Available online at www.sciencedirect.com ScienceDirect Procedia CIRP 56 (2016) 73 – 78 9th International Conference on Digital Enterprise Technology - DET 2016 – “Intelligent Manufacturing in the Knowledge Economy Era Framework and implementation of two-stage alignment for large components based on P&O and F/T Ke Wena, Fuzhou Dua,* a School of Mechanical Engineering and Automation, Beihang University, Beijing 100191, China * Corresponding author Tel.: +86-10-8231-6795; fax: +86-10-8233-8165 E-mail address: du_fuzhou@163.com Abstract A two-stage alignment framework of large components based on position and orientation (P&O) and force/torque (F/T) is put forward to solve the adjustment difficulties of accurate P&O during the alignment process, considering the geometric and physical characteristics of large components The basic environment and enabling technologies of the framework are introduced The P&O-guided alignment and the F/Tdriven alignment of the framework are set up Then, a mathematical model of P&O and assembly relationship model of P&O for P&O-guided alignment are proposed Moreover, a six-dimensional F/T analytical algorithm based on screw theory for F/T -driven alignment is given The geometric and mechanical models of components are presented Based on the obtained models, the compliance assembly strategies are analyzed The alignment experiment, using aerospace products, was performed on the self-designed alignment system, and the experimental results proved the proposed the framework based on P&O and F/T is correct and effective © 2016The TheAuthors Authors Published by Elsevier © 2016 Published by Elsevier B.V B.V This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the Scientific (http://creativecommons.org/licenses/by-nc-nd/4.0/) Committee of the “9th International Conference on Digital Enterprise Technology - DET 2016 Peer-review under responsibility of the scientific committee of the 5th CIRP Global Web Conference Research and Innovation for Future Production Keywords: P&O-guided; F/T-driven; Compliance assembly; Large components alignment Introduction In recent years, with the rapid development of the assembly technology toward being digital, flexible and intelligent, Germany and the United States as the representative of the world's leading industrial countries are moving towards a new round of industrial revolution which uses intelligent equipment and information communication Various information sensing technologies and data collection methods are the premises of industrial automation and information technology integration in the intelligent assembly Measurement Aided Assembly (MAA) [1]-[2] is one of its forms of implementation, also is the inevitable trend of large component intelligent assembly technology development The large components such as airframes, satellites and rockets typically have similar characteristics i.e., large size, easily deformed, complex coordination relationship, etc In the conventional assembly process, large scale fixtures, which consist of large steel structures configured for a special component or structure, are used to position the components, with manual operation to realize the geometrical relationships and constraints between components, and to ensure their variation within acceptable tolerances With such an assembly process it is difficult to accurately adjust the position and orientation (P&O) Moreover, it needs a large work space and a lot of manpower So, the conventional low accuracy and inefficient assembling process cannot meet the demand of advanced, flexible, accurate and high-efficient large components assembly [3]-[4] The development of large scale metrology technologies, which are based on the high-precision and efficient digital measurement systems, have become the key technologies during assembly for process control and quality assurance To improve the efficiency and precision of assembly, a novel assembly system based on MAA is proposed by manufacturers and researchers which has been employed in level docking system in large component assembly of spacecraft [5], digital alignment system for large component assembly of aircraft [6]-[7], vertical docking system in large component assembly of satellite[8], etc 2212-8271 © 2016 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the 5th CIRP Global Web Conference Research and Innovation for Future Production doi:10.1016/j.procir.2016.10.019 74 Ke Wen and Fuzhou Du / Procedia CIRP 56 (2016) 73 – 78 The MAA technology uses the digital measurement system (such as electronic theodolite, laser tracker, indoor GPS, camera systems) to measure the P&O of components and assist the adjustment of P&O [9]-[10] However, the electronic theodolite and laser tracker are by-point measurement way Indoor GPS can be used in a fixed space, and be susceptible to signal interference Camera systems are more sources of error With the improvement of the manufacturing accuracy, the accuracy of the measurement system will be lower than that of assembly design, which will fails the assembly In robotic assembly applications (such as peg-in-hole, surface grinding), when the assembly objects are in contact during assembling, the compliant control methods that make the interaction forces