Design and Implementation of a Contact Aerial Manipulator System for Glass-Wall Inspection Tasks IROS2019国际学术会议论文集 0740

Abstract Glass curtain walls have been widely used in modern architecture This makes it urgent to inspect and clean these glasses at regular intervals Up to now most of these work is performed by workers which is expensive and inefficient Therefore a novel robot the contact aerial manipulator system is developed The new designed system presents priorities in the aspects of high flexibility and easy operation In this paper the system mechanical structure is first introduced Subsequently the hybrid force motion control framework is utilized to realize the precise and steady motion on the two dimensional plane and maintain a certain sustained contact force simultaneously Finally two flight experiments including continuous square wave trajectory tracking and aerial drawing task are performed and the results indicate that the developed contact aerial manipulator works and presents good performance I INTRODUCTION A large number of high rise buildings with glass curtain walls appear in modern cities nowadays These glass walls need inspection at regular intervals 1 to ensure safety This task is mainly done manually by workers and relies on using gondola systems which is dangerous high cost and low efficiency Several kinds of climbing robot systems are developed by researchers to conduct this task such as vacuum gripper robot 2 sliding suction cup robot 3 electroadhesion robot 4 and legged robot 5 Although they can achieve climbing the glass wall there still exist some difficulties and limitations in practical applications for these climbing robots 6 For instance the poor flexibility of movement is the disadvantage of this class of adhesion robots In addition the non uniform building structure e g depression in the surface and diverse task demands including detect cracks and mark and locate defects makes the glass wall inspection task a challenging problem for robotization This work is supported by the Science and Technology Planning Project of Guangdong Province Grant No 2017B010116002 and the National Natural Foundation of China Grant Nos U1608253 and 61433016 Xiangdong Meng is a Ph D candidate at the State Key Laboratory of Robotics Shenyang Institute of Automation Institutes for Robotics and Intelligent Manufacturing Chinese Academy of Sciences Shenyang 110016 China and University of Chinese Academy of Sciences Beijing 100049 China Corresponding author e mail xdmeng09 Yuqing He is with the State Key Laboratory of Robotics Shenyang Institute of Automation Institutes for Robotics and Intelligent Manufacturing Chinese Academy of Sciences Shenyang 110016 China and Shenyang Institute of Automation Guangzhou Chinese Academy of Sciences Guangdong 511458 e mail heyuqing Jianda Han is with the State Key Laboratory of Robotics Shenyang Institute of Automation Chinese Academy of Sciences Shenyang 110016 China and Nankai University Tianjin 300071 China e mail jdhan A novel robot the contact aerial manipulator system is studied and tried to be applied to this scenario An aerial manipulator usually consisting of a rotorcraft unmanned aerial vehicle UAV and a robotic manipulator can conduct interactive operations while in flight as shown in Fig 1 Compared with the general wall climbing robots 2 5 it has high flexibility and strong mission adaptability which can avoid obstacles or even jump over grooves on wall surfaces The application of an aerial manipulator in the glass wall inspection task is a suitable choice However in addition to the physical system implementation three key control problems need to be addressed the aerial manipulator is required to maintain steady contact with wall surfaces track the trajectory on a 2 dimensional 2D plane and in the meanwhile control the contact force Figure 1 An aerial manipulator is moving on the surface of a glass wall and marks the traveled trajectory the black lines Rotorcraft UAVs are under actuated systems with strong nonlinearity and coupling and sensitive to disturbance making these aforementioned control issues complicated In addition it is difficult to move on glass curtain walls for the surface is usually smooth and rigid Some existing research has tried to solve this class of problem An approach combined with motion planning and stabilization control is applied to leg wall climbing motion of a quadrotor robot and a ground test bed is used for theoretical analysis 7 In 8 the quadrotor dynamics is transformed into that of the tool tip position a passive decomposition method is designed to realize the force and position control in simulation environment In 9 a control framework combined path planning and physical interaction control is proposed for contact based inspections and evaluated by flight experiment A multi directional thrust octorotor with an inspection arm is Design and Implementation of a Contact Aerial Manipulator System for Glass Wall Inspection Tasks Xiangdong Meng Yuqing He and Jianda Han 2019 IEEE RSJ International Conference on Intelligent Robots and Systems IROS Macau China November 4 8 2019 978 1 7281 4003 2 19 31 00 2019 IEEE215 applied to contact inspection 10 11 and a three layer control architecture is tested 12 These research has shown their feasibility in simulation or UAV body contact only scenarios and has not been evaluated on the contact aerial manipulator systems in glass wall inspection task This paper presents an aerial manipulator system and a hybrid force motion control is further implemented to conduct the glass wall inspection on a 2D plane The contact force and position are controlled separately in two decoupled spaces constrained space the normal direction of the glass wall surface and free flight space tangential direction w r t the wall surface as indicated in Fig 1 The closed loop UAV is theoretically proven to behave dynamically as a spring mass damper system Based on this characteristic an inverse dynamics based controller is used to