Magnetostatic switches — switches that open or close in response to magnetic fields — are being developed and fabricated using micromachining (MEMS) technology. These switches operate similarly to traditional magnetic reed switches, but can be made much smaller. While capable of serving as direct replacements for traditional relays and switches, these MEMS devices open up many new application areas. For example, the magnetostatic switches are being used as compact, lightweight, energy-efficient replacements for bulky electronic commutation circuitry in brushless dc electric motors. For another example, an array of MEMS switches, each of which opens or closes at a different magnetic-field strength, could serve as a rotary encoder or magnetometer.
The components of a basic micromachined magnetostatic switch are (1) a cantilever-beam spring and supporting structure made of an electrically insulating material, (2) a small plate of magnetic material attached to the cantilever, and (3) contact points and electrical leads made of an electrically conductive material. The switch can be normally open or normally closed. The basic principle of operation is illustrated in the figure. In the presence of a suitably oriented magnetic field, the magnetic force on the soft magnetic plate either bends the cantilever to bring the contacts together (in the case of normally open) or else pulls the contacts apart (in the case of normally closed). When the magnetic field exceeds a threshold value, the switch becomes closed (in the case of normally open) or open (in the case of normally closed).
The large variety of potential designs and materials precludes a complete description in this article. In a typical case, the electrically insulating structural material is oxidized single-crystal silicon. Silicon is chosen both for its attractive mechanical properties and for the potential of integrating the MEMS switches with electronic components in a single monolithic process. A micromachined magnetostatic switch or an array of such switches can be produced on a single silicon wafer or else assembled from two or more substrates. Electromagnet coils can be integrated on the substrate along with the switches to form fully integrated electromagnetic relays.
The soft magnetic material chosen for the prototype micromachined magnetostatic switches is the alloy Ni80Fe20 (permalloy). High-quality specimens of this alloy exhibit relative permeability as large as 5,000; this is desirable because the higher the permeability the more sensitive the switch. High-quality films of permalloy can be formed by electroplating.
Gold has been chosen as the electrical-contact material for the prototype switches. Gold can be deposited easily, resists oxidation, and exhibits contact resistances.
Experiments on the prototype switches have yielded some approximate performance figures. Switch contact forces >5 mN, and contact resistances 5 to 106 cycles at high currents (0.45 A). At low currents (1 mA), switch lifetimes have exceeded 5 × 108 cycles, with no failures observed.
This work was done by Yu-Chong Tai and John A. Wright of Caltech for NASA's Jet Propulsion Laboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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Micromachined magnetostatic switches
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