as a constraint condition are used to recognize and change the contact state, and finish the assembly [11] These methods provide a new assembly idea for the MAA technology The force measurement and control technology will depend on two important parts: sensors and force control x Six-dimensional F/T sensors: The isotropic configuration of the six-dimensional F/T sensor based on SP, the task-oriented design method of the six-dimensional F/T sensor and a six-dimensional F/T sensor to complete peg-inhole assembly task have been introduced in [12]-[13] A sixbeam sensor based on SP and an idea of “joint less” structure and beam sensors to improve the precision and sensitivity in measuring a small F/T have been proposed in [14] A sixdimensional heavy F/T sensor with high stiffness and good linearity based on SP has been presented in [15] Experimental results verified the feasibility and validity of the sensor by the established calibration platform [15] x Force control techniques A shape recognition algorithm based on a six-dimensional F/T sensor and a hole detection algorithm have been reported in [16] The sixdimensional F/T sensor to estimate the contact phases and designed the assembling strategy to achieve the force-guided robotic assembling in [17]-[19] The admittance characteristics for a force-guided robotic assembly have been studied and analytical derivations for different contact states are presented in [20] A modified control scheme for SP with compensation for interaction force control and positional error recovery is introduced in [21].A novel strategy of the highprecision chamferless peg-hole insertion with a sixdimensional F/T sensor is introduced in [22] The MAA technology for large component assembly so far presented only considers the geometrical characteristics and doesn't give any attention to the physical characteristics Focusing on the geometric and physical characteristics of large components, this paper proposes a two-stage alignment framework for large components based on P&O and F/T This paper is organized as follows: Section introduces the development and application of MAA technology, highlights the measurement of interaction force and its significance in assembling Section provides the two-stage alignment framework of large components based on P&O and F/T in detail, Section provides the P&O-guided alignment based on model Section provides the F/T-driven alignment based on six-dimensional F/T feedback A practical alignment system is designed to verify the effectiveness of the proposed framework, and the obtained results are discussed in section Section concludes the paper and assesses the presented framework Framework of large components two-stage alignment based on P&O and F/T As shown in Fig 1, the two-stage alignment framework of large components based on P&O and F/T can be divided into two sections: (1)The base environment for aligning large components: The software and hardware that are used to support the twostage alignment of large components based on P&O and F/T are shown in Fig The software system includes Product Data Management (PDM) system, database, integrated control platform, etc The hardware system includes a digital measurement system, force sensors, P&O adjustment platform, fixed platform, control cabinet, assembly fixtures, etc The software and hardware will communicate with each other Fig The two-stage alignment framework of large components based on P&O and F/T (2)The enabling technologies for aligning large component: The alignment based on P&O and F/T has two stages as illustrated in Fig.1 ķ The P&O-guided alignment The measurement process model is integrated with the assembly planning to instruct the deployment and planning of digital measurement systems, enabling automation Through processing and analyzing measurement data in the alignment process, the geometrical information of components viz., P&O, dimension, variation and others are calculated and compared with the design requirements Then, the feedback from the analysis is used to adjust the P&O adjustment platform ĸ The F/T-driven alignment The compliant assembly model is integrated with the assembly planning to follow the six-dimensional F/T feedback, enabling automation Through processing and analyzing measurement data in the alignment process, the interaction force between the components is calculated Then, the feedback from the 75 Ke Wen and Fuzhou Du / Procedia CIRP 56 (2016) 73 – 78 analysis is used to adjust the P&O adjustment platform The two-stage alignment of large component based on P&O and F/T in this paper can be described by the flowchart shown in Fig where R3h3 is called the rotation matrix, and M3h1 is called the displacement vector If Pi1 is known, and Pi0 is obtained though measurement, according to (1), the x,y,z,Į,ȕ,Ȗ can be calculated by measuring the coordinates of at least three points which are mutually non-collinear 3.2 Assembly relationship model of P&O Fig The two-stage alignment of large component P&O-guided alignment based on model 3.1 Mathematical model of P&O Schematic diagram of the P&O adjustment platform is shown in Fig It consists of a moving platform and a stationary platform, which are connected using six stretchable limbs through spherical