While vehicles in general are now full of IoT sensors and devices, and thus subjected to a continuous exchange of information, tires are still rather “silent”. In fact, to date the only sensor used on large scale is the Tire Pressure Monitoring System - TPMS - which measures pressure and often temperature. These data are important in their own right, however, several other information can be gathered from them, such as a slow and somewhat insidious deflation due to a small leak. The need for reliability, even when sourcing energy, explains the proliferation of research in the field of “energy harvesting”: energy obtained from wheel rotation. This need for reliable power supply will be further intensified by the arrival of autonomous vehicles (which is expected to be somewhat slower than electric vehicles), which will make these sensors even more important, making reliability and consistency of operation a crucial factor: the presence of a human driver to act upon any alarm may not be the norm in the future. However, sensor reliability will be crucial even before the arrival of self-driving vehicles because "smart" tires will be a key element for ADAS and the Internet of Vehicles (IoV), a network that allows vehicles, infrastructure and humans to communicate with each other. Last but not least, sensors placed in the wheels (not exactly easy as these are rotating components separated from the rest of the vehicle) makes it possible to reduce tire wear, emissions and dependence on raw materials.
Searching for energy
Tire manufacturers are busy with creating increasingly “smart” products so that they can measure and quantify tire-road interactions. This information would produce useful estimates such as stopping distance, grip, drift angle, road conditions and tire wear: fundamental data that imply even more advanced sensor technology. A study of the prospects for collecting "vibrational energy" has been conducted by Mohammed V University in Rabat, Morocco. It involves converting vibrations (tires produce plenty of them) into electrical energy that can power the very sensors that make our tires “smart”. Today's TPMSs contain a pressure sensor (possibly combined with a temperature sensor), an interface to the control unit (ECU), a radio transmitter that sends signals to the vehicle's ECU, and, of course, a battery.
The whole thing consumes an average of 0.45 mW if signals are sent at a frequency of 1 Hz (once per second); if the transmission rate increases so does the power consumption. Piezoelectric devises, which produce a potential difference if they are mechanically stressed, can be used as possible "converters" of mechanical vibrational energy into electrical energy. Thus, currently studies are being made on piezoelectric elements stressed by the rotational forces exercised on a wheel. Oscillating elements are calibrated according to the rotation of the wheel, with a process that starts by analysing the vibration spectrum of the wheels breaking down the overall vibration into its elemental components. Among the most pronounced components are those at low frequencies (6 to 16 Hz) which correspond to the rotation of the tires at speeds between 40 and 105 km/hour: these must be calibrated to oscillate at these frequencies in order to maximize energy transfer to the piezoelectric elements.
Energy through deformation
Power generation using tire deformation is being studied through a variety of avenues. One includes placing a piezoelectric ring-like element between the tire's metal belts: the compression and release of the casing during rotation will stress the ring, generating energy. Another idea looks at piezoelectric elements housed between the tire and the rim. Piezoelectric elements have been envisioned also on the inner surface of tread: they stretch as the tire flattens against the road surface and compress as the casing regains its circular shape, generating an electrical charge. The materials generally used are PZT (titanium lead-zirconate) ceramic and polyvinylidene fluoride polymer. Sumitomo Rubber Industries, however, has used a different approach, creating energy harvesters consisting of wave-shaped membranes, one negatively and one positively charged, placed opposite each other. Deformations generated by the contact area of the tire, "flatten" the membranes, and this relative movement generates electrical charges by friction which, once collected by conductive rubber electrodes will then flow to a sensor. Other research involves microgenerators based on magnets oscillating inside coils of copper wire with piezoelectric material and a magnet at the end that, interacting with a magnet placed nearby, ensures oscillations at any speed. Magnetic systems look very promising in terms of power generated.
The future of ADAS sensors
Let us now try to see how sensors in tires might evolve, starting with Sumitomo itself and its static electricity microgenerators. By arranging several of them in a line from the centre of the tread to the sidewalls and analysing the waveforms of electricity produced by each one (this is alternating voltage), tire wear can be calculated. When the tread is new the energy harvesters closer to the sidewalls of the tire suffer minor deformation and therefore the voltage produced by them is limited compared to the amount generated by the devices closer to the centre. This processed is reversed once the tread start flattening due to wear and the energy created is substantially the same. From this is possible to detect the wear of the tread itself. From Izze-Racings, on the other hand, we have a Tire Temperature and Pressure Monitoring System that is TPMS to the nth degree. In fact, its sophisticated infrared sensor detects the temperature distribution inside the tire in 16 different points. The digital pressure sensor is also very advanced since, working at 24 bits, it is extremely accurate. Energy harvesting, therefore, promises greater reliability to next generation sensors. This short review, admittedly brief and limited in scope, showed us the future prospects for tire sensors and the systems designed to power them. Although energy harvesters are still in the experimental stage, it is more than likely that they will soon equip our tires along with highly evolved sensors. Clearly, industry operators will have to be trained to maintain and repair increasingly “smart” tires, which will also require more elaborate diagnostics and calibration methods than what is required by current TPMS.
