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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control apparatus for controlling a
vertical vibration elimination table which is used as a component in a
semiconductor production unit and which is of the type having four
supporting leg structures. More particularly, the present invention
relates to a vertical vibration eliminator control apparatus which
effectively performs control of motion of four degrees of freedom: three
degrees of freedom in a rigid motion mode including one degree of freedom
for translational motion and two for rotational motion, plus one degree of
freedom for flexible motion.
2. Description of the Related Art:
In general, a pneumatic vibration elimination table carries apparatuses or
instruments which are to be kept isolated from vibration. An example of
such an apparatus is an exposure XY-stage which is used in the production
of semiconductors. In order to achieve precise and quick exposure, the X-Y
stage has to be used on a vibration elimination table which is free as
much as possible from vibration. This is because the exposure has to be
conducted while the exposure X-Y stage is completely still. The operation
of the X-Y stage essentially includes intermittent motion of step and
repeat. Thus, the X-Y stage itself constitutes a source of vibration as it
generates vibration during a repeated step operation to vibrate the
vibration elimination table. Exposure operation cannot be commenced until
the vibration caused by the X-Y stage remains in the vibration elimination
table. Thus, the vibration elimination table is required to have such
characteristics that provide good balance between isolation from external
vibration and damping of vibration forcibly caused on the table by, for
example, an X-Y stage.
As well known in the field, a control apparatus for a pneumatic vibration
elimination table employs an active feedback control in which the table is
driven in such a manner as to cancel detected displacement of the
vibration elimination table caused by vibration. FIG. 2 is a block diagram
of a conventional control apparatus for a pneumatic vertical vibration
elimination table of a type in which vertical position control of a
vibration elimination table 8 is effected by means of four pneumatic leg
structures. The pneumatic supporting leg structures are arranged at four
corners of the vibration elimination table 8. Referring to FIG. 2, there
are shown a servo valve 1a which controls supply discharge of air as the
working medium to and from a pneumatic spring 2a, a position sensor 3a for
measuring vertical displacement of a pedestal 4a, an acceleration sensor
5a for measuring vertical acceleration of the pedestal 4a, a pre-loading
mechanical spring 6a and a viscous element 1a which inclusively represents
the viscosity of the entire mechanism including the pneumatic spring,
mechanical spring and other elements which are not shown. The mechanism
constituted by the components or elements 1a to 7a is collectively
referred to as a pneumatic spring supporting structure. As shown in this
Figure, four pneumatic spring supporting leg structures are used to
vertically support the vibration elimination table 8. Suffix symbols "a",
"b", "c" and "d" are attached to numerals indicating the elements or
components for the purpose of discrimination.
A description will now be given of the construction and operation of a
feedback apparatus 15a for the pneumatic spring supporting leg structure
having the components 1a to 7a. Output from the acceleration sensor 5a is
negative feedback, fed through a low-pass filter 9a having a moderate
amplitude and time constant, to the primary side of a voltage-current
converter 10a which is used for operating the servo valve 1a. This
acceleration feedback loop serves to stabilize the operation of the entire
mechanism and produces a damping effect. The output from the position
sensor 3a is input to a comparator circuit 12a trough a displacement
amplifier 11a so as to be compared with a command voltage 13a which is
equivalent to a command position of the pedestal 4a relative to the ground
level, and the difference is output as a position error signal e.sub.a.
This position error signal e.sub.a is supplied through a PI compensator
14a to excite a voltage current converter 10a. Consequently, the servo
valve 1a is opened and closed in accordance with the position error
signal, so that the pressure inside the pneumatic spring 2a is
correspondingly adjusted so as to hold the pedestal at the command
position with zero steady-state error P and I respectively indicate
proportional and integrating operations. The other three pneumatic spring
supporting leg structures are controlled by feedback systems 15b, 15c and
15d similar to the feedback system 15a described above.
