Task-oriented mechanical design of the AApe Power line robots

Task-oriented mechanical design of the AApe Power line robots Hongguang Wang, Yifeng Song and Lie Ling the State Key Laboratory of Robotics Shenyang Institute of Automation Chinese Academy of Sciences Shenyang, China e-mail:hgwang@sia.cn Abstract The mobile robots have increasingly played an important role in the power line inspection and maintenance work. Amongst research on this kind of robots, the robot mechanical design is basic but essential for the entire robot system. This paper presents the task-oriented mechanical design of the AApe series power line robots, AApe-B and AApe-D respectively. Corresponding to the locomotion, obstacle-crossing, maintenance and protection requirements, the mechanical design and analysis is carried out. Experiment results demonstrate the effectiveness of the task-based mechanical design. I. INTRODUCTION To ensure the smooth running of power grid, the regular inspection and maintenance plays an irreplaceable role in obtaining running conditions, finding damages and repairing the faults in time. Nowadays, the inspection and maintenance work is mainly carried out by specialized workers, but the work involves high risks and high working intensity. As a logical substitute of specialized workers, power line inspection and maintenance robots are thus strongly practically required for the normal running of power grid systems. Research on power line inspection and maintenance robots began twenty-five years ago [1]. So far, a number of mobile robots with different structures have been developed for the power transmission line inspection and maintenance [2-4]. The inspection and maintenance of an overhead power line are difficult tasks with the unstructured environment, changeable climate conditions and protection requirements for power line equipments. Therefore, appropriate techniques should be used to make robots successfully implement the inspection and maintenance tasks. The rational mechanical design is surely indispensable and essential for power line inspection and maintenance robots. So far, there have been some literatures on robot mechanical design. Based on the function of the designed mechanism, we can categorize the relevant literatures into two kinds: the mechanical design of mobile platform and the mechanical design of maintenance tools. The mobile platforms need to process a good adaptivity to the environment for a long-distance inspection. [5] presented the design of a teleoperated robot called LineScout. The paper described both the conceptual design and geometric design. The geometric parameters of the robot were optimized by considering both obstacle-crossing sequences and the specific features of the electric power network. [6] introduced an inspection robot with three feasible arms, which could finish inspection task autonomously between strain towers. In [7], an inspection robot named Expliner was designed by combining the structure of manipulator and vehicle, and could apply two different acrobatic modes to cross obstacles in the wire. In [8], the robot, composed by a central body with three serial arms, can complete the obstacle-crossing through the multi-joint motion. For the mechanical design of maintenance tools, the capability of fast and effective repair for power line faults and convenient integration with mobile platform are quite important. [9] introduced a robot, which can install and remove the warning sphere in the wire both in autonomous and tele-operation ways. The team at Hydro-Quebec research institute developed several different maintenance robots. LineROver was developed for the de-icing maintenance task. LineScout, which was with high performance on locomotivity, was designed as a mobile platform, and a series of modularized maintenance tools can be integrated to LineScout for fault sensing and repair, e.g. tightening screws and temporally repairing broken strand [10-11]. In [12], a multi-sensor system, namely a sensor-unit, was constructed for conductor strand inspection. With two cameras and a mirror, all strands of the conductor could be inspected simultaneously. Furthermore, the laser sensors were used to detect obstacles by measuring the variance of the conductor diameter. In this paper, we present the mechanical design of AApe-B and AApe-D robots based on the inspection and maintenance tasks, which consists of the mechanical design based on 978-1-4799-6422-2/14/$31.00 2014 IEEE

locomotion, obstacle-crossing, maintenance requirements in detail. The remaining paper is organized as follows: Section II introduces the background of inspection and maintenance tasks. Section III describes mechanical design based on locomotion, obstacle-crossing, maintenance requirements for AApe power line inspection and maintenance robots. In Section IV, we show the experimental results. Finally, the conclusion and future work are given in Section V. II. THE BACKGROUND OF ROBOT INSPECTION AND MAINTENANCE As shown in Fig. 1, extra-high voltage transmission lines mainly consist of overhead ground wire(ogw), towers, single, double or quad bundled conductors and fittings. Conductors are installed for power transmission, while OGW installed above the conductors is mainly used for the power grid