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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to the field of control systems for actuating
optical elements.
Electrically controlled movable optical elements such as tip-tilt mirrors
are often employed for telescopic tracking or viewing of various objects.
In accordance with the prior art, a substantial plurality of mirror
actuators are affixed to one side of a mirror to be pointed in various
directions. The actuators move the mirror by different amounts so that the
mirror as a whole points in a given selected direction, and such amounts
are proportional to electrical control signals applied to the input
circuits of the actuators. Since there are often many actuators affixed to
the movable mirror, numerous different control voltages are individually
applied to the actuators to cause a given selected mirror orientation.
This means that a large number of control voltages which differ from one
another must be individually generated.
SUMMARY OF THE INVENTION
It is an object of the present invention to eliminate the application of a
large number of control voltages which generally differ from one another
and are applied individually by driver amplifiers to the actuators,
thereby to simplify the electrical control circuitry. It is a further
object of the invention to eliminate the necessity for a substantial
number of such driver amplifiers.
It is a further object of the present invention to automatically compensate
for variations in the responsiveness or sensitivity of individual
actuators; thus actuators which have a different degree of responsiveness
to a particular voltage may be readily employed in a mirror actuation
system without detrimental effect, in contrast with the prior art.
In accordance with a preferred embodiment of the invention, a two
dimensional resistive mat has a single but variable control voltage
gradient applied thereto for controlling the motion of all actuators in
the two dimensional actuator array. Each actuator is electrically coupled
to the surface of the resistive mat by an adjustable contact means. The
appropriate response of each actuator is established by testing a number
of given points with the adjustable contact until a particular point is
found for each actuator, which results in the appropriate responsiveness
of each actuator with respect to the single control voltage gradient
applied across the mat.
Other objects, features and advantages of the present invention will become
apparent upon study of the following description taken in conjunction with
the drawings in which:
FIGS. 1 and 2 illustrate a preferred embodiment of the invention.
SPECIFIC DESCRIPTION
Referring now to FIG. 1, a movable mirror 1 is schematically illustrated,
having a lower surface 2 affixed to six electro-mechanical actuators 3, 7,
8, 9, 12', and 13'. Each actuator could comprise a plurality of
piezo-electric layers, separated by electrodes which produce expansion or
contraction of the layers depending upon the applied voltage polarity, as
is well known to those skilled in the art. Let it be assumed that it is
desirable to tip movable mirror 1 in a counterclockwise direction as
indicated by arrows 16' and 17'. Under these circumstances, the right hand
portion 20' of movable mirror 1, would be displaced upwardly to a greater
extent relative to mirror portion 18'. In like manner, mirror portion 19'
would be displaced by an appropriate lesser amount in order to cause the
mirror to change its axial direction without bending, which generates
detrimental internal stress. In like manner, mirror portion 21' at the
extreme left hand portion of FIG. 1 would be moved downwardly as indicated
by arrow 17' to an extent greater than mirror portions 22' and 23'.
In accordance with the invention, the inherent sensitivity or
responsiveness of an individual actuator to a given applied voltage value
could be the same, or surprisingly could differ substantially, with
respect to the responsiveness of the other actuators without adversely
affecting operation of the system by inducing bending stresses in the
mirror. This is due to the provision of a two dimensional impedance
device, preferably a resistive mat 5 and adjustable actuator input
terminal contacts 6, which function as explained later in greater detail.
These contacts further eliminate the need to apply a plurality of precise
different control voltages to individual actuators.
