Magnetic crawler climbing detection robot basing on metal magnetic memory testing technology

Robotics and Autonomous Systems 125 (2020) 103439

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Magnetic crawler climbing detection robot basing on metal magnetic memory testing technology ∗

Fumin Gao, JianChun Fan , Laibin Zhang, Jiankang Jiang, Shoujie He China University of Petroleum-Beijing, Fuxue Road 18, Changping, Beijing 102249, PR China

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Article history: Received 17 July 2019 Received in revised form 5 December 2019 Accepted 13 January 2020 Available online 16 January 2020 Keywords: Climbing robot Overcoming obstacles Nondestructive testing High payload

a b s t r a c t Failure detection of high facilities always presents a tremendous challenge. Climbing-wall robot with detection capacity has become a main approach. But owing to their limitations in overcoming obstacles and complicated wall situation, reliable wall-climbing property and precise detection abilities are the most basic demand for achieving this function. Hence, further research is required to enhance robot capabilities in overcoming obstacles and accurate detection signal. The paper presents a new climbing-wall detection robot mechanism. The wall-climbing robot consists of two climbing modules. The two climbing modules are connected by anti-overturning mechanism to provide a capacity of anti-overturning during overcoming obstacle. The detection mechanism is installed at the bottom of the robot. Detailed design issues are presented with analyses of the design parameters. Transition displacement of anti-overturning mechanism and force transfer equation are derived, and stable operating conditions are verified. The abilities of flat surface locomotion, antioverturning, preload and detection capacity are validated by using experiments. Experiment results show that the prototype achieves 10kg payload capacity on vertical surfaces and can overcome 10mm obstacle. 1mm×1mm circular groove can be found. © 2020 Elsevier B.V. All rights reserved.

1. Introduction At present, some high facilities such as gas and oil tanks, wind turbines and marine vessels, which are impossible to inspect manually, may exist significant security threats. Therefore, developing a remote and automatic climbing detection mechanism is impending. However, the walls have some obstacles such as weld and rust. Meanwhile, there is not any support on vertical wall during climbing. Therefore, design of climbing robot is not easy. On the other hand, metal magnetic memory testing technology (MMM) has a certain sensitive property for failure and region of stress concentration of ferromagnetic materials. Through measuring surface magnetic field signal on the failure area and region of stress concentration, failure condition of equipment can be estimated. However, basing on principle of metal magnetic memory testing technology, MMM senor must be kept a close and constant distance to detected surface. In order to achieve precise MMM signal, stable and reliable locomotion and enough adhesion capacity are necessary and significant. Certainly, for complex wall condition, the reliable climbing ability is important problem need to be resolved firstly. ∗ Corresponding author. E-mail address: [email protected] (J. Fan). https://doi.org/10.1016/j.robot.2020.103439 0921-8890/© 2020 Elsevier B.V. All rights reserved.

So far, wall-climbing robots have been developed in various forms of adhesion technologies. As Table 1 shows, we demonstrate some common climbing methods such as magnetic absorption [1–16], negative pressure [17–26], elastomer [27–31], mechanical biomimetic [32–35] and electrostatic [36–38]. Each climbing method has different types of adhesion. For example, magnetic climbing method have wheel-type [1–10], tracktype [11–13] and plate-type [15,16]. Negative pressure has vacuum [17–20], propeller [21–23] and multilink [24–26]. Elastomer has adhesive footpads [27,28] and track-type [29–31]. Mechanical biomimetic has small spines [32–34] and claw [35]. Each climbing type has its own characteristics and a certain limitation. Some important aspects are compared as shown in Table 1. ‘‘+’’ or ‘‘−’’ marker means that the selected type is suitable or unsuitable, respectively, for use in meeting a requirement. ‘‘ ’’ marker means that the selected type exhibits average performance with respect to the requirement. In terms of adhesion technologies as shown in Table 1, elastomer and electrostatic can be used to attach to various materials. But their payload and reliability are low due to influence of external factors such as dirt. Pneumatic adhesion technology seems to compensate the shortcoming if the vacuum is well maintained. But high energy consumption will limit ability of work. In contrast, magnetic, mechanical biomimetic have strong and reliable adhesion forces. However, Mechanical biomimetic method

