J. Cent. South Univ. Technol. (2010) 17: 1043-1048
DOI: 10.1007/s11771-010-0596-z
Unilateral self-locking mechanism for inchworm in-pipe robot
QIAO Jin-wei(乔晋崴), SHANG Jian-zhong(尚建忠), CHEN Xun(陈循),
LUO Zi-rong(罗自荣), ZHANG Xiang-po(张详坡)
School of Mechatronics and Automation, National University of Defense Technology, Changsha 410073, China
? Central South University Press and Springer-Verlag Berlin Heidelberg 2010
Abstract: A unilateral self-locking mechanism (USM) was proposed to increase the tractive ability of the inchworm in-pipe robots for pipeline inspection. The USM was basically composed of a cam, a torsional spring and an axis. The self-locking and virtual work principles were applied to studying the basic self-locking condition of the USM. In order to make the cooperation between the crutch and telescopic mechanism more harmonical, the unlocking time of the USM was calculated. A set of parameters were selected to build a virtual model and fabricate a prototype. Both the simulation and performance experiments were carried out in a pipe with a nominal inside diameter of 160 mm. The results show that USM enables the robot to move quickly in one way, and in the other way it helps the robot get self-locking with the pipe wall. The traction of the inchworm robot can rise to 1.2 kN, beyond the limitation of friction of 0.497 kN.
Key words: unilateral self-locking mechanism; traction; inchworm; in-pipe robot
1 Introduction
Pipelines have been widely used for transforming oils, gases or something else for a long time. But recently many troubles occurred in pipelines. It was reported that most of them were caused by aging, corrosions, cracks, and chemical damages [1]. Therefore, many kinds of in-pipe robots were developed to solve the in-pipe maintenance problems of inspections and repairs [2-3], such as wheel structure [4], wall-press type [5-7], inchworm like configuration [8-12] and screw type [13].
Only one pneumatic line used, was an inchworm like micro robot invented in Korea in 2007. Its main components were only three chambers, which were controlled orderly to obtain a step forward or backward. Finally, a micro robot with the diameter of 10 mm was finished [14-15]. In 2009, a wheel type robot called MRINSPECT V was developed for the inspection of pipelines with a nominal inside diameter of 200 mm. The robot could act flexibly under different conditions based on the status of clutches set in the driving system. Therefore, it could pass through a complicated tubular structure easily [16]. By utilizing an electromagnetic actuator as the servo driver for the movement in biomedical fields, a micro robot was accomplished by GUO et al [17]. The robot’s power was obtained outside and its locomotion direction might be reversed by changing the frequency of its magnetic field. An in-pipe micro robot actuated by piezoelectric bimorphs was presented by LIU et al [18] in 2009, which could provide a stable and accurate locomotion inside pipes with diameters ranging from 16 to 18 mm.
The tractive ability of the robots mentioned above is limited because of their complete dependence on the static friction. And the friction will convert to resistance force when the robots move forward. When the pressure against the pipe wall increases to improve the robot’s tractive ability, the forward resistance will also magnify accordingly. In this work, a unilateral self-locking mechanism (USM) was proposed to arm the inchworm in-pipe robots. The mechanism enables the in-pipe robot to move quickly in one locomotion direction and to keep self-locking with the pipe wall in converse, with a high load capability beyond the limitation of friction.
2 Configuration design
Fig.1(a) shows the inchworm type robot composed of three parts including two crutches and one telescopic mechanism. As a crutch leg of the robot, USM is located at the crutch circumferentially 120? apart from each other. As illustrated in Fig.1(b), the gait cycle of the robot contains three phases. From state 1 to state 2, crutch 2 keeps self-locking with the pipe wall while crutch 1 slides along with the pipeline. Then, from state 2 to state 3, the telescopic mechanism stretches out, and it is the turn of crutch 1 to get locked and crutch 2 begins to slide.
Fig.1 Schematic diagram of inchworm in-pipe robot (1—Crutch 1; 2—Telescopic mechanism; 3—Crutch 2): (a) Prototype; (b) Locomotion principle
After a gait cycle the robot makes a step forward of ΔL.
USM is composed of a cam, a torsional spring and an axis, as depicted in Fig.2. The cam and the torsional spring can rotate around the axis connected with the crutch body. The torsional spring links the cam with the body, which ensures the cam to contact with the pipe wall all the time.
