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Byline: Jinhai Yu, Qiang Fang, Yinglin Ke In the manufacturing engineering field, a multi-robot coordination platform is often used to adjust a large subassembly's location (including position and orientation). This paper introduces an open spherical joint coupling between the subassembly and every robot manipulator and presents a B-spline interpolation trajectory planning method for the manipulators. The location of the subassembly can be described with an element of the group of spatial displacement in Euclidean space. The trajectory of every manipulator can be obtained through finding a smooth motion of the subassembly and then mapping it to the corresponding axes of every manipulator. Unit quaternion is adopted for describing a given set of orientations, and then a cubic B-spline interpolation method is applied to calculate the position and orientation curves, respectively. Finally, the two curves are fused. This paper proves that all the motion points of the subassembly are continuous in the same order as the B-spline function.

Autoclave curing process is one of the most frequently used manufacturing techniques of thermosetting composite materials. An efficient curing process requires good understanding of the thermal behavior of molds and composites during autoclave processing. In this paper, the effect of thermal gradients on curing performance of laminates is investigated through numerical approaches. In the first section, a computational fluid dynamics–finite element method numerical model is established to simulate the temperature field and the process-induced deformation of laminates. Then, a curved composite part with two different structures of mold is introduced to exhibit different temperature and degree of cure gradients during the autoclave process. Furthermore, by analyzing the position errors of measurement points, the deformation of the composite parts in different molds is evaluated. The results suggested that more synchronous curing process and less deformation of the composite part can be achieved by reducing the thermal gradients. In this specific case of a curved part, the range of position errors in X direction (the length direction) is reduced by 86.9% with the redesigned mold. [ABSTRACT FROM AUTHOR]

Purpose: The riveting process is a metal forming process involving complex elastic-plastic deformation, which will induce a compressive residual stress field and cause local distortions in the connecting areas. Regarding to the aircraft panel assemblies with plenty of rivets, the global deformation is inevitable and undesired, leading difficulties to downstream assembly processes. This paper aims to present a new method for the local distortion calculation and the global deformation prediction of sheet panel assemblies during the automated riveting process. Design/methodology/approach: In this paper, a simplified algebraic study is presented to analyze the local distortion of single countersunk rivet joint with the consideration of the barrel-like shape of the driven head and the through-thickness variations along the rivet shank. Then, an equivalent rivet unit is proposed based on the result of the algebraic study and embedded into the global-level model for the prediction of the overall distortions of riveted panels. Findings: The algebraic study is able to reach a more precise contour of the deformed rivet than the traditional assumption of cylindrical deformations and rapidly determine the equivalent coefficients of the riveting unit. The result also shows an industrial acceptable accuracy of the prediction for the global deformations of the double-layered panel assemblies widely used in the aircraft panel structures. Originality/value: A new local-global method for predicting the deformations of the riveted panel assembly based on the algebraic study of the local distortions is proposed to help the engineers in the early design stages or in the assembly process planning stage. [ABSTRACT FROM AUTHOR]

In order to investigate the effect of shim compensation for nonuniform gaps in aircraft assembly, the influence of the shims with different material and parameters on bolted joints is studied in this paper. According to the real material and assembly conditions of the aircraft joint structures, the specimen and experiment are designed to obtain the tensile performance of the joint structures with different shims. A three-dimensional finite element model, which incorporates the Johnson–Cook material property of the alloys, traction-separation law of liquid shims, contact relationships between the joint elements, and boundary conditions of the tensile process, is established with the specimen configurations. After validating through comparing with the experimental results, the modeling method is adopted to simulate the tensile response of the bolted joints with shims. Furthermore, both the influence of the shim material and thickness on the mechanical behaviors of bolted joints is investigated in detail. Shims can considerably reduce the assembly stress of joint structures and improve the joint stiffness and load capacity, and this effect is more remarkable with the increase of gap values. Liquid shims improve the joint stiffness due to its cohesive ability, while solid shims improve the joint load capacity. Hybrid shims possess a composite shimming effect of liquid and solid shims. Whatever the shim material is applied, the joint stiffness and strength drop with the growth of shim thickness, so strict deviation control method should be taken to ensure the assembly gaps as small as possible. The research results enhance the knowledge of shimming effect on joint structures, and thus offer positive guidance for practical application in aircraft assembly. [ABSTRACT FROM AUTHOR]

