportive electronic device with an internal energy consumption sufficiently low to keep the required capacitor size to a minimum. 
     It is a further object of the present invention to provide an energetically self-supportive electronic device for an efficient converting of capacitor energy into mechanical work done on the actuator. 
     The above objects and advantages, as well as numerous improvements attained by the apparatus and method of the invention are pointed out below. 
     SUMMARY OF THE INVENTION 
     The objects and advantages of the invention are achieved by a self-sufficient electronic device to retract and park a voice coil actuator on a resistive parking ramp. The device is preferably used in miniaturized hard disk drives, in which the kinetic energy of the rotating disk stack is not sufficient to be utilized as the energy source for the power-off retract and parking of the actuator. The invention therefore provides a simplified electronic circuit with low internal energy consumption that uses electric energy stored in one or more capacitors to perform the necessary tasks. 
     An initiation signal received by the device can be an operational signal from the computer or an internally generated signal resulting from a power loss in the computer. The initiation signal initiates a pulsed discharge of a stored energy from a capacitor through a voice coil (VCM) attached on the actuator. 
     The actuator has an operational velocity range defined by a maximum velocity toward and away from a resistive parking ramp, which is adjacent to the disk stack. After initiation of the pulsed discharge, the retraction velocity of the actuator depends on the operational velocity immediately prior to the application of the pulsed discharge. Further factors influencing the retraction velocity are mainly related to an actuator torque constant and a bias torque of flex cables connected to the actuator. 
     The retract velocity varies between a value sufficient to have the actuator reach the resistive parking ramp from the most distant position, and a maximum value that causes the actuator to bounce off a crash stop and jump back onto the disk surface. The crash stop is a physical barrier that prevents the actuator from moving past the end of the parking ramp. 
     The pulsed discharge is a sequentially interrupted energy flow from a capacitor charged during the regular operational use of the hard disk drive. The pulsed discharge can extend the discharge period of the capacitor to provide enough remaining energy at the moment the actuator reaches the resistive parking position. The remaining energy presses the actuator against the crash stop to avoid eventual rebound of the actuator. The pulsed discharge converts the stored electrical energy with higher efficiency than with a single capacitor discharge. This is achieved by reducing the period of each discharge pulse to a fraction of the theoretical rise time of the current in the VCM&#39;s self-inductance. 
     The maximum velocity of the actuator retraction results in a short retraction time and a short pulsed discharge. The remaining energy is sufficient at the moment, the actuator would bounce off the crash stop and compensates the high kinetic rebound energy of the actuator. 
     The minimum velocity of the actuator retraction results in a long retraction time and a long pulsed discharge. The remaining energy has a low value at the time the actuator reaches the resistive parking position without bouncing off the crash stop. The approximately proportional relation between the maximum velocity and the high remaining energy value on one hand and the minimal velocity and the low remaining energy value on the other hand results in an advantageous response characteristic of the retract device. Hence, the broad range of actuator conditions at the beginning of the emergency retraction is dealt with by a simplified electronic circuit. 
     The internal energy consumption of the electronic circuit is kept low by using few or no resistors and relying mostly on transistors functioning as switches to provide the pulsed discharge by the conduction and alternating interruption of the energy flow from the capacitor. 
     In addition, the VCM&#39;s self-inductance (SI) is utilized as an energy buffer during each alternating interruption interval (AII) of the energy flow. The energy in the buffer is consumed to maintain the torque imposed from the voice coil on the actuator, even during periods when no energy is drawn off the capacitor. The period to load the SI is voltage dependent. To utilize the buffer function of the shortened VCM to achieve an optimal current distribution over the retract event, the invention also reduces the AII dependent on the decaying voltage of the drained capacitor energy. As a result, the average current flow remains high to compensate a declining voltage on the discharging capacitor and to provide sufficient energy to push the actuator onto the ramp under all varying initial retract conditions. The energy drain from the capacitor remains also above a minimum level, necessary to maintain the minimum velocity of the actuator. 
     The electronic device of the invention embodies a simple logical concept and has small demand for information gathering and processing. These features make the device space and energy efficient, and hence suitable for use in miniaturized hard disk drives. 
    
    
     BRIEF DESCRIPTION OF FIGURES 
     FIG. 1 shows a fragmental perspective view of a simplified hard disk drive. 
     FIG. 2 shows a fragmental schematic view of a simplified hard disk drive. 
     FIG. 3 shows a simplified retract circuit with one alternating connecting and disconnecting switch. 
     FIG. 4 shows a simplified diagram of the average current flow of the retract circuit of FIG. 3 compared with an uninterrupted capacitor discharge. 
