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Analog Angle Blog

Sensors Without Wires, But Not Wireless

We know that much of the analog world is related to sensors for physical variables such as temperature, pressure, or acceleration. The physical realization of these sensors takes on many forms, of course, depending on the specifics of the application. As a result, there's incredible diversity in the sensors themselves. Can there be a need for yet more?

That's why I was curious when I read about a sensor developed at NASA which uses no wires but is wireless, though not in the conventional RF-link way. This may seem like a contradiction, but it's not. NASA call it the SansEC, (sans , or without, electrical connection), and the idea is remarkably simple — always a good thing.

The SansEC is really almost nothing. It's an open-circuit conductive spiral designed to self-resonate and couple to a near-field magnetic probe antenna. It works on the underlying physics principle that changes in surrounding physical factors (such as a stress) cause subtle but detectable changes in the resonating spiral's fundamental characteristics.

“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,” Kenneth Dudley, a researcher in the Electromagnetics and Sensors Branch at NASA's Langley Research Center, said in a NASA Tech Briefs article. “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.”

What's interesting about the SansEC approach is that it is small, costs little, and can be embedded into the material you wish to monitor. Clearly, it's not for many applications, and it may not work out for various reasons, but it might be a viable solution in some situations, such as detecting cracks in an aircraft's skin due to flexing or lightning.

The brief article doesn't say it, but we all know that the transducers by themselves are not the whole story for successful sensing. In most cases, the change in a sensor's characteristic due to the change in the sensed parameter is quite small — much smaller than the original signal level itself, which is already at a fairly low level. This means that any practical system needs low-noise, high-resolution analog interface circuitry that works well with fairly low magnitudes.

Coincidentally, the same issue of NASA Tech Briefs also reported on progress in yet another common yet very difficult sensing situation: measuring humidity (specifically, for car air conditioning systems).

We all know what humidity is, and we can all sense relative humidity, but it is surprisingly difficult to assess accurately. Factors such as temperature affect sensor range and performance. Thus, many humidity sensors have been developed, each targeted at a niche application. (Capacitance is being tested elsewhere to look for hidden tire flaws, but that's being done a via a wired interface; though it's simple enough in concept, it is obviously more challenging in practice.)

In many ways, sensors are an area of great technical challenge. When you peel back simple terms such as temperature, pressure, or acceleration, the real-world challenges of building and installing reliable, accurate sensors for a given application class, range, and situation are often quite complicated. That's why there are so many very different implementations for each parameter. Just look at all the ways you can measure air velocity — from a basic bendable cantilever with mounted strain gauge to a more sophisticated hot-wire anemometer circuit or the late Jim Williams' design for an acoustic thermometer.

Have you ever seen or used sensors for apparently simple physical parameters but still required cleverness and subtlety?

23 comments on “Sensors Without Wires, But Not Wireless

  1. Netcrawl
    May 26, 2014

    @Bill that was great! thanks for that, NASA developed wireless technology that is damage tolerant and requires no electrical connection, SansEC technology was developed as a method that does not have any conventional electrical connections making it highly resilient to damage, it a huge achievement for NASA and US.    

  2. samicksha
    May 28, 2014

    The SansEC sensor can provide lightning strike protection, damage protection, and to some degree, shielding effectiveness against the electromagnetic environment around the aircraft.


    NASA is known for their premier devolopment and research but this sounds little more than challenging. Curious to learn more in depth.

  3. RedDerek
    May 28, 2014

    I had one project years ago where the goal was to measure gas purity. It involved a combination of temperature, pressure, humidity and acoustics. Trick was that there was no formula set up for reading the values and converting to purity. Thus the first step was to create the formula and then use it in reverse to make the product that provided the purity number.

  4. Netcrawl
    May 29, 2014

    Its a big achievement for NASA, sansEC would definitely help make NASA's next generation of space vehicles to become much safer and smarter, able to protect the vehicle against lightning strike and sense any damage that may occur in a lightning strike. The new technology serves as multifunctional material, something that NASA really need.

  5. eafpres
    May 29, 2014

    Hi Bill–thanks for pointing out this article.  This is a great example of how much effort and resource it takes to get to useful results.  NASA, although their budgets have been hugely cut, is one of a few organizations in the world that could support such an activity.  I note that Kenneth describes his work as a sub-project within a program.  Like a lot of basic reserch and fundamental advancements, it is hard to envision this work going on in the private sector until enough proof of concept was shown that some entrepreneur decided they could try to make money by further development.

     

  6. eafpres
    May 29, 2014

    The description of the application of this technology by Kenneth Dudley is interesting–essentially the spirals are coupled to each other with enough effectiveness to not allow the energy of a lightning strike to get through, and therefore act as a Faraday cage.  He also describes the response as a resonant frequency–I had the image in my mind of some low level excitation and a spectrum analyzer doing FFTs and watching for shifts.  Essentially the whole aircraft is covered by a mesh network of these sensors.  I would like to know if Kenneth thinks they could sense location of the damage as well as the detection of the damage itself.

  7. chirshadblog
    May 30, 2014

    @eafpres1: Yes true but just because you have enough resources you should not use them all at once. Should try to mitigate and save as much as possible and get the work done efficiently. I think that is what they too are focusing on. 

  8. chirshadblog
    May 30, 2014

    @RedDerek: Well that seems to be a very complicated process isn't it ?