Purpose: To gain accurate support for large aircraft structures by ball joints in aircraft digital assembly, this paper aims to propose a novel approach based on visual servoing such that the positioner's ball-socket can automatically and adaptively approach the ball-head fixed on the aircraft structures. Design/methodology/approach: Image moments of circular marker labeled on the ball-head are selected as visual features to control the three translational degrees of freedom (DOFs) of the positioner, where the composite Jacobian matrix is full rank. Kalman–Bucy filter is adopted for its online estimation, which makes the control scheme more flexible without system calibration. A combination of proportional control with sliding mode control is proposed to improve the system stability and compensate uncertainties of the system. Findings: The ball-socket can accurately and smoothly reach its desired position in a finite time (50 s). Positional deviations between the spherical centers of ball-head and ball-socket in the X-Y plane can be controlled within 0.05 mm which meets the design requirement. Practical implications: The proposed approach has been integrated into the pose alignment system. It has shown great potential to be widely applied in the leading support for large aircraft structures in aircraft digital assembly. Originality/value: An adaptive approach for accurate support of large aircraft structures is proposed, which possesses characteristics of high precision, high efficiency and excellent stability. [ABSTRACT FROM AUTHOR]

In composite manufacturing, large composite parts usually exhibit high heating gradients during the autoclave process, which may intensify the process-induced residual stresses and deformations. As the thermal behavior of molds is of crucial importance to the curing performance of composites, a heat-balance method is presented to reduce the heating rate on overheated areas of molds, thus providing a more homogeneous curing process. The method is based on a local-isolation structure installed under the mold plate, which is used to change the local heat transfer coefficient of the mold. In the local-isolation structure application, an optimization process combining numerical simulations with a greedy genetic algorithm is developed to find the optimal layout and geometry of local-isolation structure in molds. The optimization results suggest that more uniform heating condition and more synchronous curing process can be achieved with the optimal design of local-isolation structure. In the case of a typical mold for C spar component, the maximum temperature difference in the composite part is reduce by 45.69%, while the maximum difference in degree of cure is decreased at a rate of 40.16%. [ABSTRACT FROM AUTHOR]

Due to small production volume in aircraft industry, the available information of variation sources is often not enough to make assumptions on their probabilistic characteristics, especially in the stage of prototype manufacturing. To deal with the problem, an assembly variation modeling and analysis method based on the elasticity mechanics and interval approach is proposed for aircraft assembly. First, variation sources are modeled as bounded convex sets, which are defined as interval structural parameters in interval arithmetic. Then, variation modeling and analysis are successively implemented using the method of influence coefficient and interval arithmetic. After that, a uniform-splitting method is applied to achieve the refinement of the interval extension in variation analysis. To reduce the complexity of the finite element analysis and assembly variation computation, part deformation forms including warpage and torsion are concisely characterized with angle instead of the deviations of isolated key points on the part. The comparison of the assembly variations estimated with the proposed variation analysis method and actual experiment results verifies the effectiveness of the constructed assembly variation model and the proposed method. The interval approach–based assembly variation analysis method is a good complement to traditional probabilistic approach–based methods for compliant assembly systems, which is suited for linear and linearized nonlinear assembly systems. The proposed method provides an improved understanding of the application of compliant assembly variation analysis methods in aircraft manufacturing. [ABSTRACT FROM AUTHOR]

