
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)13406-8
10.1016/j.heliyon.2024.e37375
e37375
Research Article
Exploratory research of intelligent gecko-inspired robot based on integrated design and experiment
Qiu Haifei qhf8386@163.com
⁎
Zhang Jiayou
Zhao Jing
Wang Chaohui
Li Chengchuang
School of Mechanical Engineering of XIJING University, Xi'an, 710123, China
⁎ Corresponding author. qhf8386@163.com
04 9 2024
15 9 2024
04 9 2024
10 17 e373754 4 2024
21 8 2024
2 9 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
A quadruped robot with intelligent properties is developed using a bionics approach to explore the potential value of gecko-like machinery. The robot structure incorporates mechanical links, steering engines, and wheel groups, which can expand the movement function of its leg joint. A pneumatic control circuit that can generate negative pressure is built by a vacuum pump, electromagnetic valve, sucker, hose, and others, to enable mobile climbing and adsorption of the gecko-inspired robot. By integrating gait planning, program compilation, Arduino board development, theoretical calculation, and digital modeling, the robot incorporates several practical functions such as “adsorption climbing, ultrasonic obstacle avoidance, remote control, Bluetooth communication, WiFi wireless image transmission, and multi-terrain maneuvering,” which give a basis to realize the multi-dimensional integrated design of “machine, electricity, gas and intelligence” of biomimetic gecko. The experimental prototype of the gecko-inspired robot is designed and manufactured with 3D printing, combined with virtual prototype development, mechanism trajectory verification, finite element analysis, and CFD hydrodynamic simulation. Test results indicate that the biomimetic body has ideal characteristics of intelligent control and maneuvering response in the natural environment, which specifically manifested as that the robot can carry out stable adsorption and climb on the vertical wall, respond quickly to avoid obstacles intelligently, and detect and monitor the external environment in real-time with the help of a mobile phone control terminal. This work is promising for solving high-risk social production and engineering operation challenges.

Highlights

• In this paper, a smart and controllable biomimetic mechanical gecko has been designed and developed through the organic integration of functions such as visual remote control, suction climbing, ultrasonic automatic obstacle avoidance, and WiFi wireless image transmission.

• It is suitable for high-altitude climbing, dangerous rescue, real-time detection, and peripheral environment monitoring, and has promising potential for practical exploration and application prospects.

Keywords

Gecko
Intelligence
Biomimetic
Adsorption
Airflow field
Experiment
Prototype
==== Body
pmc1 Introduction

The gecko is a common reptile in nature that has withstood the survival test of natural selection since ancient times. Its unique, innate functions and body characteristics, such as adsorption climbing, tail-breaking rebirth, and quick response, have earned it a place of interest in modern scientific research and engineering production [1]. In particular, since the 21st century, with the rapid progress of industrial digitization and computer-aided technology, biomimetic machinery inspired by geckos has been widely used in civil, military, and aerospace fields.

Gecko biomimetics currently focuses on two significant aspects: adsorption mode and mobile technology. Several scientists, universities, and research institutions have significantly progressed in this field. Some countries like the United States, Japan, and Germany are at the forefront of developing and applying gecko-inspired machinery [2]. For instance, Stanford University uses tiny artificial rubber hairs for gecko toe adsorption and a 4-bar mechanism with the motor to lift gecko legs. The University of California, Berkeley, and a robotics laboratory have collaborated to create a gecko-inspired robot on wheels that can climb walls using pre-installed and peeling adhesives. Mitsubishi Heavy Industries in Japan has developed a magnetic wall-climbing spraying robot to move on reinforced walls and ceilings. This robot has already been successfully sold and applied in the market. Besides, the Japan Institute of Applied Technology has introduced a wheel-type magnetic adsorption wall climbing robot for large structures, such as oil tanks, spherical gas tanks, and ships, which can replace manual maintenance operations [3,4].

Additionally, Gorb et al. from the Max Planck Institute for Metals in Germany successfully prepared a micro-adhesion array using silicone rubber as a casting material, and they tested the contact adhesion strength on a glass plane. Hoon Eui Jeong et al. of South Korea have proposed a manufacturing method for a microrod array with a multi-level branch structure and a large aspect ratio. Meanwhile, they obtained a micron polymer array by a molding method. Mei Tao's research group at the Hefei Institute of Intelligent Machinery, Chinese Academy of Sciences, used silicone rubber and polyimide as castable materials to prepare micro-adhesive arrays with various aspect ratios by the ICP deep etching method [5].

