introduction
With the rapid development of science and technology, the functional requirements of products are increasing, the complexity is increasing, the life span is shortened, and the replacement and replacement speed is accelerated. However, the design of products, especially the design methods of mechanical products, is incapable of following the needs of the development of the times. At present, computer-aided product design, drawing, design and calculation, processing and manufacturing, and production planning have received relatively extensive and in-depth research, and have achieved initial results. However, computer-aided design of early product development programs is far from meeting design needs. For this reason, after reading a large amount of literature, the author summarizes the methods used by design scholars at home and abroad for designing programs, and discusses the organic links between various methods and the development trends of computer product design and computer implementation.
According to the main characteristics of the methods used by domestic and foreign designers for designing mechanical product solutions, the modern design methods of the solutions can be summarized as the following four types.
1 systematic design method
The main features of the systematic design method are: Considering the design as a system composed of several design elements. Each design element is independent. There are organic links among the elements and they are hierarchical. All the design elements are combined. After that, you can accomplish the tasks that the design system needs to accomplish.
The systematic design thought was put forward by German scholars Pahl and Beitz in the 1970s. Based on the system theory, they formulated the general mode of design, and advocated that the design work should be organized. On the basis of this design concept, the German Institute of Engineers worked out the standard VDI2221 "Development and design methods for technical systems and products". The design process is shown in Figure 1.
Figure 1 Development and design methods for technical systems and products
(German Engineering Society Standard VDI2221)
The formulating process model for the design of mechanical products basically follows the design method of the German standard VDI2221. In addition, many design scholars in our country have borrowed and quoted the systematic design ideas of other developed countries when designing product plans. Representative examples are:
(1) The user needs are taken as the basis for product function feature design, structural design and part design, process planning, job control, etc. Starting from the macroscopic process of product development, the quality function deployment method is used to systematically make the user demand information reasonable and effective. The method of converting the ground to the technical goals and operational control procedures at each stage of product development is illustrated in Figure 2 for an overview of the quality function layout method.
Figure 2 Overview of Quality Function Arrangement Method
(2) The product is regarded as the life system at the organism level, and with the aid of the life system theory, the product design process is divided into the functional requirement level, the concept level of realizing the functional requirements, and the specific design level of the product. At the same time, the life system icon is used to abstractly express the functional requirements of the product and form the functional system structure of the product.
(3) Generalize the application of system science in mechanical design into two basic problems: first, treat the product to be designed as a system, determine its component parts (units) and their mutual relations optimally, and secondly, design the products. The process is seen as a system, and the various aspects of the design work and various design stages are correctly and reasonably determined based on the design goals.
Due to the different perspectives of each designer's research problem and the different focus of the consideration of the problem, there are also differences in the specific research methods used in the program design. Here are some representative systematic design methods.
1.1 Design Element Method
A “product solution†is described with five design elements (functions, effects, effect vectors, shape elements, and surface parameters). After the five design element values ​​of a product are determined, all the product's characteristics and feature values ​​are determined. There are also design scholars in China who have used similar methods to describe the original understanding of the product.
1.2 Graphical Modeling
The KALEIT “Design Analysis and Guidance System†was developed to describe the functional structure of the product and its related abstract information with clear and hierarchical graphs, and to achieve a graphical modeling of the system structure and functional relationships, as well as the connection between functional layers.
The design is divided into two aspects: the auxiliary method and the information exchange. The Nijssen information analysis method can use the graphic symbols, the structure of the semantic model with rich content, can describe the integration conditions, can classify the constraints, and can achieve any combination of relationships. , Integrate design method solutions with information technology to achieve graphical modeling of information relationships between different abstract layers in the design process.
The semantic design network is used as a design tool. In the active semantic design network ASK developed by it, the network description design is composed of nodes and lines, and the nodes represent elemental units (such as design tasks, functions, components, or processing equipment). Lines are used to adjust and define the different semantic relationships between nodes, thereby pre-modeling all activities and results in the design process, so that the definition of early design requirements to each structure's specific description can be expressed by the definition of relationships. , to achieve a computer-aided design process from abstract to specific leap.