As stated before, the vibration elimination table carries instruments or
apparatuses which are to be kept as much as possible from vibration. The
vibration elimination table 8 therefore must be constructed to have high
rigidity. Such a rigid construction causes the weight of the whole system
including the vibration elimination table 8 and instruments and
apparatuses carried by the table to be increased tremendously. In
addition, transportation and installation of the vibration elimination
table is dangerous and requires great care because the table including the
associated control apparatuses is large and heavy and because the table is
designed and constructed delicately to carry various types of precision
equipment. Consequently, a specific transportation facility is required
for the purpose of transportation of the vibration elimination table. It
is also necessary that the construction of the site where the vibration
elimination table and associated apparatuses are to be situated has a high
level of rigidity and strength to bear the large weight. More
specifically, a house having a floor with a large load carrying capacity
is required for accommodating the vibration elimination table.
Thus, the rigid construction of the vibration elimination table 8 not only
raises the cost of the table 8 itself but also incurs increased overall
costs inclusive of the costs for transportation, installation and building
of the accommodation facility such as a house.
Japanese Unexamined Patent Publication No. 3-28910 discloses a control
method for a pneumatic vibration elimination table which uses a state
feedback control. The state feedback method, however, too strongly relies
upon physical parameters of the controlled object, so that this technique
cannot suitably be used in practical commercial or industrial systems.
SUMMARY OF THE INVENTION
In view of the above-described problems of the known arts, the present
invention is directed to an improvement in a control apparatus of the type
which independently has a feedback system including a minor loop for
acceleration and a loop containing compensation for position error. These
features make this type of control apparatus adaptable to both practical
and industrial uses.
More specifically, an object of the present invention is to provide a
control apparatus for a vertical vibration elimination table which can be
safely and easily handled and which can be produced and installed at
reduced costs.
In accordance with one aspect of the invention, a control apparatus for
controlling a vertical vibration elimination table comprises four
supporting leg structures arranged at four corners of the vibration
elimination table, with each supporting leg structure including a position
sensor for producing a position signal indicative of a vertical position
of the vibration elimination table, an acceleration sensor for producing
an acceleration signal indicative of the vertical acceleration of the
vibration elimination table and an actuator for vertically supporting the
vibration elimination table. A feedback system receives the position
signals and the acceleration signals and feeds back the position signals
to form drive signals for driving the actuators and feeds back the
acceleration signals to the drive signals. The feedback system comprises a
4-degree-of-freedom motion mode extraction circuit which extracts motion
mode error signals of motion modes for four degrees of freedom of the
table, including one degree of freedom for translational motion, two
degrees of freedom for rotational motion and one degree of freedom for
twist motion, based upon error signals determined as deviations of the
position signals produced by the position sensors and corresponding
reference position signals. Compensation means produces compensated motion
mode signals by compensating the respective motion mode error signals such
that each motion mode error signal is compensated independently of other
motion mode error signals, and a motion mode distribution circuit produces
drive signals corresponding to the respective actuators based on the
compensated motion mode signals and delivers the driving signals to the
respective actuators.
In accordance with another aspect of the invention, a control apparatus
controls a vertical vibration elimination table apparatus supported by
actuators at four portions thereof. The actuators are operable in response
to signals which are fed back thereto and which are indicative of vertical
positions and vertical accelerations of the four portions of the table.
The control apparatus comprises a 4-degree-of-freedom motion mode
extraction circuit for extracting motion mode error signals of four
degrees of freedom of the table, including one degree of freedom for
translational motion, two degrees of freedom for rotational motion and one
degree of freedom for twist motion. Compensation means produces
compensated motion mode signals by compensating the respective motion mode
error signals such that each motion mode error signal is compensated
independently of the other motion mode error signals, and motion mode
distribution circuit means produces drive signals based on the compensated
motion signals and delivers the drive signals to the respective actuators.
In addition, control means drives the actuators based on the drive signals
supplied by the motion mode extraction circuit.
In accordance with yet another aspect of the invention, a vibration
elimination apparatus comprises a base and a plurality of actuator units
supporting the base at different positions. Each actuator unit includes an
actuator for moving the base, a position sensor for measuring the position
of the base and an acceleration sensor for measuring the acceleration of
the base. In addition, control means controls the actuators based on
outputs of the position sensors and the acceleration sensors. The control
means includes a motion mode extraction circuit which extracts, based on
deviations of the outputs from the respective position sensors from
corresponding different positions, error signals of motion modes of four
degrees of freedom of the base, including one degree of freedom for
translational motion, two degrees of freedom for rotational motion and one
degree of freedom for twist motion. The control means controls the
actuators based on independent motion mode error signals.
