Learning theoretical knowledge when there is no actual work experience often finds it very obscure and boring. After working for a few years, I have accumulated a certain amount of practical experience. When I look back at the relevant theories, I will feel a bit late.
With the rapid development of science and technology, the requirements of product functions are increasing, the complexity is increasing, the life span is shortened, and the speed of upgrading is accelerated. However, the design of products, especially the design methods of mechanical product solutions, appears to be incapable of keeping up with the needs of the development of the times. At present, the design and drawing, design calculation, processing and manufacturing, and production planning of computer-aided products have been relatively extensive and in-depth research, and initial results have been achieved. However, the computer-aided design of the initial product development plan is far from meeting the needs of design. For this reason, on the basis of reading a large number of documents, the author summarized the methods adopted by domestic and foreign design scholars when designing the scheme, and discussed the organic connection between various methods and the development trend of computer realization of mechanical product scheme design.
According to the main characteristics of the methods used by domestic and foreign design scholars to design mechanical products, the modern design methods of the scheme can be summarized into the following four types.
1. Systematic design method
The main feature of the systematic design method is: to regard design as a system composed of several design elements, each design element is independent, there is an organic connection between each element, and it is hierarchical, and all design elements are combined Then, the tasks required to complete the design system can be achieved.
Systematic design thought was put forward by German scholars Pahl and Beitz in the 1970s. Based on system theory, they formulated a general design model and advocated that design work should be organized. On the basis of this design idea, the German Association of Engineers formulated the standard VDI2221 “Technical System and Product Development and Design Method.
The design process model of the mechanical product scheme developed basically follows the design method of the German standard VDI2221. In addition, many design scholars in my country also draw on and quote the systematic design ideas of other developed countries when designing product schemes. The representative ones are:
(1) Use user needs as the basis for product functional feature conception, structural design and part design, process planning, operation control, etc., starting from the macro process of product development, using quality function layout methods to systematically make user demand information reasonable and effective Land conversion into the technical goals and operation control procedures at each stage of product development.
(2) Regard the product as a life system at the organism level, and with the help of the life system theory, divide the product design process into functional demand levels, conceptual levels to achieve functional requirements, and specific product design levels. At the same time, the life system icon is used to abstractly express the functional requirements of the product, forming a product functional system structure.
(3) The application of system science in mechanical design is summarized into two basic problems: one is to treat the product to be designed as a system, and to best determine its components (units) and their mutual relations; the other is to design the product The process is regarded as a system. According to the design goals, the various aspects of the design and the various design stages are correctly and reasonably determined.
Because each designer has different perspectives on the problem and the focus of considering the problem, there are also differences in the specific research methods used when designing the scheme. Here are some representative systematic design methods.
1.1 Design element method
Five design elements (functions, effects, effect carriers, shape elements and surface parameters) are used to describe the “product solution”. After the five design element values of a product are determined, all the features and characteristic values of the product have been determined. There are also design scholars in my country who have used similar methods to describe the original understanding of products.
1.2 Graphical modeling method
The developed “Design Analysis and Guidance System” KALEIT uses clear graphics to describe the functional structure of the product and its related abstract information, and realizes the graphical modeling of the system structure and functional relationships, as well as the connection between functional layers.
Divide the design into two aspects: auxiliary method and information exchange. Using Nijssen information analysis method, graphical symbols can be used, semantic model structure with rich content, integration conditions can be described, constraint types can be divided, and arbitrary combinations of relationships can be realized. , Integrate the design method solution with information technology, and realize the graphical modeling of the information relationship between different abstract layers in the design process.
Literature [11] uses the semantic design network as a design tool. In the active semantic design network ASK developed by it, a network description design composed of nodes and lines is used. The nodes represent componentized units (such as design tasks, functions, components or Processing equipment, etc.), lines are used to adjust and define different semantic relationships between nodes, thereby pre-establishing models for all activities and results in the design process, so that the definition of early design requirements to the specific description of each structure can be determined by the relationship The definition and expression of time has realized the leap of the computer-aided design process from abstract to concrete.
1.3 “Concept”-“Design” method
The product design is divided into two stages: “conception” and “design”. The task of the “conception” stage is to seek, select and combine the original understanding that meets the requirements of the design task. The work in the “design” phase is the concrete realization of the original understanding of the conception phase.