The previous developments and applications of biomimetic geckos have flaws, such as single functionality and poor environmental adaptability due to the complexity of micro-mechanism and the difficulty of biomimetic realization. As a result, the innate advantages of geckos in biomimetics have yet to be fully utilized [6]. However, with the emergence of 5G mobile communication and the digital intelligence era, biomimetics has undergone a revitalization in its development and application. Even so, the ongoing research on biomimetic geckos still concentrates on creating new biomimetic adsorption materials and innovating mobile technology. So far, few have combined intelligent control with the design and development of gecko-inspired robots. Thus, it is significant to enhance the comprehensive performance and practical value of the biomimetic machinery through intelligent empowerment for the development and innovation of gecko biomimetics.

The purpose of our study is to advance the development of gecko-inspired robots for use in social production and engineering applications. The focus is on integrating various practical functions such as “adsorption climbing, automatic obstacle avoidance, mobile expansion, visual detection, and remote control” into a mechanical system. By doing so, it provides practical solutions for aerial work, danger rescue, real-time monitoring, or similar biomimetic research. Moreover, the work in this paper is based on extensive research, including investigation, analysis, design, modeling, simulation, programming, experiment, and others, which are all beneficial for the technological innovation and exploration of gecko biomimetics.

2 Ideas and methods

As we all know, the gecko is a nocturnal creature and is naturally cautious. It can break its tail to escape in times of crisis and be good at preying on mosquitoes, flies, moths, spiders, and insects. Besides, it is even used as a component in Chinese herbal medicine, as it is considered a beneficial and harmless animal [7]. From a physical standpoint, geckos are small, flat in the back and abdomen, have extended limbs, and are densely covered with setae at their toe ends, as shown in Fig. 1. This unique physique gives geckos strong adhesion ability and reaction mobility, allowing them to crawl freely on vertical walls, eaves gaps, ceilings, and cliffs.Fig. 1 Microstructure of gecko toe [8].

Fig. 1

In order to replicate the biological form of a gecko as accurately as possible, the gecko is divided into seven distinct modules, including the head, eyes, torso, thigh, shank, sole, and tail [9], as shown in Fig. 2. Each module possesses its unique biological structure and function. Table 1 indicates that various biomimetic modules have been developed based on the body structure and movement characteristics of the gecko. The leg joint of the biomimetic gecko is simulated by a mechanical link, wheel group, and steering engine. At the same time, its expected crawling gait is created through the program development of a single-chip microcomputer. Components such as an electromagnetic valve, vacuum pump, hose, and sucker are used to achieve the gecko's sole adsorption. Subsequently, the adsorption and climbing movement of the biomimetic body on a vertical wall is accomplished by orderly controlling the negative pressure on and off of the pneumatic circuit.Fig. 2 Gecko body decomposition.

Fig. 2

Table 1 Ideas and methods of biomimetic design.

Table 1No	Biomimetic function	Implementation method	Remarks	
1	Crawling movement	Mechanical link, steering engine, single chip microcomputer	Simulated leg joint and mobile	
2	Visual exploration	Wireless image transmission, real-time camera shooting	Analog vision	
3	Adsorption	Vacuum pump, electromagnetic valve, sucker, hose, etc.	Simulated microscopic array and setae adsorption	
4	Obstacle avoidance	Ultrasonic sensor	Simulated real-time judgment and site adaptability	
5	Multi-terrain adaptation	Detachable wheel group	Maneuverability expansion	
6	Human-computer interaction	Visual interface, remote Bluetooth control	Production service and engineering application	

On the other hand, to ensure that the biomimetic gecko can work in various environments and be highly maneuverable, Bluetooth technology is used for remote control. Inside the body, ultrasonic ranging and wireless image transmission camera modules are installed to simulate the automatic obstacle avoidance and visual function of the gecko. It enables the biomimetic gecko to respond quickly and avoid obstacles intelligently. Additionally, it can detect, monitor, and analyze the external environment in real-time by employing the display terminal and man-machine interaction interface.

3 Biomimetic mobile joint

3.1 Mechanism design

A leg joint based on the link structure has been designed to make the biomimetic body more light, flexible, and reliable in movement, as shown in Fig. 3(a) and (b). The thigh component is driven by steering engine 1, allowing it to rotate back and forth around the E point, and can enable crawling steering and moving transposition of the biomimetic body. The upper and lower links connect the thigh component and the shank component, forming a planar 4-bar mechanism. The lower link is driven by steering engine 2 to reciprocate around the C point, thus allowing for the lifting and landing of the shank joint. By comprehensively controlling parameters such as rotational speed, steering, and rotation angle of steering engine 1 and steering engine 2, the expected crawling gait can be achieved based on the movement characteristics and mechanism conditions of the gecko.Fig. 3 Biomimetic structure of leg joint.