In operation, when the vibration elimination table is deviated from the
reference position, actuators are driven in accordance with drive signals
based on the error signals, so that the table is reset to the reference
position. In this operation, error signals of various motion modes of one
degree of freedom for translational motion, two degrees of freedom for
rotations and one degree of freedom for twist motion, are extracted from
the error signals. Compensation is adequately effected on the error signal
for each of the motion modes. Compensated error signals are again
distributed to driving components for the respective actuators. Thus,
compensation is effected also on twist motion of the vibration elimination
table so that an effective vibration elimination is achieved. Thus, the
requirement for high rigidity of the table structure is less strict. Any
twist motion in vibration mode, which may occur due to lack of rigidity,
is effectively eliminated. Any structure has an aspect of being flexible
from the view point of high-frequency vibration, however it may be rigid.
Therefore, the invention also can be applied to existing rigid vibration
elimination tables so as to further improve the accuracy of the posture
control.
The above and other objects, features and advantages of the present
invention will become clear from the following description when the same
is read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an embodiment of a control apparatus for a
pneumatic vertical vibration elimination table in accordance with the
present invention;
FIG. 2 is a block diagram of a known control apparatus for a pneumatic
vertical vibration elimination table;
FIGS. 3(a)-3(d) are illustrations of motion modes of a vibration
elimination table; and
FIG. 4 is a waveform chart illustrative of convergence of a motion mode
error signal as obtained when a stepped twist is applied as a disturbance
to the pneumatic vertical vibration elimination tables of FIGS. 1 and 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment of the present invention will be described with
reference to the accompanying drawings.
FIG. 1 is a block diagram illustrative of the construction of an embodiment
of the pneumatic vertical vibration elimination table.
Each of the elements shown in block outline in FIG. 1, as well as in FIG.
2, is well known per se, and a specific type of construction is not
critical to carrying out the invention or to the disclosure of the best
mode for carrying out the invention. As will be seen from this Figure, a
control apparatus for a pneumatic vertical vibration elimination table has
four pneumatic supporting leg structures arranged at four corners of a
tabular vibration elimination table 8 and a feedback system for locating
the table 8 at a command vertical position without steady-state error. The
feedback system comprises a 4 degree of freedom motion modes extracting
circuit 16 which is connected between comparator circuits 12a to 12d and
PI compensators 14a to 14d. The circuit 16 extracts, upon receipt of
position error signals [e.sub.a, e.sub.b, e.sub.c, e.sub.d ].sup.T from
respective pneumatic supporting leg structures, motion mode error signals
[z.sub.g, z.theta..sub.x, z.theta..sub.y, z.theta..phi.].sup.T including
three kinds of rigid motion modes (one degree of freedom for translational
movement and two degrees of freedom for rotations) and one kind of
flexible motion mode (one degree of freedom for twisting). The feedback
system also comprises a motion mode distribution circuit 17 which is
connected between the PI compensators 14a to 14d and voltage-current
converters 10a to 10d corresponding to servo valves 1a to 1d. The motion
mode distribution circuit 17 performs a computation using motion mode
compensation signals [z.sub.g ', z.theta..sub.x ', z.theta..sub.y ',
z.theta..phi.'].sup.T so as to produce drive signals [S.sub.a, S.sub.b,
S.sub.c, S.sub.d ].sup.T and distributes these drive signals to the
actuators of the four supporting leg structures. The superscript suffix T
indicates transposition. The sums of the drive signals [S.sub.a, S.sub.b,
S.sub.c, S.sub.d ].sup.T and the negative feedback signals through
low-pass filters 9a to 9d are supplied to voltage current converters 10a
to 10d. Other portions are materially the same as those of the known
apparatus described before in connection with FIG. 2.
According to this arrangement, it is possible to detect four types of
motion modes assessed by the vibration elimination table 8, through the
computation which is performed by the motion mode extraction circuit 16
based on the signals output from the comparator circuits 12a to 12d.