1.3 "Conceiving" - "Design" Act
The program design of the product is divided into two phases: "conception" and "design". The task of the "conception" phase is to seek, select, and combine the original understanding that meets the requirements of the design task. The "design" phase of the work is the original understanding of the concrete realization of the concept phase [12].
The "conception" of the program is specifically described as follows: According to the appropriate functional structure, it seeks to meet the original understanding of the design task requirements. That is, the function in the functional structure is realized by "structural elements", and the physical connection between "structural elements" is defined as "functional carrier". The interaction between "functional carrier" and "structural element" forms a functional schematic diagram ( Mechanical movement diagram). The "design" of the program is based on the functional diagram, first qualitatively describes all "functional carriers" and "structural elements", and then quantitatively describes the shapes and positions of all "structural elements" and couplings ("functional carriers"). Schematic. Roper, H., using graph theory, with the aid of the "general design element (GE)", "structural element (KE)", "functional structural element (FKE)", "connected structural element (VKE)", as defined by him, Concepts such as "KT" and "KET", as well as several types of diagrams depicting the relationship between the size, position, and drive parameters of structural elements, formalize the design experts' intuitive design approach. The description forms a method for effectively applying existing knowledge and applies it to the "conception" and "design" stages.
From the point of view of design methodology, the design work after the design task is clearly defined is divided into three steps: 1) the function and function structure (referred to as "function"), 2) the search effect (referred to as "effect"), and 3 ) Find the structure (referred to as "formation rule"). The following four strategies are used to describe the workflow of the mechanical product ideation phase: Strategy 1: Consider the "function", "effect" and "formation rule" separately. Therefore, it is possible to create variants in each work step separately, thereby producing a broad original understanding spectrum. Strategy 2: "Effect" is associated with "formation rules" (including the rules created by the designer), considering functions separately (usually related to design tasks). At this point, it is necessary to have a wealth of experience in distinguishing the typical configuration rules and their subordinate effects, and the spectrum of the generated programs is much less than that of Strategy 1. Strategy 3: "Function", "Effect" and "Formulation Rule" are closely related. Applicable to areas where there are no choices between functions, effects, and configuration rules, and have special requirements, such as ultra-small machines, oversized machines, high-value functional parts, parts with special function requirements, and so on. Strategy 4: Structured solution to design requirements. The strategy starts from the existing parts and achieves the desired function through different sorting and connection of parts.
1.4 Matrix Design
In the project design process, the "requirements-functions" logic tree ("and-or" tree) is used to describe the relationship between requirements and functions, and functional design solutions that meet the requirements are obtained to form different design schemes. According to the "Requirements - Function" logic tree, a "Requirements - Function" correlation matrix is ​​established to describe the complex relationship between the functions required to meet the requirements and to show the one-to-one correspondence between requirements and functions.
Kotaetal uses matrices as the basis for the design of mechanical systems. It decomposes the design space of the mechanical system into functional subspaces. Each subspace represents only one module of the solution design. At the high level of the abstraction stage, each design module uses a motion conversion matrix and one. Constrained vector representations that can be manipulated; at the lower levels of the abstraction stage, each design module is represented as a parameter matrix and an equation of motion.
1.5 Bonding Chart
Divide the functions of the components that make up the system into various types such as energy generation, energy consumption, energy conversion, energy transfer, etc., and borrow the functional solution of the bond diagram to express the elements. We hope to combine the function-based model with the bond diagram to achieve a functional structure. The automatic generation and automatic conversion between functional structures and bond diagrams seeks ways to generate multiple design alternatives from bond diagrams.
2 Structural Modular Design Method
From the perspective of planning products, we propose that when defining a design task, based on a functional product structure, refer to existing product solutions (such as general parts, etc.) to describe the design task, that is, consider whether each subtask has a correspondence when decomposing the task. Product solutions, so that in the product planning stage, the possible contradictions in the design tasks can be eliminated, the production capacity and costs can be predicted in the early stage, and the adjustability of the plans in the development and design process can be improved, thereby improving design efficiency and design reliability. It also reduces the cost of new products. Feldmann divided the functional product structure describing the design task into four layers, (1) product→(2) functional composition→(3) main functional component→(4) functional element. The application-oriented structured feature catalog is used to provide more specific qualitative and quantitative descriptions of functional components. At the same time, STRAT, a tool software suitable for early product development and early design use, was developed.
