Automobile test is an important means to find out all kinds of problems in automobile design and development. According to the test results, we can make an objective evaluation of all kinds of automobile performance. As one of the basic projects of automobile industry, automobile test plays an important role in the overall development of automobile industry. Automobile performance test system is a key part of automobile test engineering. It is an organic whole composed of a number of interrelated and interacting sensors, instruments and equipment to realize the test of automobile performance. The performance of automobile test system often has an important impact on the effectiveness of the whole automobile test.
Overall structure design of the system
Vehicle test mainly includes power performance, fuel economy, handling stability and emission characteristics. The main performance parameters include speed, acceleration, fuel consumption, temperature and dynamic motion parameters in handling stability test. The test signals of these parameters obtained by sensors are processed by front-end processing module (shaping, filtering, amplification, etc.) After that, it is sent to c805l-f020 microprocessor. After analog-to-digital conversion and data processing in the MCU, it is connected with ZigBee terminal node through serial port. Then, the terminal node sends the data in WLAN. After receiving the data, the ZigBee central node communicates with the host computer through serial port. The central node can also send commands from the upper computer to the terminal node to control the execution of the terminal node. The overall structure block diagram of the system is shown in Figure 1.
Front end processing module
The sensor converts all kinds of common non electric signals into electric signals, which are generally weak. The front-end processing module processes these signals and sends them to the A / D conversion port of MCU. The system has 8 channels of sensor signals, including 2 channels of voltage sensor signals, 2 channels of – 5 ～ + 5 V voltage signals, 2 channels of 4 ～ 20 mA current signals and 2 channels of thermocouple signal front-end processing. The front-end processing hardware circuits of the voltage sensor signal and thermocouple signal are shown in Fig. 2 and Fig. 3 respectively.
AD620 is a low-cost, high-precision instrument amplifier, only one external resistor is needed to set the gain, and the gain range is 1 ~ 10 000. The front-end processing of the voltage sensor signal adopts the amplification circuit composed of ad62-0 and ad705, which is powered by a single power supply. Ad705 is a voltage follower, which provides the zero point of the output voltage for AD620. VREF and agnd are sent to ainl. 0 and ainl. 1 ports of 8-bit precision ad-cl of MCU, and the reference voltage and analog ground of the signal are calculated by software program. The thermocouple sensor is used to measure the temperature of the key parts of the automobile, and its front-end processing circuit adopts OP07’s adjustable gain amplifier circuit. OP07 is a low noise, non chopper zero stabilized bipolar operational amplifier IC, with very low input offset voltage, low offset, high open-loop gain characteristics, making OP07 particularly suitable for high gain measurement equipment and amplifying weak signal of sensor. The offset voltage of OP07 can be ignored when the accuracy is not required. R25 and R24 are used to adjust the amplification factor of the system in this circuit. When selecting different types of thermocouples, the resistance values of the two can be adjusted appropriately.