These four types of motion modes are: (a) translational motion mode in
which to whole vibration elimination table 8 is evenly displaced in a
vertical direction, (b) rotation-about-X-axis mode in which the Whole
table 8 rotates about an X-axis, (c) rotation-about-Y-axis mode in which
the whole table 8 rotates about a Y-axis, and (d) twist mode, as shown in
FIG. 3. The motion of three degrees of freedom presented as the motion
modes (a), (b) and (c) is a rigid motion mode which is not accompanied by
deformation of the vibration elimination table 8, while the twist mode (d)
is a flexible motion mode which is attributable to the fact that the
vibration elimination table 8 is not perfectly rigid, i.e., it has a
certain degree of flexibility.
The motion mode error signals [z.sub.g, z.theta..sub.x, z.theta..sub.y,
z.theta.f].sup.T which are output from the motion mode extraction circuits
16 are delivered to the respective PI compensators 14a to 14d. In
contrast, in the conventional control apparatus shown in FIG. 2, the PI
compensators 14a to 14d receive local position error signals [e.sub.a,
e.sub.b, e.sub.c, e.sub.d ].sup.T of the respective pneumatic supporting
leg structures. Each of these position error signals includes all motion
components concerning the motion of the vibration elimination table 8,
i.e., translational motion, rotations and twist, so that the output signal
from each of the PI compensators 14a to 14d contain various motion
components. In contrast, the embodiment of the invention shown in FIG. 1,
each of the PI compensators 14a to 14d perform compensation of only one
motion component. More specifically, the PI compensator 14a conducts
compensation of the translational motion component alone. Similarly, the
PI compensators 14b, 14c and 14d conduct compensations of the about-X-axis
rotation motion, about-Y-axis rotation motion and twist motion component,
respectively.
The motion mode compensation signals [z.sub.g ', z.theta..sub.x ',
z.theta..sub.y ', z.theta..phi.'].sup.T output from the PI compensators
14a to 14d of FIG. 1 are delivered to the motion mode distribution circuit
17, and the outputs [S.sub.a, S.sub.b, S.sub.c, S.sub.d ].sup.T from the
motion mode distribution circuit are used as signals which activate the
respective pneumatic supporting leg structures arranged at four corners of
the vibration elimination table 8. Namely, the motion mode compensation
signals [z.sub.g ', z.theta..sub.x ', z.theta..sub.y ',
z.theta..phi.'].sup.T corresponding to translational, rotational and twist
motion components are made to pass through the motion mode distribution
circuit 17 so as to be transformed again into the signals corresponding to
the drive of the respective pneumatic supporting leg structures arranged
at the four corners of the vibration elimination table 8.
The computation performed by the motion mode distribution circuit is
represented by the following formula (1), while the computation performed
by the motion mode extraction circuit 16 is represented by the following
formula (2). These computations can easily be realized by using an
operation amplifiers and resistors.
##EQU1##
The superiority of the control apparatus of the embodiment shown in FIG. 1
over the known apparatus shown in FIG. 2 will be demonstrated by comparing
operation performances of both apparatuses with each other. In each of
these apparatuses, adder terminals were provided in input stages of the
voltage current converters 10a to 10d and stepped voltages were applied to
these terminals to apply such a disturbance as to cause stepped twist
motion of the vibration elimination table 8. The control apparatus shown
in FIG. 2 showed such control characteristics that significant excitations
appear not only in the twist error signal z.theta..phi. but also in the
error signals of other motion modes, in response to the disturbance which
caused twisting motion. In contrast, in the case of the control apparatus
shown in FIG. 1, error signals of operation modes other than the twist
error signal z.theta..phi. showed no excitation. In addition, the
excitation amplitude of the twist error signal z.theta..phi. itself is
significantly smaller than that in the control apparatus shown in FIG. 2,
and the setting time is also shortened as compared with that offered by
the apparatus shown in FIG. 2. Thus, the described embodiment of the
control apparatus for pneumatic vertical vibration elimination table can
effectively suppress flexible twist motion mode. The control apparatus of
this embodiment, therefore, allows the use of a vibration elimination
table having reduced rigidity.
