A magnetically suspended, motor-driven light-chopping wheel has been developed as an alternative to a traditional light-chopping wheel suspended on ball bearings. This chopper (see figure) is designed to satisfy application-specific requirements to fit into a small volume, dissipate relatively little power, modulate light with only small errors, and have a long operational life (at least 5 years) at a speed of 5,000 rpm. Relative to ball bearings, the magnetic bearings in this chopper are subject to no wear, and unlike ball bearings, magnetic bearings present no need for lubrication.

This chopper satisfies additional stringent requirements for full redundancy of magnetic-bearing coils, motor coils, and sensors. The magnetic bearings are controlled, in all six degrees of freedom of the chopper wheel, by use of a digital signal processor (DSP) operating in conjunction with sensor and driver electronic circuits. The DSP and the other circuitry are contained on a printed-circuit board with overall dimensions of 6 by 8 in. (≈ 15 by 20 cm).

Unique features of the design and its implementation include the following:

  • Homopolar magnetic bearings are made in a novel (patent pending) configuration to minimize rotational losses;
  • High-resolution, highly linear optical position sensors with feedback (patent pending) provide automatic gain and offset control;
  • An ironless stator motor assembly, optimized to minimize eddy-current losses, is fabricated by use of a modified printed-circuit-board manufacturing technique;
  • The DSP is used to compute 6-axis classical and adaptive control algorithms; and
  • At the time of reporting the information for this article, this chopper is the smallest known mechanism that incorporates fully active magnetic bearings with full redundancy of electrical components.

This specific application for which this chopper has been developed is an infrared-sensing instrument aboard an Earth-observing satellite. Magnetic-bearing systems like those of this chopper, but larger, are under consideration for use in diverse outer-space and terrestrial applications in which rotating wheels would be used to store angular momentum and/or kinetic energy.

This Optical Chopper Wheel is magnetically suspended, offering certain advantages over traditional light-chopping wheels, which are suspended on ball bearings.

This work was done by Ken Blumenstock, Carlos Bernabe, Carlos Hernandez, Ken Lee, Joe Schepis, Clarence Johnson, Maurice Lewis, and Paul Haney of Goddard Space Flight Center.

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Photonics Tech Briefs Magazine

This article first appeared in the April, 2002 issue of Photonics Tech Briefs Magazine.

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