joints Such arrangement offers 6Degrees-Of-Freedom (DOF) motion due to the movements of six limbs as a whole The cartesian coordinate system o0-x0y0z0 is located in the center of the top surface of the stationary platform and the cartesian coordinate system o1-x1y1z1 is located in the center of the bottom surface of the moving platform The centers of the spherical joints are denoted as Ai and Bi (i=1,…,6) The position of Ai in o2-x2y2z2 can be expressed in the form of a vector as Pi2=[Ai2,1]T, where Ai2=(xi2,yi2,zi2) is the coordinate in o2-x2y2z2 As shown in Fig 4, the P&O of the moving platform with respect to o0-x0y0z0 can be expressed by T1-0, the P&O of the digital measurement equipment in relation to o0-x0y0z0 can be expressed by T2-0 and the P&O of the moving platform with respect to o2-x2y2z2 of measurement coordinate system is expressed by T1-2 T1-0, T2-0 and T1-2 are related as T1-0gPi1 = Pi0, T2-0gPi2 =Pi0, T1-2gPi1 =Pi2 The relationship between these P&Os is as follows: T1-0= T2-0gT1-2 When there are multiple P&Os in the alignment process, their relationship can express the final assembly relationship and determine the assembly parameters The relationship between P&Os can be obtained following the method so far discussed F/T-driven alignment based on six-dimensional F/T feedback 4.1 Analytical algorithm of six-dimensional F/T According to screw theory, the external load [Fs Ms]T on the moving platform in o1-x1y1z1 as shown in Fig 3, can be calculated by applying the force equilibrium equation as follows F [G ] f (2) where F=[Fs Ms]T=[Fx Fy Fz Mx My Mz]T, f=[f1 f2 f3 f4 f5 f6]T Fig Schematic diagram of the P&O adjustment platform >G @ Fig The relationship between P&Os The P&O of the moving platform with respect to o0-x0y0z0 can be expressed using a six-dimensional variable T1-0= {x,y,z,Į,ȕ,Ȗ}, which includes the amount of rotation and displacement of o1-x1y1z1 with respect to o0-x0y0z0 Where Į,ȕ,Ȗ are the rotational angles of each axis of o1-x1y1z1; x,y,z are the displacements of the original point of o1-x1y1z1 Ai is one of the points on the moving platform and the position of Ai in o0-x0y0z0 and o1-x1y1z1 can be expressed in the form of vectors as Pi0 = [Ai0,1]T, Pi1 = [Ai1,1]T Where Ai0 = (xi0,yi0,zi0) is the coordinate in o0-x0y0z0 Ai1 = (xi1,yi1,zi1) is the coordinate in o1-x1y1z1 The two vectors are different descriptions of the same point; hence they can be related using a linear transformation as follows: T1 Pi1 Pi , T1 [ R3u3 M 3u1 01u3 ] (1) ê S1 ôS 01 S2 S02 S3 S03 S4 S04 S5 S05 S6 º S06 ằẳ A6 B6 ê A1 B1 ô A B A6 B6 ! 1 « A6 u S6 ơô A1 u S1 ằ ằ ẳằ (3) where Ai and Bi are coordinates in o1-x1y1z1 fi is the measured force of limbs Hence, the external load [Fs Ms]T can be calculated using (2) 4.2 Geometric and mechanical models of components The components which are studied in this paper have certain rigidity, and their P&Os are adjusted for alignment at low speeds Therefore, the alignment process for the large component can be described as a typical peg-in-hole assembly The contact state of three points on the upper circle surrounding the hole is determined by the P&O-guided alignment process, which is the beginning of the F/T-driven alignment process The geometric and mechanical models of components are analysed as shown in Fig.5 In (b), F and M are calculated by using the analytical algorithm f1 and f2 are 76 Ke Wen and Fuzhou Du / Procedia CIRP 56 (2016) 73 – 78 the supporting forces and ȝ is the friction coefficient Following equations can be established l sin T d cos T D ® ¯(d s )sin T l cos T (4) ¦ Fyp F sin E P f f1 ° ®¦ Fzp F cos E f P f1 ° ¦M M P f1 L f1 d / s P f d / f ( L l ) xp ¯ The P&O of peg can be adjusted until the next contact state satisfying the above relationship The same analytical method is used for adjusting other contact states as well From the change of contact state, F/T-driven alignment process is described in Fig (5) Fig F/T-driven alignment process from the change of contact state Experimental results and Discussion As shown in Fig and Fig.9, the designed experimental system includes laser tracker, force sensors, P&O adjustment platform, fixed platform, control cabinet, assembly fixtures and an integrated control platform As shown in Fig 10 and Fig.11, the GUI for integrated control platform includes functional areas, navigation tree, a graphical display for status monitoring and functional dialogs Experimental process is divided into two stages: Fig (a) Geometric analysis; (b) Mechanics analysis 4.3 Compliance assembly strategies When the three contact points on the upper circle surrounds the hole, the P&O of peg should be adjusted Say, Fh and Mx are applied (as in fig.6) Fig.8 Experimental environment of P&O-guided alignment process Fig Mechanics analysis of P&O adjustment process Fyh P f1 cos T f1 sin T f cos T P f sin T ma yh ° ® Fzh P f1 sin T f1 cos T f sin T P f cos T mazh °M P f L f d / s P f d / f ( L l ) ID 1 2 xp ¯ xp (6) When the peg is in uniform motion and ayh=0, azh=0, Įxp=0, f1> 0, f2> 0, then Fyh Fzh M xp Fh ! P cos T sin T cos T P sin T P (7) §P d d · L L d P s P l sin T ¨ s l P ¸ cos T â cos M 1 P2 (8) Đ P 2d d · s P l ¸ sin T P L L d P s P l cos T ă 2 â sin M 1 P Fig Experimental environment of F/T-driven