Physical Sciences

Relative motions can be suppressed or imposed on demand.

Optoelectronic sensors and magnetic actuators have been developed as parts of a system for controlling the relative position and attitude of two massive optical tables that float on separate standard air suspensions that attenuate ground vibrations. In the specific application for which these sensors and actuators were developed, one of the optical tables holds an optical system that mimics distant stars, while the other optical table holds a test article that simulates a spaceborne stellar interferometer that would be used to observe the stars.

ImageThe control system is designed to suppress relative motion of the tables or, on demand, to impose controlled relative motion between the tables. The control system includes a sensor system that detects relative motion of the tables in six independent degrees of freedom and a drive system that can apply force to the star-simulator table in the six degrees of freedom.

The sensor system (see Figure 1) includes (1) a set of laser heterodyne gauges and (2) a set of four diode lasers on the star-simulator table, each aimed at one of four quadrant photodiodes at nominal corresponding positions on the test-article table. The heterodyne gauges are used to measure relative displacements along the x axis.

ImageThe most innovative part of the sensor system is the quadrant-photodetector subsystem, which differs from commercial quadrant-photodetector relative-position-measuring systems. The active area of each quadrant photodiode is 1 cm in diameter. The 635-nm-wavelength output of each laser diode is coupled via an optical fiber to a collimator, which focuses the laser beam to a spot centered at the nominal center of the corresponding quadrant photodiode. The laser spots on two of the quadrant photodiodes are 5 mm in diameter; these quadrant photodiodes provide low-resolution, high-dynamic range measurements of relative displacements and tilts in the y-z plane. These measurements are used to bring the tables into coarse alignment. The laser spots on the remaining two quadrant photodiodes are 0.55 mm in diameter; these quadrant photodiodes provide high-resolution, low-dynamic range measurements of relative displacements and tilts in the y-z plane, thereby enabling finer alignment. This quadrant-photodiode subsystem offers two advantages over commercial quadrant-photodetector relative-position-measuring systems: it is much quieter (its position noise output is about a hundredth of that of a typical commercial system) and it can be constructed at relatively low cost.

The drive system includes eight magnetic linear actuators—two more than the minimum of six needed for the six degrees of freedom. Each actuator partly resembles a traditional voice-coil actuator in that it includes a permanent magnet, upon which an axial force is exerted by a nominally concentric electromagnet coil. The force exerted by each actuator depends on the current applied to its coil and typically lies in the range of ±10 N (for some degrees of freedom, the typical range is ±40 N). Unlike a traditional voice-coil actuator, which cannot withstand a lateral displacement of more than 0.1 mm without breakage, each of these magnetic actuators can withstand a displacement of ±5 mm in any direction without breakage. The electromagnet coils are wound on aluminum forms that contain water cooling channels to remove excess heat from the coils when necessary. The motions of the permanent magnets in the aluminum forms also provide eddy-current damping of rapid motions (e.g., vibrations) of the table.

This work was done by Yekta Gursel and Elizabeth McKenney of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www. under the Physical Sciences category.


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