During vehicle operation, tire condition directly impacts driving safety and performance, and tire pressure is a key indicator of tire condition. Proper tire pressure ensures excellent vehicle handling, fuel economy, and comfort, while also reducing tire wear and extending tire life. As a crucial automotive safety feature, the tire pressure monitor (TPM) monitors tire pressure in real time and promptly alerts the driver when pressure is abnormal, effectively preventing accidents caused by tire pressure issues. Therefore, a thorough understanding of the working principles of a TPM is crucial for ensuring driving safety.

How a TPM Measure Tire Pressure in Real Time

Direct TPM System Detection Principle
1. Pressure Sensor Installation Location: The pressure sensor in a direct TPM system is typically installed inside the tire or at the valve stem. When installed inside the tire, the sensor can more directly sense the actual pressure inside the tire, resulting in more accurate measurements. Sensors installed at the valve stem are relatively simple to install and easier to maintain. 2. Conversion of Pressure Signals into Electrical Signals: When the pressure inside the tire changes, the sensitive elements within the pressure sensor undergo corresponding physical changes, such as the piezoelectric effect and the piezoresistive effect. These physical changes cause changes in the sensor's internal electrical parameters (such as resistance, capacitance, and voltage), thereby converting the pressure signal into an electrical signal. For example, a piezoresistive pressure sensor utilizes the piezoresistive effect of semiconductor materials. When pressure acts on the semiconductor material, its resistance changes. By measuring this resistance change, the corresponding pressure value can be obtained.
3. Initial Processing of the Electrical Signal: The electrical signal output by the sensor is typically weak and may contain some noise. Therefore, the electrical signal requires initial processing, such as amplification and filtering. The amplifier circuit amplifies the weak electrical signal to an appropriate amplitude for subsequent signal processing and analysis. The filter circuit removes high-frequency noise and interference from the electrical signal, improving signal quality and stability. After initial processing, the electrical signal can be transmitted to subsequent circuits for further processing and transmission.

Indirect Tire Pressure Monitoring System Detection Principle
1. Working Principle Based on Wheel Speed Sensors: Indirect tire pressure monitoring systems primarily rely on the vehicle's existing wheel speed sensors. Wheel speed sensors are typically mounted on the wheel hub or transmission output shaft. They monitor wheel speed in real time and transmit speed signals to the vehicle's electronic control unit (ECU).
2. Inferring tire pressure changes by analyzing wheel speed differences: When tire pressure changes, the tire diameter also changes accordingly. For example, when tire pressure decreases, the tire diameter decreases, causing the tire's speed to increase relatively during driving; conversely, when tire pressure increases, the tire diameter increases and the speed to slow relatively. Indirect tire pressure monitoring systems analyze the signals from the four wheel speed sensors and compare the speed differences between the wheels to infer whether tire pressure has changed and the approximate extent of the change.
Source: Textbooks on automotive electronics technology, such as "Automotive Electronic Control Technology" (author: Wang Shaoxuan et al., Machinery Industry Press), which provide a detailed explanation of the basic detection principles of direct and indirect tire pressure monitoring systems.

What are the Differences Between Direct and Indirect Tire Pressure Monitoring Systems?

Working Principle Differences

Direct tire pressure monitoring systems directly measure tire pressure using pressure sensors installed inside the tire or at the valve stem, providing real-time and accurate information on each tire's specific pressure. Indirect tire pressure monitoring systems, on the other hand, monitor wheel speed using wheel speed sensors. They indirectly infer changes in tire pressure by exploiting the principle that changes in tire pressure cause changes in tire diameter, which in turn causes differences in wheel speed. They cannot directly determine actual tire pressure.

Accuracy and Real-Time Comparison

Direct tire pressure monitoring systems offer high accuracy and accurately display the specific pressure of each tire in real time. The error range is typically small, typically between ±0.1 and ±0.2 bar. Furthermore, they update pressure data in real time, allowing the driver to monitor tire pressure status at all times.

Indirect tire pressure monitoring systems have relatively low accuracy; they can only determine whether tire pressure is abnormal, but cannot accurately display the actual pressure. Furthermore, because they infer tire pressure changes by analyzing differences in wheel speed, there is a certain time delay, resulting in relatively poor real-time performance.

