Improved design and implementation of axle identification in highway toll collection system

Since the implementation of the self-weight toll collection system, the highway administration has not only high requirements for the vehicle axle recognition rate, but also requires a long service life of the equipment to ensure that the vehicle passes through the toll station normally. The original wheel axle identifier requires frequent maintenance, which is not only costly, but also the circuit board is often burned out, and the recognition rate is relatively low, affecting the efficiency of the toll station. In response to the problem of frequent circuit board burnout, the author started to improve the design from the power supply hardware circuit, such as adding a varistor and a thermistor. In response to the problem of low recognition rate, the hardware circuit is improved, such as adding an analog amplifier circuit module.

1 Improvement of the power supply circuit

In field use, the circuit board often burns out, especially after thunder and lightning. The cause is analyzed and the power supply circuit is improved.

The original circuit diagram is shown in Figure 1, and the improved circuit diagram is shown in Figure 2.

By comparing the two circuit diagrams, it can be found that the improved power supply has a varistor connected to one end of the socket instead of directly connected to the ground, and a positive thermistor is connected in series with a varistor at the other end and capacitor C1. A varistor is a resistor whose resistance value changes with voltage within a certain current and voltage range. When the voltage across it exceeds its threshold, the current flowing through it surges, which is equivalent to the threshold gate opening, thereby suppressing the abnormal high voltage that often appears in the circuit and protecting the circuit from damage caused by excessive voltage. During operation, the temperature of a positive thermistor is similar to that of other components. When connected in series in a circuit, it will not hinder the passage of current. However, when an overcurrent suddenly occurs in the circuit, the temperature of the thermistor rises due to the increase in heating power. When the temperature exceeds the switch temperature, the resistance will increase sharply in an instant, causing the current in the circuit to quickly decrease to a safe value, thereby protecting the circuit from damage caused by the sudden increase in current.
From the above, it can be seen that circuits are often burned out, basically due to a sudden increase in voltage or current provided by the circuit power supply, especially thunder and lightning. This design is to avoid damage caused by such an increase in voltage and current from the source.

2 Improvement of the hardware circuit of the wheel axle identifier
In the field, the number of axles identified by the wheel axle identifier often shows the phenomenon of missing axles or missing axles, and the recognition rate is relatively low. When there are many vehicles, the axle often gets stuck, which affects the work efficiency. The whole circuit working process is analyzed and the hardware circuit is improved. 2.1 Introduction to the original wheel axle identifier
The overall structure of the original wheel axle identifier is relatively simple, as shown in Figure 3. Its circuit mainly consists of an analog switch and a microprocessor. The signal detected by the sensor is sent to the microprocessor for processing through the analog switch. After the data processing is completed, it is sent to the host computer or instrument for use by the host computer or instrument.

2.2 Improved wheel axle identifier
The overall structure of the improved wheel axle identifier is shown in Figure 4. Its circuit mainly includes analog amplifier module, microprocessor module, AD converter module, electronic analog switch and output indication. Compared with the original circuit, the main additions are analog amplifier module, AD converter module and output indication.

The function of the amplifier is to amplify the input tiny voltage by a certain multiple, so that it is not easily consumed by other components such as resistors in the circuit. There are two amplifiers in the improved wheel axle identifier amplifier module, one is the OP07 operational amplifier and the other is the AD620 instrument amplifier. OP07 is a high-precision operational amplifier with extremely low input offset voltage, extremely low offset voltage temperature drift and long-term stability. AD620 is a high-precision instrument amplifier with low offset voltage and low offset drift characteristics. It is the first choice for precision data acquisition systems such as electronic scales and sensor interfaces.
Electronic analog switches are selected with high performance, such as the switching speed must be fast enough to ensure that the signal generated by the vehicle when it passes through the sensor quickly can pass through it completely.
The A/D converter converts analog quantities into digital quantities through a certain circuit. The analog quantity can be electrical signals such as current and voltage, or non-electrical signals such as humidity, displacement, and sound. Before A/D conversion, the signal input to the A/D converter must be converted into voltage signals by various sensors. Its selection must also meet the requirements of field measurement, especially in terms of conversion speed and conversion accuracy. It is the key part of the whole circuit, and its performance will directly affect the overall performance of the whole circuit board.
The output indication is mainly displayed by LED lights.
Under normal circumstances, the output signal of the sensor is relatively small, only a few millivolts, which is very weak. After being amplified several times by the AD620 instrument amplifier and OP07 operational amplifier in the amplification module, it is sent to the AD converter through the electronic analog switch. At this time, the AD converter is directly connected to the electronic analog switch. The CPU is the core processor of the whole circuit board. It not only processes the data signal, but also coordinates the work of the AD converter and the electronic analog switch. The output indication circuit is mainly for the convenience of on-site observation and debugging.
By comparing the two circuits, it can be seen that the original wheel axle identifier has no amplifier module, and the signal collected by the sensor is likely to be consumed by the resistor and disappear, and its data processing method is relatively simple.
Actual measurements have proved that the improved wheel axle identifier has faster processing speed, greatly improved recognition rate, and rarely stuck.
3 Software implementation of wheel axle identifier
The system software is programmed in C language, which was launched by Bell Research Institute in the United States. C language has been successfully transplanted to large, medium, small and micro computers. It can be used as a working system design language to write system applications, and it can also write applications that do not rely on computer hardware. It has a very wide range of applications and has strong data processing capabilities. Specific applications include embedded systems and single-chip microcomputers. The main program code is as follows:



4 Conclusion
Field tests have proved that after the improved design of the wheel axle identifier, in general, the design of this article has added lightning protection and other functions, and has improved the speed and ability to process data. After being applied to the highway weight toll collection system, the circuit board rarely burns out and gets stuck, and the recognition rate is greatly improved, and the application is stable.