2.1. Wheeled type UGV platform

Wheeled UGV platform is well suited for completing transportation and supply activities in a field setting because of their quick acceleration and straightforward control scheme. One classic example is the rigid suspension unmanned platform, which has a specific design but has poor high-frequency ground vibration absorption due to the absence of a suspension system.

The Squad Mission Support System (SMSS) vehicle [Reference Sabatta5] (shown in Fig. 5(a)), developed by Lockheed Martin in 2005, was a 6×6 amphibious UGV platform, within 1.7 ton weight, 3.6 m long, 1.8 m wide, and 2.1 m height. It could go past barriers that are 0.55 m deep or 1.7 m wide. Besides, it had adequate dynamic performance even if it could not alter ground clearance. The Guardium vehicle [Reference Ni, Hu and Xiang6] (shown in Fig. 5(b)), developed by Israel G-UNIUS company in 2005, was 1.4 ton weight, 2.95 m long, 1.8 m wide, and 2.2 m height. Its speed could reach 80 km/h in the off-road environment. Zaporizhzhia company Infocom had developed a robotic structure Laska 2.0 [Reference Nevliudov, Yanushkevych and Ivanov7] (shown in Fig. 5(c)), designed for patrolling, surveillance, demining, delivery of ammunition, and evacuation of the wounded. The Laska 2.0 URP was based on a 4 $\times$ 4 wheeled chassis with fixed and mobile platform configurations. Laska 2.0’s base platform was 2.27 m long, 1.3 m wide, and 0.95 m high. The amount of ground clearance of this UGV was 220 mm. In 2018, during the EUROSATORY arms exhibition, the German company Rheinmetall AG presented a relatively mature modular UGV platform demonstrator called Mission Master [Reference Ieva Bērziņa8] (shown in Fig. 5(d)), which could be used as a platform for multiple applications. It could be used for tactical surveillance, chemical, biological, radiological, and nuclear detection, medical evacuation operations, and communications relay missions. This all-terrain UGV was built on an 8 $\times$ 8 platform that is 2.95 m long and weighs around 750 kg. In amphibious operations, it could carry up to 400 kg and had a maximum load capacity of 600 kg.

Furthermore, there are other studies on the UGV platforms with wheels. Lee [Reference Lee, Ryu, Kim and Seo9] proposed an angled spoke-based wheel design to enhance the driving speed of a mobile robot. Chen [Reference Chen, Wang, Zhong, Zhu, Yang and Wang10] researched an all-terrain mobile robot with a linkage suspension, and its complex kinematics and dynamic model are studied. Aaron [Reference Tan, Peiris, El-Gindy and Lang11] demonstrated the design and development of a novel custom-built, autonomous scaled multi-wheeled vehicle that features an eight-wheel drive and eight-wheel steer system. Daniel [Reference Gonzalez, Lesak, Rodriguez, Cymerman and Korpela12] presented a Four-Wheeled Independent Drive and Steering (4WIDS) robot named AGRO (Agile Ground Robot) and a method of controlling its orientation while airborne using wheel reaction torques. Nikitenko [Reference Nikitenko and Kulikovskis13] proposed an eight-wheel platform. The platform’s wheels are coupled in pairs using movable joints. Each of the joints is independent of others thus allowing to maintain excellent contact with ground or obstacles. Son [Reference Son, Shin, Kim and Seo14] demonstrated a mobile robotic platform that uses a normal wheel and a curved-spoke tri-wheel (CSTW). The normal wheel is used for driving on flat terrain, and the CSTW is used for stair climbing. Edgar [Reference Martinez-Garcia, Lerín-García and Torres-Cordoba15] developed a general kinematic control law for automatic multi-configuration of four-wheel active drive robots. Jia [Reference Jia, Cheng, Ye, Xie and Wu16] presented an amphibious soft-rigid wheeled crawling robot consisting of a soft-rigid body actuated by two soft pneumatic actuators (SPAs), four wheels, and four annular soft bladders (ASBs) as brakes. Kim [Reference Kim, Lee, Lee, Kim, Kim and Seo17] researched a new mobile platform with two degree of freedom (2-DOF) transformable wheels for service robots, which can overcome steps and stairs of various sizes encountered in indoor environments.

