《1 Engineering research fronts》

1 Engineering research fronts

《1.1 Trends in Top 10 engineering research fronts》

1.1 Trends in Top 10 engineering research fronts

The ten most-researched engineering topics in the field of mechanical and vehicle engineering include mechanical, transportation, ship and marine engineering; weapon science and technology; aeronautical and astronautical science and technology; and power and electrical equipment engineering and technology (as listed in Table 1.1.1). Among these, “low-carbon and zero-carbon fuel engine technologies”, “flexible self-powered wearable sensors”, “multi-material 4D printing”, “state-of-the-art vascular robotic system development”, “transfer learning-based machine fault diagnosis”, and “robot-assisted milling and polishing” are extensively studied traditional topics. “Dynamically reconfigurable mobile micro-robot swarms”, “intelligent performance test for automatic driving in an adversarial environment”, and “wireless charging system for underwater autonomous vehicles” are considered as emerging topics. The annual number of core papers published during the years 2017–2022 is listed in Table 1.1.2.

(1) Hypersonic flight vehicle technology

Hypersonic aircraft refer to aircraft that can fly in the atmosphere at speeds exceeding Mach 5. Compared with traditional subsonic, supersonic, and supersonic aircraft, hypersonic aircraft have high speeds, fast response capabilities, and high penetration success rates. They are also hard to intercept. Moreover, hypersonic aircrafts have tremendous military and potential economic value. The development of a hypersonic vehicle technique involves multiple disciplines, such as high- speed aerodynamics, computational fluid dynamics, high-temperature gas dynamics, chemical dynamics, guidance and control, electronics, and material and structure fabrication. In their more than half a century of development, hypersonic

《Table 1.1.1》

Table 1.1.1 Top 10 engineering research fronts in mechanical and vehicle engineering

《Table 1.1.2》

Table 1.1.2 Annual number of core papers published for the Top 10 engineering research fronts in mechanical and vehicle engineering

vehicle techniques have achieved great success in missile weaponry but slight progress in reusable launch vehicles and aircraft. The major challenges focus on four aspects. The first aspect is the development of a new theory and technique to design new configuration with a high lift-to-drag ratio and a high payload-to-structural weight fill-in ratio under a wide-speed band. The second is to develop new propulsion system in addition to scramjet engines that work well in high-speed conditions. This new system may be the new technology that combines aero-engines and ramjets. The third is to develop extremely high- temperature resistant materials and thermal protection cooling systems and methodologies. The fourth aspect involves control and navigation systems. The complicated environment and extremely hard work states demand cutting-edge techniques for control and navigation systems. Analysis shows that the above four research directions are the key to developing reusable hypersonic launch, hypersonic aircraft, and ground take-off for space flight vehicles.

(2) Low-carbon and zero-carbon fuel engine technologies

Low-carbon and zero-carbon fuel engine technologies refer to the use of fuels with low or zero carbon content to partially or fully replace conventional high-carbon-content gasoline, diesel, and other fuels, with the goal of reducing the specific carbon dioxide emissions of engines from the source. It involves the cross-integration of power engineering and engineering thermophysics, energy science and technology, chemistry and chemical engineering, traffic and transportation engineering, materials science and engineering, and other disciplines. Related research includes the manufacturing of various low-carbon and zero-carbon fuels, their safe onboard storage and delivery, clean and effective utilization through novel engine combustion modes and control, and the analysis and optimization of their full life cycle carbon emissions. At present low-carbon and zero-carbon fuels that have received extensive public and research attention include natural gas, biomass-derived methanol, ethanol, dimethyl ether and biodiesel, green hydrogen, green ammonia, and e-fuel, which are synthesized using renewable electricity. Significant technological advancements of low-carbon and zero-carbon engines have been achieved in recent years. Various companies and research institutions have developed prototype engines that can run using carbon-free fuels, including hydrogen and ammonia. Future research directions will focus on the development of novel engine combustion and emission control technologies that can further increase the thermal efficiency of engines running on low-carbon and zero-carbon fuels while approaching zero pollutant emissions. Meanwhile, with the continuous development of renewable electricity, efficient and low-cost production of biofuel, green hydrogen, ammonia, and other related technologies, the use of low- and zero-carbon fuels in engines will eventually become widespread.