The “concept” of the scheme is specifically described as: seeking the original understanding to meet the requirements of the design task according to the appropriate functional structure. That is, the sub-functions in the functional structure are realized by the “structural elements”, and the physical connection between the “structural elements” is defined as the “functional carrier”, and the interaction between the “functional carrier” and the “structural element” forms a functional schematic diagram ( Mechanical movement diagram). The “design” of the scheme is based on the functional schematic diagram, first qualitatively describe all “functional carriers” and “structural elements”, and then quantitatively describe the shapes and positions of all “structural elements” and couplings (“functional carriers”), and get Schematic. Roper, H. Using graph theory, with the help of “general design unit (GE)”, “structural element (KE)”, “functional structural element (FKE)”, “connected structural element (VKE)”, Concepts such as “Structural Parts (KT)” and “Structural Element Parts (KET)”, as well as several simplified diagrams describing the relationship between the size, position and transmission parameters of structural elements, have formalized the intuitive design method of design experts The descriptions form a method to effectively apply existing knowledge and apply it to the “conception” and “design” stages.
From the point of view of design methodology, the design work after clarifying the design task is divided into three steps: 1) Obtain function and functional structure (referred to as “function”); 2) Find effect (referred to as “effect”); 3 ) Look for structure (referred to as “configuration rules”). The following four strategies are used to describe the working process of the mechanical product conception stage: Strategy 1: Consider “function”, “effect” and “configuration rule” respectively. Therefore, variants can be created separately in each work step, thereby generating a broad spectrum of original understanding. Strategy 2: “Effect” is associated with “configuration rules” (including rules created by the designer), and functions are considered separately (usually related to design tasks). At this time, to distinguish the typical configuration rules and their effects requires a wealth of experience, and the resulting plan spectrum is far less than the plan spectrum of Strategy 1. Strategy 3: “Function”, “Effect”, and “Configuration Rule” are closely related. It is suitable for fields that have no choice between functions, effects and configuration rules and have special requirements, such as ultra-small machinery, extra-large machinery, high-value functional parts, and parts with special functional requirements. Strategy 4: Structured solution to design requirements. This strategy starts from the existing parts and obtains the expected functions through different sorting and connection between the parts.
1.4 Matrix design method
In the program design process, a “requirement-function” logic tree (“and-or” tree) is used to describe the interrelationship between requirements and functions, and a functional design solution set that meets the requirements is obtained to form different design schemes. Then build a “requirement-function” association matrix based on the “requirement-function” logic tree to describe the complex relationship between the functions required to meet the requirements and show the one-to-one correspondence between requirements and functions.
Kotaetal regards the matrix as the basis of the mechanical system scheme design, and decomposes the design space of the mechanical system into functional subspaces. Each subspace only represents one module of the scheme design. At the high level of the abstract stage, each design module uses a motion conversion matrix and a Operational constraint vector representation; at the lower level of the abstract stage, each design module is represented as a parameter matrix and a motion equation.
1.5 Bond graph method
The functions of the system components are divided into various types such as energy generation, energy consumption, energy conversion, energy transfer, etc., and the functional solution of the components is expressed by the bond graph. It is hoped that the function-based model and the bond graph can be combined to realize the functional structure The automatic generation and the automatic conversion between the functional structure and the bond graph, seeking a method to generate multiple design schemes from the bond graph.
2. Structural modular design method
From the perspective of product planning: When defining the design task, it is based on the functional product structure, and the existing product solutions (such as general parts, etc.) are used to describe the design task, that is, when the task is decomposed, it is considered whether each subtask has a corresponding In this way, possible conflicts in the design task can be eliminated in the product planning stage, and the production capacity, cost, and the adjustability of the plan during the development and design process can be predicted early, thereby improving design efficiency and design reliability. It also reduces the cost of new products. Feldmann divides the functional product structure describing the design task into four layers, (1) product → (2) functional composition → (3) main functional components → (4) functional elements. And use the application-oriented structured feature catalog to describe more specific qualitatively and quantitatively the functional elements. At the same time, a tool software STRAT suitable for early product development and early design was developed.
