These days when the Internet of Things (IoT) is mentioned one of the first things that come to mind are sensors. This is actually “unfair” in a few different ways – first, the IoT is more than just sensors and second, if you do talk about that foundational layer you should also include actuators/controllers, which are especially relevant for the Industrial IoT. In this sense, it would be better to talk about transducers – superset of sensors and actuators. It is true that sensors are a major part of the IoT and therefore it is worth spending some time (and text) to talk about these components.
We all know that sensors are devices that can monitor and measure the physical world and everything in it (including ourselves) – how does this “magic” happen? The basic answer is Physics – physical phenomena happen all the time: electromagnetic radiation (i.e., light, RF, etc.), mechanical and kinematic (i.e., motion, force, etc.), thermal, acoustic (i.e., sound, ultrasonic), chemical reactions, biological and more. These phenomena reflect things and processes that are happening in world around us (and even in us), so that if we measure these physical quantities we learn about what is going on. This is not unlike what we do as humans – we sense and “measure” the world with our senses (our eyes are sophisticated light sensors, our ears are sound sensors, etc.) and we then use our brain to make sense of this data.
Sensors typically convert the raw physical phenomena being measured into electrical current or voltage, because these are easy to “manipulate”. For example, many light sensors (not just visible light) are constructed of an appropriate semiconductor material such that when a specific type of light illuminates it the photons (light particles) are absorbed by the semiconductor and transformed into electrons (and an aptly named photocurrent, that is an electrical current created by the absorbed photons). This electrical current is related in a known way to the amount of light; the electrical current can then be easily manipulated and processed by electrical circuitry (including, for example, analog-to-digital conversion). This processed signal can then be transmitted by another appropriate communication device (using an antenna for wireless transmission, a laser for optical transmission, etc.). Clearly, the sensor itself, that part of the device that transforms a physical phenomenon into an electric current, is just the very front end of a chain that produces a signal that can be transmitted. By the way, actuators work the other way around – typically an electrical signal is sent to the actuator, which causes the end device to produce a physical phenomenon (e.g., a loudspeaker that produces sound related to the electrical signal sent).
So let’s move to a more complex scenario – in many cases, the direct measurement in itself is not very interesting but by proper transformation can tell us something that is interesting. For example, a pyroelectric sensors are based on materials that create an electric voltage when there is a change in temperature around them. However, if you have ever dealt with motion sensors of security purposes then you know that the end objective of a motion sensor is to detect motion. The pyroelectric sensor (used in many security applications) senses the temperature change in the room associated with a (warm) object moving in front of the sensor. So the motion sensor not only does transforms a temperature change into a voltage and then performs processing of this voltage, it also needs one more level of processing to yield an indication whether there is motion in front of it (e.g., it has crossed some temperature change threshold).
Here is another, more complex, example –
Many wearables, specifically smart watches and bracelets, include a heart rate monitor (HRM). One of the most common approaches is to use an optical HRM and it goes beyond just a sensor. In fact, it requires an active element to work – the HRM includes a light emitting diode (LED) that shines light onto the wrist of the wearer, which penetrates the skin and reaches the blood vessels (turns out our skin is not as opaque as it looks). A light sensor (not unlike the one I described earlier) measures the reflected light. Because the blood absorbs more of this light than the surrounding tissue and the amount of blood at any given point changes with the heart rate, the (measured) reflected light is actually modulated by the heart rate. If you are really interested, check out photoplethysmography (after you have practiced saying it a few times; the common acronym is PPG, which is a lot easier to pronounce). This waveform needs to undergo some signal processing before a reliable and robust heart rate is extracted. So now we have the heart rate as a function of time, but really we are interested in things like intensity of physical activity, fatigue, calories burned, etc. In other words, we need to further analyze the heart rate waveform and how it changes and infer (or calculate) these quantities of interest. Quite often, other sensors embedded in the smart watch (like accelerometers, see this for example) are also monitored and fused with the HRM to get more accurate information.
What I am trying to highlight here is the fact the term “sensor” is sometimes used rather loosely and can range from the actual sensing element (measuring light or temperature) to a complete “sensing system” like the optical HRM (that actually senses reflected light but delivers the wearers heart rate, by applying signal processing and analytics). One of the reasons for the broadening of the term “sensor” is directly related to the commoditization of both the basic sensing element as well as the computational resources that can be attached to it. Advances in semiconductor manufacturing technology has driven the price of these components down by orders of magnitude over the last two decades (driven, in turn, by the demand created by smart phones and wearables). So these days, one can buy 3-axis accelerometer chips for significantly less than $1 (quantities of 1000) but on the other end of the spectrum you can get a system on chip (SoC) with multiple sensors and an ARM processor. In fact, in 2015, ST Microelectronics came out with iNEMO advanced inertial module, Invensense came out with Firefly and Bosch Sensortec came out with their sensor hub solutions – all similar in specs, single package (about 3x3x1 mm) SoCs that one would be hard pressed to call simply sensors. Maybe these should be called Smart Sensors (implying integrated compute capabilities) or Sensing Engines (implying more of a system rather than a discrete component); in any case these are some of the new building blocks of the IoT (especially the consumer or Human IoT), which are already being embedded in smart phones, smart devices and wearables.
Now what about the Industrial IoT sensors?
Turns out that here too, sensors as discrete components are also fading away. GE is using an additive technology (similar to 3D printing) called Direct Write. One application GE is developing is to build small devices such as sensors on (machine) parts to measure things like temperature or strain – the sensor is basically “sprayed” onto the part. The notion of printing sensors is not new and is certainly part of the trend of sensor price and size reduction. Admittedly, these printed or “written” sensors are usually the bare sensing elements (without the intelligent part, i.e., the ARM processor is not printed together with the sensor), on the other hand a lot of them can be produced and integrated inexpensively.
Smaller, cheaper, lower power, less obtrusive (quoting Mark Weiser – “The most profound technologies are those that disappear. They weave themselves into the fabric of everyday life until they are indistinguishable from it.”) and more intelligent are the major trends associated with sensors. These also lead to new challenges like security issues, especially when your sensor looks more like a small system.
So where are else are sensors headed?
I think the already emerging frontier is related to the sensors that sense us – wearable sensors coupled with body area networks (BANs). The human individual as an intelligent network node – again, not a new concept (the military has been talking about such ideas as part of its network centric warfare concept for at least 2 decades), but one that can now be taken to new extremes. Here are some examples of the exotic (some already here and some not necessarily very far off…) for you to look up: the Biostamp, the ingestible sensored pill, the ingestible pill camera, smart contact lenses, implantable RFID chip (a little bit creepy for my taste…), to name a few. You can wear them, ingest them and implant them (let us not forget things like pacemakers, which have become incredibly sophisticated and continue to make advances by leveraging some of the same trends mentioned earlier).
Indeed, sensors, sensors everywhere…Read less