Wireless Approach Monitors Tire Pressure

March 1, 2003
Several methods are available for remote monitoring the air pressure of vehicle tires, but highly integrated wireless circuits may offer the most reliable and accurate solutions.

Tire-pressure monitoring (TPM) is among the more useful, albeit humble, of wireless applications. Although application-specific integrated circuits (ASICs) have been commonly used for TPM functions, several integrated solutions from Atmel (San Jose, CA) can simplify the addition of wireless TPM functions to automotive telematics systems. These ICs are available with adequate on-board memory to support tire identification for production, service, and control purposes. This level of integration even helps keep vehicle tires in balance.

Most tire air-pressure losses occur from slow leaks in which the pressure may escape over hours, days, or even months; only a few percent of air-pressure losses are due to immediate incidents because of contact with road hazards. Slow leaks may be caused by different factors such as natural leakage, permeation, or seasonal climatic changes. Furthermore, slight damage to a tire by a road hazard that punctures small holes in the tire have been detected as a typical reason of slow air-pressure losses. When a tire is used while significantly under-inflated, its sidewalls flex more and the air temperature inside increases, making the tire more prone to failure. In addition, significantly under-inflated tires lose lateral traction, making handling more difficult. High-quality tires with optimum air pressure are equivalent to a life-insurance policy for drivers, passengers, and the vehicles themselves. Tire-related problems account for many vehicle breakdowns. Most drivers do not regularly monitor their tires, inviting dangerous conditions.

To identify slow pressure leaks and to guarantee that the driver will be warned, two automotive TPM methodologies have evolved during the past 15 years: indirect sensing systems and direct sensing systems. In the first type, the tire pressure is evaluated based on the rotational speed of a vehicle's wheel relative to the other wheels. These systems—usually linked to an automatic braking system (ABS)—compare the rotation of the wheels to determine if a wheel is under-inflated. In direct sensor systems, encapsulated tire module boards contain pressure and temperature sensors to perform an on-going series of measurements on the tire pressure and temperature. These modules are mounted in the wheel and transmit an RF signal containing the tire-pressure and temperature information to a central receiver (Rx) in the vehicle.

Early TPM systems were introduced with the indirect sensing method. The differential speed detection of the indirect pressure sensing system has several inherit weaknesses, including the fact that the loss of tire pressure cannot be detected for all four tires at a similar rate. Even monitoring two tires on the same side or axis can result in ambiguous results. In addition, the correlation of tire temperature to air pressure is not included in the indirect method, and drop in tire pressure of less than 15 percent cannot be detected with the indirect method. A system calibration time of several minutes and up to hours would be needed to teach the variables associated with distinct tire types at differential driving preconditions. The detection of under-inflated tires itself requires several minutes. The detection and determination of which tire is under-inflated is an unsolved problem. An incorrect indication of under-inflated tires has been found in certain circumstances.

Direct-sensor TPM systems have been developed to overcome the limitations of indirect sensing systems. Dedicated tire-pressure monitoring is now possible, even when all four wheels are involved. The tire-pressure monitoring can also operate when the vehicle is stationary. Smooth pressure decrease can be monitored easily. Advances in direct-sensor circuitry has enabled these modules to run under battery power with low current consumption, and new technologies that use RF energy supplied from the car body module, thus avoiding a battery supply, are in development.

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Currently, microcontrollers are becoming more relevant to improving the tire-pressure monitoring solution in tire modules. Flexibility of the system definition is becoming an important requirement for production and service resulting in the integration of the electronically erasable programmable-read-only memory (EEPROM). The EEPROM can also be used for the programming of the tire-identification (ID) code during tire installation.

Due to battery-life requirements, the power consumption of the tire-pressure module is a key performance requirement. Currently, standby or sleep modes along with duty cycling of the RF transmission are used to obtain the lowest power consumption. However, no matter how successful these different techniques are, market focus will continue to be on the development of a TPM IC with even lower current consumption. Furthermore, TPM module systems must be optimized for minimum weight, size, and cost. These improvements will lead to a higher acceptance of the direct TPM approach, allowing the introduction of direct TPM into new vehicle models with a high adoption rate.

All of the silicon (Si)-based ICs and components needed to construct TPM ultra-high-frequency (UHF) communications channels are available from Atmel. The firm currently supplies components to several commercial TPM module platforms. For example, the company's model AT86RF401 is an AVR-based 8-b microcontroller with integrated UHF phase-locked-loop (PLL) transmitter (Tx) designed for amplitude-shift-keying (ASK) modulation supplied as a single complementary-metal-oxide-semiconductor (CMOS) device. Ideal for direct-sensing TPM applications, the device is housed in a TSSOP20 package. It is capable of maximum transmit power of +6 dBm, and offers a 36-dB RF output-power control range, adjustable in 1-dB steps. The device is specified for reliable operation over typical automotive temperature ranges. It is available as a flash version and requires only a crystal resonator, three capacitors, an inductor, and a tuned loop antenna to implement a complete on-off-keyed (OOK) UHF Tx operating in the 264-to-456-MHz frequency range. The IC is typically powered by a single lithium (Li)-type coin-cell battery and designed to operate with minimum voltage of +2 VDC.

The company also offers the model ATAR862 integrated TPM module (Fig. 1) as a customized mask read-only-memory (ROM) version supplied in a SS024 package. The device's EEPROM enables ID programming for optimum production and service flexibility (Fig. 2). The device, with integrates control and Tx functions, supports both ASK and frequency-shift-keying (FSK) transmit modulation formats for further flexibility. It meets extreme TPM operation temperature conditions up to +125°C combined with lowest possible current consumption of 0.4 µA in sleep mode supported by the unique power-down modes of the 4-b Atmel microcontroller. The silicon bipolar Tx achieves +10-dBm maximum output power and beneficial spurious emission parameters of −52 dBm. It is also available in a pin-compatible flash version.

In addition, Atmel's T5753/52 PLL Tx IC is available in an eight-pin TSSOP8 package for all direct-sensing TPM frequency ranges. The PLL, which is well suited for used with a single-ended antenna, provides frequency stability and accuracy at UHF when operating with a low-cost crystal resonator; it requires only seven additional external components. The Tx is flexible enough to operate with voltages from +2 to +4 VDC; a version for use at +1.9 VDC is also available.

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