《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 Top 10 engineering research fronts 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 them, “flexible robotic endoscopy systems for minimally invasive surgery”, “robotized additive manufacturing”, “quasi-zero stiffness vibration isolation method”, “accurate path tracking control for unmanned vehicles”, and “hybrid renewable energy power generation” are extensively studied traditional topics. “Origami-inspired metamaterials”, “micro insect-inspired flapping-wing vehicle”, “aircraft digital twin technology”, “unmanned underwater vehicle”, and “deep learning-based methods for intelligent urban traffic flow prediction” are considered as emerging topics.

Papers annually published in 2015–2020 are listed in Table 1.1.2. “Accurate path tracking control for unmanned vehicles”and “aircraft digital twin technology” are the most rapidly growing topics in terms of paper publications in recent years.

(1)   Flexible robotic endoscopy systems for minimally invasive surgery

Flexible endoscopy typically involves advancing slender surgical instruments along narrow and tortuous anatomical paths to reach the target disease sites through natural orifices, implementing dexterous observation and exploration, and further performing complex and delicate operations. It has mainly gone through three stages: endoscopic exploration, manual endoscopic operation platform and robot-assisted endoscopic minimally invasive surgery (MIS) system. Related research can be mainly categorized into three aspects, which are studies on the design methods of minimally invasive surgical instruments, the implementation and integration of multimodal sensing units, and the control strategies of movement coordination. In terms of investigating minimally invasive surgical instrument design methods, research focuses on the exploration of the motion and deformation mechanism of continuum surgical instruments and the development of multiple types of surgical instruments with hybrid rigid and flexible functions. Regarding the implementation and integration of multimodal sensing

《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

units, key exploration includes the development of high- precision self-sensing/perceptive (force/shape) units, the cross-modulus integration of different sensing units, and visual guidance-based technology. With respect to the control strategies of movement coordination, further studies should address advanced control strategies for movement coordination based on multimodal information fusion with constrains of the human body cavity environment and provide a solid theoretical foundation for the dexterous movement and stable operation of flexible surgical instruments during operation. With its advantages of high flexibility and excellent compliance and adaptability, a flexible robotic endoscope system can well respond to the challenges associated with the constraints of complicated anatomical structures and narrow cavities. Therefore, it represents the developmental trend of being “scar-free” in modern minimally invasive surgery. Broad application prospects can also be envisaged for disease diagnosis in different human cavities and tracts, and integrated diagnosis and treatment for early cancer.

(2)  Unmanned underwater vehicle

An unmanned underwater vehicle is a device that can assist or substitute people to complete some underwater interventions. It has a perception system, and several tools, including manipulators, can be operated autonomously or through remote control. Studies on unmanned underwater vehicles have explored the interdisciplinary integration of naval architecture and ocean engineering, computer science and technology, artificial intelligence and automation, and materials science and engineering. It is a kind of special equipment belonging to the high-end manufacturing industry. Unmanned underwater vehicles and their industry chain are strategic emerging industries for maritime power construction. They have shown good application prospects in marine scientific research, marine resources investigation, marine security, and defense. The main research directions of unmanned underwater vehicles include system design, structural design, power and propulsion system, electrical control, navigation and positioning, path planning, motion control, emergency safety, deployment and docking, and cluster formation among others. In response to the major needs of deep-sea technology development, Chinese scholars have successively broken through key technologies such as overall design, structure optimization, mechatronics, underwater acoustic positioning and navigation, autonomous path planning, and motion control of deep-sea vehicles at theoretical and application levels. We have the capability to develop small and medium scaled unmanned underwater vehicles that meet certain engineering application requirements (such as “Qianlong” series and “Haidou” remote control/autonomous underwater robots and “Haiyi” and “Haiyan” underwater gliders). However, given the complicated marine environment, accurate operation tasks, and diversified operation requirements, unmanned underwater vehicles still face many challenges, including the development of large- scale and intelligent unmanned underwater vehicles, path planning and ADRC, cluster formation, underwater real- time positioning and mapping, underwater detection and target recognition, underwater energy supply and wireless communication, bionic software underwater robot, and operational underwater vehicles. These domains will be the frontier and hot spots of unmanned underwater vehicle technology. Through vigorously carrying out basic scientific research in these domains, scientific and technological self- reliance and self-control in high-end marine equipment can be further strengthened in China.

(3)  Micro insect-inspired flapping-wing vehicle

A micro insect-inspired flapping-wing vehicle is a new type of unmanned aerial vehicle that mimics the flight of insects. Its flight efficiency and maneuverability can be increased by observing an insect’s body structure and locomotion mechanism, combining engineering system designs, and exploring bionics. Related research includes four aspects. First, the design methodology for a high-efficiency flapping wing system involves the analysis of the influence of insects’ body structure and locomotion parameters related to the aerodynamic performance of a flapping wing system, the discovery of the unsteady aerodynamic coupling mechanism on the excellent flying ability of insects, and the parametric modeling and optimization design of the flapping wing system under the coupling of structure and aerodynamic condition to achieve high-aerodynamic performance and low-power consumption. Second, a high-efficiency energy and transmission micro-system covers research on the development of a multi-type energy battery based on metal ion, two-dimensional material, optoelectronic film and polymer, and design of intelligent energy management strategy to achieve high-efficiency, lightweight, and high- stability energy system; the development of micro drivers and actuators based on smart piezoelectric, artificial muscle materials, and precision gear; and studies on preparation and molding fabrication methods to achieve high-precision, high-efficiency, and high-reliability micromechanical drive and transmission system. Third, high-density lightweight micro airborne avionic system includes designing a MEMS- based information-fusion intelligent micro mission payload system to sense environment information, such as voice, light thermal source, and electric magnetic signal; establishing a high-performance intelligent fight-control self-learning strategy and the corresponding control system under the coupling condition between multisource disturbances and nonlinear systems; creating long-distance, low-power, and high-efficiency information communication systems linking airborne equipment and ground station system; and developing chip-level flexible fabrication methodology to integrate a mission payload and information communication system with a flight control system for high-density and lightweight micro airborne avionic systems. Fourth, the integration of whole aircraft and experimental platforms involves research on the integrated modeling and design strategy that focuses on bionics under multidisciplinary coupling conditions, the development of a comprehensive experimental test platform, and the establishment of data acquisition and analysis systems for bionic MAV. With the development of bionics, unsteady aerodynamics, micro- electro-mechanical systems, flexible materials, control theory, artificial intelligence, and other technologies, studies have widely explored miniaturization, lightweight, and swarm intellectualization for micro insect-inspired flapping-wing vehicles.