system protection from lightning strikes. According to facility installation requirements, several kinds of fittings, e.g. counterweights, insulators, etc., are mounted on both OGW and conductors. The power line faults occur randomly in OGW, conductors, fittings or towers. and other practical technologies are strongly required under these conditions. III. MECHANICAL DESIGN BASED ON TASKS OF AAPE Supported by the State High-tech Development Plan, the team in Shenyang Institute of Automation, Chinese Academy of Sciences, has been focusing on the development of a series of inspection and maintenance robots AApe. Amongst AApe robots, AApe-B robots and AApe-D, as shown in Fig. 2, have been developed for multi-span inspection and broken strand repair respectively. The AApe-B robot is designed to have a good locomotion and obstacle-crossing performance, while AApe-D robot is designed to meet the maintenance (the broken strand repair) requirements. (a) AApe-B Figure 1. Robots inspection and maintenace Due to the special working environment of power line inspection and maintenance robots, power transmission lines owns some characteristics that need to be under consideration during robot design: Unstructured environment: the fittings, towers and faults in the power line systems are treated as obstacles for the robots. To work in this environment, the robot needs to own a good performance on locomotion and obstaclecrossing. Protection requirements: the safety of power grid systems is the first priority at any time. Therefore, robots are forbidden to cause any damage to the transmission line. The weight of robots are limited due to the load capability of wire, and the force applied to power line fault by the maintenance tool should be in a safety range. Besides, the safety of robot itself should be also taken into account during the period of robot design. Geography and climate: most power grid systems are built in complex geographical environments, (e.g., forests, marshes, and mountains, etc), and under changeable climate conditions. Waterproof, endurance (b) AApe-D Figure 2. AApe-B and AApe-D Robots We set AApe-B and AApe-D robot as examples for the robot mechanism introduction [14-15]. As shown in Fig. 2-a, the mechanism of the inspection robot AApe-B consists of fore gripper 1, rear gripper 5, fore revolute joint 2, rear revolute joint 6, fore prismatic joint 3, rear prismatic joint 7, prismatic joint 4, body 8 and mass center adjustment mechanism 9. The fore/rear grippers 1 and 5 are respectively coupled with fore/rear wheels to increase the adhesion force of driving wheel. With the fore/rear revolute joints 2 and 6, of the grippers 1 and 5 can complete revolute movements. With revolute joints 2 and 6 and prismatic joints 3 and 7, the inspection robot can cross obstacles such as counterweight and clamp. Through prismatic joint 4, the distance between the rear and fore arms can be adjusted for the robot to cross obstacles in different

size. During obstacle-crossing, the flexibility of the inspection robot can be assured with the robot centroid adjustment by joint 9. At the bottom of the body 8, a camera is installed to capture images for inspection. For simplification, the combination joints 1-3 are called the fore arm, while the combination joints 5-7 are called the rear one. As shown in Fig. 2-b, AApe-D is a power line maintenance robot designed for the broken strand repair task in OGW. The robot consists of mobile platform and two specialized tools for the repair task, Looper and Clamper. The specialized tools are installed on the mobile platform. With the wheel structure, the robot can move continuously along OGW. The gripper wheels installed under the driving wheels can increase the adhesion capability of the driving wheel and guarantee the safety of the robot during the maintenance task. A passive revolute joint is installed on each arm to improve the obstacle crossing capability. Through these mechanical designs, the mobile platform can run along OGW, move across obstacles, e.g. counterweights and splicing sleeves, and carry repair tools to the broken strand. A pan-tilt-zoom camera is installed on the electrical box for image acquisition. The specialized tools, Looper and Clamper, play different roles during the repair period. During the maintenance, Looper is first used to put the broken strand back to the original position of OGW, and then Clamper is used to install a clamp at the fault location in case that the broken strand unravels again. The robot operator can operate AApe-D through humancomputer interface at the ground control station, where the robot state and environment state are transferred by wireless data and image modems. A. Mechanical design based on locomotion Optimization design of driving wheel For the inspection robots with the structure of driving wheels, the adhesion force provided by the driving wheels has a strong influence on the robot performance on climbing. As the diameter of wire is quite small, the contact area between the wire and the driving wheel is limited to a small region. The initial design is to enlarge the contact area, as shown in Fig. 3. However, the deign causes a constraint slip between the wire and driving wheel due to the varied radius of driving wheel. And the