Vehicle load detection system based on differential capacitance sensor

Abstract: Aiming at the shortcomings of traditional vehicle load detection system, a capacitive vehicle load detection system is proposed. The load detection sensor of this system adopts differential structure, which greatly improves the sensitivity and nonlinearity of measurement. The capacitance measurement circuit adopts differential pulse width modulation integrated measurement circuit. The data acquisition and processing adopts STC89LE516AD single-chip microcomputer chip with built-in A/D converter, and the data communication adopts wireless communication mode. This load detection system has simple structure, low cost, easy installation, reliable performance, simple measurement circuit, good anti-interference, can be used for traffic data collection and portable measurement, and has good application prospects.

0 Introduction

With the continuous development of highway transportation and commercial trade, vehicle load detection technology has become the focus of research in the field of measurement. At present, the commonly used vehicle dynamic load detection sensors are mainly bent plates, piezoelectric shafts, single sensors, vehicle-mounted capacitive sensors and optical fiber sensors. These load detection sensors are mostly suitable for fixed installation and have high requirements for road conditions. Even some portable vehicle load detection sensors cannot truly achieve portable measurement due to their heavy weight and large size. At the same time, some sensor measurement technologies are too complicated and the sensor prices are too expensive. Therefore, in order to reduce the installation and maintenance costs and improve the portability of the vehicle dynamic load detection system, this paper proposes a capacitive vehicle load detection system. The load detection sensor in this system adopts a differential structure, which greatly improves the measurement sensitivity and nonlinearity. The capacitance measurement circuit adopts a differential pulse width modulation integrated measurement circuit. The data acquisition and processing adopts the STC89LE516AD single-chip microcomputer chip with built-in A/D converter, and the data communication adopts the wireless communication mode. This load detection system has a simple structure, low cost, and easy installation. The differential capacitive load sensor has strong anti-interference ability, good dynamic response, wide measurement range, high sensitivity, and good stability.

1 Differential Capacitor Vehicle Load Detection System

The differential capacitance vehicle load detection system is shown in FIG1 .

The vehicle load detection device is portable and is laid on the road surface when in use. The handheld device is the control unit of the measurement system, which sends instructions to the detection device and receives data through wireless communication. The load detection sensor adopts a differential capacitive load sensor, which converts the change of load into the change of capacitance value. The capacitance measurement circuit adopts a unique differential pulse width modulation integrated circuit to collect the extremely weak capacitance signal from the differential capacitive load sensor and convert it into an easy-to-detect voltage signal. The data processing module adopts the voltage input type STC89LE516AD microcontroller chip with an internal 8-channel 8-bit A/D converter. The data processing module performs A/D conversion, data acquisition, and data processing on the signal, and then outputs the processed load result. In order to reduce the trouble of line laying and increase the safety of staff, the data communication of the detection system adopts a wireless communication device.

2 Structure and working principle of differential capacitive load sensor

The structural diagram of the differential capacitive load sensor is shown in Figure 2. It is mainly composed of a measuring head, a housing, a sensitive element (elastic body), a fixed pole, a moving pole, an electrode, an equipotential ring, and lead wires. Its characteristics are: a wide measuring range; high sensitivity, easy to pick up signals; no contact, no deformation, no wear between the plates, small mechanical loss, and long life; the capacitive sensor is less affected by temperature; good dynamic performance; simple structure, and adaptable to various harsh environments and occasions.

The measuring head and the housing of the sensor are clearance-matched, and they can slide relative to each other. There are positioning screws to locate the initial position of the measuring head. The positioning screws also play a directional role when the measuring head slides, and can also keep the force-applying object relatively stable. The measuring head is supported by a sensitive element (elastic body), which transmits the force to the sensitive element after being acted upon by an external force. The sensitive element (elastic body) is located between the measuring head and the housing, and plays the role of sensing the external force and converting it into mechanical displacement according to a certain relationship. The moving and fixed poles are hollow cylindrical, and their surfaces are plated with electrodes. The moving pole is bonded to the measuring head as a whole and slides with the measuring head. The fixed pole is bonded to the housing as a whole and is relatively fixed. Equipotential rings are provided at both ends of the moving and fixed pole electrodes to reduce the capacitance edge effect and improve the measurement accuracy.

When the differential capacitive load sensor is subjected to an external force F, the measuring head transmits the force to the sensitive element. The sensitive element is an elastic body with an elastic coefficient of k. It undergoes elastic deformation under the action of this force, and its deformation d is proportional to the external force. The deformation of the sensitive element causes the measuring head and the electrodes on the moving pole to move the same distance d. At this time, the capacitance value of the differential capacitive load sensor will change accordingly, and the change is △c. The distance d moved by the measuring head is proportional to the change in the output capacitance △c of the sensor. It can be seen that the external force F exerted on the object under test is proportional to the change in the output capacitance △c of the differential capacitive load sensor, that is: (where k is the elastic coefficient of the sensitive element; L is the length of the initial covering part of the moving pole and the fixed pole; c0 is the initial capacitance between a single capacitor electrode). As long as the change in capacitance △c is detected by the measuring circuit, the external force F exerted on the object can be known.