(4)  Robotized additive manufacturing

With the growing diversification and complexity of additive manufacturing objects, traditional three-axis Cartesian coordinate machines or gantry additive manufacturing equipment is increasingly limited by their operating space, degrees of freedom, and strict layer-by-layer manufacturing, which cannot meet the needs of manufacturing quality, efficiency, and cost. Robotized additive manufacturing is one of the future trends of additive manufacturing. Combining highly flexible robotics with additive manufacturing technology provides the possibility of multi-axis additive and complex geometry manufacturing in nonstructural environments; thus, it has become a hot research topic in the field. The main research directions of robotized additive manufacturing include multidirectional additive manufacturing, which uses robots with multiple degrees of freedom to achieve multidirectional printing; conformal layer additive manufacturing, which aims to use robots for curved surface conformal manufacturing; prefabricated component assembly, which involves robot dexterity to achieve the embedding of complex prefabricated parts; supportless additive manufacturing, which utilizes robot dexterity to avoid the time-consuming and material-intensive printing of support structures; and large-scale additive manufacturing, which involves robots that can be used to achieve large- scale additive manufacturing because of its large working space. Next-generation robotized additive manufacturing will achieve breakthroughs in the following directions: using robots to switch manufacturing tools flexibly for the additive manufacture of multimaterial components; employing robot accessibility to balance large- and small-scale details for the in situ manufacture of oversized components with small-scale structures; reaching a robot’s own precision limits for high- precision additive manufacturing; developing multirobot collaboration technology to improve manufacturing efficiency; and seeking breakthroughs in microrobot technology for the additive manufacture of microscale parts.

(5)  Quasi-zero stiffness vibration isolation method

With the development of high-end equipment to extreme working conditions or ultimate performance, such as ultra-precision manufacturing and measurement, space exploration, carrier, and weapon equipment, environmental vibration is complex, and low frequency is prominent. Ultra- low frequency vibration isolation requires that the natural frequency of vibration isolation is near zero, that is, stiffness is near zero, which cannot be achieved through traditional vibration isolation methods. In quasi-zero stiffness vibration isolation method, near-zero vibration transmission can be achieved by developing vibration isolators that have a high load capacity and quasi -zero stiffness. Its main idea is to introduce a negative stiffness mechanism to counteract the positive stiffness of isolators. Quasi-zero stiffness can be obtained through passive and active ways. Passive ones include mechanical and magnetic negative stiffness. The principle of mechanical negative stiffness is simple, but its nonlinearity and contact friction are significant; therefore, its working area is small, and its performance is limited. With magnetic negative stiffness, a noncontact force performs negative stiffness characteristics by using specially arranged magnets. This process is a good way to realize quasi-zero stiffness vibration isolation. Studies have focused on improving the density, linearity, and working range of magnetic negative stiffness via magnetic structure and array optimization. The active ways usually adopt vibration control force via displacement feedback to exhibit negative stiffness characteristics. If the accuracy of a sensor and an actuator is enough, negative stiffness can be quite accurate; theoretically, the best quasi-zero stiffness effect can be obtained. It is an important research area of quasi-zero stiffness vibration isolation. Another way of quasi-zero stiffness vibration isolation is to connect the precision displacement servo mechanism in series with an ultra-low (or quasi-zero) stiffness vibration isolator, and the follow-up servo control is adopted to reduce comprehensive stiffness and expand the working area by an order of magnitude. It is a potential development area.

(6)  Origami-inspired metamaterials

Metamaterials/metastructures are man-made materials or structures whose internal constituents and topologies allow them to have unusual emergent properties. Origami- inspired metamaterials have been developed with various advanced functions. Notably, traditional origami and modular origami structures provide metamaterials with enriched 3D surfaces or internal topologies so that they exhibit unusual electromagnetic, thermal, acoustic, or mechanical properties. The folding/unfolding mechanism motion or deformation of origami structures allow metamaterials to generate wide morphing scopes on their properties. Therefore, research frontier on origami-inspired metamaterials has entered a new arena, incorporating programmability and tunability, making properties that can be adjustable through design parameter selection and tunability via actively controlled or passively deformed topology transformation. Hence, the major challenge is to set up a fundamental relationship among the structural topological geometry, base material parameters, and physical properties of metamaterials. As a result, the development trends of origami-inspired metamaterials can be extended into three stages. First, studies should focus on the synthesis of origami unit cells based on the requirement of property programmability rather than taking existing origami structures as unit cells to analyze the corresponding properties. Second, studies should aim at the accurate deformation control of metamaterials by the embedded actuation or external stimuli, which fulfils the task of property tunability. Third, studies should integrate unit cell topologies and constituent materials corresponding to different types of physical properties into one metamaterial to generate a super metamaterial with multiple tunable properties. Origami- inspired metamaterials, which are directly proposed on the basis of application requirements, will be theoretically novel, multidisciplinary, and engineering practical, thus becoming one of the highlights on metamaterial research.

(7)  Accurate path tracking control for unmanned vehicles

Accurate path tracking control for unmanned vehicles to follow a desired path is the key technology to guarantee the safety, stability and riding comfort in autonomous driving. Traditional methods are mostly based on the static linear model or massive manual tuning of algorithm parameters, including PID control, feedback-feedforward control, optimal control, etc. These methods can operate stably under the designed working conditions, but they are usually sensitive to changes in working conditions and model parameters and have poor performance in actively adapting to dynamic environmental changes. For high-speed dynamic scenes, control algorithms that comprehensively consider uncertainty, model generalization and adaptability in working conditions are the main research direction to achieve accurate path tracking control in all working conditions, such as model predictive control or intelligent control based on model learning or parameter optimization. Dynamic models are usually highly nonlinear with numerous model parameters, so model-based methods should balance between the model fidelity and real-time performance of the algorithm, while dynamically adjust the model parameters according to changes in working conditions. Therefore, a hybrid control strategy can be constructed by combining adaptive control, intelligent control and optimal control to meet the requirements of these two aspects. In addition, machine learning can use offline experience for learning and fitting complex nonlinear models, so the identification of model parameters and online parameter updating through machine learning methods can significantly reduce difficulties in tuning algorithm parameters, improve model adaptability, and increase control accuracy. Model-free intelligent control methods based on direct model learning, such as approximate optimal control, end-to-end neural network control, and life- long policy learning, have become one of the most important research directions because of their extensive advantages in self-updating, self-adaptation, and generalization ability.