(3) Dynamically reconfigurable mobile micro-robot swarms

A mobile micro-robot swarm refers to an autonomous task execution swarm in which a large number of dispersed, low- cost isomorphic/heteromorphic autonomous mobile micro-robot individuals with local perception, decision-making, and action capabilities are self-organized according to the task objectives in wide-area and complex task scenarios by means of the mechanisms of information interaction, cooperation, and coordination among the individuals. A micro-robot swarm is characterized by a stable and orderly topology, individual coordination and collaboration, a consistent behavioral goal, and mutually complementary functions. Dynamic reconfigurability indicates the ability of a mobile micro-robot swarm to dynamically change its composition, topology, coordination relationships, and individual task loads according to changes in mission objectives, capability requirements, mission execution states and situations, environmental events, individual robots’ capabilities and others uncertainties to quickly respond to changes in the swarm and in the environment. A dynamically reconfigurable mobile micro- robot swarm is a vivid reflection and physical embodiment of a multiagent system with self-organization, self-adaption, and intelligence emergence characteristics, involving not only individual intelligence (autonomous control, autonomous perception, autonomous planning, and autonomous decision making) but also a swarm-level architecture, communication network, coordination mechanism, information fusion, state estimation, task allocation, path planning, formation control, multiagent consistency, and other multilevel, multifaceted theories and technologies. A dynamically reconfigurable mobile micro-robot swarm is also an advanced form of multi-robot system collaborative control technology. Traditional multi-robot cooperative control is usually realized through the use of a centralized control and global coordination mode, incurring difficulties in swarm scale, coordination efficacy, and expandability. Dynamically reconfigurable mobile micro-robot swarms are significantly different from traditional multi-robot cooperative control technology in terms of the number of robots involved, complexity of tasks, environment, and division of labor and collaboration as well as self-adaptation and robustness to tasks, environmental events, and so on. Furthermore, adapting existing technology to the development requirements of dynamically reconfigurable mobile micro-robot swarms is difficult. The swarming behaviors of many swarming animals in nature can serve as an inspiration for the development of dynamically reconfigurable mobile micro-robot swarms. Exploring the emergence mechanisms of swarm intelligence in swarming animals and mapping it into the field of control of dynamically reconfigurable mobile micro-robot swarms and constructing a task-oriented self-organization-based dynamically reconfigurable architecture and self-organized collaboration mechanism based on localized information propagation under complex and restricted communication conditions. Control of key technologies such as the wide-area, distributed collaborative dynamic allocation of large-scale swarm multimodal tasks; large-scale multisource heterogeneous information fusion and decentralized situational awareness under weak communication conditions; path planning and replanning, considering dynamics; formation dynamic control, including motion uncertainty information and event-triggered multi-robot coherent tracking; and so on is the key direction and a research hotspot for the development of dynamically reconfigurable mobile micro-robot swarms.

(4) Flexible self-powered wearable sensors

A flexible self-powered wearable sensor refers to a specialized device that can be attached to the surface of the human body. Its main function is to collect and monitor physiological parameters or environmental information relevant to the human body. A distinguishing characteristic of this sensor is its inherent capability to generate power autonomously, eliminating the need for an external power source. This self-powering feature contributes significantly to the sensor’s capacity to ensure wearer comfort and convenience during usage. Typically, such sensors are fabricated using flexible materials, enabling them to accommodate various body shapes and movements effectively. The key innovation lies in the incorporation of energy harvesting technology, which empowers the sensor to derive power from bodily motions or alternative ambient energy sources, such as light or temperature gradients. This self-sustaining mechanism obviates the reliance on conventional batteries or external power supplies, enhancing the overall efficiency and practicality of the wearable sensor. Flexible self-powered technology enables the conversion of micro-energy from the human body and the surrounding environment into electric energy. This technology provides active wearable sensors for human physiological and motion monitoring or prolonged working life for wearable sensors. Energy harvesting and conversion methods for flexible self-powered wearable sensors primarily encompass electromagnetic, piezoelectric, triboelectric, photovoltaic, thermoelectric, and others. Relevant research principally involves three aspects. The first one is the study of mechanisms, materials, structures, and performance enhancement for energy conversion. This study includes the development of energy harvesting devices that can be flexibly attached to the human body to enhance energy conversion efficiency. The second aspect is the combination of various environmental energy collection methods to design and fabricate flexible composite energy harvesting systems, thereby optimizing the environmental energy utilization efficiency. The third one is the energy management and signal processing technologies used to develop integrated flexible circuits to improve the utilization rate of electric energy, including the sensitivity and accuracy of active wearable sensors. Flexible self-powered wearable sensors provide an attractive direction for the development of wearable electronics with their capabilities for environmental energy harvesting, active sensing, and miniature integration. These capabilities represent the development trend of self-powered technology and body area networks, with broad application prospects in fields such as motion sensing, health monitoring, and personalized medicine.

