The aircraft consists of tens of thousands or even hundreds of thousands of parts and components, which are structurally divided into fuselage, wing, vertical tail, horizontal tail, flaps, ailerons, elevator rudder, engine rudder, hatch, door cover, etc. The structure of the aircraft is large in size and complex in appearance. Its structural parts are mainly thin-shell structures composed of sheet metal parts or composite wall panels. It is characterized by complex shapes, multiple connecting surfaces, and low process rigidity, which will be produced during processing and assembly. Deformed.

      Aircraft assembly is the main link in the aircraft manufacturing process. The aircraft assembly process is the process of assembling and connecting a large number of aircraft parts according to drawings, technical requirements, etc., and is divided into partial assembly (a combination of parts, one piece, one component) and final assembly (each component to complete the fuselage). The design and manufacture of aircraft is difficult and the cycle is long, which is not only reflected in the large amount of CNC machining of its parts, but also in the complexity and difficulty of its assembly. The assembly workload of aircraft accounts for about 40% to 50% of the total aircraft manufacturing labor (general machinery manufacturing only accounts for about 20%). The manufacturing and installation accuracy of the assembly frame is of great significance for ensuring the assembly quality of the aircraft.

      The quality of each link in the processing and assembly of aircraft parts will directly affect the performance of the entire aircraft. With the introduction of a large number of advanced CNC machine tools in my country's aviation factories in recent years, the CNC machining capabilities of aircraft parts have been greatly enhanced, the manufacturing cycle of aircraft parts has been greatly shortened, and the processing quality has been greatly improved. However, the assembly of aircraft components and the assembly of the entire fuselage still use the original traditional methods, which is in sharp contrast with the advanced numerical control processing methods. Aircraft assembly technology has become a huge obstacle restricting the rapid development of my country's aircraft. With the improvement of aircraft performance requirements, the shortening of the delivery cycle, and the more frequent replacements, higher requirements are put forward on the assembly technology of the aircraft.

Development status at home and abroad

     Since Boeing’s B777 adopted a paperless design in the early 1990s and began to fully implement digital aircraft manufacturing technology, digital assembly technology has developed rapidly in western aviation developed countries. It is a manufacturing and assembly-oriented approach proposed by advanced foreign aircraft manufacturing companies in recent years. A key part of the concept of concurrent design. Digital assembly technology integrates automated assembly technologies such as virtual assembly technology, frameless assembly technology, flexible assembly tooling technology, laser positioning technology and automatic drilling and riveting technology.

      In the 1990s, people paid more and more attention to the assembly technology of airplanes. The most famous research projects include European JAM (Jigless Aerospace Manufacture) and European ADFAST (Automation for drilling, fastening, assembly, systems integration, and tooling).

      In the development of military aircraft, Boeing has applied digital assembly technology. By adopting the determinant assembly technology (Determinant Assembly) that quickly locates different parts and components based on the key feature information of the parts and components, the huge assembly frame is eliminated. A universal bracket is used as the main tooling to support the prototype, and 4 Zeiss laser trackers are used to position the components in space to complete the assembly work.

       In terms of digital connection assembly technology, domestic digital connection assembly technology, some domestic models use digital models as templates, and use laser tracking and CAT (Computer Aided Theodolite) technology based on digital models in frame installation and maintenance. In military aircraft development and subcontract production projects, various OEMs have adopted this technology to varying degrees.

       Lockheed and Martin have adopted "flexible assembly technology" in the development of advanced military aircraft, using laser positioning, electromagnetic drive and other technologies to achieve precision drilling, which not only reduces the drilling error rate, but also greatly reduces tools and tooling. During the development and production of the F-35, Lockheed Martin adopted an advanced modular flexible automated assembly process and system. The assembly system is a seamlessly integrated whole system as part of the assembly process. Including assembly technology and equipment such as hole making, countersinking, riveting, etc. It also includes functions such as data collection, equipment safety assurance, control process monitoring, assembly simulation modeling, etc., shortening the aircraft assembly manufacturing cycle by 2/3, and the single machine takes 15 months It was shortened to 5 months, while the process equipment was reduced from 350 pieces to 19 pieces. At the same time, the manufacturing cost was reduced by half. It only took one year from order to delivery. The ultimate goal is to produce one aircraft per day.

