Currently, the EU is mid-way through mandating its installation in all vehicles — commercial and passenger alike.
A TPMS is an important vehicle safety feature that was first mandated as a legal requirement in the United States in 2008. Since then, it’s also become mandatory in South Korea, Japan and the European Union. The latter is the most recent to update its legislation, which now makes TPMS a legal requirement for commercial vehicles, as well as passenger vehicles, which will be fully introduced by July 2024.
The benefits of a TPMS
A TPMS ensures the correct tyre pressure is maintained, which is essential to the safe operation of a vehicle. Improving safety and reducing CO2 emissions were the key areas targeted by EU and US legislators. The system identifies and immediately informs the driver any time that the tyre pressure drops below a predetermined threshold, whether that’s through natural deflation or a slow puncture.
A substantial number of vehicles drive with underinflated tyres each day, which by doing so negatively impacts vehicle safety and running costs. A report conducted by the UK’s National Highway Traffic Safety Administration (NHTSA) revealed that vehicles with tyres that are underinflated by 25 per cent are three times more likely to be involved in a road accident, since underinflation results in poorer handling of a vehicle, which in turn makes it less safe and reliable to drive. So having a TPMS, which immediately notifies the driver of a tyre pressure drop, ensures it’s handled as soon as a risk emerges.
But it does more than that. A TPMS is a valuable tool to keep maintenance and fuel costs down too as correct tyre pressure inflation will optimise the tyre’s surface contact throughout its performance. As a result, this in turn will optimise the tyre’s traction, ensure a shortened braking distance, improve the fuel economy, and ensure a slower and even wear along the whole tread area.
The direct measurement system
There are two options of TPMS — a direct and indirect system. Indirect TPMS works by using data from the vehicle’s antilock braking system (ABS) wheel speed sensors. This method detects a drop in tyre pressure by comparing wheel speeds, since a lower-pressure tyre has a smaller diameter.
However, the more reliable and accurate, and Swindon’s choice, of TPMS is the direct system. To monitor temperature and pressure, the direct system has a sensor within each tyre that is mounted on the valve.
The sensor takes a direct pressure reading, which is transmitted via a radio link back to the central electronic control unit (ECU) in real time, where it is analysed, anomalies are immediately identified, and the system provides a warning if the reduction threshold has been reached.
Direct TPMS sensors are battery powered, and the system must be designed to have a long battery life, which is mainly achieved by utilising impressive low current usage techniques. Each sensor is designed to last 10 years or 160,000 kilometres. As a result, it must be designed for ultra-low power and be able to detect when the vehicle is moving to enable deep hibernate mode during stationary periods.
A key component of a TPMS sensor is an integrated circuit (IC), which performs the crucial signal conditioning and processing functions that convert, manipulate and process the analogue signals from the tyre pressure data into a digital form for use within the control system, and transmit the data via radio link to the car. Because of the unique system requirements, it becomes desirable to use a mixed signal application specific integrated circuit (ASIC) to perform these functions in order to provide a technical and commercial edge over other design philosophies.
The Swindon approach is to co-package the bespoke ASIC together with a micro-electromechanical systems (MEMS) pressure sensor and a MEMS accelerometer, all designed by Swindon, in a System in Package (SiP) configuration, which is calibrated and production tested at Swindon’s test facilities.
The ASIC within
Unlike commercial off the shelf integrated circuits (COTS ICs), an ASIC is designed to meet the customer’s unique performance specification, providing commercial and technical competitive advantages.
The ASIC is a complex device that provides an optimised, high quality and reliable system to conduct the signal conditioning, signal processing and communication functions required for direct TPMS sensors. The ASIC receives information from both the MEMS pressure sensor and accelerometer to calculate the pressure within the tyre and determine the motion of the wheel.
The ASIC and both MEMS are custom designed, adhere to strict automotive standards, provide accurate pressure measurement and communicate the information to the vehicle over an extended period. The ASIC developers integrate as much of the system as possible into a single package, which reduces size and power consumption — helping to extend the TPMS battery life. This compact size is essential too, since every millimetre of space equates to weight within the tyre.
There’s no doubt that the legal appetite for TPMS is increasing across the globe, and with this brings challenges of increasing functionality over and above the traditional tyre pressure measurements. An ASIC is the crucial component in the evolution of TPMS. Its unique attributes, combined with the knowledge and experience of its designers, is driving the future technology within this sector, by providing bespoke optimal functionality and performance differentials greater than its COTS alternatives.
Author details: Richard Mount, Director of Sales at Swindon Silicon Systems
