FUHAR: A transformable wheel-legged hybrid mobile robot

https://doi.org/10.1016/j.robot.2020.103627Get rights and content

Highlights

  • This paper introduces a mobile robot with a new type of transformable wheel legs that can be used for flat and rough terrain.

  • It integrates the stability and maneuverability of a wheeled robot and the legged robot’s obstacle climbing capacity using a transformable mechanism with wheel legs.

  • Dynamic modeling and design of a control system were obtained. Simulation of robot’s prototype was designed and produced.

Abstract

This paper introduces a mobile robot with a new type of transformable wheel legs that can be used for flat and rough terrain. It integrates the stability and maneuverability of a wheeled robot and the legged robot’s obstacle climbing capacity using a transformable mechanism with wheel legs. With a transformation structure based on a four-bar mechanism, these two modes can be easily changed. This paper analyzes the movements for the proposed robot in wheeled and legged mode. Dynamic modeling and design of a control system were obtained. Then, the obstacle climbing strategies under legged modes were carried out. Finally, on the basis of the simulation, a prototype of the proposed robot was designed and produced. The results from the experiments validate the efficiency of the designed hybrid mobile robot.

Introduction

Recently, in the military industry, the use of mobile land robots (MLR) is rapidly increasing in applications such as exploration, observation, target detection, destruction, search and rescue [1]. Different mechanical designs are developed for mobile robotic land robots to have high maneuverability in variable and challenging environment conditions [2]. Wheeled robots are fast-acting and high-performance as they move on straight roads. However, the wheel structure is not effective for the robot to pass over the obstacle in rough terrain [3], [4]. For this reason, robotic land robots with different types of working principles (wheeled, tracked, footed) are designed by researchers [5]. If the movements of the search and rescue robots according to the working area are analyzed, the robot will have to move on flat ground, for a relatively long time However, it must climb on rough terrains and stairs [6]. For this reason, for MLR, where we have encountered wheeled and tracked designs in previous years, different designs have been made in the last years with different types of legs and hybrid wheel legs [7], [8].

Hybrid and legged mobile land robots are basically robotic systems built from biomimetic (animal modeling) experiments. Their designs are inspired by the physiological and anatomical characteristics of living creatures on land [9]. Movement mechanisms of these creatures are examined and modeled. Basic studies in this field started with understanding the evolutionary development process of land’s creatures [10]. Studies have shown that most of the land animals’ legs emerged after a lengthy evolution. Active and strength legs allowed the animals to move quickly and comfortably on rough terrain [11]. Thus, the idea of applying, designing, and producing legged MLRs to the robotic field was formed. This concept has become possible [12], [13], with the development of advanced mechatronic modules and the manufacturing technologies of bio-heat-treated robots. Research on the design of legged robots has been increasing in recent years [14], [15], [16].

Legged robots are more advantageous than wheeled robots in moving on rough terrains, opening fields and overcoming obstacle surfaces such as curbstone and escalators inside. However, the legged robot, do not have an effective performance on flat surfaces like wheeled robots. For this reason, the researchers have focused on hybrid robot projects, which incorporate rough terrain (with legs). Mobile hybrid robotic platforms are generally classified by their morphology [17]. Hybrid robots could be divided into three groups: the benefits of action mechanisms (legs, convertible wheels) in these two robots. Hybrid robots are basically robots that have great mobility both on the ground (with wheels) and on legged and wheeled robots, rotary-legged robots, and transformable wheel-legged robots.

Legged and wheeled hybrid robots are the easiest kind of hybrid robots with their designs both the leg mechanism and the wheel mechanism (Fig. 1).

The Roller Walker robot (Fig. 1a) is a 4-wheeled, 4-legged (with 3-degrees of freedom) hybrid robot. The wheels are attached to the end of the legs. The robot acts like a four-legged robot on rough ground, with a passive wheel (like a foot walks on the floor). The wheels are activated on flat ground and act as a mobile 4-wheeled robot [18]. The PAW robot (Fig. 1b) combines the benefits of the leg and wheel motion system to increase mobility. The PAW body has a compact and lightweight design. The legs have a limited rotation angle. In wheeled mode, feet turn and provide movement on the wheel. In the legged mode, the wheels are locked and made passive. The dynamic behavior of walking is performed with legs [19]. A similar strategy exists in Hylos robot (Fig. 1c) [20]. The Chariot III robot (Fig. 1d) has 2 big wheels and 4 DOF legs [21]. The Wheeleg robot (Fig. 1e) has two front legs with 3-DOFs pneumatic actuators and two rear wheels powered independently [22]. The Robot Octal Wheel (Fig. 1f) features a wheel–leg mechanism and the robot can climb obstacles such as stairs [23]. The HANZO robot (Fig. 1g) adopted a similar method [24]. HANZO comprises of three main components: the main body, the right, and the left wheel. The main body can be moved upright and in parallel to the right and left wheels. With its mobile body and wheel construction, it has high driving performance on flat grounds due to the HANZO robot design and also can overcome high obstacles. Space Rover robot (Fig. 1h) has a specific system configuration with wheels and intermediate self-adjusting tie rods to ensure proper body posture and enhance stability on the stairs and highland [25]. The Loper robot (Fig. 1i) has a unique combination of Tri-lobe wheels and a very similar chassis [26]. This design allows it to easily cross uneven terrains. Each Tri-lobe wheel is connected directly to high torque, highly sensitive AC servo actuator, and has a simple but robust mechanical design. By turning four Tri-lobe wheels Loper can climb stairs. The robots Epi.q-1 and Epi.q-2 (Fig. 1j) are equipped with three wheels to the lever system which can be extended and shortened on each leg. Thus, it can adjust to rough terrain and move rapidly [27].