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F. Gao, J. Fan, L. Zhang et al. / Robotics and Autonomous Systems 125 (2020) 103439 Table 1 Qualitative comparison of adhesion technologies and locomotion mechanisms related to introduced requirements.

requires some protruded features for grip and may damage material surface. It is not also permitted for detection equipment. Relatively, although magnetic adhesion can be applied only to ferromagnetic materials, Magnetic adhesion technology seems to be best satisfy the stated requirements for application object, reliability and energy consumption. At present, climbing and locomotion function of climbing robots on a vertical wall have been accomplished. But the obstacle-overcoming abilities is still a big challenge for wallclimbing robots [2,27,28]. In order to overcome obstacles and achieve stable climbing capabilities, some reliable wall-climbing mechanisms are proposed. In terms of wheel-type mechanism, speed and stability of wall-climbing robot can meet requirements. However, the obstacle-overcoming abilities of wheel-type mechanism is limited by adhesion structure. Potential of failure is still very large. Although Wolfgang Fischer [8] designed ◦ ridges and angular 135 transitions to solve obstacle-overcoming ability, the line contacting part of magnetic wheel is still insufficient to support whole robot-structure to overcome obstacle. For walking mechanism, Guan [17] presented a biped robot basing on vacuum adsorption to overcome obstacle. The pose of robot legs is controlled by programming to complete strong transition and obstacle-overcoming ability under the premise of

enough energy. TaeWon Seo [27,28] designed a new linkagetype, wall-climbing robot, called WCR. The function of repeated walking and obstacle-overcoming is achieved based on fourbar mechanism and VytaFlex-10 elastomer. However, due to limitation of walking mechanism, the speed and stationarity of the walling-climbing robot is not enough perfect for performing detection task. For track-type mechanism, improvement of obstacle-overcoming ability is mainly through increasing of track units. Seo and Sitti [30] extended Tankbot mechanism and increased the reliability of Tankbot mechanism by connecting two modules, and an active tail was adopted to reduce pitch-back moment. J Krahn [31] utilized TBCP-II to connect two tank-like modules. The transition ability from horizontal to vertical or vertical to horizontal was achieved. In addition, Giuk Lee [24] employed a multilinked track mechanism and pneumatic adhesion technology to perform wall-to-wall transitions by dexterous motion. Although the method by increasing module of track-type mechanism can effectively increase reliability, active joint which is used to connect with track unit cannot support and recover the pitch-back moment during locomotion. Meanwhile, the common failure way of track-type is peeling off from head of track. But this problem has not really been solved. Therefore, preventing pitchback behavior of track and providing corresponding support are crucial.

F. Gao, J. Fan, L. Zhang et al. / Robotics and Autonomous Systems 125 (2020) 103439

Although existing technology realized a certain the obstacleovercoming abilities. However, in order to achieve and complete detecting function, climbing robots not only need to have a certain ability to overcome obstacles, but also have high locomotive stability to ensure that detection system fits wall. Considering to payload capacity and motion process of wall-climbing robot, we thought track type climbing mechanism with magnetic adsorption could meet this requirement for rough ferromagnetic wall. But it is very critical to prevent the robot from overturning when climbs over obstacles such as welding line along vertical wall. In this study, a wall-climbing robot is designed for vertical structure detection. The wall-climbing robot performs climbing ability by using tracks with multiple magnetic units. And a detection system is installed on the wall-climbing robot. The wall-climbing robot consists of two climbing modules. The two modules are connected by anti-overturning mechanism. When wall-climbing robot crosses the obstacles, the two modules interact to prevent the front of one from lifting-up by using antioverturning mechanism. Thereby, the normal adsorption area of the robot crawler belt can be ensured, and the crawler belt of the robot is prevented from falling off due to the front lifting-up. In addition, the two tracks in the first module are independent of each other. When encountering horizontal and vertical obstacles, the second module can provide support for the first module and ensure that the first module passes through the obstacles stably. Reversely, the first module can provide enough adsorption force for the passage of the second module. Detection mechanism is composed of four-bar linkage mechanism and limited by a couple tension springs. The whole detection mechanism is installed at the bottom of the second module. When wall-climbing robot is moving on the wall, detection area can adhere to the wall under the action of the tension springs tightly.