It is believed that most of the functions in the special machinery can use the existing product solutions, and there are only a few special functions with new solutions. Therefore, the use of a functional product structure in the design of special machinery is very advantageous for evaluating the design and manufacturing risks of special machinery.
It is advocated that on the basis of product functional analysis, the product is decomposed into one or several modular basic structures with certain functions, and different products are built by selecting and combining these modular basic structural groups. These basic structures can be parts, components, or even a system. The ideal modular basic structure should have a standardized interface (connection and coordination), and be serialized, generalized, integrated, hierarchical, flexible, economical, interchangeable, compatible, and relevant. China combines software component technology and CAD technology to combine deformation design and combination design. According to the hierarchical modular principle, the machining center machine tool is divided into product level, component level, component level and component level from big to small, and utilizes expert knowledge. And CAD technology will combine them into different varieties, different specifications of the functional modules, and then by these functional modules combined into different machining center overall program.
Taking the design as a catalog as a tool for selecting a variational mechanical structure, a complete and structured arrangement of the solution elements of the design is proposed to form a solution set design catalogue. And the additional information of each solution of the review is listed in the solution collection design directory, which is very favorable for the design engineer to select the solution element.
According to the connection characteristics of mechanical components, they are grouped into four types: 1) Direct positioning between components and components with self-adjustability; 2) Compositions with common components; 3) Nested structures and inlays The coupling of the sleeve elements; 4) The coupling of the modular structure and the modular elements. The quasi-symbols are used to represent the connection rules between typical components and components, thereby realizing the algorithmization and the visualization of concepts between components.
In the design of the mechanical system, the "function establishment" module is used to decompose the function, and the optimal "graininess" degree of the function decomposition is a one-to-one correspondence between the function and the organization type. The "Structural Creation" module serves as a selection object for the functional solution in order to implement the mapping algorithm.
3 Design Method Based on Product Feature Knowledge
The main features of product-based knowledge design methods are: using computer-recognizable languages ​​to describe the characteristics of products and the knowledge and experience of experts in the design field, establish corresponding knowledge bases and inference engines, and reuse the stored domain knowledge and established Reasoning mechanisms implement computer-aided product design.
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 push and decide, and complete the type and number of institutions. To achieve this stage of computer-aided design, we must study the automatic acquisition, expression, integration, coordination, management, and use of knowledge. To this end, domestic and foreign design scholars have done a lot of research on automated processing of mechanical system design knowledge. The methods used can be summarized as follows.
3.1 Coding method
According to the "motion conversion" function (referred to as functional element), the organization is classified, and the function element and the organization category are described by the code, thereby establishing the knowledge base of the "institution system design expert system". Based on this, combining the binary logic reasoning with the principle of fuzzy comprehensive evaluation, the reasoning mechanism of this “expert system†was established and used in the design of the four-station special machine tool.
Using biological evolution theory, the principle of natural evolution and sexual reproduction to enable organisms to evolve, in the design of institutional plans, the use of network graph theory to express the structure of institutions as a topological map, and then through the coding technology, the structure of the organization and The performance is converted into binary strings of individual chromosomes, and the fitness value is compiled according to the design requirements. The biological evolution theory is used to control the reproductive mechanism. Through selection, crossover, sudden mutation and other means, the unadapted individuals with low fitness values ​​are eliminated and the evolution is extremely fast. The process obtains the most adaptable individual, ie, the institutional solution that best meets the design requirements.
3.2 Mixed Expression of Knowledge
For the scheme design of complex mechanical systems, it is particularly suitable to use a hybrid knowledge representation to describe various types of knowledge in the design. This has been agreed by many design scholars in China.
In the development of complex product design intelligent decision support system DMDSS, rules, frameworks, processes, and neural networks and other knowledge representation methods are organically combined to fit the description of different types of knowledge in the design. A variety of single knowledge representation methods (rules, frameworks, and processes), according to object-oriented programming principles, use the slot of the framework to represent the properties of the object, use rules to represent the dynamic characteristics of the object, and use the process to represent the processing of knowledge to form a kind of A mixed knowledge expression model, and successfully developed the "object-oriented CNC portal milling machine gearbox design intelligent system GBCDIS" and "transmission structural design expert system GBSDES".
