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The Principle of Measuring and Weighing the Entire Vehicle On the Highway Scale

     
 
 
    The development direction of highway weighing and toll collection is the use of vehicle scale measuring equipment on highways. This product replaces axle measuring equipment and can effectively prevent various cheating behaviors. This article introduces the principles of static and dynamic weighing respectively, and starts from the system composition. Through the analysis of the advantages and disadvantages of various components, detailed design selection suggestions are proposed.
 

1. Overview

      At the beginning of the implementation of weight based toll collection on highways, it played a good role in curbing over limit transportation and significantly increased toll revenue. However, axle measuring equipment is commonly used for weight measurement, and the principle of measuring equipment requires vehicles to pass at a uniform speed in order to measure accurately. Some car owners are driven by profit, resulting in various cheating phenomena. The problem of vehicle cheating and reducing tolls seriously disrupts the normal operation of toll stations, causing the loss of tolls on highways and easily leading to traffic accidents. In order to curb the behavior of non-standard weighing and evading vehicle tolls, reduce toll disputes, improve traffic efficiency, reduce safety hazards, and reduce the difficulty of toll management on site, currently highways generally update axle measuring equipment to whole vehicle measuring equipment. The whole vehicle measurement can prevent various cheating behaviors, which is the development direction of weight based toll collection on highways.

2. Principle of whole vehicle measurement and weighing

2.1 Principle of Static Weighing

      The weighbridge uses the principle of strain electrical measurement for weighing. A strain gauge is attached to the elastic body of the weighing sensor, forming a Wheatstone bridge. When under zero load, the bridge is in a balanced state and the output is zero. When the elastic body is subjected to a load, each strain gauge generates a strain proportional to the load, and the magnitude of the applied load can be measured by the output voltage.
      Install multiple weighing sensors below the weighing platform, connect multiple sensor cables in parallel into the junction box, and then connect one cable to the instrument. After the car drives onto the weighing platform, the platform transfers the force it receives to various weighing sensors, causing a change in the resistance of the strain bridge and causing a change in the output voltage. This electrical signal is transmitted to the instrument panel, and after digital filtering, linear amplification, A/D conversion, and CPU processing, the final weighing value is displayed.
 
 

2.2 Principle of Dynamic Weighing

When the first vehicle reached the weighing platform for weighing, the vehicle separator was obstructed, and the system sensed that the vehicle was starting to weigh. The axles of the vehicle are sequentially analyzed by an embedded axle load gauge to determine the number of axles and direction of travel. The weighing sensor senses the pressure of the vehicle and converts it into an electrical signal. It is amplified by the weighing circuit and AD converted into a digital signal. The weighing unit calculates the weight of the vehicle on the weighing platform based on the digital signal. When the vehicle leaves the light screen, the system perceives that the vehicle is completely weighed, automatically determines the vehicle's model, speed, direction of travel, and other data, and calculates the vehicle's axle weight and total weight based on the weight of the weighing unit. The system will upload the calculated weighing information to the billing computer, and the weighing process will be completed.

Before the first vehicle is fully weighed, if a second vehicle is loaded onto the scale, the system uses the same calculation principle as the first vehicle to calculate the number of axles, vehicle type, speed, and other data. After the second vehicle is fully loaded onto the scale, the system will confirm whether the first vehicle has started to load based on the data from the embedded axle recognizer at the weighing end. If it has already started to load, the system will calculate how many axles the first vehicle has loaded onto the scale, thereby determining the remaining weight of the first vehicle on the scale body. The total weight of the weighing platform minus the remaining weight of the first vehicle is the weight of the second vehicle, and the weighing data of the second vehicle is also calculated. The system uploads the data of the second vehicle.

Before the first vehicle is fully weighed, if a third vehicle is weighed, the system uses the same calculation principle as the first vehicle to calculate the number of axles, vehicle type, speed, and other data. After the third vehicle is fully weighed, the system will confirm whether the first vehicle has started weighing based on the data from the embedded axle recognizer at the weighing end. If the weighing has already started, the system will calculate how many axles the first vehicle has weighed, and thus determine the remaining weight of the first and second vehicles on the weighing body; The total weight of the weighing platform minus the remaining weight of the first and second vehicles is the weight of the third vehicle, and the weighing data of the third vehicle is also calculated. The system uploads the data of the third vehicle.

If there is a fourth vehicle weighing before the first vehicle is weighed, the principle is similar.

 

3. Composition of vehicle type metering and toll collection system

      The vehicle type metering and charging system consists of a vehicle type weighing platform (weighbridge, lower scale end axle recognizer, upper scale end axle recognizer, weight control system, vehicle separator, anti slip, tire barrier anti friction, etc.).