Purpose The purpose of this paper is to design, analyze and optimize a new type of inner-side working head for automatic horizontal dual-machine cooperative drilling and riveting system. The inner-side working head is the key component of automatic drilling and riveting system, and it is a challenge to design an inner-side working head which must be stiffness and stable with a compact structure to realize its functions.Design/methodology/approach According to the assembly structure features of large aircraft panels and riveting process requirements, a new type of inner-side working head is designed for pressure riveting. The force condition of the inner-side working head during the riveting process is analyzed and the deformation model is established. Design optimization is performed based on genetic algorithm and finite element analysis. The optimized inner-side working head is tested with automatic horizontal dual-machine cooperative drilling and riveting system.Findings The deformation model provides the precision compensation basis for control system. Application test results show that the automatic drilling and riveting system can realize assembly of large aircraft panel with high efficiency and quality through the inner-side working head.Research limitations/implications The inner-side working head has been used in aircraft panel assembly.Practical implications The inner-side working head has been used in aircraft panel assembly.Originality/value This paper presents the design, analysis and optimization of a new type of inner-side working head which can realize automatic riveting for aircraft panel. The research will promote the automation of aircraft panel assembly. [ABSTRACT FROM AUTHOR]

Purpose Assembly variation analysis generally demands probability distributions of variation sources. However, due to small production volume in aircraft manufacturing, especially prototype manufacturing, the probability distributions are hard to obtain, and only the small-sample data of variation sources can be consulted. Thus, this paper aims to propose a variation analysis method driven by small-sample data for compliant aero-structure assembly.Design/methodology/approach First, a hybrid assembly variation model, integrating rigid effects with flexibility, is constructed based on the homogeneous transformation and elasticity mechanics. Then, the bootstrap approach is introduced to estimate a variation source based on small-sample data. The influences of bootstrap parameters on the estimation accuracy are analyzed to select suitable parameters for acceptable estimation performance. Finally, the process of assembly variation analysis driven by small-sample data is demonstrated.Findings A variation analysis method driven by small-sample data, considering both rigid effects and flexibility, is proposed for aero-structure assembly. The method provides a good complement to traditional variation analysis methods based on probability distributions of variation sources.Practical implications With the proposed method, even if probability distribution information of variation sources cannot be obtained, accurate estimation of the assembly variation could be achieved. The method is well suited for aircraft assembly, especially in the stage of prototype manufacturing.Originality/value A variation analysis method driven by small-sample data is proposed for aero-structure assembly, which can be extended to deal with other similar applications. [ABSTRACT FROM AUTHOR]

Purpose Aircraft assembly demands high position accuracy of drilled fastener holes. Automated drilling is a key technology to fulfill the requirement. The purpose of the paper is to conduct positioning variation analysis and control for an automated drilling to achieve a high positioning accuracy.Design/methodology/approach The nominal and varied connective models of automated drilling are constructed for positioning variation analysis regarding automated drilling. The principle of a strategy for reducing positioning variation in drilling, which shortens the positioning variation chain with the aid of an industrial camera-based vision system, is explored. Moreover, other strategies for positioning variation control are developed based on mathematical analysis to further reduce the position errors of the drilled fastener holes.Findings The propagation and accumulation of an automated drilling system’s positioning variation are explored. The principle of reducing positioning variation in an automated drilling using a monocular vision system is discussed from the view of variation chain.Practical implications The strategies for reducing positioning variation, rooted in the constructed positioning variation models, have been applied to a machine-tool based automated drilling system. The system is developed for a wing assembly of an aircraft in the Aviation Industry Corporation of China.Originality/value Propagation, accumulation and control of positioning variation in an automated drilling are comprehensively explored. Based on this, the positioning accuracy in an automated drilling is controlled below 0.13 mm, which can meet the requirement for the assembly of the aircraft. [ABSTRACT FROM AUTHOR]

Robotic boring is an effective way to implement finish machining of intersection holes. However, to a certain extent, its application is limited due to the low rigidity of the robot, whose stiffness brings on high vibration levels. In this study, a new approach based on an equivalent stiffness is proposed to gain a fundamental understanding for the cutting mechanism and vibration performance of a robotic boring system. In the approach, the robotic boring system in one direction is regarded as a mass–spring–damping unit according to the structure characteristics of the robot. Thus, the whole robotic boring system is equivalent to a mass–spring–damping group in three-dimensional space. The stiffness and natural frequency of the robot system were measured by stiffness identification and a modal test on an ABB IRB 6600-175/2.55 robot. An equivalent three-dimensional finite element model based on this approach was established to simulate the robotic boring process, and a verification experiment was conducted to determine the accuracy of the finite element simulation. The results show that the simulated cutting force components and the amplitude in the feed direction are in good agreement with the experiment under different cutting conditions, and this proposed approach is feasible. [ABSTRACT FROM AUTHOR]

