Research on passive tire pressure monitoring system with integrated pressure sensor

1 Introduction 

In 2002, due to the quality problems of Firestone tires, more than 100 people died and 400 people were injured, which attracted great attention from the automobile industry and the US government. Bridgestone/Firestone was forced to recall 6.5 million tires.

According to a recent survey by the Society of Automotive Engineers of the United States, 75% of tire failures each year are caused by tire leakage or insufficient inflation. According to statistics from the Ministry of Public Security, 70% of traffic accidents on Chinese highways are caused by tire blowouts, while in the United States, this proportion is as high as 80%. How to prevent tire blowouts has become an important issue for safe driving.

According to the analysis of the National Rubber Tire Quality Supervision Center, maintaining standard tire pressure and timely detecting tire leaks are the keys to preventing tire blowouts, and the automobile tire pressure monitoring system will be an ideal tool for preventing tire blowouts. Since tire pressure changes are usually a gradual process, even if tire deflation caused by foreign objects puncturing the tire is a continuous process, real-time monitoring of tire pressure and the immediate alarm after abnormal tire pressure can buy precious time for drivers to correctly handle emergencies, thereby ensuring driving safety. To this end, the U.S. Department of Transportation and the National Highway Traffic Safety Administration have formulated relevant policies, stipulating that from November 2003 to October 31, 2006, newly manufactured light vehicles will gradually introduce tire pressure monitoring systems.

2 Basic principles of tire pressure monitoring

At present, there are two main solutions for tire pressure monitoring systems, direct systems and indirect systems. The direct tire pressure monitoring system uses the pressure sensor installed in each tire to directly measure the tire pressure, and displays and monitors the tire pressure of each tire. When the tire pressure is too low or there is leakage, the system will automatically alarm. The indirect tire pressure monitoring system uses the wheel speed sensor of the automobile ABS system to compare the speed difference between tires to achieve the purpose of monitoring tire pressure. The main disadvantages of this type of system are:

① It cannot display the accurate instantaneous pressure value of each tire;

② It cannot alarm when the pressure of the same axle or the same side wheel or all tires drops at the same time;

③ It cannot take into account factors such as vehicle speed and detection accuracy at the same time. Obviously, the direct sensing system is more effective. The

direct tire pressure monitoring system is divided into two types: active and passive.

The active system uses a capacitive or piezoresistive pressure sensor made using the MEMS process on a silicon base. The pressure sensor is installed on each wheel rim and the signal is transmitted through wireless radio frequency. The wireless receiving device installed in the cab receives the pressure-sensitive signal and displays the current tire pressure after certain signal processing. The advantage of active technology is that the technology is relatively mature and the developed module can be applied to tires of various brands, but the disadvantage is also prominent. Its sensing module needs to be powered by a battery, so there is a problem with the service life of the system.

The sensor of the passive tire pressure monitoring system is designed using a surface acoustic wave (SAW). This sensor generates a surface acoustic wave through a radio frequency electric field. When this surface acoustic wave passes through the surface of the piezoelectric substrate material, it will change. By detecting this change in the surface acoustic wave, the tire pressure can be known. Although this technology does not require battery power, it requires the transponder to be integrated into the tire, and it is possible to implement it only when all tire manufacturers establish common standards.

There is currently no unified standard for the real-time monitoring and alarm system of tire pressure. Companies are working hard to develop competitive products in order to be invincible in future competition. Tire pressure monitoring systems with high resolution, passive and small size will be the future development trend. The

tire pressure monitoring system needs to detect abnormal tire pressure. Only with high resolution can it have high accuracy. Battery life is limited, and capacity is also affected by temperature. To improve the reliability of the system, the sensor is best to perform passive detection. Whether the tire can work properly is not only related to air pressure, but also to temperature, wheel speed and load. In the future, the pressure sensor should be able to measure the temperature and load inside the tire while measuring tire pressure. Many studies have shown that the information collected by the tire pressure sensor can be used to monitor vehicle suspension system faults and correct the navigation system. Therefore, the future sensor should be a passive intelligent sensor with various functions.