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Fig. 1. Configuration of the wall-climbing robot. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2. Design of modular climbing robot The objective of the design is to develop a wall-climbing detection robot based on metal magnetic memory technology. The wall-climbing robot will be used to detect giant facilities such as storage tank, ship and so on. Besides having enough payload and stability, the wall-climbing robot has a stable thin-wall transitioning and relatively large obstacle overcoming ability to adapt uneven surface and welding seams during detecting. In general, height of welding seam of giant storage tank is below 4 mm. To meet the stated requirement, obstacle-overcoming ability of robot is set 10 mm. The distance of remote control is set as about 1 km. The weight of detection system is about 3 kg. Considered that it may bear other loads, payload capacity of robot is set as 10 kg. In particular, the wall-climbing robot should provide enough adhesion force to ensure detection system can tightly adhere to the uneven wall. Even when the wall-climbing robot is climbing over the weld, accurate data is acquired. In addition, in order to obtain accurate detection signal and keep high detection efficiency, Speed of locomotion is demanded to set within 0.5 m/s. Although track-type method has strong adhesive and transition ability among the magnetic adhesion technologies, if the head of robot’s track raises and front part of track separates from wall during climbing, wall-climbing robot is easy to peel off from head. The wall of the oil tank or marine vessels is uneven, and some welding lines are existed. It is possible and common that the head of wall-climbing robot will raise during vertical climbing. Therefore, a double-linked module mechanism and an anti-overturning mechanism are designed to suppress or recover impending failure state. The wall-climbing robot is illustrated as shown in Fig. 1. Entire structure frame is connected to the rear axle of the first and second module. The front axle of the first and second modules

are linked with anti-overturning mechanism. Anti-overturning mechanism is marked by color lines as shown in Fig. 1. The first module adopts a design like antennae, which is composed of two independent tracks. Track is made of polyurethane. Each track installs 62 rectangular parallelepiped magnetic units. Because the track is soft, each magnet can adapt to the condition of the wall to achieve maximum fit. Each track of the first module is linked with the second module through anti-overturning mechanism. The rear axle of the first and second module is installed in structural frame. The two modules form a closed loop system. It is helpful for recovering balance of the two modules and increasing the reliability of adhesion. The anti-overturning mechanism is linked with the front axle of the first and second module. It is a linkage mechanism and installed two springs. The linkage mechanism makes the first and second modules each interact to prevent the front of the robot track from lifting-up to lead to peel off. Two compression springs of anti-overturning mechanism are to maintain a balance between first and second module and recover the lifting-up state when wall-climbing robot collides weld joint. At the same time, it is helpful to slow the collision. The structural frame is mounted on the rear axle of the first and second module. Some electronic control chips and lithium battery are installed inside. Detection system is installed at the bottom of the structural frame. Detection system is four-bar linkage. Tension spring is mounted on the four-bar linkage to ensure that the sensor can fit the tank wall and make some selfadjustments during climbing over obstacle. Sensitivity of magnetic sensor of detection mechanism is 12 mV/V/Oe.

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F. Gao, J. Fan, L. Zhang et al. / Robotics and Autonomous Systems 125 (2020) 103439 Table 2 Specifications of the prototype. Parameter

Size

Parameter

Size

lAE lAB lGK lKC lCD lGD lGF lMN lOP

113 mm 180 mm 29 mm 114.21 mm 180 mm 157.65 mm 94.18 mm 1.14 mm 105 mm

β γ θ

30◦ ◦ 120.78 45.79◦ 56 mm 75.5 mm 70◦ 118.34◦ 240

h l

ψ ψ lNP

function above. We assume that the height of the weld is K. The center distance of the track of first module is lAB . The inclined angle α of the first module is defined as follows:

α = arcsin

2K

(1)

lAB

When the first track overcomes an obstacle of height K, displacement of the hinge E in the X direction (Fig. 3) is calculated as follows: XE =