implement the force control Trajectory planning and position controller are applied to controlling system position in the free flight space This control scheme does not transform UAV dynamics into that of the tool tip position as in 8 The aerial manipulator force control problem is transformed into a position control problem by using spring mass damper like characteristics making the developed system reliable to track the desired trajectory on a 2D glass plane and in the meanwhile draw out the traveled trajectory i e perform the aerial drawing The focus of this paper is to present the novel aerial manipulator system the implementation of the hybrid force motion control framework the flight experiment process and experimental results analysis The theory and formula derivation is briefly introduced for an overall understanding The outline of this paper is as follows The system structure is presented in Section II Section III describes the closed loop UAV system characteristics Section IV introduces the hybrid force motion control framework Section V is the flight experiment process and results analysis Section VI is the conclusion II AERIAL MANIPULATOR SYSTEM DESIGN A Structure Design Force sensor Pixhawk autopilot Servo 2 Link rod Universal ball structure Servo 1 Force sensor data processing board Universal balls Battery Figure 2 The aerial manipulator system overview A one degrees of freedom DOF aerial manipulator system equipped with an end universal ball structure is designed as depicted in Fig 2 Its body platform is a hex rotor The system total mass is 2 516kg A link rod made of carbon fiber tube extends outside the rotor effect area for contact operations A single axis force sensor is installed to measure the contact force exerted on the surface in interactive process A battery container is designed to be moved forward and back to balance the system center of gravity Two servos are used on this system as shown in Fig 2 Servo 1 is installed to drive the universal ball structure to move forward and back introduced in the following The other large torque servo Servo 2 is used to drive the manipulator link providing a rotational DOF for this manipulator and decoupling the pitch of the UAV from the pitch of the robot manipulator Linear bearing Support frame Drilling for weight reductionInner rod Link rod Fixed plate Universal ball plate Figure 3 The universal ball structure design The detailed universal ball structure design is illustrated in Fig 3 These balls are free and passive in motion To limit the balls movement and also for maintaining the steady contact in the initial contact process a brake like square mechanism is designed as depicted in Fig 3 A support frame contacts with the wall surface in the beginning of interaction The inner rod could slide along its axis in a linear guide When the steady contact is achieved the inner rod can enable these universal balls to maintain contact with the wall surface to track the trajectory The two operation modes disable movement and enable movement are illustrated in Fig 4 A linear bearing is used to reduce the friction resistance The inner rod movement is driven by Servo 1 The fixed plate is full of drilling holes to reduce the weight Besides the contact area of this support frame is adhered with soft layer material to absorb vibration in contact operation On the universal ball plate a marker pen can be attached to perform the aerial drawing task The main dimensions are presented in TABLE I Enable movementDisable movement Figure 4 Two operation modes of the universal ball structure B System Function The force sensor is interfaced with autopilot via an onboard data processing module and its real time force data can be recorded online If the contact force exceeds a certain safety threshold a warning will be sent out on ground station for safe flight and operation Servo 2 is designed to be automatically 216 controlled to compensate pitch angle in contact operations keeping the manipulator link orthogonal to the wall surface to avoid the sliding motion on a vertical smooth glass surface The other servo 1 is able to be controlled in two modes automatically or manually TABLE I MAIN DIMENSIONS OF UNIVERSAL BALL STRUCTURE Item Value Support frame 100 100 mm Universal ball plate 80 80 mm Distance between support frame and fixed plate 60 mm Inner rod outside diameter 15 mm Link rod inside diameter 16 mm III CLOSED LOOP UAV SYSTEM MODELING The definitions of inertial north east down NED frame I body fixed frame B and Euler angle are shown in Fig 5 The UAV position in frame I is p x y z the velocity v the mass m the arm length l the inertia matrix I diag Ix Iy Iz and gravity acceleration g 9 81ms 2 The UAV dynamics is presented as 1 1 1 cos sin cossin sin cos sin sinsin cos cos cos xum yum zg um 22 2 33 3 44 4 abu abu abu 1 where ai and bi are constants a2 Iy Iz Ix a3 Iz Ix Iy a4 Ix Iy Iz b2 l Ix b3 l Iy and b4 1 Iz u1 is the UAV thrust u2 u3 and u4 are torque inputs for each attitude axis m xB zB yB mm m m m m N ED I xB OB zB yB B Forward Down Right p l k1zk2z k1x k2x k1y k2y Figure 5 Frame and variable definition ki1 2 i x y z are UAV position controller gains These mass weighted values can be respectively treated as stiffness and damping of the equivalent spring mass damper system A Spring Mass Damper Like System Dynamics When attitude and position controllers are designed for a UAV system 1 the stable closed loop UAV will behave as a spring mass damper system in each dimension as shown in Fig 5 The detailed formula derivation has been presented in our previous work 13 For a quick understanding the corresponding results are provided in the following That is the relationship between the UAV position response X and the exerted external force Fext is ext 2211 2 21 1 x y z iiii ii ssKk m Kk m i s m sKK XF 2 where X Xd X X x y z T represents the actual position and the desired position Xd xd yd zd T ki1 2 i x y z are UAV position controller gains Remark 1 For a spring the exerted force is known