Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
1997-06-19
2001-10-16
Sniezek, Andrew L. (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C360S078040
Reexamination Certificate
active
06304408
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to hard disk drive systems and, more particularly to the control system determining the position of the read/write head over the storage disk. The read/write head is mounted on an arm, the arm in turn is driven by a servo-actuator unit relative to the tracks on the disc. The control system is designed to minimize the time to move the read/write head from an initial position over the tracks of the storage disk to a final position over the tracks of the storage medium, a mode of operation referred to as a “seek” operation.
2. Description of the Related Art
The reduction of the seek time is becoming more difficult as the tracks on the storage disk become more dense. The arm, upon which the read/write head is mounted, and the servo actuator motor (or dc motor) driving the arm are referred to as the hard disk drive “plant”. The plant is characterized by electrical and mechanical systems which can be non-linear, which can be unknown, and which can be a function of environmental (and/or time-dependent) parameters. Prior art plant control units which perform the seek function are unable to achieve the minimum seek time within the parameters and constraints of the end point accuracy and settling requirements.
Referring to
FIG. 1
, a block diagram of a hard disk drive closed-loop plant control unit for a hard disk drive plant, according to the prior art, is shown. Signals representing the desired and the actual position of the read/write head are applied to input terminals of difference amplifier
10
. Position loop component
11
receives the output signal from difference amplifier and applies the filtered signal to a first terminal of difference amplifier
12
. A second terminal of difference amplifier
12
has a signal representing the read/write head velocity applied thereto. (The head velocity is frequently referred to herein as the head delta position.) The output signal of difference amplifier
12
is applied to an input terminal of velocity loop component
13
. The output signal of velocity loop component
13
is applied to a first terminal of difference amplifier
14
. A second input terminal of difference amplifier
14
receives a signal representative of the actuator current. The output signal of difference amplifier
14
is applied to current loop component
15
. The output signal of current loop component
15
is applied to a power supply saturation analog unit
16
. The output signal from analog unit
16
is applied to one input terminal of difference amplifier
17
. A second input terminal of difference amplifier
17
receives a signal indicative of induced emf from an output terminal of back-emf component
102
. The output signal from difference amplifier
17
is applied to the actuator impedance analog unit
18
. The output signal from the analog unit
18
is applied to the second input terminal of difference amplifier
14
and to the torque/inertia analog unit
19
. The output signal from analog unit
19
is applied to the second input terminal of difference amplifier
12
, to an input terminal of back-emf component
102
, and to 1/s component
101
.
The output signal of 1/s component
101
is applied to the second terminal of difference amplifier
10
and determines the position of the read/write head. The position is also referred to herein as the actual position or relative position, i.e., measured with respect to the final track position.
Difference amplifier
17
, actuator impedance analog unit
18
, torque/inertial analog unit
19
, 1/s component
101
and back-emf component
102
represent a model of the hard disc drive plant. Actuator impedance unit
18
provides the analog of the actuator unit impedance (resistance and inductance). Torque/inertial analog unit
19
provides the analog of the actuator torque constant (K
t
), the rotational inertial (J) and the integration (1/s), providing the velocity from the acceleration. 1/s component
101
provides the analog of integration from velocity to position. And back-emf component
102
provides the analog of the back emf constant K
b
of the voltage generated by the actuator motor.
Conventional closed-loop controllers typically perform the seek-to-track function by using estimates of the position, velocity and acceleration (plant states) information as feedback information to close the loop. The closed-loop controllers can be implemented using either analog or digital technology. The closed-loop controllers employing the analog technology, attempting to provide a minimum seek time can be complicated by uncontrollable variables such as loop gain variation, power supply variations, and large plant variations. Closed-loop controllers in digital technology provide improved performance through the use of more complex mathematical algorithms to compensate for offset and to compensate for some loop parameter variations. Closed-loop analog controllers of the prior art provide an average seek time of approximately 15 milliseconds (msec). Using digital technology, this seek time can be reduced to a range of between 10 and 12 msec. It is anticipated that future requirements, derived from the need for higher positional accuracy resulting from the higher track density on the storage disks, will require seek times of approximately 6 msec.
Referring to
FIG. 2
, a model of the hard disk drive unit plant expressed in terms of Laplace transform variables is shown. A difference amplifier
20
has voltage drive signal applied to a first input terminal and a signal from the back-emf component
201
applied to a second input terminal. The output of difference amplifier
20
is amplified to first input terminal of difference amplifier
21
while an output signal from actuator resistance component
202
is applied to the second input terminal of difference amplifier
21
. The output signal from difference amplifier
21
is applied to the actuator impedance component
22
. The output signal from the output impedance component
22
is applied to 1/s analog component
23
. The output signal from the 1/s analog component
23
is applied to the actuator resistance filter
202
and to the actuator torque/inertia component
24
. The output signal from the actuator torque/inertia component
24
is applied to a first input terminal of difference amplifier
25
, while an output signal from coulomb friction analog unit
29
is applied to a second input terminal of difference amplifier
25
. The output signal of difference amplifier
25
is applied to resonance analog unit
26
. The output signal from the resonance analog unit
26
is applied to 1/s analog component
27
. The output signal for 1/s analog component
27
is applied to an input terminal of back-emf component
201
, to and input terminal of coulomb friction analog component
29
, and to an input terminal of 1/s analog component
28
. The output signal from 1/s analog component
28
is indicative of the position of the read/write head. The major components of the plant model of
FIG. 2
represent the rotary actuator (dc torque) characteristics, the head and actuator inertia, the mechanical resonance of the plant, and the mechanism friction. The model includes a non-linear representation, i.e., the saturation of the power supply at its limits (±12 volts), mechanism resonance, and friction on the seek time performance. Typical values for the parameters of the actuator plant unit are: the actuator resistance R=8.0 ohms, the actuator inductance L =1.0 mH, the actuator torque constant K
t
=13 oz-in/amp, the back-emf constant K
b
=0.092 v-sec/rad, and the actuator inertia J=0.0009 oz-in-sec
2
.
Referring once again to
FIG. 2
, the transfer function for input voltage to output velocity (neglecting mechanism resonance and friction can be described by the Laplace equation:
&ohgr;(s)/V(s)=(1/K
b
)/{1+JRs/K
b
K
t
}{1+Ls/R} (1)
Assuming that JR/K
b
K
t
>>L/R, an assumption normally interpreted that the mechanical tim
Brady III W. James
Sniezek Andrew L.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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