A tire-pressure-monitoring system (TPMS) provides early warning of abnormal tire pressure or failure. Used separately or in conjunction with other vehicle electronic systems, a TPMS serves an essential role for passenger safety, vehicle handling, and tire lifetime. At the heart of most TPMS designs, RF devices serve a fundamental function in communicating tire pressure data used by vehicle safety systems to alert drivers. For building integrated, standalone, and add-on TPMS devices, engineers can take advantage of available devices from manufacturers including Atmel, Maxim Integrated Products, Silicon Laboratories, and Texas Instruments, among others.
TPMS systems function directly by monitoring tire pressure or indirectly by using the vehicle's anti-lock system to detect changes in rotational speed associated with the decreased radius of a deflated tire. Direct methods (the subject of this article) rely on low-frequency RF devices to transmit tire pressure measurements to vehicle safety management systems.
Mounted on the inside rim, direct TPMS units connect to the valve stem to measure tire pressure, transmitting measurements wirelessly to a dedicated TPMS controller or to the vehicle's electronic systems controller. Built with non-replaceable batteries, the units are designed with careful consideration to power consumption. To conserve power, most TPMS architectures typically use an initiator, which transmits a low-frequency RF signal that causes each TPMS unit to wake up and transmit updated tire pressure data before returning to low-power mode (Figure 1).
Figure 1: In this TPMS design, a low frequency RF initiator wakes up each tire-mounted unit, which in turn transmits current data to a receiver in a dedicated or centralized subsystem. (Courtesy of Panasonic.)
Designed specifically to support this initiating function, the Atmel ATA5276 IC drives 125 KHz LC resonant tank circuits that initiate this wake-up process. Controlled by an MCU through a simple one-wire interface, the ATA5276 combines control logic with a VCO to generate the 125 KHz signal used to drive the initiator LC coil circuit. By driving the device's DIO pin with data rather than using a simple "enable" signal, engineers can also use the ATA5276 to transmit ASK-modulated data to TPMS units (Figure 2).
Figure 2: Besides providing simple wake-up signals, the Atmel ATA5276 can transmit data to TPMS units: Driving DIO with data toggles DRV, resulting in ASK modulated signals at the coil. (Courtesy of Atmel.)
The actual TPMS measurement unit mounted on each tire comprises a pressure sensor, signal-processing stage, and RF transmitter. When an initiator is used, the wake-up circuit on the tire side can be as simple as an analog comparator, such as the Maxim MAX9075. In this approach, the comparator would sense the output of a coil designed with a resonant frequency matched to the initiator (Figure 3). When the comparator reaches threshold, it can drive a single transistor that drives a startup signal to the TPMS measurement unit.
Figure 3: Responding to the signal detected by its resonant circuit, a tire-side TPMS wake-up circuit can send the rest of the TPMS circuit an enable signal, using a single transistor switched by an analog comparator. (Courtesy of Maxim Integrated Products.)
Actual tire pressure measurement relies on pressure sensors that provide a temperature-dependent differential output signal, typically highly nonlinear with a large offset and offset drift. Signal-conditioning circuits are required to provide the needed linearization, calibration, and temperature compensation. To simplify this stage of TPMS design, engineers can draw on integrated devices such as the Texas Instruments PGA309 or Maxim MAX1452, among other such ICs designed specifically with the on-chip subsystems needed for sensor signal conditioning (Figure 4).
Figure 4: Specialized signal-conditioning ICs such as the TI PGA309 provide the functionality required to achieve linearization, compensation, and calibration of nonlinear, temperature-dependent output from pressure sensors. (Courtesy of Texas Instruments.)
The TI PGA309 includes a complete sensor-conditioning signal chain, integrating an input multiplexer, programmable gain instrumentation amplifier, linearization circuitry, voltage reference, control logic, and output amplifier. Engineers can calibrate the device through its one-wire digital interface and store calibration parameters on off-chip storage such an SOT23-5 EEPROM.
The Maxim MAX1452 is a precision signal conditioner that integrates a programmable gain amplifier, digital-analog converters (DAC), temperature sensor, and internal EEPROM. The device uses analog amplification to initially boost input signals, followed by analog temperature correction, and finally using digital circuitry to complete correction. Intended specifically for sensor-conditioning applications, the device allows engineers to calibrate and correct sensor signals by programmatically varying sensor bridge excitation current or voltage.
For the communications core of TPMS units, engineers can select from diverse ISM RF devices. For TPMS tire-mounted unit designs requiring only transmission capability, devices such as the Maxim 7044, Micrel MICRF112, Silicon Labs Si4020/Si4021, and Texas Instruments CC1070 among others, offer a complete solution requiring a minimum of external components, including crystal, blocking capacitors, and appropriate matching components between the power amplifier output and the antenna.
The Maxim MAX7044 offers OOK/ASK transmission in the 300 MHz to 450 MHz range. As with other devices in this class, the MAX7044 needs minimal external components. The MAX7044, however, sets itself apart with its ability to provide up to 13 dBm output power while drawing only 7.7 mA at 2.7 V.
The Micrel MICRF112 transmitter provides ASK/FSK modulation in the 300 to 450 MHz frequency band with output power up to 10 dBm. The MICRF112 differentiates itself with the ability to operate with a supply voltage as low as 1.8 V, in contrast to most devices in this class with minimum supply voltage typically 2.0 to 2.2 V.
The Silicon Labs Si4020/Si4021 ISM transmitter provides a fully integrated solution, requiring only an external crystal and bypass filter. The pin-compatible devices include an integrated PLL with fast settling time for operation in the 433, 868, and 915 MHz bands. The Si4020 also operates in the 315 MHz band, while the Si4021 offers higher output power (8 dBm at 433 MHz compared to 3 dBm for the Si4020).
The Texas Instruments CC1070 allows engineers to set the operating band programmatically, using its integrated VCO and frequency divider to reach the desired operating frequency. The device features a flexible, programmable power management capability that allows engineers to easily set the device for minimum power consumption when the tire-mounted TPMS unit returns to low-power mode after a data transmission event.
TPMS devices serve an essential safety function and contribute to overall passenger comfort and vehicle handling. To achieve success in integrated, standalone, or third-party TPMS products, engineers need to combine sensor signal-conditioning circuits with flexible RF communications capabilities. The availability of highly integrated ISM devices such as those described above can help simplify TPMS designs and speed deployment of more sophisticated vehicle safety functionality.
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