to linearly relate to its position change According to 2 the spring mass damper like aerial manipulator force control issue can also be transformed into a position control problem B Force Relationship Analysis in Interaction Scenario Owing to the under actuation property of UAV platforms when a UAV exerts a horizontal force on wall surfaces the dynamic balance relationship in physical interaction can be analyzed 13 The contact force Fx could be expressed with UAV attitude angle and system gravity mg and it is x tanFmg 3 Corresponding with the tilt angle the forward exerted force has a matching pitch angle If the contact force is the desired value Fxd in force control the matching is thus the desired pitch angle d which can be obtained based on 3 and it is d dx arctan Fmg 4 Remark 2 An aerial manipulator system could act as a force sensor as in 3 which means the force control can be implemented without direct force feedback And it also can exert force on environment where the desired horizontal force Fxd can be quantified from the UAV attitude angle and it can be achieved as the attitude reaches the desired value d IV HYBRID FORCE MOTION CONTROL FOR CONTACT INSPECTION TASK A Decoupled Dynamics Based on Remark 1 the aerial manipulator force control can be transformed into a position control issue Remark 2 also indicates that the contact force control can be achieved as the attitude angle reaches the desired value It means that the UAV attitude subsystem can be treated as its internal dynamics Thus an attitude controller has already been implemented for the UAV system making the attitude subsystem act as a low level servo tracking module The hybrid motion force control idea depicted in Fig 6 is to decompose the robot dynamics along the contact surface and normal to that surface 14 Define the normal direction control input un u1 x and the tangential direction control input ut u1 y u1 z T The relationships between them are n1 1 y1 1 z1 cos sin sin coscos uum uum ugum 5 Then the translational dynamics is decomposed as Normal direction subsystem nx xuFm 6 Tangential direction subsystem 1 y 1 z yu zu 7 B Normal Contact Force Control According to subsection III A a position controller is firstly designed for 6 without considering the external force and the controller general form is expressed as 217 xx nd1x2x uxk ek e 8 where ex and x are state error ex xd x and ex d kx1 and kx2 are their gains Thus a stable closed loop x position subsystem is obtained which obviously has the spring mass damper like system features So the relationship between position change error x and the exerted force Fxe is e x 2xx 21 1 x sF s s m sk m k m 9 Then based on Remark 1 the x position error x will be controlled to enable the UAV to produce the corresponding contact force The inverse dynamics based control is applied as shown in Fig 6 The closed loop x subsystem is considered as the original system and the inverse system dynamics can be obtained by using the aforementioned relationship 9 A PD controller is applied to deal with the uncertainties and perturbations in application In addition based on Remark 2 the desired attitude is introduced into the control system as a feedforward term to improve the contact force control performance as indicated in Fig 6 C Tangential Motion Control The tangential motion space is a 2D surface yoz in which the aerial manipulator could conduct free flight For the dynamics 7 already decoupled with x direction motion controllers are designed yy 1 yd1y2y zz 1 zd1z2z uyk ek e uzk ek e 10 where the parameters have the same definition as in 8 An aerial manipulator needs to track a desired scanning trajectory in the glass wall inspection Thus the task trajectory planning is necessary as shown in Fig 6 V EXPERIMENT The introduced hybrid force motion control algorithm has been implemented on the aerial manipulator system to perform glass wall inspection task Two flight experiment are conducted the first is the continuous square wave trajectory tracking and the other is the aerial drawing operation A vertical glass plate is used as the glass curtain wall as shown in Fig 7 The experiment is carried out in Motive motion capture system which provides position information indoors The onboard inertial measurement unit IMU provides the attitude estimates 1 2 3 4 Figure 7 Flight experiment process 1 Approach 2 Contact 3 Move in z direction 4 Move in y direction A Continuous Square Wave Trajectory Tracking When an aerial manipulator is applied in glass curtain walls inspection a basic capacity is to track a desired 2D trajectory Thus the aerial manipulator is commanded to track a continuous square wave trajectory and in the meanwhile the traveled trajectory is drawn out on the glass surface The main process is depicted in Fig 7 and the experimental results 41 160 s are analyzed below in which real indicates the actual value and desd is the desired value The square wave in one period is illustrated in Fig 8 z m y m 1 1 1 6 0 5 0 4 0 3 16s 5s 3s 3s 5s Figure 8 The square wave in one period Figure 6 Hybrid force motion control framework for the aerial manipulator in operation process 111 d Attitude controller 2 3 4 u 1 u Robotic manipulator UAV system d y P 222 11 x1 y1 z g um uuu Y and Z position controller d z d y t u Target position Trajectory planning 1 u Tangential motion control z P d 2xx 21 1 s m sk m k m d x F x F tanmg PD controller PD FF ksk e x Fx r x d x x P d x arctan Fmg d x F X position controller n u Normal contact force control Aerial manipulator system Glass wall surface 111 218 The three dimensional position changes are depicted in Fig 9 The contact force control is based on position adjustment Thus the x axis position error appears in Phase 1 Contact and Phase 2 Track the trajectory on the surface Phase 1 is in the static contact scenario and no position tracking Thus the x axis position error maintains a constant value While the aerial manipulator tracks the square wave trajectory in Phase 2 the y and z position tracking er