Loosely coupled rotary transformer having resonant circuit

Details Loosely coupled rotary transformer having resonant circuit Loosely coupled rotary transformer having resonant circuit

1999-09-14

2001-09-18

Bettendorf, Justin P. (Department: 2817)

Wave transmission lines and networks

Coupling networks

Nonreciprocal gyromagnetic type

C280S735000, C307S010100, C340S646000

Reexamination Certificate

active

06292069

ABSTRACT:

TECHNICAL FIELD
The present invention is directed to rotary transformers, and more particularly to loosely coupled rotary transformers that transfer both power and data between two structures.
BACKGROUND ART
Rotary transformers, and particularly loosely coupled power transformers, are often used for transmitting both data and power between two structures that rotate relative to one another, such as between a vehicle tire and its corresponding wheel axle in a tire pressure sensor system, or for coupling data and power to a steering wheel. As is known in the art, loosely coupled power transformers do not conduct power efficiently between the primary and secondary of the transformer. Instead, a part of the input current into the primary coil stores energy in the leakage inductance of the coil. Prior art structures often include a Zener diode across the primary to absorb the energy of the voltage spike that occurs in the transformer when the current to the primary coil is turned off. More particularly, the Zener diode will conduct current before the drive transistor in the primary side breaks down. However, under this approach, the stored energy is dissipated as heat, thereby wasting the energy built up in the primary coil's leakage inductance and lowering the power coupling efficiency of the transformer.
To overcome this problem, conventional rotary transformer designs tend to focus on methods of increasing the coupling efficiency by constructing a magnetically efficient structure for power transmission, such as by using more expensive, high-efficiency core materials, and then adding a complex load impedance mechanism for providing limited two-way communication through the transformer. This results in an overly complicated structure requiring close mechanical tolerances, which increases the manufacturing cost of the system. Further, the bandwidth for these structures tends to be relatively narrow, which limits the amount of data or the speed at which data can be transmitted between the primary and secondary sides of the transformer.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a loosely coupled rotary transformer structure that includes a resonant circuit, such as a resonating capacitor and a drive transistor coupled, to the primary coil in the transformer. In one embodiment, the drive transistor connects the capacitor to the transformer during a power transfer mode and disconnects the capacitor during a data transfer mode. As a result, the energy stored in the primary coil's leakage inductance is coupled to the capacitor when the drive transistor is turned off, allowing the energy to continue being coupled to the secondary side of the transformer. Thus, the inventive structure uses the stored energy in the primary leakage inductance for coupling instead of wasting the energy as dissipated heat, thereby increasing power coupling efficiency. Also, by disconnecting the resonating capacitor during the data transfer mode, the inventive transformer structure avoids the decrease in bandwidth that would ordinarily be caused by the resonating capacitor if it remained connected to the circuit. Preferably, the transformer continuously cycles between the data transfer mode and the power transfer mode via time-sequenced multiplexing.
An embodiment of the invention also includes a full wave rectifier coupled to the secondary coil of the transformer to extract the power being coupled to the secondary side. The rotary transformer according to the invention therefore combines efficient power transfer characteristics with a wide bandwidth for two-way data transfer while eliminating the need to use high-cost, high-efficiency magnetic structures in the transformer; the inventive structure is equally as effective for air core transformers as well as for rotary transformers using a high efficiency magnetic structure.


REFERENCES:
patent: 5594176 (1997-01-01), Kiefer
patent: 5608771 (1997-03-01), Steigerwald
patent: 5636863 (1997-06-01), Reid et al.
patent: 5906392 (1999-05-01), Reid et al.
patent: 6133741 (2000-10-01), Mattes
patent: 6175461 (2001-01-01), Fukuda
SAE document “Contactless Air-Bag Firing and Signal Transmission on the Steering Wheel with an Inductive Contact Unit” by Erich Zabler, Anton Dukart & Thomas Herrmann, Robert Bosch GmbH, 1998.
Martin Scheck document “Dynamic simulation of a transformer for a contactless clockspring design used for functional information exchange of airbag, cruise control and other systems.”, 1998.

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