It is believed that most functions in special machinery can be solved by existing products, while special functions with new solutions are only a few. Therefore, the use of functional product structure in the design of special machinery is very beneficial for evaluating the design and manufacturing risks of special machinery.
It is advocated to decompose the product into one or several modular basic structures with a certain function on the basis of product function analysis, and build different products by selecting and combining these modular basic structures. These basic structures can be parts, components, or even a system. The ideal modular basic structure should have standardized interfaces (connection and coordination parts), and be serialized, generalized, integrated, hierarchical, flexible, economical, interchangeable, compatible and relevant. China combines software component technology and CAD technology, combines deformation design with combination design, and according to the principle of hierarchical modularization, the machining center machine tools are divided into product level, component level, component level and component level from large to small, and use expert knowledge Combine them with CAD technology into functional modules of different varieties and specifications, and then combine these functional modules into different overall plans for machining centers.
Taking design as a catalogue as a tool for selecting variant mechanical structures, it is proposed that the solution elements of the design should be arranged in a complete and structured manner to form a solution set design catalogue. And the additional information of each solution is listed in the solution set design catalog, which is very helpful for design engineers to choose solution elements.
According to the coupling characteristics of mechanical parts, they are divided into four types: 1) Directly positioned between components and self-adjustable parts; 2) Structurally common components; 3) Nested structure and embedded parts The connection of sleeve elements; 4) The connection of modular structure and modular elements. And use quasi-symbols to represent typical components and the connection rules between components, so as to realize the algorithmic connection between components and the visualization of concepts.
In the design of the mechanical system, the function is decomposed by the “function establishment” module, and it is stipulated that the optimal degree of function decomposition is the one-to-one correspondence between the function and the mechanism type. The “structure building” module is used as the selection object of the functional solution in order to realize the mapping algorithm.
3. Design method based on product feature knowledge
The scheme design of the mechanical system is mainly based on the characteristics of the product, as well as the knowledge and experience of experts in the design field, to estimate and make decisions to complete the type and number of the mechanism. To realize this stage of computer-aided design, we must study the automatic acquisition, expression, integration, coordination, management and use of knowledge. For this reason, domestic and foreign design scholars have done a lot of research work on the automatic processing of mechanical system scheme design knowledge, and the methods used can be summarized as the following.
3.1 Encoding
According to the “motion conversion” function (referred to as the function element), the organization is classified, and the code is used to describe the function element and the organization category, thereby establishing the “mechanism system scheme design expert system ” knowledge base. On this basis, the dual logic reasoning and fuzzy comprehensive evaluation principle are combined to establish the reasoning mechanism of the “expert system” and used in the design of the four-station special machine tool.
Using the theory of biological evolution, the principle that organisms can evolve through natural selection and sexual reproduction, in the design of the organization scheme, the network graph theory method is used to express the structure of the organization as a topological map, and then the structure of the organization is combined with the coding technology. The performance is transformed into a binary number string of individual chromosomes, and fitness values are compiled according to design requirements. The biological evolution theory is used to control the reproduction mechanism. Through selection, crossover, sudden mutation and other methods, individuals with low fitness values are eliminated and unsuitable individuals with low fitness values are eliminated to evolve extremely fast The process gets the most adaptable individual, that is, the institutional plan that best meets the design requirements.
3.2 Mixed expressions of knowledge
For the scheme design of complex mechanical systems, it is particularly suitable to describe various types of knowledge in the design using a mixed knowledge expression method. This has been the consensus of many design scholars in my country.
In the development of the intelligent decision support system DMDSS for complex product scheme design , knowledge representation methods such as rules, frameworks, processes and neural networks are organically combined to adapt to the description of different types of knowledge in design. Combining a variety of single knowledge expression methods (rules, frameworks and procedures) according to the principles of object-oriented programming, using framework slots to represent the attributes of objects, rules to represent the dynamic characteristics of objects, and processes to represent the processing of knowledge to form one A hybrid type of knowledge expression, and successfully developed the “Object-oriented CNC gantry milling machine gearbox design intelligent system GBCDIS” and “Gearbox structure design expert system GBSDES”.
