What are TPMS Sensors?

Catalog

The tire pressure monitoring module of TPMS is composed of five parts: (1) a smart sensor SoC with a combination of pressure, temperature, acceleration, voltage detection, and post-signal processing ASIC chip; (2) 4-8 bit single-chip microcomputer (MCU); (3) RF radio frequency transmitting chip; (4) Lithium battery; (5) antenna. The shell is made of high-strength ABS plastic. All devices and materials must meet the automotive temperature range of -40℃ to +125℃.

I. Features

Tire electronic sensors are relatively small measurement parts of the TPMS system. This module has the following characteristics and functions:

- Measure the absolute pressure in the car tire (see if it’s greater than the relative pressure 101Kpa).

- Measure the temperature inside the car tire.

- Measure the acceleration in the car tire.

- With an integrated microprocessor, it has decision-making functions.

- Contains low-frequency receiver for diagnosis and mode change.

- Contains a radio frequency transmitter, used to transmit data from the tire electronic sensor to the receiving module.

- Durable battery (5 years).

- Installed on the tire hub through a valve.

Figure 1: Image of the finished tire pressure monitoring module of TPMS

II. Working Principle

The smart sensor is a digital signal processing ASIC chip, consisting of a pressure sensor made by integrating silicon micromachining (MEMS) technology, an acceleration sensor chip, and a temperature sensor, battery voltage detection, internal clock and analog-to-digital converter (ADC), sample/hold (S/H) ), SPI port, sensor data calibration, data management, ID code, and other functions. It has mask programmability, that is, it can be configured with customer-specific software. It is made up of MEMS sensors and ASIC circuits in several chips, which are made in a package with integrated circuit technology (Figure 2). A pressure/temperature introduction hole is left above the package (Figure 3), which directly introduces the pressure into the stress film of the pressure sensor (Figure 4), and this hole also introduces the ambient temperature directly into the semiconductor temperature sensor.

The MEMS silicon piezoresistive pressure sensor adopts a fixed circular stress silicon film inner wall and uses MEMS technology to directly engrave four high-precision semiconductor strain gauges on the surface where the stress is greatest, forming a Wheatstone measuring bridge, which acts as a force electric For the conversion measurement circuit, the physical quantity of pressure is directly converted into electricity, and its measurement accuracy can reach 0.01-0.03% FS. The structure of the silicon piezoresistive pressure sensor is shown in Figure 4. The upper and lower layers are glass bodies with silicon wafers in the middle. There is a vacuum chamber above the stressed silicon film, making it a typical absolute pressure sensor.

In order to facilitate the identification of the TPMS receiver, each pressure sensor has a unique 32-bit ID code, which can generate 400 million unique numbers.

Figure 2: The combination of pressure, acceleration, and ASIC/MCU is packaged in one package

Figure 3: The combination of pressure, acceleration and ASIC/MCU is packaged in one package

Figure 4: The structure of silicon piezoresistive pressure sensor

Figure 5: Plane structure diagram of acceleration sensor

Figure 6: Sectional structure diagram of acceleration sensor

Similarly, the acceleration sensor is also made with MEMS technology. Figure 5 is a plan structure diagram of a MEMS acceleration sensor. Figure 6 is a cross-sectional structural diagram of an acceleration sensor. In the middle of the figure is a piece of MEMS technology that can swing up and down with the force of motion. For the silicon island mass block, a strain gauge is engraved on the silicon beam connected to the surrounding fixed silicon, and three strain gauges engraved on the fixed silicon form a Wheatstone measurement bridge. When the acceleration force swings, the balance of the Wheatstone measuring bridge is destroyed, and the Wheatstone measuring bridge outputs a changing voltage △V that is linear with the force.

The pressure sensor, acceleration sensor, and ASIC/MCU are three independent bare chips, which are integrated into a packaged unit by the chip's integrator, as shown in Figure 7. The NPX2 of the American GE company, Figure 8 shows that the three bare chips can be clearly seen after removing the packaging materials, and the connection and matching between the three chips are in it.

Figure 7: NPX2 of GE, USA

Figure 8: after removing the packaging material

The acceleration sensor can make the transmitter module automatically wake up. The SP12/30 and NPX2 series of smart sensors include an acceleration sensor. The acceleration sensor uses the sensitivity of its mass to motion to realize the instant startup of the car when it moves, and enters the system for self-check and automatic Wake up, according to the speed of the car when driving at high speed.