  9. geek
    May 30, 2014

    “Essentially the whole aircraft is covered by a mesh network of these sensors.  I would like to know if Kenneth thinks they could sense location of the damage as well as the detection of the damage itself.”

    @Eafpres1: That seems to be an interesting concept. I think the sensors would be capable of that. What I'm more concerned about is the fact that whether these sensors are able to transmit this information through a wireless medium to the ground so that the authorities on the ground can also be aware of this and not just the pilots and staff.

  10. geek
    May 30, 2014

    “Trick was that there was no formula set up for reading the values and converting to purity. Thus the first step was to create the formula and then use it in reverse to make the product that provided the purity number.”

    @RedDerek: Without any values to benchmark, it must have been very difficult to use the sensor readings and make sense out of them I believe. How did you eventually reach to the formula to assess the purity?

  11. RedDerek
    May 30, 2014

    @tzubair – How did you eventually reach to the formula to assess the purity?

    That there was the trade secret. Suffice it to say one has to gather a ton of data of results by varying know impure gas at different temperatures and pressures and other conditions. Then it becomes a huge simultaneous equation program to identify the terms necessary to create the final equation to use. Some terms are known such as the gas law of PV=nRT, etc.

  12. geek
    May 30, 2014

    “SansEC technology was developed as a method that does not have any conventional electrical connections making it highly resilient to damage, it a huge achievement for NASA and US.”

    @Netcrawl: I think that can have a huge application in quite a few areas. One of the most important ones that I can think of are nuclear reactors. These type of sensors would be very useful there as they can still work and trigger the control system even under a disaster.

  13. Netcrawl
    May 31, 2014

    @tzubair I agree with you about the potential of this technology, the recent Fukushima nuclear disaster triggered the need for a new method to monitor the status of fuel rod that doesn't rely on electrical power, SansEC could be the right technology for this. During the disaster electrical power connection to the nuclear reactor failed and sensor useless, operators were unable to monitor the fuel rod and control the reactor. Its like a “huge bomb” waiting to explode.   

  14. geek
    May 31, 2014

    @RedDerek: Sounds like a lot was achieved after a series of trials and errors and experimenting with the readings and threshold values 🙂

  15. geek
    May 31, 2014

    “During the disaster electrical power connection to the nuclear reactor failed and sensor useless, operators were unable to monitor the fuel rod and control the reactor. Its like a “huge bomb” waiting to explode.   “

    @Netcrawl: That's exactly the kind of incidents I am reffering to and there have been quite a few cases where the sensors fail to work because they got deactivated even before they could sense anything. Even in smaller power plants this can be often a problem.

  16. fasmicro
    May 31, 2014

    >> NASA is known for their premier devolopment and research but this sounds little more than challenging. Curious to learn more in depth.

    Is it not ironous that NASA is gutted of money while government at the same time is expanding bucrecracy and administration with so many administrators in government and government agencies and labs? That is our priority and it is unfortunate for long-term competitiveness of America.

  17. fasmicro
    May 31, 2014

    >> The new technology serves as multifunctional material, something that NASA really need.

    Hope they can license the technology to SpaceX, the “new NASA”. Yes, I think SpaceX has hired many NASA engineers and getting a lot of funding from NASA. The implication is that NASA continues to support them as though SPACEX will evolve to become a private sector version fo NASA.

  18. goafrit2
    May 31, 2014

    That there was the trade secret. Suffice it to say one has to gather a ton of data of results by varying know impure gas at different temperatures and pressures and other conditions.

    It is analogous to the same project Intel spent years to find the best material composion on metals that can improve chip production through special alloys. People asked Intel in conferences and they will tell you that it is part of the competition- they will not tell anyone. Trade secret is the business.

  19. goafrit2
    May 31, 2014

    @Netcrawl: I think that can have a huge application in quite a few areas. One of the most important ones that I can think of are nuclear reactors.

    At least, besides the nuclear reactors, there are many applications this can go into. This includes aeroplanes, rockets, space vehcicles and even drones. It is what happens when bright people meet resources to change the face of science.

  20. geek
    May 31, 2014

    “At least, besides the nuclear reactors, there are many applications this can go into. This includes aeroplanes, rockets, space vehcicles and even drones.”

    @goafrit2: Yea the list is countless because virtually every type of application has the risk of being exposed to an extreme environment like high or low temperature, pressure or anything else that can hamper the working of a normal sensor and disrupt the transmission.

  21. fasmicro
    June 2, 2014

    The more I think about the title of this piece, the more I imagine a revolutionary technology. In the automotive industry, in gyroscopes and XLs, the highest cost factor is not the sensor which is MEMS but the wires. So, if you can get this type of sensors, you can be riding a new disruptive technology.

  22. geek
    June 8, 2014

    “In the automotive industry, in gyroscopes and XLs, the highest cost factor is not the sensor which is MEMS but the wires. So, if you can get this type of sensors, you can be riding a new disruptive technology.”

    @fasmicro: I don't think a wireless technology really helps in reducing the costs. As far as I've seen, the wireless provides convenience along with other benefits but the cost generally goes up. Wire, by far, is the cheapest medium across all applications.

  23. fasmicro
    July 6, 2014

    >>  Wire, by far, is the cheapest medium across all applications.

    You missed the point – you get electronics for cents as Moore's law works there but in wires, you have no reduction capability. In automotive electronics, wiring cost is huge, never the “cheapest medium”.

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