       In terms of civil aircraft development, in order to realize the monthly production of 38 wings in 2005, Airbus considered a lot in the second phase of the wing box automatic assembly research project (AWBA II, started in 1998, two years). A flexible assembly technology in order to reduce the cost of wing assembly and shorten the assembly cycle. The AWBA II project integrates technologies such as measurement, positioning, clamping, feeding, and robot drilling. A prototype of a flexible assembly unit with a height of 8.5 m has been developed, and it has passed the processing test of a certain wing rib on the A380.

       Boeing’s precise fuselage assembly AFA and fuselage assembly improvement team FAIT use Snap-together assembly technology to make the “super siding” and the “super siding” through the precision holes on the “super siding” and laser installation and positioning adjustments. Other parts realize automatic positioning, which simplifies the assembly tooling and fixture. In November 1999, Boeing delivered the first B747 passenger aircraft using this technology.

       At present, a large number of assembly tooling based on the analog coordination system and traditional assembly frames are still used in China. The design and application level of digital chemical equipment is still relatively low, especially for the assembly of military aircraft. Fixed tooling frames and manual assembly are also used. Based on the main method, the research and application of aircraft digital flexible assembly are basically blank.

       Through the tracking and learning of foreign advanced technology during the "Ninth Five-Year Plan" and "Tenth Five-Year Plan" period, China has made significant progress in digital design and manufacturing technology, achieved certain results in product digital definition technology, and gradually established a " Design for manufacturing" design concept. Research work has been carried out on some advanced single process technologies, such as automatic hole making and riveting technology, automatic positioning and detection technology, etc., and appropriate improvements have been made in the tooling, and part of the civil aircraft subcontracting and civil aircraft production have been adopted. Automated assembly technology, these works provide a certain research foundation for the research of flexible assembly technology in our country.

       The main technical reasons for the long delivery cycle and poor assembly quality of my country’s current aircraft are:

      (1) There are many types of aircraft components, complex structures, and numerous component connections, which require a large number of assembly frames. Each new aircraft development or modification will produce a large number of component assembly design and manufacturing tasks, and size adjustment is complicated and difficult, which seriously affects the delivery cycle of the aircraft;

      (2) Due to the low level of automation in the assembly of aircraft components and the entire fuselage, it has seriously affected the effective assembly time of the aircraft and the consistency of processing quality, and the assembly efficiency is low;

       (3) There are many problems in the coordination of analog displacement technology, accounting for about 80% of the technical problems in the development of the new machine, which seriously affects the cycle;

       (4) Since the precision of tooling manufacturing is 2 times higher than that of the product, the cycle is about 2/3 of the development cycle of a new aircraft. There are about 50,000 to 60,000 tooling for an aircraft, and the manufacturing time is about 6 times that of a trial-produced aircraft. , The cost accounts for about 25% of the new machine development cost, and the management is complicated.

The key technology of flexible assembly

       Digital flexible assembly is a fully digital coordination system built on a computer digital information processing platform that integrates aircraft. It applies computer information technology. Digital control technology and uses various CNC assembly tools to realize automatic clamping, hole making, and riveting.

Connect and seamlessly align and connect to complete the comprehensive system engineering of assembly and connection of components, parts and fuselage. Flexible assembly technology can adapt to the changing requirements of aircraft component specifications, batch sizes, assembly technology, site and time, and quickly complete assembly tasks in a limited space, achieving high quality, high efficiency, low cost, and time saving. Advanced aircraft flexible assembly technology is one of the key technologies to ensure the overall performance of aircraft components and aircraft. The aircraft assembly has the following characteristics.

        (1) A large number of thin-walled plate parts have elastic deformation, but the strict aerodynamic shape of the aircraft must be ensured;

        (2) Complex structure, huge number of parts and connections;

        (3) Special process equipment is required to ensure the accuracy of assembly;

       (4) The technical coverage is wide, and the amount of information to be managed is large.

        The digital flexible assembly of aircraft is a system engineering, and its digital technology runs through the entire process of aircraft assembly, and computer information technology occupies a very important position. The application of digital flexible assembly technology is the current general trend of digital manufacturing in the aircraft manufacturing industry at home and abroad. It can not only overcome the single application of assembly tooling under the traditional aircraft manufacturing (mold line_model method) analog quantity coordination system, long manufacturing cycle, high manufacturing cost, Coordination requires a large number of physical tooling and other shortcomings, and through integration with automated hole-making equipment, CNC drilling and riveting machines or automatic riveting equipment and industrial robots and other automated equipment, a digital flexible assembly system can be formed.

