Patent Application
20140061467
1. Field of the Invention
This invention relates generally to variable apertures, and more particularly to variable apertures capable of operating in a vacuum and cryogenically-cooled environments.
2. Description of the Related Art
Some photodetectors, such as those commonly found in infrared (IR) sensors, require very low temperatures for optimum performance. For example, typical IR sensors are designed to operate in an environment that has been cryogenically-cooled to temperatures below 150° K; the sensors are also typically operated in a vacuum.
Such sensors are typically part of an imaging system which includes optical elements that affect the electromagnetic radiation that reaches the sensor. One such element is a variable aperture mechanism (VAM), which is mounted in front of the IR sensor and is used to control the sensor's field-of-view (FOV). Since the target and the VAM are both in the sensor's FOV, to improve the overall system sensitivity the VAM needs to be cryogenically cooled so that the thermal signature of the VAM does not compete with that of the target.
However, maintaining reliable operation of the VAM in a vacuum and at cryogenic temperatures can be problematic. One known design employs three motors to operate the moving elements of the VAM; however, this arrangement requires that the motors be precisely aligned, and fails to operate if any of the motors fails. Other approaches require complex mechanical designs that thermally isolate the drive motor from the moving elements; however, the required complexity can result in unreliable operation.
A variable aperture mechanism (VAM) is presented which addresses the challenges noted above, providing reliable operation in a vacuum and cryogenically-cooled environment.
The present VAM comprises a cam assembly, a single motor capable of rotating the cam assembly, and a pair of aperture members which are coupled to the cam assembly and arranged to form an aperture, the size of which varies with the position of the cam assembly. The motor is preferably a piezoelectric motor, and the cam assembly preferably includes a ceramic disc cam. The VAM is preferably arranged such that the aperture members are decoupled from the motor.
The VAM would typically be used with a sensor having an associated optical field-of-view (FOV), with the aperture members moving in and out of the FOV with the rotation of the cam assembly such that the VAM provides an aperture capable of providing multiple f-numbers. There is preferably a thermal link between the aperture members and a cryogenically-cooled surface such that the aperture members are also cryogenically-cooled. Several illustrative VAM designs are discussed, including an embodiment in which the aperture members comprise two scissors-like blades which rotate about a common pivot point, and another in which the aperture members comprise two flat blades that move linearly in opposite directions.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and claims.
a and 1b are perspective views of one possible embodiment of a VAM per the present invention, showing the aperture members at their fully open and fully closed positions, respectively.
c is a cutaway view of the VAM shown in
a and 2b are perspective views of another possible embodiment of a VAM per the present invention, showing the aperture members at their fully open and fully closed positions, respectively.
One possible embodiment of a VAM suitable for use in a vacuum and cryogenically-cooled environment is shown in
In a preferred embodiment, the cam plate has at least two pins which extend from the cam plate in a direction normal to the plane in which the cam assembly rotates, and each of the aperture members has a hole which surrounds a respective one of the pins, such that the aperture members move with the pins. For the exemplary embodiment shown in
Drive cam 12 preferably has at least two drive cam pins 29 which extend from the drive cam in a direction normal to the plane in which the cam assembly rotates, and cam plate 13 preferably has corresponding cam plate holes 30 which surround respective ones of the drive cam pins such that cam plate 13 rotates with drive cam 12. Cam plate holes 30 are preferably oversized with respect to drive cam pins 29 such that aperture members 16 and 18 are decoupled from motor 14 and its associated vibration.
Motor 14 is preferably a commercial piezoelectric motor. To reduce the amount of heat conveyed to aperture members 16 and 18 via the drive mechanism, the cam assembly preferably includes a ceramic disc cam and cam pins 22 and 24 preferably have a low thermal conductivity.
