Mechanical Writings

This article focuses on how the tire pressure monitoring system (TPMS) circuit design and related technical issues. TPMS system by the pressure sensor module and the central receiver, of which the pressure sensor module consists of a pressure sensor SP12, micro-controller and work in the 2.4GHz ISM band RF transceiver chip nRF2401 composition of the tire pressure to complete the acquisition and launch tasks, while receiving machine receives pressure information, and processing.
1.TMPS Introduction
TPMS tire pressure monitoring system is used in the car driving in real time, automatic monitoring of tire pressure, low pressure on a flat tire and the police to ensure traffic safety.
TPMS is divided into two types, one is indirect WSB TPMS, the system through the vehicle speed sensor ABS system compare the speed difference between the tire in order to achieve the purpose of monitoring tire pressure, the main drawback is that not on two or more tires At the same time, anaerobic conditions and faster than 1OOkm / h to judge the situation.
The other is Direct PSB TPMS, tire mounted on each sensor can directly measure the pressure inside the tire pressure, and to display the tire pressure and monitoring, when tire pressure is too low or there is leakage, the system will automatically alert . Direct TPMS can be divided into basic and Bus Direct TPMS TPMS.
2. System solutions
TPMS detection principle is to install sensors in the tire / transmitter module through the electrical bridge electronic air temperature sensors, precision sensors, measuring each tire's air pressure, temperature. And air pressure, temperature signal is converted to electrical signals, then through the wireless transmitting devices will be installed in the driving signal to the indoor receiver and digitally displayed in real time. When a tire pressure of any system temperature exceeds the set value of the normal temperature region, will be issued to the police.
2.1 MSP430 microcontroller
Transmitter module installed in the car because the tire inside, the system can only use battery power, so the TPMS system, the main problem is how to minimize / system power consumption. Ultra-low power MSP430 microcontrollers have the characteristics of its internal serial port module with multiple USRAT. Taking into account the cost, this paper has two serial ports used the low end of any MSP430 microcontroller as a receiver and transmitter modules can MCU.
2.2 Pressure, Temperature Sensor
Is a piezoelectric pressure sensor SP12T resistance sensor, can be used to 50kPa ~ 1400kPa range of pressure measurement. It has a digital serial communication port. SCM can be easily carried out with the transmission of data and control commands, the work does not require other external device, the interface circuit shown in Figure 1.
Shown in Figure 1, SDO is the serial data output pin, and the MSP430 microcontroller USART0 input pin P4.5 / SOMll connected to measure pressure, temperature, voltage and chip ID sent to the MCU; SDI as a string line data input pin, connected to the USART1 output pins P4.6 / SIMO1, MCU micro controller output SP12 8-bit control command, the corresponding action specified SP12T: SCLK for the serial synchronous clock, and the MCU serial port clock UCLK1 connected synchronous serial data communications.
In addition, SPl2T WAKEUP and RESET to provide two outputs, each output can be in a certain time interval after a pulse, provide MCU interrupt or wake-up signal.
2.3 RF transceiver chip nRF2401
Single Chip RF Transceiver nRF2401 to work in the 2.4GHz-2.5GHz ISM band of UHF, the maximum speed of up to 1Mb / s, power supply voltage is 1.9V ~ 3.6V, low power design needs.
nRF2401 circuit connection shown in Figure 2, through the SPI port and MCU micro controller. nRF2401 only one SPI port ATA, the pin of the SPI data transceiver and MCU pin SlMO0 and SOMIO connected, while the SPI MCU series with 10k resistor. DR1 ready to receive data pin, nRF2401 set DR1, that there is data coming, and then DRl pin voltage is low, DR1 and P1.2 is connected, while the P1 port has MCU interrupt only when the data are ready , MCU to exit the interrupt service routine, perform the appropriate send and receive data communications.
nRF2401 has four operating modes, that is, send and receive mode, configuration mode, idle mode and shutdown mode, nRF2401 by CE, CS and PWR_UP three pins control the working status of the chip, respectively, and the MCU, P1.0, P1.1, and P1 .3 connection. For example, when PWR_UP = 1, CE = 1, CS = 0  , nRF2401 into receive mode.
nRF2401 provided by connecting an external crystal clock required, as a result of the band for the 2.4GHz, so the use of 16MHz crystal. In addition, nRF2401 an external antenna to send and receive data, the antenna design is related to an important factor in system performance.
