Electricity: motive power systems – Positional servo systems – Plural servomotors
Reexamination Certificate
1999-09-02
2001-02-13
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Positional servo systems
Plural servomotors
C318S626000, C250S229000, C355S035000, C355S083000
Reexamination Certificate
active
06188193
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to light valves, and more particularly, to a light valve with rotatable vanes controlled by separate integral servo motors that provide a fast response time for the light valve. One use of the light valve is in high speed film printing. The invention is directed to a shutter control system having improved zero-light position control.
2. Description of the Prior Art
A light valve is a device that varies the size of an aperture through which a light beam passes. A common use of light valves today is in film printing. The light valve is adjusted to control the cross-sectional size of the light beam passing through it for adjusting the exposure in accordance with the light levels of scene changes in the film being printed. Typically, separate light valves are used for each of three principal colors in the film printing process (red, green and blue), and the exposures from each light valve are independently controlled.
A common form of light valve includes a pair of vanes carried on a pair of rotating shafts. The vanes protrude into the path of the light beam, and the shafts are rotated to move the vanes toward or away from each other to form a variable width optical slit for controlling the amount of light passing through the vanes. Usually, the shafts are counter-rotated relative to each other and through approximately the same phase angle so the center of the aperture formed by the vanes does not appreciably shift laterally during operation of the light valve.
Light valves of various forms have been developed in the past for controlling the phase angle of the shafts that rotate the vanes. These have included use of a stepper motor, or means for rotating the shafts against stops which have been preset prior to each new aperture change. When the shafts are rotated by a common motor, mechanical coupling means such as linkages, gears, or the like are used to rotate both shafts simultaneously through the same angle. Such mechanical coupling increases the inertia of the moving parts which, in turn, slows down the response time of the light valve. Also, it is difficult to avoid backlash and added elasticity which can cause oscillations in the mechanical system that controls the vanes.
Moreover, these problems are amplified when attempting to speed up the response time of the light valve. U.S. Pat. No. 4,594,539 discloses a light valve having a vane control system which can effectively operate at higher speeds than previously known light valves. For instance, in the past, the fastest light valves with mechanical coupling had a response time on the order of about 5 milliseconds. In a film printing system, “response time” is the time required for the vanes to assume a new position to produce a new sized aperture for each scene change in the film. Previous light valves with their connecting linkages and gearing experienced oscillation problems when operated at higher speeds. Film printing speed with such devices had been limited to about 600 feet per minute. The light valve disclosed in '539 patent fulfilled the need to provide a film printing system that could operate at higher speeds. The slow response time of the light valves with mechanical coupling as used in previous film printing systems had been a principal obstruction to higher speed printing.
In the '539 patent (see
FIG. 8
) the vanes
18
move in unison either toward or away from the light source. As the left vane rotates counter-clockwise toward the light source, the right vane rotates clockwise also toward the light. In normal operation there are instances in which the vanes may have to be totally closed instantaneously to completely block passage of light. In the
FIG. 8
embodiment, the vanes do not close entirely. When moved to the closed position they leave a tiny narrow gap between their adjacent edges (through which one can see a narrow slit of light). The vanes must never touch each other because the power of each servo motor driving the vanes is approximately one horsepower; and the acceleration of the vanes is on the order of 1,000,000 radians/sec
2
. If the vanes were long enough to touch each other (to block all light transmission) they could irrecoverably jam or damage the vanes or the servo motors.
There are certain applications that require a light valve that closes completely so that no light is transmitted. The present invention solves the problem of closing the vanes, while operating at high speeds, so that the vanes and servo motors are not damaged in the zero-light position.
SUMMARY OF THE INVENTION
Briefly, as described in the '539 patent, the light valve includes a pair of rotating vanes for controlling the cross-sectional size of a light beam passing between the vanes. The vanes rotate on corresponding shafts, each controlled by its own integral servo motor. Separate feedback control systems produce electrical control signals to operate each motor for rotating each shaft independently of the other to control the desired phase angle of the light-admitting vanes.
In one embodiment of the light valve described in the '539 patent, the shafts are supported on pre-loaded backlash-free bearings, and both motors are permanent magnet direct current servo motors in which the wound armature is an integral part of each shaft. In another embodiment, each servo has a variable gain amplifier controlled so as to switch from a low gain mode to a high gain mode with each new command signal. The controls switch back to the low gain mode after the light valve aperture stabilizes.
The light valve has a fast response time and greatly reduced tendency to oscillate at higher speeds because of absence of backlash; because of absence of elastic coupling between the two shafts and between the two shafts and the motor; and because of short, stiff elastic paths between the various masses located on each shaft.
Because of the lack of mechanical linkages or gearing between the two shafts, and because of using a separate motor for each shaft, response time of the light valve is greatly reduced, and accuracy is improved. Use of the separate variable gain amplifier in each servo system also produces faster response time, reduced tendency toward oscillations, and improved accuracy.
The present invention provides an improvement to the light valve disclosed in the '539 patent. According to the present invention, a light valve for varying the size of an opening through which a light beam passes comprises a pair of spaced apart rotatable shafts, and a corresponding vane mounted on each shaft so the vanes extend side by side into the space between the shafts. Light is directed toward the vanes so that rotation of the shafts causes the vanes to move through an angle toward or away from each other for varying the size of the light beam passing between the vanes. A separate servo motor coupled to each shaft rotates the shafts in unison to adjust the amount of light passing between the vanes. Each servo motor includes a closed loop servo system responsive to a vane position input signal representing a desired position of each vane for varying the size of the light beam passing between the vanes. A separate position feedback signal is generated as a function of the actual position of the vane on each rotatable shaft throughout operation of the light valve. Separate motor control signals, responsive to the vane position input signal and to each position feedback signal, are produced for independently controlling operation of each servo motor. Each shaft and vane is rotated to constantly pass a selected amount of light between the vanes in response to the motor control signals.
In one embodiment, (1) the vanes (when closed) are aligned on a common plane, (2) the vanes both rotate about parallel axes in the same rotational direction away from opposite sides of the plane to enlarge the light opening between the vanes, (3) the vanes both rotate in the same rotational direction toward the plane to reduce the light opening between the vanes, and (4) the
Christie Parker & Hale LLP
Ro Bentsu
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