     FIG. 5 shows a simplified retract circuit with two alternating connecting and disconnecting switches. 
     FIG. 6 shows a simplified diagram of the current flow at various positions of the retract circuit of FIG. 5 with a relatively long duty cycle. 
     FIG. 7 shows a simplified diagram of the current flow at various positions of the retract circuit of FIG. 5 with a relatively short duty cycle. 
     FIG. 8 shows a simplified diagram of the average current flow of the retract circuit of FIG. 5 with various duty cycle lengths. 
     FIG. 9 shows a simplified electric circuit of the preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Although the following detailed description contains many specifics for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the following preferred embodiment of the invention is set forth without any loss of generality to, and without imposing limitations upon, the claimed invention. 
     FIG. 1 shows a fragmental perspective view of a simplified hard disk drive with an array of parking ramps  10  on a ramp block  9  as an example of a parking ramp structure. A chassis base  11  carries an actuator  1  with a number of actuator arms  4 , each of them carrying one or two read and write heads  5  and a friction contact  6 . Each read and write head  5  is held in close flying height to one disk surface  3  of the disk stack  2 . A voice coil motor shown in FIG. 1 works according to the principle of a moving coil-motor. The voice coil  23  as the moving part is thus attached to the actuator  1 . A magnet assembly  22  is mounted on the chassis base  11 . A current applied by a controller through voice coil  23  in the magnetic field of magnet assembly  22  results in a force on the voice coil  23  and a torque on the actuator  1 . The voice coil  23  has inductance; as a result, whenever a current is applied to voice coil  23 , a certain energy is stored in the form of an additional magnetic field eminating from the voice coil  23 . When the voice coil  23  moves through the magnetic field from magnet  22 , across voice coil  23  a back electromotive voltage develops, which is proportional to the angular velocity of actuator  1 . This stored energy is utilized in the present invention by simultaneously disconnecting the OV and shorting the voice coil  23 . The stored energy is then consumed while creating a mechanical torque on the actuator  1 . The efficiency of this process depends on an end-voltage, at which the voice coil  23  is opened again. The efficiency is at the highest level, when the voice coil  23  remains shorted exactly to the moment when the end-current reaches zero. If the voice coil  23  remains shorted beyond that moment, the Vbemf of the moving actuator brakes the previously accelerated actuator  1  by generating a current in the opposite direction. During regular operation the disk stack  2  spins at a predetermined rotational speed. The read and write head  5  has an air bearing surface which flies on the air film and keeps the read and write head  5  at a constant height above the disk surface  3 . 
     During regular operational shut down of a computer, the voice coil  23  moves the actuator  1  with the attached friction tabs  6  to a parking ramp  10 , which consists, in the example shown in FIG. 1, of the following: an inclining portion  24   a,  a flat section  24   b,  a downhill section  24   c  and a flat parking area  24   d.  The parking ramp  10  extends onto the disk surface  3  such that each retracting read and write head  5  is lifted out of it&#39;s movement plane  26 . During the operational retraction each friction tabs  6  glides along the flat section  24   b  and the downhill section  24   c  into the flat parking area. It is appreciated that anybody skilled in the art may vary the appearance of the parking ramp  10  without diverting from the core of the invention. 
     A crash stop pin  8  is positioned within the movement range of the voice coil  23  on the chassis base  11  to prevent further movement of the actuator  1  which would force the lift tabs  6  beyond the flat parking area  24   d.  To avoid a situation where the actuator  1  is pushed out of the parking position from a peripheral induced shock impulse such that it might land on the disk surface, it has to be secured onto the flat parking area  24   d.  Two types of latches are typically used for this purpose: 
     1) a passive mechanical latch, in which the actuator arm  4  has to be pushed with a significant amount of torque to engage with a snapping mechanism belonging to this latch. 
     2) an inertia latch, which is initiated only by an externally induced shock, whereby the actuator arm  4  or the side of the actuator  1  at the voice coil  23  is grabbed and secured in the parking position. The inertia latch is energetically independent from the retract event, which makes it preferable in combination with low energy consuming emergency retract devices. The inertia latch has a certain grabbing range within which the actuator arm  4  has to be parked. 
     In an emergency situation, e.g., when the functioning of the computer is interrupted, or when the power supply level changes beyond acceptable limits, an emergency retract command is issued by the computer or by an pulsed discharge circuit  28 , which recognizes the power level change of the power supply. In a pulsed discharge, a capacitor  29 , which is charged during regular operation of the hard disk, discharges its energy through the voice coil  23  to generate a torque therein and force the actuator  1  towards the parking position. 