      Based on the toll length and lane width of the toll station, the size of the weighing platform is designed in various specifications, such as 3mx 21m, 3.4mx 21m, etc.

3.1 Whole vehicle weighing platform

      The whole vehicle weighing platform, also known as the weighbridge scale body, includes weighing sensors. At present, there are two forms of car scales on the market: steel shaped structures and U-beam structures. The common practice is to use U-beam structures for normal lanes and steel shaped structures for ultra wide lanes. The panel of a car scale is generally made of two steel plates welded together or a whole plate. The joint welding method of two steel plates has low cost, but the rigidity of the weighing platform is low, and long-term use is prone to deformation of the weighing body. The whole plate welding weighing platform has strong rigidity, but the cost is relatively high. The weighing platform of the weighbridge is generally welded with CO2 gas protection. After welding, shot blasting and sandblasting pretreatment are required to ensure the adhesion strength of the paint film. Adding patterned steel plates on the surface effectively improves the wear resistance and anti slip performance of the equipment.

      There are two main types of weighbridge weighing sensors: bridge type and column type. The advantage of bridge sensors is their strong resistance to lateral forces, mainly used for static weighing scales. The advantages of cylindrical sensors are fast dynamic response, small size, and easy maintenance, mainly used for dynamic weighing scales. Therefore, column sensors are commonly used in highway vehicle measurement products, which have superior performance and higher reliability.

3.2 Vehicle separator

      The function of a vehicle separator is to separate the vehicle and provide a final signal to the weight control system, in order for the system to perform vehicle data calculations. There are two types of vehicle separators used for toll collection on highways, one is an infrared grating, also known as a light curtain; One type is ground sensing vehicle detector. The characteristic of a light curtain is fast and accurate detection, which can separate vehicles with a distance of more than 0.1 meters. The disadvantage is that it is installed on a safety island and may be damaged by vehicles. The characteristic of ground sensing vehicle detectors is that they are buried inside the road and are not easily damaged, but their detection accuracy is not high. If the distance between two vehicles is less than 1.5m, it is easy to misjudge.

      These two devices have distinct characteristics, so they are generally used in conjunction with the weight control system of highway toll collection. When the light curtain is working normally, the system uses the light curtain signal as the vehicle separation signal. When the light curtain malfunctions, the system automatically switches to the ground sensing vehicle detector signal and issues a light curtain fault alarm.

3.3 Lower scale end axis recognizer

      The lower scale axle recognizer is located at the toll booth end and generally consists of two pressure sensors. It is installed as a whole and can automatically record the number of axles, axle weight, and pressure ratio of the weighing vehicle. Compared with traditional tire recognizers, it can significantly improve the recognition rate of the number of axles and provide accurate weighing axle data for the weight control system. The length of the axle recognizer is adapted to the weighing platform, with a detection width of 2110mm for ordinary lanes and 21500mm for ultra wide lanes. When the shaft recognizer malfunctions, a fault alarm can be issued through hardware and software.

      3.4: Compared to the lower scale end axis recognizer, the upper scale end axis recognizer has a heavier task in the measurement control system. In addition to automatically recording the number of axles, axle load, and pressure ratio of the weighing vehicle, it also bears the responsibility of judging the vehicle model, driving direction, and driving speed. Because one of the important data for toll collection on highways is vehicle type information, which directly affects the toll amount, the design of the weighing end is more complex. It generally consists of 6 pressure sensors, with a width consistent with the width of the weighing platform and divided into two output signals, providing accurate weighing axis data for the weighing control system. According to different toll stations, the threshold value can be adjusted separately for higher adaptability. When the axle recognizer malfunctions, it can issue a fault alarm through hardware and software.

      The metering control system is the control core of the entire vehicle weighing and toll collection system on highways. The system can automatically receive various information from components such as weighing sensors, upper scale end recognizers, lower scale end axle recognizers, vehicle separators, etc. It integrates and calculates various data signals to obtain data such as vehicle weight, speed, vehicle model, number of axles, axle weight, and axle spacing. It is seamlessly integrated with the high-speed highway weighing and toll software system.

      Due to the characteristics of multiple toll lanes and high traffic volume on highways, it is required that the weight control performance be stable, easy to set parameters, easy to operate, and have strong adaptability to working environments. In the early days of shaft measurement products, the control system was relatively simple and the display information was limited, which could no longer meet the current usage needs. The vehicle weight control system requires the use of an LCD display screen with a fully Chinese operating interface.