2.2. Tracked type UGV platform

The wheeled platform’s high grounding-specific pressure makes it vulnerable to sinking on soft surfaces. Instead, the tracked construction has a low grounding-specific pressure, making it ideal for usage in soft soil and typical in the wild [Reference Wong18]. A typical tracked type UGV platform usually includes double-track forms and multi-track ones. The double-track form is the traditional double-track walking configuration. Additionally, the multi-track one builds on the double-track design and adds a deformable mechanism that helps the platform perform better while navigating obstacles.

Foster-Miller’s TALON [Reference J.González19] (shown in Fig. 6(a)) was a remote-controlled reconnaissance platform that could be outfitted with a rifle, grenade launcher, or incendiary weapon. The system was made of several sensor components and is modular in design. It could be utilized in all terrain and environmental circumstances and had an excellent payload-to-weight ratio. A more straightforward method than the SMSS, the Titan [Reference Williams20] (shown in Fig. 6(b)), consisted of a platform supported by two diesel-electric hybrid tracks, with mission-specific controls and automation being contained in modular payload frames. The multi-mission UGV Titan could be rearranged to increase mission effectiveness. It had a cargo bay area that was 72 in by 48 in, 79 in long, 83 in broad, and 40 in high. A maximum payload of 1500 pounds was possible. The new generation of the Russian unmanned ground vehicle Uranus-9 [Reference Zhang, Fan, Wang, Zhao, Zhang and Ma21] (shown in Fig. 6(c)) had been tested on the Syrian battlefield. It could march and hunt for targets on its own. The body of the Uranus-9 was a small, track-shaped armored vehicle that was 4.5 m long, 2 m wide, and 1.4 m high. With a total weight of 10 tons, it had a top speed of 40 km/h. Additionally, it could climb barriers up to 1.2 m high. The American iRobot company developed a four-track unmanned platform Packbot [Reference Yamauchi22] (shown in Fig. 6(d)) with double swing arms. Packbot could traverse ditches and ascend stairs with its swing arm while still keeping its feet firmly planted on uneven roadways. Mourikis and Yunwang researched active obstacle-surmounting control and obstacle-surmounting kinematics mechanism of this type of platform, respectively. Their findings showed that tracked platforms with swing arms performed better while navigating obstacles [Reference Mourikis, Trawny, Roumeliotis, Helmick and Matthies23, Reference Yunwang, Shirong, Hua and Jian24]. Besides, there are other researches on tracked type UGV platform [Reference Yuting, Baoling, Qingsheng and Kailing25–Reference Hardouin27].

Figure 6.

Tracked type UGV platform: (a) the Foster-Miller’s TALON [Reference J.González19]; (b) the Qinetiq Titan [Reference Williams20]; (c) Russia UGV of Uranus-9 [Reference Zhang, Fan, Wang, Zhao, Zhang and Ma21]; (d) American iRobot of Packbot [Reference Yamauchi22].

2.3. Legged type UGV platform

It is common knowledge that walking on bumpy roads is best done on wheeled or tracked platforms. However, the effectiveness of these two platforms will be much diminished in rugged hilly terrain with numerous barriers like weeds and plants. Based on the bionic mechanism of the footed animal, the legged type UGV platform can realize autonomous identification and overcome obstacles by controlling the discrete gait of the legs [Reference Ko, Chen, Li and Lin28].