Fig.2 Schematic diagram of USM: (a) Three-dimensional model; (b) Substance photograph
3 Self-locking condition analysis
The USM can be simplified, as shown in Fig.3. Cartesian coordinate system Oxy is located in the centre of the axis. B is the contact point of the cam and pipe wall, and F is a force applied to the robot.
Fig.3 Simplified model of USM
It is assumed that all the elements are rigid enough, and the axis can only move along the pipe line. The friction coefficient between the cam and the pipe wall is f, and the friction coefficient between the axis and the cam is μ. When the direction of F is shown in Fig.3, pressure angle θ will decrease and contact point B will move rightward. If the parameters involved meet the basic locking condition, the USM will get self-locking and keep still with the pipe wall. In order to keep self-locking, the intersecting point of the composite force and x-axis needs to lie between original point O and point (rμ, 0), where r is the radius of x-axis.
Torsional spring torque Mk and friction resistant torque Mμ can separately be substituted by force Fk and Fμ at contact point B as
(1)
where R is the distance from point B to original point O.
If the intersecting point of the composite force and x-axis just falls at point O, there will exist
(2)
where Fn is the pressure on the cam at point B; and Ff is the corresponding friction.
And if the intersecting point moves to point (rμ, 0), there will be
(3)
where L is the distance from the pipe wall to original point O.
Combining Eq.(2) with Eq.(3), the self-locking inequation can be obtained:
＜
＜
(4)
To use the principle of virtual work, θ is selected as the generalized coordinate with original value of θ0. The initial forces involved in this system are F, Mk, Mμ, Ff and Fn. There exists
(5)
For δx=Rδθ and δy=0, Eq.(5) can be simplified as
(6)
Besides, Mf can also be described as
(7)
where 
then
(8)
Putting Eq.(8) into inequation (4), the result can be expressed as
＜
＜ (9)
where 
So, it can be inferred that if all the parameters involved satisfy inequation (9), the USM will realize the function of self-locking with the pipe wall.
4 Calculation of unlocking time
When the direction of F is reversed, angle θ will increase and contact point B will move leftward. Then, the cam will get loss and slide along the pipe. Viscous friction coefficients here are b1 and b2, corresponding to μ and f, respectively.
Torques Mn and Mμ generated by pressure Fn and friction Fμ are respectively
(10)
Then, the torque equilibrium equation will be
(11)
where J is the rotary inertia of the cam; K is the torsional spring’s rigidity; b1 is the viscous friction coefficient between the axis and the cam; and b2 is the viscous friction coefficient between the cam and the pipe wall.
The boundary conditions are
(12)
Then, the derivative of θ(t) can be obtained as
(13)
where 
and 
Supposing
then, unlocking time tp is computed as
(14)
Additionally, if 0.3≤ξ≤0.8, a much more precise parameter called rising time tr that also describes the responding time can be obtained as
(15)
where K is the torsional spring’s rigidity. As an example, a set of parameters that meet inequation (9) are selected, as shown in Table 1, which can be used to make prototype, simulations and experiments.
Table 1 Parameters of USM
According to Table 1, the unlocking time (tp) and responding time (tr) can be respectively calculated as
tp=0.04 s, tr=0.02 s (16)
Therefore, when the telescopic mechanism takes larger tp and tr illustrated in Eq.(16) to draw back or stretch out, the inchworm in-pipe robot will move forward.
5 Simulation and experimental tests
5.1 Simulation and discussion
The model to be simulated is illustrated in Fig.4, with all disturbing factors eliminated. The simulation was executed in the environment of software ADAMS.
Fig.4 Simplified model for simulation
With all parameters set as listed in Table 1, force F was applied to the crutch body, as shown in Fig.3. A static balance was verified before dynamic simulation. The simulation consisted of two stages. Some essential results are shown in Figs.5-6. During the first stage, force F was initially 1.0 kN and then rose to 2.5 kN.
The velocity of the crutch body along the pipeline is almost equal to zero with a traction of 1.0 kN, as shown in Fig.5(a). It can be considered as keeping still. So, the USM’s anticipative function of unilateral self-locking is achieved successfully. The departure from zero is due to the problem of the software itself, because its result cannot be odd with a positive input. The absolute value shown in Fig.5(b) is just equal to F, whose negative sign means the inverse of force F. When force F increases from 1.0 to 2.5 kN, the force acting on the cam goes up accordingly, as shown in Fig.5(c), which proves its powerful tractive ability. At this time the USM keeps still with the pipe wall whose result is just the same as the curve, as shown in Fig.5(a).