Fig. 3

From the leg joint structure, it can be observed that the planar 4-bar mechanism is composed of the steering engine 2, the upper link, the lower link, and the shank joint. Equation (1) demonstrates the method of calculating its degree of freedom [10], where the number of moving members n = 3, the low pair PL = 4, and the high pair PH = 0. Therefore, the 4-bar mechanism degree of freedom is F = 1. As the steering engine 2 provides a rotational degree of freedom to the thigh joint, the leg joint's degree of freedom is 2, consistent with the mechanism design conditions and the number of steering engines that provide power.(1) F=3n‐2PL‐PH

3.2 Gait analysis and planning

The Trot gait is a commonly used symmetrical movement pattern for quadruped robots. It is characterized by diagonal consistency, meaning that the two legs on the diagonal line move as a group, with legs in the same group moving at the same pace [11]. The two groups of legs move in half-cycle difference, making it suitable for running at medium and low speeds with high energy efficiency [12]. This gait is often used in the design and development of quadruped robots due to its efficiency and reliability.

A quadruped robot was designed to move like a gecko, as shown in Fig. 4. The robot uses a Trot gait planning method, where a motion period 1 is set, and the duty cycle is set to 0.5. The coordinate output controls the gait timing of the supporting and swinging phases during the biomimetic body's motion and transposition [13]. During Trot gait, diagonal legs (i.e., front left #1 and rear right #3, front right #4 and rear left #2) move simultaneously and switch when approaching half cycle (1-ρ). It is ideal for both leg joints at diagonal positions to lift or land simultaneously.Fig. 4 Timing planning of Trot gait.

Fig. 4

3.3 Trajectory simulation

The motion of a biomimetic gecko's mobile joint is decomposed into three actions - lifting, stepping, and falling [14]. The shank joint is controlled by steering engine 2 to lift and fall, while the thigh joint is controlled by steering engine 1 to move and swing. In an attempt to verify the feasibility of gait planning and mechanism design, the leg joint's moving trajectory was simulated in ADMAS/View software.

Since the rotation speed of the steering engine is between 53 and 62r/min, the drive speed at the joint of the thigh and shank is set to 60r/min. During the mechanism trajectory simulation, the velocity driving at the leg joints is constructed by a Step function to realize the independent moving control of their forward and reverse. The Step function approximates the Heaviside function by a cubic polynomial, which is shown in equations (2), (3) [15]. Where: t-independent variable; t0-independent variable value at step start point; h0-function value at step start point; t1-independent variable value at step endpoint; h1-function value at step endpoint.(2) Step(t,t0,h0,t1,h1)={h0(t≤t0)y(t)(t0<t<t1)h1(t≥t1)

(3) y(t)=h0+(h1−h0)[(t−t0)(t1−t0)]2[3−2(t−t0)(t1−t0)]

During the crawling process of a biomimetic gecko, its shank joint moves by reciprocating lifting. When the steering engine 2 outputs a constant torque, the shank component and the sucker are lifted due to the joint action of upper and lower links. The movement tracks of moving points A, B, and C are shown in Fig. 5. Upon comparing the movement trajectory of the shank components, it can be seen that there are no interferences or collisions when the sucker is lifted from the bottom to the highest position. The angle interval of the circular arc trajectory where points A, B, and C are located is about 0–43°. The sucker rises to a height of about 34 mm, which meets the practical requirements of the gecko's step transposition.Fig. 5 Motion track of shank joint.

Fig. 5

The thigh joint primarily moves back and forth in a Trot gait. Points A, B, D, and E are all in motion during this process, and their trajectories are shown in Fig. 6. Each moving point of the thigh joint follows a circular arc trajectory during the swing, with a swing angle between 0 and 73°. The trajectory lines from the starting to the ending position are A, B, E, and D in that order, with the length decreasing from longest to shortest. The trajectory lines of each segment are smooth and free from interference during the movement, indicating the thigh joint has a reasonable structure, stable movement, and good mechanism characteristics.Fig. 6 Motion track of thigh joint.

Fig. 6

3.4 Finite element analysis

3.4.1 Boundary conditions

The biomimetic joint, which includes a thigh joint, shank joint, and upper and lower links, is a multi-body component manufactured using 3D printing technology with high-strength photosensitive resin. The leg joint's finite element model is created using Workbench software based on setting the actual bearing conditions and a bonded contact between the connecting members [16]. The finite element model of the leg joint is discretized using the hexahedral element Solid186 and tetrahedral element Solid187, which includes 113289 nodes and 51021 elements, as shown in Fig. 7, while adsorbing on the vertical wall.Fig. 7 Finite element model.

Fig. 7

To accurately simulate the bearing capacity of the biomimetic joint, several cylindrical motion pairs are placed at the hinges of moving components like the upper and lower links and the thigh and shank joints. These motion pairs only allow rotation around the central axis [17]. It is worth noting that the selected steering engines only have a single-axis power output. The maximum driving torque of these engines is T1 = 13 kg cm(1.274N·m) and T2 = 2 kg cm(0.196N·m), respectively. Considering the safety of the biomimetic joint, its boundary condition is defined based on the maximum torque. Thus, torque T1 is set along the Y-axis at the hinge of the thigh joint and steering engine 1, and torque T2 is set along the Z-axis at the hinge of the lower link and steering engine 2.

