An assembly that includes electromechanical rotary actuators has been developed specifically for use as the shutter mechanism of a cryogenic infrared camera that will be part of an astronomical telescope. The camera will be cooled, by use of superfluid helium, to an operating temperature of 1.4 K. On command, the shutter mechanism rotates a mirror to one of two angular positions, denoted open or closed, at opposite ends of a 38° arc (see Figure 1). When the mirror is in the open position, light gathered by the telescope proceeds unobstructed to the focal plane of the camera; when the mirror is in the closed position, it obstructs the incoming light and provides a dark environment for calibration of the infrared photodetectors in the camera. The shutter mechanism is designed to be rugged, to have relatively low mass (<1.6 kg), and to satisfy several requirements that pertain to mechanical and electrical performance in the cryogenic environment. A primary requirement is that the power dissipation averaged over time not exceed 5 mW.
Figure 2 depicts the components of the mechanism. The mirror is mounted on an arm that extends radially outward from an aluminum shaft. A tantalum counterweight mounted on the shaft opposite the mirror minimizes the offset of the center of gravity of the shaft, thereby minimizing moments that could be affected by gravitation, acceleration, and vibration. The shaft is supported by bushings that allow free rotation. The bushings fit into holes in end caps. The mating surfaces in the end caps are anodized and impregnated with poly(tetrafluoroethylene) to minimize friction. The shaft is machined to provide a central hollow that accommodates a beryllium copper wire, which serves as a torsion spring to bias the mirror in the open position.
The shaft supports two rotors that are magnetically soft and that are constrained in fixed angular positions relative to the shaft. These rotors are the moving parts of two variable-reluctance electromagnetic actuators. The stationary parts of each electromagnetic actuator include an electromagnet coil plus two magnetically soft stator plates and a magnetically soft closeout cylinder that completes the magnetic circuit. Electric current in the electromagnet coil of each actuator generates a magnetic field that is focused by the stators and passes through the rotor.
The geometry of the rotor and stators is such that the reluctance of the magnetic circuit varies with the angular position of the shaft, decreasing toward the closed position. As in any such actuator, this arrangement gives rise to a torque in the direction of decreasing reluctance. Hence, the application of current to the electromagnet coil gives rise to a torque that opposes the spring bias, turning the mirror toward the closed position.
A magnetic latch is essential for satisfying the requirement of low average power dissipation. The magnetic latch comprises a set of magnetically soft tabs that are affixed to the stators and extend from the stator faces. These tabs make contact with the rotors in the closed position. This contact effectively completes the magnetic circuit, reducing all airgaps to nearly zero, thereby effecting a large decrease in magnetic reluctance. In the low-reluctance condition, the mechanism can be held in the closed position, fighting the spring-bias restoring torque with a lower current than is needed in the noncontact, higher-reluctance condition. The net result is that whereas a current of ≈55 mA is needed to close the shutter, a current of <1 mA is needed to hold it closed.
Of course, the naturally low electrical resistance of the electromagnet coil at the low operating temperature also helps to limit the power dissipation. A further reduction in power dissipation is obtained by use of an angular-position sensor and associated control circuitry: Inasmuch as the time taken in closing the shutter is about half a second, the control circuitry initially sends a high pull-in current pulse to the electromagnet, then quickly reduces the magnitude of the current to the holding level. The angular-position sensor informs the control circuitry when the mechanism reaches the closed position, making it possible to minimize the time spent at the higher pull-in current.
This work was done by David Scott Schwinger, Claef Hakun, George Reinhardt, and Clarence S. Johnson of Goddard Space Flight Center.