Analysis and research of digital measurement syste

Analysis and Research on digital measurement system in large aircraft assembly

modern aircraft design is developing towards the direction of structural integration, large parts and manufacturing precision. For example, the fuselage diameter of large aircraft can reach 5m, the wingspan is more than 50m, and the core stage diameter of carrier rocket has already exceeded 5m, which not only increases the difficulty of parts manufacturing, but also improves the quality requirements of assembly

the development of foreign assembly technology is very rapid. Advanced digital technology is used to achieve its precise assembly, and a large number of digital definition models and optical measurement and positioning technology and equipment are used. The measurement requirements that cannot meet the standards are still weak in domestic aircraft manufacturing and assembly, mainly because there is no systematic research and application under the high assembly accuracy requirements, and a complete system has not been formed. Therefore, this paper will discuss the digital measurement system (DMS) in the digital assembly of large aircraft in detail

characteristics and principles of digital measurement system

digital measurement system is an organization system that uses digital equipment and technology and computer control to complete automatic, fast and accurate measurement purposes, tasks and work. Its functions and advantages mainly include the following []:

(1) it has the ability to carry out large-scale measurement work. This is more valuable for the increasing size of aircraft, spacecraft, carrier rockets and other aircraft

(2) although DMS is relatively complex and expensive, its service life cycle is long. It has been used in the high-cost aircraft manufacturing industry for a long time. In terms of the whole cycle cost, its cost has been greatly saved

(3) it can simplify the chemical equipment and make it more versatile and flexible. For example, the laser tracker is used to realize the assembly scheme without positioning parts

(4) it can complete more complex shape and position measurement tasks. Its dynamic real-time measurement ability can complete the simultaneous feedback and control of multi-target point position data

dms adopts digital measuring equipment such as laser tracker, photogrammetry, radar scanner and IGPS. Their measurement principle is to determine the spatial pose of the measured object by controlling its six degrees of freedom. In the process of assembly design and manufacturing, three optical tooling points (OTP) are designed in advance on the main plane of the product or tooling, and the theoretical coordinate values of these three points when the product or tooling is in the correct position are given. Then the three points are measured by digital measuring equipment, and the actual coordinate values of each point are calculated by computer. Due to the error between the actual value and the theoretical value, properly adjust the position of the assembly object so that the measured actual value gradually approaches the theoretical value. When the actual coordinate value of each point reaches the tolerance range of the theoretical coordinate value given by the design, the correct spatial position of the product or tooling in the assembly is determined

the digital measurement system also needs to adhere to four principles when working, namely, the principle, the principle of separation of degrees of freedom, the principle of tolerance distribution and the principle of selecting the main plane []

dms integrates computers, digital measurement equipment and other software and hardware, as well as digital measurement means, to work together to complete the assembly task. Its application principle in assembly is shown in Figure 1. It can be seen from the workflow framework that DMS roughly includes three parts:

(1) computer aided measurement mock up design system environment (cammdse)

(2) measuring system environment (MSE)

(3) meas using exploitation system environment (MESE)

computer aided measurement prototype design system environment

cammdse is mainly based on the needs of digital measurement, taking products (parts, tooling) as the core, expanding and reorganizing the three-dimensional digital model library of product design, generating a three-dimensional digital model (digital printer) for the purpose of measurement, including the definition of tooling measurement model, and preliminarily establishing parameters and documents related to measurement scheme, programming, etc. The whole set of work relies on computers to form a complete design environment. It mainly includes the following work contents:

1 establish a measurement digital model library

it is a set of measurement data sets containing digital models, which are applied to all links of digital measurement, and can also be used as the basis of inspection data sets in the later stage of assembly. The establishment process is shown in Figure 2, which is based on the engineering data set

two types of model information are separated from the engineering data set: geometric information and non geometric information. Geometric information is the geometric model, which is mainly the three-dimensional spatial graphics of products, including auxiliary points, lines and surfaces

when generating the 3D digital model for measurement, you can add or delete these information as needed, extract the required point, line and surface information, and generate the necessary 3D digital information for measurement based on this. All non geometric information related to measurement appears in the form of attributes. According to the different objects to be described, the content of non geometric information can be divided into the following aspects: attribute information reflecting the structure of assembly products, attributes reflecting the requirements of assembly technology, attributes reflecting the measurement environment, inspection text and other attribute information (such as attributes reflecting the measured materials of products, etc.)

in addition, the established and derived measurement information must be checked by the review department and reviewers. It can be put into storage only after it is verified to be correct, and the assembly digital prototype for measurement is established. In the process of establishing the measurement digital model base, the engineering data set and measurement digital model can also be modified