      The weighing information includes total weight, number of axles, axle weight, axle spacing, speed, etc., to facilitate equipment maintenance; The system interface also requires standard PS/2 keyboard interface, Ethernet interface, RS-232C serial port, vehicle separator interface, axle recognition sensor interface, traffic light control interface, railing machine control interface, etc., to facilitate later functional upgrades.

3.5 Anti slip

      Anti slip is a key issue in the overall measurement of highways, especially when it comes to rainy days. If the anti slip issue is not handled properly, it can easily cause the vehicle to slip out of control or rear end collision, resulting in many unnecessary troubles. At present, the main anti slip measures for vehicle products on the market include coating anti slip, welded steel bar anti slip, full patterned plate anti slip, intermittent patterned plate anti slip, etc. Their advantages and disadvantages are as follows:

      1. Anti slip coating: If commonly used modified epoxy resin with diamond sand curing or specialized anti slip coating roughening process is used for road surfaces and ship decks, the initial anti slip effect is relatively good, but the sharp edges after roughening cause significant damage to the tires, and its processability is poor. This material needs to be cleaned by sandblasting on the surface of the steel plate before coating, and the remaining uncovered parts can only be manually painted instead of automatic painting and paint drying, which greatly reduces the anti-corrosion, anti rust performance, and aesthetics of the remaining exposed parts. In addition, the adhesion with the metal tabletop is poor, making it easy to fall off. Moreover, the on-site recovery after falling off is greatly affected by environmental temperature and humidity, making it difficult to recover, and the maintenance cost is high. The construction cycle is long (at room temperature). It will take about 15 days to use.

      2. Anti slip steel bars: refers to welding steel bars along the direction of the vehicle on the surface of the weighing platform. The characteristics are: low weld strength, easy welding and detachment of steel bars, strong sense of vehicle weighing and bumping, and poor driving experience.

      3. Full coverage of patterned steel plates for anti slip: If the weighing platform is fully covered with patterned steel plates, it cannot be replaced on site after local pattern wear.

      4. Intermittent laying of patterned plates for anti slip: High quality bean shaped patterned steel plates are laid on the surface of the scale body. After leveling, all patterned steel plates are placed on the scale body panel, compacted with heavy objects, and continuously welded with carbon dioxide gas shielded welding. The patterned board is laid on four roads and can cover the driving area of general road vehicles, ensuring that all types of vehicles can achieve ideal anti slip effects. The patterned plate adopts an intermittent distribution structure, which improves the final anti slip effect by changing the friction coefficient between the vehicle tires and the anti slip plate; The intermittent layout structure facilitates local replacement on site, reduces replacement time, greatly improves maintenance efficiency, and facilitates surface water drainage.

3.6 Tire barrier and edge cleaning

      In vehicle measurement products, in order to prevent cheating by wheels crushing the foundation road surface, tire lateral friction island stones, etc., vehicle tire barriers, commonly known as anti friction edges, are generally installed. There are three common anti scratch structures in the market, namely: reverse C-shaped, reverse V-shaped, and reverse U-shaped. Among them, the anti C type appeared earliest, characterized by a simple structure and low cost, but the disadvantage is poor strength and easy deformation during heavy vehicle compression. Currently, it is basically eliminated. The characteristic of inverted V-shaped is simple processing and low cost. Two steel plates can be placed in an inverted V-shaped shape and welded together. The disadvantage is that the welding joint is sharp, which can easily cut the tire and cause controversy. The inverted U-shaped plate is made of hot-rolled sheet metal, which is integrated and bent to form an inverted U-shaped section. It is then connected to the weighing platform using peripheral continuous welding technology.

      The integrated bending process improves the overall stiffness of the vehicle tire barrier through the principle of cold work hardening. Compared with the vehicle tire barrier formed by welding steel plates, it has the characteristics of less welding residual stress, high strength, good reliability, and beautiful appearance. The vehicle tire barrier has an inverted U-shaped cross-section and features a wide bottom and a round top, effectively avoiding possible tire scratches and ensuring driving safety. At the same time, the tire barrier of the vehicle is intermittently arranged on the weighing platform to facilitate the drainage of surface water.

4. Conclusion

      The working principle and structural characteristics of vehicle weighing products determine the accuracy and non cheating of vehicle weighing. Practice has also proven that it meets the new needs of the development of weight based toll collection on highways, solves many practical problems that axle measuring equipment cannot solve, and will be the mainstream equipment for weight based toll collection for a long time in the future.

 
Release time: 2024-08-15