1.  Technical Field:
The invention relates to control of drive circuits for linear or rotary actuators in direct access storage devices, and, more particularly, to protecting electrical components of the direct access storage device from voltage transients associated with operation of either type of actuator.  Still more particularly, the invention relates to limiting flyback voltage generated by inductive components of an actuator during operation while minimizing decay time of actuator current. 
2.  Description of the Related Art:
Direct access storage devices such as disk drives store data on rotating magnetic media disks.  Recovery of data from or writing of data to a location on the disk requires positioning of a read/write transducer over the selected location.  The transducer is typically carried on an arm which is moved to position the transducer.  The arm may be of a type moved linearly across the disk or it may be of a type rotated in a fashion similar to the movement of the tone arm for record player.  The actuators used to move the arms are also of two types, linear actuators and rotary actuators. 
Certain features are common to both types of actuator.  The actuators are direct current electric motors.  Current is passed though the motor in one of two directions to generate a magnetic field to accelerate movement of the arm in a selected direction.  Reversal of the current results in generation of an opposed magnetic field to accelerate the arm in the opposite direction.  The magnetic field results from the current passing through a voice coil within the motor. 
The prior art discloses numerous schemes for the control of acceleration, torque, velocity and position in a mechanical system driven by a direct current motor.  Control comes through control of current through the motor.  Though many control schemes exist, most rely on an H-bridge circuit connected at its input terminals across a direct current power source.  The motor coil provides a load connected across the output terminals of the bridge.  Each arm of the H-bridge circuit includes a switch.  The output terminals of the bridge can be connected by closure of the appropriate switch to either input terminal.  Closure of one pair of opposed switches supports a current through the motor in one direction.  Opening of the first pair of switches and closure of the remaining pair of opposed switches supports a current through the motor in the opposite direction. 
One problem is always encountered in any attempt to change the direction of current through a direct current electrical motor.  A motor is an inductive load and accordingly a current through it cannot be interrupted.  A change in the opposed pair of switches connecting the terminals of the load to the input terminals of the bridge will not instantly change the direction of current through the load.  If such a change is attempted, the voltage on the terminals of the motor will rise to whatever level is required to create a path for the motor "flyback" current. 
In some applications, return of the flyback current into the power source is acceptable.  Guzik, in U.S. Pat.  No.  4,710,686, teaches a motor control scheme using an H-bridge circuit.  The inductive load is essentially connected across the output terminals f two bridge circuits, where one bridge circuit has a switching transistor in each arm, and the second bridge circuit has a diode in each arm.  The diodes are oriented to provide a path for the flyback current back through the power source.  A modified pulse width modulation scheme is used for closing opposed pairs of transistor switches to control current through the motor.  The diodes "freewheel" after interruption of a current path through one of the two sets of transistor switches, passing current back to the power source while it decays.  The scheme minimizes current decay time, which is generally desirable, but introduces a considerable power supply ripple, which in some applications is unacceptable. 
U.S. Pat.  No.  4,581,565 to Van Pelt et al.  is another H-bridge type control circuit, directed, among other things, to reducing the ripple effects associated with changing motor current direction.  A flyback current route is provided through a current switching transistor, which is isolated from the power source, and a freewheeling diode.  However, decay of the flyback current is uncontrolled.