The present VAM would typically be used with a sensor, such as an IR sensor, having an associated optical field-of-view (FOV). In this application, the VAM is positioned such that aperture members 16 and 18 are moved in and out of the optical FOV with the rotation of the cam assembly, thereby enabling aperture 20 to be set to one of at least two different f-numbers. The VAM may be arranged such that the cam assembly can be rotated into either of two possible positions, thereby enabling aperture 20 to be set to one of two different f-numbers. The VAM might also be arranged such that the cam assembly can be rotated into more than two positions, or arranged such that position is infinitely variable by, for example, moving the blades out of plane so that they can overlap; these embodiments would further increase the available number of f-numbers.
As noted above, the present VAM is suitably employed with an IR sensor designed to operate in an environment that has been cryogenically-cooled and in a vacuum. To improve system sensitivity, the VAM should also be cryogenically-cooled to minimize the VAM's contribution to the total signal. Thus, there is preferably a thermal link between each of the aperture members and a cryogenically-cooled surface. For example, the VAM in
A cold shield 31 as might be used with an IR sensor equipped with a VAM per the present invention is suitably made from copper, or nickel. Aperture members 16, 18 preferably have a high thermal conductivity, as would be suitably provided by materials such as molybdenum, magnesium, aluminum or beryllium copper.
One possible embodiment of aperture members 16 and 18 is two scissors-like blades which rotate about a common pivot point; an example is shown in
The distal ends of blades 16 and 18 are preferably arranged such that they lie in the same optical plane, which allows more precise control of aperture f-number and enables the tolerance allowance for the placement of the aperture relative to the image plane to be increased. The blades would typically be placed between atop plate 42 and a VAM base 44 through which pivot point 40 extends, and which may include one or more physical stops 46 which limit how far each blade can move when they are moving together, and one or more physical stops 48 which limit how far each blade can move when moving apart. Top plate 42 and VAM base 44 should also have a high thermal conductivity, as would be suitably provided by materials such as molybdenum, magnesium, aluminum or beryllium copper.
a shows an embodiment of a VAM which uses scissors-like blades 16, 18 as aperture elements, in which the blades are fully open and aperture 20 is at its maximum possible size.
A cutaway view of the VAM of
Another possible embodiment of a VAM suitable for use in a vacuum and cryogenically-cooled environment is shown in
Here, however, aperture members 108 and 110 comprise two flat blades that move linearly in opposite directions. The proximal ends of flat blades 108 and 110 are coupled to pins 114 and 116, respectively, such that the distal ends of the flat blades move together and apart in a push-pull arrangement to affect the aperture size in response to the rotation of cam assembly 102, 104.
As noted above, when used with a sensor that must be operated at low temperatures, there is preferably a thermal link between each of the aperture members and a cryogenically-cooled surface. For example, the VAM in
Flat blades 108, 110 would typically be placed between atop plate 128 (shown transparent) and a VAM base 130, which may include one or more blade stops 132 which limit how far each blade can travel.
a shows an embodiment of a VAM which uses flat blades 108, 110 as aperture elements, in which the blades are fully open and aperture 112 is at its maximum possible size.
A VAM as shown in
Note that the blades of a flat blade embodiment as shown in
Drive cam 102 preferably has at least two drive cam pins 140 which extend from the drive cam in a direction normal to the plane in which the cam assembly rotates, and cam plate 104 preferably has corresponding cam plate holes 142 which surround respective ones of the drive cam pins such that cam plate 104 rotates with drive cam 102. Cam plate holes 142 are preferably oversized with respect to drive cam pins 140 such that aperture members 108 and 110 are decoupled from motor 106 and its associated vibration.
An embodiment employing flat blades might also include a sliding track module located between the proximal ends of the blades and arranged such that the force applied to the blades by the rotation of the cam is applied to the blades near their respective centerlines, which may help to maintain proper position and alignment of the blades when they are sliding. One possible implementation of such a module 150 is shown in
One beneficial aspect of the present VAM design is that it is very compact, and able to fit within a standardized Dewar package. As such, the present VAM is likely to be backward compatible.
The embodiments of the invention described herein are exemplary and numerous modifications, variations and rearrangements can be readily envisioned to achieve substantially equivalent results, all of which are intended to be embraced within the spirit and scope of the invention as defined in the appended claims.