3.4.2 Load calculation

The study focuses on a biomimetic gecko, which has a mass of approximately 1.35 kg. One of the gecko's legs has an effective supporting area of 99 mm2, and the gravitational acceleration is around 9.8 m/s2. When the gecko stands on two feet on flat ground, the reverse pressure on the supporting surface of one leg can be calculated using equation (4), which results in P1 ~ 66818Pa.

However, in another scenario where the gecko crawls on the ceiling, it must generate a more vital reverse adsorption force than its weight [18]. Under this condition, the minimum static pressure acting on the supporting surface of the shank joint should be greater than or equal that on flat ground (P2≥P1). In order to ensure sufficient adsorption safety, the static pressure can be calculated based on equation (5), where P2 = 113591Pa and the safety factor is set to f1 = 1.7.(4) p1=12·mgs

(5) p2≈f1p1

In the third scenario, when the biomimetic gecko clings to and climbs a vertical wall, the maximum static friction between the sucker and wall surface must be equal to or greater than the gecko's dead weight [19]. To ensure safe adsorption on the wall, a safety factor f2 = 1.8 is set while providing a static friction factor μ = 0.45. Equation (6) shows that the minimum supporting static pressure (P3) is approximately 267273Pa.(6) p3=f2p1μ

3.4.3 Stress analysis

The finite element analysis of stress results in the mobile joint of this biomimetic gecko was conducted in different conditions, such as flat ground support, upside-down adsorption, and vertical wall adsorption, as shown in Fig. 8. When the stress distribution of the leg joints in Fig. 8(a), (b), and (c) was compared, it was observed that there is a similar stress distribution in all three states, and the maximum stress occurs near the hinge area between the thigh joint and steering engine 1. The highest Von-Mises stress was produced by flat support (11.4614 MPa), followed by vertical wall adsorption (11.4566 MPa), while the lowest was produced by upside-down adsorption (11.4481 MPa). These results indicate that when both legs of the gecko are supported or adsorbed, the maximum stress exerted on a single-leg joint is approximately 11.45–11.46 MPa.Fig. 8 Stress nephogram of leg joint.

Fig. 8

Photosensitive resin materials used in 3D printing have a tensile strength of about 50∼60 MPa and a bending strength of about 75∼110 MPa. According to the Von Mises stress criterion, as shown in equation (7) [20]. The maximum static stress (σmax) of the biomimetic joint is significantly less than the yield strength (δlimit) of photosensitive resin, which indicates that the stress analysis results comply with the requirements of structural safety and the leg joint has sufficient strength reserve.(7) δVonMises≤[δlimit]

4 Pneumatic adsorption realization

4.1 Formation of negative pressure

The adsorption system in the biomimetic body is divided into four paths, each corresponding to a specific position and orientation layout on the body. These positions are the front left, front right, rear left, and rear right, as shown in Fig. 9. Before conducting the adsorption experiment, it is essential to ensure that the four suckers located at each position are in contact with the vertical wall surface. Next, start the vacuum pump under the control of electromagnetic valves A and B, and pump the air out of the suckers through the tee joint, hose, and other components to create a vacuum negative pressure, which enables the four suckers to simultaneously squeeze and adsorb onto the wall surface under the external atmospheric pressure.Fig. 9 Aerodynamic layout of adsorption system.

Fig. 9

As the biomimetic body adheres to and climbs a vertical wall, a pair of diagonal pneumatic circuits is closed using either electromagnetic valve A or B. Because the closed adsorption circuit is connected to atmospheric pressure, the two vacuum suckers on the diagonal will regain their original shape due to internal and external pressure. Meanwhile, the biomimetic joints will perform adsorption separation and movement by driving program. It is important to note that during the process of adsorption and movement, at least one diagonal sucker must remain in an adsorption state [21].

4.2 Vacuum level analysis

A double-layer corrugated sucker made of silica gel was used for the biomimetic body, taking into account the wall adaptability and adsorption load. This sucker can be easily assembled and connected with the pneumatic circuit through the upper joint pipe, as shown in Fig. 10. Its height is h = 26 mm, and the effective adsorption diameter at the bottom is d = 33 mm. Compared to a regular sucker, the corrugated sucker has a more significant vertical deformation and compression amount, which enables it to achieve a certain swing angle and is particularly useful for adsorbing on slightly inclined and larger radian surfaces in a more stable manner.Fig. 10 Sucker connection diagram.