Furthermore, hybrid robots with sophisticated original segmented wheel design are the Rotary-Legged Hybrid Robots (Fig. 2) [35]. Due to this structure, rotary-legged hybrid robots are able to overcome obstacles more easily than wheeled robots. For instance, Whegs (Fig. 2a) [28], [29], [30], [31], [32] and Asguard (Fig. 2b) are good examples [33]. These robots’ wheels have no rims. The split wheel structure supports overcome obstacles such as leg robots. In comparison to legged and wheeled robots, all the configuration and control systems of these rotary-legged robots are relatively simple. However, they are quite less stable, and less maneuverable compared to traditional wheeled robots [2]. The researchers developed different split wheel designs [36], [37], [38]. Whegs (Fig. 2c–d) has rimless triple-split wheels. It is successful on rough ground as well as on flat surfaces [29]. On the other side, the Impass robot has a roller structure with two rotating roll mechanisms at a 90 degree. It has original ‘frameless’ rollers with bars that actively change their length to maintain their position and balance when obstacles are climbed [34].

Rotary-legged robots are not suitable for all ground conditions. Therefore, the researchers have sought solutions for both moving on flat surfaces and designing a robot with a high ability to climb obstacles. The third hybrid robot design method, the Transformable Wheel-Legged Robot (Fig. 3), has been developed for this reason. Furthermore, the Hybrid Robots of the transformable wheel-legged robot, have a convertible component wheel mechanism. Due to the convertible mechanism, while the robot runs on flat ground in wheel mode, the mechanism is opened in uneven terrain and moves to foot mode and the robot climbs by holding on the obstacles. Such robots have a high speed on flat ground and an effective ability to climb on uneven terrain. The transformable system design is essential for such robots. Research into this type of robot has flourished in the last few decades [2].

For example, Tadakuma divided each robot wheel into three continuous parts which were connected by two joints. Then he attached motors to each joint and created the transformable wheel–leg robot: the Armadillo-Inspired Wheel–Leg Robot (Fig. 3a) [39], [40]. Because of its motor and connected elements, its wheel mechanism is large. Similarly, a convertible four-wheeled MLR (TurboQuad —  Fig. 3b) was developed by Chen. By designing the wheel in two sections, Chen enabled the wheel, if desired, to be transformed into a mechanism for the leg [41]. The biggest disadvantage of this design is that the inclusion of actuators in the wheel system raises energy consumption. To reduce energy usage, Kim made the transition from wheeled mode to legged mode with a special non-motorized trigger and named this version a Transformer Robot (Fig. 3c) [42], [43]. The researchers used axial thrust force to switch between operating modes. Shen and Chen developed a mechanism that folds each wheel in the axial direction into two semicircles and transforms a Qu-type wheel into a C-type leg (Fig. 3d) [44]. Lee designed a robot with the origami transformer wheel, inspired by the origami folding technique (Fig. 3e) [45]. Chou derived his four-circle leg from the double-circle wheels instead of modifying the wheel configurations by converting the whole robot body and calling it the Transformable Claw Wheel Robot (Fig. 3f) [46]. Assuming that the most important factor of the reversible wheel design is post-conversion management of the working condition. That is wheel stability safety. Yu She and his colleagues developed a robot that determines the mode of operation by the physical contact force (Fig. 3g) [3]. The Wheg robot [47] has two wheels and each wheel has a six-split part’s form that opens in the shape of a star (Fig. 3j). The robot is very efficient in mobility maneuverability and control of trajectories. It is capable of moving on smooth, uneven terrain and steps. The biggest disadvantage of this robot is that the wheel does not have an independent mechanism for opening and closing. Since the mechanism of the wheel is driven by motors that provide the mechanism of motion, the mechanism of the wheel opens and closes only after the robot is stopped.

