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10/11/2017
Smart and connected tires, energized through motion

Motion and energy

 

Future “smart” vehicles will have to collect a lot of data, even from tires, with a consequent increase in the number of sensors: where will the energy come from? One idea is to collect and use the mechanical energy generated by tire vibration and deformation, and many are in fact the studies pursuing this idea.

Nicodemo Angì

Tires heat up, and this is a known fact, but this phenomenon does not depend solely on the temperature of the road surface scorched by the summer sun. Our trusted tires are constantly subjected to stress; braking, acceleration, steering and, at times, bumps and holes. But even if the road was as straight and smooth as a snooker table and the vehicle moved at constant speed, deformations would still affect the tires, as shoulders and sidewalls are compressed by the weight of the car around the contact area to be released once again as the vehicle moves forward.

Heat is a clear indication that Energy is being wasted: just a small fraction of it would be enough to feed many sensors and processors that, thanks to recent developments in electronics, need little power to work. However, their sheer number makes battery power limited which would cause the battery to be replaced more often.

 

How to condense energy

Though the purpose is the same, that is to turn into electricity the mechanical energy and heat created by the moving wheels of a vehicle, the means available to reach the target are many and of different types. We can mention, for example, a research carried out by Visteon engineers, a well-known micro-chip manufacturer, along with researchers from the Vestfold University College in Norway. The study is aimed at powering classic TPMS systems, but the solution is suitable also for other “power-hungry” sensors. The experiments concerned electrostatic Energy harvesters that function as variable capacitors. Vibrations separate the plates of a charged variable capacitor, and mechanical energy is converted into electrical energy. Connecting the plates to a battery, creates a current flow that gradually decreases to zero: the capacitor is charged and the plates have accumulated opposing charges which attracts the plates towards each other: this force is generated by an electrostatic field. The process is reversible: by moving the plates further apart or closer by an external force (pre-charged by a special manufacturing process), a small electric flow can be generated able to power a microchip; it is also appropriate to add that the force in question comes from the vibrations and stress to which a tire is subjected.

 

A bit of everything

On the other hand, a similar device takes advantage of electromagnetism rather than an electrostatic field: in this case the movement of a magnet, rather than a conductor coil, replaces the movement of the two conductive plates, working the other way round compared to a loudspeaker. A coil close to a magnet will experience a variable force with the same frequency as the current flowing through it: this is the power, for example, that moves the cone of the loudspeaker. By moving a magnet near a wire coil, taking advantage of the vibrations of the wheel/tire unit, we can pick up an alternating voltage at both ends of the coil itself: this can power, once converted into a continuous voltage, the microchips. Another promising front involves piezoelectric materials, used, for example, in kitchen igniters that work by pressing a button. Their main feature is the production of electricity when compressed and expanded, and this kind of movement is rather generous in moving tires.

Take for example the thesis that was discussed by Otso Jousimaa for his Master of Science and Technology at the Finnish Aalto University. A series of piezoelectric elements were placed inside the tire so that they were subject to deformation and vibration as the tire moved.

 

Compressed and electrified

The AC power thus produced was collected, converted into DC and then stored in a super-capacitor (a component that is something in between the above mentioned condenser and a battery) at a voltage that can be used by modern microprocessors. The Energy production was not really impressive, 13 microwatts at low speed and 50 microwatts at a higher speed, but this energy is relative to a few elements and it is likely to increase by using more units. The same conclusions, namely the use of piezoelectric materials, was reached by the Centre for Process Innovation (CPI) in the UK during a joint project with the Bath University and Silent Sensors. This partnership aims to develop a key component for future smart tires, i.e. Energy for their sensors and processors, a definite step forward towards the development of traceable tires monitored in any working conditions, with beneficial effects on safety, reduced fuel consumption, and consequently, CO2.emissions.

An evolution of these materials is also found in the BH 03 concept by Goodyear, a tire that has a kind of network of piezoelectric units that transform both mechanical stress and heat into electricity. The high-energy tread of the tire collects solar heat and generates electricity by heating the thermo-piezoelectric network. As the car starts to move the process is increased as friction and deformation increase. As the vehicle moves faster, more electricity is generated through the thermo-piezoelectric units.

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