constraint slip is very harmful to the increase of adhesion force. De on adhesion capability can be expressed in Fig. 4. The increase of R at beginning can enhance the adhesion capability. However, the constraint slip plays a more important role of influencing the adhesion capability with a further increase of constraint. On the base of the initial design, the driving wheel has been optimally designed with an increase of wheel diameter De and a reduction of the radius difference R Posture adjustment Figure 4. Analysis of adhension capability Figure 5. AApe-B climbing with a posture of equal length arms When AApe-B robot implements a climbing task with a posture of equal length arms and no centroid adjustment, as shown in Fig. 5, there is a difference between the support forces of two driving wheels. The unbalanced distribution of support forces could result in an overload of the fore driving motor and a slip of the rear one. Furthermore, the unbalanced distribution of support forces will hinder the motors' normal operation on driving wheels. Figure 3. Driving wheel design After the modeling of driving wheel and the adhesion force calculation, the influence of the radius difference of contact area R and the equivalent diameter of driving wheel Figure 6. AApe-B climbing with a regulated posture The unbalance distribution of support forces is because the centroid is closer to the fore wheel than the rear one in the horizontal direction. Based on the mechanical design of

AApe-B, the unbalanced distribution of support forces can be greatly improved by elongating the fore arm and adjusting the robot centroid to the rear wheel, as shown in Fig. 6. B. Mechanical design based on obstacle-crossing Biomimetic design The design of AApe-B robot was inspired by agile apes which could easily cross the obstacles in the jungle, as shown in Fig.7, and that is the reason the robots were named as AApe. wire. On each arm, there is a passive joint with reset spring. After the driving wheel meets the obstacle, the contact between the driving wheel and obstacle can result in some deformation on the passive joint. The joint deformation can regulate the robot into an appropriate configuration for the obstacle-crossing. With this design, the mobile platform has good performance on obstacle-crossing for counterweights and splicing sleeves, which are the main obstacles for the robot in one span range. C. Mechanical design based on the task of broken strand repair Mechanism design of Looper: broken strand reposition Figure 7. Biomimetic design inspired by apes According to the configuration of the inspection robot AApe-B, the process of obstacle-crossing can be realized by two modes, namely rotate mode and cankerworm mode. In the cankerworm mode, the inspection robot can cross the obstacles like counterweights by lifting the fore/rear arms. The rotate mode is used for the obstacles like clamps, which robot cannot cross in cankerworm mode. Fig. 8 and 9 shows the detailed counterweight-crossing process in both of the modes. Figure 10. Mechanical design of AApe-D Figure 8. AApe-B cankerworm mode With a combination of both obstacle-crossing modes, AApe-B can cross clamps and counterweights. After installing a specialized accessorial rail, it can even cross the strain towers. Figure 9. AApe-B climbing rotate mode Passive joint design for obstacle-crossing After the broken strand fault is detected, AApe-D is installed at the tower nearest to the fault. Therefore, the working range of the platform can be defined in one span. With driving and gripper wheels, the mobile platform can move along the wire smoothly, even if in the 30 inclined Looper is designed to put the broken strand back to the original position in OGW so that a specialized clamp can be installed to fix the broken strand in next repair step. Looper can regulate its state into opening or closing through driving the screw, as shown in Fig. 10-a. Looper always stays the opening state except for the situation of putting the broken broken back to original position in OGW. Inner ring, outer ring and connecting plate are three important components of Looper. Each half inner ring and connecting plate can rotate with respect to outer ring and they are connected with some pins on the connecting plate, while the outer ring is fixed with the frame. In the situation of putting the broken back, Looper is in the closing state. With the robot moving forward, the spring is compressed and the inner ring moves along the red arrow direction, as shown in Fig. 10-b, due to the friction force between OGW and inner ring. As the pin on connecting plate is inserted into the hole of the other half inner ring, the two half inner rings are integrated into an integral ring through the connecting plate. On the integral inner ring, there are the same number of beads as the OGW strand, so each strand can be located between two beads, as shown in Fig. 10-c. With the robot going forward, beads of Looper moves along a spiral line with respect to the axis of OGW. Therefore, the broken strand is forced to the original position by its two lateral beads. Mechanism design of Clamper: broken strand fixing