2.4 TPMS transmitter and receiver interface circuit
Broom transmitter and receiver interface circuit shown in Figure 3. Transmitter module consists of a pressure sensor SP12, MSP 430F 449, RF transceiver nRF2401, battery and antenna. The module on the tire pressure, temperature, battery voltage and acceleration of data acquisition, and wirelessly transmitting the data.
TPMS module received by the RF transceiver chip nRF2401, MSP 430F 449, dot matrix LCD12864, LED, speaker and antenna. TPMS transmitter module receiving module will send the information demodulation, decoding, and data received through the LCD display.
3.TPMS receiving the software design process
This receiver module of the software design flow as shown in power, the receiver has initialized and configured, when the confirmation receiver configuration is completed, all the LED flashes once, this module is ready; to receive a data frame, the re-calculated checksum with the data has been received frame comparison: the data frame is verified, a tire and ID are stored in the MCU memory 4 ID value for comparison, when detected ID values match, then further process the data, and light the corresponding LED; Finally, the data frame is sent through the serial port for external data receiving and storage.
Since each wheel module to send data in the same format, the receiver controller can receive all data frames in the interrupt enable low-power MCU enter the sleep state.
4. Conclusion
FSK modulation used in the text transmission, data transmission reliability, sensitivity better than-1OOdbm. Tire pressure warning system with high function, under-voltage warning function, excessive leakage early warning early warning capabilities and features such as temperature, monitor module can work in -40 ℃ ~ 125 ℃ temperature range, 0kPa ~ 450kPa pressure range. This system is compact and reasonable structure design, low cost.

In 2002, the Firestone (Firestone) the quality of the tire, resulting in more than 100 people were killed and 400 injured in the incident, causing the auto industry and the U.S. government attach great importance, Bridgestone / where WorldCom was forced to recovery of 6.5 million tires. Society of Automotive Engineers, according to a recent survey, 75% of annual tire failure was due to leakage or inflated tires cause. According to Ministry of Public Security statistics, in China highway accident that occurred on 70% is due to a puncture caused in the United States the proportion is as high as 80%. How to prevent puncture has become an important topic of safety.
According to the National Center of Quality Supervision, rubber tires, tire pressure and maintain the standards to detect leaks is to prevent puncture tires the key, and tire pressure monitoring system is the ideal tool to prevent puncture. As the tire pressure is usually a gradual process, even if as a result of foreign body punctured tire deflated tire has an ongoing process, so by real-time monitoring of tire pressure and abnormal pressure in the tire after the first alarm, the driver can correctly handle unexpected situations for members of valuable time, to ensure traffic safety. To this end, the U.S. Department of Transportation and the U.S. National Highway Traffic Safety Administration to develop the relevant policies, regulations from November 2003 to October 2006 31 new factory will be gradually introduced for light vehicles tire pressure monitoring system.
The basic principle of tire pressure monitoring
Currently, the tire pressure monitoring system are two solutions, direct and indirect system of systems. Direct tire pressure monitoring system is installed in each tire using a pressure sensor in the direct measurement of tire pressure, tire pressure and the display and control, when the tire pressure too low or there is leakage, the system will alarm automatically. Indirect tire pressure monitoring system is the system through the vehicle ABS wheel speed sensors to compare the difference in speed between the tire in order to achieve the purpose of monitoring tire pressure, the main disadvantage of this type of system are: â‘  the tire can not accurately show the instantaneous air pressure; â‘¡ the same side of the same axle or wheel or all the tire pressure drops can not be alarm at the same time; â‘¢ not taking into account the speed, precision and other factors. Obviously, the direct sensing system more effective.
Direct tire pressure monitoring system is divided into active (active) and passive (passive) two.
Active system is used on the use of MEMS technology in the production of silicon capacitive or piezoresistive pressure sensors, pressure sensors installed in each wheel, by way of radio frequency signals sent out, the installation of wireless in the cab receiver receives the signal of the pressure-sensitive, after a certain amount of signal processing, display the current tire pressure. The advantages of active technology is relatively mature technology, developed the module suitable for all brands of tires, but the disadvantage also more prominent, the need for battery-powered sensor module, so there is a systematic problem of life.
Tire pressure monitoring system of passive sensors is the use of surface acoustic wave (SAW) to design, this sensor by radio frequency electric field to generate a surface acoustic wave, when the surface acoustic wave through the piezoelectric substrate material surface, will produce changes by detecting this change in surface acoustic wave, you can know the tire pressures. Although this technology without battery power, but it needs to be integrated into the tire transponder, the tire manufacturer needs to establish common standards for possible implementation.
