The newly promulgated highway safety regulations in the United States make tire pressure monitoring system (TPMS) the fastest developing field in the automobile industry. In April 2005, the National Highway Traffic Safety Administration (NHTSA) issued a final regulation, requiring all vehicles (except single-axle two-wheeled vehicles) with gross weight less than or equal to 563 kg to be equipped with TPMS. By September 1 2007, all light vehicles produced by manufacturers must meet the requirements of this standard. The United States produces about170,000 cars every year, which creates a fast-growing market for TPMS. It is estimated that by 2008, the penetration rate of TPMS in the global automobile market will exceed 30%. Figure 1:TPMS tire module assembly block diagram.
It is worth noting that this provision's attitude towards direct and indirect systems is technically neutral. However, the forecast of market research organization Strategy Analytics shows that direct system technology will become the mainstream technology, and its share will exceed 95% after 2008. Direct TPMS has a tire pressure sensor in each tire, which transmits pressure and other information to the central receiver through radio frequency (RF) signals. However, indirect TPMS has no pressure sensor and needs to rely on the speed sensor in ABS system to detect and compare the difference of tire speed. Although the cost of direct TPMS is high, the system has obvious performance advantages, such as high sensitivity, zero speed measurement and multi-tire deflation detection. This paper focuses on direct TPMS, and will discuss the design challenges faced by engineers, including component selection, power management, media compatibility, system cost and RF design. The tire module of TPMS consists of MEMS pressure sensor, temperature sensor, voltage sensor, accelerometer, microcontroller, RF circuit, antenna, low frequency interface, oscillator and battery. Automobile manufacturers require direct TPMS batteries to last for more than ten years. The operating temperature of the battery must be-40 C to125 C. The battery is light in weight, small in size and high in power. Due to these limitations, button cell is usually chosen instead of a large battery. The new button cell weighs only 6.8 grams and can reach the standard power of 550mAh. In addition to the battery, in order to achieve a working life of more than ten years, the components must have integrated functions and low power consumption. Infineon SP30 is such an integrated product, which integrates pressure sensor, temperature sensor, voltage sensor, accelerometer, LF interface, microcontroller and oscillator into one component. A complete tire module system has only three components-SP30, RF transmitter chip (such as Infineon's TDK5 10xF) and battery. When the power management module goes on the market, it is unrealistic to replace the battery. Because of this limitation, power management is particularly important for design engineers. There are four ways to reduce power consumption. One of them is to choose low-power components and use components with integrated functions to reduce the number of components. The second method is to use software. Efficient algorithm can adjust the transmission and measurement frequency. In software design, efficient algorithms may have to consider these questions: What is the frequency of data transmission and measurement? Is it necessary to send duplicate data? What is the system display mode? The power consumption of radio frequency transmission is the largest, so extending the transmission interval, reducing the number of calculations and repeating data transmission will naturally extend the life of the battery. However, we must strike a balance between all these factors, data reliability and users' timely access to information. The third way to reduce power consumption is to use sensors with built-in accelerometers (such as SP30) to detect the operation of wheels. When the vehicle is parked, TPMS can stop running to save power. In most cases, the parking time of vehicles is much longer than the actual driving time. Therefore, this method can greatly save electricity. Using low frequency (LF) interface is also a way to reduce power consumption. Through the low frequency interface, the central receiver module can send instructions and data to the tire module that is always in standby mode. Only after receiving the "wake-up" signal will the tire module measure and send data. In this way, the battery life of the tire module can be greatly prolonged. In addition to power saving, the low-frequency interface also brings flexible design and extra functions. For example, low frequency communication allows the system to automatically identify the tire after the tire is changed. SP30 with low frequency input interface can solve this design problem. Media Compatibility Figure 2: Infineon's TPMS solution.