     The capacitance of capacitor  29  is chosen to provide a predetermined energy to overcome a number of varying opposing forces and effects. These are mainly due to the slope and height of the load/unload ramps, which the heads must climb in the parking process, a flex bias of supply cables  31  running from the hard disk controller  30  onto the moving actuator  1 , mechanical friction resistance in the pivot bearing of the actuator  1 , varying positions of the actuator  1  and varying velocities of the actuator  1  at the beginning of the emergency retraction. Further opposing effects are: electrical resistance in the voice coil  23 , friction of the friction tabs  6  on the parking ramp  10 , a pull-down force on the read and write heads from the air bearing surface during lift off on the parking ramp  10 , drive-to-drive component differences and time dependent component characteristics. 
     The size of the capacitor  29  has to be kept at a level that is low enough to fit the capacitor  29  into miniaturized hard disks. 
     FIG. 2 shows a schematic fractional top view of an exemplary parking ramp structure consisting of the same exemplary components as shown in FIG.  1 . Further shown is an inner and outer actuator range limit  12 ,  16  defining an operation angle  13  within which the actuator  1  swings at various velocities. A parking position  15  and a ramp contact position  16  define a mechanical ramp friction range  17 . 
     Since a power failure of the disk drive has to be anticipated at any time, actuator  1  is considered moving within the operation range  13  at an unknown velocity and at an unknown location at the retraction initiation. Retract current is imposed onto the voice coil  23  to exert a torque on actuator  1  in order to move it to the parking position  15 . A minimum and maximum limit of the retract current level are defined in regard to the unknown initial state of actuator  1 . The minimum current limit is dictated by a worst-case of the opposing effects and the highest required torque to move actuator  1  from a resting position at the bottom of the inclining portion  24   a  into the parking position  15 . The maximum current limit is dictated by the lowest torque required to move actuator  1  to parking position  15 . This case commonly occurs when the actuator  1  is at the retract initiation far away from the parking position  15  close or at the inner actuator range limit  12 . When a torque is applied to the actuator  1 , it has time to accelerate and build up angular momentum to a high velocity before striking parking ramp  10  and crash stop pin  8 . In a situation, when the velocity of the actuator  1  exceeds a maximum limit, it strikes the crash stop  8  with excessive velocity, and actuator  1  bounces back onto the inclining portion  24   a,  or the read and write heads  5  land back on the disk surface  3 . 
     FIG. 3 shows a circuit that accomplishes a direct discharge of an energy storage capacitor through a voice coil motor VCM as it is the state of art. Switch S 0  closes in response to a retract initiation, and a current I 0  flows continuously while capacitor C 0  discharges. The electrical equivalent of the voice coil motor VCM is an inductor Lvcm in series with a resistance Rvcm and a back electromotive voltage Vbemf. 
     FIG. 4 shows a simplified diagram of the average current flow  57  of the pulsed discharge circuit  28  compared with a direct and continuous capacitor discharge as described in FIG. 3. A vertical axis  53  represents the current level and a horizontal axis  54  represents the time duration. Curve  51  is an exemplary waveform for I 0  in FIG.  3 . Depending on the position of actuator  1  within operating range  13  and kinetic status of actuator  1  at the retract begin  40 , the actuator  1  reaches the resistive parking position  15  in a time range between a minimum retract period  55  and a maximum retract period  58 . In the case where the actuator  1  reaches the ramp during minimum retract period  55 , there is still torque exerted on the actuator by retract current  51 , since current  51  does not diminish to zero until the end of time  56 . If the actuator  1  reaches the ramp after period  56 , during maximum retract period  58 , current  51  has decayed and thus no torque is exerted on the actuator  1 . In a situation when the velocity of the actuator  1  exceeds a maximum limit, it strikes the crash stop  8  with excessive velocity and actuator  1  bounces back onto the inclining portion  24   a,  or the read and write heads  5  land back on the disk surface  3 . 
     The pulsed discharge circuit  28  provides a pulsed discharge of capacitor  29  and takes advantage of temporary energy storage in the inductance of the VCM. As a result, the average current flow  57  is lower than the peak current of a theoretical, proportional expanded discharge current (EDC), which is shown in FIG. 4 with the curve segments  61 . The EDC is defined by the theoretical peak values  75  of an instantaneous current  86 , which correspond to a supply interval  63   a,    63   b,  (see FIGS. 6,  7 ) and a supply interruption interval  64   a,    64   b  (see FIGS. 6,  7 ) of the pulsed discharge circuit  28 . The curve segments  61  are shown in FIG. 4 for the sole purpose of visualization. Since the inductance of the voice coil  23  charges asymptotically, the theoretical peak values  75  are never reached in praxis such that they are used only as a theoretical reference. The average current flow  57  typically extends beyond minimum and maximum retract periods  55 ,  58  to an extended duration  60 . 