Purpose This paper aims to propose a calibration model for kinematic parameters identification of serial robot to improve its positioning accuracy, which only requires position measurement of the end-effector.Design/methodology/approach The proposed model is established based on local frame representation of the product of exponentials (local POE) formula, which integrates all kinematic errors into the twist coordinates errors; then they are identified with the tool frame' position deviations simultaneously by an iterative least squares algorithm.Findings To verify the effectiveness of the proposed method, extensive simulations and calibration experiments have been conducted on a 4DOF SCARA robot and a 5DOF drilling machine, respectively. The results indicate that the proposed model outperforms the existing model in convergence, accuracy, robustness and efficiency; fewer measurements are needed to gain an acceptable identification result.Practical implications This calibration method has been applied to a variable-radius circumferential drilling machine. The machine's positioning accuracy can be significantly improved from 11.153 initially to 0.301 mm, which is well in the tolerance (±0.5 mm) for fastener hole drilling in aircraft assembly.Originality/value An accurate and efficient kinematic calibration model has been proposed, which satisfies the completeness, continuity and minimality requirements. Due to generality, this model can be widely used for serial robot kinematic calibration with any combination of revolute and prismatic joints. [ABSTRACT FROM AUTHOR]

Purpose The purpose of this paper is to introduce a robotic boring system for intersection holes in aircraft assembly. The system is designed to improve the boring quality and position accuracy of the intersection holes.Design/methodology/approach To improve the boring quality of intersection holes, a robot posture optimization model is established. The target of the model is to maximize the robot stiffness and the variate is location of the robot on the guideway. The model is solved by the iterative IKP algorithm based on the Jacobian matrix. To improve the position accuracy of intersection holes, a robot positioning accuracy compensation method is introduced. In the method, a laser tracker is used to measure the actual position and orientation of the boring bar. Combined with the desired position and orientation, the error can be obtained and compensated.Findings In practical case of the robotic boring system, the robot stiffness is effectively improved and the surface roughness of intersection holes achieves a grade of Ra0.8. Besides, the robot end achieves a position accuracy of 0.05 mm and an orientation accuracy of 0.05°.Practical implications The robotic boring system has been applied successfully in one of the aircraft assembly projects in northwest China.Originality/value The robotic boring system can be applied for machining intersection holes in an aircraft assembly. With the robot posture optimization method and accuracy compensation method, the boring quality and position accuracy of the intersection holes can be guaranteed. [ABSTRACT FROM AUTHOR]

The paper introduces a new automatic drilling and riveting system for aircraft panel assembly which is composed of two five-axis machines in coordination. Kinematic modeling and base frame calibration are fundamental problems for the dual-machine system to improve its positioning accuracy. In this paper, a comprehensive kinematic model for the dual-machine system is established based on the modularization concept, and a new base frame calibration method is proposed which consists of the direct calibration based on laser tracker and calibration error compensation based on a specially designed instrument. In the kinematic modeling stage, the kinematic model of the dual-machine system is divided into three parts and three corresponding sub-models are established. With the pose constraints of coordinated machines, a comprehensive kinematic model of the dual-machine system is built. In the base frame calibration stage, the base frames of dual machines are firstly established by laser tracker. Due to many error sources in the calibration process, the relative pose errors of dual machines exist with the relationship between the base frames of coordinated machines. Then, the calibration error is measured by a specially designed instrument and used to modify the primary base frame calibration results. To verify the validity of the proposed method, experiments have been performed and the results have shown great improvement in base frame calibration accuracy. With the advantage of easy operation and high accuracy, this method can be extended to other multi-machine and multi-robot systems. [ABSTRACT FROM AUTHOR]

