Tire Pressure Monitoring Systems (TPMS) have become a research hotspot in China's automotive electronics industry. It is estimated that 710 million tires worldwide will require tire pressure monitoring sensors in the next five years. US law requires all passenger cars and light trucks sold in the US to have TPMS installed since August 2007, and Europe has also enacted similar regulations. China, a major automobile consumer country, is expected to formulate similar regulations in the near future. The demand for TPMS is fueling the rise of a new industry. TPMS, short for Tire Pressure Monitoring System, is primarily used to automatically monitor tire pressure in real time while a vehicle is in motion, issuing alarms for tire leaks and low pressure to ensure driving safety. It serves as a life-saving early warning system for drivers and passengers. During high-speed driving, tire failure is the most worrying and difficult-to-prevent issue for all drivers, and a major cause of sudden traffic accidents. Statistics show that 70% to 80% of traffic accidents on highways are caused by tire blowouts. Preventing tire blowouts has become a crucial issue for safe driving. TPMS (Tire Pressure Monitoring System) is currently one of the most effective scientific methods for tire pressure prevention and early warning. A TPMS system mainly consists of two parts: a remote tire pressure monitoring module installed on the tires of a vehicle, and a central monitor (LCD/LED display) installed on the dashboard. The module directly installed in each tire measures tire pressure and temperature, and the measured signals are modulated and transmitted via high-frequency radio waves (RF). A car or van typically has 4 or 5 TPMS monitoring modules (including the spare tire), while a truck has 8-36. The central monitor receives the signals transmitted by the TPMS monitoring modules and displays the pressure and temperature data of each tire on the screen for the driver's reference. If the tire pressure or temperature is abnormal, the central monitor issues an alarm signal to remind the driver to take necessary measures. Since most car tires are now tubeless radial tires, it is very convenient and easy to install the TPMS tire pressure monitoring module inside the tire. However, the internal environment and temperature conditions of the tire are extremely harsh when a car is driving at high speed, with significant changes in pressure, temperature, and humidity. Therefore, the design of this module must select components according to military-grade product requirements and formulate manufacturing processes according to industrial product requirements. However, it is a widely used automotive safety product and should be priced like a consumer electronics product. The tire pressure monitoring module consists of five parts: an intelligent sensor MCM with pressure, temperature, acceleration, and voltage detection and post-signal processing ASIC chip combination; an 8-16 bit microcontroller (MCU); an RF radio frequency transmitter chip; a lithium-ion battery; and an antenna. The housing is made of high-strength ABS plastic. All components and materials must meet the operating temperature range of -40℃ to +125℃ (Figure 1). The tire pressure monitoring module is a tire pressure and temperature real-time monitoring and data transmission system installed on the tire. Its internal structure is shown in Figure 2. The TPMS transmitter composed of the tire pressure monitoring module is divided into two types: internal to the tire and external to the tire. TPMS smart sensors integrate pressure, temperature, and accelerometer functions, battery voltage detection, internal clock, and a digital signal processing ASIC or MCU unit (SoC) containing an analog-to-digital converter (ADC), sample/hold (S/H), SPI port, calibration, data management, and ID code, all fabricated using silicon micromachining (MEMS) technology. The module features mask-based programmability, allowing configuration using customer-specific software. It consists of two chips: a MEMS sensor and an ASIC/MCU circuit, all packaged together using integrated circuit technology. The smart sensor is an MCM module that integrates pressure/temperature, acceleration, and MCU (Figure 3). For easy identification by the TPMS receiver, each pressure sensor has a unique 16-32 bit ID code. The MEMS sensors used in TPMS are extremely small, lightweight, and have high functional requirements, therefore they can only be designed and manufactured using MEMS technology. Currently, the MEMS sensors used in TPMS mainly fall into two categories: silicon piezoresistive sensors, such as GE NovaScnsor's NPX1 and NPX2, and Infincon SensoNor's SP12, SP30, and SP35; and silicon integrated capacitive sensors, such as Freescale's MPXY8020, MPXY8040, and MPXY8300 series. The NPX and SP30, SP35, and MPXY8300 series smart sensors all include pressure sensors and accelerometers. The pressure sensor is mainly used to measure tire pressure changes, while the accelerometer utilizes the sensitivity of its mass block to motion to achieve real-time power-on during vehicle movement, system self-testing, automatic wake-up, and automatic intelligent determination of the detection time cycle based on the vehicle's speed during high-speed driving. Software-defined safe, sensitive, and dangerous periods are used to gradually shorten the cycle detection time, improve early warning capabilities, and