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Abstract A kind of tracked pipeline robot with three-axis driving structures and radially adjustable characteristic is designed. The structure and the working principle of robot are introduced, the mechanical model of adjustable mechanism and its obstacle states are established, the mechanical properties of adjustable mechanism are analyzed, the relationship between traveling-capability and obstacle-climbing capability of robot and the driving force of robot is given, the influence factors of tractionforceandobstacleheightareanalyzed.The experimental results of prototype show that the pipeline robot designed in this paper has a good adaptability to the pipeline environment. I. INTRODUCTION Whether the drainage and the exhaust of urban life, or oil, gas and other industrial fields of material transport, pipelines are widely used as an effective means of transport. In order to make the long-term safe operation of pipeline, the pipeline robotsequippedwithvarioussensorsandoperating mechanical devices to detect , cleaning, welding and a series of pipeline operations by people1-3. As a research hotspot at home andabroad, onlyfrom the driving modesto division, the pipeline robot has developed the wheeled, tracked, legged, creeping, snake-like and other types4-7. However, the differenttypesofpipelinerobotshavecorresponding limitations for the different pipeline environments, so there is a higher requirement for the adaptability of pipeline robot in the pipeline environment. Due to the problems of processing technology and material, there are some conditions such as the irregular shape ofpipelinesectionandtheinconsistentofphysical characteristics of inner wall, which are caused by the welding and other human causes8-9. In addition, the depressions or bulges formed by corrosion, rust, material accumulation and other reasons during the pipeline using, will produce a certain impact on pipeline robot when it travels in the pipeline10. Therefore, the traveling-capability of pipeline robot, the adaptabilityofdifferentpipelinediametersandthe obstacle-climbing capability are the focus of study, it is indispensable to improve the adaptability of pipeline robot in harsh working environment of pipeline. Based on the large contact area of tracked and the pipeline wallhasthestabilitycharacteristics,fromthe traveling-capability of pipeline robot and its adaptability to the pipeline environment, a kind of three-axis tracked pipeline robot with radially adjustable is designed, and the mechanical *Research supported by the National Natural Science Foundation of China (Grant No.51279185). Shan Meng is with the School of Engineering, Ocean University of China, Qingdao,China. (Corresponding author,e-mail: xiaoyinlongmeng). Lei Zhang iswith the School of Engineering, Ocean University of China, Qingdao, China. (e-mail:zhanglei1107). properties of adjustable mechanism, the traveling-capability of pipeline robot and the obstacle-climbing capability are analyzed in detail. II. THESTRUCTURE OFPIPELINEROBOT The overall structure of robot is shown in Fig. 1, it mainly consists of tracked driving mechanism, the adjustable mechanism and the detachable additional power module. Three-axis driving machines form 120 spatial symmetric distribution, its driving force is provided by three motors separately, the motors drive the driving wheels through a bevel gear and a gear set, to drive the rotate of tracked . By controlling the direction of rotation and speed of three motors, the robot can moves forward, backward and steering. The adjustable mechanism drives the lead screw through the step motor, to drive the parallel connecting rods which is connected with the screw nut to adjust the size of three-axis supporting angle, to control the scaling of three-axis. So when the pipeline robot is walking and working in the pipeline within the scope of design, not only it can change the size of robots outer diameter by controlling, but also increase the positive pressure between the tracked and the inner wall of pipeline, to improve the motion characteristics of robot in different posture. The additional power module provides power source for the robot, which satisfies the robots need of cableless operation inpipeline. Inthe case ofshorteroperation time, can also remove the additional power module, use the