Tech Briefs

Contactless Rotary Electrical Couplings

Efficient inductive couplings are used in place of slip rings.

Rotary electrical couplings based on induction (transformer action) rather than conduction between rotating and stationary circuitry have been invented. These couplings provide an alternative to slip rings and contact brushes.

Mechanical imperfections of slip-ring and brush contact surfaces and/or dust particles trapped between these surfaces tend to cause momentary interruptions in electrical contact and thereby give rise to electrical noise. This source of noise can be eliminated in the inductive rotary couplings because no direct contact is necessary for transformer action.

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Figure 1. Transformers, Instead of Slip Rings and Brush Contacts, are used to couple ac power and data signals between stationary and rotating circuits.
Figure 1 shows an example of the use of a rotary inductive coupling. In this application, it supplies power to a rotating digital data acquisition/ transmission system test bed. The rotating data system is shown under the transparent dome and the data is transmitted via free-space optical data transmission through the dome. The rotary inductive coupling is shown in the lower half of the photograph. As in the case of conventional stationary transformers, the levels of power that can be transferred via inductive rotary couplings are limited only by considerations of operating frequencies, sizes, and provisions for dissipation of heat. Typical operating frequencies for transfer of power lie in the same range as that for conventional stationary power transformers — tens to hundreds of hertz. Typical frequencies for coupling of digital data signals tend to lie at the low end of this frequency range.

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Figure 2. This Inductive Rotary Coupling resembles a conventional stationary transformer, except that one coil rotates with a shaft that passes through the middle, and the core is shaped to accommodate the rotating parts. The hollow shaft accommodates the wires that connect the rotating coil with the rotating circuit at one end of the shaft.
Figure 2 shows a simplified cross section of a typical inductive rotary coupling between a stationary circuit and a rotating circuit mounted on a motor-driven shaft. The transformer windings comprise a stationary and a rotating coil, both concentric with the axis of rotation. Either coil can serve as the primary winding, with the other serving as the secondary winding. The stationary assembly includes a transformer core that, like a conventional stationary power transformer, is made by stacking sheets of silicon steel or other suitable magnetically "soft" material.

The transformer core partly resembles the core of a common type of conventional power transformer; it includes an inner part framed in a generally rectangular outer part. However, unlike the core of a conventional power transformer, this core includes a clearance hole for the rotating shaft, and the inner core is a round cylinder (instead of a rectangular solid) with the clearance hole lying along its axis. Moreover, there is a gap at one end of the inner core to provide clearance for a clamp that attaches the rotating coil to the shaft. Both coils are annular, with inner radii and outer radii to fit the recess between the inner and outer core. To provide clearance for rotation, the inner radius of the stationary coil must be made somewhat greater than the outer radius of the inner part of the core.

This work was done by Hiroyuki Kumagai of Aerospace Computing, Inc., for Ames Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Computers/Electronics category.

This invention has been patented by NASA (U.S. Patent No. 5,691,687). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Ames Research Center; (650) 604-5104. Refer to ARC-12072.

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Contactless Rotary Electrical Couplings (reference ARC-12072) is currently available for download from the TSP library.

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