Electronic Design

Tire-Pressure Monitoring Pumps Up Performance

Manufacturers of tire-pressure monitoring systems (TPMSs) are striving to meet the demands of what will be a universal application. Driven mostly by legislation, designers are struggling to overcome key technical challenges such as integrating the TPMS unit within the tire itself (versus in the tire stem) and lowering the cost of such systems.

TPMSs have been used for many years on high-end vehicles, particularly in Europe and Japan, where their high cost wasn’t an issue and since manufacturers of luxury cars see such items as product differentiators. But that is about to change. Many IC manufacturers want to get in on the potentially large market for TPMSs with a low-cost system that can be mounted in all cars, not just high-end ones.

On Sept. 1, 2007, the U.S. National Highway Traffic Administration (NHTSA) mandated that all new passenger cars sold in the U.S. must include TPMSs as spelled out in the U.S. Transportation Recall Enhancement Accountability and Documentation (TREAD) Act. Such systems must include a “direct” measurement approach, instead of the older and lower-cost “indirect” method in which variations in wheel rotation speed tied to the anti-lock braking system (ABS) are sensed.

The direct method allows both temperature and pressure measurement in a tire. There is no way for temperature to be measured in the indirect method. In Europe and Asia, there is no legislation for TPMSs, at least not yet, though some automotive experts believe that may change in the coming years.

Presently, U.S. auto manufacturers are employing TPMSs that meet the minimum TREAD Act requirements of monitoring a tire to sense improper inflation within 25% of the tire’s rated pressure level and detecting this condition within 20 minutes. In this minimal implementation, a simple warning light on the dashboard lights up, indicating only a tire inflation problem. Nearly all direct-type TPMSs use microelectromechanical-system (MEMS) pressure sensors.

IC manufacturers are trying to overcome several obstacles to produce low-cost TPMSs. One key challenge is producing a TPMS that can be readily integrated into a tire, not just in the tire’s stem valve. Stems have been known to break off and degrade, causing damage to the TPMS molded to the stem and rendering the part useless. Another key challenge lies in developing a low-power battery-operated TPMSs where the battery can last for many years before replacement.

Strong Incentives

Besides legislation, there are also strong incentives for developing more advanced TPMSs. Statistical data shows that improperly inflated tires reduce car fuel efficiency, decrease tire lifetimes, and contribute to roadway accidents. Major players developing advanced TPMSs include Freescale Semiconductor, Infineon Technologies, and Texas Instruments. Melexis, Kavlico, VTI Technologies, GE Sensing, LV Sensors Inc., and Sensonor are also working on TPMSs.

General Motors estimates that 83% of tire pressure loss occurs gradually, often without being noticed by the driver, with many negative consequences. Tire maker Goodyear estimates that fuel efficiency is reduced by 1% for every 3 psi of tire under-inflation. Both Goodyear and Michelin Tire Co. say that running tires at 20% under-inflation can reduce tire life by up to 50%. And, the NHSTA says that 27% of passenger cars and 33% of light trucks are driven by one or more substantially under-inflated tires. The result is an estimated 23,000 crashes (535 of them fatal) annually due to blowouts or flat tires.

Speaking at the MEPTEC 2008 Conference, LV Sensors Inc. chief technology officer Janusz Bryzek and Axept Inc. chief executive officer Joseph R. Mallon Jr. projected a “green killer application” for wireless tire-pressure monitoring. They estimated that the installation of TPMSs in all cars in the U.S. alone could have a large environmental and economical impact for the country.

The executives based their findings on an average tire under-inflation pressure of 26% for all cars on the road in the U.S., which is a conservative figure given other studies that showed higher percentages (see the table). LV Sensors develops advanced TPMSs, while Axept designs, develops, and manufactures custom-engineered MEMS modules.

TPMS modules use wireless links that operate in the 315-MHz RF band in the U.S. and Japan (433.92 and 868 MHz in Europe), simplifying communications circuitry and contributing to lower costs. A battery-operated transceiver in each tire, usually mounted in the tire stem, communicates wirelessly with a dashboard controller.

The 315 MHz is the same frequency at which remote keyless entry (RKE) systems operate for cars. RKE systems, which generally use surface-acoustic-wave (SAW) or phase-locked loop (PLL) devices, consist of a transmitter in the key fob and a receiver inside the car.

The TREAD Act requires the TPMS to have a long battery life of at least 10 years. Freescale Semiconductor’s MPXY8300 uses a capacitve pressure sensor and several low-power techniques to surpass that requirement (Fig. 1). The highly integrated unit also features an 8-bit microcontroller, a dual-axis accelerometer, and the RF transmitter with a charge pump, all in a 20-pin small-outline IC (SOIC) package.

Infineon Technologies was one of the earliest companies to offer an integrated TPMS product. Its SP35 includes an 8-bit microcontroller, a wireless communications module, and pressure, acceleration, and temperature sensors all within a P-DSOP-14 package (Fig. 2).

Teaming up with Texas Instruments, Transense Technologies plc is offering the first piezo-electric SAW TPMS product, using TI’s TMS320F28x DSP chip. Unlike some MEMS-based products, this TPMS doesn’t need a battery. The passive SAW sensor incorporates a three-element die within a small gas-tight capsule. Pressure is transmitted via a diaphragm to deform the die and mechanically strain one of its elements. An RF signal interrogates the sensor. Signals are transmitted via resonant SAW frequencies, which are used to determine pressure and temperature.

Bosch has made improvements in lowering power consumption with its SMD400 tire-pressure and temperature monitor (Fig. 3). This self-calibrating TPMS component includes a user-programmable ASIC designed for ultra-low-power operation, consuming just 0.5 µA in the idle mode.

The NXP1 remote sensor from GE NovaSensor represents the next-generation TPMS (Fig. 4). It includes the sensor element and an 8-bit RISC ASIC microcontroller in a single small-outline IC (SOIC) 14-pin package. Its output is fully signal-conditioned, calibrated, and temperature-compensated. The operating-temperature range is –40°C to 125°C. Standard absolute pressure ratings are 15, 30 and 100 lb/in.2 (psi).

Smart Sensing Tire

A team of researchers at Purdue University led by professor Gary W. Krutz has developed a tire system that senses failures in real time. Although no details about the system have been released, it works on the concept that the entire tire acts as a sensor that sends information to on-board computers (Fig. 5). “We test loaded rolling tires and have two patents in the works,” Krutz says.

The system consists of two conductive layers that surround a dielectric layer within the tire’s structure. These three layers form either a capacitive, resistive, or semi-conducting sensing circuit that produces an electrical signal in response to several tire status conditions when it’s excited. These include tire structure cuts, punctures, manufacturing quality, imbalance, impact, rubber hardening or degradation, or improper mounting and repair. The system can also be used during tire manufacture before mass production. The patented technology is available for licensing through the Purdue University Research Foundation’s Office of Technology Commercialization.

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