(8)  Aircraft digital twin technology

Aircraft digital twin is the seamless integration of digital twin technology with the key features, steps, and scenarios of aircrafts. It uses high-fidelity models and historical and real- time data to portray, simulate, and monitor physical aircraft. It also uses cloud computing, big data, artificial intelligence, and other techniques to analyze, evaluate, predict, and optimize aircraft for improving design efficiency, monitoring operating status, reducing operation and maintenance costs, and extending aircraft service life. Related research can be mainly divided into four aspects. First, high-fidelity model construction for aircrafts fully considers a coupling relationship between various disciplines and fields. Aircraft digital twin models integrate multi-domain models, such as structure, aerodynamics, and control, to describe the attributes of aircraft accurately. Second, to realize comprehensive perception and processing of aircraft data, sensors should be arranged properly according to specific objects, and aircraft-related information, such as structural status and load changes, as well as environmental information, such as temperature, humidity, and electromagnetic radiation, should be obtained. Through data fusion between physical data and virtual data, rich information can be mined to provide support for aircraft fault diagnosis and life prediction. Third, the consistency of virtual and real interaction should be maintained to ensure that an aircraft digital twin model is consistent with the actual operating state of the physical aircraft, the technologies of virtual–real mapping, data–model linkage, and interaction control are under development. Fourth, research has been performed on the service of aircraft digital twin, including efficient aircraft design, virtual testing and verification, real-time status monitoring, remaining life prediction, maintenance strategy optimization, etc. In the future, digital twin aircraft will continue to be developed toward more realistic models, more comprehensive data, more real-time interaction, and more convenient services.

(9)  Hybrid renewable energy power generation

Hybrid renewable energy power generation refers to a power generation method that involves two or more distributed renewable power generation sources (e.g., wind, solar, tidal, and geothermal energy), energy storage systems, and various electric or electronic control devices. The electrical output of hybrid renewable power generation systems is more stable, reliable, and sustainable. The distributed systems can reduce costs by sharing grid connections. To maximize the benefits of multi-energy complementarity, current research frontiers have focused on targeted capacity optimization, system design, and planning. In particular, their development can be promoted through the construction of comprehensive mathematical abstraction models, research on expression paradigms for multi-energy complementarity, and the establishment of comprehensive, general, and standardized descriptions of complementarity. Current problems involve the integration of currents generated by different renewable energy sources, the selection and deployment of geographic location, the stochastic simulation of renewable energy generation systems, the selection of equipment types, more comprehensive system modeling, and efficient optimization- seeking algorithms to provide new breakthroughs for the enhancement of the in-depth optimization of hybrid renewable energy systems.

(10)  Deep learning-based methods for intelligent urban traffic flow prediction

As a key enabling technology of intelligent transportation systems, urban traffic flow prediction can provide technical support for intelligent urban mobility and dynamic transportation planning to alleviate road traffic congestion, facilitate people mobility, and form innovative services of intelligent transportation systems. Current studies on urban traffic flow prediction have not yet formed a complete technical solution, which needs to overcome many difficult problems. These problems include the following. ① The data of road traffic flow have rich information and high- degree nonlinearity and complexity. Thus, utilizing the data fully is difficult. ② Efficient data-driven prediction models, methods, and technologies are lacking. Current intelligent traffic flow prediction technologies include long-term prediction (time series analysis) and short-term prediction (dynamic real-time traffic prediction). Many research results based on deep learning for traffic flow prediction have been published: ① deep learning models, including temporal graph convolutional networks (T-GCN), spatiotemporal fusion graph deep neural networks, diffusion convolutional recurrent neural networks (RNN), and multi-branch prediction models; ② deep learning frameworks, including spatiotemporal multi-task learning frameworks, two-way long- and short- term memory (LSTM) artificial neural networks, and temporal information-enhanced LSTM; and ③ hybrid deep learning methods, including stacked learner + fully connected networks (FCN) and hybrid CNN + RNN + attention mechanisms. This active research topic has been widely explored. Future research directions on this topic will involve the analysis of the temporal and spatial relationships of traffic data, the integration of long- and short-term prediction technologies, new deep learning models, and creative solutions and services in specific traffic scenarios.

《1.2 Interpretations for three key engineering research fronts》

1.2 Interpretations for three key engineering research fronts

1.2.1 Flexible robotic endoscopy systems for minimally invasive surgery

Cancer has become the main threat and killer of human health. The number of cancer incidences and deaths in China ranks first in the world. Among them, the incidence and mortality of respiratory and digestive tract cancers are relatively high, accounting for more than 50% of all cancer incidences and deaths. Due to the influence of the air environment and dietary habits, the prevalence rate of gastrointestinal and respiratory cancers in China is particularly high, accounting for about 50% of which in the world. Compared with the disadvantages of traditional rigid endoscopes, namely the large size and the difficulty during operation in narrow body cavities and tracts, flexible robotic endoscopic systems are widely applied to disease diagnosis and cancer treatment in digestive tracts (esophagus, stomach and colorectal system), respiratory tracts (bronchial tubes), and the genitourinary system (vaginal, urethra, and bladder), because of their advantages in terms of high flexibility and excellent compliance and adaptability. These merits also provide strong support for the timely and integrated diagnosis and treatment of early cancers in various types of human natural cavities. In recent years, the emergence of multimodality (image, ultrasound, molecular imaging, etc.) and cross-scale endoscopic imaging technology (from centimeter to molecular level) has paved the way for more accurate exploration and detection, timelier diagnosis, and more minimally invasive operations, which represent the developmental trend of “scar-free” in modern minimally invasive surgery.

