Electricity: electrical systems and devices – Control circuits for electromagnetic devices – Including means for using – or compensating for – the induced...
Reexamination Certificate
2002-01-31
2004-08-03
Toatley, Jr., Gregory J. (Department: 2836)
Electricity: electrical systems and devices
Control circuits for electromagnetic devices
Including means for using, or compensating for, the induced...
C360S075000
Reexamination Certificate
active
06771480
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to control of an actuator and, more particularly, to a method and apparatus accurately controlling the velocity of the actuator member by monitoring the back electromotive force (“EMF”) of a actuator coil, and driving the coil with a voltage.
BACKGROUND OF THE INVENTION
Conventional actuators sometimes referred to as “motors”, have a movably supported member and a coil. When a current is passed through the coil, a motive force is exerted on the member. A control circuit is coupled to the coil in order to controllably supply current to the coil. One example of such an arrangement is found in a hard disk drive, where the movable member of the actuator supports a read/write head adjacent a rotating magnetic disk for approximately radial movement of the head relative to the disk. There are situations in which it is desirable to move the member to one end of its path of travel at a predetermined velocity which is less than its maximum velocity. An example of such a situation is a power failure. In such a situation, it is desirable to move the member to a parking location, where it is held against potentially damaging movement which could occur if the member were not so parked. The movement of the member to the parking location is commonly referred to as a retract of the member.
When a current is applied to the coil of the actuator, the member is subjected to a force tending to accelerate the member at a rate defined by the magnitude of the current, and in a direction defined by the polarity of the current. Consequently, in order to accelerate or decelerate the member until it is moving at a desired velocity and in a desired direction, it is important to know the actual direction and velocity of the member. In this regard, it is known that the back-EMF voltage on the coil of the actuator is representative of the velocity and direction of movement of the member. Specifically, the following relationship applies to actuators:
V
M
=I
M
*R
M
+K
e
&ohgr;
V
M
=voltage across actuator (motor),
I
M
=current through actuator
R
M
=internal resistance of actuator
K
e
=torque constant of actuator, and
&ohgr;=velocity of actuator.
The term, K
e
&ohgr;, represents the back-EMF of the actuator coil.
Apparatus have been provided that control such actuators by providing a drive current to the coil of the actuator in response to the provision of a target speed voltage signal having a voltage corresponding to the target speed of the moveable member. For example, co-pending patent application U.S. Pat. No. 6,040,671, issued on Mar. 21, 2000, and entitled “CONSTANT VELOCITY CONTROL FOR AN ACTUATOR USING SAMPLED BACK EMF CONTROL,” discloses such an apparatus. However, such apparatus does not lend itself readily to providing such control in cases where the drive transistors for the actuator are power MOSFETs external to the integrated circuit (“IC”) containing the control circuitry. In such cases, it is difficult and/or expensive to implement a current mode output. To do so would require current feedback. To process this feedback, additional circuitry would be required. This additional circuitry would add expense and would be difficult to operate at low voltages such as experienced with the power failure.
FIG. 1
is a diagrammatic view of a system including an actuator
10
under control of a control circuit
12
. The particular system shown is that of a hard disk drive, in which the actuator
10
controls the movement of a member
20
on which a read/write head
34
is mounted. The control circuit
12
applies drive signals DRV+ on line
14
and DRV− on line
16
in response to a move command voltage signal V
C
on line
18
. The drive signals DRV+ and DRV− cause motion in a member
20
of actuator
10
by setting up a force field in a coil
22
on the member
20
. The force field thus set up in coil
22
interacts with the magnetic field of a permanent magnet
24
disposed nearby. Member
20
is constrained to move about a shaft
26
, resulting in pivoting motion as shown by arrow
28
. The member is constrained in its movement between a first stop
30
and a second stop
32
. The result is that a magnetic head
34
is caused to move about a magnetic disk (not shown in this figure) in conjunction with the reading and writing of data from and to the magnetic disk in a hard drive system.
FIG. 2
is a high-level block diagram of a control unit and the actuator it controls, such as is used in the system shown in
FIG. 1. A
control circuit
90
receives a move command signal V
C
on line
92
and provides drive current DRV+ and DRV− to an actuator. In
FIG. 2
, the actuator shown is an idealized model
65
of an actuator. It will be appreciated that the control circuit
90
would be unable to “see” a significant difference between the actuator model
65
and an actual actuator, were an actual actuator connected to control circuit
90
.
The actuator model
65
includes an ideal current sensor
66
, an inductance
68
, a resistance
70
and an ideal voltage-controlled voltage source
72
, all coupled in series between the two terminals
94
,
96
of the actuator model
65
. The output
67
of the ideal current sensor
66
is a signal representing the current flowing through the actuator. This signal
67
is coupled to an input of an amplifier
74
, which has a gain K
t
that represents a torque constant of the moveable member
20
(FIG.
1
). The output of the amplifier
74
is coupled to the input of a junction
76
, which adjusts the amplifier output using a signal representing a load torque. The output of junction
76
is coupled to the input of a circuit
78
, which makes an adjustment representative of the inertia J, of the member
20
.
The output
80
of the circuit
78
is a signal which represents an acceleration of the member
20
. The signal
80
is integrated at
82
, in order to obtain a signal
84
which represents the velocity of the member
20
. The signal
84
is applied to the input of an amplifier
86
having a gain K
e
that represents an electrical constant for the back-electromotive force (EMF) of the actuator. The output
88
of the amplifier
86
is a voltage V
be
which represents the back-EMF voltage of the actuator. This voltage is applied to an input of the ideal voltage-controlled voltage source
72
, which reproduces this same voltage V
be
across its output terminals. Since the voltage source
72
is ideal, it produces the output voltage regardless of whether there is any current flowing through source
72
.
Since the signal
84
represents the actual velocity of the member
20
, and since the back-EMV voltage V
be
present at
88
and across source
72
is proportional to the magnitude of signal
84
, it will be appreciated at the magnitude of the back-EMF voltage V
be
across source
72
is an accurate representation of the actual velocity of the member
20
. However, when a current is flowing through the actuator model
65
, the resistance
70
produces a voltage which is added to the voltage V
be
across the voltage source
72
. Consequently, so long as current is flowing through the actuator model
65
, it is not possible to accurately measure the voltage V
be
alone, in order to accurately determine the actual velocity of the movable member.
Therefore, the system of
FIG. 2
independently measures the back-EMF voltage V
be
, and thus determines the actual velocity of the member
20
. It does this by interrupting the current flow through the actuator coil
68
so that the voltage across the resistance
70
goes substantially to zero, after which the back-EMF voltage V
be
is measured across the two terminals
94
,
96
, of the actuator model
65
. It is a characteristic of the actuator that the back-EMF voltage V
be
does not change rapidly after the current flow through the actuator model
65
is decreased to zero, once short term transient effects have died down.
The control circuit
90
includes the following components. A juncti
Brady W. James
Nguyen Danny
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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