To study the development of intelligent autonomous legged mobility in challenging and unstructured terrain, the LLAMA [Reference Nicholson, Jasper, Kourchians, McCutcheon, Austin, Gonzalez, Pusey, Karumanchi, Hubicki and Clark29] (shown in Fig. 7(a)) quadrupedal robotic platform was developed. The LLAMA could move at 1.2 m/s in all directions. Moreover, LLAMA could move swiftly and carry weights thanks to its revolutionary designs and specially made, customizable movement systems. ANYmal [Reference Hutter, Gehring, Jud, Lauber, Bellicoso, Tsounis, Hwangbo, Bodie, Fankhauser and Bloesch30] (shown in Fig. 7(b)) was a quadrupedal platform designed to be highly mobile and durable for autonomous operation under challenging conditions. ANYmal was created with exceptional mobility and dynamic motion capabilities, allowing it to sprint and easily climb enormous barriers. “ANYmal” denoted that the platform could move steadily and any place to assist people in hazardous industrial settings. Atlas [Reference Nelson, Saunders and Playter31] (shown in Fig. 7(c)), a bipedal humanoid robotic platform created by Boston Dynamics, had a stable control strategy and environmental awareness technologies that enabled it to carry objects and execute movements like climbing stairs and leaping. Additionally, in certain unique situations, it could do tasks that people would typically perform. MIT researched the quadrupedal robotic platform named Cheetah [Reference Park, Wensing and Kim32] (shown in Fig. 7(d)), which had real-time planning of gait through model predictive control based on force feedback. It had excellent real-time control capabilities and can react swiftly to ongoing impediments like stairs and walking patterns.

Figure 7.

Legged type UGV platform: (a) the quadrupedal robotic platform LLAMA [Reference Nicholson, Jasper, Kourchians, McCutcheon, Austin, Gonzalez, Pusey, Karumanchi, Hubicki and Clark29]; (b) the quadrupedal robotic platform ANYmal [Reference Hutter, Gehring, Jud, Lauber, Bellicoso, Tsounis, Hwangbo, Bodie, Fankhauser and Bloesch30]; (c) the bipedal humanoid robotic platform Atlas [Reference Nelson, Saunders and Playter31]; (d) the quadrupedal robotic platform Cheetah [Reference Park, Wensing and Kim32].

There are other research on UGV platforms with legs as well. Dennis [Reference Goldschmidt, Hesse, Wörgötter and Manoonpong33] developed a neural control mechanism for hexapod robots which generates basic walking behavior and especially enables them to effectively perform reactive climbing behavior. Xiong [Reference Xiong and Ames34] presented the design and validation of controlling hopping on the 3D bipedal robot Cassie. A spring-mass model is identified from the kinematics and compliance of the robot. In ref. [Reference Ficht and Behnke35], a comprehensive review of the technologies crucial for bipedal humanoid robots was performed. Different mechanical concepts have been discussed, along with the advancements in actuation, sensing, and manufacturing. Liu [Reference Liu, Shen, Zhang, Zhang, Zhu and Hong36] researched a miniature bipedal robot named Bipedal Robot Unit with Compliance Enhanced (BRUCE). Each leg of BRUCE has five degrees of freedom (DoFs), which includes a spherical hip joint, a knee joint, and an ankle joint. In ref. [Reference Yöngül and Kavlak37], an eight-legged spider type mobile robot was designed. Klann type walking mechanism with one degree of freedom has been designed for the robot to overcome the obstacles and move in a balanced way. In ref. [Reference Kavlak and Kartal38], the ideal coupler curve was drawn using the “Cinderalla” program for the mobile robot with “Strandbeest” walking mechanism to move on a smooth surface and the link distances that provided this curve were determined. Huang [Reference Huang and Grizzle39] demonstrated a reactive planning system for bipedal robots on unexplored, challenging terrains. Daniel [Reference Blackman, Nicholson, Ordonez, Miller and Clark40] explored improvements to both physical and control methods to a quadrupedal system in order to achieve fast, stable walking, and trotting gaits. Kiss [Reference Kiss, Gonen, Mo, Buchmann, Renjewski and Badri-Spröwitz41] developed a 0.49 m tall, 2.2 kg anthropomorphic bipedal robot. Roennau [Reference Roennau, Heppner, Nowicki and Dillmann42] presented the design and development of the new six-legged walking robot with its improved kinematics and robust mechanical structure. Katz [Reference Katz, Di Carlo and Kim43] proposed a small and inexpensive, yet powerful and mechanically robust quadruped robot, intended to enable rapid development of control systems for legged robots. Ding [Reference Ding, Pandala, Li, Shin and Park44] researched a novel representation-free model predictive control (RF-MPC) framework for controlling various dynamic motions of a quadrupedal robot in three-dimensional (3D) space. Dettmann [Reference Dettmann, Planthaber, Bargsten, Dominguez, Cerilli, Marchitto, Fink, Focchi, Barasuol, Semini and Marc45] demonstrated a navigation and locomotion control system that enables legged robots to be able to perceive the terrain, to plan a path to a desired goal, and to control the path execution while traversing unconsolidated, inclined, and rugged terrain. Hendrik [Reference Kolvenbach, Hampp, Barton, Zenkl and Hutter46] explored a 22 kg quadruped robot exploits lunar gravity conditions to perform energy-efficient jumps. The robot achieves repetitive, vertical jumps of more than 0.9 m and powerful single leaps of up to 1.3 m. Kolvenbach [Reference Kolvenbach, Arm, Hampp, Dietsche, Bickel, Sun, Meyer and Hutter47] presented experimental work on traversing steep, granular slopes with the dynamically walking quadrupedal robot.