portable lithium battery to reduce the body weight. Figure 1.The structure of robot. III. THEMECHANICALPROPERTIES OFADJUSTABLE MECHANISM A.Adjustable Mechanism Adjustable mechanism adopts the way of screw nut and parallelconnectingrodstoconnectwiththedriving mechanism to meet the needs of adapt to different pipelines diameter.Sothat when thepipelinerobotwalks inthe pipeline, the tracked of adjustable mechanism can be attached closely to the pipeline wall to generate the sufficient adhesion, and Analysis of Traveling-capability and Obstacle-climbing Capability for RadiallyAdjustable Tracked Pipeline Robot* Lei Zhang and Shan Meng Proceedings of the 2016 IEEE International Conference on Robotics and Biomimetics Qingdao, China, December 3-7, 2016 978-1-5090-4364-4/16/$31.00 2016 IEEE1748 driving the pipelinerobot tomove forwardorbackwardstably. Its basic principle is shown as Fig. 2. When the screw nut pipeline robot crossing obstacle, the pipeline wall is required to have proper positive pressure to the driving mechanism. Therefor, the motor can adjust the screw nut in real time, so it requires a high accuracy of motor. Based on the characteristics of accurate angular displacement and non-accumulative error of stepper motor, the motor selects the stepper motor which connect with the lead screw by coupling. By using photoelectric encoder to measure the turns number and the rotations angle of stepper motor, can calculate the screw nut within the scope of process in the AO accurately. The component AD is driven by the connecting rod BC fixed on the screw nut, to change the corresponding angle size of and . Due to the characteristics of parallel connecting rods, the front and back components have the same action, also make the spatial symmetric distribution of three-axis can be expanded or shrank simultaneously, so that the diameter of pipeline can be calculated currently. At the same time, the pressure sensor on the screw nut can collect the pressure value of tracked with pipeline to feedback, according to the difference between the positive pressure and the set value, the rotating direction of stepper motor is controlled to form a closed-loop control, and ensure there is a proper pressure between the tracked of pipeline robot and the pipeline wall. Not only avoid the problems of impede the movement of tracked, the motor locked-rotor and the tracked abrasion caused by excessive pressure, but also solve the problem of insufficient traction caused by week pressure or even the tracked out of pipeline wall, to provide a stable and reliable driving force for the robot. Figure 2.Mechanical analysis of adjustable mechanism. B. Mechanical Properties of Adjustable Mechanism Due to the symmetry of adjustable mechanism of robot, only need to analyze one of driving mechanisms. Set the center of screw nut to O point, the coordinate system Oxy is established as Fig. 2, the x-axis is the central axis of pipeline robot, the y-axis is through the center of component O and in the symmetry plane of axis . The parameters in Fig. 2 are set as shown in TABLE 1. TABLE I.PARAMETERSSETTINGTABLE INFIG. 2 ParametersThe Meanings of Parameters N Positive pressure generated by pipeline wall to the tracked of driving mechanism FThe force of screw nut on the component BC 、The angle between components L1、L2、L3、L4 The lenght of member AD, BC, AC and lead screw AO respectively T0Output torque on the shaft of stepper motor TEffective torque on the shaft of lead screw h1、h2、h3Vertical distance If the entire tracked driving mechanism is seen as overall of quality evenly distributed, Fig. 2 shows the geometric relationship: sinsin sin coscos 23 211 234 LL hhLy LLLx E B Differential equation (1): cos costansin 1 3 Ly Lx E B According to the principle of virtual work: BE xFyN Put equation (2) into equation (3), the positive pressure produced by adjustable mechanism can be obtained: F L L N)tan(tan 1 3 If the helical pitch of screw nut is P, the relative rotation angle between the lead screw and the screw nut is , so the displacement of screw nut s can be expressed as: 2 P s So: )sin(2 sinsin coscos 321 23 234 hhLD LL LLsL The relationship between the displacement s of screw nut and the diameter D of pipeline can be calculated by equation (6): 32 3 4 43 2 3 2 2 1 22hh L sL sLL LL LD Differential equation (5): 2 P s Transfer efficiency of screw nut