3.3 Utilize knowledge-based development tools
In the CAD system of the coupling, using the knowledge-based development tool NEXPERT-OBJECT, an object-oriented design method database was created with the help of the object-oriented method, which provided the designer with the design of the coupling and the structural design of the coupling. Broad and reliable design method spectrum. Then use NEXPERT to describe the content of the linear guide design that needs to be based on knowledge, and then seek a solution based on knowledge, and develop a linear guide design expert system.
3.4 Design Directory Act
Constructed a three-level progressive design catalogue of “function moduleâ€, “functional element solution†and “mechanism groupâ€, and used this three-stage progressive design catalog as the knowledge base for the intelligent design system of the mechanical transmission principle scheme and the development design. Auxiliary tools.
3.5 Instance-based approach
In the development of the knowledge base of the design expert system, the basic predicates are used to describe the design requirements, design conditions and selected solutions. The “engineering examples†and various “conceptual entities†are described by the framework structure, and the candidate solutions are generated by instance-based reasoning techniques. To match the design requirements of the product.
4 Intelligent Design Method
The main features of the intelligent design method are: According to the design methodology theory, the use of three-dimensional graphics software, intelligent design software and virtual reality technology, as well as multimedia, hypermedia tools for product development and design, expression product ideas, describe the product structure.
In the literature [39], while taking advantage of mathematical system theory, it considered system engineering theory, product design technology, and system development methodology VDI2221 to develop MUSE, a multimedia development system software that is suitable for initial product design use.
In the literature [40], during the design of ATMs, the entire development process of the product was summarized into three stages: "product planning", "development" and "production planning", and full use of the existing CAD cutting-edge technology - virtual Reality technology. 1) Product Planning - Conceptual Product. Its task is to determine the external characteristics of the product, such as color, shape, surface quality, ergonomics, etc., and the initial vision is represented by a CAD three-dimensional model, creating a simple model that can reflect the overall product shape, which can be virtual Built in the environment, with the aid of data caps and 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 detecting the effect of the external shape, and is also the basis for displaying the design variables in the geometric figure. It is also the basis for various types of analysis in the development process. 2) Develop-design products. This stage is mainly based on the principle of “system synthesisâ€. The elements are configured and integrated on the three-dimensional model. The solution elements have different meanings depending on the design goals: they can be basic elements such as bolts, shafts, or hub connections; they can also be composites. Elements, such as mechanical, electrical, electronic components, control technology or software consisting of a drive system; can also be requirements, features, shapes, and so on. After the key solution elements of the implementation function are configured on the three-dimensional model, the configuration of the product (the relationship between the solution elements in the design model) can be analyzed, and the product configuration analysis is important for integrating the "product planning" and "development" results. means. 3) Production planning - processing and assembly products. In this stage, the literature [40] mainly discusses the application of CAD technology in the assembly process, and proposes to use the computer image to display the assembly process of the solution element in the corresponding position, that is, to reveal the relationship between the shape and the assembly through the virtual assembly model. Difficulties and problems were identified and solutions to the problems were identified. It was considered that applying CAD technology to the three stages of product development can make the synthesis and analysis of the design process “product planningâ€, “development†and “production planningâ€. Continuously alternately. Therefore, problems existing in various stages can be discovered earlier, and products can be continuously refined and improved in the development process.
China's design using virtual reality technology is still in its infancy. Using object-oriented technology, the expert system for the design of the mechanism-combination scheme synthesized in time series was studied with emphasis, and OpenGL technology with high-performance graphics and exchange processing capability was used to perform the scheme designed by the expert system from various angles in a three-dimensional environment. Observe if there is conflict between organizations in the movement.
The formulation of a standard module, the overall product structure, and its manufacturing process and use instructions (see Figure 1) are referred to as rapid prototyping. It is recommended that rapid prototyping technology, multimedia technology, and virtual expressions be used in conjunction with neural networks (applicable to the various stages of the solution process) in the product development process. 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, for example, three-dimensional graphics (three-dimensional model) instead of assembly, disassembly, and three-dimensional structure imagination required when designing connection parts, etc. Wait.