alignment process (1) P&O-guided alignment process: Firstly, measurement plan is carried out that includes laser tracker configuration and station planning, P&O measurement characteristics planning (planning results as in Fig 8) etc Then, the measurement field is constructed Secondly, Laser tracker automatically measures the P&O measurement characteristics to fit the P&O of components Then the assembly coordination is determined The automatic measurement dialog and the assembly Ke Wen and Fuzhou Du / Procedia CIRP 56 (2016) 73 – 78 coordination dialog are shown in Fig 10 Thirdly, the calculation results are used to adjust the P&O adjustment platform These steps are repeated until the P&O adjustment platform reaches the target P&O Fig 10 GUI of Integrate control platform for P&O-guided alignment process Fig 11 GUI of Integrate control platform for F/T-driven alignment process (2) F/T-driven alignment process Firstly, the experimental setup has six force sensors which are placed in each limb of P&O adjustment platform to measure the forces on each limbs and then the six-dimensional F/T is calculated based on the 77 78 Ke Wen and Fuzhou Du / Procedia CIRP 56 (2016) 73 – 78 analytical algorithm (as shown in Fig 9) Secondly, the gravity of the moving platform, assembly fixtures and components are compensated The gravity compensation dialog in GUI of integrated control platform is shown in Fig 11 Thirdly, the compliance assembly is started (as in Fig 11) The calculation results are used to adjust the P&O adjustment platform The F/T-driven alignment process is repeated until the interaction force meets the threshold value In the above experiments, the alignment process is completed successfully The precision of the system varies in different stages In the P&O-guided alignment process, the precision of the system depends on the precision of digital measurement system and the positioning accuracy of P&O adjustment platform In the F/T-driven alignment process, the precision of the system depends on the precision of force sensors The precision analysis is out of scope of this paper as this document is drafted to introduce the idea Conclusions Considering the geometry and physical characteristics of the alignment of large component, a two-stage alignment framework for large components based on P&O and F/T methods is presented in this paper It can evaluate the target alignment quickly from the geometry and mechanical parameters Two stages of framework are presented, which are the P&O-guided stage and the F/T-driven stage The measurement process model and the compliance assembly model are presented to design an intelligent alignment process The implemented framework contains functionality that supports planning and automatic measurements, P&O fitting, MAA automation, six-dimensional F/T measurement and compliance assembly automation The alignment experiment was performed on the self-designed alignment system using aerospace products and the relevant experimental results proved that the proposed two-stage alignment framework for large components is effective Future research will focus on the accuracy analysis of six-dimensional F/T measurement and more applications of artificial intelligence technology in the F/T-driven stage The proposed alignment approach can be applied to the sleeve connection of large components, just like peg-in-hole assembly It can also be applied to the alignment of surface and plurality of holes for large components But the large components should have enough rigidity Acknowledgements This work is under the support of National Defense Basic Scientific Research (No A2120132007) and Fund of National Engineering and Research Center for Commercial Air-craft Manufacturing (No SAMC14-JS-15-055) References [1] Maropoulos PG, Muelaner JE, Summers MD, et al A new paradigm in large-scale assembly-research priorities in measurement assisted assembly International Journal of Advanced Manufacturing Technology 2014; 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2135-2138 [20] Wiemer SC, Schimmels JM Optimal admittance characteristics for planar force-assembly of convex polygonal parts Robotics and Automation (ICRA), 2012 IEEE International Conference on IEEE 2012; 2578-2583 [21] Bera TK, Merzouki R, Bouamama BO, et al Force control in a parallel manipulator through virtual foundations Proceedings of IMechE, Part I:Journal of Systems and Control Engineering 2012; 226:1088–1106 [22] Qiao H, Dalay BS, Parkin RM Robotic peg-hole insertion operations using a six-dimensional force sensor Proceedings of IMechE, Part C:Journal of Mechanical Engineering Science 1993; 207: 289-305 ... components, this paper proposes a two- stage alignment framework for large components based on P& O and F/ T This paper is organized as follows: Section introduces the development and application of. .. environment for aligning large components: The software and hardware that are used to support the twostage alignment of large components based on P& O and F/ T are shown in Fig The software system... large components two- stage alignment based on P& O and F/ T As shown in Fig 1, the two- stage alignment framework of large components based on P& O and F/ T can be divided into two sections: (1)The base