(5) Intelligent performance test for automatic driving in an adversarial environment

An intelligent performance test for automatic driving in an adversarial environment is the key technology and means for solving the intelligent research and development, training, evaluation, rating, and verification of vehicles, ships, and other modes of transportation. Virtual simulation, scaled model, scenario test, and real vehicle/ship test are the basic methods for traditional autonomous driving testing. However, with the enhancement of digital twin technology and the evolution of sensor communication technology, a single-mode test method can no longer meet the requirements of intelligent performance testing in terms of environmental consistency, effectiveness, and repeatability. Virtual–real fusion testing that emphasizes mapping, interaction, linkage, and complementarity between digital space and physical space has become a research hotspot in intelligent performance testing. In particular, under the premise of ensuring safety and efficiency in intelligence testing, determining how to reflect game, confrontation, and cooperation between agents and between agents and humans and building an adversarial test environment are extremely important fundamental problems that are worthy of attention. Theories and methods of testing and evaluation, driving performance test and evaluation, safety test and evaluation, reliability test and evaluation, comprehensive tests, test tool chain design, and other issues are currently the major research hotspots in this field. Systems engineering concepts (e.g., model-based systems engineering, model-based design, digital twins, and cyber–physical systems), modeling and simulation technologies that are relevant to intelligent performance test are also receiving increasing attention.

(6) Multi-material 4D printing

Multi-material 4D printing is a technology that utilizes additive manufacturing processes to create intelligent components with “stimulus–response” characteristics. The stimuli mainly originate from external energy fields, such as heat, electricity, magnetism and light, and the response manifests as controllable changes in the shape, properties, or functionalities of the components over time. Traditional single-material additive manufacturing has encountered significant challenges in meeting the demands of high-end manufacturing industries for property, functionality, and their dynamic variations. Consequently, the integration of materials, structures, and functionalities in multi-material 4D printing has become a cutting-edge technology. This involves distributing different materials to designated regions and achieving predetermined changes in structures and properties through external stimuli. The inherent self-sensing and self-driven qualities of such components hold immense potential for aerospace deployable structures, biomedical scaffolds, and other critical applications. The current research focus in this field includes intelligent component design and topology optimization, the development of new 4D printing processes, numerical simulations and path planning for the printing process, micro/nano-level multi-material 4D printing, metamaterial printing, characterization methods for intelligent component property and its variations, and the regulation and optimization of multi-material interface properties. There is also increasing interest in designing intelligent components with various responses under multiple energy fields and their multi-material 4D printing, alongside high-precision and high-efficiency multi-material printing.

(7) Wireless charging system for underwater autonomous vehicles

Power endurance and sustainability are important indices for measuring the performance of underwater autonomous vehicles, mainly depending on the energy storage capacity of the onboard energy system. The method of recovering vehicles and quickly replacing their modular energy system suffers from high cost, inefficiency, and poor concealment. Underwater wireless charging technology has become a key landmark technology for enhancing the power endurance and sustainability of underwater autonomous vehicles because it achieves energy transmission through a non-contact method, which has the advantages of high safety and strong environmental adaptability. Wireless charging technology can be classified into magnetic field, electric field, microwave, laser, and ultrasonic modes according to the power transfer mechanism. Considering the complexity of the underwater environment and the particularity of the transmission medium, the magnetic field mode is considered as one of the optimal schemes for underwater charging due to its unique advantages in the principle and structure of power transmission, with significant advantages in the wireless charging process of underwater autonomous vehicles. In the principle design and engineering practice of underwater charging systems, the design of the magnetic coupling mechanism, seawater medium energy transmission and eddy current loss characteristics, bidirectional energy transmission circuit topology and control strategy, underwater bidirectional energy, and information synchronous transmission technology are the popular topics on underwater wireless charging technology.