2.4. Complex mobile type UGV platform

Legged type UGV platforms have a better capacity to adapt to rough terrain. Still, their motion mechanism must be built on a complicated control algorithm, which raises more complex computer configuration requirements. A novel configuration known as the wheel-leg complex mobile type platform [Reference Conduraru, Doroftei and Conduraru48] was created by fusing the fast speed of the wheeled type platform with the potent obstacle-surmounting abilities of the legged type platform. It has become a hub for UGV platform research because of its excellent mobility and adaptability. Deformable and non-deformable platforms are both included in the wheel-leg complex mobile type UGV platform, generally speaking. The deformable one alludes to the platform’s changing configuration being realized in the shape of a transmission mechanism. Next, the structural adjustment between the leg and the wheel is made following the characteristics of the terrain. In contrast, the non-deformable one relates to the selection of the structural form. Passive adaptive or attitude control will also change postures. It is often a non-deformable platform in the field of UGV platform to reduce the complex mechanism and increase dependability.

The Federal Institute of Technology in Lausanne (EPFL) researched the wheel-leg complex mobile type platform Shrimp [Reference Siegwart, Lamon, Estier, Lauria and Piguet49] (shown in Fig. 8(a)) based on a parallel four-bar suspension. The redundant link mechanism’s purpose was to maintain all-terrain contact with the ground. While doing so, it could navigate obstacles by modifying the front wheel fork’s spatial location. Its dead weight was merely 3.1 kg, but the obstacle’s maximum height was almost double that of the wheel. Tencent Robotics X laboratory proposed a novel wheel-leg complex mobile type platform Ollie [Reference Wang, Cui, Zhang, Lai, Zhang, Chen, Zheng, Zhang and Jiang50] (shown in Fig. 8(b)), which was composed of one floating-based body, two legs ending with active wheels, and one balancing tail ending with a passive revolution. Ollie was capable of using both his legs and wheeled vehicles. The wheeled frame traveled quickly and efficiently, and the portion helped it to adjust to rough terrain. The latest incarnation of ANYmal was named Swiss-Mile [51] (shown in Fig. 8(c)). Like the original ANYmal, Swiss-Mile had four legs. Additionally, these legs had wheels affixed to them for rolling and walking. It could still walk like a quadruped by locking the wheels on the ends of those legs when necessary. These three-wheeled, three-leg complex mobile platforms were highly mobile and terrain-adaptable. It could thus be used in a variety of domains, including space robotics. TALBOT [Reference Guo, Qiu, Xu and Wu52] (shown in Fig. 8(d)) was a tracked-leg transformable robot. To adapt to any terrain, the robot can convert between the tracked and legged modes thanks to its original tracked-leg transformable structure. Talbot is controlled in tracked mode via the technique of differential speed between the two tracked feet. In order to generate and convert gait, TALBOT is controlled in the legged mode using a bionic control technique of the central pattern generator. A revolutionary wheel-legged UGV known as “Dragon Horse” [Reference Miaolei, He, Ren and He53] (shown in Fig. 8(e)) was presented. The obstacle-surmounting strategy was inspired by a horse crossing the fence. The platform included four swinging, horse-like arms that enabled the UGV to quickly climb a vertical barrier by carefully organizing the position of its components. Equipped with a hydraulic drive system, the Horse Dragon had outstanding cargo capacity and enough power to handle a climbing scenario. Ascento [Reference Klemm, Morra, Salzmann, Tschopp, Bodie, Gulich, Küng, Mannhart, Pfister, Vierneisel, Weber, Deuber and Siegwart54] (shown in Fig. 8(f)) was a two-wheeled balancing robot with the ability to move swiftly over flat terrain and jump over obstacles. The mechanical design of the system, which was 3D-printed and topology-optimized, had proven to be both light and impact-resistant.