Real-time tire pressure monitoring and alarm system there is no uniform standard, companies are trying to develop competitive products to compete in the future in an invincible position. High resolution, passive, small three characteristics of tire pressure monitoring system will be the future trend of development.
Tire pressure monitoring system to detect abnormal tire pressure condition, and only with high resolution in order to have high accuracy. Battery life is limited, and the capacity is also affected by temperature. To improve system reliability, the sensor is best to be passive detection. Tire pressure can work not only with, but also with temperature, wheel speed and load weight, respectively, and the future of the tire pressure sensors measure the pressure at the same time, the tire should be able to measure the temperature and carrier quality. Many studies show that use of tire pressure sensors collected information, the vehicle suspension system fault monitoring and correction of the navigation system. Therefore, the future of the sensor should be set a variety of functions in a passive smart sensors.
Passive TPMS design field electromagnetic coupling
a. Principles
Inductive coupling is a transformer model, high-frequency alternating magnetic field through space to achieve coupling. Based on the law of electromagnetic induction, in fact, tire through the alternating magnetic field measurements in the transmitter module of the coil induces a voltage and current measurement to the transmitter module inside the tire to provide energy. Inductive coupling in general suitable for low frequency radio frequency identification system to work closely.
b. Design
The system includes at least the tire measured in two parts, modules and readers.
In general, tire measurement module coupled by the low-frequency antenna (large coil), micro-chip and high-frequency transmitter-specific antenna. Low-frequency alternating magnetic field coupling antenna from the energy required to obtain work, ASIC is responsible for measuring the pressure and the pressure information into RF signals, high-frequency RF signal transmitting antenna will be launched into space.
Reader includes a receiver, controller and low-frequency drive circuit, low-frequency antenna, high frequency receiving antenna. Controller emits low-frequency antenna for the tire to the space within the measurement circuit using electromagnetic waves, launching a small portion of the magnetic field through the magnetic field lines from the reader antenna coil within a certain distance of the tire coupled low-frequency antenna coil measurement module, by induction, in the low-frequency antenna coupling coil produces a voltage Ui, be rectified as a receiver module measuring tire power.
Abstract: The United States, NHTSA TREAD regulations have been issued a new round automotive TPMS pressure sensors and tide product development boom. For the product development and design of domestic species, we developed a tire pressure sensor chip, which is based on silicon MEMS technology, bulk micromachining technology manufacturing, the use of silicon - silicon vacuum bonding greatly reduced cell size, is conducive to reduce costs. Has been successfully developed using this pressure sensor chip manufacturing performance has reached the practical requirements.
Introduction: the new century, the Firestone Tire (Fire-tone) of quality problems, a lot of tire and rollover accident caused more than 100 people dead and 400 injured, causing the car industry and the U.S. government's attention. The following year in August the company had to recall 6.5 million tires. According to statistics, there are 260,000 cases of traffic accidents each year due to low tire pressure or leakage caused by traffic accidents accounted for 75%. And China 70% of highway accidents caused due to a puncture.
Tire quality experts believe that, "to maintain a standard tire pressure tire vehicle and to detect leaks is to prevent puncture of the key." Armed with the current prevalence of low-cost digital tire pressure the tires can not guarantee timely detection of leakage, currently used for more than 200 million cars on the GM and Ford indirect measurement techniques - with snatches dead brake speed sensor to measure the tire comparison speed of each tire, the tire pressure to conclude that the method is less cumbersome and inaccurate it is.
Installed in each tire on the tire pressure monitoring sensors and radio transmitters intelligent tire is causing great concern, intelligent tire with the vehicle dashboard with a receiver and display, which constitute a tire pressure measurement system device, the TPMS (Tire Pressure Monitoring System), it can also be directly referred to as tire pressure monitoring sensor (Tire Pressure Monitoring Sensor).
The National Highway Traffic Safety Administration last century, the mid-seventy mandatory federal law, contributed to the popularity of the fuel injection system automatically and the first climax of automotive sensor applications. NHTSA 2002, another federal law, requires that U.S. cars from 2003 to 2006 in each of the 15%, 35%, 65%, 100% of the proportion of assembly TPMS system, which will trigger a new round of the climax of automotive pressure sensor applications .