√

l2AE − (lAE sin β − lAB sin α)2 − lAE cos β + lAB (1 − cos α) (2)

where lAE is the length of linkage AE, β is the acute angle between linkage lAE and lEF . Then rotation angle γ ′ of the hinge lGK is calculated as follows:

γ ′ = arctan

Fig. 2. Schematic diagram of the wall-climbing robot.

l + XE h

− arctan

l

(3)

h

where h is the height from hinge G to linkage lEF , l is the distance from the hinge F to hinge G along X direction (Fig. 3). Then rotation angle θ ′ of lCD is calculated as follows:

√

θ = 180 − θ − 2 tan( ′

3. Parametric design As mentioned in Section 2, the wall-climbing robot is composed of two modules. Each module is made of two track units. The track is made of Polyurethane. We designed that each track installs 28 pairs of magnetic units. Therefore, there are 96 magnetic units adhered on wall during locomotion at any time. Theoretical adhesion force of each magnetic units is 28 N along the vertical wall. If the friction coefficient of the wall surface is assumed to be 0.2, the two modules can afford 537.6 N load, which will be enough to achieve wall-climbing function. However, in order to ensure reliability of wall-climbing and prevent robot falling from height, obstacle-overcoming ability must be considered. When the wall-climbing robot crosses the vertical wall weld or other convex obstacles, the body of wallclimbing robot will be inclined, and the front half of the body will be separated from the wall. The absorb capacity of magnetic units will fail. At the same time, the weight of the body will produce a tilting moment with respect to the rear axle, and the crawling track of the first module will be separated from the front. These would be a main reason why the robot falls from a high altitude. In order to prevent the track detaching wall from the head in turn, the wall-climbing robot adopts the front and rear dualmodule design. When the front module is lifted, we expect the front part of the second module could be subjected to a pressure to break down the detaching behavior from the head. At the same time, the front part of the first module which has been slanted can restore to the initial state. Based on the principle, the mechanism is designed as shown in Fig. 2. An anti-overturning mechanism is designed and installed between the first module and the second module. It can achieve the

◦

A = − sin(γ − γ ′ ) C =

B−C

B=

l2GK + l2CD + l2GD − l2KC 2lGK lCD

A2 + B2 − C 2

A+

lGD lGK

−

)

(4)

− cos(γ − γ ′ )

lGD lCD

cos γ − γ ′

(

)

where lGD is the distance between the hinges G and D, lGK is the distance between the hinges G and K, lKC is the distance between the hinges K and C, lCD is the distance between the hinges C and D. Displacement of the hinge C in the y direction is calculated as follows: YC = lCD sin θ ′

(5)

In general, the weld height of oil and gas storage tanks is less than 4 mm. the maximum obstacle which we design is 10 mm (K = 10 mm). Based on Eq. (5) and Table 2, displacement of the hinge C in the Y direction is 2.98 mm when the first module climbs over 10 mm obstacle. The specific transition relationship between the displacement of the second module and height of obstacle is shown in Fig. 4. When the first module of robot encounters a large obstacle, anti-overturning mechanism can convert it into a smaller displacement on the second module to close to the wall. The proportion of this conversion gradually increases from 5. Fig. 5 shows posture transition of wall-climbing robot when wall-climbing robot climbs along vertical wall. Fig. 5(a), (b) and (c) show the first module is across the obstacle. When the first module crosses the obstacle, the posture of the second module also correspondingly changes. As soon as the head of the first

F. Gao, J. Fan, L. Zhang et al. / Robotics and Autonomous Systems 125 (2020) 103439

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the middle of the first module climbs over the obstacle, the head of the first module begins to attach to vertical surface under the action of the springs of anti-overturning mechanism as shown in Fig. 5(c). Correspondingly, the head of the second module also follows this change and lifts. But the height of lifting-up is limited as a result of transition relationship of the anti-overturning mechanism. So, absorption force of the second module is enough to provide support for climbing. Fig. 5(d) and (e) show the second module begins to overcome the obstacle. The first module begins to provide a basing adhesion force for climbing of robot. And due to existence of the anti-overturning mechanism, peeling behavior from head will be prevented. Therefore, failure behavior of wall-climbing robot can be counteracted. 4. Quasi-static analysis Fig. 3. A sketch of anti-overturning mechanism of the wall-climbing robot. A, B, C, D, E, F, G respectively represent initial position of hinge. A′ , B′ , C′ , D′ , E′ , F′ represent updated position for corresponding hinges (A, B, C, D, E, F, G) when the robot climbs over K height of obstacle.