     The pulsed discharge integrates overall more current flow during the retraction than it is achievable with a direct capacitor discharge, since it reuses the energy stored in the inductance of the voice coil  23 . 
     Even after the maximum retract period  58  a compensation period  59  is available, during which sufficient average current is flowing through the voice coil  23  to built up compensatory torque against an eventual rebound of the actuator  1  from the crash stop  8 . 
     FIG. 5 shows a simplified schematic of the pulsed discharge circuit  28  to illustrate the switching necessary for the pulsed discharge of capacitor  29 . Capacitor C 1  is charged during regular operational use of the hard disk to an initial voltage Vc, while the switches S 1  and S 2  are open. At the initiation of the retract event, switch S 1  is first closed for the supply interval, while switch S 2  remains open. After this, the first switch S 1  is opened, and switch S 2  is closed simultaneously for the supply interruption interval. The alternating opening and closing of switches S 1  and S 2  continues until the energy to power the retract event is exhausted. 
     FIG. 6 shows exemplary current waveforms for the circuitry shown in FIG.  5 . The horizontal axes  61  represent the elapsed time. The vertical axis  62   a  shows the current Ivcm through the VCM. The vertical axis  62   b  shows the current flowing through S 1 . The supply intervals  63   a,    63   b  represent the on-time of switch S 1  and supply interruption intervals  64   a,    64   b  represent the off-time of switch S 1 . The waveforms in FIG. 6 represent a low duty cycle of the supply intervals  63   a,    63   b  relative to a total cycle period, which is the sum of one of the supply intervals  63   a,    63   b  and one of the consecutive supply interruption intervals  64   a,    64   b.  In FIG. 6 the duty cycle is shown as being approximately 33%. During the supply intervals  63   a,    63   b  current is flowing from the capacitor C 1  through the voice coil VCM. The current flow curves  65   a,    65   b  rise asymptotically and in accordance to the time constant of the voice coil motor VCM, which is given by the ratio of the inductance Lvcm to the resistance Rvcm. The supply intervals  63   a,    63   b  are approximately one fifth of a period  77  for a 99% saturation level  78  of the theoretical exponential curve  76  with the theoretical current peak level  75   a.  During Interval  63   a,    63   b  second switch current I 2  has a zero level  67   a,    67   b.  At the end of interval  63   a,    63   b  switch S 1  opens and switch S 2  closes, whereby the voice coil VCM is shorted. As a result, the voice coil current Ivcm is maintained by the energy stored in the inductance Lvcm and flows in a decaying curve  70   a,    70   b  through second switch S 2 . During supply interruption interval  64   a,    64   b  the capacitor current I 1  is zero. The length of the supply interruption interval  64   a,    64   b  is optimal for high energy efficiency, when the supply interruption intervals  64   a,    64   b  last to the moment, where the current Ivcm reaches zero, which is shown by the voice coil decaying curve  68   a,    68   b.    
     Shorting the voice coil VCM during supply interruption intervals  64   a,    64   b  utilizes the inductance Lvcm as an energy buffer. As a result, the magnetic energy stored in the inductance of the voice coil VCM at the beginning of each supply interval  63   a,    63   b  is utilized. Therefore, in the circuit shown in FIG. 5 more energy is converted into torque than can be converted in the circuit shown in FIG.  3 . 
     A first average current level  72   a  is the integrated average of the current flow curves  65   a,    65   b  and the voice coil decaying curve  68   a,    68   b.  Due to the low duty cycle with the voice coil decaying curve  68   a,    68   b  going down to nearly zero, the average level height  74   a  is relatively low compared to the theoretical peak level  75   a.  This first discharge characteristic is desirable at the beginning of the retraction where the high initial voltage in the capacitor  29  results in a high theoretical peak current  75   a.  As a result, the average current flow  72   a  is tuned to a minimal level, which is necessary to exert sufficient torque under worst conditions. 
     FIG. 7 shows an example of the current waveforms of the circuit shown in FIG. 5 with a high duty cycle of approximately 50%. The supply interruption intervals  64   a,    64   b  have a significantly shorter duration than those of FIG. 6 with the following two main effects: 
     1) the constant voice coil decaying curve  68   a,    68   b  does not decay to nearly zero anymore. 