Extensive research has been conducted to target cancer treatments in different human natural cavities by developing flexible endoscopic robotic technology. Related technology has mainly undergone three developmental stages from endoscopic exploration to manual endoscopic operating platforms and robot-assisted endoscopic minimally invasive surgery robot systems. Current related research mainly lies in three aspects: developing design theories for minimally invasive surgical instruments, devising and integrating multimodal sensing units, and realizing advanced control strategies for movement coordination. In terms of developing design theories for minimally invasive surgical instruments, exploring the motion and deformation mechanism of continuum surgical instruments, establishing the kinematics and mechanical models of flexible surgical instruments, and studying the mechanism of variable stiffness and the transformation mechanism of rigid-flexible configuration have received great attention. In the meantime, realizing the integrated design of materials/structures/variable stiffness, and summarizing the design criteria methods of minimally invasive surgical instruments are also the foci of future research. Regarding the design and integration of the multimodal sensing units, key investigations involve the design criteria of tactile sensing units and self-perceptive shape sensing sensors, the cross-modulus integration of different sensing units, multitype image fusion, and visual guidance-based technology to achieve the fusion of visual and tactile information and the integration of macro-micro information. With respect to advanced control strategies for movement coordination, realizing multimodal information fusion with constrains of the human body cavity environment, combining the application of dynamic feedforward and feedback controllers, accomplishing the coordinated control of motion and stiffness, and providing a solid theoretical basis for the dexterous motion and stable operation of flexible surgical instruments during minimally invasive surgery are considered worthy of scholarly attention. As the next challenging and technical acme in the field of surgical robots, flexible endoscopic surgical robots offer solid technical support to realize the integration of diagnosis and treatment of early cancers in various types of narrow cavities and tracts, which further meet the actual requirements for MIS with safe contact, flexible reachability, rigid operation, and precise control. The associated merits provide essential tools and technical approaches for exploring new surgical procedures which combine the merits of internal medicine and surgery. The development of flexible endoscopic minimally invasive surgical robots can improve our country’s independent technical levels in the field of surgical robots, promote the popularization and application of narrow cavity and tract surgery, and achieve the leading position in global high-end medical equipment research and development.

Countries with the highest number of core papers published on “flexible robotic endoscopy systems for minimally invasive surgery” are Singapore and China, and countries dominant in citations are Germany and the USA as seen in Table 1.2.1. Among the Top 6 countries with the most published papers, China has more cooperation with Singapore, as shown in Figure 1.2.1. Institutions with the highest number of core papers published are the Chinese University of Hong Kong, National University of Singapore, and Nanyang Technological University. Top institutions on citation frequency are Carnegie Mellon University, Leibniz University Hannover, and the University of Tennessee, as shown in Table 1.2.2. National University of Singapore and the Chinese University of Hong Kong have a lot of cooperation, as shown in Figure 1.2.2. The top countries for citing core papers are China and the USA, as shown in Table 1.2.3. The main output institutions for citing core papers are the Chinese University of Hong Kong, National University of Singapore, and Harbin Institute of Technology, as shown in Table 1.2.4.

1.2.2 Unmanned underwater vehicle

Unmanned underwater vehicles are important for exploring oceans. To explore marine resources, the international community began the development of unmanned underwater vehicles as early as the 1950s. In the 1970s, underwater autonomous vehicles began to be used in military tasks, such as searching for wrecked submarines and anti-mine operation by remote control. Since the 21st century, with the development of computer science and technology, integrated navigation, acoustic communication, and other technologies, the autonomy degree of unmanned underwater vehicles has improved, and their industrial application has gradually matured. This technology gradually plays a more important role in civil and military fields. For example, since 2000, the US Department of Defense has successively issued six roadmaps for the development of unmanned underwater

《Table 1.2.1》

Table 1.2.1 Countries with the greatest output of core papers on “flexible robotic endoscopy systems for minimally invasive surgery”

《Table 1.2.2》

Table 1.2.2 Institutions with the greatest output of core papers on “flexible robotic endoscopy systems for minimally invasive surgery”

《Figure 1.2.1》

Figure 1.2.1 Collaboration network among major countries in the engineering research front of “flexible robotic endoscopy systems for minimally invasive surgery”

《Figure 1.2.2》

Figure 1.2.2 Collaboration network among major institutions in the engineering research front of “flexible robotic endoscopy systems for minimally invasive surgery”

《Table 1.2.3》

Table 1.2.3 Countries with the greatest output of citing papers on “flexible robotic endoscopy systems for minimally invasive surgery”

《Table 1.2.4》

Table 1.2.4 Institutions with the greatest output of citing papers on “flexible robotic endoscopy systems for minimally invasive surgery”

vehicles and fully confirmed the important military value of unmanned underwater vehicles. It has also proposed strengthening relevant research on key technologies, such as large-scale underwater vehicles, special operation vehicles, and underwater distributed networks. The European Defense Agency has also issued the roadmap and methodology of maritime unmanned systems to coordinate the forces of European countries and jointly promote the technological development of unmanned underwater vehicles.

An unmanned underwater vehicle is an underwater carrier platform with high efficiency, low cost, and strong stealth. Hence, unmanned underwater vehicles have been recently applied in marine resource exploration, seabed topographic and geomorphologic investigations, marine intelligence, surveillance and reconnaissance, underwater anti-submarine, and anti-mine. However, several key technologies are needed before unmanned underwater vehicles can be applied in complicated marine environment and diversified operation tasks. At present, the main research domains can be divided into the following six aspects:

1)   Planning and control: uncertainty and disturbance compensation control of a single unmanned underwater vehicle and cooperative path planning and cluster formation control of multiple unmanned underwater vehicles.

2)   Operation tool control: cooperative control strategy of unmanned underwater vehicles and manipulators, including neural network control, fuzzy adaptive control, and model predictive control.

3)  Coupling effects: coupling dynamic modeling of unmanned underwater vehicles and manipulators, redundant degree of freedom motion planning, soft manipulator/claw design, and robust control.

4)   Perception and recognition: environmental situation awareness of unmanned underwater vehicles, including underwater real-time positioning and mapping, and underwater target detection and recognition.

5)   Energy and communication: high-energy-density lithium battery, hydrogen energy, underwater docking charging, network and navigation technology, and underwater optical wireless communication.

6)   New concept underwater vehicle design and modeling: bionic and soft robots.

