All new cars sold in the EU must now have a Tire Pressure Monitoring System (TPMS) installed by the manufacturer. Aftermarket systems are also available, which take advantage of proven sensing and wireless technologies to bring the safety and economy advantages of TPMS to owners of older vehicles as well as trucks, caravans and motorcycles.
TPMS for safety and savings
A car’s tires can lose as much as 6% of their pressure every month. If the owner does not notice, or check, this can lead to unpredictable or dangerously deficient handling. It can also impair fuel economy and increase tire wear, which is bad for the environment and drives up the cost of vehicle ownership.
The safety implications of tire under-inflation have been a major driver of legislation mandating the fitment of tire-pressure monitoring systems. Vehicle inspections carried out in the US revealed that as many as seven out of every ten cars on the country’s roads would likely have underinflated tires. Seeking to reduce the number of accidents attributed to incorrectly inflated tires, US legislators implemented the TREAD (Transportation Recall Enhancement Accountability and Documentation) Act, which requires fitment of TPMS. This has applied to all new vehicles in the US since September 2007.
In Europe, legislation based on the US laws has required TPMS to be fitted to all new types of vehicles since November 2012, and all new vehicles since November 2014. As far as the economic and environmental benefits of TPMS are concerned, figures in a 2008 report on vehicle type approval by the European Commission suggest that underinflated tires can increase fuel consumption by 4% and reduce tire lifespan by 45%.
The European technical regulations governing TPMS require the system to alert the driver if the pressure on any tire deviates by more than 20% from the recommended pressure. An additional 5% tolerance on the measurements is allowed. The TPMS must also be able to self-test in order to detect any internal malfunctions.
Operating Principles
TPMS can work in either of two ways. A controller can compare information from the vehicle’s wheel-speed sensors and generate an under-inflation alert if excessively fast rotation is detected on one wheel only. This approach is relatively inexpensive, but can become inaccurate if the tires wear unevenly or if the owner changes to a larger or smaller tire. This is sometimes referred to as Indirect TPMS.
The alternative approach is to fit a battery-powered pressure sensor/transmitter unit inside the wheel. This may be either attached directly to the valve stem or banded to the center of the wheel. The sensor module incorporates a wireless receiver that manages actuation of the sensor, and a transmitter to send internal pressure, tire-ID and other data to the controller. This approach, sometimes called Direct TPMS, offers greater accuracy, but the sensor must be changed when the battery runs low. Careful power management can allow the sensor to last for around 10 years, which is significantly longer than the typical lifespan of a tire in regular use.
Aftermarket opportunity?
An OEM TPMS can be fully integrated with the vehicle’s electronics and instrumentation. The commonly used dashboard warning comprises a yellow illuminated symbol showing a tire in cross section with an exclamation mark in the center, as shown in Figure 1. Although, in Europe, all car types introduced since 2012 have an OEM TPMS, aftermarket TPMS can allow drivers of older cars to benefit from the safety and economy advantages that these systems can bring.
Figure 1: The TPMS dashboard warning light.
Replacement valves with built-in mechanical pressure indicators have been available for some time. These provide a visual good/bad pressure reading using color-coded indicators in the valve cap (Figure 2). One drawback of this type of system is that the indicators must be inspected visually but cannot be seen from inside the car. The driver must check each wheel individually to assess the status of the indicator.
Figure 2: Mechanical in-valve system for tire-pressure monitoring.
Aftermarket electronic TPMS comprising wireless replacement valves and a controller/user-interface unit that can be mounted to the dashboard (Figure 3) are now available. These can be fitted to cars and other vehicles such as motorhomes, caravans and trailers without modification to existing wiring. Similar systems are also available for motorcycles.
Figure 3: Aftermarket TPMS comprising pressure-sensing valve caps, wireless base-station and user-interface unit.
TPMS reference design and sensors
Whether the TPMS is an OEM or aftermarket fitment, similar functional blocks and operating principles apply. Figure 4 shows the main functional blocks of a TPMS reference design by Microchip. The central controller connects to the Low-Frequency (LF) actuators via LIN, and issues commands to each actuator in turn to wake up the associated pressure-sensor/transmitter unit. The 125 kHz low-frequency wireless connection has a short range of around one meter, which enables the actuator to connect with the sensor unit in its immediate vicinity. After sending the command to wake the sensor, the controller then waits to receive the wireless transmission from the sensor containing the detected tire pressure, temperature and sensor-battery condition.
Figure 4: Main TPMS functional blocks.
The major functions of the sensor/transmitter module include the pressure sensor, microcontroller, LF receiver and RF transmitter. A wireless microcontroller such as the Microchip RFPIC12F675F combines a PIC12F microcontroller and UHF ASK/FSK transmitter in a single package. Other valuable integrated features of this device help further minimize component count, including an internal RC oscillator, a comparator that enables the LF decoding to be performed on-chip, and a 10-bit ADC that gives the designer freedom to choose an analog pressure sensor with a suitable sensing range, such as 100-450 kPa (15-65 psi), without needing to design-in an external A-to-D converter. The reference design utilizes a separate analog front-end IC to detect the 125 kHz LF signal.
Alternatively, a sensor/transmitter unit can be built using a fully integrated TPMS pressure sensor that combines the sensing element, microcontroller, LF receiver, RF transmitter and other functions in a single package. Only a minimum of external components are needed to complete the system. A good example is the Freescale FXTH870511DT1, as shown in Figure 5. This device is a dedicated TPMS sensor that integrates an 8-bit S08 core, 125 kHz LF receiver and 315-434 MHz radio transmitter, and also features a MEMS accelerometer and a temperature sensor in addition to the pressure sensor. The accelerometer can be either a single-axis sensor for measuring radial acceleration only, or a dual-axis sensor that is also capable of supporting tire localization capability.
Figure 5: The Freescale FXTH87 integrates all the major functions needed for a TPMS sensor module.
Conclusion
TPMS is now mandatory on all new cars sold in the US and Europe. As the safety and environmental benefits become more widely appreciated, and the operating principles understood, the owners of an even wider variety of vehicles may be able to take advantage of reliable, timely tire-pressure information.
