Avoid the deflated feeling
11 March 2008
An intelligent, single-package chipset helps car manufacturers achieve precise tyre pressure sensing.
The industry’s first tyre pressure monitoring system (TPMS) containing a capacitive pressure sensor for low power consumption and precise sensing has been introduced by Freescale Semiconductor.
Freescale’s MPXY8300 system-in-package device provides an integrated, cost-effective solution that features pressure and temperature sensors, eight-bit MCU with 512Mb RAM and 16KB flash, XZ-axis accelerometer, 20-pin SOIC wide body package, RF transmitter, -40°C to +125°C operating temperature range, over-temperature shutdown, and low-frequency input for receiving diagnostic commands. In addition, the MPXY8300 incorporates a number of low-power techniques engineered to help extend TPMS battery life beyond 10-year requirements. It has been designed to address global motor vehicle safety requirements for accurate tyre pressure monitoring. Properly inflated tyres can help reduce the potential for life-threatening blowouts, enhance road handling, improve fuel efficiency, prolong tyre life and lower the cost of vehicle ownership.
The national highway traffic safety administration (NHTSA) estimates that 23,000 accidents and more than 500 fatalities a year in the United States involve flat tyres and blowouts stemming from improperly inflated tyres. Indeed, under the transportation recall enhancement, accountability and documentation (TREAD) act, such systems have been compulsory on all cars sold in the USA since 2006. The TREAD act was passed in November 2000 in a bid to make tyres safer for the public, following the Ford/Firestone fiasco/crisis. This came to a head in August 2000 with claims that 15-inch Wilderness AT, ATX and ATX II tyre treads were separating from the tyre core. As a result, Bridgestone/Firestone recalled 6.5 million tyres that were original equipment on the Ford Explorer. However, Firestone claimed that the problem was not merely with the tyres, but also with the Explorer model and that something was causing it to roll over, no matter which tyres were fitted.
Freescale’s MPXY8300 TPMS solution is designed to help drivers maintain correct tyre pressure by instantly notifying them when any individual tyre (including the spare) is not at the optimal pressure level. Engineered for wheel-mounted applications, the MPXY8300 system helps automotive engineers reduce development cycle time, component count and system cost by combining the necessary digital, analogue and sensor functions of a TPMS in a single, compact package. As well as offering real-time tyre pressure measurement using direct monitoring at each wheel, Freescale’s single-package solution has an integrated accelerometer for motion detection. The system can be programmed to transmit measurements at certain tyre rotation speeds and even when the tyres are not rotating at all.
Rospa (Royal Society for Prevention of accidents) argue that it is difficult to spot an under-inflated tyre, especially without a fully inflated tyre for comparison. Due to the rigidity of tyre walls, a drop in pressure will only lead to slight increased flexing of the wall when the tyre is viewed. This is why a TPMS can be advantageous, as it can warn drivers that their vehicle has an under inflated tyre. Under inflation can lead to deformation in the tyre wall as it concentrates the load upon the tread shoulders and reduces the amount of surface contact the tyre has with the road. The consequences of this are increased tyre tread wear, which can result in a high chance of aquaplaning, reduced control of the vehicle, longer stopping distances, and a high chance of the tyre de-laminating, which could lead to a sudden tyre failure. Furthermore, fuel consumption increases when tyres are incorrectly inflated and tyres that are under-inflated by 6PSI can increase fuel consumption by two per cent and tyre wear by 25 per cent.
There are three types of TPMS; direct, indirect, and hybrid. Arguably the most accurate and reliable form of TPMS is the direct system. This uses sensors to monitor tyre pressure and has the advantage that it can take into account factors such as the tyre temperature when calculating the pressure. Indirect TPMS is an addition to the wheel speed sensors used as a component of the ABS. A decrease in tyre pressure will lead to a decrease in the wheel’s radius; meaning that it will rotate faster compared to the other tyres and the speed sensors detect this change. This system is cheaper to implement and quicker to introduce onto new vehicles, but has disadvantages due to the fact that it cannot detect a slow and equal decrease in pressure on every tyre. The system also needs to be calibrated frequently, which could be a difficulty for end users. The concept of a hybrid TPMS is to combine the advantages of both systems; the accuracy of the direct system and the cost savings of the indirect system. The pressure sensors are on two of the wheels instead of four and the wheel speed sensor compare the differences in speed to these wheels so as to detect a dip in pressure.
A spokesman for BMW told EPD: “Our systems work on comparing the speed of each wheel against the expected speed given the steered angle of the car. When a tyre deflates, the circumference effectively decreases resulting if a faster rotation. This is compared to the other wheel speeds via a running algorythm of ‘is and should be’ figures and a warning is issued when the two don't agree. This indicates that the tyre has deflated.”
Installing a TPMS unit allows tyre pressures to be constantly monitored, which will lead to improved braking efficiency and a reduced risk of an accident caused by under or over inflated tyres. Although TPMS is a useful tool in reminding drivers that tyres require frequent checks, it should not be seen as a replacement. Drivers should still perform regular tyre checks that involve testing the pressure with a gauge, checking the tyre wall for damage, and making sure that the tread is not worn.
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