A sense of safety

Michelin has launched a wireless and battery-free tyre monitoring system that marks the commercial debut of UK-developed pressure sensor technology.

The global tyre giant’s eTire II sensor patch, designed for use on trucks and other commercial vehicles, monitors the pressure and temperature of tyres via surface acoustic wave (SAW) technology licensed from Oxfordshire-based Transense Technologies.

The system uses the unique physical properties of SAWs to take continuous measurements and transmit the readings via an integrated radio frequency identification tag.

At 11g and the thickness of a few sheets of paper, the sensor patch is small and lightweight enough to be embedded in the interior of a tyre’s sidewall without affecting the wheel balance.

Because SAW monitoring is a passive technology the Michelin system requires no power source, eliminating the need to periodically remove wheels and change batteries. This also boosts the system’s environmental credentials compared with other products, say its developers.

In the eTire system, the measurements taken by the sensor are read at the vehicle’s depot or a drive-by unit by a hand-held device that retrieves the data automatically. Fleet operators can also track the performance of their tyres via the internet.

Tyre-pressure monitoring has long been a focus for the automotive industry and regulators in the US and EU. Inaccurate or infrequent manual checks on pressure are the cause of thousands of accidents and breakdowns as tyres either blow out (sometimes with catastrophic consequences) or run flat overnight.

US studies suggest that more than a quarter of vehicles on the road at any time have at least one tyre that is significantly under-inflated. In 2000 a wave of fatal accidents in the US were caused by tyre blow-out on sports utility vehicles, prompting new safety legislation.

The arrival of eTire marks the culmination of five years of work by Transense to develop SAW applications for tyre-pressure monitoring and get them into a commercial product.

James Perry, Transense chief executive, said the launch was a result of the company’s strategy of licensing and technical collaboration with key players in the industry. ‘Although Transense holds the worldwide core patents for the technology, it has taken the support of two major global automotive suppliers to commercialise our invention.’

Engineers from the UK group worked with colleagues from Michelin and Honeywell Sensing & Control, the co-developer of eTire and the direct licensee of Transense’s SAW technology.

While the launch of eTire into the North American commercial market is a major breakthrough for Transense, the biggest prize will be a mass-market system for passenger vehicles. The company has equipped several demonstrator vehicles with a SAW-based monitoring system and is in talks with manufacturers in the US, Europe and Asia.

SAW can also be used to measure torque and Honeywell is developing products for use in automotive powertrain and driveline systems.

The core of the technology is the Rayleigh wave, named after the physicist Lord Rayleigh who discovered SAW propagation in the 1880s and predicted the properties of the waves.

Piezo-electric SAW sensors use an oscillating electric field to create an acoustic wave that propagates on a surface before being transformed back to an electric field for measurement.

Because the propagation path length and surface wave velocity are affected by mechanical strain, the measurements taken can be used to detect pressure and temperature within tyres.

The sensors operate at a high frequency of around 433MHz at wavelengths of 1-100 microns. According to Transense, because most acoustic energy is contained within one wavelength of the surface the SAW sensors can achieve unrivalled sensitivity.

Another major advantage of SAW-based devices is that they are low-cost products for volume manufacturing. Millions are already produced for use in consumer electronics products such as mobile phones, where the technology is used within RF filters, and televisions.