The top country with the maximum number of core papers published in the engineering research front of “unmanned underwater vehicle” is China. The top countries cited by frequency are Japan, New Zealand, and China, as shown in Table 1.2.5. China has published cooperative studies with Japan, New Zealand, Australia, and Saudi Arabia, and Australia has published cooperative studies with Saudi Arabia, as shown in Figure 1.2.3. Institutions with the maximum number of core papers are Nanjing University and Nanjing University of Information Science & Technology. Institutions with the maximum frequency of citations are Kyushu Institute of Technology, Northwestern Polytechnical University, Shanghai Jiao Tong University, University of Electronic Science and Technology of China, Hangzhou Dianzi University, Southwest Forestry University, and Zhongnan University of Economics and Law, as shown in Table 1.2.6. More cooperation is observed between Nanjing University and Nanjing University of Information Science & Technology, as shown in Figure 1.2.4. The top three countries for citing core papers are China, the USA, and Saudi Arabia, as shown in Table 1.2.7. The main output institutions for citing core papers are Nanjing University of Information Science & Technology, Qufu Normal University, and Anhui University, as shown in Table 1.2.8.

1.2.3 Micro insect-inspired flapping-wing vehicle

Compared with the traditional fixed-wing and rotorcraft

《Table 1.2.5》

Table 1.2.5 Countries with the greatest output of core papers on “unmanned underwater vehicle”

《Figure 1.2.3》

Figure 1.2.3 Collaboration network among major countries in the engineering research front of “unmanned underwater vehicle”

《Table 1.2.6》

Table 1.2.6 Institutions with the greatest output of core papers on “unmanned underwater vehicle”

《Figure 1.2.4》

Figure 1.2.4 Collaboration network among major institutions in the engineering research front of “unmanned underwater vehicle”

《Table 1.2.7》

Table 1.2.7 Countries with the greatest output of citing papers on “unmanned underwater vehicle”

《Table 1.2.8》

Table 1.2.8 Institutions with the greatest output of citing papers on “unmanned underwater vehicle”

unmanned aircraft, micro insect-inspired flapping-wing vehicles feature small size, lightweight, low cost, strong concealment, and superb agility. Such vehicles can operate missions such as low-altitude reconnaissance, urban combat, electronic warfare, chemical and nuclear threat detection, geographical survey, natural disaster monitoring and support, environmental pollution measurement, and border patrol. They have fundamental and comprehensive application prospects in civil and national defense. Considering the advantages and excellent application potential of micro flapping-wing aircraft, researchers and engineers from the USA, Germany, Japan, and South Korea have studied bionic micro bionic flapping-wing vehicles at various scales, such as dragonflies, butterflies, bees, and mosquitoes. Harvard University, Carnegie Mellon University, University of California, Berkeley, Northwestern Polytechnical University, Shanghai Jiaotong University, and other institutions developed piezoelectric driven insect-like flapping-wing vehicles. Delft University, Carnegie Mellon University, Purdue University, Konkuk University, Free University of Brussels, Northwestern Polytechnical University, Harbin Institute of Technology, Beijing University of Aeronautics and Astronautics, Beijing University of Science and Technology, and other institutions developed motor-driven insect-like flapping wing aircraft. Other driving forms of insect-like flapping wing aircraft, such as electromagnetic drive, chemical muscle drive, and memory material drive, have also attracted attention at home and abroad. Insect-inspired flapping-wing aircraft has become the focus of attention in unmanned micro aircraft. Its theoretical research and technical breakthrough are important in promoting the miniaturization and intelligence of the UAV industry and in expanding the application of bionic technology in aviation.

The related research includes four aspects. The first aspect is the design methodology for the high-efficiency flapping wing system. This aspect includes analysis on the influence of insects’ body structure and the locomotion parameters related to the aerodynamic performance of the flapping wing system, the unsteady aerodynamic coupling mechanism behind the excellent flying ability of insects, and the parametric modeling and optimization design of the flapping wing system under the coupling of structure and aerodynamics condition to achieve high aerodynamic performance and low power consumption. The second aspect is the micro-high-efficiency energy and transmission system. This aspect includes the research for the development of multi-type energy battery based on metal ions, 2D materials, optoelectronic films and polymers; the design of an intelligent energy management strategy to achieve a high- efficiency, light-weight, and high-stability energy system; the development of the micro driver and actuator based on smart piezoelectric, artificial-muscle materials and precision gear; and the study of the preparation and molding fabrication methods to achieve a high-precision, high-efficiency, and high-reliability micromechanical drive and transmission system. The third aspect is the high-density light-weight micro airborne avionics system. This aspect includes the design of a MEMS-based information-fusion intelligent micro mission payload system to sense environment information, such as voice, light thermal source, and electric magnetic signal; the design of a high-performance intelligent fight learning strategy and corresponding control system under the coupling condition between multi-source disturbance and nonlinear system; the design of a long-distance, low-power, and high- efficiency information communication system linking airborne equipment and ground station system; the development of a chip-level flexible fabrication methodology to integrate mission payload and information communication system with a flight control system to achieve a high-density and light- weight micro airborne avionics system. The fourth aspect is the integration of whole aircraft and experimental platforms. This aspect includes the research on integrated modeling and the design strategy focused on bionics under multidisciplinary coupling conditions, the development of the comprehensive experimental test platform, and the data acquisition and analysis system for bionic MAV. With the development of technologies such as bionics, unsteady aerodynamics, micro- electro-mechanical systems, flexible materials, control theory, and artificial intelligence, miniaturization, lightweight, and swarm intellectualization have become current research hotspots for micro insect-inspired flapping-wing vehicles.

With the development of bionics, unsteady aerodynamics, micro-electro-mechanical systems, flexible materials, and other technologies, miniaturization, intellectualization, and engineering for micro insect-inspired flapping-wing vehicles have become the research frontier and development trend of such vehicles. With the introduction of artificial intelligence, advanced control, and data fusion, swarm and cooperative control are expected to become the focus of attention in the future.

The top two countries with the largest number of core papers published on the engineering research forefront of “micro insect-inspired flapping-wing vehicle” are the USA and China; the top two countries with the highest citations per paper are China and USA, as shown in Table 1.2.9. No cooperation has been observed among these top two countries with the largest number of publications. Among the Top 5 institutions with the largest number of publications, the top organization with the largest number of core papers is Harvard University. The institutions with the highest citations per paper are Shanghai Jiao Tong University, University of Washington, and Massachusetts Institute of Technology, as shown in Table 1.2.10. More cooperation is observed between Harvard University and Massachusetts Institute of Technology, and also between Harvard University and University of Southern California, as shown in Figure 1.2.5. The top three countries for publishing papers that cite core papers are the USA and China, as shown in Table 1.2.11; for institutions, the top two are Harvard University and University of California, Berkeley, as shown in Table 1.2.12.