(8) State-of-the-art vascular robotic system development

Vascular diseases have emerged as a primary threat to human health. Vascular robotic systems can offer valuable support to interventionalists during vascular interventions, such as enhancing precision, minimizing radiation exposure, and reducing workload. However, most existing vascular robotic systems use a master–slave control paradigm, where tool delivery at the slave end relies entirely on the master’s instructions. These systems lack expert-level capabilities in intelligent analysis, decision- making, and manipulation, and thus, they fail to provide effective intelligent assistance to interventionalists and restrict their widespread application. To enhance the intelligence of vascular robotic systems, key challenges, such as autonomous image navigation, active force perception, and expert skill learning, must be addressed. These challenges encompass frontier issues, such as vessel segmentation in surgical images, detection and localization of interventional tools, preoperative/intraoperative multimodal image registration, modelling tool–vessel interaction, tactile perception and feedback, manipulation skill modeling and learning, and human–robot intelligent collaboration. Moreover, focus has been increasing on the mechanism design for collaborative multi-tool delivery, intelligent multi-tool control, and precise quantitative delivery in robot-assisted intervention for complex vascular lesions.

(9) Transfer learning-based machine fault diagnosis

Transfer learning refers to the process of applying knowledge learned in one field to another. In the context of fault diagnosis, transfer learning involves using diagnostic methods validated in simulations or laboratory settings. This means that insights gained in a digital or semi-physical space can be leveraged in the actual physical space. Transfer learning can be accomplished through feature- and model-based transfer, which involve techniques, such as enhancement and fine-tuning. These methods aim to enhance the consistency of samples between the source domain and the target domain, thus reducing the disparity during the transfer process. Consequently, they improve the decision accuracy and scene generalization performance of fault diagnosis models, particularly when working with limited data and varying operating conditions. Transfer learning proves especially crucial in fault diagnosis for large complex equipment, such as aircraft engines, aerospace engines, nuclear power equipment, and space station operation equipment. In such cases, obtaining real fault samples is difficult due to the destructive nature of the faults. Transfer learning is expected to offer substantial benefits in these scenarios. Research on the theory and application of transfer learning to fault diagnosis represents the current hotspots and challenges in this field. Similar to how jade can be polished using stones from other hills, transfer learning has the potential to enhance the effectiveness of fault diagnosis for major equipment.

(10) Robot-assisted milling and polishing

Robot-assisted milling and polishing is a method of using the flexibility and reconfigurability of robots to achieve high-efficiency, high-quality, and stable machining of large and complex components in the major equipment of strategic industries, including aviation, aerospace, and navigation. Large and complex components have characteristics such as large size, complex surface, and strict requirements of shapes and dimensions, and the high-performance manufacturing of these components is a recognized international problem. Traditional manufacturing mostly uses equipment such as machine tools and machining centres, which have small travel distance and insufficient mobility. The machining of large structural parts requires multiple segmentations and slicing, which result in low manufacturing efficiency. At the same time, the process of polishing relies heavily on manual operations, resulting in inconsistent machining accuracy. Moreover, the generation of polishing dust damages the operators’ health. Robot- assisted machining has the characteristics of flexible large-space manufacturing, good reconfigurability, and easy integration and coordination. It can transform the manufacturing mode for large and complex structures and shows great application potentials in fields such as aircraft structural parts milling, large wind turbine blades, and high-speed rail body polishing. The main research hotspots in this field are the precision control of large components, the machining mechanism of robot-assisted milling and polishing, the innovative design of machining robot system, and the integration of multi-robot collaborative measurement- modeling-machining.

《1.2 Interpretations for three key engineering research fronts》

1.2 Interpretations for three key engineering research fronts

1.2.1 Hypersonic flight vehicle technology

The research on hypersonic flight vehicle technology can be traced back to the development of hypersonic gliders in the 1950s. Since the late 1990s, hypersonic flight vehicle techniques have become the research focus in several science and technology powerhouses as the scramjet technology matures. Several nations have successfully conducted experiments and flight tests, and some have used hypersonic missiles in practical missions.