Tire Pressure Sensor

Tires unique working conditions, determine the real-time monitoring of the pressure sensor tire pressure high demand, requiring large temperature, wide supply voltage range of high practical total accuracy, low power requirements, the wireless signal transmission requirements, resistance to harsh environmental requirements and cost requirements.
Motorola (Motorola) is an active developer TPMS system, which uses MEMS technology based on silicon integrated capacitive pressure sensor MPXY8020A as tire pressure detection unit, with low power consumption and fully integrated features. 32-pin package MC68HC908RF2 as signal processing and emission control unit, which is an 8-bit microcontroller and integrated UHF transmitter device, using MC33594 as a receiver unit.
General Electric Company acquired in recent years to the famous sensor manufacturer Nova Sensors (GE NovaSensor) is another active developers, which uses technology based on silicon MEMS piezoresistive pressure sensor as a tire pressure monitoring unit, with complete control to have a , measurement, and adjustment and the emission signal compensation proprietary CMOS LSI. The two chips packaged in a standard 14-pin SOIC package, which constitutes the TPMS, model NPXC01746, thanks to a wake-up transient mode, the power consumption is only 9.7 microamps seconds of work to meet the battery life of ten years , the use of digital compensation, at -40 ℃ ~ 125 ℃, the battery voltage range 2.1V ~ 3.0V, pressure measurement accuracy of better than 1.5% FS, Nova model is that it can be a greater use of existing ASIC and Whiston any kind of integrated full-bridge piezoresistive pressure sensor complex integration, to produce more types of TPMS products.
SensoNor Norway-based company specializes in manufacturing specific integrated piezoresistive pressure sensor, tire pressure monitoring system suppliers such as TRW Automotive Company, SmarTire supporting the production company into a TPMS, SensoNor piezoresistive sensor and SmarTire tire combination of the success of the RF generator Siemens VDO TPMS has the U.S. auto parts companies and Goodyear Tire Company (Goodyear) used.
Tire pressure sensor stress-sensitive chip design and development

For the development of tire pressure sensor, we designed and developed a suitable absolute pressure sensor, which is based on silicon bulk micromachining MEMS technology, micro piezoresistive absolute pressure sensor, sensing element is - Integrated Full Bridge Whiston . Force-sensitive resistors located in the conventional design of the four sides of the square edge of the center of silicon film by <110> crystallographic alignment, a pair of a longitudinal layout, the layout of a pair was cut to form the Wheatstone bridge strain. Resistance stripe width 8um, full-length 60um, the average effective stress can guarantee 20mV / V output sensitivity. Resistance formed by ion implantation doping, have excellent doping uniformity and accuracy to ensure the stability of zero and sensitivity, the design of the impedance of resistor 5KΩ, silicon-silicon bonding with the formation of an absolute vacuum reference cavity pressure sensors. Than the silicon-pyrex glass anodic bonding to form an absolute pressure reference cavity system with better thermal expansion match, and thus more conducive to product thermal stability and time stability. Using this design, another major advantage of technology can greatly reduce the cell chip is the size of the design elements of the chip size of 1mm × 1mm, in a four British silicon wafer can be made on a force of seven thousand sensor unit, and silicon - the design elements of glass bonding chip size is generally 1.5 × 1.5mm to 2.2 × 2.2mm, which in a four-inch silicon wafer can be made of pressure-sensitive component unit were 3400 and 1600, it is obvious Our design is conducive to reducing unit production costs. Of course, the designs on the premise that mastering silicon - silicon bonding thin silicon membrane technology and production technology. In view of large-scale tire pressure sensors, silicon diaphragm thick, using precision machinery thinning or isotropic etching is not hard to meet the design requirements, even under the extended range of the design of automotive absolute pressure MAP and AAP sensor range, the use of Smart Cut technology or wafer electrochemical selective etching technique is not difficult to get 15 ~ 20um thick silicon film.
Nonlinear: 0.05 ~ 0.1% FS;
Hysteresis and repeatability: 0.03% FS;
Output Sensitivity: 10 ~ 20mV / V;
Range: 700Kpa;
Overload: 300%;
Temperature coefficient of zero and sensitivity: 1 ~ 3 × 10-4 / ℃ · FS.
The chip may also be used hand-held tire pressure sensor and digital tire pressure and necessary use.
Conclusion

Using silicon bulk micromachining MEMS technology to produce a successful tire pressure monitoring system tire pressure sensor chip used, the use of silicon - silicon bonding technology design and related reduced chip size, lower unit costs, product development, the first step TPMS.