Although not illustrated, comparisons between these two types of
apparatuses were conducted by applying to the vibration elimination table
8 disturbances for causing translational motion and two types of rotations
of the table 8, in the same way as that used in the experiment described
above in connection with FIG. 4.
Needless to say, the embodiment of the present invention provided much
superior convergence of motion mode error signals than the known control
apparatus.
Although the invention has been described through illustration of a
preferred embodiment, it is to be understood that the described embodiment
is only illustrative and various changes or modifications may be imparted
thereto. In the illustrated embodiment, the control apparatus is used in
combination with a pneumatic vertical vibration elimination table which
employ pneumatic springs as the actuators, so as to control not only rigid
motion modes but also flexible motion mode. The invention, however, is not
limited to the control of an active vibration elimination table using
pneumatic springs as the actuators but also to control other types of
vibration elimination tables such as an active vibration elimination
tables which use voice coil motors as the actuator.
It is also to be noted that the control system used in the embodiment shown
in FIG. 1, which is composed of analog computing circuits, may be partly
or wholly replaced with digital computing means such as an electronic
computer.
As has been described, according to the present invention, error signals
are extracted not only for three rigid motion modes including one degree
of freedom for translational motion and two degrees of freedom for
rotations but also for flexible twist motion mode, as a result of
computations which are performed based on position error signals derived
from four positions. Optimum compensations are effected on the respective
motion modes and the thus obtained compensated signals are again
distributed to driving signals corresponding to drives of the respective
actuators. It is therefore possible to apply effective controls on the
respective motion modes independently. In the conventional control
apparatus in which independent feedback system is used for each supporting
leg structures, the control of posture of the vibration elimination table
could not effectively be done due to the mutual influences of motions of
four supporting leg structures. According to the invention, this problem
is overcome by virtue of the above-described features.
Hitherto, the work for stably setting the vibration elimination table by
four supporting leg structures was rendered difficult due to interference
of effects of controls of the four supporting leg structures. Namely, when
an adjustment such as setting of a reference position signal, adjustment
of a control loop gain or adjustment of a time constant of the PI
compensator is conducted for one of the supporting leg structures, the
effect of the adjustment inevitably influences the motions of other
supporting leg structures. Consequently, complicated and troublesome work
has been necessary to stably set the vibration elimination table 8.
According to the invention, however, the adjustment work for stabilizing
the motion of the vibration elimination table 8 can be conducted on the
basis of the synthetic motion of the whole vibration elimination table
apparatus, without relying upon independent adjustment of local portions
of the table. Consequently, the adjustment work is remarkably facilitated
as compared with the conventional control technique.
Conventionally, a vibration elimination table is required to have a very
high level of rigidity in order to carry heavy instruments or apparatuses,
resulting in a tremendous increase in the weight of the whole table
apparatus including the instruments or apparatuses carried thereon.
Consequently, transportation and installation of vibration elimination
table could not be done easily and the site of installation required
special construction work such as reinforcement of the floor for bearing
the large weights of the entire vibration elimination table apparatus.
According to the invention, however, it is possible to effectively suppress
the motion of the table in a twist mode, thus reducing the strictness of
requiring that the vibration elimination table be very rigid.
Consequently, the overall weight of the entire vibration elimination table
apparatus can be reduced, thus facilitating transportation and
installation, while eliminating the necessity for stiffening the floor of
the house as the installation site, thus contributing to a reduction in
the total cost.
Obviously, any structure can more or less have a flexible motion mode such
as twist motion, however it may be rigid. Thus, the present invention can
appreciably improve the precision of posture control even when applied to
an existing highly rigid vibration elimination table.
Although a specific embodiment of the present invention has been described
above in detail, it will be understood that this description is merely for
purposes of illustration. Various modifications of and equivalent
structures corresponding to the disclosed aspects of the preferred
embodiment in addition to those described above may be made by those
skilled in the art without departing from the spirit of the present
invention which is defined in the following claims, the scope of which is
to be accorded the broadest interpretation so as to encompass such
modifications and equivalent structures.
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