4.1. Flat dry adhesion Ndfeb magnet is adopted as the material of the adhesive tread for the wall-climbing robot. Because it has a strong adhesive strength for the wall of ferromagnetic material. Theoretical adhesion strength σA , which is the vertical pull-off strength of the adhesive tread on the surface, is calculated as 0.38 Mpa. Certainly, wall-climbing capacity also is decided by the tangential friction force Ff during vertical climbing. Ff = µFN , Where µ is the friction coefficient. FN is adhesion force. As show in Fig. 3, in order to prevent the climbing robot being overturned and slipping, the adhesion force of magnetic units must overcome the total torque which arises from the gravity. Meanwhile, friction force can support the gravity of the climbing robot.

∑

MD =

∑

Fli + Mg

(6)

i=n

∑ Fig. 4. Relationship between the displacement of the second module and obstacle of height.

module lift, the head of the second module receives a pressure from the anti-overturning mechanism as shown in Fig. 5(a) and (b). It is helpful for preventing peeling off caused by the head of the first and second module lifting-up at the same time. When

F = G1 + Ff

(7)

Magnetic adhesion climbing-wall robot has a special character. It is strong in terms of resisting vertical detachment, but relatively much weaker in terms of resisting peeling detachment. Therefore, the robot’s adhesion unit should sustain the weight of the robot in the direction of the vertical surface. In other words, attachment or detachment can be easily performed along the peeling direction. However. The peeling detachment is most likely to occur when climbing-wall robot overcomes obstacle in vertical wall as shown

Fig. 5. Transition process of anti-overturning mechanism during locomotion of wall-climbing robot.

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F. Gao, J. Fan, L. Zhang et al. / Robotics and Autonomous Systems 125 (2020) 103439

in Fig. 2. The front wheel of first module will be lift. As the adsorbed magnetic unit decreases and the gravity moment relative to the adsorption surface increases, the magnetic unit of the track of the robot will fall off from the front to the rear, resulting in peeling detachment. Therefore, the critical peeling detachment need to be clearly defined. Force of magnetic unit Fm is defined as follows: Fm = Ff + Ft + Fl + FNG

(8)

where Ff is friction force. Ft and Fl are force of track. FNG is support from wall. We assume the second module is stable during overcoming obstacle. Force analysis of the first module is shown in Fig. 6. When the front wheel of the first module climbs over the obstacle, the moment of gravity will increase. The climbing-wall robot will exist dangers of peeling detachment. But there is a kind of critical condition for resisting the peeling detachment. Specially, a critical magnetic unit in the track, which is marked by red (Fig. 6), can balance the increasing the moment of gravity. If the magnetic unit is peeled, the balance condition will fail. In other words, the rest of magnetic unit will not be able to withstand the increasing torque of gravity. The climbing-wall robot will drop from vertical wall. Therefore, the special force of magnetic unit Fm is calculated as follows: √ G (9) Fm = Fp2 + G21 − 2G1 Fp cos θ · sin ϕ + nµ In general, the formulates of Fp , sin θ and cos θ are defined as follows: (XE + lAE cos β − lAB (1 − cos α )) (10) FP = kXE lAE lAE sin β − lAB sin ∂ sin θ ′ = (11) lAE XE + lAE sin β cos θ ′ = (12) lAE ′