is expressed by , according to the principle of virtual displacement: 1749 TTsF 0 Putequation(4)and(7)intoequation(8),the torque-adjusting of screw nut can be obtained: N L PL T )tan(tan2 3 1 If the friction coefficient of tracked and the pipeline wall is expressed by , the traction force Fqof robot can be expressed as: T PL L NFq 1 3 )tan(tan2 It can be known that the traction force of robot is related to the angles of structures, the friction coefficient, the length of components, the helical pitch of lead screw and so on. particularly, its closely related to the torque of stepper motor. Therefore, according to the size of traction, the robots operating speed and the transmission efficiency of system to select the appropriate motor. In the case of a certain power, the speed of stepper motor is inversely proportional to the torque, So if want to get a good traction, can reduce the speed of stepper motor appropriately. IV. ANALYSIS OFTRAVELING-CAPABILITY AND OBSTACLE-CLIMBINGCAPABILITY FORPIPELINEROBOT A.Analysis of Adjusting Torque When the robot ready to operate in the pipeline, the geometric center point O0is not coincident with the geometric center O of pipeline, in this case, its necessary to adjust the adjustable mechanism to make the tracked of driving mechanism to contact with the inner wall of pipeline, and to make the two center lines coincide. In this process, the adjusting torque is mainly to overcome the gravity, and to overcome the sideslipping of two lower driving mechanisms. When the three trackeds of driving mechanisms have been contact with the pipeline wall and produce the appropriate positive pressure, the robot has designed traction force, and the adjusting torque is mainly determined by the traction index. When the attitude angle of robot is 0, the output adjusting torque of motor reach the maximum. Therefore, as long as to analyze this situation, the maximum adjusting torque can be required. Analysis the diagram of adjusting torque to overcome the gravity and the sideslipping is shown as Fig. 3, the pipeline center is denoted as O, the center ofrobot is denoted as O0, the length of pipeline diameter is denoted as D, the current distribution radius of drive wheel is denoted as r, the robots gravity is denoted as G, Nb, Ncare the positive pressure of pipeline wall to the two axis of robot respectively. The work of adjusting motor to overcome the gravity can be obtained: rrDGOOGW 22 0 3 2 1 Figure 3.Analysis diagram of adjusting torque to overcome gravity and sideslipping. From Fig. 3: GN r D D S 2 60sinsin 2 2 Also known from Fig. 2: 321sin hhLr From the principle of virtual work: 0 B xFSN The adjusting torque to overcome the sideslipping is obtained: tantan2 3 3 1 DL GPL T By comparing equation (10) and (16), it can be seen that the adjustment torque is almost determined by the traction index equation (10). B. Force Analysis of Robot in Pipeline The force analysis diagram of robot in the pipeline as shown in Fig. 4. is the angle between the axis of upper part of pipeline and the horizontal positive direction, as the attitude angle of robot . Due to the symmetry of robots structure, as long as to study the attitude angle from 0 to 120. In general, the main body of robot is coincident with the center of coordinate, denoted as O. The robots gravity is denoted as G, Na, Nb, Ncare the positive pressure of pipeline wall to the three-axis of robot respectively. The force balance of X-axis and Y-axis shows: 0)60sin()120sin(sin 0)60cos()120cos(cos cbay cbax NNGNF NNNF 1750 Figure 4.The force diagram of pipeline robot in the pipeline. C. Analysis of Obstacle-climbing Capability There are two main obstacles for the robot in the pipeline, one is raised step type obstacle, the other one is ditch type obstacle. When the ditch type obstacles in large size, it can be divided into up and down two steps of actions. So the obstacle-climbing capability of robot is mainly performance of climbing steps. Because the speed of pipeline robot is very small when encounter obstacles, static analysis can be used to analyze the stress situation as shown in Fig. 5. The force point between the front wheel and the ground is replaced by the obstacle fulcrum when encounter obstacles. The parameters in Fig. 5 are set as shown in TABLE 2. Figure 5.The force diagram of obstacle-climbing of robot. TABLE II.PARAMETERSSETTINGTABLE INFIG. 