Transistors have reshaped the electrical communication infrastructure making it more affordable and more flexible. This latter was the result of digitalization, a trend started in the 1950ies and now ubiquitous. Bits are bits and scientists have found more and more ways to convert basically “everything” into bits, be it voice, images, sounds, strings of codons, traffic flows, diseases spread, people’s mood. I’ll consider the economic impact of this digitalisation later on. For now let’s consider the technologies that are making this digitalisation possibile because they have a strong influence on the path towards smarter cities. What you want, as a municipality, is to create a digital image of your city, of its citizens and of its processes.
Presenting an overview of current and future technologies used in “sensing” would require a long time so many and varied they are. Knowing fully well that I can only provide a glimpse into the sensors world, I will try nevertheleto provide a such an overview since getting the feeling of what is already possible and what is going on in this area is crucial to a smart city planner.
There are both analogue and digital sensors, meaning that the data provided can be in an analogue or in a digital form. The general trend is to either create data directly in a digital form or to process them locally, in the sensor, to deliver a digital data. So for our purposes I will consider digital sensors, even though in many cases the sensing technology produces an analogue sensing, but this is then translated into a digital data.
Even the classification of sensors into broad family is complex and ultimately subective. I chose to classify sensors in 11 categories as follows:
- Acustic sensors: these sensors detect sounds, in the audible and outside of the audible spectrum. There are sensors able to capture ultrasound, as bats do (this is the case of ecography where a computer reads the ultrasounds reflection to create an image of your tissues and organs). The technologies supporting these sensors are mostly electromechanical (a plane moves as sound waves hit its surface and the movement is translated into a variation of an electrical current that is then converted into streams of bits). More recently there have been progress in the area of electrospun piezoelectric nanofibers that have very high sensitivity beyond what is currently available. Usually the data provided by an acustic sensors are further analysed by signal processing applications. As an example, acustic sensors are used in agriculture to pick up the sounds in a potato field. Through signal processing it is possible to detect the presence of a white grub, a bug that eats the potato leaves. By analysing the data coming from several sensors in the field it is possible to localise the bug and spray the insecticide directly on it, thus decreasing pollution and cost. In a city we have hundreds of thousands of acustic sensors moving around: the microphones in cellphones and smartphones! A Smart City should find a way of leveraging from the data (background noise) these microphones pick up…
- ElectroMagnetic sensors detect the presence, and variations, of electrical and magnetic fields. Although we might consider these sensors as artificial (whilst the acustic ones may be felt as mimicking one of our senses) we have examples in the animal world where these kind of sensing is present (in sharks the ampulla of Lorenzini detects tiny changes in electrical fields, several birds have magnetic field detectors). The sensing is done by measuring the perturbations of a reference electric magnetic field inside the sensors caused by an external field. The study of animals able to detect these fields is leading researchers to study chemical ways of detection that can be more sensitive, working on spin properties at atomic level. We have electro/magnetic sensors everywhere, including our homes (e.g to detect the opening of a door…). A metallic object, like a vehicle, moving through an electromagnetic field perturbates it (by creating reflections). The city of Santander, Spain, uses magnetic sensors to monitor traffic congestion. Also an object that contains water molecules, like our body, moving around alters the electromagnetic field (by absorbing it). These perturbations can be used to “sense” what’s happening. Experiments have shown that by detecting the perturbation on a field generated by a WiFi network it is possible to detect the presence of people inside a building… Another possibility for a city to become “smarter”.
- Optical sensors detect .. photons. We are using optical sensors whenever we are taking a digital picture, and this happens over 4 billion times a day in 2016 (this is just about one photo per person with a smartphone, so it is not really a lot!). Optical sensors are in security cameras, in cars, in buildings…They can capture images, videos or simply the level of light in the ambient. Technology is usually based on photodiode, converting light into electricity thus generating an electrical signal that is further processed by software to render an image, to detect a face, to measure heart beat rate, temperature, pressure, humidity, strain.…The optical sensors can detect five optical properties of light: intensity, phase, polarisation, wavelegth and spectral distribution. Depending on the setting variations in one or more of these properties can tell us what’s going on. As an example an optical fibre placed in the concrete pillars of a building or glued to its iron beams gets strained as the building vibrates or stretches and these changes create a phase change in the photons flowing in the fibre, change that is measured by the sensor. A city planner may consider requesting that optical fiber are placed inside pillars of all new buildings to get data on the buildings themselves and on their surrounding thus detecting potential landslide danger.