′

Fig. 6. A sketch of anti-overturning mechanism of the wall-climbing robot. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7 shows quasi-static force analysis of magnetic unit in the different position of track. There are 12 magnetic units absorbing wall for every track during climbing process. When the climbing-wall robot is overcoming the obstacle, parts of magnetic units near the obstacle would take off. But there exist a special magnetic unit to withstand the force. The special magnetic unit is named as G point. Here, every magnetic unit is assumed as G point. Then the quasi-static force of G point is calculated and analyzed. Meanwhile, the quasi-static force of G point under different heights of obstacle are compared. The consequence is shown in Fig. 7(a). It can be observed that the force of G-point gradually increases with increasing of distance to the first magnetic unit, then rapidly increases. Based on the absorbing limit of magnetic unit, the seventh magnetic unit is chosen as a critical point. When the G-point is beyond the seventh magnetic unit, the first module would peel off the wall, that is, climbing behavior will fail. On the other hand, payload capacity is calculated. The second magnetic unit of the first module is considered as G-point. Then, payload capacity of wall-climbing robot is analyzed for different heights of the weld. From the results, it can be observed that the maximum load of the wall-climbing robot is 15 kg. The effect of load on force of G-point is line and presents positive relationship as shown in Fig. 8. 4.2. Inclining dry adhesion When wall-climbing robot locomotes along inclined surface, the force Fx paralleled to inclined surface plays an important role to prevent falling and slipping. Certainly, the force is determined by friction force and the component of gravity along the slope as

Fig. 7. Quasi-static force analysis of magnetic unit in the different position of track.

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Fig. 10. Simulation results of force of X axis under different angles of inclined surface when robot overcomes different height of obstacle K is height of obstacles..

Fig. 8. Effect of load on force of G-point.

shown in Fig. 9(a). The area which adheres the wall is two modules. So, the total friction Ff of robot under different inclination angles is made of friction force of the first module Ff 1 and the second module Ff 2 . The friction Ff is defined as follows: Ff = Ff 1 + Ff 2

(13)

The surface friction of each module is decided by the adhesion force FN and the component of gravity mi g cos θ under the constant friction coefficient µ. Therefore, force of X component is calculated as follows: Fx = Ff −

3 ∑

mi g sin θ Fig. 11. Prototype of the modular climbing robot.

i=1

=

2 ∑

µFNGi +

i=1

3 ∑ i=1

µmi g cos θ −

3 ∑

mi g sin θ

(14) FN2 is support force from inclined surface. The anti-overturning

i=1

When the robot overcomes the obstacle, the first module and the second modules will be subjected to extra forces from obstacle and anti-overturning mechanism (FN1, FN2, FN31Y and FN32Y ) as shown in Fig. 9(b). FN1 is support force from obstacle when the first module climbs over the obstacle. The relation is defined is as follows: FN1 = Fp sin θ ′

(15)

mechanism will apply a force FN2 on the front of the second module. because the displacement of hinge C is very small, the switching relation is approximately defined as follows: sin(arctan FN2 =

h 1 + XE lGK

) · KXE lGF sin ϕ sin ψ

Fig. 9. Free-body diagram for static analysis on an inclined surface. (a) locomotion without obstacle, and (b) overcome obstacle.

(16)

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307 N. The obstacle is higher and the pressure from anti-overturn mechanism is larger. 5. Experimental results and discussion 5.1. Prototype description

Fig. 12. Prototype of detection mechanism of the modular climbing robot.

where ϕ is the angle between linkages lCK and lGK . ψ is angle between linkages lCK and lCD . FN31Y and FN32Y are support forces for anti-overturning mechanism. The equation is as follows: FN31Y =

m3 g sin θ lOP − Fk lMP lNO + lOP

FN32Y = m3 g cos θ − FK −

(17)

m3 g sin θ lOP − Fk lMP lON + lOP

(18)

where FK is support force for structure frame from the first modkX (l sin β−l sin α ) ule when the front of first module rises, FK = E AE l AB . AE Because the motion of anti-overturning mechanism will affect the support force of robot on the inclined surface. Thus, friction force also changes. Finally, the force of X component is calculated as follows: Fx = µ(FNG1 − FN1 − FN31Y ) + µ(FNG2 + FN2 − FN32Y )

+

2 ∑ i=1

µmi g cos θ −

3 ∑

mi g sin θ

(19)

i=1

where FN1 is support force from the obstacle. FN2 is support force from the inclined surface. FN31X and FN32X are support forces for anti-overturning mechanism. Basing on Eq. (9), We choose the seventh magnetic unit of the first module as G-point because it is a critical condition. Force of X component Fx is shown in Fig. 10 under different angles of inclined surface. K is height of obstacle. Effect of angle of inclined surface on force of X axis surpasses the height of obstacle. The most dangerous angle of inclined surface is 90◦ and Fx is about