3 Passive TPMS magnetic field electromagnetic coupling design

3.1 Principle

Inductive coupling is a transformer model that achieves coupling through high-frequency alternating magnetic fields in space. It is based on the law of electromagnetic induction. In fact, the alternating magnetic field is used to induce voltage and current in the coil of the measurement transmitter module in the tire to provide energy to the measurement transmitter module in the tire. The inductive coupling method is generally suitable for short-range RFID systems working at medium and low frequencies.

3.2 Design of the scheme

The system consists of at least two parts: the tire measurement module and the reader.

In general, the tire measurement module consists of a low-frequency coupling antenna (a large-area coil), a dedicated microchip and a high-frequency transmitting antenna. The low-frequency coupling antenna obtains the energy required for operation from the alternating magnetic field. The dedicated chip is responsible for measuring the pressure and converting the pressure information into an RF signal. The high-frequency transmitting antenna transmits the RF signal into space.

The reader includes a receiver, a controller, a low-frequency driving circuit, a low-frequency antenna and a high-frequency receiving antenna. The controller transmits electromagnetic waves for use by the tire measurement circuit into space through the low-frequency antenna. A small part of the magnetic field of the transmitting magnetic field passes through the low-frequency coupling antenna coil of the tire measurement module at a certain distance from the reader antenna coil. Through induction, a voltage Ui is generated on the low-frequency coupling antenna coil, which is rectified as the power supply of the tire measurement receiving module.

A capacitor is connected in parallel with the antenna coil of the reader. The capacitance of the capacitor is selected based on the fact that it forms a parallel oscillation circuit with a resonant frequency that matches the reader's transmitting frequency together with the inductance of the antenna coil. The resonance of this loop causes the antenna coil of the reader to generate a very large current. This method can also be used to generate the field strength required for the operation of a long-distance transponder. The low-frequency coupling antenna coil and capacitor of the in-tire measurement module form an oscillation loop, which is tuned to the transmission frequency of the reader. Through the resonance of this loop, the voltage on the transponder coil reaches the maximum value. The structure on these two coils can also be interpreted as a transformer (coupling of the transformer). There is only a weak coupling between the two coils of the transformer. The power transmission efficiency between the antenna coil of the reader and the low-frequency coupling antenna coil of the in-tire measurement module is proportional to the operating frequency f, the number of turns n of the transponder coil, the area A surrounded by the transponder coil, the relative angle of the two coils, and the distance between them. After the in-tire measurement module obtains energy to work, it modulates the pressure information into an RF signal and transmits it. After the high-frequency receiving antenna of the reader receives the RF signal, the receiver demodulates the RF signal and transmits the pressure signal to the controller. The controller informs the owner of the pressure signal through the human-machine interface.

3.3 Difficulties and solutions

The inductive coupling system is not efficient, so it is generally suitable for low-current circuits and has a short operating distance, generally only tens of centimeters. The energy provided is limited, so the design of the sensor circuit in the module is very important. The key is: ① The chip design must be efficient and able to complete the measurement and transmission tasks under low current conditions; ② All the energy required for the microchip to work must be supplied by the reader. The high-frequency strong electromagnetic field is generated by the antenna coil of the reader, so the reader design must provide sufficient magnetic field strength.

This system requires an effective distance of about 30 to 40 cm and can provide a large current of about 20 mA, so the design is challenging. In order to achieve the expected goal, efforts can be made in two aspects: on the one hand, as much current and energy as possible should be provided for the normal operation of the chip; on the other hand, a low-power design should be adopted for the measurement chip to minimize the current required for the normal operation of the in-tire measurement module. [page]