save energy. Piezoresistive MEMS Sensors Silicon piezoresistive sensors are made by micromachining silicon wafers into peripherally fixed circular diaphragm pressure sensors. A pressure/temperature inlet is located above the TPMS sensor module package, allowing external pressure to be directly introduced onto the stress film of the pressure sensor. The inner wall of the peripherally fixed circular stress film is composed of semiconductor strain gauges forming a Wheatstone bridge (Figure 4). Silicon piezoresistive pressure sensors use high-precision semiconductor resistance strain gauges to form a Wheatstone bridge as the force-to-electricity conversion measurement circuit, offering high measurement accuracy and low power consumption. In the Wheatstone bridge, the piezoresistive sensor outputs zero when there is no pressure change, consuming almost no power. Simultaneously, this inlet also directly introduces ambient temperature into the semiconductor temperature sensor. Accelerometers, based on the same principle, require a certain amount of space for their mass block to move, making them slightly larger. They also use high-precision semiconductor resistance strain gauges to form a Wheatstone bridge as the force-to-electricity conversion measurement circuit. Accelerometers utilize the sensitivity of their mass block to motion to achieve real-time power-on during vehicle movement and enter system self-test. The internal structure of the sensor module is shown in Figure 3 after removing the module package. Capacitive MEMS Sensors Capacitive sensor technology has seen significant development in recent years. Freescale's new MPXY8300 series adopts a new generation of capacitive sensor technology, completely different from its predecessor, with lower power consumption and higher integration capabilities. The MPXY8300 integrates a silicon capacitive pressure sensor, a dual-axis accelerometer, an MCU, and an RF transmitter (Figure 5). The capacitive pressure sensor utilizes MEMS technology to fabricate a grid-like structure on a silicon wafer. The upper and lower grids form a set of capacitive pressure sensors. When the upper grid is subjected to pressure, it displaces downwards, changing the spacing between the upper and lower grids, thus changing the capacitance between the two grids, i.e., Δpressure = Δcapacitance (Figure 6). A MEMS photograph of this sensor is shown in Figure 7. Similarly, capacitive X-axis and Z-axis accelerometers are also manufactured using MEMS technology. Taking the capacitive Z-axis sensor as an example, when a car moves, the suspended torque beam of the Z-axis sensor rotates due to centripetal force. This rotation causes a change in capacitance between the rotatable beam and the silicon substrate, i.e., Δrotation speed = Δcapacitance (Figure 8). The MEMS image is shown in Figure 9. [align=center] [/align] When the car moves, the X-axis sensor causes the central movable plate to shift left and right due to acceleration. This displacement causes a change in capacitance between the central movable plate and the left and right fixed plates, i.e., Δacceleration = Δcapacitance (Figure 10). The MEMS image is shown in Figure 11. [align=center] [/align] The X and Z axis accelerometers are used simultaneously for tire position identification. Figure 12 shows the mainboard of the tire pressure monitoring module using the Freescale MPXY8300. It is quite simple. The PCB board mainly consists of the MPXY8300, a three-in-one MEMS sensor, MCU, and RF unit, with a closed-loop antenna and a 125kHz low-frequency wake-up antenna on the PCB board. Below is a lithium-ion battery. Performance Analysis of Commonly Used TPMS Sensors The next generation of TPMS design should utilize the latest, highly integrated, high-performance, cost-effective, and sustainable components, especially in the selection of the transmitter module. The separate combination of pressure, acceleration, and temperature sensors with an MCU is absolutely unacceptable due to space limitations in the transmitter module. Some transitional products, such as the SP12, are also unsuitable due to manufacturer discontinuation. Selecting appropriate components and solutions is crucial. Table 1 is a performance analysis table of commonly used TPMS sensors, which can be used as a reference for design selection. TPMS Market Forecast China is becoming the world's largest emerging automotive market. The demand and ownership of automobiles in China are showing an accelerating growth trend. The number of automobiles in China has exceeded 26 million, and annual sales are expected to exceed 6 million. In the next five years, it will become the world's second-largest automobile market after the United States. Automotive safety products will become a vibrant emerging market hotspot in China, with an annual growth rate of up to 50% expected. Among them, the market capacity of TPMS technology products alone reached 200,000 sets by the end of 2005, nearly 700 million RMB; it reached 500,000 sets in 2006, nearly 1.7 billion RMB; and it is expected to reach 700,000 sets in 2007, nearly 2.3 billion RMB. According to the 2003-2010 automobile demand forecast, the annual growth rate of automobiles is 16%-20%; the annual growth rate of passenger cars is 19.2%-24%. Based on the above analysis, it is estimated that from 2005 to 2010, China's automobile ownership will grow at a rate of over 16% (Figures 13 and 14).