The next generation of flight vehicles calls for the capability to reach any corner on earth within an hour, fast penetration ability, and launch vehicle reusability from ground to space. The hypersonic vehicle technology facilitates the objectives. Compared with traditional aircraft, hypersonic vehicles work in extremely complex environments, flights with wide speed bands, and large altitude ranges. They experience an extremely complicated flow field, and aerodynamic heating produces extremely high temperatures in the boundary layer. The combined effects make designing aerodynamic configurations and vehicle structure layouts extremely difficult. Moreover, cutting-edge propulsion techniques are required. The vehicle body structure is strongly coupled with the propulsion system. High nonlinearity and big uncertainty exist in the flight vehicle model. Thus, high accuracy in control and terminal guidance is very hard to obtain. Hypersonic aircraft technology has become a hot research frontier in the 21st century. The major challenges focus on four aspects. The first aspect involves studying the principle combining the wave rider and lift body theory to develop a configuration with a high lift-to-drag ratio. This aspect also involves studying AI- based optimization methods for lightweight structures and structures with high payload-to-vehicle fill-in ratios. The second aspect includes developing a new propulsion engine, investigating new methods to utilize the advantages of aero-engines and ram jets, studying techniques for variable sizes of engine burners, and developing adjustable flame stabilization devices. The third aspect involves studying extremely high-temperature-resistant composite material systems and ablative heat protection methods, constructing high-accuracy flow models in ablation and detachment processes, and developing accurate simulation methods and experimental test approaches. The fourth aspect includes studying the methodology for an accurate flight control dynamic model, deriving a robust intelligent control strategy to handle strong interferences and big uncertainties, and developing high-accuracy terminal guidance techniques in the hypersonic flight phase. The study in above aspects lays the foundation for developing hypersonic flight vehicle theory and technology, and is of great practical significance for China to achieve a strong aerospace force and move toward an aerospace powerhouse.

The country with the highest number of core papers published in this front is China, and the countries with the highest citations per paper are USA, Canada, and Singapore, as shown in Table 1.2.1. Among the Top 10 countries with the most published papers, China has the most collaboration with UK, Canada, and Singapore, as shown in Figure 1.2.1. Institution with the highest number of core papers published is Northwestern Polytechnical University. The institutions with the highest citations per paper are Concordia University, National University of Singapore, and Tsinghua University, as shown in Table 1.2.2. As shown in Figure 1.2.2, Northwestern Polytechnical University and Tsinghua University have the most collaboration. The top country for citing papers is China, as shown in Table 1.2.3. The Top 3 output institutions for citing papers are Northwestern Polytechnical University, Harbin Institute of Technology, and Beihang University, as shown in Table 1.2.4. Figure 1.2.3 shows the roadmap of the engineering research front of “hypersonic flight vehicle technology”.

《Table 1.2.1》

Table 1.2.1 Countries with the greatest output of core papers on “hypersonic flight vehicle technology”

《Figure 1.2.1》

Figure 1.2.1 Collaboration network among major countries in the engineering research front of “hypersonic flight vehicle technology”

《Table 1.2.2》

Table 1.2.2 Institutions with the greatest output of core papers on “hypersonic flight vehicle technology”

《Figure 1.2.2》

Figure 1.2.2 Collaboration network among major institutions in the engineering research front of “hypersonic flight vehicle technology”

《Table 1.2.3》

Table 1.2.3 Countries with the greatest output of citing papers on “hypersonic flight vehicle technology”

《Table 1.2.4》

Table 1.2.4 Institutions with the greatest output of citing papers on “hypersonic flight vehicle technology”

《Figure 1.2.3》

Figure 1.2.3 Roadmap of the engineering research front of “hypersonic flight vehicle technology”