The application of dedicated cutting tools with complex geometric structures in the orbital drilling process has promoted the rapid development of this new technology within the aircraft industry. Based on the orbital drilling kinematics, the complex machining path generated by the tool, combined with the large number of non-uniform and irregular chips formed under the varying cutting conditions, makes it a challenge to model the cutting force for the orbital drilling operation. This paper proposes the steady machining stage in orbital drilling as a representation of the whole machining process. During the chip formation period at this stage, the undeformed chip geometries produced by each tooth are analyzed and described. The undeformed chip thickness and the width of cut are then calculated. Based on these instantaneous chip geometries, a non-linear cutting force model for the orbital drilling process can then be developed. This experimental study on the orbital drilling process is conducted using a TC4 alloy, with the aim of calibrating the special cutting force coefficients using the average cutting force method, and further validating the cutting force model. The results show that this model can be used to predict the cutting force generated in the half rotational period in the steady cutting stage of the orbital drilling process. Furthermore, it can be used to guide the optimization of machining parameters and the structural design of the dedicated cutting tool by analyzing the relationship between the predicted cutting forces and the feed rate, with the aim of improving the final quality of the borehole. [ABSTRACT FROM AUTHOR]

The system of robotic boring, mainly involving an industrial robot and an end-effector, is prone to chatter during the boring process of intersection hole, which not only affects the machining surface quality, but also restricts the boring efficiency. To improve the quality and the efficiency of the intersection hole, a new approach to identify and forecast the chatter of a robotic boring system based on the measured force signal of the dynamometer is presented. The proposed approach consists of three steps. First, the measured force signal is decomposed a series of intrinsic mode functions (IMF) and a residue by empirical mode decomposition (EMD). Secondly, Hilbert transform is invoked for each IMF to obtain the instantaneous frequencies and the instantaneous magnitudes, which comprise the Hilbert-Huang spectrum of the original signal. Finally, the chatter feature is extracted by analyzing the Hilbert spectrum of each IMF and a statistical method is used to detect the chatter symptom. The experiment results show that the extracting chatter feature from the signal can gain the chatter symptom at most 0.6 s ahead of the chatter outbreak, which is beneficial to guarantee for follow-up chatter suppression and improve the surface quality of workpiece. [ABSTRACT FROM AUTHOR]

Inverse kinematics solutions are the basis for position and orientation control of automated machines in their Cartesian workspace. This paper presents an efficient and robust inverse kinematics algorithm for a new circumferential drilling machine for aircraft fuselage assembly. After a brief introduction to the circumferential drilling machine and its forward kinematics, the paper discusses the nonlinear optimization method for solving inverse kinematics problems. The objective function is defined as a weighted combination of a position error function and an orientation error function. By representing orientation error as the geodesic distance between two points on a unit sphere, the paper proposes to define the orientation error function by using faithful geodesic distance functions, which are accurate approximations to the geodesic distance when it is small. For increased efficiency, robustness, and easy setting of initial values, the inverse kinematics problem is decomposed into two subproblems. The revolute joint coordinates are obtained by nonlinear optimization, and the prismatic joint coordinates are calculated with closed-form formulas. Numerical experiments show that the objective function defined with faithful geodesic distance functions is effective, and the proposed algorithm is efficient, robust, and accurate. The algorithm has been successfully integrated into the control system of the circumferential drilling machine. Preliminary drilling experiments show that the position accuracy of drilled holes is within ±0.5 mm, which is acceptable for the assembly of large aircrafts. [ABSTRACT FROM PUBLISHER]

A large fuselage panel is always connected to a full-scale shape-preserving bracket to enhance its stiffness in digital assembly processes and reduce assembly deformation induced by factors such as gravity. However, the full-scale shape-preserving bracket’s characteristics, such as large size and weight, usually result in a complex installation process and high maintenance costs. A method for optimizing a multi load-transmitting device based discrete support layout is presented in this paper. By constructing finite element model including the fuselage panel and LTDs, the panel’s strain energy under different support layout parameters is obtained according to the principle of mixed uniform design. Using the method of partial least squares regression, the mathematical relationship between the strain energy and the support layout parameters is established, which is then used to obtain the optimum multi LTD based support layout. Furthermore, by comparing the panel deformation under the optimum multi LTD based support with that under the full-scale shape-preserving bracket-based support, we conclude that the optimum multi LTD based support can not only suppress panel deformation effectively, but also keep the panel’s stiffness comparable to that under the full-scale shape-preserving bracket-based support. [ABSTRACT FROM AUTHOR]