Then, in the second stage the direction of force F is reversed. The results obtained are illustrated in Fig.6.
Figs.6(a)-(b) show the velocity of the crutch body and the force acting at point B along the pipeline separately. Two curves both clearly consist of three steps. During step 1, the velocity keeps almost zero and the force drops dramatically. This means that the cam releases locking with the pipe in this process. The time duration of this period is the unlocking time, and its value is 0.03 s, as shown in Fig.6(b), very close to the calculated result of Eq.(16). Then, the robot moves with a constant acceleration in step 2, as shown in Fig.6(a). In this process, the USM slides along the pipe with a friction
Fig.5 Results of simulation in self-locking direction: (a) Velocity of crutch body; (b) Force along pipeline at point B with traction of 1.0 kN; (c) Force along pipeline at point B with traction of 2.5 kN
of 0.5 kN. At the last step, the robot runs out of the pipe.
5.2 Experiments and discussion
An inchworm in-pipe robot with the USM was constructed, as shown in Fig.1(a), and some experiments were carried out in a steel pipe with 160 mm in diameter and 6 m in length. The experiments included velocity and tractive ability tests.
The inchworm robot was firstly pushed into the
Fig.6 Results of simulation in sliding direction: (a) Velocity of cam; (b) Force along pipe on cam
steel pipe, as shown in Fig.7(a). Then, the air cylinder CDG1BN80-100J-B54 began to play its role in the form of extension and contraction as an inchworm muscle, as illustrated in Fig.7(b). Its effective delivery force was 1.76 kN and the needed air pressure was about 0.7 MPa. During this process, the cylinder stretched out and drew back again and again, and the crutches slid along or got locking with the pipe orderly, as shown in Fig.1(b). Finally, the inchworm robot got out of the pipe, as shown in Fig.7(c).
While the inchworm in-pipe robot moved in the pipe from one end to the other, it was timed. Some results of the velocity experiments are given in Table 2. The average velocity is 0.177 m/s, which not only verifies that the USM can help the inchworm in-pipe robot to move, but also shows the robot’s high speed capability. The good performance explains that self-locking and unlocking functions of the USM are achieved in practice.
In the tractivie ability test a windlass was connected at the tail of the robot as a load, corresponding to force F used in the simulation above. In order to get the maximum load, the load was added step by step, with a size of 0.1 kN. The digital push-pull tension gauge L520674 was employed to measure the force between
Fig.7 Photos of experiments: (a) Being Inserted; (b) Beginning to move; (c) Moving out of pipe
Table 2 Results of sliding experiment
the windlass and the crutch. Its indicating value increased with the winding up the windlass, and the robot kept moving forward when the indicating value increased to 1.2 kN. During this process, the velocity of the robot kept constant all the time. When the pull rose to 1.2 kN, it stopped. There was no moderating process between the movement and stillness. After that, the friction between the robot and the pipe was measured by pulling the robot forward and the indicating value of tension gauge was 0.497 kN.
The friction is obviously much smaller than the maximum load of 1.2 kN. Therefore, it is verified that the USM can automatically improve the traction according to the increase of the load outside and rise up without the constraint of the maximum friction. It is observed that the inchworm in-pipe robot’s maximum tractive ability has not reached up to the theoretic value of 1.76 kN. Two main reasons account for this phenomenon. One is the decrease of air pressure because of the leakage; the other is the sufficiency in pre-tightening force of the torsional spring which will be emphasized in the future research.
6 Conclusions
(1) A new mechanism USM applied to the inchworm in-pipe robot is developed. This mechanism makes the in-pipe robot obtain an inchworm motion and can automatically improve its traction with the increase of the load outside.
(2) The way to design the USM is described, and the relationship among all the parameters involved is deduced.
(3) The unlocking time from contact to loss is calculated, which is a key parameter for choosing the action frequency of telescopic mechanism.
(4) A prototype is constructed with a set of selected data, and its digital simulations and experiments are carried out. The tractive ability increases to 2.5 kN in simulations and 1.2 kN in experiments, both surpassing the friction. The results show that the USM can keep self-locking, unlock and finally help the in-pipe robot to increase its tractive ability beyond the limitation of friction.
References
[1] WANG Dian-jun, LI Run-ping, HUANG Guang-ming. Progresses in study of pipeline robot [J]. Machine Tool and Hydraulics, 2008, 36(4): 185-187. (in Chinese)
[2] CHOU C, JUNG S. Pipe inspection robot with an automatic tracking system using a machine vision [C]// Proceedings of 2006 SICE-ICASE International Joint Conference. Busan: SICE-ICASE Press, 2006: 5647-5652.
