Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
2001-12-03
2004-06-15
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C360S078060
Reexamination Certificate
active
06751043
ABSTRACT:
BACKGROUND
In a Hard Disk Drive assembly illustrated in simplified form in
FIG. 1
, the read/write R/W heads
101
are attached to one end of an actuator arm
102
that pivots about point
103
. A voice coil motor VCM
104
is located at the opposite end of the arm and, when driven, generates a torque, T
vcm
, about the pivot to control the head position, &thgr;
h
. The spindle motor
105
drives multiple platters
106
which contain the stored information. The equivalent circuit model of the simplified HDD assembly, known in this discipline as the ‘Plant’, is illustrated in FIG.
2
.
Because of errors in the servo-writing process and imperfections in the spindle bearings, the true location of a given track, y
201
, moves beneath the head. Through the servo-writing process, position information is encoded onto the disk surface at a finite number of equally spaced locations around the disk known as servo sectors. At each servo sector, the read head serves as the position transducer for the VCM servo system, and provides the signal y
m
202
, the measured position of the head with respect to the track center line. The position measurement is processed by a compensation algorithm and produces a control signal, u
203
, which is indicative of the desired VCM torque necessary to keep the R/W head on track in the presence of the various disturbance torques and disk errors. A power driver is used to transform the control signal into the power signal, V
d
204
, necessary to physically actuate the arm and move the head.
Voice Coil Motor
The VCM, consisting of a coil of wire sandwiched between two permanent magnets, produces an actuating torque on the arm carrying the R/W head. The torque is a function of the coil current and the position of the coils between the permanent magnets and can be described by the equation,
T
vcm
=k
i
(
i
,&thgr;) (1)
If we assume that the magnetic field strength, B, is constant over the full range of travel of the actuator, then the torque function can be approximated by the linear relation, k
t
(i,&thgr;)=K
t
, and (1) can be written,
T
vcm
=K
t
i
(2)
If N is the number of turns in the coil, l is the length of each turn in the coil and r is the distance from the coil center to the arm pivot, then K
t
is given by,
K
t
=NBlr
(3)
Because a moving coil in a magnetic field generates an electric potential, known as the back e.m.f. voltage, the effective voltage on the coil, V
c
, is a combination of the voltage supplied by the power driver, V
d
, and the back e.m.f. voltage, V
b
,
V
c
=V
d
−V
b
(4)
The back e.m.f. voltage is a function of the angular velocity of the moving coil &ohgr;, and is given by the equation,
V
b
=K
t
&ohgr; (5)
where K
t
is given by equation (3).
Conventional Hard Disk Drive Controllers
Voice Coil Motors in conventional hard disk drives are controlled with a current command to a transconductance loop. The principle, illustrated in
FIG. 3
, is that this high bandwidth current loop
301
is able to reject disturbances and provide more protection from system parameter uncertainty than a smaller bandwidth. control system can. The circuitry and components needed to build the current loop can be costly when a hard disk drive manufacturer is producing millions of disk drives. Conventional current mode control architectures require both a measured coil current
301
and measured position
302
components to obtain a stable configuration.
The illustration of
FIG. 4
is derived from U.S. Pat. No. 4,679,103 awarded Jul. 7, 1987 and is largely representative of presently used current mode control techniques. The position signal P
OS
(t)
400
, which is a function of time, is amplified in block
411
and then separated into primary
402
and quadrature
404
components in the demodulator
412
. These signals are converted to digital samples P
ESP
(n)
406
and P
ESQ
(n)
408
by A/D converters
413
and
414
, respectively. Note that it is technically feasible to embody A/D converters
413
and
414
using a single digitizer multiplexed between the two analog inputs. This would save circuits. Note this alternative is feasible only if the sampling rate is low enough to permit the single digitizer to form two digital words each sample period, one for the in-phase signal and one for the quadrature signal. The voice coil current signal i(t)
403
is also sampled by an A/D converter
421
, which develops the current feedback signal i(n)
405
. Desired position signal XD
409
is supplied as an input to the state estimator and summing loop function
415
which completes computation of the control input signal u(n)
410
from the three components P
ESP
(n), P
ESQ
(n), and i(n). The digital signal u(n) is then converted in D/A converter
416
to the analog control input signal u(t)
420
which drives the power amplifier
417
.
The possibility of removing the current feedback signal
403
and driving the VCM in voltage mode without any current feedback would allow for cost reduction in the electronic control module. If this could be implemented losing none of the desired performance, an extremely attractive alternative using voltage mode control could be realized. The following illustrates a current mode design using available simulation and synthesis techniques. This will provide a contrast to the present invention in which control of the voice coil motor is accomplished using a simpler voltage mode architecture.
Current Mode Control System
The conventional current mode control system makes use of a reduced order model for current mode control of a hard disk drive actuator and the related control system. This control system is divided into two parts: one for seeking mode and one for tracking mode. The design of each controller is discussed and guidelines are provided where appropriate after the model of the actuator is discussed.
Current Mode Actuator Model
The reduced order model of a hard disk drive actuator under current mode control may be shown to be of the form
{dot over (x)}=Ax+Bu
(
t
−&lgr;)
y=Cx
(6)
where the state vector, x, was defined as
x
=
[
θ
ω
b
]
and
(
7
)
A
=
[
-
0
K
spr
J
0
-
1
K
v
J
0
0
K
t
K
drv
J
0
]
B
=
[
0
K
t
K
drv
J
0
]
C
T
=
[
K
pes
0
0
]
.
(
8
)
These equations (6), (7), and (8) represent collectively the information contained in the equation of
FIG. 4
for current mode control. The states are &thgr;, the angular position of the VCM shaft in units of radians, &ohgr;, the angular velocity of the VCM shaft in units of radians/sec and b, a bias in units of D/A bits. The output is y, the position of the read/write head in units of track bits. Since the plant is positioned by a discrete-time control system running in a DSP, a computational delay, &lgr;, was incorporated into the model.
In that discussion the discrete-time model of the plant becomes
x
[n+1]=&PHgr;
x
[n]+&Ggr;
2
u
[n]+&Ggr;
1
u
[n−1]
y
[n
]=Cx
[n] (9)
where &PHgr;=e
AT
s
, &Ggr;
1
=∫
T
s
−&lgr;
T
s
e
At
B dt and &Ggr;
2
=∫
0
T
s
−&lgr;
e
AT
B dt. By creating an additional state x
a
[n]=u[n−1] and augmenting it with the original state vector, the plant model becomes
[
x
[
n
+
1
]
x
a
[
n
+
1
]
]
=
[
Φ
Γ
1
0
0
]
[
x
[
n
]
x
a
[
n
]
]
+
[
Γ
2
1
]
u
[
n
]
y
[
n
]
=
[
C
0
]
[
x
[
n
]
x
a
[
n
]
]
(
10
)
For convenience in the firmware, the units of the state variables have be subjected to the transformation
T
=
[
K
pes
0
0
0
0
K
pes
·
T
s
0
0
0
0
1
0
0
0
0
1
]
(
11
)
This transformation was applied to give the angular position state units of bits and the angular velocity state units of track bits/sample. The units of the bias state and the prev
DiRenzo Michael T.
Heaton Mark W.
Magee David P.
Brady III W. James
Hudspeth David
Marshall, Jr. Robert D.
Slavitt Mitchell
Telecky , Jr. Frederick J.
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