4 Design and Analysis of Direct Tire Pressure Monitoring System

This paper designs a direct TPMS, and its principle is shown in Figure 1. The system consists of two parts, the tire module and the vehicle-mounted receiving module. Among them, the tire module includes pressure and temperature sensors, A/D converters, controllers and radio frequency transmitters, as shown in Figure 2. The vehicle-mounted receiving module includes radio frequency receivers, controllers and display alarm devices, as shown in Figure 3. The









basic working principle of the system is as follows: the tire module is installed in the tire, and the pressure and temperature sensors detect the current stress and temperature information inside the tire. The analog signal obtained is converted into a digital signal through the A/D converter and then sent out through the radio frequency transmitter. The vehicle-mounted receiving module is installed in the cab, and the radio frequency receiver receives the stress and temperature information from the tire module. When the tire pressure is too high or too low, the alarm information is issued through the display and alarm device.

In the tire module, a new type of pressure and temperature integrated sensor is designed as a tire pressure monitoring sensor. The sensor integrates two sensors, pressure and temperature, and has the characteristics of small size, low cost and high measurement accuracy. The use of a low-noise four-channel amplifier can easily realize the amplification of both pressure and temperature signals. The controller uses Motorola's MC6S08QG8 chip, the working voltage of which is 3 V, which is consistent with the working voltage of the sensor. The RF transmitter chip uses Motorola's MC33493 chip, which has the characteristics of 3 V power supply and low power consumption.

In the vehicle receiving module, the RF receiving decoding chip uses Motorola's MC33594 chip, which is a RF receiving chip that matches the RF transmitter chip MC33493 chip, and its working voltage is 5 V. The controller uses Motorola's MC9S08AW16 microcontroller, the core of which is S08, which can use the same development environment as the MCU MC6S08QG8 of the tire module. In addition, the chip is powered by 5 V and is compatible with the RF receiving chip and LCD display chip level.

5 Analysis of a new tire pressure monitoring sensor

The pressure sensor and temperature sensor are the key components of the tire pressure monitoring system. A new sensor is designed and manufactured, which includes a piezoresistive pressure sensor and a temperature sensor.

Among them, the piezoresistive pressure sensor mainly uses the piezoresistive effect of semiconductors to make a Wheatstone bridge on a silicon cup, as shown in Figure 4. When the silicon cup is deformed by pressure, the resistance of the four bridge arms of the Wheatstone bridge will change, breaking the electrical balance of the bridge, so that there will be an output of an electrical signal. The





temperature sensor adopts a single resistor structure and makes a p-type resistor on an n-type silicon substrate. Because the p-type injection resistor has a positive temperature coefficient, when the temperature changes, the resistance value of the resistor will change accordingly. By supplying power to the resistor with a current source, the temperature change can be detected by measuring the voltage on the resistor. Figure 5

is an electron microscope photo of the sensor chip made in this paper. Figure 5 (a) is the full view of the front of the device; Figure 5 (b) is the front of the film; Figure 5 (c) is the back of the film. The pressure-sensitive element uses a 50μm silicon cup film (500 μm×500μm) that is thicker than the conventional one, which increases the area of ​​the high stress area on the bulk silicon, while reducing the process requirements and improving the linear measurement range and overload pressure. The varistor is designed as a zigzag structure with optimized geometric dimensions, and part of it is made on high-stress bulk silicon to obtain higher sensitivity. The temperature-sensitive resistor on the bulk silicon is made with the varistor using a synchronous injection process, which reduces the process complexity. The device has a simple process, high yield, and is compatible with standard IC processes.





Preliminary test results show that the integrated sensor has good performance. The full-scale output of the pressure sensor is 150 mV, the pressure measurement range is 0-500 kPa, and the sensitivity is 0.3 mV/kPa. The sensitivity of the temperature sensor is 1.24 mV/℃ and the nonlinearity is 1.6%. The pressure-temperature integrated sensor is fully suitable for the requirements of the tire pressure monitoring system.

6 Conclusion

Based on the analysis of the principles of the tire pressure monitoring system and various solutions, a new tire pressure monitoring system solution is designed. According to the requirements of TPMS, a pressure-temperature integrated sensor is designed and manufactured. The device has a simple process, high yield, and is compatible with standard IC processes.