Kenneth Dudley is a researcher in the Electromagnetics and Sensors Branch at NASA's Langley Research Center. His project team currently focuses on the testing and development of the SansEC sensor.
NASA Tech Briefs: What is the SansEC sensor?
Kenneth Dudley: SansEC is a sensor technology, a new technical framework for designing, powering, and interrogating sensors to detect various physical phenomena. It can measure anything from electrical, mechanical, thermal, and chemical phenomena.
SansEC is a very simple structure. It’s essentially a conductive spiral that can be any geometry. We tend to make them square, but they can be round or triangular. That conductive spiral is an open circuit; it’s not connected to anything directly, hence the name SansEC, meaning without electrical connection. We couple it to an antenna through a near-field magnetic probe antenna. The device self-resonates, with an electromagnetic field around the sensor, and any material that perturbs that field can be sensed, measured, and quantified.
NTB: What can it measure?
Dudley: All materials in nature have an intrinsic characteristic to them, an intrinsic electrical storage characteristic known as permittivity and an intrinsic magnetic storage characteristic known as magnetic permeability. If a change in a material’s permittivity or permeability occurs, due to say fatigue of the material or damage to the material, or stresses within the material, that electrical storage characteristic or magnetic storage characteristic can be sensed through the electromagnetic field.
NTB: How are the SansEC sensors currently being used to study lightning strikes on composite materials?
Dudley: We’re part of a program known as the Atmospheric Environment Safety Technologies (AEST) project, and we’re a sub-project within the AEST known as LEEM, or Lightning and Electromagnetics Effects Mitigation. We’re tasked with the research responsibility to come up with the next generation of lightning strike protection for composite aircraft. The SansEC sensor can provide lightning strike protection, damage protection, and to some degree, shielding effectiveness against the electromagnetic environment around the aircraft.
We envision it as a smart skin or smart replacement to the standard way of protecting composite aircraft structures. Typically an aircraft, if it’s constructed of metal, has little to no issue with lightning strike. It forms what’s known as a Faraday cage, and a lightning strike on the aircraft that’s made of metal diverts the electric charge energy around the aircraft and back to the atmosphere and eventually to ground. Composite structures that are unprotected cannot dissipate that amount of energy; we’re talking energies on the level of 200,000 amps per lightning strike. A composite structure cannot effectively dissipate that energy. Typically, to protect aircraft made of composite, a conductive metal mesh or expanded metal foil is embedded in the top layer of the composite, and therefore, you form a Faraday cage or Faraday shield; the strike will divert the energy through that Faraday shield.
That works very well, but it adds no additional functionality other than the lightning strike protection. With the SansEC sensor, we form a continuous structure of arrays of the SansEC sensors in the composite material, thereby forming a Faraday cage that protects the aircraft against lightning strikes. In addition, if the sensor is damaged or the composite structure that the sensor is embedded in is damaged, we can know and tell immediately that the aircraft has been struck, and to some degree quantify the location and the amount of damage that has occurred, and to serve as a smart skin.
For lightning strike protection, that is important because in aircraft that are coming online now, particularly unmanned aerial vehicles, you may not have a pilot aboard or the situational awareness to know that you have been struck by lightning. You may need to take some corrective action to safely navigate the aircraft. For instance, if you took a strike on an unmanned aerial vehicle, a SansEC sensor array would be able to say: “You’ve been hit, you’ve been hit in this location, and the damage is not so severe. You can continue with your mission or to your destination.” Or alternatively, the SansEC sensor could determine that you’ve been hit, the damage is quite severe, the aircraft is still flyable, but you need to find the next exit ramp and get the aircraft down. That’s a short scenario of how a SansEC sensor can serve as a lightning strike protection system as a smart skin for the aircraft.
NTB: What kinds of applications do you envision for the SansEC sensor? Is there a whole range of possibilities?
Dudley: The application space for the SansEC sensors is limited almost only to the imagination. In the aircraft example I just gave for lightning strike protection, it’s a robust, multifunctional sensor. The sensor takes damage; it still functions. You may have to acquire a new baseline for the damage condition, but you’ll still get a sensor’s signature from the damage.
I should back up for a moment and say that the signature you get from a SansEC sensor in its undamaged condition, ready to sense its environment, is a resonance response, and essentially that is a frequency signature of a certain amplitude as a function of frequency. If a material or the environment that you’re sensing around the sensor changes, you’ll get a shift in the frequency or a shift in the amplitude of the frequency signature — or a combination of the two. There are some advanced things that we can do in signal analysis to get additional information and intelligence from that signal. Fundamentally, you focus on the frequency response and the shift of that frequency response when the sensor is functioning.
For a lightning strike, you take the damage, and you get a frequency response. But it’s a multifunctional sensor that can, say, detect icing buildup on the leading edge of your wing. The ice has a specific material characteristic that I discussed earlier (permittivity or permeability, collectively known as dielectric constant. All materials have this dielectric constant. Ice has a specific dielectric constant on the order of 30 or 40). If you get a buildup of that on the sensor in the previous environment, you’ll see that frequency resonance shift from the effect of a previous dielectric constant of 1 (for air) to the dielectric constant of 30 (for the ice buildup). As you detect that shift, you can analyze that signature as a buildup of icing. In addition to the lightning strike protection and the damage detection of the composite material, which the SansEC is embedded in, you can do additional functions.
That’s a specific application still connected with the aircraft or aerospace, but we envision a number of different applications. You can embed a SansEC sensor in a device, such as a bandage. The bandage wrapping the wound of a person could have this conductive spiral sensor in it. It could be made of a conductive silver thread, and silver has known antibacterial properties to it, so that would probably be a good material to make the bandage with. As it’s applied to the wound, as the wound is changing over time, as it’s healing, or different characteristics of the wound are changing, it can be sensed with the SansEC sensor, without removing the bandage to look at the wound.
In the food industry, the simple spiral can be placed onto, say, meat, poultry, or milk products. By interrogating the signature of the SansEC sensor as milk spoils, and its dielectric constant changes, the SansEC sensor’s self-resonating electromagnetic field can detect that, and you can get an indication of food conditions.
We often sit around on the team that I lead, and we’ll take one day out of the week or every couple weeks and kick around new application spaces for this resonance sensor to be applied in.