Since the birth of the car for more than 100 years, major traffic accidents caused by car punctures have been a technical problem that has plagued the automotive industry. Especially in the high-speed driving process of the car, tire failure is the most worrying and most difficult to prevent for all drivers, and it is also an important cause of sudden traffic accidents. According to relevant statistical data [1], 70% to 80% of the traffic accidents on the expressway in China are caused by tire problems, of which nearly 50% are puncture accidents. How to prevent puncture has become an important issue in safe driving. According to expert analysis, maintaining the standard tire pressure and timely detection of tire leakage is the key to preventing puncture. To this end, various tire pressure monitoring systems [2] (TPMS) products based on solving such problems have emerged. The TPMS introduced in this paper is an active safety technology product. It adopts wireless transmission technology. Through the tire pressure monitoring module and the receiving module, the tire pressure and temperature of the car during the exercise process are automatically monitored in real time, and the tire is leaking and low pressure. If the temperature is too high, an alarm will be issued, that is, an alarm will be issued before the accident to remind the driver to take measures to prevent the occurrence of a puncture accident and provide a more reliable guarantee for the driver of the vehicle. In addition, it also has the function of extending the life of the tire and saving fuel.
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1 MC68HC908RF2 chip introduction
The MC68HC908RF2[3] microcontroller is a low-cost, high-performance 8-bit microcontroller unit (MCU) from the company FREESCALE. It is a member of the M68HC08 family and uses the enhanced M68HC08 central processing unit (CPU08). ), suitable for multiple modes, storage sizes and types. Optimized for low power operation, this MCU is housed in a small 32-pin small outline quad flat package (LQFP) consisting of two parts. The first part is a 2KB Flash HC08 MCU (part number MC68HC908RK2); Part of the RF transponder (part number MC33493), which can transmit data hundreds of meters in free space, is suitable for automotive tire pressure monitoring systems.
The main features of the MC68HC908RF2 are as follows:
(1) High performance M68HC08 architecture.
(2) Fully upward compatible with the object code of the M6508, M146805 and M68HC05 series.
(3) The maximum internal bus frequency at 3V supply is 4MHz.
(4) 2KB of on-chip FLASH memory, 128B of on-chip RAM.
(5) 16-bit 2-channel timer interface module (TIM).
(6) 12 general-purpose I/O ports, 6-bit keyboard interrupt with keyboard wake-up function.
(7) One built-in high frequency transmitting module UHF (MC33493 module).
(8) Wide operating temperature range: -40 ° C ~ +125 ° C.
2 Working principle of automobile tire pressure monitoring system
This design is based on the core chip and other optimization solutions based on FREESCALE MC68HC908RF2, MPXY8020A[4], MC33594[5] and the optimized solution. It is designed to continuously monitor the tire pressure of high-speed vehicles. And forecast, and record the last 100 seconds of tire pressure information, to provide a basis for safe driving and driving management. The working principle of the system is shown in Figure 1.
As can be seen from Figure 1, the monitoring system consists of two main parts: the tire pressure monitoring module and the central receiving module. The tire pressure monitoring module is installed in the tire of the automobile. The module is provided with a sensor and a wireless transmitting device, and is mainly used for monitoring the air pressure and temperature in the tire and transmitting the monitored data to the receiving module through the transmitting device; the central receiving module is installed at Near the cab operating panel, with wireless receiving device, sound and light alarm module and liquid crystal display module, the wireless receiving device receives the data sent by the tire pressure monitoring module, and displays the air pressure and temperature of each tire by the liquid crystal display module, the driver Through the display module, you can grasp the air pressure and temperature conditions of each tire. When the tire pressure and temperature are abnormal, there will be danger signs, and the sound and light alarm module will automatically alarm to remind the driver to slow down or check accordingly. Overhaul to ensure safe driving and keep the tires in normal operation. In addition, in order to update and maintain the system functions, the module also retains the USART communication interface with the host computer.
3 Hardware design of automobile tire pressure monitoring system
The hardware design of the automobile tire pressure monitoring system is mainly divided into two parts: the tire pressure monitoring module and the central receiving module. The specific implementation is described below.
3.1 tire pressure monitoring module hardware design
It can be seen from Fig. 2 that the hardware part of the tire pressure monitoring module is mainly composed of three parts: a sensor, a controller and an antenna matching network. Among them, the sensor uses the MPXY8020A with the function of measuring temperature and pressure. The sensor integrates a variable capacitance sensing element, a temperature sensing element and an interface circuit (with wake-up function) on the same chip; with power management and digital output functions, its special function of low power consumption is specifically for TPMS The internal media protection filter protects the silicon from environmental influences. The MPXY8020A has four operating modes depending on the configuration of the S0 and S1 pins: standby/reset mode, measured air pressure mode, measured temperature mode, and output data read mode. There are 6 pins connected to the controller MC68HC908RF2, 2 of which are used to control its operation mode, 2 are used for data serial exchange, and 1 is used to wake up the controller. The MPXY8020A also has a reset function. This pin acts as an output terminal and periodically sends a reset signal to the controller to ensure reliable operation of the module.