While a single line of actuators such as illustrated in FIG. 1 may be
employed to tip the mirror back and forth only about a single axis,
generally rings of actuators of FIG. 2 are positioned beneath the mirror,
such actuators having upper surfaces thereof affixed to the lower mirror
surface as in FIG. 1, so that the mirror axis 24' may be pointed in any
direction. Thus, actuators 11 and 21 are positioned on radial line 31,
actuators 13 and 23 are positioned along radial line 32, actuators 15 and
25 are positioned along radial line 33, and actuators 17 and 27 are
positioned along radial line 34. In like manner actuators 12 and 22 are
positioned along radial line 36, actuators 14 and 24 are positioned along
radial line 37, actuators 16 and 26 are positioned along radial line 38
and actuators 18 and 28 are positioned along radial line 39. Accordingly,
the actuators in the first or inner ring are assigned the tens digit 1,
whereas the actuators in the second ring are assigned the tens digit 2,
and the units digit, such as 1 in actuator 21 indicates that the actuator
is in the first group extending along radial line 31 along with actuator
11. In like manner radial line 37, for example, intersects actuators 14
and 24 which form the fourth group. In the illustrated example of FIG. 2,
eight actuators are positioned within each ring illustrated; a third ring
of eight actuators could also be employed. Thus the three actuators, 3, 7
and 8 of FIG. 1, would be positioned along a particular radial line,
whereas actuators 9, 12' and 13' would be positioned along a second radial
line in 180.degree. opposition to the first radial line.
The actuator arrangement in FIG. 2 illustrates a prior art configuration of
an array of actuators. Only two rings have been illustrated in the
interest of simplicity and will suffice to support an appropriate
explanation of the nature of the array. In accordance with known prior art
techniques, let it be assumed that it is desirable to tip the mirror in
and out of the plane of FIG. 2 and to rotate about horizontal radial line
32. A positive going control signal would be applied to actuator 11 having
a given value, whereas an appropriately larger control voltage would be
applied to actuator 21 so as to move the mirror portion at 21 to a greater
extent relative to that portion of the mirror affixed to actuator 11. In
like manner, actuator 15 would cause an opposite displacement of the
mirror portion adjacent 15, and actuator 25 would cause an even greater
displacement of the portion adjacent 25 in an opposite direction, so that
four different control voltages would be applied to the four transducers
for actuators 11, 21, 15 and 25. In accordance with the present invention,
the four individually applied control voltages are reduced in number by
applying a single control voltage differential across resistive mat 5.
Should a third, fourth and fifth actuator ring be employed, greater
economies are realized since all of the actuators along the radial lines
31 and 33 for example, would respond to the single control voltage
differential. The dual polarity aspect discussed before, enables the
push-pull operation of the optical element in one direction or the other
depending upon the voltage polarity applied across the mat, whereas the
degree of change in direction is a function of the magnitude of this
voltage.
While the nature of the control voltages applied to the various actuators
in the rings do not directly involve the present invention, the following
prior art material is presented in order to give the reader further
background.
For the maximum rotation of the mirror about the horizontal radial lines 32
and 34, the maximum voltage would be applied to the outermost actuators
and lesser voltages would be applied to those actuators closer and closer
to the central mirror portions. For lesser amounts of rotation, lesser
voltages would of course be applied. In the special case of rotation about
horizontal radial lines 32 and 34, no voltages would be applied to
actuators 13, 23, 17 and 27, whereas reduced voltages would be applied to
the intermediate actuators 22, 28, 24 and 26. The maximum voltage V.sub.o
is defined as equal to the maximum deflection called for divided by the
sensitivity or responsiveness of the edge actuator, in microns of
displacement/volt. In the special case of rotation about the horizontal
radial lines 32 and 34, V.sub.21 (V.sub.o) could for example equal plus
100 volts and the voltage applied to actuator 11 would be that reduced
voltage appropriate to move the mirror without tending to induce bending
stresses therein. In like manner, the voltage applied to actuator 25 could
be minus 100 volts to effect a complimentary opposite displacement,
whereas the voltage applied to actuator 15 would be the aforesaid reduced
voltage with a negative value. To effect this particular rotation about
the horizontal radial lines 32 and 34, V.sub.22 and V.sub.28 would be, for
example, positive and equal to V.sub.21 cos theta (assuming all radial
lines are angularly displaced from each other by equal angles) whereas
V.sub.24 and V.sub.26 would be equal to V.sub.25 cos theta and would of
course be negative. In the specific case of rotation of the mirror in and
out of the plane of FIG. 2 and about vertical radial lines 31 and 33, the
voltage pattern would be rotated 90.degree., such that the voltages
previously applied to actuators 11 and 21 would now be applied to
actuators 13 and 23 and so forth.
