Technical Analysis of TPMS Special Sensor Modules for Vehicles

TPMS is the abbreviation of "Tire Pressure Monitoring System". It is mainly used to automatically monitor the tire pressure in real time when the car is driving, and to alarm for tire leakage and low pressure to ensure driving safety. It is a life safety warning system for drivers and passengers.

In developed countries such as Europe and the United States, TPMS has become a standard product for automobiles, so TPMS is growing rapidly in both product variety and production output. The technology of MEMS chips and IC chips used in it has developed rapidly, and the final product technology of TPMS has also developed rapidly. The

tire pressure monitoring module of TPMS consists of five parts: (1) an intelligent sensor SoC with a combination of pressure, temperature, acceleration, voltage detection and post-signal processing ASIC chips; (2) a 4-8-bit single-chip microcomputer (MCU); (3) an RF radio frequency transmitter chip; (4) a lithium-ion battery; and (5) an antenna. See Figure 1. Figure 2 is a physical picture of the finished product. The shell is made of high-strength ABS plastic. All components and materials must meet the automotive-grade operating temperature range of -40°C to +125°C.

Figure 1: The TPMS transmitter consists of five parts.

Figure 2: The finished product of the TPMS tire pressure monitoring module

Smart sensor is a pressure sensor and acceleration sensor chip made by silicon micromachining (MEMS) technology, and a digital signal processing ASIC chip including temperature sensor, battery voltage detection, internal clock and analog-to-digital converter (ADC), sampling/holding (S/H), SPI port, sensor data calibration, data management, ID code and other functions. It has mask programmability, that is, it can be configured using customer-specific software. It is made of MEMS sensor and ASIC circuit chips in a package using integrated circuit technology (Figure 3). There is a pressure/temperature introduction hole on the top of the package (Figure 4), which directly introduces the pressure into the stress film of the pressure sensor (Figure 5). At the same time, this hole also directly introduces the ambient temperature into the semiconductor temperature sensor.

The MEMS silicon piezoresistive pressure sensor uses a circular stress silicon film inner wall with a fixed periphery. Four high-precision semiconductor strain gauges are directly engraved at the maximum stress point on its surface using MEMS technology to form a Wheatstone measurement bridge. As a force-electric conversion measurement circuit, it directly converts the physical quantity of pressure into electrical quantity, and its measurement accuracy can reach 0.01-0.03%FS. The structure of the silicon piezoresistive pressure sensor is shown in Figure 5. The upper and lower layers are glass bodies, and the middle is a silicon wafer. There is a vacuum cavity on the upper part of the stress silicon film, making it a typical absolute pressure sensor.

To facilitate identification by the TPMS receiver, each pressure sensor has a 32-bit unique ID code, which can generate 400 million non-repeating numbers.

Figure 3 Pressure, acceleration and ASIC/MCU combined in one package

Figure 4 Pressure/temperature inlet hole

Figure 5 Silicon piezoresistive pressure sensor structure

Figure 6 Acceleration sensor plan view

Figure 7 Acceleration sensor cross-sectional structure diagram

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Similarly, acceleration sensors are also made using MEMS technology. Figure 6 is a planar structure diagram of a MEMS acceleration sensor, and Figure 7 is a cross-sectional structure diagram of an acceleration sensor. In the middle of the figure is a silicon island mass block made using MEMS technology that can swing freely up and down with the motion force. A strain gauge is engraved on the silicon beam that connects it to the surrounding solid silicon, and together with the other three strain gauges engraved on the solid silicon, it forms a Wheatstone measuring bridge. As long as the mass block swings with the acceleration force, 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 magnitude.

The pressure sensor, acceleration sensor, and ASIC/MCU are three independent bare chips, which are integrated into a packaged unit by the chip integration manufacturer, such as Figure 8, NPX2 of GE Company of the United States. Figure 9 shows the three bare chips clearly after removing the packaging materials, and the connection and matching between the three chips are also made in it.

Figure 8 NPX2 of GE Company, USA

Figure 9 shows the removal of packaging materials.

The accelerometer can make the transmitter module have an automatic wake-up function. The smart sensors of SP12/30 and NPX2 series all contain accelerometers. The accelerometer uses the sensitivity of its mass block to motion to realize instant startup when the car moves, enter the system self-check, and automatically wake up. When the car is driving at high speed, it can be automatically awakened according to the speed of movement.

Automatically and intelligently determine the detection time cycle, and use software to set the safe period, sensitive period and dangerous period to gradually shorten the patrol detection cycle and improve early warning capabilities, save energy and other functions. The wake-up function setting can be completed using acceleration sensor + MCU + software design, and no other chips are needed to avoid increasing costs.

Figure 10 SP30 integrated with P2SC using PHILPS

Figure 11 NPX2 integrated with P2SC using PHILPS

The smart sensor module also integrates ASIC/MCU. Both NPX2 and SP30 use PHILPS's P2SC sensor signal conditioning ASIC chip (Figure 10, Figure 11). This unit can be clearly seen in the electrical schematic of NPX2. It includes an 8-bit RISC microcontroller for calculation and processing control, 4K EROM or FLASH for placing system firmware, 4K ROM for storing customer applications, 128Byte EEPROM for storing sensor calibration parameters and user-defined data, RAM, timing modulator, interrupt controller, RC oscillator, and low noise amplifier LNA for amplifying the signal from the sensor, ADC for converting the sensor signal into a digital signal, I/O port for communicating with the outside world, power management and watchdog, intermittent timer, 1-3 dimensional LF interface.

Figure 12 TPMS sensor module technology development trend

The development trend of TPMS sensor module technology is to develop the transmitter module towards high integration, singularization, and wireless passiveness (Figure 12). With the TPMS product market's requirements for high IC integration and high reliability, there are already smart sensor modules such as Infineon SP12/SP30 and GE NPX that combine the sensors and MCUs required to test various physical quantities. In the next few years, we will also develop modules that include three-in-one RF transmitter chips and four-in-one modules that use mechanical energy for self-power supply. By then, the tire pressure monitoring transmitter will only consist of one module and one antenna, making the customer's secondary design very simple.