Cost and Installation Complexity Comparison

Direct tire pressure monitoring systems require multiple pressure sensors and the corresponding signal processing and transmission equipment, resulting in higher costs. Meanwhile, the installation process is relatively complex, requiring tire disassembly and assembly, proper placement of the pressure sensor, and commissioning and calibration. Indirect tire pressure monitoring systems utilize the vehicle's existing wheel speed sensors, eliminating the need for additional pressure sensors and resulting in lower costs. Installation is also relatively simple, requiring only a software upgrade or configuration in the vehicle's electronic control unit to enable tire pressure monitoring.

Application Scenarios
Direct tire pressure monitoring systems are suitable for vehicles with high requirements for tire pressure monitoring and precision, such as high-end luxury cars and high-performance sports cars. These vehicles typically have extremely high demands for driving performance and safety, requiring real-time and accurate tire pressure monitoring to ensure vehicle handling and stability. Indirect tire pressure monitoring systems are suitable for cost-sensitive vehicles that do not require particularly high tire pressure monitoring accuracy, such as economy sedans and commercial vehicles. These vehicles prioritize cost control while meeting basic safety requirements. Indirect tire pressure monitoring systems can provide certain tire pressure monitoring functions at a lower cost. Source: Automotive industry research reports, such as the "China Tire Pressure Monitoring System (TPMS) Industry Research Report" (Source: Zhiyan Consulting), which provides an in-depth analysis and comparison of the differences between direct and indirect tire pressure monitoring systems in various aspects.

How Tire Pressure Monitors Transmit Data and Trigger Alarms

Data Transmission Methods
1. Direct System: Direct tire pressure monitoring systems typically use wireless transmission technologies, such as low-frequency or high-frequency wireless communication. The sensor converts the processed electrical signal into a wireless signal, which is then transmitted via a built-in wireless transmitter module. A central receiving module, installed in a suitable location in the vehicle, receives the wireless signal from the sensor and converts it into pressure data. Low-frequency wireless communication has strong penetration capabilities, enabling reliable communication over long distances and in complex environments. High-frequency wireless communication has a higher data rate, enabling fast and accurate transmission of pressure data.
2. Indirect System: Indirect tire pressure monitoring systems transmit wheel speed data to the body control module (BCM) via the vehicle's internal controller area network (CAN) bus. The wheel speed sensor transmits the collected speed signal to the vehicle's electronic control unit (ECU). The ECU processes the signal and then sends the wheel speed data to the BCM via the CAN bus. The CAN bus offers advantages such as high real-time performance, long transmission distance, and strong anti-interference capabilities, ensuring accurate and rapid transmission of wheel speed data to the BCM.

Alarm Triggering Mechanism
1. Direct System: After receiving the pressure data from the sensor, the central receiving module compares it with a preset safety threshold. The safety threshold is typically set based on factors such as tire specifications and vehicle load. When the pressure data exceeds the safety threshold, the central receiving module triggers an alarm. Alarms typically include displaying a warning light on the instrument panel or emitting an audible alarm. For example, if tire pressure is too low, the tire pressure warning light on the instrument panel will illuminate, and the vehicle may also emit a beeping alarm, reminding the driver to check tire pressure immediately.
2. Indirect System: After receiving the wheel speed data transmitted via the CAN bus, the body control module (BCM) analyzes the data using a preset algorithm to determine whether the tire pressure is abnormal. When abnormal tire pressure is detected, the BCM triggers a corresponding alarm signal. Similar to direct systems, these alerts are provided through a warning light on the instrument panel and an audible alarm.

Source: References to automotive electronic communication technology, such as "Automotive Electronic Communication Technology and Applications" (by Li Jianqiu et al., Tsinghua University Press), provide detailed technical descriptions of the data transmission and alarm triggering mechanisms of tire pressure monitoring systems.

Conclusion
As a crucial device for ensuring driving safety, the tire pressure monitor (TPM) operates through multiple steps, including real-time tire pressure detection, data transmission, and alarm triggering. Both direct and indirect TPM systems have their own advantages and disadvantages. Direct systems offer high accuracy and real-time performance, but are more expensive and complex to install. Indirect systems offer lower costs and easier installation, but are relatively less accurate and less real-time. When choosing a TPM, drivers should consider a comprehensive range of factors, including their vehicle type, usage requirements, and budget. At the same time, no matter which type of tire pressure monitor is chosen, car owners should pay attention to tire pressure monitoring, regularly check and maintain the tire pressure monitor to ensure its normal operation, so as to effectively prevent traffic accidents caused by tire pressure problems and ensure driving safety.