This article, after analyzing the structure of existing transformable mobile wheeled robots, suggests a unique design of transformable mobile four-legged robots for use in search and rescue missions. Robot’s wheel has a structure with six-split legs. With the effective conversion mechanism, the wheel framework can be opened and closed while the robot is moving. Every wheel is powered by individual motors which ensure that the robot’s torque is strong. It has a body design that is suitable for certain sloping multiple climbing tasks such as flat and uneven terrain, stair surface. While in wheeled mode, the robot can provide smooth, high-speed, and flexible mobility on flat terrain. On the other hand, in the legged mode, it climbs the obstacle quickly with its six-split wheel–leg mechanism placed at 60°. It grasps and climbs in a balanced manner. Another advantage of the six-split wheel-leg structure is that body swings are less than robots with three or two-piece wheels. In addition, the robot which is proposed with the wheel–leg transformable mechanism can perform its operating modes easily while maintaining the robot movement according to the surface conditions.

This paper is organized as follows. Section 2 presents the mechanical design of The Transferrable Wheel-Legged Robot of the proposed robot. Motion Analysis and Dynamic Modeling of Transferrable Wheel-Legged Robot are detailed in Section 3. Section 4 introduces Simulation and Experiment, the strategies of obstacle avoidance under wheeled mode and legged mode respectively, whereas simulation and experiment validation is conducted. Conclusions are shown in Section 5.

Section snippets

Conceptual design

Within the scope of this research, a four-wheel mobile land robot with a transformable wheel structure was designed and manufactured. This robot is named FUHAR (Firat University Hybrid Mobile Robot). The maneuverability and trajectory control ability of the designed robot has novelty features. FUHAR moves on its wheel structure on a flat road. In the uneven terrain, the six-piece wheel mechanism is opened to ensure the ability to climb stairs and high obstacles.

The 3D model of the mechanism was

Motion analysis and dynamic modeling of transferrable wheel-legged robot

In order to analyze the movement of the robot, which is designed physically, under rough, and stair surface conditions, a dynamic model has been created. The system has a total of 5 DoFs (degree of freedom). These are the translational movements of the body on the horizontal and vertical axis, the rotational movement of the body, and the rotational movement of the front and rear wheels. The dynamic model of the system consists of three torque and two force equations that must be applied to the

Motion simulation of robot on different obstacle condition

Motion simulation (Fig. 12) was created for three different surface condition scenarios of the robot in the Matlab environment by using the dynamic equations in Chapter 3. Dynamic responses of the body and wheel fingers of the robot were observed.

  • 1.

    While the robot is moving terrain condition.

  • 2.

    While the robot is climbing over obstacles of different heights and widths

  • 3.

    While the robot is climbing a three-step ladder.

As seen in Fig. 12, the transformable wheels of the robot are in the open position. In

Conclusion

This paper proposes a structured design and control simulation experimental result for a novel active transformable wheel-legged mechanism. A prototype for search and rescue missions is also conducted. According to the design, analysis, and the experiments of the prototype, the conclusions can be drawn as follows:

(1) This unique transformable wheel-legged robot (FUHAR) integrates the stability and maneuverability of a wheeled robot and obstacle climbing capability of the legged robot by

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study was conducted as a part of the Master thesis titled “Design and Prototype of a Mobile Robotic Land Robot” carried out under the supervision of Assist. Prof Dr. Beyda TAŞAR in the Department of Mechatronics Engineering, Faculty of Engineering at Firat University. This project was supported by Firat University Scientific Research Projects Management Unit within the scope of the Master Thesis Project number MF-18-64.

İrem Mertyüz is a Research Assistant of Department of Mechatronics Engineering in Firat University. She received the degree in 2016 in Mechatronic Engineering Department, Firat University, and she graduated the M.Sc. degree Mechanical Engineering University of Firat in 2019. Currently she is interested in autonomous mobile robot, control application.

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    İrem Mertyüz is a Research Assistant of Department of Mechatronics Engineering in Firat University. She received the degree in 2016 in Mechatronic Engineering Department, Firat University, and she graduated the M.Sc. degree Mechanical Engineering University of Firat in 2019. Currently she is interested in autonomous mobile robot, control application.

    Alper Kadir Tanyıldızı is a Dr. Research Assistant of Department of Mechatronics Engineering in Firat University. He received the degree in 2007 in Mechanical Engineering Department, Yeditepe University, and he graduated the M.Sc. degree and Ph.D. degree Mechanical Engineering, University of Firat in 2011 and 2018 respectively. Currently he is interested in multibody control, dynamic system identification, robotic, control application.

    Beyda Taşar is an Assistant Professor Dr. of Department of Mechatronics Engineering in Firat University. She received the Ph.D. and M.Sc. degree respectively 2016 and 2012 in Electrical and from the University of Firat, Turkey. Currently she is interested in prosthetic and biomedical.

    Ahmet Burak Tatar graduated in Mechatronics Engineering from Firat University, Turkey in 2012. He received the Master’s degree in Mechatronics Engineering from Firat University, in 2015. Since 2014, he has been a research assistant in Department of Mechatronics Engineering, at Firat University.

    Oğuz Yakut received the BAs., M.S. and PhD degrees in Mechanical Engineering from Firat University respectively in 1998, 2001 and 2007. He is teaching and researching at Mechatronics Engineering department as an Associate Professor since 2014. His research interests include artificial intelligence, robotics and vibration control.

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