Electricity: motive power systems – Positional servo systems – With compensating features
Reexamination Certificate
2000-02-14
2002-05-28
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Positional servo systems
With compensating features
C318S610000
Reexamination Certificate
active
06396234
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a servo control apparatus for controlling a subject to be controlled (i.e., controlled system) defined by a characteristics equation including at least a second order term, a first order term and a zero order term by the feedback control, more specifically, it relates to a servo control apparatus for estimating the disturbance to be applied on the controlled system and compensating an actual disturbance on the basis of the estimated disturbance.
2. Description of the Related Art
Recently, studies have been conducted actively on the technique for preliminarily estimating the disturbance to be applied on the controlled system in the feedback control of a controlled system, and compensating an actual disturbance by applying the manipulated variable considering the estimated disturbance to the controlled system.
As a preferable device for predicting the disturbance, a so-called state observer is attracting the attention recently.
The observer will be explained.
The observer is a device for estimating the state that cannot be detected actually from the detectable state. The state that cannot be detected actually is, for example, the state of a controlled system with a disturbance applied. The disturbance predicted to be applied on the controlled system is estimated by the observer so that the manipulated variable to be corrected is calculated based on the estimated disturbance variable, and the result thereof is added to the manipulated variable in the above-mentioned feedback control system. Accordingly, the disturbance actually applied to the controlled system is compensated.
The process for estimating the disturbance in the observer will be explained with reference to FIG.
12
.
FIG. 12
shows the case of adopting the observer to a focus servo control system for a focus actuator included in an information reproducing apparatus such as CD (Compact Disk) player as the controlled system, specifically it shows a feedback servo loop formed in the focus servo control system.
In the information reproducing apparatus, information recorded on an optical disk is reproduced by irradiating the information recording surface of the optical disk with a light beam. In this process, the focus actuator is used for setting a focal position of the light beam. The focus actuator is an actuator for moving an objective lens in the direction perpendicular to the information recording surface of the optical disk to focus the light beam on the information recording surface.
The focus actuator supports the objective lens by an elastic member such as a plate spring. A characteristics equation showing the controlled system having such a configuration, in general, includes at least a second order term, a first order term and a zero order term. Moreover, the characteristics equation can include also a term of three order or more. Hereinafter, such a controlled system will be referred to as a spring-mass system.
In
FIG. 12
, the controlled system U(s) is the focus actuator, and the controlled variable y is the position of the objective lens to be moved by the actuator.
The characteristics equation (transfer function) of the actuator can be represented in the second order delay system as:
U
(
s
)=
A×wa
2
/(
s
2
+2
×ka×wa×s+wa
2
) (1).
Herein, A is the gain of the actuator (m/Ampere), ka is the viscosity braking coefficient of the actuator, and wa is the natural frequency of the actuator (rad/sec).
With the conversion sensitivity for outputting the focus error signal in the actuator (that is, the conversion sensitivity determined by the sensitivity of the photo detector in the information reproducing apparatus and the amplifying ratio of the error generation amplifier) considered to be the position detecting sensitivity Ke (volt/m), it can be represented as:
REF−
y×Ke=er
(2).
Herein, REF is the desired value wherein the objective lens should be disposed, and er is the error in the above-mentioned feedback control system. As shown in
FIG. 12
, the error er obtained by the formula (2) is input to one of the input terminals of the observer.
On the other hand, the relationship between the manipulated variable (voltage value) u and the drive current i for driving the actuator in
FIG. 12
can be represented as:
i=Kdr×u
(3).
Herein, Kdr (Ampere/Volt) is the voltage/current conversion sensitivity of the driver (to be driven by the manipulated variable u) for generating the drive current i. The drive current i is converted to the input voltage v by the current/voltage converter with the current/voltage conversion sensitivity Kiv (Volt/Ampere) so as to be input to the other input terminal of the observer as shown by the following formula (4):
v=Kiv×I
(4).
Herein, the above-mentioned current/voltage conversion sensitivity Kiv corresponds to the conversion sensitivity in feeding back the drive current i to the observer, that is, so-called return resistance.
In order to simplify the explanation, the disturbance at a certain position is considered to be the only disturbance to be applied on the actuator. As shown in
FIG. 12
, with the disturbance variable defined as d, it can be represented as:
i×U
(
s
)+
d=y
(5).
Herein, in the case the desired value REF is zero (REF=0) in the above-mentioned formula (2), it can be represented as:
y×Ke=−er.
Therefore, from the above-mentioned formula (4), it can be represented as:
i=v/Kiv.
By erasing i and y from the above-mentioned formula (5), it can be represented as:
(
v/Kiv
)×
U
(
s
)+
d=−er/Ke.
By rearranging the formula, the disturbance variable d can be represented as the following formula (6) by using the input voltage v to be input to the observer corresponding to the drive current i and the error er:
d=−er/Ke
−(
v/Kiv
)×
U
(
s
) (6).
Herein, the parameters showing the inside of the observer as a model is represented as nominal values. In order to distinguish this from the actual control element, it is shown with an additional letter n. That is, the position detecting sensitivity Ke is represented as the position detecting sensitivity nominal value Ken, the voltage/current conversion sensitivity Kdr is represented as the voltage/current conversion sensitivity nominal value Kdrn, the current/voltage conversion sensitivity Kiv is represented as the current/voltage conversion sensitivity nominal value Kivn, and the controlled system U(s) is represented as the controlled system nominal value Un(s). The nominal value corresponding to the controlled system may be referred to, in general, as the internal model of the observer.
Incidentally, the nominal value is, for example, the torque rated value of the spindle motor for rotating the optical disk in an information reproducing apparatus, which is the value shown in the performance indication of the information reproducing apparatus, or the like. In the case, the performance indication is not provided, it is determined preliminarily by an experiment, or calculated by the theoretical calculation. The nominal value and the actual control element cannot always be equal due to factors such as insufficient accuracy of the determination or the calculation, aging, and temperature change.
From the above-mentioned formula (6), the estimated disturbance variable DOBS, which is the estimated variable of the disturbance d can be represented using the nominal values as:
DOBS=−
er/Ken
−(
v/Kivn
)×
Un
(
s
) (7).
Accordingly, the estimated disturbance variable DOBS can be calculated from the input voltage v and the error er using the observer without detecting the actual disturbance d.
In
FIG. 12
, the disturbance d is suppressed by generating the manipulated variable u by multiplying the calculated estimated disturbance variable DOBS by the inverse transmission characteristic
Nixon & Vanderhye
Pioneer Corporation
Ro Bentsu
LandOfFree
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