The developed prototype is shown in Fig. 11. The robot structure is made of aluminum alloy and stainless steel. The material of track is polyurethane. Magnetic units are fixed by bolts. The weight of robot is 7.5 kg. The max size of robot is 430 × 145 × 234 mm. The detection mechanism is installed at the bottom of the second module as shown in Fig. 12. The mechanism can achieve self-adjusting to keep it close to the wall, which can ensure acquire accurate signal. The motion of robot is controlled by STM32F427. There are four brushless motors to drive the whole mechanism. 5.2. Climbing on the different inclination walls In order to test the climbing property of robot, the assembled prototype is tested by assigning three different tasks. Fig. 13 shows the robot climbs on the flat vertical wall. The wall is welded by Q235 small plates (250 mm × 250 mm). The climbing path of robot has 4 mm and 6 mm high welds. The robot first passes through a flat wall. The flat area is set kinds of prefabricated defect as shown in Fig. 13(a) and (b). Fig. 13(c) shows robot will overcome 4 mm seam obstacle. Because the length of the robot is greater than the distance between two welds. When robot will overcome the second obstacle (6 mm high weld), the body still locates in the first 4 mm weld. The detection mechanism keeps attaching on wall and weld. Fig. 14 shows the wall-climbing robot performs on the inclination of the wall. The angle of inclination is about 100 degrees. Fig. 15 shows steering process of robot on the inclined surface. The robot crawls on the 30 degrees slope. The steering principle is different direction motion between left-side track and right-side track. The robot has a certain steering ability.

5.3. Payload capacity on a vertical surface In order to verify wall-climbing reliability, the payload capacity of the robot is tested on a vertical surface. Fig. 16 shows climbing process of climbing-wall robot with payload 5 kg at a speed of 40 mm/s. Note that robot can carry the payload and overcome 4 mm and 6 mm high weld. Fig. 17 shows climbing

Fig. 13. Experiment of vertical climbing on a flat surface.

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Fig. 14. Experiment of climbing on the inclined surface.

Fig. 15. Experiment of steering on the inclined surface (Angle of inclined surface is 30◦ ).

process of climbing-wall robot with 10 kg payload at a speed

5.4. Transitions and obstacles

of 40 mm/s. Robot also stably passes through 4 mm and 6 mm

In order to test the robot capacity of obstacle-overcoming, we set different obstacles to verify this property. Fig. 19 shows the transition process of climbing-wall robot during overcoming weld obstacle. When the tenth magnetic unit of first module climbs over 10 mm weld, the first module lifts. But anti-overturning mechanism can provide a certain support to recover the state of impending failure. If the second module of robot can stably

weld. Fig. 18 shows climbing process of climbing-wall robot with 15 kg payload at a speed of 40 mm/s. the robot just crawls on wall without welds.

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Fig. 16. Payload capacity test process with 5 kg weight.

Fig. 17. Payload capacity test process with 10 kg weight.

adsorb the wall, the function of anti-overturning is able to be achieved. Based on payload capacity of the magnetic units of the second module, the adhesive force could provide the support for the body during locomotion. So, the lifting-up of the first module will be recovered quickly as shown in Fig. 19. Fig. 20 shows wallclimbing robot overcomes 16 mm height obstacle. The obstacle is a 16 mm thick steel plate. When the first module overcomes

the obstacle, the second module still adheres the wall to provide enough support for climbing process as shown in Fig. 20(a), (b) and (c). Correspondingly, when the second module overcomes the obstacle, the first module supports the whole prototype as shown in Fig. 20(d) and (e).

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Fig. 19. Transitions and obstacles process of climbing-wall robot for 10 mm weld.

Fig. 18. Payload capacity test process with 15 kg weight.