Clamper is a 2-DOF manipulator mounted on the electrical box to install the clamp at the broken strand end tail to prevent it unraveling again. Fig. 11-a shows the structure of clamper. The Prismatic joint A is used to regulate the installation position of clamp at the same height of OGW and joint B is used to install the clamp in OGW. A finite element analysis is carried out toward the working condition of clamper installation, as shown in Fig. 11-b. Based on the analysis result, some parameters of teeth shape of the clamp is also optimized for the strength of the repair. Furthermore, the connected clamps on broken strand tail end can maintain both mechanical strength and conductivity of OGW. (a)mechanism design of Clamper (b) Deformation of Clamper In order to test the performance and verify the effectiveness of the developed power line robot prototypes AApe-B and AApe-D, a lot of field experiments have been carried out on 500kV extra-high voltage transmission line as shown in Fig. 12. The results of experiments have demonstrated the effectiveness of the mechanical design based on locomotion, obstacle-crossing and maintenance requirements. V. CONCLUSIONS This paper presented the mechanical design of AApe-B and AApe-D robots based on tasks during inspection and maintenance, which facilitates the practical application of power line inspection and maintenance robots. The proposed mechanical design is respectively based on locomotion, obstacle-crossing, and maintenance. The effectiveness of the proposed mechanical design is verified and demonstrated by numerous experimental studies. Aim at the practical application, the future work should mainly be oriented toward the optimum design of the mechanical structure, the reliable obstacle detection and identification, the battery technologies, and the sensor fusion to improve the autonomous level. (c)clamp before optimization (d) Clamp after optimization Figure 11. Mechanical design of AApe-D IV. EXPERIMENTS Figure 12. Experiments References [1] J. Sawada,K. Kusumoto, Y. Maikawa, T. Munakata, and Y. Ishikawa, "A mobile robot for inspection of power transmission lines", IEEE Trans. Power Delivery, vol.6, no.1, pp.309-315, Jan, 1991. [2] S. Montambault and N. Pouliot, "The HQ LineROVer: contributing to innovation in transmission line maintenance", Proc. IEEE Int. Conf. Transmission and Distribution Construction, Operation and Live-Line Maintenance, 2003, pp. 33-40. [3] Y.F. Song, H.G. Wang, and F.R. Jing, "Obstacle performance analysis for a novel inspection robot with passive joints", in Proc. IEEE Int. Conf. Robotics and Biomimetics, 2011, pp. 926-931. [4] H.G. Wang, F. Zhang, Y. Jiang, G.J. Liu, and X.J. Peng, "Development of an inspection robot for 500 kv EHV power transmission lines", in Proc. IEEE Int. Conf. Intelligent Robots and Systems, 2010, pp.5107-5112. [5] N. Pouliot and S. Montambault. "Geometric design of the LineScout, a teleoperated robot for power line inspection and maintenance." IEEE Int Conf. Robotics and Automation, 2008, pp. 3970-3977. [6] M. Nayyerloo, S. Yeganehparast, A. Barati, and M. Foumani, "Mechanical implementation and simulation of monolab, a mobile robot for inspection of power transmission lines", Int. Journal of Advanced Robotic Systems, vol.4, no.3, pp. 381-386, 2007. [7] P. Debenest, M. Guarnieri, K. Takita, E.F. Fukushima, S. Hirose, K. Tamura, A. Kimura, H. Kubokawa, N. Iwama, and F. Shiga, "Expliner - Robot for inspection of transmission lines, Robotics and Automation," in Proc. IEEE Int. Conf. Robotics and Automation, 2008, pp.3978-3984. [8] J. Rocha and J. Sequeira,"The development of a robotic system for maintenance and inspection of power lines", IEEE Int. Symposium on Robotics, Paris, ISR 2004. [9] M.F.M. Campos, G.A.S. Pereira, S.R.C. Vale, A.Q. Bracarense, G.A. Pinheiro, and M.P. Oliveira, "A mobile manipulator for installation and removal of aircraft warning spheres on aerial power transmission lines", in Proc. IEEE Int. Conf. Robotics and Automation, 2002, pp.3559-3564. [10] N. Pouliot and S. Montambault, "LineScout technology: from inspection to robotic maintenance on live transmission power lines", in Proc. IEEE Int. Conf. Robotics and Automation, 2009, pp.1034-1040. [11] N. Pouliot, P. Richard, and S. Montambault, "LineScout power line robot: Characterization of a UTM-30LX LIDAR system for obstacle detection", Proc. IEEE Int. Conf. Robotics and Automation, 2012, pp.4327-4334. [12] P. Debenest, M. Guarnieri, K. Takita, E. F. Fukushima, S. Hirose, K. Tamura, A. Kimura, H. Kubokawa, N. Iwama and F. Shiga, "Sensor-

arm: Robotic manipulator for preventive maintenance and inspection of high-voltage transmission lines" Proc. IEEE Int. Conf. Intelligent Robots and Systems, 2008, pp.1737-1744. [13] J. Katrasnik, F. Pernus, and B. Likar, "A Survey of Mobile Robots for Distribution Power Line Inspection", IEEE Trans. Power Delivery, vol.25, no.1, pp.485-493, Jan, 2010. [14] Y.F. Song, H.G. Wang, Y. Jiang, and L. Ling, "AApe-D: a Novel Power Transmission Line Maintenance Robot for Broken Strand Repair," in Proc. IEEE Int. Conf. Applied Robotics for the Power Industry, 2012, pp.117-122. [15] Y.F. Song, H.G.Wang HG, L. Ling. Research On The Influence Of The Driving Wheel And Robot Posture On Climbing Capability Of A Transmission Line Inspection Robot. 2011 IEEE Conference On Industrial Electronics And Applications NJ, USA.IEEE.2011,1632-1639