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Delta Voltage Control During Actuator Retract

IP.com Disclosure Number: IPCOM000108794D
Original Publication Date: 1992-Jun-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 3 page(s) / 148K

Publishing Venue

IBM

Related People

Carpenter, GD: AUTHOR [+3]

Abstract

A typical actuator power driver circuit and voice coil are shown in the figure. The driver is designed in such a way that the differential voltage across the voice coil (voltage between nodes 3 and 4) is measured and controlled during retract. That means that the four FET power devices (devices 2A-2D) will operate in such a way that a predefined differential voltage will be forced across the voice coil whenever retract is activated.

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This is the abbreviated version, containing approximately 47% of the total text.

Delta Voltage Control During Actuator Retract

       A typical actuator power driver circuit and voice coil
are shown in the figure.  The driver is designed in such a way that
the differential voltage across the voice coil (voltage between nodes
3 and 4) is measured and controlled during retract.  That means that
the four FET power devices (devices 2A-2D) will operate in such a way
that a predefined differential voltage will be forced across the
voice coil whenever retract is activated.

      By forcing this predefined differential voltage across the
voice coil, the actuator will then be forced to move in a desired
direction at a desired velocity.  If the actuator was already moving
in the correct direction, then the driver will speed it up or slow it
down.  If it was moving in the wrong direction (away from the landing
zone), then this circuit will apply enough current to reverse the
direction of the actuator so that it will retract.

      Whenever a retract condition is initiated, a differential
measurement circuit contained in the voice coil driver, shown in the
figure, will measure the differential voltage across the voice coil
(node 3 to node 4).  Normally the actuator will not be in motion, and
so there will not be any differential voltage across the coil.  If
the actuator is in motion, then there will be a back EMF (Vb) being
generated by the voice coil.  Moving a coil of wire in a magnetic
field generates a voltage across the nodes of the coil.  The amount
of voltage generated is proportional to the velocity of the coil and
the back EMF (torque) constant of the magnetic circuit.

      In the three cases described below, FET 2D is always completely
turned on.  FET 2A or FET 2C will be operated in regulation mode
(never both 2A and 2C at the same time) by the FET control circuit to
maintain the desired differential voltage between nodes 3 and 4.
CASE 1.
ACTUATOR NOT IN MOTION OR IN MOTION TOWARD THE ID TOO SLOW.

      Assume that in order to retract the actuator, a current must be
applied from node 3 through the voice coil and into node 4.  In order
to apply this current, FET 2A and 2D must turn on.  The operation of
the FETs recommended by this invention is to completely turn on
(saturate) the ground FET (2D) and to linearly regulate the supply
FET (2A).  A differential voltage measuring circuit will detect the
voltage between nodes 3 and 4.  This voltage is the current (I) times
the voice coil impedance (L1 and R1) plus the back EMF (Vb) generated
due to the coil movement.  The FET control circuit compares the
desired differential voltage to the measured differential voltage and
applies a control signal to the supply FET (2A) such that the
measured voltage matches the desired voltage.  As the current causes
the voice coil to move or speed up, the back EMF increases.  As the
back EMF increases, the current is reduced by the FET control
circuit.  Eventually, the back EMF equals the desired volt...