《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

The Top 10 engineering development (as opposed to research) fronts in the field of mechanical and vehicle engineering include mechanical, transportation, ship and

《Table 1.2.9》

Table 1.2.9 Countries with the greatest output of core papers on “micro insect-inspired flapping-wing vehicle”

《Table 1.2.10》

Table 1.2.10 Institutions with the greatest output of core papers on “micro insect-inspired flapping-wing vehicle”

《Figure 1.2.5》

Figure 1.2.5 Collaboration network among major institutions in the engineering research front of “micro insect-inspired flapping-wing vehicle”

《Table 1.2.11》

Table 1.2.11 Countries with the greatest output of citing papers on “micro insect-inspired flapping-wing vehicle”

《Table 1.2.12》

Table 1.2.12 Institutions with the greatest output of citing papers on “micro insect-inspired flapping-wing vehicle”

《Table 2.1.1》

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

marine engineering, weapon science and technology, aeronautical and astronautical science and technology, and power and electrical equipment engineering and technology (as listed in Table 2.1.1). Six of these frontiers are characterized as in-depth traditional research: “development of coexisting-cooperative-cognitive robots”, “big data-driven decision optimization technology for distributed intelligent manufacturing”, “technology of integrating ultra-precision machining and measurement for manufacturing complex curved components”, “spacecraft orbital threat perception and autonomous avoidance technology”, “3D printing technology of multifunctional gradient composite material for high-performance metal components”, and “multi-modal sensing and autonomous decision-making technology of urban intelligent passenger vehicle”. Four other fronts that are newly emerging include “self-organized collaboration of multiple unmanned surface systems”, “reusable spaceplane”, “lifecycle digital twin technology”, and “bionic underwater vehicle propulsion and control technology”. Table 2.1.2 shows the annual publication rate of core patents from 2015 to 2020. “Self-organized collaboration of multiple unmanned surface systems”, “lifecycle digital twin technology”, and “3D printing technology of multifunctional gradient composite material for high-performance metal components” are the most significant directions of patent disclosure in recent years.

(1)  Reusable spaceplane

A reusable spaceplane is a reusable launch vehicle (RLV) that can liberally enter into and fly outside the atmosphere to complete certain given space missions. With the rapid expansion of space activities, such as low-earth-orbit constellation, space stations, and deep-space exploration, the demand for a cheap space transportation system has been increasing dramatically. Reusing launch vehicles is an efficient way to reduce the space access cost. RLVs can be partially or fully reusable. In terms of boost method, RLVs can be single staged, two staged, or multi-staged.

RLVs can be classified into three categories according to their take-off and landing methods: vertical take-off and landing, horizontal take-off and landing, and vertical take-off with horizontal landing. Moreover, RLVs can be rocket powered or combined powered. The missions of RLV include upward or downward personal and cargo transportation between the earth and the space stations, the launch or recovery of the satellites, and independent space experiments. SpaceX’s vertical landing launch vehicle, which is partially reused, has achieved great success. It has effectively reduced the cost of space launch, which has spurred the increase of the space launch capabilities of the USA. However, the traditional rockets have high requirements of launch sites, and the

《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

propelling efficiency of rocket engine is difficult to be highly upgraded. Thus, a reusable spaceplane, which is equipped with combined circle power and can take off from and land on a civilian airport, should be the vehicle used for frequent space transport missions in the future.

(2)  Development of coexisting-cooperative-cognitive robots

Given their large inertia and high rigidity, traditional industrial robots need to work in an environment that is physically isolated from humans for safe consideration. Different from traditional industrial robots, coexisting-cooperative-cognitive robots (Tri-Co Robots) can be integrated into the normal living environment of humans, can interact with humans, and have human-like dexterity and intelligent decision-making capabilities. Their core characteristics are as follows: humans and machines are in the same natural space, and robots and humans naturally interact, learn human skills, coordinate and complement each other, and augment people’s motion ability. How to accurately understand human movement intentions in an unstructured environment and operate flexibly according to human intentions to complete complex and various tasks while ensuring human safety is a major challenge in robotics. The key technologies include rigid-flexible coupling mechanism design, flexible drive and control, mechanical implementation of human natural movement, robot’s perception and understanding of the environment, and human–robot natural interaction. The USA, Europe, Japan, and other countries have initiated research on Tri-Co Robots and have developed prototypes of Tri-Co Robots, such as compliant robotic arms for industrial applications, endoscopic surgical robots for surgical operations, rehabilitation robots, and biomechatronic prostheses.

(3)  Self-organized collaboration of multiple unmanned surface systems

The self-organized collaboration of multiple unmanned surface systems refers to achieving specified target missions coordinatively via complex, macroscopic, and ordering behaviors on the aspect of perception, decision making, planning, and control through interactions within swarm systems composed of unmanned surface vehicles (USVs). On the one hand, in sophisticated and dynamic environments, a single USV fails to achieve complicated ocean tasks because of weak anti disturbance and perception capability. On the other hand, USV swarm formation effectively promotes task scope and efficiency while elevating autonomous control, decision making, and coordination. Therein, collaboration of multiple unmanned surface systems becomes an important enabling tool for managing national water resources and safeguarding national marine rights. Major research topics include the mechanism and controllability of self-organized cooperation of multiple unmanned surface systems; multi- source information fusion, situation awareness, environment perception, and target tracking of multiple unmanned surface systems; collective decision making and optimization, task assignment, and path planning of unmanned surface swarm systems to cope with channel constraints, international maritime obstacle avoidance rules, wind, wave, current, and surge interference; and the efficient collective control method for unmanned surface swarm systems in response to water emergencies.