Using the intelligent CAD system SIGRAPH-DESIGN as a development platform, the product development process is divided into concept design, assembly design and part design. Based on the variable design technology, a conceptual model of the cam link mechanism of the offset press is established. From the research work introduced in the literature, the conceptual model is based on the establishment of the institutional type, the number of integrated, based on the SIGRAPH-DESIGN software to provide the variable design function, so that the schematic changes with the structural parameters of the organization and change, and Pass the parameters of the conceptual model to the next level assembly model and part design.
5 Review and Development Trends of Various Design Methods
To sum up, the systematic design method divides the design task from abstract to concrete (from the task requirements of the design to the scheme or structure for realizing the task), and formulates the goals and methods to achieve in each layer, from shallow to deep. From the abstract to the concrete, the layers are organically linked together to systematize the entire design process, so that the design has regular rules, methods can be followed, and the computer-aided implementation of the design process is easy.
The structure modularization 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 certain kind of mechanical products, because the functions of its components are relatively clear and relatively stable, the division of structural modules is relatively easy. Therefore, it is more appropriate to adopt a structural modular approach for program design. Since there is not a one-to-one relationship between entities and functions, an entity can usually implement several functions, and one function can often be implemented by several entities. Therefore, if the structural modular design method is used for the design of general-purpose product solutions, the division and selection of structural modules are more difficult, and the designers are required to have considerable design experience and extensive multidisciplinary domain knowledge.
The scheme design of mechanical products usually cannot be performed by pure mathematic calculation methods, and it is difficult to use a mathematical model for complete description. It is necessary to make formal descriptions based on product features and to reason and make decisions based on the knowledge and experience of design experts. Therefore, in order to realize the design of computer-aided product solutions, it is necessary to solve the problems related to computer storage and application of product design knowledge and expert design decisions, thereby forming a design method based on product feature knowledge.
At present, the intelligent design method is mainly designed using three-dimensional graphics software and virtual reality technology. The intuitiveness is good. In the early stages of development, users can directly participate in the design to a certain extent, but the system is poor and the structure of parts and components is Reasonable determination of shape, size, and location requires that the software has a high degree of intelligence or designers with extensive experience participate.
It is worth mentioning that: the above methods are not completely isolated, and there is a certain degree of connection between various methods. For example, in the method of structural modularization design, when the structural module is divided, it contains systematic ideas and establishes products. Features and design methods Knowledge bases and inference engines often use systematic and structural modularization methods. In addition, design based on product feature knowledge is at the same time one of the foundations of intelligent design. In the mechanical product design, the general parts, components, or common mechanisms that can implement specific functions are considered as structural modules, and they are applied to the specific design of the systematic design hierarchy, that is, the structural modularization method is integrated into the systematic design method. This not only ensures the standardization of the design, but also simplifies the design process, improves design efficiency and quality, and reduces design costs.
The flourishing development of network technology, collaborative design and manufacturing in different places, and the realization of concurrent engineering from the user's functional requirements of the product → design → processing → assembly → finished products become possible. However, one of the important prerequisites for achieving these goals is to achieve a three-dimensional visualization of the product design effects. For this reason, not only three-dimensional graphics software and intelligent design software are increasingly used in product design, but virtual reality technology, as well as multimedia and hypermedia tools, are also emerging in the design of products. At present, developed countries such as Germany 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 Intemet) in product design.
The design of mechanical products is moving toward the direction of computer-aided implementation, intelligent design, and meeting the demand for collaborative design and manufacturing in different places. Due to the late start of the study of computer-implemented methods for product design, it is not yet mature enough to achieve these goals. Scenario design tool software. The author believes that the comprehensive use of the four types of design methods in the text is an effective way to achieve this goal. Although the comprehensive application of these methods involves many fields, not only related to the field of mechanical design, but also related to system engineering theory, artificial intelligence theory, computer software and hardware engineering, network technology and other aspects of the field of knowledge, but still Product design must be the direction of effort. Foreign studies in this area have achieved initial success. Chinese design scholars have also realized the importance of CAD technology and international exchanges and cooperation and the measures that should be taken.