Resonant Inductive Signal Coupler

The motion of neutrally buoyant tracer particles in a flowing fluid is commonly used to experimentally measure fluid velocities.  Typically, this is done optically, but in opaque flows, scattered ultrasound can be a substitute.  Rotating fluid experiments have always posed something of a challenge for instrumentation as signals must be coupled from the rotating frame into the lab. Instruments with network connections have alleviated some of these problems for experiments large enough to carry the necessary equipment.  WiFi is an excellent rotating flow data acquisition tool.  But smaller, more rapidly rotating experiments cannot physically carry the required equipment.  And there's useful older equipment that can only be operated under local control.

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A non-contact resonant inductive coupler for ultrasound velocimetry.

Coupling analog signals from the instrument to a transducer rotating with an experiment is challenging.  The signals are typically at a few hundred kilohertz to a couple tens of megahertz and the returning echoes from particles in the flow are very weak, resulting in microvolt-level electronic signals. Mercury-wetted mechanical slip rings are a viable option, but are expensive and typically require mounting directly on the axis of rotation.  

The resonant inductive coupler shown above is a solution to both problems.  Two rings of copper tubing are close-spaced and inductively coupled.  The coupling is greatly enhanced and noise immunity is increased substantially when capacitors are used to resonate the rings at the operating frequency, 4MHz in the case of the rings above.  This sort of resonant near-field coupling is the same approach proposed for some "wireless power" products.  

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Schematic of resonant inductive coupler.  

 In the schematic above, capacitors C1 and C2 are trimmer capacitors chosen to resonate with the inductance of an isolated ring at the operating frequency.  The actual capacitance is adjusted to provide a 50 ohm impedance to the instrument.  

This system has very low phase shift, good amplitude fidelity, and good noise immunity.