Figure 8.

Complex mobile type UGV platform: (a) EPFL wheel-leg type platform Shrimp [Reference Siegwart, Lamon, Estier, Lauria and Piguet49]; (b) Tencent Robotics X laboratory Ollie [Reference Wang, Cui, Zhang, Lai, Zhang, Chen, Zheng, Zhang and Jiang50]; (c) the latest incarnation of ANYmal: Swiss-Mile [51]; (d) tracked-leg transformable robot platform TALBOT [Reference Guo, Qiu, Xu and Wu52]; (e) a horse inspired eight-wheel ugv Dragon Horse [Reference Miaolei, He, Ren and He53, Reference He, Ren, He, Wu, Zhao, Wang and Wu55]; (f) wheel-leg jumping robot platform Ascento [Reference Klemm, Morra, Salzmann, Tschopp, Bodie, Gulich, Küng, Mannhart, Pfister, Vierneisel, Weber, Deuber and Siegwart54].

Furthermore, there are other researches on complex-type UGV platforms as follows. In ref. [Reference Lim, Ryu, Won and Seo56], a modified rocker-bogie mechanism that improves mobility using only two actuators and a damper is proposed. Choi [Reference Choi, Kim, Jung, Kim and Kim57] suggested a new performance metric for a mobile platform called as a posture variation index to evaluate the smoothness of its movement, which is an important factor in predicting undesired oscillations or shocks on a mobile platform while traveling on rugged terrain. Avinash [Reference Siravuru, Shah and Krishna58] discussed the development of an optimal wheel-torque controller for a compliant modular robot. The wheel actuators are the only actively controllable elements in this robot. For this type of robot, wheel-slip could offer a lot of hindrances while traversing on uneven terrains. Tobias [Reference Klamt and Behnke59] proposed a navigation planning method that generates hybrid locomotion paths. The planner chooses the driving mode whenever possible and takes into account the detailed robot footprint. Fahad [Reference Raza, Zhu and Hayashibe60] presented a control framework to improve the stability and robustness of an underactuated self-balancing wheel-legged robot using its upper limb arm. Edo [Reference Jelavic, Farshidian and Hutter61] demonstrated a combined sampling and optimization-based planning approach that can cope with challenging terrain. The sampling-based stage computes whole-body configurations and contact schedule, which speeds up the optimization convergence. Munzir [Reference Zafar, Hutchinson and Theodorou62] addressed a whole-body control framework for Wheeled Inverted Pendulum (WIP) Humanoids. WIP Humanoids are redundant manipulators dynamically balancing themselves on wheels. Victor [Reference Klemm, Morra, Gulich, Mannhart, Rohr, Kamel, de Viragh and Siegwart63] presented a hierarchical whole-body controller leveraging the full rigid body dynamics of the wheeled bipedal robot Ascento. In ref. [Reference Sun, Zhang, Li, Shi and Wang64], a novel articulated wheel-legged forestry chassis is presented. To balance the terrain mobility and stability, a serial suspension system which is a combination with the active four-bar linkage articulated suspension and passive V shape rocker-bogie is proposed. In ref. [Reference Kameduła and Tsagarakis65], a reactive control scheme that exploits wheels steering and robot articulated legs is proposed to continuously adjust the robot support polygon in response to unknown disturbances. Marko [Reference Bjelonic, Sankar, Bellicoso, Vallery and Hutter66] researched an online trajectory optimization framework for wheeled quadrupedal robots