There is a large proportion of serious traffic accidents on the freeway caused by underage of the car tires. In the future car to install tire pressure monitoring system, and ABS, airbags, as the inevitable trend of development. TPMS system is divided into direct TPMS and indirect TPMS two. One indirect TPMS is through the car ABS system wheel speed sensor to compare the speed difference between the wheels in order Sensor to achieve the purpose of monitoring tire pressure, its low accuracy. Direct TPMS works by using a pressure sensor and a temperature sensor installed in each tire to measure tire pressure and temperature directly and to display and monitor the tire pressure. Its advantage is that when the tire pressure is too high, too low, slow tire leakage or abnormal temperature changes, you can promptly to the car wireless receiver alarm, effectively prevent the tire, while monitoring the status of all tires, and the system of the car Travel speed is not required.
The direct TPMS system requires four transmit modules and one receive module. 4 transmission modules need to receive data transmission with the receiving module, so how to make their communication between the coordinated and orderly manner, as the design needs to pay attention to the problem. For GE's solutions, temperature and pressure sensors, A / D converters and 8-bit MCUs are integrated into Temperature Sensor the NPX series of chips, making data collection and transmission in the communication design greatly simplified. For the NPX I and NPX II chips, only the appropriate RF transmit circuit is required to implement the full functionality of the transmit module.
GE has been committed to the development of TPMS sensor products, and its products NPX I chip integrates pressure sensors, temperature sensors and an 8-bit RISC microprocessor. The NPX I chip has 4 KB of user-programmable space, 4 KB of custom ROM, and a 2D LF input stage. The signals of the various types of sensors are converted to the 12-bit ADC and are provided to the user and the system for further processing. In 4-KB custom ROM, GE's unique pressure, temperature and voltage measurement, compensation and calibration procedures, and other useful subroutines are implemented. Users can save complex programming and simply call to get Of the state value. During the development phase, GE can provide a programmable version of the sensor, the user can download the program through the emulator program Throttle Position Sensor to the device's 4 KB user-programmable space. After downloading the program, the sensor can be run directly, or real-time simulation through the simulator single-step debugging. Debugging environment and the general microcontroller is very similar. Customers in mass production versions of the sensor can be produced by masking ROM processes to further reduce costs.
The receiving module of the TPMS system is mainly composed of an antenna, a radio frequency receiving circuit, a master chip MCU and a keyboard and a display for receiving the tire temperature and pressure data transmitted by each transmitting module, displaying the ID and measurement data of each tire, Abnormal situation occurs when the sound and light alarm. As the receiving module installed in the car compartment, so the choice of all aspects of the device requirements are not high, industrial grade can be. RF receiver chip selection requirements for higher acceptance, the choice of Maxim's MAXl473 chip. The MAXl473 is a fully integrated, low-power, CMOS superheterodyne receiver with an input signal range of 114 to 0 dB dB above the 50 dB image carrier suppression for receiving a range of 300 to 450 MHz frequency range Key (ASK) data signals are ideal. The current consumption in this mode is less than 2.5μA in shutdown mode and 5.2 mA in receive mode, allowing data rates up to 100 kbps.
In order to achieve reliable wireless communication between the system tire module and the receiving module, the transmitter and the receiver must communicate with a certain protocol. The MCU in the transmitting module sends data in the data frame mode, and the receiving module is awakened by the leading bit of the data frame. The preamble function is to wake up the receiver and indicate the actual data to follow; the tire ID is used to identify and determine the tire position; The pressure value and the temperature value carry the actual measured value; the status bit indicates the current system state; the checksum is used to verify the correctness of the data; the stop bit indicates the end of the data frame. Because the transmission module is designed to take into account the power saving function, the module is normally in the dormant state. When the acceleration wake-up signal is received, the module is measured and transmitted. The program flow is shown in Fig. The program flow of the receiving module is shown in Fig. After receiving the module power, first configure the MCU and RF receiver chip, and then began to receive data. When the data is received, the MCU checks the tire 1D. When the tire ID is valid, the MCU processes Pressure Switch the data and displays the data. Finally, the MCU according to set the threshold to determine whether to make overpressure, undervoltage and leakage and other alarm action.
Tire pressure and temperature real-time monitoring and alarm system is one of the essential functions of automotive safety system, in which direct TPMS system is the main implementation of the current way. In this paper, the NPX I chip is designed for the NPX I chip. The transmitter module adopts the NPX I chip and the transmitter chip PCH7900. The receiving module is realized by using the MAXl473 receiver chip and the C8051F410 single chip microcomputer. Compared with other TPMS system solutions, the design of the NPX I chip integrates the pressure sensor, temperature sensor and microprocessor MCU, high integration, stable performance, can achieve sensor data and microprocessor between the efficient and reliable communication.