         Aircraft assembly is generally divided into 4 stages: assembly design, assembly preparation, assembly proceeding, and assembly testing. Each stage has corresponding key technologies to support it to ensure the high efficiency and high quality of the entire aircraft assembly.

        1 Aircraft assembly process technology

        Aircraft assembly technology is the technology to study the basic structure of aircraft assembly, such as wings, skin panels and fuselage, etc. It analyzes several assembly construction modes, such as skin packaging, long truss parts, and partition suspension rails. Assembly lines, pneumatic tools, reconfigurable modular tooling, modular sub-assembly systems and direct acting wing boxes, etc. The research proposed a geometric feature classification method, which specifically includes shape control, contour control, direction control, avoidance control, positioning control and so on. Summarized 3 key characteristics:

• Product key features (PKCs)

The geometric characteristics and material properties of the product, this feature greatly affects the performance, function and assembly shape of the product.

•Assembly key features (AKCs)

The characteristics of the product, tool, and fixture assembly stage, which greatly affects the realization of the key features of the product in the next assembly process.

• Manufacturing key features (MKCs)

Manufacturing process parameters or workpiece tooling characteristics, which greatly affect the realization of key features of the product or assembly process at the part characteristic level.

 2 Flexible assembly tooling technology

      Before the aircraft is assembled, the components must be fixed to maintain a good aircraft dynamics appearance. All this is achieved by tooling. Therefore, tooling technology is the basis of aircraft assembly technology. According to the structure and performance of the assembly tooling, it can be divided into: conventional tooling, modular tooling, flexible tooling, CNC control tooling.

    Multi-point forming technology is the basis of flexible tooling design. The basic idea is to use discrete points to fit the three-dimensional surface of aircraft assembly parts, that is, points to replace surfaces. The use of flexible tooling can greatly shorten the manufacturing cycle of aircraft assembly frames and can replace most fixed assembly frames. With its reconfigurability, a set of flexible tooling can assemble a variety of aircraft parts.

       Fixed aircraft thin-walled panel skins generally use two types of tooling, one is X, F plane fixed dot matrix static tooling, and the other is row and column movable dynamic tooling. The basic element of the two is the length-adjustable support and clamping unit, whose height position is controlled by CNC servo, and finally the tooling system is distributed in the theoretical shape of the workpiece.

       The flexible tooling system is composed of many height-adjustable support units, and its positioning support body can be installed with different operating heads to meet different component fixing requirements. In order to reliably support and clamp the assembly parts of the aircraft, the clamping force must be adjustable, and the opening and closing of the clamping state must be controlled by the computer program to adapt to different aircraft panel assembly parts.

3 Laser tracking measurement technology

      In the preparation stage of aircraft assembly, the accuracy of the position measurement of assembly components and assembly tooling systems directly affects the success or failure of aircraft assembly. The assembly parts of the aircraft are large in size and complex in structure, making it difficult to use three-dimensional position measurement. Laser tracking measurement system is the best way to solve this problem. It has the advantages of high measurement accuracy, large measurement size, convenient installation and operation, and mobility. It can be widely used in the assembly of aircraft.

      The laser tracking and positioning measurement system can be connected to the system computer via Ethernet. The laser tracking and positioning system measures some reference points of the assembled parts after clamping. The obtained measurement data is processed by the processing unit and directly fed back to the system computer. The actual assembly position is compared with the position of the precise mathematical model to obtain the correction value of the component assembly position. In this way, the mathematical model of the part and its actual assembly position can be unified, and the assembly reference coordinate system can be determined.

4 Computer digital control technology

        Computer digital control technology is the basis for the realization of digital flexible assembly of aircraft. It will complete the precise positioning, hole making and riveting of various moving parts required in the aircraft assembly process, realize the reliable fixation of the aircraft assembly components by the tooling system, and ensure the aircraft assembly shape Consistency with digital prototype. The flexible assembly system includes assembly-oriented design system, process data preparation system, man-machine interface, axis and logic control, three-dimensional laser measurement, hole making and riveting unit, mobile positioning platform, fastener feeding unit and flexible tooling system.