capable of executing hybrid walking-driving locomotion strategies. In ref. [Reference Xin, Chai, Li, Rong, Li and Li67], a whole-body dynamic model is built. It consisted of the torso dynamic model, the wheel-leg dynamic model, and the contact force constraint between the wheels and the ground. Pico [Reference Pico, Park, Luong, Medrano and Moon68] presented a mobile robot for delivery services that use laser scanning sensors to recognize the local geometry of the terrain, using the contact angle parameter that gives information regarding the surface that the wheel touches the ground. Sun [Reference Sun, You, Zhao, Adiwahono and Chew69] proposed a control framework to tackle the hybrid locomotion problem of wheeled-legged robots. Medeiros [Reference Medeiros, Jelavic, Bjelonic, Siegwart, Meggiolaro and Hutter70] demonstrated a trajectory optimization formulation for wheeled-legged robots that optimizes over the base and wheels’ positions and forces and takes into account the terrain information while computing the plans. Viragh [Reference de Viragh, Bjelonic, Bellicoso, Jenelten and Hutter71] addressed a trajectory optimizer for quadrupedal robots with actuated wheels. Du [Reference Du, Fnadi and Benamar72] developed a more general dynamics controller to generate whole-body behaviors for a quadruped-on-wheel robot.

Table I is summarized to present the worldwide research status of UGV platforms.

Table I.

Summary of worldwide research status of UGV platforms.

3.1. Overview of a typical obstacle-surmounting mechanism

According to the type of interaction with the ground, typical obstacle-surmounting techniques are often categorized into continuous and discrete mechanisms. Continuous mechanism, which is often based on wheeled and tracked designs, refers to a walking mechanism that keeps continuous contact with the ground. Meanwhile, discrete mechanisms, which are often based on leg shape, are in discrete contact with the ground.

The articulated mechanism moves like an inchworm and has degrees of freedom for pitch and roll. On sloping roads, it performs superbly in terms of contact retention. Nowadays, all-terrain mobile platforms with articulated mechanisms are frequently utilized in tandem. The University of Paris VI (UPMC) developed a three-body couple-wheeled robot RobuROC6 [Reference Lucet, Grand, Sallé and Bidaud73] (shown in Fig. 9(a)) with a hydraulic cylinder and roll joint. The hydraulic cylinder’s telescopic drive propelled the robot’s pitching motion, enabling obstacle crossing. The roll joint was a weakly actuated joint that could automatically adjust to challenges like ground depressions or bumps. Sunward Intelligent Equipment Company and Central South University applied the hydraulic articulation mechanism to the heavy-duty UGV platform, which developed a two-body articulated serial unmanned platform named “Longma No.1” [Reference Zhou, He, Ren, Zhao and Peng74] (shown in Fig. 9(b)). The obstacle-surmounting height of this platform could reach 1.5 times the tire’s diameter. The Shenyang Institute of Automation, Chinese Academy of Sciences, and Beihang University applied the articulated mechanism to the tracked robot [Reference Li, Wang, Ma, Li and Wang75, Reference Wang, Yu and Zhang76] (shown in Fig. 9(c,d)). They created a reconfigurable robot with the ability to adapt to the ground environment and modify its overall design in response to the terrain.

