Why Electromagnetic Fields?

EM field and wave properties have been used for years and have benefited society from the development of radio and television to cutting edge medical devices to cell phones and modern day electronics. The uniqueness of EM waves is their ability to transport energy from one point to another. This energy transfer can be in the form of sending power to our homes, broadcast radio signals, and even the transmitted digital data that allows you to read this web page. However, in order to have these EM fields transmit the energy in the most productive and efficient manner, they must be guided from point A to point B. Such guides are called transmission lines and take the form of waveguides, fiber optics, coax cables, etc. In order to guide the EM fields to deliver their energy where needed, tight control of the fields and how they interact with their surroundings is of utmost importance. EM fields that “stray” from their transmission guiding structure waste energy, are inefficient, and can be costly for transmitting energy—but are great for sensor design.

The Leaky Wave Concept

Remember those “stray” fields we were just reading about? Well now let’s look at an excellent transmission structure, the coax cable, and see how it can be used as a sensor. This transmission structure, when designed properly, confines all the EM fields within the coax cable and allows the energy to be transferred, without much loss, over miles of cable length. This is because the EM field is trapped within the coax cable and all its energy is delivered to the user. The EM field is purposefully shielded from the environment and has no interaction with it. This is why cables can be buried under ground with minimal effect on performance.
But what if we somehow let the EM fields leak out of the coax cable? For example, we cut the end of the coax cable as shown in the video below. For sure it would ruin our TV reception and we wouldn’t get any internet information. However, it would allow the EM fields to interact with their surrounding environment. If we can control how the fields “leak out” and control which environment they interact with, we just might be able to make a useful sensor.

How Our Sensors Work

So how do we get a displacement sensor from a non-sensor transmission structure? Let’s look at the coaxial cable in more detail. First, lets build up a target that can be displaced ( as shown in the video).The displacement target is caused to move by the stimulus that is to be detected (i.e. weight, force, pressure, flow, speed, etc). Now let’s cut a specially designed slot in the coax to let the fields leak out.
The slot now allows the EM fields to leak out and interact with the target. To be sure, we have degraded the performance of the coaxial cable due to the slot, but we have also realized a displacement sensor as the movement of the target now affects the transmission of energy through the coaxial cable as can be seen in the video.
So, by turning the pristine energy transfer concept of a transmission line on it’s head – purposefully allowing energy to leak out – we have converted the transmission line into a sensor. Clever, no?
What this now means is that we can use any transmission topology at any suitable frequency, engineer out some leaky energy fields and create a multitude of sensor types serving different applications and purposes. We are not restricted to only a coax topology, any transmission line topology that we can have EM fields leak out of will work as a sensor.

Innovative Characteristics

Although this coax sensor has very limited practical uses, it only serves here as a jump off point to illustrate our sensor technology’s powerful application design possibilities.

Transmission Line Topology Choices

We have only used a simple coaxial cable to illustrate our sensing technology. There are many other transmission line topologies that are useable in an analogous manner to realize sensing capability. Such topologies as waveguide, microstrip, coplanar waveguide, planar (spiral inductor and interdigitated capacitor) and slotline have all been used in the design of our sensors. There is virtually an unlimited supply of topologies available, and typically the application itself will dictate which topology is most appropriate.

Frequency Independence

There is no particular frequency of operation that must be chosen in order to realize a sensor. Although a frequency needs to be established as an operating point, the frequencies can vary from 100’s of KHz to 10’s of GHz without loss of generality, provided the transmission line structure can support these frequencies. As a typical guideline, higher frequencies allow for sensor miniaturization such as for accelerometers, while lower frequencies allow for macro sensors such as load cells and proximity sensors.

Target/ Transmission Line Design Optimization Independence

Since the target and the transmission line only interact through the leaky EM field and never physically contact each other, each can be separately optimized for the particular sensing application. It’s quite simple —as long as the transmission line sees a target displacement, it will produce a desired signal regardless of how that target displacement occurs; as long as the target displaces it could care less that a transmission line is in close proximity to it, —only the EM fields join the two, and they of course, are without any physical substance.

Target Material Choice

Since EM fields interact with everything to a certain degree, any material can be used to construct the target. Metal, insulator, semiconductor, wood, bone, cement, even ice has been used as a “target”. To be sure, each target material should be optimized for its counterpart transmission line topology, but since there are so many topologies to choose from, the designer can readily optimize the sensor for a particular application.

Non Contact Interface Between Target and Transmission Line

The interface region between the target and the sensing portion of the transmission line can be filled with any material that allows for the target to still displace. In the case of the coax example, the slot can be filled with epoxy, the entire structure can be immersed in oil, a thin glass encapsulation of the target can be formed, etc. Encapsulation is particularly useful when liquid materials such as blood and plasma need to be maintained in a high state of sterility and yet sensing must occur.