2 definition of tooling and tools

definition of tooling and tools is a supplementary design and improvement of the assembly model of tooling and tools that have been completed by technicians. There are special process structure design departments or departments in the aircraft manufacturing plant. Generally, there are teams composed of process personnel responsible for manufacturing and assembly between factories. One of their jobs is to design tooling for the product engineering data issued by the design unit. Here is the need to complete the measurement, check the digital models of these designed tooling and tools, and supplement relevant necessary measurement information, mainly including the OTP acquisition of tooling/tools and the structural inspection of assembly tooling, as shown in Figure 3

(1) OTP acquisition

for the assembly process completed by assembly tooling, OTP is mainly set on the tooling frame and positioner, and can be adjusted appropriately according to the measurement needs

take the central wing box and the middle and rear oil tank components of Airbus new a as an example, extract the engineering data set of the central wing box connecting the truss and the skin, and design and install the locator according to the needs of the truss assembly. Due to the very compact space, the OTP measurement plans to use the extension rod. For the method of direct connection and assembly between parts, OTP should be set on the parts. For locating pin installation parts, set 2 OTP points near the pin position. The layout and positioning are similar to the layout of OTP when tooling and positioner are positioned. The first point should be 8mm away from the pin surface, and the minimum deviation from the second point is 25mm, that is, the line between the two points is in the normal plane of the direction of the pin pressing hole (pin axis). The second point is required only when the part positioning effect occurs in the direction of the pin axis. Of course, after adopting the digital assembly system, it is not recommended to use the pin positioning. In order to improve the accuracy and easy detection, it is best to use the positioning surface on the part for direct positioning

(2) assembly tooling structure inspection

this link is mainly completed by the tooling design department, workshop technicians and product designers. Check according to the design standards, process requirements and technical conditions, including the measurement benchmark of assembly tooling and measurement auxiliary tooling, OTP recheck, poor contact of geometric and non geometric junction sensor lines required for measurement, recheck of sensor line structure after unplugging the controller, etc. For example, in order to avoid unnecessary movement of the measuring equipment, the tooling can be numbered in a certain order to facilitate measurement. The inspection results will have a certain impact on the measurement data model

3 guiding scheme documents

the main purpose is to generate electronic or paper documents required for various follow-up work and provide them to reviewers, experts and leaders for review. These include the following categories

(1) operation specification documents

this document is a technological document with detailed operation methods, skills and experiences, precautions and other contents provided in the form of text, which is used to describe the more complex, cumbersome or important operation items in the DMS measurement process

(2) parametric file

mainly records the digital information transmitted by DMS, which can be divided into two categories. One is parameterized geometric information. Due to the complexity and repeatability of the measurement process, sometimes the data of some geometric information must be extracted and recorded in a list in the form of files. The other is the specification file needed for programming. In DMS, programming mainly has two aspects: on the one hand, it is the secondary development or re programming of the ready-made software system provided by the measurement equipment manufacturer; On the other hand, the post-processing of the measurement data and the preparation of the interface control program connected with the electromechanical control system

(3) graphic file

graphic files are three-dimensional digital models or two-dimensional drawings in the computer stored in disk media, printed, archived, distributed and transmitted, or shared through the network

measurement system environment

mse is mainly based on the previous cammdse work results and preliminary measurement plan, and according to the various provisions of the guiding plan document, the working steps of DMs are formulated in detail, and the relevant procedures are prepared. In addition, the digital measurement process can also be simulated, and a certain degree of qualitative analysis can be carried out in advance to generate some predictable results. The workflow of this part is shown in Figure 4

1 detailed measurement program design

(1) improve the measurement plan

every step of the measurement process must be arranged and designed in detail when using DMS. This content requires the detailed design and improvement of the measurement workflow. Self compiled software program can be used to realize the automatic measurement, and the program can be changed according to different measurement contents, which has strong pertinence and autonomy. In fact, this demand factor has been taken into account in the existing large-scale measurement software, so the "measurement planning" (MP) function has been generated. That is, first list the schedule of each step of function, then add each step of function operation to the MP list of the software, and finally save and run the "execute MP" command to work [7]

(2) programming design of assembly measurement software

due to the complexity and specificity of the measurement work of large aircraft, the major Aerospace Manufacturers in the world will hire professional design companies or personnel to design special program systems for each aircraft type, including the programs used by various hardware. Although this method lacks versatility, it can better complete some specific work, especially for the mass production of large aircraft manufacturing, which is more reasonable and cost-effective. This kind of program can generally be compiled on the basis of existing measurement software by using the interface function provided by the manufacturer of measurement equipment

(3) electromechanical control software interface program design

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