A New Paradigm in Electrical Sensing
When a snake is cut in half, it dies.
This illustrates how electrical devices can respond to damage. Since they are at their core closed circuits, devices will fail whenever the circuit is interrupted. However, if these devices could be more like earthworms, intriguing new possibilities arise. Unlike their reptilian counterpart, one functioning worm can become two functioning worms when cut in half.
The worm-snake illustration helps explain the recent innovation by a Duke engineering alumnus that has overturned the conventional wisdom guiding the development of electronic devices for over a century.
The story begins at NASA Langley Research Center in Hampton, Virginia. “Since 2000, we had been developing methods to remove many of the shortcomings of measurement systems,” said Stanley Woodard (engineering Ph.D. ’95). “We had been doing a lot of work on developing new techniques to power and interrogate wireless sensors as well as designing them.”
In 2005, there was an opportunity for Woodard and colleague Bryant Taylor of ATK Space Systems, to apply their work to create a system that would detect when the outer skin of an inflatable structure for space has been pierced by space debris.
Ideally, any type of sensors would need to be resilient, self-sufficient and have the capability to determine the scope and location of any damage. Another goal was to make arrays of these sensors using the highly reflective thermal protection skin layers. These layers already had a thin film metal coating thus the damage detection system would not add weight. They started designing the system using wireless resonant closed circuits sensors similar to the ones they had previous developed. These respond with harmonic RF energy when electrically powered via exposure to harmonic magnetic fields. But since the sensors were closed circuits, when something happened to any part of the circuit, the sensor no longer functions.
“We tried to develop different geometric configurations of the sensors but each had functional shortcomings for detecting subsequent surface penetrations after the first one,” Woodard said. “After much time and effort looking at the problem and coming close to throwing in the towel, we tried one last approach that went against certain rules we are taught regarding electrical systems. One rule was major – only closed electrical circuits are functional. This rule was basically an unquestionable truth.”
Their previous sensor designs were primarily two components – inductor and capacitor, electrically connected forming a closed circuit. They could provide power to the sensor using harmonic magnetic fields. Each sensor, when powered, responded with its own harmonic magnetic field whose signature depended upon the measurement. Thus there was no physical connection to the sensor but the sensor had electrical connections.
Woodard’s strategy was far-fetched to the point that he felt uncomfortable sharing it with his colleague. Basically, instead of the two components, they got rid of one of them by taking advantage of a common nuisance that plagues to some degree all electrical engineers as they design electrical devices, namely unwanted “parasitic” attributes of an electrical element. For example, the unwanted capacitance when one wants a pure inductor. Since it cannot be eliminated, engineers try to minimize these parasitic attributes when designing electrical devices.
“Even though this parasitic capacitance is always present, the goal was to try the opposite tack – instead of trying to eliminate it, we wanted to see if we could use it to our advantage by increasing it as much as possible,” Woodard explained.
The goal was to use a pattern previously used for an inductor and redesign it so that it intentionally had the functions of an inductor and capacitor. But even more radical was doing it without any electrical connections, thus leaving the circuit open. The result - the sensor was a single component that had the functions of an inductor, a capacitor and a resistor but it was also an open circuit.
Even though Taylor was initially skeptical that such a device could work, he made the electrical drawings for the first prototype. The prototype was 18 inches by 24 inches with thickness less than a sheet of paper. They exposed it to a series of increasing magnetic harmonics – still not sure if it would work because it went against many of the rules they were taught. At approximately 900 KHz, the prototype responded with its own harmonic magnetic field. It worked.
They punched holes in the sensor and it continued to work. Because it was an open circuit, there was no place on it that if punctured, it would not work. In many cases, the puncture resulted in two working circuits. They had ripped a third off some of the sensors and they still worked.
The Achilles Heel of electrical devices had been engineered out of the sensing system.
They had developed a sensor that can “take a licking and keep on ticking” by turning on its head the accepted rules of designing electrical devices. In essence, they devised a sensor that acts more lake a worm than a snake when damaged.
Over the next year, Woodard and Taylor tested an array of “open-circuit” sensors at the NASA White Sands Test Facility in New Mexico, subjecting the sensors to numerous conditions they would likely experience in space. It passed with flying colors.
As they continued their work on the new sensor, they found that it could make and report more than one type of measurement simultaneously. For example, a single sensor placed on a container could at the same time monitor what is happening inside and outside the container.
Since this new sensor approach does not need an electrical connection to a power source or solder to connect any circuit elements, Woodard said it would theoretically be possible to employ some version of the sensor in biomedical settings including inside the human body.
Much of the sensors functional flexibility lies in its design and how it is powered and interrogated. It can literally be placed on surfaces like a decal. “One could imagine placing it on a non-conductive surface – with different patterns each having a different sensing function.” Woodard said.
These accomplishments did not go unnoticed. Recently, Research &Development Magazine named Woodard and Bryant’s invention as one of the top 100 technical innovations for 2008. The technology has been dubbed SansEC (without electrical current). NASA began applying for patents in 2007 and expects that within two to three years to see the technology in use more widely.
This new approach to electrical sensing also has welcome environmental impacts. Since there are no electrical connections to be made, manufacturing costs and wastes are minimized, Woodard said. Also, for the same reasons, the environmental “footprint” left in landfills is much smaller.
“In a certain way, the ability of the sensor to survive damage emulates the natural ability of some animals to regenerate lost limbs or appendages,” Woodard said. “If you sever a sensor you end up with two separate sensors that could still be used for detecting damage. That means that in inhospitable environments – like space – you would have a device that would give you a much better chance of surviving and completing the mission at hand.”