       The electrical control of aircraft flexible assembly is a typical networked digital control system, which uses TCP/IP industrial Ethernet and fieldbus technology. The upper layer of the aircraft flexible assembly electrical control system is the assembly-oriented aircraft design system and the assembly process data preparation system. The lower level has mobile positioning platform, tooling system, hole-making riveting unit, etc. The mobile positioning platform itself is a five-axis servo control device, while the flexible tooling system uses more position servo axes, ranging from dozens to hundreds. This system is a fusion of information technology, drive technology, automation technology, logic control, and process control.

5 Robot fully closed loop positioning technology

        Introducing robots in aircraft assembly can improve the flexibility and accessibility of the assembly system. With various terminal actuators, various assembly operations can be realized, such as automated drilling and riveting.

catch. However, because the positioning of the robot is a semi-closed loop control, the positioning accuracy is not high, and it is not suitable for high-precision hole making in aircraft assembly. In recent years, due to the use of embedded control methods, the 3D laser tracker can be integrated with the computer and the robot to quickly monitor the space position of the robot in real time, determine the absolute position of the robot, and achieve the effect of full closed-loop control of the robot. This greatly improves the positioning accuracy of the robot, and its full working space accuracy can reach 0.05mm, which is fully capable of aircraft.

6 Mobile positioning platform technology

     The mobile positioning platform is used to place the hole-making riveting unit, realizing the position of the hole-making riveting unit and positioning along the normal direction of the skin, and solving the precise positioning of large-size spaces. It should have good rigidity. On the premise of satisfying the function of the system, the influence of the size of the assembled parts on the structural performance of the system should be considered as much as possible. The positioning platform generally requires more than 5 degrees of freedom.

7 Assembly simulation control technology

      The process of aircraft flexible assembly is complicated, with the participation of tooling, hole making, riveting, feeding and other devices. Most of the assembly paths are complicated and the assembly surface is not open. Therefore, the entire process of aircraft assembly must be carried out before the formal assembly of aircraft components. Necessary simulation and verification to check the accuracy and reliability of the automated assembly process to ensure that the assembly is foolproof. The assembly simulation system should have the following functions:

      (1) Feature modeling of equipment objects;

      (2) Movement trajectory modeling;

      (3) Post-processing of CNC program;

      (4) Dynamic three-dimensional simulation;

      (5) Collision and interference check.

      Generally, the input data of the simulation system includes three-dimensional mathematical models of aircraft components, flexible tooling models, assembly process parameters, assembly feature trees and other data.

       After the simulation system is processed, a three-dimensional dynamic simulation of the entire assembly process will be generated, the types and layouts of the flexible tooling support and clamping units and their adjustment position height data, the assembly process control program is generated, and the generation time of the assembly tooling system\aircraft parts The assembly time and the inspection and evaluation report of assembly operability and safety. Assembly simulation control technology is the digital support platform technology of the aircraft flexible assembly system, which can ensure the maximum performance of the assembly system, such as high flexibility, high reliability, and high efficiency.

Outlook

        Digital flexible assembly is an important link in the digital manufacturing of aircraft. It ensures the uniqueness and consistency of the geometrical mathematical model of the aircraft's three-dimensional digital prototype during the aircraft assembly process, which can improve assembly efficiency and assembly quality, reduce error rates, and reduce aircraft Manufacturing and assembly costs and shortening the development cycle are of great significance. Under the control of the target cost of the aircraft, the digital flexible K technology can meet all the performance indicators required by the user and achieve a leap in the performance-to-cost ratio of the aircraft. With the comprehensive application of computer information technology, numerical control technology and intelligent robot technology in aircraft assembly, the assembly process technology for digital assembly is becoming more and more perfect and mature, and various efficient and advanced automated assembly equipment have emerged. The emergence of digital flexible assembly tooling has greatly reduced the space occupied by aircraft assembly and manual intervention in the assembly process, thereby reducing assembly costs, improving assembly quality, and shortening assembly cycles. The application of digital flexible assembly technology will promote the development of the global aircraft manufacturing industry in the direction of modularization, integration, intelligence, economy, and greenness.

        my country's digital flexible assembly technology has just started, and some aspects are still blank. Digital flexible assembly technology will greatly promote the development of my country's aircraft manufacturing industry. Specifically in:

(1) Analog quantity coordination will be replaced by digital quantity coordination;

(2) The area of the assembly site and the number of assembly tools will be greatly reduced;

(3) Digitization and automation of aircraft assembly are realized.