1.2.2 Low-carbon and zero-carbon fuel engine technologies

Combustion engines are the primary power sources for road and maritime transport, off-road construction, agricultural machineries, and national defense equipment in our country. At present, most combustion engines use petroleum gasoline and diesel as fuel. While providing the necessary power, these engines emit a considerable amount of CO₂ into the atmosphere. Statistics indicate that carbon emissions from the transportation industry accounts for 10% of all carbon emissions in the country. Under the background of the “dual carbon” strategy, reducing and finally achieving zero engine carbon emissions through technological innovations is an imperative task. This goal is a prerequisite for the existence and sustainable development of combustion engines and related industries. Two technological routes are available for low-carbon engines: ① further increasing the thermal efficiency of conventional engines to reduce their specific fuel consumption, and ② partially or fully substituting conventional high-carbon-content gasoline and diesel fuels with new low-carbon-content fuels to stop carbon emissions from the source. Zero-carbon engines can only be realized through the full adoption of carbon-free or carbon-neutral fuels.

Increasing the thermal efficiency of conventional gasoline and diesel engines cannot only reduce specific carbon emissions but also help increase fuel economy. Therefore, this issue has always been the focus of research and development. For diesel engines, technological pathways for increasing thermal efficiency include boosting fuel injection pressure, compound turbocharging, exhaust heat recovery, and smart control. For gasoline engines, gasoline direction injection, variable compression ratio, stratified lean burn, gasoline compression ignition, electrification, and novel thermodynamic cycle are potential technologies for high-efficiency enablers. Zero-carbon fuels that can be used in engines are mostly hydrogen and ammonia, while life cycle carbon-neutral fuels include biomass-derived methanol, ethanol, dimethyl ether and biodiesel, and e-fuel, the latter of which are synthesized using renewable electricity combined with CO₂ captured from the air. Although carbon element is present in many carbon-neutral fuels and its combustion in engines will surely emit CO₂, its synthesis consumes CO₂, such that carbon neutrality can be achieved through the whole life cycle of these fuels. Future research is expected to focus on two aspects: ① the development of high-efficiency and low-cost techniques for the large-scale manufacturing of various carbon-neutral fuels; and ② fuel design based on the complementary combustion features of various carbon-neutral fuels, which will support the realization of zero-carbon emission engines when combined with the development of novel combustion and emission control techniques.

The combustion of hydrogen and ammonia does not produce any CO₂ emissions, and engines powered by these carbon- free fuels have been a research focus in recent years. Hydrogen engines mostly employed a spark ignition mode, and major research directions include techniques to avoid flashback and knocking, NOx emission mitigation techniques, and safe and efficient onboard hydrogen storage methods. Considering the low reactivity and low flame speed of ammonia, achieving its stable combustion across all engine conditions is challenging. Current strategies primarily include mixing ammonia with more reactive fuels, such as natural gas, gasoline, and diesel. The co-combustion of ammonia and hydrogen is also possible, with the potential of obtaining the required hydrogen from the onboard cracking of ammonia. Future research is expected to focus on high-efficiency and clean combustion modes for ammonia, highly effective onboard selective catalytic reduction, and ammonia cracking systems.

Low-carbon and zero-carbon fuel engine technology involves the fields of power engineering and engineering thermophysics, energy science and technology, chemistry and chemical engineering, traffic and transportation engineering, materials science and engineering, and other related disciplines. It is a key technology for ensuring the achievement of the “dual carbon” strategy in the transportation industry and promoting the sustainable development of combustion engines and other related industries under the “dual carbon” background. It is an area of high research value and societal impact.

The top countries with the maximum number of core papers published in this front are China, UK, and India, and the countries with the highest citations per paper are Saudi Arabia, UK, and Canada, as shown in Table 1.2.5. China has the most collaboration with UK and with Ireland, as shown in Figure 1.2.4. Institutions with the maximum number of highest citations per paper are University of Oxford, Tsinghua University, and National Institute of Technology, as shown in Table 1.2.6. There is cooperation between Dalian Maritime University and Trinity College Dublin, as well as between Xi’an Jiaotong University and Beijing Institute of Technology. Cooperation also exists between Tsinghua University and the University of Oxford, as shown in Figure 1.2.5. The top country for citing papers is China, as shown in Table 1.2.7. The Top 3 output institutions for citing papers are Xi’an Jiaotong University, Chinese Academy of Sciences, and Beijing Institute of Technology, as shown in Table 1.2.8. Figure 1.2.6 shows the roadmap of the engineering research front of “low-carbon and zero-carbon fuel engine technologies”.