5 ParametersThe Meanings of Parameters OThe center of robots gravity F1、F2 The reaction force of the front and the rear wheels respectively f1、f2 The friction force of the front and the rear wheels respectively H1、H2 The driving forces of the front and rear wheels respectively dThe diameter of wheel hThe height of obstacle The angle between the reaction force of obstacle on the wheel and the horizontal direction The maximum resistance of obstacle-climbing of robot : sincos 11 fFR By the balance of force: 0)()( 0cossincos 0sincossin 222 2111 22111 cbaFhfhHbaGM FGfFHF fHfFHF y x Assuming the biggest propulsion and resistance can be expressed approximatively as: FH Ff Where : H is the vector sum of H1and H2, F is the vector sum of F1and F2, f is the vector sum of f1and f2, by the equation (18) - (21) can obtain: )(tan)()1( )(tan)1 (tan)( 2 cbah cbabahcG R Where : the size of and a can be expressed by h and d: d h d hd2 1 2/ 2/ sin 2222 )2( 2 1 ) 2 () 2 (hddh dd a The successful conditions of obstacle-climbing of robot is the output torque of motor greater than the resistance moment of obstacle: ) 2 (h d RM On the contrary, if the output torque of motor is known, the maximum obstacle height h of robot can be calculated. According to this way, the other obstacle types can be made an analytical analogy one by one. Through analysis, the other obstacle types suffered maximum resistance are less than above, so the analysis of obstacle-climbing above is the basic requirement. V. EXPERIMENT The prototype of pipeline robot is shown in Fig. 6, of this prototype can suit the diameter of pipeline from 200mm to 300mm. Its lengthis 340mm, andthe total qualityis about 4kg. Similarly, with the adjustment mode of parallel connecting rods and screw nut, the robot can be extended to adapt the different rang of diameter to meet the needs if replace its parallel connecting rods and lead screw. The experiments select the straight pipeline with the diameter of 250mm to carry out the traction experiment of robot. The critical friction coefficient =0.4, the attitude angle of robot =90. 1751 Figure 6.The prototype of robot. A.Traction Experiment of Robot The fixed force sensor is connected to the end of robot, and control the robot to move forward in the straight pipeline, until the robot cant go ahead with the tracked slide on the contact surface. At this time, the measured value of force sensor is the maximum traction value of robot on the contact surface. The experimental data are shown in TABLE 3, the maximum traction value is 124.5N. Traction experiment of robot is shows in Fig. 7. TABLE III.DATASHEET OFTRACTIONEXPERIMENT Types of Contact Surface Results of Traction Experiment(N) Mean Value(N) Friction Coefficient 123 PVC pipeline 0.38124.8122.5126.2124.5 Figure 7.Traction experiment of robot. The size of components in Fig. 2: L1=80mm, L2=60mm, L3=50mm, L4=160mm, the helical pitchofleadscrewP=5mm, the transfer efficiency =0.6, the torque of stepper motor T0.4Nm, the size oftraction is determined by equation (11): Fq128.87N, there is almost no gap with the actual result. Because the regulating mechanism is the multi-parameters system with many parameters constrained by each other, the small error of each parameters can cause the change of final result. Analysising the error source is mainly from the measurementerroroffrictioncoefficientandmotor transmission efficiency. Because the measuring instruments is required higher when measuring these two values in the actual process, the certain error range is allowed. B. The Experiment of Obstacle-climbing Capability Put the obstacle in front of the tracked of driving mechanism in straight pipeline, the height of obstacle is increasedeverytimesfortheexperimentof obstacle-climbing capability, until the robot can not pass through. At this time, with the caliper which precision is 0.02mm to measure the height of obstacle, the obstacle indicator of robot can be measured. The experiment of obstacle-climbing capability as shown in Fig. 8. Figure 8.The experiment of obstacle-climbing capability of robot. Experimentalmeasurement,whenthedriving mechanisms of three-axis contact with the pipeline wall closely, the maximum height of robot can crossing is 8mm, shows that the robot has the good obstacle-climbing capability. As can be seen from equation (22) and (25), if the contact forc