5.5. Reliability evaluation

angle of inclined surface were shown in Table 3. Orthogonal experiment method is used to design experimental content. Here, A, B and C respectively represent load, height of obstacle and angle of inclined surface. 1, 2, 3 and 4 respectively represent specific parameter for corresponding item (A4B3C2 represents payload 15 kg, height of obstacle 8 mm and angle of inclined surface 90◦ ). Successful and failure times of experimental results are counted as shown in Fig. 21. When robot climbs over 6 mm or 8 mm height of obstacle with 15 kg load along 90◦ or 120◦ inclined surface, the success rate is very low (A4B3C2 and A4B2C3). However, robot can have high success rate for the rest of work condition. 5.6. Detection result

In order to evaluate the robot’s performance for climbing over obstacle, we let robot locomote under different condition. Payload capacity, over-obstacle capacity and locomotion capacity are tested. Specific parameters of Load, height of obstacle and

The detection experiments are conducted in the plate. The plate is made of Q235B and made of many small unit welds. One of units has artificial defects. The defects are divided into two area, defecting area 1 and defecting area 2 along detection path

Fig. 20. Transitions and obstacles process of climbing-wall robot for 16 mm obstacle.

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F. Gao, J. Fan, L. Zhang et al. / Robotics and Autonomous Systems 125 (2020) 103439

Table 3 Specific parameters of experiment. Parameters

A-Load (kg)

B-Height (mm)

C-Angle(degree)

1 2 3 4

0 5 10 15

4 6 8 10

60◦ 90◦ 120◦

in the defecting area 2. Defecting area 2 has some 4 mm circular groove. The depth of circular grooves is increased from 0.1 mm to 0.8 mm every 0.1 mm. The climbing-wall robot climbs along the detecting path as shown in Fig. 22(d). The detection result of climbing-wall robot in the vertical plate is shown in Fig. 22. Fig. 22(a) shows original magnetic field signal. In order to clearly characterize defects, the original magnetic field signal is subjected to gradient processing as shown in Fig. 22(b). Fig. 22(c) shows color graphic of detection results. It can be observed that two detecting area and weld joint can be characterized. 6. Conclusion In this paper, a new climbing-wall robot was developed for detecting some high facilities such as gas and oil tanks. Parametric design, analysis, prototype and experiments were presented. The robot was designed based on a double-linked modules mechanism, anti-overturning mechanism, detection mechanism, magnetic adsorption mode. The climbing-wall robot has a 10 kg payload capacity on a vertical surface and climbs over the maximum height of 10 mm weld. And detection capacity is also verified by the artificial defects, the circular groove (diameter is 4 mm and depth is 1 mm) can be found. Declaration of competing interest

Fig. 21. Robot performance for multivariate work conditions. A, B and C respectively represent experimental item. 1, 2, 3 and 4 represent corresponding parameter.

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

as shown in Fig. 22. There are six oval long groove defects with 4 mm width and 2 mm depth in the defecting area. Their length is increased from 10 mm to 60 mm uniformly. There are eight circular grooves with a diameter of 4 mm along detection path

This work was supported by National Key Research and Development Project of China (No. 2017YFC0805803) and Science Foundation of China University of petroleum, Beijing (No. 2462018YJRC035).

Fig. 22. Detection result of wall-climbing robot during locomotion along vertical plate. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

F. Gao, J. Fan, L. Zhang et al. / Robotics and Autonomous Systems 125 (2020) 103439

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Fumin Gao is a lecture of the Safety and Ocean Engineer of the China University of Petroleum (Beijing). His research interests include Nondestructive Testing, inspection robots and climbing robots.

Jianchun Fan is a Professor of the Safety and Ocean Engineer of the China University of Petroleum (Beijing). He mainly focuses on State monitoring and intelligent diagnosis, safety testing and early warning, oil well tube integrity testing and evaluation, tribology.

Laibin Zhang is a Professor of the Safety and Ocean Engineer of the China University of Petroleum (Beijing). He is mainly engaged in the field of oil well pipe and oil and gas production large power units for safety testing, diagnosis and dynamic evaluation of oil and gas production processes and early warning.

Jiankang Jiang is a postgraduate student majoring in safety at China University of Petroleum (Beijing). His research interests include wall climbing robot, Non-destructive testing, Tank safety and intelligent diagnosis and monitoring.

Shoujie He is a postgraduate student majoring in mechanical engineering at China University of Petroleum (Beijing). His research interests include wall climbing robot, Non-destructive testing.