[3] XIA Qin-xiang, SHANG Yue, ZHANG Shuai-bin, RUAN Feng. Numerical simulation and experimental research on the multi-pass neck-spinning of non-axisymmetric oblique tube [J]. Chinese Journal of Mechanical Engineering, 2008, 44(8): 78-84. (in Chinese)
[4] TACHE F, FISCHER W, CAPRARI G, SIEGWART R, MOSER R, MONDADA F. Magnebike: A magnetic wheeled robot with high mobility for inspecting complex-shaped structures [J]. Journal of Field Robotics, 2009, 26(5): 453-476.
[5] LEE Y P, KIM B, LEE M G, PARK J. Locomotive mechanism design and fabrication of biomimetic micro robot using shape memory alloy [C]// Proceedings of International Conference on Robotics and Automation. Barcelona: IEEE Press, 2004: 5007-5012.
[6] WAKIMTO S, NAKAJIMA J, TAKATA M, KANDA T, SUZUMORI K. A micro snake like robot for small pipe inspection [C]// IEEE Proceedings of International Symposium on Micromechatronics and Human Science. Nagoya: IEEE Press, 2003: 303-308.
[7] TANG De-wei, LIANG Tao, JIANG Sheng-yuan, DENG Zong-quan, YU Wei-zhen. Mechanism and simulation analysis of mechanical self-adaptive pipe-robot [J]. Robot, 2008, 30(1): 29-33.
[8] OMORI H, NAKAMURA T, YADA T. An underground explorer robot based on peristaltic crawling of earthworms [J]. Industrial Robot, 2009, 36(4): 358-364.
[9] ZHANG Rong-yi, WANG Ke-jun, SHI Wei, BEN Xian-ye. Development of new peristalsis micro in-pipe robot based on SMA [J]. Journal of Nanjing University of Science and Technology, 2009, 33(S1): 252-256. (in Chinese)
[10] GUO Tong, KE Ying-lin, LI Jiang-xiong. Inchworm-like in-pipe micro-robot actuated by piezoelectric slotted cymbal [J]. High Technology Letters, 2005, 15(11): 35-38. (in Chinese)
[11] CHOI C, JUNG S, KIM S. Feeder pipe inspection robot with an inch-worm mechanism using pneumatic actuators [J]. International Journal of Control, Automation, and Systems, 2006, 4(1): 87-95.
[12] NAKAZATO Y, SONOBE Y, TOYAMA S. Development of an In-pipe micro mobile robot using peristalsis motion [J]. Journal of Mechanical Science and Technology, 2010, 24(1): 51-54.
[13] LI Peng, MA Shu-gen, LI Bin, WANG Yue-chao. Design of a mobile mechanism possessing driving ability and detecting function for in-pipe inspection [C]// Proceedings of International Conference on Robotics and Automation. Pasadena: IEEE Press, 2008: 3992-3997.
[14] LIM J, PARK H, MOON S, KIM B. Pneumatic robot based on inchworm motion for small diameter pipe inspection [C]// Proceedings of the 2007 IEEE International Conference on Robotics and Biomimetics. Sanya: IEEE Press, 2007: 15-18.
[15] LIM J, PARK H, AN J, HONG Y S, KIM B, YI B J. One pneumatic line based inchworm-like micro robot for half-inch pipe inspection [J]. Mechatronics, 2008, 18(7): 315-322.
[16] ROH S G, KIM D W, LEE J S, MOON H, CHOI H R. In-pipe robot based on selective drive mechanism [J]. International Journal of Control, Automation, and Systems, 2009, 7(1): 105-112.
[17] GUO S X, PAN Q X, KHAMESEE M B. Development of a novel type of micro robot for biomedical application [J]. Microsyst Technol, 2008, 14: 307-314.
[18] LIU Pin-kuan, WEN Zhi-jie, SUN Li-ning. An in-pipe micro robot actuated by piezoelectric bimorphs [J]. Chinese Sci Bull, 2009, 54: 2134-2142.
(Edited by CHEN Wei-ping)
Foundation item: Project(2007AA04Z256 ) supported by the National High-Tech Research and Development Program of China
Received date: 2009-12-30; Accepted date: 2010-03-29
Corresponding author: QIAO Jin-wei, PhD; Tel: +86-13298654596; E-mail: joe613722@126.com