It can also automatically and intelligently determine the detection time period and use software to set the safety period, sensitive period and dangerous period, so as to gradually shorten the touring detection period, improve the early warning capability, and save power. The acceleration sensor + MCU + software design can be used to complete the wake-up function setting, no other chips are needed, in order not to increase the cost.

Figure 9: SP30 integrated P2SC using PHILPS

Figure 10: NPX2 integrated P2SC using PHILPS

The smart sensor module also integrates ASIC/MCU. Both NPX2 and SP30 are ASIC chips that use PHILPS's P2SC sensor signal conditioning (Figure 9, Figure 10). This unit can be clearly seen in the electrical schematic diagram of NPX2. Including an 8-bit RISC single-chip microcomputer for arithmetic processing control, 4K EROM or FLASH for setting the system firmware, 4K ROM for storing customer applications, 128Byte EEPROM, RAM for storing sensor calibration parameters and user-defined data , Timing modulator, interrupt controller, RC oscillator, and low noise amplifier LNA that amplifies the signal from the sensor, ADC that then converts the sensor signal into a digital signal, I/O port that communicates with the outside world, power management, and monitoring Door dog, intermittent timer, 1-3 dimension LF interface.

Figure 11: TPMS sensor module technology development trend

The development trend of TPMS sensor module technology is to develop the transmitter module to a highly integrated, singular, and wireless passive direction (Figure 11). As the TPMS product market requires high IC integration and high reliability, there are smart sensor modules such as Infineon SP12/SP30 and GE NPX that combine sensors and MCUs that need to test various physical quantities into one. In the next few years, a three-in-one module containing RF transmitter chips will be developed, including a four-in-one module that uses the mechanical energy of motion to be self-powered. The tire pressure monitoring device will only consist of one module and one antenna. Sub-design becomes very simple.

III. Functions

TPMS mainly detects the changes in tire pressure and alarms abnormal pressure. Therefore, the basic functions of TPMS include low tire pressure alarm function, high tire pressure alarm function, fast air leak alarm function, and system self-check function. In addition, extended functions can also include functions such as high-temperature alarm, real-time tire pressure, and temperature display.

1. Low tire pressure alarm

When the pressure value of one or more tires is lower than the under-pressure alarm threshold, the TPMS should send out the under-pressure alarm message within the specified time when the system is running, and indicate the position of the under-pressure tire. Through this function, the driver can detect the failure of the tire pressure shortage in time, and carry out the correct treatment, so as to ensure that the car tires work within the normal pressure range.

2. High tire pressure alarm

When the pressure value of one or more tires is higher than the overpressure alarm threshold, the TPMS should send out a high-pressure alarm signal within the specified time under the system operation state and indicate the location of the overpressure tire. Through this function, the driver can detect the danger of excessively high tire pressure in time, and perform speed reduction or other effective treatment to avoid the abnormal increase in tire pressure and possible tire burst accidents.

3. Quick leak alarm function

In the operating state of the system, when the pressure of one or more tires in a car tire decreases at a rate greater than the specified rate, the TPMS should issue a rapid air leakage alarm signal before the specified air leakage rate of 1min before the pressure drop corresponding to the air leakage And indicate the location of the flat tire. This function can detect the rapid air leakage failure after the tire is pierced by screws or other sharp objects in time, and remind the driver to control the driving direction of the car and stop as soon as possible to avoid traffic accidents caused by rapid air leakage.

4. Self check

Self check is a necessary function of TPMS. After the system is powered on, it starts self-check and lights up the fault light at the same time. The self-check ends within the specified time. If there is no fault in the system, the fault light will go out. If there is a fault, a fault alarm signal will be issued and the fault light will not go out until the fault is removed. For cars with more than four wheels, for each additional tire, the self-check time is allowed to increase by 1 second. This function is to detect the TPMS fault in time after the car is started. If there is a problem with the TPMS, it will remind the driver to repair it in time.

5. High tire temperature alarm

The tire temperature is closely related to the tire pressure, and the monitoring of the tire temperature is also very important. However, the current indirect TPMS cannot realize the monitoring of the tire temperature. Therefore, the tire temperature is too high alarm is an extended function. The direct TPMS can monitor the temperature inside the tire through the temperature sensor, and realize the alarm function of excessive tire temperature. When the temperature of one or more tires is higher than the alarm value of 85°C, TPMS will send out a high-temperature alarm signal within a specified time and indicate the location of the high-temperature tire. Because the tire temperature is too high, it is very easy to cause a puncture. Therefore, this function is of great significance to remind the driver to reduce the speed when the tire temperature is too high, thereby improving the safety of driving.