《Table 1.2.5》

Table 1.2.5 Countries with the greatest output of core papers on “low-carbon and zero-carbon fuel engine technologies”

《Figure 1.2.4》

Figure 1.2.4 Collaboration network among major countries in the engineering research front of “low-carbon and zero-carbon fuel engine technologies”

《Table 1.2.6》

Table 1.2.6 Institutions with the greatest output of core papers on “low-carbon and zero-carbon fuel engine technologies”

《Figure 1.2.5》

Figure 1.2.5 Collaboration network among major institutions in the engineering research front of “low-carbon and zero-carbon fuel engine technologies”

《Table 1.2.7》

Table 1.2.7 Countries with the greatest output of citing papers on “low-carbon and zero-carbon fuel engine technologies”

《Table 1.2.8》

Table 1.2.8 Institutions with the greatest output of citing papers on “low-carbon and zero-carbon fuel engine technologies”

《Figure 1.2.6》

Figure 1.2.6 Roadmap of the engineering research front of “low-carbon and zero-carbon fuel engine technologies”

1.2.3 Dynamically reconfigurable mobile micro-robot swarms

As the application requirements of intelligent mobile robots expand toward the direction of complex multimodal tasks, three-dimensional wide-area operation, complex and changeable working environments, and interference and confrontation, meeting the functional and performance requirements of single complex and expensive robots has become difficult. The use of a large number of dynamically reconfigurable, and relatively low-cost isomorphic/heterogeneous autonomous mobile micro-robots to form a task execution swarm, giving full play to the advantages of group division of labor, can not only improve the efficiency and robustness of task completion in complex environments but also reduce the cost of developing special complex system equipment and obtain significant economic benefits. Dynamically reconfigurable mobile micro-robot swarms have become an important development direction in the field of intelligent mobile robots and have broad application prospects in intelligent manufacturing factories, area coverage exploration, wide-area target search, and military operations.

Dynamically reconfigurable mobile micro-robot swarms include aerial intelligent unmanned aerial vehicle swarms, ground mobile robot swarms, water surface and underwater unmanned vehicles, and other forms, as well as the mixed swarms of the aforementioned forms. The individual robots in a swarm can be isomorphic or heterogeneous. The overall behavior of a dynamically reconfigurable mobile micro-robot swarm derives from the mutual coordination and cooperation of the individual robots with autonomous abilities based on the task goal, which is a vivid reflection and physical embodiment of the self- organization, self-adaptability, and intelligence emergence of a multiagent system and an advanced form of the collaborative control technology of a multi-robot system.

Dynamically reconfigurable mobile micro-robot swarm technology involves not only individual intelligence (autonomous control, autonomous perception, autonomous planning, and autonomous decision-making) but also multilevel and multifaceted theories and technologies such as a swarm architecture, communication network, coordination mechanism, information fusion, state estimation, task assignment, path planning, formation control, and multiagent consistency at the swarm level. Traditional multi- robot cooperative control mainly involves a hierarchical control architecture, strongly connected communication network, collaboration and coordination based on global information, centralized task allocation and scheduling, and pilot-following formation control. Improving a swarm’s scale, collaborative efficiency, and scalability is difficult, and a swarm’s scale is generally limited to several dozens of individuals. A dynamically reconfigurable mobile micro-robot swarm usually has a number of individuals reaching several hundred or several thousand or more, and the requirements for complex tasks, environments, division of labor and cooperation as well as adaptability and robustness to tasks and environmental events are significantly different from those of traditional multi-robot cooperative control technology. Meeting the development requirements of dynamically reconfigurable mobile micro-robot swarms is difficult for existing collaborative technology; thus, swarm theory and technology breakthroughs are urgently needed.