     2) only a fraction of the energy pumped into the inductance Lvcm is consumed during supply interruption intervals  64   a,    64   b  and during the supply intervals  63   a,    63   b,  the current reaches after several cycles a level closer to the theoretical peak level  75   b.  As a result of both, the average voice coil current  72   b  is at a higher level relative to the theoretical current peak level  75   b  than it is the case in FIG.  6 . The decrease in theoretical current peak level  75   a,    75   b  during the retract event is in proportion to the gradually decreasing voltage during discharge of the capacitor C 1 . The average torque on the actuator is proportional to the average current in the coil. As the capacitor discharges, the current would fall unless the duty cycle is increased to compensate. By increasing the duty cycle as the capacitor discharges, an approximately constant torque can be applied to the actuator until the capacitor discharges beyond a certain point. 
     This second discharge characteristic is desirable for the late period of the retraction, where the capacitor energy has been mostly consumed and the voltage has dropped. The preferred embodiment of the invention combines first and second discharge characteristic to adjust the duration of the supply interruption interval  64   a,    64   b.  Additional circuitry as it is well known to those skilled in the art is used thereby to monitor the capacitor voltage and to take its level as parameter to modulate the first and second duration Dl and Dh of a clock signal Vclock (see FIG.  9 ), which initiates synchronously the switches S 1  and S 2 . 
     In an alternate embodiment of the invention the additional circuitry is left out to reduce the complexity of the pulsed discharge circuit  28 . The duty cycle is set at a fixed level to balance the energy efficiency of the retract event with the average current necessary to move actuator  1  safely to parking position  15 . 
     FIG. 8 shows a simplified graphic of a number of current levels for various duty cycles to visualize the relation between duty cycle and average current through the voice coil motor VCM. All elements shown in the graphic are for the sole purpose of visualization without any accuracy. 
     The vertical axis  53  represents the magnitude of the average current, the horizontal axis represents a retract duration  54  and the origin  40  is the zero point of both. A first curve  91  shows a duty cycle of 100%, which equals a direct single pulse discharge. A second curve  92  shows a duty cycle of approximately 80%. A third curve  93  shows a duty cycle of approximately 60%. A fourth curve  94  shows a duty cycle of approximately 40%. First, second, third and fourth curve  91 ,  92 ,  93  and  94  show that even a fixed duty cycle significantly reduces the differences of the retract current at the beginning and the end of the retraction. As further results, the retract current duration is typically extended in case the retract current level is lower and the total integrated current increases with decreasing duty cycle. 
     FIG. 9 shows a simplified electric circuit of the preferred embodiment of the invention. The exemplary logical concept of the shown circuit together with the graphical representation of metal-oxide-semiconductor transistors (MOS) is for the sole purpose to describe the working principle of the invention. A person with average skill in the art will appreciate that the switching can also be accomplished with bipolar transistors, insulated gate bipolar transistors, diodes, relays, or other current switching elements. A person with average skill in the art will appreciate that the switching effect described for this figure is also achievable with other known switching configurations without diverting from the scope of the invention. Additional circuit elements necessary to compensate specific characteristics of the involved components are not shown. These additional elements are well known to a person of average skill in the art, as are the electronics techniques for further optimizing the circuitry. 
     A capacitor C 1  is charged during regular operation of the hard disk from a power source Vdd and has an operational voltage Vc. A diode D 1  blocks an eventual discharge of C 1  into Vdd during operation of the circuit. When Vdd drops below a predetermined value, or when an emergency retract command is issued by the computer, a signal Vr goes to a high value. As a result, transistor M 3  is closed and clock generator Cg is initiated, both for the duration of the retract event. In the preferred embodiment Cg receives Vc as parameter to provide a frequency signal Vclock in the form of an alternating control voltage. Vclock is repeatedly transmitted with a low value for the first duration Dl and a high value for the second duration Dh. In the preferred embodiment, the lengths of the second duration Dh is altered in dependence to Vc to dynamically adjust the average current level  72   a,    72   b.  In an alternate embodiment the gathered voltage information is utilized to alter second duration Dh and/or first duration Dl. In a further alternative embodiment the lengths of first and second duration Dl, Dh are fixed. 
     During Dl, M 1  is in a conducting or low-resistance state, while M 2  is in non-conducting, or high-resistance state. As a result, a first current Im 1  flows from C 1  through M 1 , the VCM and M 3 . During Dh, M 1  in a non-conducting state, while M 2  is in a conducting state. A current Im 2  flows through M 2 , the VCM and M 3 . 
     Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents: