Abstract:This paper applies digital twin technology to the dynamic loading process of cargo, communicates digital twin technology with real-time data of cargo loading optimization, and realizes the integration of data and business process. On the basis of analyzing the dynamic constraints of loading and the application value of digital twins, the twin model involved in dynamic cargo loading is established by considering the dynamic constraints in the process of loading operations and cargo assembly. After the initial loading scheme is generated, these models can verify the rationality of the loading, evaluate the balance and smoothness of the loading, and realize the adjustment and optimization of the loading plan based on the digital twin.
Keywords: cargo loading; load optimization; dynamic constraints; Digital twin technology.
Introduction. Cargo loading usually refers to the layout of goods in two-dimensional or three-dimensional space such as carriages, containers, pallets, etc., that is, to determine the coordinate position of each cargo under the conditions of meeting relevant constraints, so as to make full use of space or load capacity1. Due to the dynamic nature of the operation, the loading of goods needs to meet the corresponding dynamic constraint 2. The layout scheme generated by cargo loading guides the actual packing or loading operation, and the result is often static, and the execution process of the actual loading operation is more complex, so with the advancement of the logistics and distribution process, the layout scheme of cargo loading is faced with the requirements of continuous adjustment and optimization. The dynamic operational nature of cargo loading still receives little attention in traditional load optimization work. Verma et al.3 summarized the constraints in the loading of the robot**, the item cannot be placed below the existing item and cannot be reloaded, the robotic arm should be avoided from touching the loaded item, the robot can only rotate and place the item along the vertical Z axis, and the robot only has the ability to operate with an accuracy of about 1 cm. Ramos et al.4 proposed a "lower left corner" heuristic construction method to generate a physical packing sequence in a given container loading mode, which can meet the non-interference constraints in the actual filling process, but its stability is still static.
The combination of digital twin technology and logistics business activities can build a bridge between logistics solution optimization and practical operations, and achieve the purpose of improving the feasibility of solutions and the efficiency of logistics activities. At present, digital twin technology is mainly used in product design, engineering and manufacturing, and is rarely used in the freight transportation field. Eugene et al. 5 realized the optimization of the air cargo cabin layout scheme and the actual loading operation based on the optimization characteristics of the combination of virtual and real digital twin technology. Deng Jianxin et al.6 applied digital twin technology to the process of goods distribution. Based on the virtual visualization, traceability and adjustment characteristics of digital twin technology, this paper applies it to the dynamic cargo loading process, and uses the digital twin technology to interact and feedback real-time data with the cargo loading layout plan and operation process, so as to realize the integration and application of data and the whole process of business, solve the problem of insufficient consideration of factors such as the dynamics of cargo loading operations in the actual shipment process, and achieve the purpose of improving the loading quality and efficiency.
1 Cargo dynamic loading constraints.
Cargo dynamic loading constraints include the following scenarios.
1) Constraints on the execution process of the loading operation. In all cargo loading, small and medium-sized goods account for a relatively large proportion of the loading in the carriage, and the packing or palletizing is generally completed by hand in actual operation. Due to physiological factors such as the height and arm length of the operator, and the problem that the loaded goods may block the line of sight during the loading process, it is difficult or impossible for the operator to pack the box according to the generated static layout scheme. In addition, the size of the entrance and internal access may limit the handling of some cargo.
2) Dynamic stability constraints. Cargo loading stability includes two categories: static stability and dynamic stability. The static stability category refers to the fact that the underside of the cargo is supported by the lower cargo or the bottom of the carriage, so that there will be no vertical drop or slide; The category of dynamic stability refers to the lateral movement and fall of the vehicle due to the bumps in the road surface or the acceleration and braking of the vehicle in the process of driving, and the goods are subjected to inertia without a stable side support, and even cause the collapse of the entire stack. Static stability is easy to be described and limited by mathematical methods such as the proportion of the supporting area of the bottom surface, while dynamic stability is caused by complex road conditions or unloading and other variable factors, which is difficult to track and determine by mathematical methods.
3) Dynamic equilibrium constraints. The cargo loading balance constraint means that the center of gravity of the cargo loading layout should be located near the center of the vehicle, so that the center of gravity of the load is located within the set range of the longitudinal and transverse aspects of the vehicle, so as to ensure the safety of driving and loading and unloading operations. Some truck loads also need to consider axle load constraints, i.e., the weight limit of each vehicle bearing, which is also related to the position of the center of gravity of the load 7. The requirement of the position of the center of gravity belongs to the category of static equilibrium conditions, and because there is a cargo loading and unloading operation in the middle, the original equilibrium layout state will be broken, so there is a requirement to restore the equilibrium state.
2 The application value of digital twin technology in cargo shipment.
1) Rationality inspection of loading scheme. In the generation of cargo loading schemes, it is necessary to formulate different assembly plans for different products according to factors such as cargo materials, shape, weight, and distribution order. In the process of loading, unloading and handling, the difference in packaging methods will cause changes in product quality, resulting in the stability of products in the transportation process will be affected. Therefore, in order to verify the rationality of the packaging scheme in advance, it is necessary to support logistics technology based on simulation** technology.
2) Visualization of loading operations. The digital twin technology has the characteristics of virtual and real symbiosis, and this feature is applied to the loading and distribution process of goods, and the cargo model is constructed in the digital space through the digital twin technology, so that the goods in the real world and the digital in the digital space realize the two-way mapping, data connection and data state interaction of the twin, so as to achieve the purpose of loading carriage visualization 8.
3) Real-time control of the status of goods. Digital twin technology can obtain physical entity multi-state data through real-time sensing technology such as the Internet of Things, and the twin of the digital space can comprehensively and accurately reflect the changes in the distribution status of the cargo compartment, and the distribution personnel can clearly understand the cargo distribution situation of the carriage and the appearance, status and location of the carriage cargo through the system interface, so as to intelligently and dynamically adjust the loading plan.
4) Control and optimization of the whole process of shipment. After the cargo assembly plan and distribution route plan are generated, the physical data such as the appearance, volume, weight, and attributes of the goods are inputted, and the objectives and constraints of the carriage are set, and the digital twin technology can output the assembly plan 4 through the data collection and fusion analysis capabilities. At the same time, in the process of cargo transportation, the changes and trends of the cargo status can be tracked along with the path trajectory, and the influencing factors of the physical environment on loading can be revealed through the best analysis, and the distribution itinerary and assembly plan can be optimized.
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