Fig. 10

According to the theory of vacuum adsorption, there is a correlation between the vacuum level inside the sucker cavity and the time taken to pump gas, as shown in equation (8), where: V-vacuum degree; t-time variable; K1, K2-constants related to vacuum generator, container volume, and environmental pressure; K3-limit vacuum [22]. Obviously, the vacuum degree, V, has an exponential relationship with gas pumping time, t, which indicates that the pumping response time is crucial in achieving a stable adsorption state.(8) V=K1e−K2t+K3

As time passes during the gas pumping process, the internal pressure of the sucker reduces exponentially, as described by equation (8). Simultaneously, the vacuum level within the sucker increases as the internal pressure decreases. Eventually, it approaches the maximum vacuum level of the vacuum pump until the negative pressure within the cavity stabilizes [23]. Under these conditions, an empirical formula can estimate the vacuum degree, expressed as equation (9).(9) V=4Gmfπd2n

In this study, the dead weight of the gecko biomimetic body is Gm = 13.23N, while the effective adsorption diameter of the sucker is d = 33 mm, and the number of suckers is n = 4. To ensure strong and stable adsorption, a safety factor of f = 10 (f > 4 for horizontal adsorption and f > 8 for vertical adsorption) was used. After substituting each parameter value into equation (9), the theoretical vacuum level is approximately V ~ 38.69 kPa. Therefore, to guarantee the absolute safety of adsorption, a vacuum generator with a negative pressure of V0 = −55kPa is selected when building the pneumatic circuit.

4.3 Calculation of adsorption force

During the biomimetic gecko climbs a vertical surface, it maintains balance by ensuring that at least two feet touch the surface simultaneously. To securely stick to the surface, the adhesion force created by the feet should be equal to or greater than the gecko's weight (Gm) [24]. As shown in equation (10), the maximum static friction force (Fmax) between a single sucker and the surface should be equal to or greater than Gm/2, which is approximately 6.62N. Fig. 11 demonstrates the state of adhesion to a vertical wall.(10) Fmax=μFa≥Gm2

Fig. 11 Mechanics model of vertical adsorption.

Fig. 11

On a smooth vertical wall, the strength of the sucker adsorption force (Fa) mainly depends on the vacuum level (V0) inside the sucker cavity. Where: S-effective adsorption area (πd2/4); λ-safety factor (≥2.5); V0-actural vacuum level(-55kPa) [25,26]. Considering the safety design margin, a safety factor is set to λ = 2.8, which is then substituted into equation (11) along with the values of P0 and S to calculate the adsorption force of the sucker: Fa = 16.79N.(11) Fa=V0Sλ

Equation (11) defines the variable Fa as the positive pressure on the wall surface. When the maximum static friction coefficient μ = 0.45, the maximum static friction force between a single sucker and the wall is approximately Fmax≈7.56N. It is important to note that there is a relationship: Fmax > Gm/2, which indicates that the vacuum pump with negative pressure of −55kPa can effectively meet the adsorption requirements of the sucker.

5 Fluid dynamics simulation

5.1 Flow field modeling

The three-dimensional model of the sucker is imported into ICEM CFD software for feature editing and topology reconstruction. The fluid calculation domain is discretized using an unstructured mesh. As shown in Fig. 12(a) and (b), the mesh model of the airflow field consists mainly of triangular and tetrahedral elements. There are more than 1.73 million elements and over 310 thousand nodes inside the sucker cavity, and the mesh sizes are between 0.2 mm and 1 mm, which can ensure no negative volume element in the flow field. Therefore, the mesh distortion also meets the accuracy requirements of numerical simulation. Besides, to better adapt to the boundary conditions of the flow field and simulation operation, a boundary layer was set near the wall of the sucker, in which the first layer grid's height is 0.05 mm, the enhancement ratio is 1.3, and the number of divided layers is 22.Fig. 12 Modeling of fluid computational domain.

Fig. 12

5.2 Iterative calculation

On the assumption of complete turbulence (Re > 104), a steady-state single-phase turbulence model based on the standard k-ε and two-equation has been built in FLUENT software. The k-ε model is a commonly used turbulence model for engineering fluid calculation [27]. According to the Reynolds stress viscosity mode, the turbulent eddy viscosity is a function of the turbulent kinetic energy and turbulent dissipation rate, which is shown in equation (12) [28], where μt-turbulent eddy viscosity, Cμ-empirical constant, fμ-attenuation function on near-wall, ρ-fluid density, k-turbulent kinetic energy, ε-turbulent dissipation rate.(12) μt=Cμfμρk2/ε

As the fluid medium used in the sucker is air, the pressure outside and at the inlet area is considered to be one standard atmospheric pressure (101325Pa) [29]. According to the calculation results obtained from equation (9), it is evident that the actual vacuum level (P0) in the sucker cavity should be greater than the theoretical vacuum level (P). Therefore, the pressure at the outlet area of the flow field is set as −55 kPa to ensure the desired vacuum level.