Media compatibility and reliability of sensors are very important for TPMS. Without these key features, the accuracy and reliability of the whole system will become a problem. The tire module is placed inside the tire, so the electronic components face the harsh environment inside the tire. Electronic components will work in the temperature range of-40 C to125 C, and will be attacked by moisture, dust, brake oil and other media. Media compatibility ensures that the sensor is fully protected. The sensor of TPMS is particularly susceptible to corrosion because its pressure inlet must be in contact with air to monitor the surrounding pressure. Infineon's sensors include pressure sensor and acceleration sensor (optional). The sensor module is a three-layer stacked module (glass-silicon-glass). The absolute pressure reference value is obtained from the vacuum chamber of the top glass. Because the pressure inlet faces the back of the silicon layer, the sensor assembly has good medium compatibility. Using this patented three-layer stacking technology, Infineon sensors (such as SP30) have achieved very good media compatibility. System cost The system consists of a receiver module and four tire modules, which plays an important role in commercial success. A good solution is to use components with integrated functions to reduce the number of components, working power consumption and PCB size, and finally reduce the system cost. Making TPMS and RKE receiver share the same platform can also reduce the system cost. However, this is not always feasible in business. RF Design The transmitting chip of the first generation TPMS is designed by ASK modulation technology, and the appropriate transmitting frequency is generated by SAW*** oscillator. Such an ASK system is very cheap, but the rotation of the wheel carrying the transmitter will cause the received field strength to change. Therefore, the current TPMS uses FSK modulation to generate center frequency and frequency traction on the basis of crystal oscillator and PLL frequency synthesizer. In many OEM applications, FSK can provide reliable RF communication even when the wheels are running at high speed. Infineon UHF transmitter series TDK5 10xF can provide the best solution for this TPMS application. TDK5 10xF series devices can be used in different frequency bands (3 15, 434, 868 and 9 15MHz), and can perform ASK and FSK modulation. The device includes a fully integrated PLL frequency synthesizer and an efficient power amplifier to drive the loop antenna. When the RF output power is 5dBm and the resistance is 50Ω, the typical power consumption is 7mA (which can be regarded as the lowest power consumption). The operating temperature range of the device is-40 C to125 C, and it is packaged in a small P-TSSOP- 10 package. In addition to transmitter products, Infineon also has a variety of receivers and transceiver chips in different frequency bands, such as TDA52 1x receiver chips, which are widely used in TPMS and RKE applications. Infineon's wireless chip only needs few external components, and the loop filter and VCO circuit are integrated together. Its characteristics also include low operating current and good sensitivity. Infineon's TPMS solution Infineon provides a complete TPMS system solution by integrating sensors and RF wireless chips. As can be seen from Figure 2, SP30 or SP 12 (without MCU) can be used together with TDK5 10x transmitter chip in tire module. At the receiver, TDA52 1x can be used with any 8-bit or 16-bit MCU. 1997, Infineon's TPMS sensor was put into production, and the total output has reached tens of millions so far. With a large number of products being used and tested in the market, Infineon's mass production capacity has been recognized. Infineon's TPMS sensor has many advantages, including: digital interface (SPI) brings flexible design for independent TPMS sensor (SP 12/SP 12T);
Large-scale deployment has been realized for various OEM applications;
Excellent medium compatibility and reliability;
Carry out comprehensive compensation and calibration in the production process;
Low power consumption;
Unique sensor id that is easy to identify;
Ultra wide working temperature range (-40 C to125 C); );
On-chip accelerometer monitors wheel rotation (optional);
On-chip microcontroller reduces PCB size and system cost (SP30);
On-chip temperature and voltage sensors;
On-chip LF (low frequency) input interface realizes two-way communication;
Accurate barometric measurement (7 kPa);
Use EROM for mass production to reduce costs;
Shut down at abnormal temperature to avoid inaccurate data measurement;
It can be used with RF wireless chip to form a complete TPMS solution.
Future solutions with integrated RF transmitter and low power consumption;
This paper summarizes the main challenges that engineers face when designing direct TMPS, including component selection, power management, medium compatibility, system cost and RF design, which are also the keys to commercial success. The solution based on SP30 and TDK5 10xF sets the benchmark for the number of components and PCB size. At the same time, it meets the endurance requirements of the automobile industry 10. The new generation of TPMS will have some revolutionary breakthroughs. While the performance is improved, the power consumption, size, weight and cost will also be continuously reduced. Infineon plans to launch a single electronic module named SP35 in 2007, which integrates sensing and transmitting functions, which means that MCU, sensor and RF transmitter will be integrated. By 20 10, Infineon is expected to introduce a system with lower power consumption.