3.3 Utilize knowledge-based development tools
In the CAD system of the coupling, the knowledge-based development tool NEXPERT-OBJECT is used to create an object-oriented design method database with the aid of the object-oriented method, which provides designers with the scheme design and structural design of the coupling Extensive and reliable design method spectrum. Then use NEXPERT to describe the content that needs to be designed based on knowledge in the linear guide design, so as to find a knowledge-based solution, and develop a linear guide design expert system.
3.4 Design catalog method
Constructed a three-level progressive design catalog of “functional module”, “functional element solution” and “mechanism group”, and used these three-level progressive design catalogs as the knowledge base and development design of the intelligent design system of mechanical transmission principle scheme Auxiliary tools.
3.5 Case-based approach
In the knowledge base of the design expert system, basic predicates are used to describe the design requirements, design conditions and selected schemes, the frame structure is used to describe “engineering examples” and various “concept entities”, and candidate solutions are generated through case-based reasoning techniques Come to match the design requirements of the product.
4. Intelligent design method
The main characteristics of the intelligent design method are: according to the theory of design methodology, with the help of three-dimensional graphics software, intelligent design software and virtual reality technology, as well as multimedia and hypermedia tools to develop and design products, express product ideas, and describe products structure.
While using the mathematical system theory, considering the system engineering theory, product design technology and system development methodology VDI2221, the multimedia development system software MUSE suitable for the initial use of product design was developed.
When designing the ATM, the entire product development process was summarized into three stages: “product planning”, “development” and “production planning”, and it made full use of the existing CAD cutting-edge technology-virtual reality technology.
1) Product planning-conceive the product. Its task is to determine the external characteristics of the product, such as color, shape, surface quality, ergonomics, etc., and express the initial idea with a CAD three-dimensional model, and establish a simple model that can reflect the shape of the entire product. Established in the environment, with the help of a data cap and a three-dimensional mouse, users can also participate in this environment to a certain extent, and can quickly generate different shapes and colors. The three-dimensional model is the basis for the detection of external shape effects, the basis for geometric graphics to display design variables, and the basis for various analyses in the development process.
2) Development-design products. This stage is mainly based on the principle of “system synthesis” to configure and integrate solution elements on the three-dimensional model. The solution elements have different meanings according to different design goals: they can be basic elements, such as bolts, shafts or hub connections, etc.; they can also be composite Elements, such as transmission systems composed of mechanical, electrical, electronic components, control technology or software; it can also be requirements, characteristics, shapes, etc. After the key solution elements that realize the function are configured on the three-dimensional model, the product configuration (relationship between the solution elements in the design model) can be analyzed. Product configuration analysis is an important part of integrating the results of “product planning” and “development”. means.
3) Production planning-processing and assembling products. In this stage, it mainly discusses the application of CAD technology in the assembly process, and proposes to use computer images to display the assembly process of the solution elements at the corresponding positions, that is, to reveal the relationship between the shape and the assembly through the virtual assembly model, and thus find the difficulties and problems , And find out the method to solve the problem, and believe that the comprehensive application of CAD technology to the three stages of product development can enable the synthesis and analysis of the design process to alternate continuously in “product planning”, “development” and “production planning” get on. Therefore, the problems existing in each stage can be found earlier, and the product can be continuously refined and improved in the development process.
my country’s use of virtual reality technology for design is still in its infancy. Using object-oriented technology, the focus is on the design of the expert system of the mechanism combination scheme synthesized in time sequence. With the help of OpenGL technology with high-performance graphics and exchange processing capabilities, the scheme designed by the expert system is carried out from various angles in the three-dimensional environment. Observe, such as whether there are conflicts between the linkages between organizations in the movement, etc.
The drafting of structural standard modules, product overall structure and its manufacturing process and instructions for use (see Figure 1) is called rapid prototyping technology. It is recommended to combine rapid prototyping technology, multimedia technology, and virtual expression with neural networks (applied to the occasions required by the solution process at various stages) during product development. It is pointed out that with the continuous improvement of computer software and hardware, multimedia graphics processing technology should be applied to product development as much as possible, such as three-dimensional graphics (three-dimensional model) instead of the three-dimensional structure imagination required for assembly, disassembly and design of connectors, etc. Wait.
