Coded data generation or conversion – Analog to or from digital conversion – Analog to digital conversion
Reexamination Certificate
2003-03-17
2003-11-25
Jeanglaude, Jean Bruner (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Analog to digital conversion
C360S077040, C360S078040, C341S050000
Reexamination Certificate
active
06653965
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a digital servo control method, digital servo controller, storage device, and head position control method for reading analog observation signals of the control target, performing digital processing, and generating control signals for the control target.
BACKGROUND ART
Servo controllers for servo-controlling a control target are widely used. Servo controllers are divided into analog servo controllers and digital servo controllers. An analog servo controller has many components, and a change in characteristics tends to occur due to the dispersion of characteristics of the components and age deterioration.
A digital servo controller, which performs digital processing using a processor, has fewer components, has an advantage in cost, has almost no characteristic changes, and allows characteristic resetting by rewriting the program. Therefore today digital servo controllers are used everywhere.
FIG. 29
is a block diagram depicting a conventional digital servo controller, and
FIG. 30
is a block diagram depicting the processing of DSP thereof.
FIG. 29
is a diagram depicting a configuration of a digital servo controller of a disk device where the observation signals for control are analog signals.
In
FIG. 29
,
15
are analog observation signals, such as head position information and acceleration information, and
10
is an A/D converter for converting analog signals into digital signals.
16
are digital observation signals converted by the A/D converter
10
, and
11
is a DSP (Digital Signal Processor) which performs operations according to the program which has been set in advance.
17
are digital control signals for determining current to drive the head, and
12
is a D/A converter for converting digital signals into analog signals.
18
are analog control signals converted by the D/A converter
12
,
14
is an amplifier for flowing predetermined current to the drive device of the head according to the voltage of the analog control signals, and
19
is a head drive current which flows to the head actuator.
Operation of the digital servo controller will now be described with reference to the processing blocks of the DSP
11
in FIG.
30
. At first, the analog observation signals
15
are converted into the digital observation signals
16
by the A/D converter
10
at a certain computation period Ts. Then the DSP
11
performs processing in FIG.
30
. In other words, the control signal
17
is computed by the following output equation and state equation.
Output equation:
yd[k]=C·x[k]+D·ud[k]
(1)
State equation:
x[k+
1]=
A·x[k]+B·ud[k]
(2)
Here ud[k]: an observation signal to be input to the controller
13
,
yd[k]: a control signal to be output to the amplifier
14
,
x[k]: an internal variable called a “state variable”,
A, B, C, D: constant matrixes (set in advance),
k: a parameter to indicate the sampling time, which is k=0 at the start of control, and increases as k=1, 2, 3, . . . at each computation period Ts.
In other words, the DSP
11
calculates the above mentioned output equation (Formula 1) from the digital observation signal
16
and the C·x[k], which has been calculated at the previous computation period, and determines the digital control signal
17
. Then the D/A converter
12
converts the digital control signal
17
into the analog control signal
18
, outputs the analog control signal
18
to the amplifier
14
to be the drive current
19
of the head. Finally, the DSP
11
calculates the above mentioned state equation (Formula 2) from the digital control signal
16
and the internal variable x[k], determines x[k+1] to derive C·x[k+1] that is used at the next computation period.
By repeatedly executing the above operation at each computation period Ts, the position and the speed of the head are controlled according to the observation signal.
For the equation to implement the servo control system by a processor, a method for using a transfer function for internal processing of the DSP is available, in addition to the above mentioned method for using the state equation.
FIG. 31
shows a block diagram depicting the internal processing of the DSP
11
when the following transfer function (Formula 3) is used.
y
(
z
)
u
(
z
)
=
α
0
·
z
⋀
n
+
α
1
·
z
⋀
(
n
-
1
)
+
⋯
+
α
n
-
2
·
z
⋀
1
+
α
n
-
1
z
⋀
m
+
β
1
·
z
⋀
(
m
-
1
)
+
…
+
β
m
-
2
·
z
⋀
1
+
β
m
-
1
(
3
)
In the formula (3), y(z): a control signal to be output to the amplifier
14
, u(z): an observation signal to be input from the A/D converter
10
, &agr;i, &bgr;i: constants, n, m: arbitrary integers, and z: a discrete time operator.
In the case of using this transfer function as well, in the digital controller in
FIG. 29
, the analog observation signal
15
is converted into the digital observation signal
16
by the A/D converter
10
at each predetermined computation period Ts. Then the DSP
11
calculates the above Formula (3) based on the digital observation signal
16
, and determines the digital control signal
17
. Then the digital control signal
17
is converted into the analog control signal
18
by the D/A converter
12
, and the analog control signal
18
is output to the amplifier
14
to be the drive current
19
of the head.
Compared with an analog servo controller, which is composed only of such analog electric circuits as resistors and capacitors, this digital servo controller has such advantages as (1) the analog controller is comprised of many components, and is high in cost compared with a digital controller, (2) characteristic changes occur easily in the analog controller due to the dispersion of characteristics of the components and age deterioration, but such characteristic changes do not occur in a digital controller, and (3) with a digital controller, the characteristics of the controller can be reset by merely rewriting the program of the DSP, so the digital controller is flexible.
However, as
FIG. 32
shows, the delay time (&tgr;+Ts/2) on average where Ts is the operation period of the DSP
11
and &tgr; is the operation time from when the observation signal is detected to the output signal, is always generated in the digital servo controller, compared with an analog controller. The cause thereof will be described below.
For the term of &tgr;, in the case of the Formula (1), the second term D·ud[k] of the Formula (1) is called the “direct term”, and is supposed to be reflected to the control signal y without a time difference with respect to the observation signal u. In the case of a digital controller, however, the control signal is output by the steps of [converting the observation signal u into a digital signal]→[operation by DSP]→[converting the control signal y into an analog signal]. Therefore time &tgr;, required for this processing, becomes the delay time.
For the term of Ts/2, in the digital controller, the analog control signal y(t), which is output at a certain time t, is held at a predetermined value during the operation period Ts of the DSP, so the analog control signal y(t) is not updated until (t+Ts). Therefore compared with an analog controller, which is sequentially updated according to the observation signal, the digital controller delays only for the operation time &tgr; immediately after output, but delays &tgr;+Ts at a point immediately before update. As a consequence, the delay time Ts/2 on average is generated.
In conventional control fields, delay time, which the digital controller has, has not been a major problem. This is because demand for control accuracy is not very high, and even an inexpensive DSP can implement operation period Ts and operation delay time &t
Fujitsu Limited
Greer Burns & Crain Ltd.
Jeanglaude Jean Bruner
LandOfFree
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