Figure 9.

Articulated mechanism type UGV platform: (a) UPMC three-body tandem wheeled robot RobuROC6 [Reference Lucet, Grand, Sallé and Bidaud73]; (b) two-body articulated serial unmanned platform Longma No.1 [Reference Zhou, He, Ren, Zhao and Peng74]; (c) a transformable tracked robot Amoeba2 [Reference Li, Wang, Ma, Li and Wang75]; (d) a mobile multi-robot system JL2 [Reference Wang, Yu and Zhang76].

In addition, the Japan Defense University proposed a wheeled platform based on a complex wheel sets mechanism [Reference Takita, Shimoi and Date77] (shown in Fig. 10(a)). Using planetary gears or hydraulic actuators, the independent wheels were joined together to create wheel sets, increasing the wheel diameter in a theoretical enveloping and enhancing obstacle-surmounting performance. The rocker arm suspension system [Reference Thueer, Krebs and Siegwart78, Reference Alamdari and Krovi79] (shown in Fig. 10(b)) could keep complete contact with the ground on bumpy roads and adjust to terrain characteristics since it was an underactuated mechanism. As a result, planetary exploration spacecraft frequently deployed it. This design had been investigated and used by several research organizations worldwide. Mechanism and transmission mechanics concepts were used to achieve changeable structural behavior in the deformable obstacle-surmounting mechanism. To increase versatility, multiple setup strategies were used for various terrains. Deformable wheel [Reference She, Hurd and Su80, Reference Sun, Xiang, Su, Wu and Song81] (shown in Fig. 10(c)) based on the crank-slider principle researched by Ohio State University and Tianjin University, which could convert from a wheeled configuration to a legged design when obstacle-surmounting. The ability to obstacle-surmounting significantly improved. Additionally, research organizations have put forward various bionic obstacle-surmounting processes based on researching animals’ motion mechanisms and their capacity to adapt to multiple terrains in the field. The classic example was a crawling robot powered by electromagnetic forces that mimics the movements of inchworms [Reference Lee, Kim, Paik and Shin82] (shown in Fig. 10(d)).

Figure 10.

Other typical obstacle-surmounting mechanism: (a) a complex wheel sets mechanism [Reference Takita, Shimoi and Date77]; (b) a rocker arm suspension mechanism [Reference Thueer, Krebs and Siegwart78, Reference Alamdari and Krovi79]; (c) a deformable Mechanism [Reference She, Hurd and Su80, Reference Sun, Xiang, Su, Wu and Song81]; (d) an inchworm mechanism [Reference Lee, Kim, Paik and Shin82].

The direction of research in obstacle-surmounting mechanisms progresses from the general-purpose, mobile obstacle-surmounting tool with large load capacity to the micro-miniature agent at the practical level.

3.2. Research status of obstacle-surmounting performance

Analysis of the mathematical and physical model of the UGV platform’s kinematics and dynamic performance across obstacles is referred to as obstacle-surmounting performance. The UGV platform’s ability to overcome obstacles is then objectively examined. The essential elements that determine the performance of obstacle-surmounting are kinematics and dynamic performance; hence, research on the obstacle-surmounting version of the UGV platform primarily focuses on the following aspects:

3.2.1. Mechanism of centroid movement in obstacle-surmounting

The UGV platform’s method of overcoming obstacles involves moving across space. The centroid must reach the location of the obstacle height to realize the obstacle surmounting [Reference Pijuan, Comellas, Nogués, Roca and Potau83, Reference Wang, Du and Sun84]. The centroid position has an impact on the obstacle-surmounting performance. When the centroid position reaches the height of obstacle, the surmounting process is finished. Since the centroid position of the “Dragon Horse” would change with the motion of swing arms, He [Reference He, Ren, He, Wu, Zhao, Wang and Wu55] studied the centroid kinematic model of the UGV “Dragon Horse” (shown in Fig. 11). Based on the walking control stability of bipedal robots, Yao [Reference Yao, Wang, Yao, Ding and Xiao85] proposed a speed-tracking control strategy based on the motion state of the centroid. Controlling the centroid’s displacement output would enable steady walking to occur. This study demonstrated the effect of centroid control on the stability of platform motion.

