Application of low frequency RFIC technology in tire pressure monitoring system

The Tire Pressure Monitoring System (TPMS) provides early warning of abnormal tire pressure or malfunctions. TPMS used alone or in combination with other vehicle electronic systems plays a vital role in passenger safety, vehicle handling and tire service life. As most TPMS designs, RF devices play a basic role in conveying tire pressure data used by vehicle safety systems to remind the driver. In order to build integrated, stand-alone and additional TPMS equipment, engineers can use available equipment from manufacturers, including Atmel, Maxim Integrated Products, Silicon Labs and Texas Instruments.

The TPMS system works directly by monitoring the tire pressure, or indirectly by using the vehicle's anti-lock braking system to detect changes in the speed associated with the reduction in the radius of the reduced tire. The direct method (the subject of this article) relies on low-frequency RF equipment to transmit tire pressure measurements to the vehicle safety management system.

TPMS architecture

The direct TPMS unit installed on the inner ring is connected to the valve stem to measure the tire pressure, and the measurement is wirelessly transmitted to the dedicated TPMS controller or the vehicle's electronic system controller. These devices are manufactured with non-replaceable batteries and are designed with power consumption in mind. In order to save power, most TPMS architectures usually use an initiator, which sends a low-frequency RF signal, so that each TPMS unit wakes up and sends updated tire pressure data before returning to low-power mode (Figure 1).

Application of low frequency RFIC technology in tire pressure monitoring system
Figure 1: In this TPMS design, a low-frequency RF initiator wakes up each unit mounted on the tire, and then sends these units to a dedicated receiver or receiver. Centralized subsystem (provided by Panasonic.)

Designed specifically to support this start-up function, Atmel ATA5276 IC drives a 125 KHz LC resonant tank circuit to start this wake-up process. ATA5276 is controlled by the MCU through a simple single-wire interface, combining the control logic with the VCO to generate a 125 KHz signal, which is used to drive the transmitter's LC coil circuit. By using the DIO pin of the data-driven device instead of using a simple "enable" signal, engineers can also use the ATA5276 to transmit ASK modulated data to the TPMS unit (Figure 2).

Application of low frequency RFIC technology in tire pressure monitoring system
Figure 2: In addition to providing a simple wake-up signal, Atmel ATA5276 can also transmit data to the TPMS unit: use data to drive DIO to switch DRV, thereby generating ASK modulation signal coils. (Courtesy of Atmel.)

The actual TPMS measurement unit installed on each tire includes a pressure sensor, a signal processing stage and an RF transmitter. When using a starter, the wake-up circuit on the tire side can be as simple as an analog comparator, such as Maxim MAX9075. In this method, the comparator will detect the output of a coil designed to match the resonant frequency of the transmitter (Figure 3). When the comparator reaches the threshold, it can drive a single transistor to drive the start signal to the TPMS measurement unit.

Application of low frequency RFIC technology in tire pressure monitoring system
Figure 3: In response to the signal detected by its resonant circuit, the tire-side TPMS wake-up circuit can send the rest of the TPMS circuit to use a single transistor enable signal switched by an analog comparator. (Courtesy of Maxim Integrated Products.)

Pressure measurement

The actual tire pressure measurement relies on pressure sensors, which provide temperature-related differential output signals, which are usually highly non-linear, with large offsets and offset drifts. A signal conditioning circuit is needed to provide the required linearization, calibration, and temperature compensation. To simplify this stage of TPMS design, engineers can use integrated devices such as Texas Instruments (TI) PGA309 or Maxim MAX1452, as well as ICs specifically designed for on-chip subsystems required for sensor signal conditioning (Figure 4).

Application of low frequency RFIC technology in tire pressure monitoring system
Figure 4: TI PGA309 and other dedicated signal conditioning ICs provide the functional sensors required to achieve pressure nonlinearity, temperature-dependent output linearization, compensation and calibration. (Provided by Texas Instruments.)

TI PGA309 includes a complete sensor conditioning signal chain, integrated input multiplexer, programmable gain instrumentation amplifier, linearization circuit, voltage reference, control logic and output amplifier. Engineers can calibrate the device through its single-wire digital interface and store the calibration parameters on an off-chip memory (such as SOT23-5 EEPROM).

Maxim MAX1452 is a precision signal conditioner that integrates a programmable gain amplifier, digital-to-analog converter (DAC), temperature sensor and internal EEPROM. The device uses analog amplification to initially boost the input signal, then performs an analog temperature correction, and uses a digital circuit to complete the correction. This device is specifically used for sensor adjustment applications, allowing engineers to calibrate and correct sensor signals by programming the sensor bridge excitation current or voltage.

RF link

For the communication of TPMS unit, engineers can choose from different ISM RF equipment. For TPMS tire mounting unit designs that only require transmission capacity, Maxim 7044, Micrel MICRF112, Silicon Labs Si4020/Si4021 and Texas Instruments CC1070 provide complete solutions, requiring fewer external components, including crystals, blocking capacitors, and power amplifiers. Appropriate matching components between output and antenna.

Maxim MAX7044 provides OOK/ASK transmission in the 300 MHz to 450 MHz range. Like other devices in its class, the MAX7044 requires very few external components. However, the MAX7044 can provide up to 13 dBm of output power while only providing 7.7 mA of current at 2.7 V.

The Micrel MICRF112 transmitter provides ASK/FSK modulation in the 300 to 450 MHz frequency band with an output power of up to 10 dBm. The MICRF112 can operate at a supply voltage as low as 1.8 V, which is usually 2.0 to 2.2 V compared to most devices of this type.

Silicon Labs Si4020/Si4021 ISM transmitters provide a fully integrated solution, requiring only an external crystal oscillator and a bypass filter. The pin compatible device includes an integrated PLL with fast settling time and can work in the 433, 868 and 915 MHz frequency bands. Si4020 can also work in the 315 MHz frequency band, while Si4021 offers higher output power (8 dBm at 433 MHz, while Si4020 is 3 dBm).

Texas Instruments (TI) CC1070 allows engineers to use the integrated VCO and frequency divider to programmatically set the operating frequency band to achieve the desired operating frequency. The device has a flexible programmable power management function, allowing engineers to easily set the device to achieve power consumption when the tire-mounted TPMS unit returns to low-power mode after a data transmission event.

TPMS equipment has important safety functions, helping to improve the overall comfort of passengers and vehicle handling. In order to be successful in integrated, independent or third-party TPMS products, engineers need to combine sensor signal conditioning circuits with flexible RF communication functions. The availability of highly integrated ISM equipment (described above) helps simplify TPMS design and accelerate the deployment of more complex vehicle safety features.