It seems obvious that a Tire Pressure Monitoring System (TPMS), required by several governments, would need a pressure sensor. But that’s not true. Alternative approaches take advantage of other sensing capabilities in a vehicle, computing, and algorithms to implement a solution for TPMS that meets legislated requirements.  The alternative, indirect, approach has been used for many years instead of direct tire pressure measuring and monitoring. Car manufacturers have implemented both design approaches.

Indirect vs. direct TPMS

For example, NIRA Dynamics’ calls its indirect tire pressure monitoring system (iTPMS) the tire pressure indicator (TPI). Using information from several different physical sensors it computes new, virtual sensor signals. It was introduced in Europe in 2006 and the United States in 2008 to meets Europe’s ECE-R 64 and the U.S. FMVSS 138 requirements for TPMS. The software-based solution calculates the tire inflation pressure from the wheel speed signals without requiring a pressure sensor, RF components, and other circuitry at each wheel. As a result, TPI is very cost-effective. Sensing an underinflated tire, the system alerts the driver accomplishing the safety goal of the regulations as well as providing a fuel economy improvement from avoiding driving on underinflated tire(s). The TPI system can detect the pressure drop in one, two, three, or all four tires. It can also isolate which tire(s) are underinflated.

As a tire manufacturer, Sumitomo Rubber Industries Ltd. (SRI) tire-sensing technology can detect road conditions, tire load, and other information by analyzing wheel-speed signals generated by the tires’ rotation. Using the tires as its sensors, Sumitomo’s Deflation Warning System (DWS) requires no additional sensor installation and is completely maintenance-free. Over 50 million systems have been sold to 25 car companies, including 3.6 million in 2022.

In 2023, Sumitomo announced a partnership with a connected vehicle analytics company to use Sensing Core, built on its DWS, for predictive failure services. With projected available in 2024, DWS and Sensing Core both use an indirect tire pressure monitoring system to analyze tire rotation signals and detect changes in tire air pressure. The tire manufacturer works with brake manufacturers to install Sensing Core software on the brake electronic control unit (ECU). The Sensing Core software can be installed in one of three ways: (1) as a stand-alone system on the vehicle ECU, (2) as a cloud-based system that adds shared information from other vehicles, or (3) as a cloud installation that uploads essential signals from the vehicle for analysis.

Other potential problems that a Sensing Core system can detect include torsional force that could cause wheel detachment, the load on the tire, and the degree of tire friction (mu slip) occurring on a circuit. By taking indirect system samples every 20 milliseconds, the system can distinguish between poor road conditions (grip level), which show up as transient events, and tire wear, which appear as a more gradual decrease over time.

Warning

For sensor manufacturers, virtual sensing, based on adding algorithms rather than adding more sensors, could become more common as the number, types, and intelligence of sensors in the system increase for vehicles and locations, such as smart homes, buildings, cities, factories, and more.

References

Understanding Smart Sensors, 3^rd Edition, Ch 14
Tire Pressure Indicator