After initializing the fluid computing domain, a pressure-velocity coupling solution of the flow field is carried out, which is based on several computational fluid algorithms such as simple algorithm, standard wall function, and second-order upwind scheme [30]. During the iterative calculation process, continuity residual (c), turbulent kinetic energy (k), and turbulent dissipation rate (ε) are monitored in real time. As shown in Fig. 13, at the end of 235 iterative calculations performed by FLUENT software, the three residual curves all tend to be stable. At the same time, the mass flow difference curve of the flow field at the inlet and outlet approaches a straight line near the 0 coordinate, as shown in Fig. 14, which indicates that the inlet air volume and outlet air volume are equivalent in the adsorption process. In other words, the internal flow field of the sucker is in a stable state. These results demonstrate that the numerical simulation satisfies the CFD convergence criteria.Fig. 13 Residual iteration curve.

Fig. 13

Fig. 14 Mass flow difference curve.

Fig. 14

5.3 Pressure field

The pressure distribution within the sucker cavity is extracted from the post-processing module of the FLUENT, as shown in Fig. 15. The dynamic pressure of the flow field fluctuates in the central area of the inlet region, which maximum pressure is about 6162.47Pa, as shown in Fig. 15 (a). In contrast, the dynamic pressure distribution near the marginal watershed in the inlet area is more uniform, and the pressure value is relatively small (about 0.05Pa). The difference is that the dynamic pressure distribution at the outlet region presents a noticeable gradient, and the pressure value decreases layer by layer from inside to outside near the central area; as shown in Fig. 15 (b), the maximum dynamic pressure near the center of the outlet area is about 154019.64Pa, which is much larger than the dynamic pressure in the central area of the inlet. It can be seen that the dynamic pressure will make the airflow continuously flow from the low-pressure inlet area to the high-pressure outlet area until the saturated negative pressure and a stable adsorption force are formed in the sucker cavity.Fig. 15 Dynamic pressure inside the sucker.

Fig. 15

As shown in Fig. 15 (c), the pressure state in the inner basin beneath the outlet pipeline is stable, hovering close to 0Pa, indicating a significant vacuum area. In sharp contrast, the dynamic pressure in the central area of the outlet pipeline is as high as 154019.64Pa, which is far greater than that in the vacuum basin of the inner cavity. The resulting pressure difference is helpful to the formation of negative pressure and airflow, which plays a positive role in shortening the response time of air pumping and achieving stable adsorption quickly. To sum up, there is a noticeable pressure difference between the inner basin of the sucker and the inlet and outlet of the airflow, which is conducive to generating sufficient negative pressure in a short time.

5.4 Velocity vector field

Upon comparing the airflow velocity vector fields at the inlet and outlet surfaces, it was observed that the airflow velocities gradually decrease from the central area to the periphery of the two circular areas. The difference in airflow velocities is pronounced, as shown in Fig. 16(a) and (b), in which the airflow velocity in the central area of the outlet surface is as high as 502.81 m/s, which is about five times the velocity in the central area of the inlet surface (about 100.38 m/s), indicating that the negative pressure flow field has a high air pumping response speed, which is beneficial to the sucker to reach a stable adsorption state quickly.Fig. 16 Velocity vector field inside the sucker.

Fig. 16

As a result of vacuum pumping, there are numerous vortex turbulent velocities present in the longitudinal section basin of the suction device. In this state, the direction of the vector arrow causes the airflow to converge from the bottom to the outlet area. The airflow velocity gradually increases from the bottom to the top, particularly in the outlet pipeline basin, as shown in Fig. 16(c). The airflow velocity in this area is as high as 502.13 m/s, which is significantly greater than the airflow velocity in other cavities of the suction device (<200 m/s). This suggests that the airflow in the suction device cavity will be quickly pumped out from the outlet pipeline once the vacuum pump begins.

5.5 Airflow streamline

The airflow streamline is a reliable way to describe the direction and path of airflow in the airflow field [31]. Fig. 17 displays the airflow's trajectory and path inside the suction device, indicating that during negative pressure formation, most of the airflow trajectory and flow direction are regular, with the airflow gradually moving from the bottom of the suction device to the upper pipeline outlet in a curved path. Additionally, the suction device's negative pressure flow field exhibits typical turbulent characteristics. For instance, in the basin of the suction device wall surface and its central cross-section, not only is the airflow direction and trajectory very disordered, but noticeable vortex airflow distributions are also present. These results are consistent with the turbulence simulation expectations of the standard k-ε two-equation physical model.Fig. 17 Airflow trajectory of the field.