(4)   Big data-driven decision optimization technology for distributed intelligent manufacturing

As the development direction of future manufacturing, distributed manufacturing is an advanced manufacturing mode that aims at rapidly responding to market demand, improving the competitiveness of enterprise group, reducing production cost and risk, and enhancing the responsiveness and flexibility of the manufacturing system. The manufacturing of aviation and aerospace products is a typical distributed manufacturing mode. It is more suitable for mass customization, multi-species and small-batch production, and emergency production (such as the current COVID-19 pandemic), and its decision optimization technology greatly impacts the operational efficiency of the system. Distributed manufacturing poses great challenges to efficient decision optimization technology because it involves many complicated factors, including cross-regional multi-shop multi-job collaboration, multi-objective optimization, and changing manufacturing environment. Big data technology provides a new way of solving this problem. Big data-driven decision optimization for distributed intelligent manufacturing becomes the research frontier. The main research directions in distributed manufacturing include massive multi-source heterogeneous manufacturing data-aware technology across regions/enterprises/systems, industrial Internet architecture based on new-generation information technology, decision model intelligent establishing, joint model- and data-driven multi-objective optimization method, and the proactive scheduling theory for uncertainty information. The aim is to realize the overall collaboration optimization of distributed manufacturing and improve the operational efficiency of the distributed manufacturing system.

(5)  Lifecycle digital twin technology

Lifecycle digital twin technology introduces the digital twin technology into the life cycle, which refers to the length of time a physical object turns from being designed to scrapped. The digital twin is explored to realize the accurate digital modeling for entire elements, processes, and businesses in various stages of the whole life cycle, such as design, production, sales, operation, maintenance and recycling, data and model interaction across different stages, and lifecycle data integration, fusion, and sharing. It aims at connecting the lifecycle process by using data and models, and thus breaking the barriers between different stages. This technology allows the real-time monitoring, virtual debugging, real- time simulation, and dynamic prediction driven by the data and models to realize the accurate prediction and control for key steps, performances, and behaviors during the life cycle. The main research directions supporting the lifecycle digital twin technology include real-time lifecycle data collection, interaction, integration and sharing, multi-dimension and multi-scale virtual model construction, assembly, fusion, correction, verification and management, multi-dimension data association and fusion, real-time linkages among data, models, services and entities, service systems management and value adding. In the future, the comprehensive integration of data and models will be the development trend for the lifecycle digital twin technology, and it will also be an important means for enterprises to improve product quality and production efficiency. It includes the comprehensive integration of models and data for the entire processes, including design, production, sales, operation, maintenance, and recycling; the comprehensive integration of models and data for factors, such as products, production systems, and supply chains; and the integration of models and data for businesses, such as planning, control, scheduling, and prognosis, to realize digitalization and support smart operations.

(6)  Technology of integrating ultra-precision machining and measurement for manufacturing complex curved components

With the rapid development of aerospace, IC, and optoelectronic information industries, the requirements for multi-functional and lightweight devices are continuously increasing, and the demand for ultra-precision complex curved surface components is also growing. The integration of ultra-precision machining and measurement, which contains technologies from various areas, such as machining, measurement and error compensation, is an effective way to achieve the high-efficiency and high-quality machining of such components. At present, research for ultra-precision machining of complex curved surfaces is focused on the surface generation principle, tool design and fabrication method, and intelligent tool path planning algorithm. For ultra-precision measurement of complex curved surfaces, research is concentrated on high-precision measurement technology and instrument, surface error compensation principle and closed-loop part accuracy control method. In response to the high-performance manufacturing requirements, such as extreme structural scales, complex shape, difficult-to-machine material, and near-zero surface damage and stress, research trends for the integration of ultra-precision machining and measurement lie in three aspects. The first aspect includes multi-energy field-assisted ultra-precision machining technologies, which implement the multi-field coupling effects of force, heat, light and electricity, to achieve high-efficiency, high-precision and low-damage manufacturing of workpieces. The second aspect includes novel ultra-precision machine tools, which control the complex trajectory of the cutting tool and the cutting conditions for machining workpiece surfaces with complex features. The third aspect is combination of online and offline in-site measurements into the ultra-precision machining process, which aims to achieve high-precision error compensation for complex curved surfaces via on-machine measurement data feedback.

(7)    Bionic underwater vehicle propulsion and control technology

Bionic underwater vehicles are mainly developed on the basis of the movement mechanism, biological structure, and group behavior of natural organisms, especially marine organisms. Relative to traditional underwater vehicles, bionic underwater vehicles have low noise and high stealth characteristics. Fish are the most common swimming creatures in the ocean, and their low-resistance shape and efficient and flexible swimming mode are important references for the design of underwater unmanned equipment.

The swimming propulsion mode of fish can be divided into body or caudal fin (BCF) propulsion mode and medium or paired fin (MPF) propulsion mode. The acceleration performance of bionic propulsion based on BCF mode is better than that of bionic propulsion based on MPF mode, and the maneuverability of the MPF mode is better than that of the BCF mode.

At present, the main research highlights in this field include biomimetic biological analysis technology based on biological observation and anatomy, new material or composite multidrive bionic structure design technology, bionic flexible- body large-deformation fluid simulation technology, and multidegree-of-freedom system motion coordination bionic control technology.

This research is a multidiscipline and multifield study. The future research trend in this field is to perform a closed-loop iterative optimization of multidisciplinary cross integration, including multimodal kinematics analysis, structural bionics, neural control bionics, similarity evaluation system, and experimental verification.

(8)   Spacecraft orbital threat perception and autonomous avoidance technology

With the intensification of space competition and the massive increase of space debris in recent years, the safety of orbiting spacecraft has been greatly threatened. To deal with space strikes or collisions with orbiting objects, spacecraft should be equipped with the ability to perceive potential orbital threats and to avoid space threats by means of autonomous adjustment of orbits on the basis of perception information. The orbital threat perception and autonomous evasion technology of spacecraft uses ground-based monitoring and space-based monitoring methods to track and monitor space debris and noncooperative targets; determines the orbit of spacecraft and large space debris through orbit calculations; and calculates the collision probability between spacecraft and debris. Once the collision probability reaches the warning threshold, the spacecraft is allowed to perform evasive maneuvers. The main technical directions include space target detection technology, precision orbit calculation technology, orbit error propagation technology, and early warning and evasion technology. The main development trends include strengthening the top-level design and strategic layout, developing a space–Earth monitoring system, and improving the perception of space targets. Other trends involve advancing the construction of space–Earth integrated sensing network, focusing on the development of microsatellite networking technology, improving the flexibility of satellite platforms, and enhancing the protection of satellites to build a world-ground integrated antijamming communication network system and thereby forming an all-weather, full- coverage situational awareness system. These trends will also accelerate the development of big data processing and fusion technology in the field of situational awareness, spatial big data technology, artificial intelligence technology, etc. to ensure “data sovereignty” and improve the orbital threat perception and autonomous evasion capabilities of spacecraft.