Figure 11.

Centroid kinematic model of the UGV “Dragon Horse” [Reference He, Ren, He, Wu, Zhao, Wang and Wu55].

3.2.2. Dynamics performance of obstacle-surmounting

The ideal condition of the ground is typically used as the foundation for the kinematics study of an UGV platform. But in practice, the state of slippage, instability, flameout, etc., will occur inevitably. It depends on many factors, such as ground adhesion coefficient and platform power. Therefore, the dynamic performance determines whether an obstacle may be efficiently overcome. Based on kinematics, Zhu [Reference Yan, Minghui and Hui86] took into account the slip factor. The dynamic constraint relation for obstacle-surmounting was derived by utilizing the quasi-static dynamic model of the obstacle-surmounting process. Additionally, more precisely measure the crawler robot’s capability of overcoming obstacles. In ref. [Reference Miaolei, He, Ren and He53], to guarantee the “Dragon Horse” had the maximum ability of obstacle surmounting, the authors performed a dynamic analysis of surmounting an obstacle. There was an assumption that the surmounting process was slow, and the process could be divided into two main stages (shown in Fig. 12). In the study of the obstacle-surmounting capability of a light-weight six-wheeled UGV platform, Dabrowska [Reference Dąbrowska87] abstracted the tire as a discrete spring-loaded damping element, connected by a limited quantity of spheres. These discrete spheres would pulsate in contact with the ground, which was described as simulating the elastic deformation effect of the tire. Consequently, it could affect the dynamic performance of obstacle-surmounting more accurately.

Figure 12.

Obstacle-surmounting dynamics model of the UGV “Dragon Horse” [Reference Miaolei, He, Ren and He53, Reference He, Ren, He, Wu, Zhao, Wang and Wu55]: (a) adaptation process; (b) crossing process.

3.2.4. Autonomous motion planning and control of obstacle-surmounting

Many UGV platforms presently overcome obstacles via line-of-sight remote control due to the complexity of the obstacle-surmounting process. However, autonomous obstacle-surmounting is crucial for enhancing its effectiveness of obstacle-surmounting. Numerous academic organizations have investigated autonomous obstacle-surmounting as intelligent technology has advanced. Motion planning and obstacle detection are fundamental technologies [Reference Qiao, Zhong, Chen and Wang90, Reference Su, Qi, Hu, Karimi, Ferrigno and De Momi91]. Lim [Reference Lim, Kang, Yoon, Lee and Kang92] of the Korea Academy of Science and Technology proposed an obstacle classification and scene management algorithm for a 6 $\times$ 6 unmanned ground platform in an unstructured environment, which could automatically obtain the critical point information of obstacles to construct terrain parameters. The platform could then carry out autonomous obstacle-surmounting by the obstacle information (shown in Fig. 13). Obstacle movement for wall-climbing robots, Li [Reference Li and Fu93] from the Chinese Academy of Sciences analyzed the geometric constraints of the obstacle-surmounting robot. Then, a motion planning system based on genetic algorithms was developed to overcome obstacles.

Figure 13.

Autonomous obstacle-surmounting algorithm [Reference Lim, Kang, Yoon, Lee and Kang92].

Figure 14.

Mobility assessment of various action system configurations: (a) wheeled type UGV platform assessment of mobility; (b) tracked type UGV platform assessment of mobility; (c) legged type UGV platform assessment of mobility; (d) complex mobile type UGV platform assessment of mobility.