Inductive Interrogation
Magnetic field response measurement sensors
One of the many great things about working behind the fence at Wright-Patterson Air Force Base is the diversity of truly amazing technology under development. Shuttling among the Air Force Research Laboratory facilities as an on-site research contractor is an experience that will not soon wear off. A palpable gee-whiz ether permeates the storied collection of hangers and test cells that house some of the most sophisticated state-of-the-art instrumentation in existence — the hybridization of academic smarts and military budgets.
My employer was charged with the development of optical diagnostics for the measurement of temperature, pressure and flow fields above, on and within aerospace test articles. Probing these values with light allowed us to gather our data without the cumbersome configuration of lead wires and tubing required by traditional electronic sensors that often modify the aerodynamics of the system under study. Owing to the human sensory apparatus, over time the models of sleek, futuristic flying machines melt into the background of perception causing the mundane to appear out of place.
Such was the case when a large-scale Boeing 747 center fuselage model was wheeled in by the U.S. National Transportation Safety Board (NTSB). An engineering marvel in its own right, the white, bulbous passenger plane was overtly obvious among the collection of sinister stealth. Upon closer inspection, the model contained a burst panel to study the debris trajectory from an explosion in the center wing tank, as proposed to have precipitated the 1996 TWA Flight 800 tragedy off of Long Island, NY, resulting in the loss of all 230 persons aboard. There are many conspiracy theories as to the actual cause of the calamity, most likely due to the layman’s disbelief in the ability of a short-circuit in a simple fuel gauge sensor to cause such loss of life. A research group lead by Stanley E. Woodard, Ph.D., at the NASA Langley Research Center also reports the discovery of wire damage grounding the entire space shuttle fleet, and being the probable cause of the onboard fire resulting in the 1998 loss of the MD-11 Swissair Flight 111 out of New York and all 229 persons aboard.
In addition to improving the safety of wiring materials, a research team lead by Dr. Woodard is investigating the use of wireless sensors aboard aerospace vehicles to eliminate the danger due to arcing, fraying and chemical degradation of the wires providing power and data links to sensors. Traditional wireless laboratory measurement systems incorporate processors and transmitters that often need their own wired power supplies. Radio frequency transceiver (RFID) tags can transmit their data after receiving power from a radio wave, but the required silicon processors are often incompatible with their harsh measurement environment, and the radio frequency range available for aviation use limits the separation between the antenna and RFID tag to around 15 centimeters. In our optical diagnostics laboratory, we used infrared and visible electromagnetic radiation to interrogate our sensors remotely; however, we also had the luxury of mounting high-power lasers or banks of light emitting diode (LED) lamps external to our test articles.
To avoid the shortcomings of radio and optical electromagnetic radiation, NASA Langley has developed a wireless sensor system based on oscillating magnetic fields. The sensors are comprised by a spiral induction (L) antenna coil and a plate or interdigitated capacitor (C) that is in contact with the quantity to be measured. When the L-C sensor is stimulated by the magnetic field, a resonant current is established having an oscillation frequency proportional to the dielectric environment of the sensor. After the driving magnetic field is removed, the antenna coil of the L-C sensor emits a field at its characteristic frequency that is received by an interrogation antenna. The research team reports separation distances of up to 3.3 meters when powering the sensors at 1.5 watts.
Because different L-C sensors have individual resonant frequencies, several sensors can be used simultaneously in conjunction with a single control unit by sweeping the frequency over the range utilized by the sensors, typically 1 to 10 megahertz. Various sensors have been developed for the measurement of position, level, load, angular orientation, material phase transition, moisture, specific chemicals, rotation/displacement, bond separation, proximity, contact, pressure, strain and crack detection. While the patent application is still under review, NASA Langley is actively seeking partnerships and collaborations to commercialize the technology. Let’s hope the induction time of its adoption is low so that the frequency of arcing events can approach zero.
Bill Weaver is an assistant professor in the Integrated Science, Business and Technology Program at La Salle University. He may be contacted at [email protected].