Fig. 17

6 Virtual prototype

The bionics method combines the mechanical structure, circuit hardware, and aerodynamic layout of a gecko to create a lightweight three-dimensional model. Fig. 18(a), (b), and (c) display the virtual prototype of the gecko-like robot created in SolidWorks software from different perspectives. The model includes relevant fasteners such as screws, bolts, and nuts, which are directly obtained from SolidWorks/Toolbox to enhance the efficiency of digital prototype modeling and assembly. Furthermore, to guarantee the design accuracy and motion reliability of the biomimetic mechanical gecko, the digital prototype must be free of interference and collision [32,33], which can be achieved through mechanism analysis and motion simulation.Fig. 18 3D model of biomimetic gecko.

Fig. 18

7 Experimental prototype

A control board based on Arduino has been designed and developed to control the electromechanical and pneumatic systems of the biomimetic gecko. As shown in Fig. 19, the development board integrates several electrical components, including a step-down module, motor drive, Bluetooth module, air pump switch, and power terminal, among others. It can communicate with the steering engine control system via the I2C protocol [34]. The HC-05 module enables real-time Bluetooth data transmission to the Arduino control board. The motor drive is primarily used for maneuverability expansion in wheel group mode. To ensure sufficient power supply to the motors, the biomimetic body uses a series system consisting of three 5C lithium batteries. The step-down module reduces the voltage from 12V to 6V to supply power to the 9-way motors, while the other module reduces the voltage to 5V to power the Arduino board and HC-05 module, respectively.Fig. 19 Development board of circuit hardware.

Fig. 19

Due to the weight and development cost limitations of the biomimetic gecko, certain non-standard parts such as body structure, leg joints, head, tail, and neck are created using 3D printing. On the other hand, parts like bearings, steering engines, and fasteners are directly selected based on the required specifications. To enhance the biomimetic gecko's maneuverability and flexibility, a detachable wheel group is also included at the shank joint. It means that the vacuum sucker is replaced by the detachable wheel group, allowing for rapid maneuvering response in various terrains such as flat land, sloping land, or potholes. Refer to Fig. 20 (a), (b), and (c) to see the experimental prototypes in two maneuvering types.Fig. 20 Experimental prototype of biomimetic gecko.

Fig. 20

8 Performance test

Performing a performance test on the biomimetic mechanical gecko involves using a rechargeable lithium battery. The essential steps to initiate the control program of the body system are as follows:(1) Start by pressing the power switch. It will put the functions of movement, adsorption, obstacle avoidance, and visual transmission in a ready state.

(2) At the same time, establish a Bluetooth connection between the control terminal and the Arduino system.

(3) Select and execute various biomimetic function drivers through I2C serial communication.

The performance test includes specific items that cover the following aspects:

8.1 Adsorption and climbing

According to Trot gait planning, the biomimetic gecko needs to maintain a minimum of a group of diagonal suckers in an adsorption state when climbing or staying still [35]. Before initiating the adsorption program, the bottom of the four vacuum suckers must be attached to the vertical wall in parallel. The control terminal is then used to start the program, which opens both sets of electromagnetic valves, generating suction in all four suckers simultaneously. In this condition, the biomimetic body static adsorbed onto the vertical wall, as demonstrated in Fig. 21 (a) and (b).Fig. 21 Adsorption and climbing test.

Fig. 21

When climbing a vertical wall, the biomimetic body uses two electromagnetic valves to generate negative pressure suction by controlling its switches. Meanwhile, each steering engine controls the lifting, stepping, and falling of the leg joints, following the Trot gait timing plan. This allows the biomimetic mechanical gecko to move, change steps, and climb through adsorption. Fig. 21 (c) displays the desorption state of the front right sucker and the adsorption state of the rear right sucker. From the test results, the adsorption and desorption actions are highly sensitive to the switch control of the electromagnetic valves.

8.2 Obstacle avoidance

The biomimetic gecko uses ultrasonic ranging to avoid obstacles. If the ultrasonic sensor's barrier-free sensing distance exceeds 20 cm, the path forward is free of obstacles. In such a situation, the controlling system immediately initiates the straight command to prepare for straightforward movement, as shown in Fig. 22.Fig. 22 Obstacle avoidance test.

Fig. 22

When the sensing distance of obstacles is less than 20 cm, the steering engine in the head swings left and right to detect and select the walking path. If there is a walking space more prominent than 20 cm on the left side, the body executes a left turn instruction. If the distance of barrier-free space on the left side is less than 20 cm, the obstacle avoidance module executes the distance detection on the right side. If the barrier-free space distance on the right side is greater than 20 cm, the body executes a right turn to avoid obstacles. However, if the distance on the right side is less than 20 cm, the path is impassable, and the body turns around and returns.