3 Capacitance measurement circuit

The differential capacitive load sensor converts the change of the measured load into the change of capacitance output, while the capacitance generated by the capacitive sensor is very small, and the stray capacitance generated between the capacitor plate lead and the ground is often larger than the measured capacitance. Therefore, the small capacitance conversion measurement technology has always been valued by people. However, the general detection circuit structure is relatively complex and has low accuracy, which cannot meet the measurement requirements. In order to improve the measurement sensitivity, for the differential capacitive load sensor, on the basis of the charge transfer capacitance detection circuit based on the four-phase detection technology, a differential pulse width modulation integrated measurement circuit is designed. The circuit has the characteristics of high integration, active capacitance sensor head, output pulse square wave, elimination of high-frequency excitation signal source, low power consumption, strong anti-interference ability, high resolution, etc., which is especially suitable for the measurement of differential capacitance sensors. Its internal structure block diagram is shown in Figure 3, and the dotted box in the figure is the two variable capacitors C1 and C2 of the differential capacitance sensor.

The working principle is as follows: when the DC power supply is turned on, the Q terminal is at a high level and the C terminal is at a low level, then the signal control unit enables the charge and discharge network 1 to charge the capacitor C1, and the voltage on C1 gradually increases. Once the circuit control level is reached, the signal processing unit immediately changes the Q terminal to a low level, and the C terminal is at a high level; at this time, the voltage on the capacitor C1 is rapidly discharged to zero through the charge and discharge network 1, and at the same time, the signal control unit enables the charge and discharge network 2 to charge the capacitor C2, and the voltage on C2 gradually increases. Once the circuit control level is reached, the signal processing unit again makes the Q terminal high level and the C terminal low level; then the next cycle of C1 charging and C2 discharging begins again, and so on, and so on, and a series of rectangular square waves whose width is controlled by the change in the capacitance of C1 and C2 are generated at the output end of the differential pulse width modulation integrated circuit. When C1=C2, the voltage waveforms of Q and C terminals are anti-phase symmetrical, and the difference between the two average voltages taken from the Q terminal and C terminal will be equal to zero. When the load being detected makes the capacitance C1>C2, the difference between the average voltages of the two output terminals is: (where V1 is the voltage value of the charging network input), and better linearity can be obtained.

4 Data Collection and Processing

The data acquisition and processing unit uses the STC89LE516AD single-chip microcomputer chip with built-in A/D converter to complete data acquisition, analog-to-digital conversion, data processing and drive the display unit. When the clock is below 40 MHz, an A/D conversion can be completed every 17 machine cycles. The STC89LE516AD single-chip microcomputer is combined with a differential pulse width modulation integrated circuit to complete the detection of the capacitive sensor. The main program and A/D conversion program flow chart are shown in Figure 4 and Figure 5.

5 Data Communication

The data transmission adopts wireless communication module. The wireless communication in the differential capacitance vehicle load detection system is realized by using nRF401 wireless transceiver chip and control microcontroller 89C52, which has the advantages of simple hardware circuit, low cost, easy programming and high communication reliability. The application of wireless communication technology in the vehicle load detection system enables law enforcement personnel to conveniently conduct non-stop load detection on road vehicles through handheld instruments, greatly improving work efficiency.

The wireless communication device includes a load detection device and a handheld device. The load detection device receives instructions from the handheld device, transmits the load results to the handheld device, and transmits the vehicle type and license plate number data to the handheld device when necessary for error correction; the ultrasonic signal transmitting and data receiving device in the handheld device sends instructions to the load detection device and receives data from the load detection device; the single-chip microcomputer system sends the data to the display device after receiving it, and can establish data connection with the PC; after the PC forms a local area network, it can complete information collection, display, query, retrieval, data analysis, statistics, processing, storage and other tasks.

As can be seen from Figure 1, the load detection device detects and processes the load of the vehicle, and the single-chip microcomputer receives the data of the vehicle's load detection device according to the control command and communicates data with the host. Figure 6 is a hardware block diagram of the display and transceiver control system composed of a single-chip microcomputer, which mainly includes the acquisition and data processing module, watchdog, reset circuit, power supply monitoring circuit, real-time clock circuit, wireless transceiver module, control single-chip microcomputer, information output unit and other parts. The control single-chip microcomputer uses Atmel's 89C52.

The handheld instrument in Figure 1 is the host. The hardware structure block diagram of the host is shown in Figure 7, which consists of a control microcontroller, a display circuit, a watchdog, a reset circuit, a power monitoring circuit, a real-time clock circuit, a button, a wireless transceiver module, and a serial communication circuit.

When the road management personnel press the control button of the handheld instrument to read the data, the host receives the command and sends a command to the slave, receives the slave load data through the wireless transceiver module, and then displays the load information and vehicle-related information on the display screen of the handheld instrument, and can upload it to the computer of the road management department through serial communication as needed. Compared with the slave, the host has an additional serial port for communicating with the computer. This serial port adopts the RS 232 standard and can be implemented with the MAX232 chip.

6 Conclusion

The vehicle load detection system based on differential capacitance sensor has the characteristics of simple mechanical structure, reliable performance, simple measurement circuit, good anti-interference, small size, high cost performance, etc. The actual test results show that the vehicle load detection system performs dynamic load detection on the vehicle, and the measurement error of the total load of the vehicle is within 10%, and its accuracy is better than the Class I WIM (Weigh-in-Motion) system accuracy given by ASTME131-02 (total weight error ±10% at 95% confidence), which can be used for traffic data collection, especially suitable for highway inspectors to perform portable measurements, and has a good prospect for use.