Using the intelligent CAD system SIGRAPH-DESIGN as the development platform, the product development process is divided into conceptual design, assembly design and part design, and based on variable design technology, a conceptual model of the cam link mechanism of the offset printing press is established. Judging from the research work introduced in the literature, the conceptual model is based on the determination of the mechanism type and number integration, with the help of the variable design function provided by the software SIGRAPH-DESIGN, so that the schematic diagram changes with the structural parameters of the mechanism. Transfer the parameters of the conceptual model to the next level of assembly model and part design.
5. Comments on various design methods and development trends
To sum up, the systematic design method divides the design task from abstract to concrete (from the design task requirements to the plan or structure to realize the task), and draws up the goals and methods to be achieved at each level, from shallow to deep , From the abstract to the concrete, the various layers are organically linked together, so that the entire design process is systematized, so that the design has rules and methods to follow, and it is easy to realize the computer-aided design process.
The structural modular design method regards the realization of a certain function as a structural module, and realizes the product design through the combination of structural modules. For a specific type of mechanical product, because the functions of its component parts are relatively clear and relatively stable, the division of structural modules is relatively easy, so it is more appropriate to adopt the structural modular method to design the scheme. Since there is not a one-to-one correspondence between entities and functions, one entity can usually realize several functions, and one function can often be realized by several entities. Therefore, if the structural modular design method is used in the general product design, the division and selection of structural modules are more difficult, and designers are required to have considerable design experience and extensive knowledge of multidisciplinary fields.
The scheme design of mechanical products usually cannot be carried out using pure mathematical calculations, and it is difficult to use mathematical models for complete descriptions. Instead, formal descriptions must be made based on product features and reasoning and decision-making with the help of design experts’ knowledge and experience. Therefore, in order to realize the program design of computer-aided products, it is necessary to solve the problems of computer storage and application of product design knowledge and expert design decision-making, so as to form a design method based on product feature knowledge.
At present, the intelligent design method mainly uses three-dimensional graphics software and virtual reality technology to design. It is intuitive. At the initial stage of development, users can directly participate in the design to a certain extent, but the system is poor, and the structure of parts is The reasonable determination of the shape, size, and location requires the software to have a high degree of intelligence or the participation of designers with rich experience.
It is worth mentioning that: the above-mentioned methods are not completely isolated, and there is a certain degree of connection between the various methods. For example, in the structural modular design method, the division of structural modules contains systemic thinking and product creation. Feature and design method knowledge bases and reasoning engines usually need to use systematic and structural modularization methods. In addition, the design based on product feature knowledge is also one of the foundations of intelligent design of schemes. In the design of mechanical product schemes, general parts, components or common mechanisms that can achieve specific functions are regarded as structural modules, and they are applied to the specific design of the relevant levels of systematic design, that is, the structural modular method is integrated into the systematic design method. It can not only ensure the standardization of design, but also simplify the design process, improve design efficiency and quality, and reduce design costs.
With the vigorous development of network technology, collaborative design and manufacturing in different places, as well as the realization of concurrent engineering from users’ functional requirements for products→design→processing→assembly→finished products have become possible. However, one of the important prerequisites for achieving these goals is to realize the three-dimensional visualization of the product design effect. For this reason, not only three-dimensional graphics software and intelligent design software are increasingly used in product design, but also virtual reality technology, multimedia and hypermedia tools are also emerging in product design. At present, Germany and other developed countries are focusing on the application of hypermedia technology, product data exchange standard STEP, and standard virtual reality modeling language VRML (standard exchange format based on virtual environment on Internet) in product design.
The scheme design of mechanical products is moving towards the direction of computer-aided realization, intelligent design and meeting the requirements of remote collaborative design and manufacturing. Due to the late start of the research on the computer realization method of the product scheme design, there is no mature one that can achieve the above goals. Scheme design tool software. The author believes that the comprehensive application of the four types of design methods in the article is an effective way to achieve this goal. Although the comprehensive application of these methods involves many fields, not only related to the domain knowledge of mechanical design, but also related to the domain knowledge of systems engineering theory, artificial intelligence theory, computer software and hardware engineering, network technology, etc., but it is still The direction of product design. Foreign research in this area has achieved initial results, and Chinese design scholars have also realized the importance of CAD technology and international exchanges and cooperation, and the measures that should be taken.