The controller MC68HC908RF2 integrates an 8-bit Flash microcontroller (MCU) and a UHF transmitter (MC33493) on the same chip. The MCU is connected to the sensor and periodically checks the air pressure and temperature data. Although the transmitter is integrated with the MCU, its operation is completely controlled by the MCU, and its operating frequency and mode are set to control the transmission of data. The matching network is mainly to ensure that the data transmitted by the MC33493 can be sent out in the standard of wireless communication.
3.2 Central Receiver Module Hardware Design
The central receiving module informs the driver whether the current pressure and temperature of each tire of the vehicle are abnormal by blinking lights or liquid crystal display. The hardware structure of the central receiving module is shown in Figure 3. It is mainly composed of a wireless receiving part, a main controller and related peripheral devices. The wireless receiving part consists of a highly integrated UHF receiver MC33594 and its matching network; the MC33594 has the function of receiving and demodulating OOK or FSK modulated Manchester (manchester encoded) data, and can exchange data with the main controller through its SPI bus interface. If FSK modulation is used, the MC33594 can support the data manager - Manchester data decoder. If there is a matching antenna, the sensitivity can reach -103dBm.
The choice of the main controller is independent of the TPMS. As long as the controller has an SPI bus interface, it can receive and process the data sent by the MC33594 in sequence. This design uses ATmega16, a low-power 8-bit CMOS microcontroller based on the enhanced AVR RISC architecture. The controller features fast operation, pipelined execution of instructions, online programming, high confidentiality, and low price. For peripheral devices connected to the main controller, the function is to alert the driver through sound, light and liquid crystal display when the tire pressure or temperature of the car is abnormal. In addition, in order to query, set parameters and facilitate the analysis of tire pressure data through the computer, the module also designed the keyboard module and serial communication interface with the computer.
4 Software design of automobile tire pressure monitoring system
For the hardware design of the automobile tire pressure monitoring system, the software design is divided into two parts: tire pressure monitoring and central receiving.
4.1 tire pressure monitoring module software design
The function of the tire pressure monitoring module is to accurately and timely monitor the internal air pressure and temperature data of each tire and send it. The software flow chart is shown in Figure 4. After the module is powered up, the controller MC68HC908RF2 first initializes itself and the sensor MPXY8020A. In order to reduce energy consumption and extend battery life, the controller configures itself into STOP mode without data processing and transmission, and configures the sensor to STANDBY mode. In this mode, the sensor has its OUT pin every 3 The second wakes up (falling edge trigger) and the controller responds to a keyboard interrupt. In the interrupt processing program, the controller reads the collected tire pressure and temperature data by modifying the working mode of the sensor, and then starts the RF transmitting module to send the data in Manchester encoding mode, and enters the power saving mode, waiting for a new response. The interruption.
4.2 Central Receiver Module Software Design
The function of the central receiving module software is to complete the receiving, processing, discriminating, displaying and alarming of data. The flow is shown in Figure 5.
The module uses ATMEGA16 as the main controller. After completing the initialization, it configures itself as the host mode, initializes the RF receiver MC33594 through the SPI port, configures it to operate in the 315MHz FSK mode, and then configures itself into the slave mode. , waiting to receive data. After receiving the pressure and temperature data sent from the tire pressure monitoring module, the MC33594 is transmitted to the controller by the SPI port. The controller also checks the data and performs complex algorithm processing while receiving the data. The result is judged. If the current internal pressure and temperature of the tire exceeds the predetermined range, the sound and light alarm will be immediately activated, and the buzzer and the light-emitting diode will prompt the driver to have an abnormality, and the abnormal tire number and the tire information will be displayed by the LCD; Otherwise the car is running normally. In addition, the controller can also use the keyboard to set, update the tire pressure, temperature upper/lower limit value and tire ID number, and use the serial port to realize communication with the PC, which is convenient for querying and analyzing the recorded data, so as to optimize the control algorithm and analysis. Provide a basis for traffic accidents.
At present, the TPMS products on the market are mainly divided into two types: one is the indirect TPMS, which compares the rotational speed difference between the tires through the wheel speed sensor of the automobile ABS system to achieve the purpose of monitoring the tire pressure; The other is direct TPMS, which uses a pressure sensor installed in each tire to directly measure the air pressure and temperature of the tire, and monitor the pressure and temperature of each tire. When the tire pressure is too low or leaks. The system will automatically alarm. This design is a direct TPMS. The experimental test proves that the system can identify tires (including spare tires) by means of chipset and software optimization algorithms, detect excessive or insufficient pressure, compensate for changes in cargo weight, and monitor tire temperature. , timely alarm for abnormal situations. In addition, the system is compatible with a variety of automotive platforms and tire technologies, and module-level energy management extends battery life and signals when battery power is too low.
With the increasing demand for active safety of automobiles and the positive effect of TPMS on preventing major traffic accidents, the demand for high-performance TPMS will further increase. Therefore, the car tire pressure monitoring and alarm system has broad development space and very good market prospects.
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