In the general case, rotation for a given positional command could be about
some axis perpendicular to line 41, for example, displaced angularly from
line 31 by some angle phi shown in FIG. 2.
The maximum voltage for maximum mirror displacement to be applied to
actuator 22, instead of being V.sub.o cos theta for rotation about 32 and
34, would now be +V.sub.o cos (theta-phi); the maximum voltage applied to
actuator 23 would be V.sub.o cos (2 theta-phi) and so on. This voltage for
the general case equals V.sub.o cos (n theta-phi) where n is the number of
the radial line starting with 36, and theta is the angular separation
between radial lines.
Let it be assumed that voltage source 4 produces a positive 100 volt
potential at the right hand edge of resistive mat 5 and a negative 100
volt potential at the left hand edge of resistive mat 5. With this
connection, it should be apparent that a voltage gradient will be produced
along the length of the mat such that for example, central point 51 will
have a potential of 0 volts with respect to ground. In accordance with the
above mentioned teachings of the prior art, the appropriate voltage to be
applied to each actuator, as discussed herein before, is "picked off" of
the surface of the resistive mat, so that for a given voltage applied
across the mat, the appropriate responsiveness of each actuator is
attainable. The actuators would be placed in the approximate positions
estimated in accordance with the above prior art teachings and the
adjustable contact of each actuator is positioned upon various points on
the surface of the mat until a particular point is located to produce an
appropriate response to each actuator with respect to the application of a
given control voltage across the mat. When the particular point is
located, the adjustable contact is permanently affixed to the surface of
the resistive mat.
It may be shown that the voltage at any particular point x along the length
of the mat may be given by the formula V.sub.x equal V minus 2Vx/X where x
is equal to the distance from the right hand edge of the mat to the
particular point. Thus if x equals 0, at the right hand edge, the voltage
is plus V, if the left hand edge of the mat is examined, x equals X and
the resulting voltage is minus V. If x is one half of X, V.sub.x equals 0
at the central portion of the mat. Accordingly any applied actuator
voltage may be selected by moving the contact point along x.
Now let it be assumed the right hand actuator 3 in FIG. 1 is somewhat
"sluggish" so that a greater voltage must be applied to it to obtain the
same mechanical response as the "average" actuator in the array. Under
these circumstances the contact at 52 would be displaced toward the right
to increase the voltage to be applied to the actuator to compensate for
this sluggishness. Should the actuator be more highly responsive than
required, the adjustable contact 6 would be moved to the left to reduce
the applied voltage, to compensate for this fact. Thus it should be
appreciated that actuators having various degrees of responsiveness to a
particular voltage may be employed in the array without detrimental
effects, and as a result, tight manufacturing tolerances are unnecessary
to reduce costs. Also the application of particular control voltages to a
large number of actuators has been eliminated in accordance with the
invention since only one voltage gradient need be applied across the
resistive mat. Because of this calibration feature, the resistive mat
itself need not be necessarily homogenous, and any non linearity with
respect to the voltage potential along the length of the mat may be
automatically compensated for by the process of selecting the appropriate
particular point of contact with the mat. While a precision network of
individual resistors could be employed, a continuous mat having an
infinite number of "pickoff" points is preferred.
It should be apparent that the teachings above may be expanded whereby a
single potential gradient may be applied across the mat in any given
direction, to enable changing the direction of mirror tilt. For example, a
circular mat may be provided, and the potentials may be applied at
diametrically opposite edges of the mat to establish the axis of rotation
of the mirror as explained above in connection with the prior art array of
FIG. 2. A single pair of diametrically opposite movable slip contacts
could be provided, whereby such contacts function as brushes to establish
the direction of the potential gradient across the circular mat.
Alternatively, a substantial number of pairs of permanent contacts could
be employed, where one selected pair at a time is energized.
While particular embodiments of the present invention have been shown and
described, it is apparent that various changes and modifications may be
made, and it is therefore intended that the following claims cover all
such modifications and changes as may fall within the true spirit and
scope of the invention. For example, the number of groups of actuators and
the number of actuators in each group may vary widely.
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