Modern biological studies revealed that some typical swarming animals, such as birds, fish, ants, wolves, and so on, can perceive the situation of and changes in the swarm and make decisions that are consistent with the behavioral goals of the swarm by using only the local information of the swarm and information of several neighboring individuals, thereby facilitating the emergence of the overall behavior of the swarm. Animal swarm behavior can serve as an inspiration and reference for the development of dynamically reconfigurable mobile micro-robot swarms. The intelligence emergence mechanism of swarm animals should be explored and mapped in the field of swarm control of dynamically reconfigurable mobile micro-robots, and a task-oriented dynamically reconfigurable architecture based on self-organization and a self- organization cooperation mechanism based on local information propagation under complex limited communication conditions should be built. Control of key technologies such as the dynamic assignment of large-scale distributed cooperative tasks in wide-area swarms; large-scale multisource heterogeneous information fusion and decentralized situation awareness under weak communication conditions; path planning and replanning, considering dynamic characteristics; dynamic formation control, including motion uncertainty information and event-based multi-robot consistency tracking; and so on has become a research hotspot for the development of dynamically reconfigurable mobile micro-robot swarms.

The top country with the maximum number of core papers published in this front is China, and the country with the highest citations per paper is USA, as shown in Table 1.2.9. Collaboration exist between China and USA, as well as between China and Switzerland. Collaboration can also be found between Germany and Turkey, as shown in Figure 1.2.7. Institution with the maximum number of highest citations per paper is the Chinese University of Hong Kong, and the top three institutions with the highest citations per paper are Beijing Institute of Technology, Michigan State University, and Harbin Institute of Technology, as shown in Table 1.2.10. Collaboration exists between Harbin Institute of Technology, Beijing Institute of Technology, and Michigan State University. Koç University and Max Planck Institute for Intelligent Systems also have a collaboration. The Chinese University of Hong Kong has collaboration with Chinese Academy of Sciences, University of Missouri, the Hong Kong Polytechnic University, and Shenzhen Institute of Artificial Intelligence and Robotics Society (wherein Chinese Academy of Sciences collaborates with University of Missouri), as shown in Figure 1.2.8. The top country for citing papers is China, as shown in Table 1.2.11. The Top 3 output institutions for citing papers are the Chinese University of Hong Kong, Chinese Academy of Sciences, and Harbin Institute of Technology, as shown in Table 1.2.12. Figure 1.2.9 shows the roadmap of the engineering research front of “dynamically reconfigurable mobile micro-robot swarms”.

《Table 1.2.9》

Table 1.2.9 Countries with the greatest output of core papers on “dynamically reconfigurable mobile micro-robot swarms”

《Figure 1.2.7》

Figure 1.2.7 Collaboration network among major countries in the engineering research front of “dynamically reconfigurable mobile micro- robot swarms”

《Table 1.2.10》

Table 1.2.10 Institutions with the greatest output of core papers on “dynamically reconfigurable mobile micro-robot swarms”

《Figure 1.2.8》

Figure 1.2.8 Collaboration network among major institutions in the engineering research front of “dynamically reconfigurable mobile micro-robot swarms”

《Table 1.2.11》

Table 1.2.11 Countries with the greatest output of citing papers on “dynamically reconfigurable mobile micro-robot swarms”

《Table 1.2.12》

Table 1.2.12 Institutions with the greatest output of citing papers on “dynamically reconfigurable mobile micro-robot swarms”

《Figure 1.2.9》

Figure 1.2.9 Roadmap of the engineering research front of “dynamically reconfigurable mobile micro-robot swarms”

《2 Engineering development fronts》

2 Engineering development fronts

《2.1 Trends in Top 10 engineering development fronts》

2.1 Trends in Top 10 engineering development fronts

Top ten development (as opposed to research) fronts in mechanical and vehicle engineering are listed in Table 2.1.1. Some of these fronts are characterized by in-depth traditional research: “multi-robot collaborative operation optimization technology”, “unmanned aerial vehicle path planning technology”, “precision guidance technology for MUAV”¹, “AI-based precise target recognition”, “multi-functional high-performance aerospace composites technology”, “micro high-performance combinational sensing technology”, and “control and perception system of intelligent mobile robot”. There are also other fronts that are newly emerging: “low-cost reusable spacecraft”, “underwater unmanned rescue vehicle”, “energy integration and propellant management technology for space transportation systems”. Table 2.1.2 shows the annual number of core patents published from

《Table 2.1.1》

Table 2.1.1 Top 10 engineering development fronts in mechanical and vehicle engineering

《Table 2.1.2》

Table 2.1.2 Annual number of core patents published for the Top 10 engineering development fronts in mechanical and vehicle engineering