8.3 Visualization and remote control

In order to accommodate outdoor aerial work and long-distance movement, the biomimetic gecko can be remote-controlled using Bluetooth communication. The effective measurement and control distance of the gecko is around 25m–40m. The gecko's body can be maneuvered in real-time using intelligent mobile terminals such as mobile phones, tablet computers, or handles. The test site of the human-computer interaction interface of mobile phone control terminals is shown in Fig. 23. In addition, the head of the biomimetic body is fitted with a high-definition camera, as shown in Fig. 24 (a). The camera can transmit peripheral images in real-time to the display terminal such as a mobile phone or notebook computer using WiFi wireless image transmission to allow for visual monitoring of the working environment, as shown in Fig. 24(b).Fig. 23 Remote control test.

Fig. 23

Fig. 24 Wireless graphic transmission test.

Fig. 24

8.4 Results summary

The key performance indicators based on the experimental prototype test are summarized in Table 2, specifically including θ1-thigh joint angle; θ2-shank joint angle; h-obstacle crossing height; α-climbing angle of the slope; v-speed of wheel group; t-initialize response time; s1-ultrasonic induction distance; s2-effective distance of Bluetooth control; m-total mass of biomimetic body.Table 2 Performance test data.

Table 2Indicators	θ1/deg	θ2/deg	h/mm	α/deg	v/m·s−1	t/s	s1/cm	s2/m	m/kg	
Values	0∼65	0∼38	30	35∼60	1.2–1.8	1∼2	20	20∼45	1.35	

The test results in different scenarios demonstrate that this biomimetic body can successfully adhere to vertical walls and perform stable walking, transposition, and climbing movements using the Trot gait. Additionally, the negative pressure production, obstacle avoidance, Bluetooth connectivity, and wireless image transmission response times are fast and stable, which enhances the robot's controllability and intelligence. It is worth mentioning that, while experimenting, specific parameters were set lower than their theoretical values to prevent motion collisions or mechanism conflicts. These parameters, namely θ1 < 73°, θ2 < 43°, and h < 34 mm, were attainable by controlling the steering engine while taking into account the actual structure of the experimental prototype.

9 Conclusions

Our study aims to achieve the biological shape and natural function of a gecko using biomimetic technology. The resulting biomimetic gecko robot possesses several intelligent characteristics that enhance its movement reliability, maneuverability, and multi-terrain adaptability. The biomimetic gecko robot also integrates several practical functions, such as adsorption climbing, automatic obstacle avoidance, visual exploration, and remote control. This multi-dimensional integrated design of “machine, electricity, gas, and intelligence” improves the technological innovation of gecko-like machinery and has promising application value.

Compared with similar research, this biomimetic structure of the robot is more lightweight and stable. Besides, the study merges interdisciplinary theories with intelligent digital elements to offer practical solutions for various tasks such as aerial work, hanging wall exploration, danger search and rescue, and real-time monitoring. Overall, the work has immense practical significance for reducing work risks and labor costs in high-risk industries.

Future work

The work presented in this paper is currently in the laboratory testing stage, and some shortcomings still need improvement for the biomimetic gecko, such as the power supply and safety of aerial work, which must be paid more attention. Moreover, negative pressure adsorption requires a substrate with a specific surface quality. Currently, the biomimetic gecko's climbing and adsorption capabilities are only effective for smooth or slightly rough walls. In contrast, its adsorption performance could not improve on substrates with greater roughness or in worse environments. Therefore, upcoming research will focus on the following areas:(1) Replacing the lithium battery with a solar photovoltaic power supply to improve the biomimetic gecko's endurance and outdoor adaptability.

(2) Configuring an intelligent parachute to eliminate the risk of falling from high altitudes and ensure safety when working at high altitudes.

(3) Improving the biomimetic gecko's practical service functions by expanding its technology and improving its structure, such as adding remote voice dialogue, a light mechanical arm, or other operable auxiliary devices.

(4) Exploring the microscopic array and adhesion mechanism of gecko toe to develop new biomimetic adsorption materials that can adapt to different adsorption environments.

Ethical approval

We declare that this study was conducted in compliance with the ethical principles of the Declaration of Helsinki and the requirements of our Institutional Review Board. All data generated during the study were securely stored and accessible only to authorized personnel. No animals were used in this study, and all experimental procedures followed the relevant national and international guidelines for animal welfare. We certify that this research study fully complies with the ethical principles of the Declaration of Helsinki.

Data availability statement

The data are available from the corresponding author on reasonable request.

CRediT authorship contribution statement

Haifei Qiu: Writing – original draft, Formal analysis. Jiayou Zhang: Validation, Software. Jing Zhao: Writing – review & editing, Investigation. Chaohui Wang: Validation, Software. Chengchuang Li: Validation, Software, Investigation.

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.

Appendix A Supplementary data

The following is the Supplementary data to this article:Multimedia component 1

Multimedia component 1

Acknowledgements

We like to thank all the participants and institutions for their technical support as well as all further partners supporting our research work within this application area.

Appendix A Supplementary data to this article can be found online at https://doi.org/10.1016/j.heliyon.2024.e37375.
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