(9)   3D printing technology of multifunctional gradient composite material for high-performance metal components

Metallic multifunctional gradient composite materials (GCMs) undergo continuous or quasi-continuous changes in chemical composition, microstructure, and pore structure such that their physical and chemical properties are continuously graded according to design requirements. GCMs belong to a new type of composite materials with special functions. Therefore, they can be applied to the extremes of major national strategic development areas, such as aerospace, deep-sea ships, and nuclear physics engineering with extreme service environments. 3D printing, belonging to additive manufacturing (AM) technology, uses the principle of layer- by-layer manufacturing and superimposition to directly form complex parts through CAD models. By placing different materials in different positions, setting different process parameters, or designing different structures, 3D printing technology can directly produce finely customized GCMs. Among 3D printing technologies, the processes that can be used to prepare metal multifunctional GCMs include laser engineering net shaping (LENS), selective laser melting (SLM), and electron beam selective shaping, selective electron beam melting (SEBM), and wire arc additive manufacturing (WAAM). The main development trends include gradient and functional design theory of GCMs, interface metallurgy principle and composition control mechanism, new additive process theory for GCMs, and new heat treatment methods for GCMs. Through specific design, the shape, property, or functionality of additively manufactured GCMs can controllably vary with time, space, or environment. This new idea of 4D printing is becoming a cutting-edge research field.

(10)  Multi-modal sensing and autonomous decision-making technology of urban intelligent passenger vehicle

With the rapid development of emerging technologies, such as the Internet of Things, big data, and artificial intelligence, the intelligence level of vehicles has been significantly improved, and cars with high-level autonomous driving functions are moving toward mass production. The key technologies of autonomous driving are perception, decision making, and control. As the traffic environment in urban roads is more complex than that in expressways, autonomous decision making needs to be realized by integrating multisource sensing information and machine learning methods. In terms of perception, multisource perception technology should be introduced to effectively integrate the data collected by different sensors on the same target or scene through acquisition of vehicle-on-board, roadside, and cloud data. Its application solves the defect in which the current single- sensor perception technology cannot adapt to complex urban traffic scenes. It also enhances the reliability of perception. Decision making requires the construction of an autonomous driving brain, the use of machine learning methods to perform humanoid decision making, and the forwarding of control instructions to executive components so as to realize autonomous decision making and control of urban intelligent passenger vehicles. The main development trends include making full use of urban transportation infrastructure, constructing digital transportation environment and digital twin platforms for intelligent driving of passenger vehicles, and realizing multiple perceptions of traffic information with low delay and high precision. The cloud decision-making framework of urban intelligent vehicles needs to be studied using collaborative cloud–edge–end processing technology to realize multivehicle collaboration in complex urban traffic environment. The multiscale scene understanding technology of a generalized human–vehicle–road transportation system is another research trend. We need to break through the online evolutionary learning technology of autonomous driving and realize the self-learning of urban intelligent passenger vehicles.

《2.2 Interpretations for three key engineering development fronts》

2.2 Interpretations for three key engineering development fronts

2.2.1 Reusable spaceplane

A reusable spaceplane can radically reduce the cost of space access, and realize an airline-flight-mode space transportation. Furthermore, it also greatly promotes the advance of space science and the exploration of space resources, and leads a new industry revolution.

Space shuttle is the first RLV, which consists of two solid boosters and one orbiter. The two solid boosters can be recovered by the parachute after separating with the orbiter. The orbiter is propelled by the fluid engine and will complete the mission in space. Thereafter, the orbiter can glide into the atmosphere and horizontally land on an airport. Given the flaws in the design and the limited technologies at that time, the maintenance cost of space shuttle is highly expensive. Therefore, the reusability had not reduced the launch cost of space shuttle, which finally retired in 2011.

However, with the expansion of human space activities, the demand for low-cost space launch is still strong. The USA commercial aerospace companies, such as SpaceX, have developed the vertical landing rocket system, which can land the first stage of a rocket vertically on a platform with the help of the retro-jet of rocket engine. The reuse of the first stage has evidently reduced the launch cost and achieved great success in the commercial launch market. It has also contributed to the vigorous development of low-orbit Internet satellites in the USA. However, the efficiency of the rocket system has an upper limit because of the limited specific impulse of the rocket engine. In addition, rocket launches have strict requirements for the launch site. Thus, the reuse of rocket stages cannot change the high cost of space launch. The spaceplane, which can take off from and land on a civilian airport, is the trend of future reusable space transportation systems.

The key techniques of developing the spaceplane include the design of an aero-propulsion configuration with large scale airframe and wide speed range ability, thermal protection and management, combined circle propulsion, control of complex spacecraft, ground experiment, and flight test. Scientific challenges that must be conquered include hypersonic aerodynamic, supersonic combustion, nonlinear control, and high-temperature materials.

The top two countries with core patent disclosures on the engineering development front of “reusable spaceplane” are the USA and China. The top three countries in terms of citation frequency are the Russia, the USA, and the UK, as shown in Table 2.2.1. Cooperation is observed between the USA and the UK, and between the USA and Canada. The top two institutions with core patent disclosures are Boeing Company and Beijing Aerospace System Engineering Research Institute, as shown in Table 2.2.2. Major institutions with the most public core patents do not cooperate with each other.

2.2.2 Development of coexisting-cooperative-cognitive robots

The development of Tri-Co Robots and the establishment of a new generation of robot technology with intrinsic safety, human–robot collaboration can provide support for emerging new application scenarios and needs in the fields of industry, rehabilitation, and national defense. It can meet the requirement of automation and intelligent equipment from green manufacturing, flexible manufacturing, and customized manufacturing; it can repair or rebuild the motor ability of patients with impaired or missing motor functions, which shorten the rehabilitation period and allow the natural control of the prosthesis by the amputee. Moreover, it can reduce the metabolic cost of individual load-carriage walking and enhance individual combat effectiveness.

The Tri-Co industrial robots that can be integrated into the “production line with human” combine the performance of

《Table 2.2.1》

Table 2.2.1 Countries with the greatest output of core patents on “reusable spaceplane”

《Table 2.2.2》

Table 2.2.2 Institutions with the greatest output of core patents on “reusable spaceplane”