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A prototype expert system for embodiment design of mechanisms and articulated systems
Artificial Intelligence m Engineering 8 (1993) 57-65
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A prototype expert system for embodiment design of mechanisms and articulated systems Leonardo Bertini Dipartimento di Construzioni Meccaniche e Nucleari, University of Pisa, Via Diotisalvi 2, 56126 PISA, Italy
(Received 17 May 1991; revised version received 17 November 1992; accepted 19 November 1992) The embodiment phase plays a significant role in mechanical design, particularly in the case of mechanisms and articulated systems. In order to investigate the capabilities offered by artificial intelligence techniques for the automation of this design stage, a prototype expert system (ES), called EMBMEC, was developed. The ES possesses a 'hybrid' structure, making use of both symbolic inference and numerical calculation techniques. At present, it can operate with plane mechanisms, selecting materials, link geometry and bearings. In the paper, the most relevant features of the system are discussed and a simple application example is reported. Key words: expert systems, mechanical design, embodiment design, mechanisms
required to make an integrated use of two distinct types of knowledge:
1 INTRODUCTION The application of knowledge based expert systems (KBESs) in the area of design (particularly mechanical design) are significantly less frequent than in other engineering fields (e.g. fault diagnosis, process planning, monitoring and real time control), which can be regarded as natural extensions of the earlier applications to the medical area. ~'2 KBESs for mechanical design were generally developed to operate in single highly specialized fields, such as valve, 3 composite laminates 4 or gear 5 design, finite element analysis 6'7 and material selection, s In addition to these single-domain applications, usually operating more as area consultants than as designers, there were attempts 9-~1 to develop KBESs based on general purpose design models. These systems, although still at the research stage, could, at least in principle, be applied to a rather large variety of engineering domains. Most of them try to define frameworks to solve the whole design problem, from the conceptual stage to detailing, so emulating the capabilities of actual human designers (or design staff). The experience gained with the development of such KBESs led to the recognition that, during the resolution of a mechanical design problem in contrast to classical artificial intelligence (AI) applications, it is usually
- - ' S y m b o l i c ' knowledge, i.e. facts, relationships among them and rules represented in symbolic form; this type of knowledge usually also includes empirical rules, expertise and practical criteria applied by the experienced designer in a given application domain (heuristic knowledge). - - ' A n a l y t i c a l ' knowledge, i.e. the knowledge on how to solve problems, which are stated as mathematical models, by means of 'algorithmic' techniques. KBESs employing both types of knowledge (i.e. making use of both symbolic inference and numerical calculation techniques) in order to solve problems are usually called 'hybrid', and actually constitute the great majority of modern AI applications to mechanical engineering design. The present paper illustrates a prototype 'hybrid' expert system (ES), which was developed for a relatively new application field, i.e. the 'embodiment' design of articulated systems. The general features, data structure and design model are discussed, emphasizing the main difficulties encountered during the implementation. 2 P R O B L E M STATEMENT The design process of mechanisms or articulated systems involves two main distinct phases:
Artificial Intelligence in Engineering 0954-1810/93/$06.00 © 1993 Elsevier Science Publishers Ltd.
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Leonardo Bertini
'synthesis': in this phase, given the required space and time motion law, the overall geometry of the mechanism, inclusive of the number and type of degrees of freedom and links, is defined; --'embodiment': in this phase, all the actual components of the articulated system are defined, as regards both geometry and materials. -
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Although several applications of AI techniques to mechanism synthesis have been developed in recent years, some of which gave appreciable results, 12 nearly no attempt was made, with the author's knowledge, to apply such techniques to the 'embodiment' phase. A research program was then set up at the University of Pisa, in order to analyse the capabilities offered by AI tools in this second design stage. As a result of this research work, a prototype KBES for the automatic embodiment of plane mechanism was developed, which was called EMBMEC ('embodiment of mechanisms'). Its capabilities are presently limited to the treatment of rather simple problems, i.e. articulated systems composed of an arbitrary number of links, each carrying two or three aligned hinged joints. Nevertheless, the results obtained can give useful indications for the development of KBES aimed at the automation of this design phase, both for mechanisms and for other types of mechanical systems. 3 KNOWLEDGE ACQUISITION AND SOFTWARE TOOLS The 'analytical' knowledge used by the ES was easily derived from machine design or stress analysis manuals, or directly included through the use of external analysis programs. As regards 'symbolic' knowledge, it can be classified as:
making use of the graphics capabilities of the ANSYS Finite Element program. ~3 To this end, FORTRAN code was developed which, based on the results of the EMBMEC system (which are made available as ASCII files), generates a command input file for the ANSYS code, so allowing the automatic creation of a solid model of the mechanism. Besides being useful for presentation purposes, the interface with the ANSYS program is actually thought to be the first step toward an automated finite element analysis of the links, which could be employed, in the future, to extend the capabilities of the KBES.
4 STRUCTURE OF THE KBES AND MODEL OF THE DESIGN PROCESS The EMBMEC system, whose general structure is shown in Fig. i, embodies a typical 'top-down' design process model with iterative modifications. It includes a supervisor module, called General Design Manager (GDM), and three area modules for the solution of specific subproblems, i.e. material selection module (MSM), rod geometry design module (RGDM) and hinged joint design module (HJDM). As a first step, general specifications and requirements for the current design problem are processed by the GDM, which defines the specifications for subproblems. These are solved, in the sequence given in Fig. 1, by the area modules, which embodies both heuristic and analytical specific knowledge. Subproblem solution is based both on the specifications given by the GDM and on the results previously obtained by other area modules (e.g. mechanical properties of the material selected by the MSM are employed by the RGDM to define rod dimension).
general (or domain-independent) knowledge, which includes commonly available facts and rules related to mechanical design and was mainly derived from machine design manuals; specific (domain-dependent) knowledge, which includes heuristic rules and facts typical of the particular field considered and was derived partly from experienced designers and partly by a critical examination of the system responses, as compared with known correct solutions. The EMBMEC system was developed, on an IBM personal computer, making use of the PROLOG-2 language. This allowed one to codify facts and rules easily, according to the usual PROLOG syntax, and also to perform simple calculations (e.g. the evaluation of the stress in a rod under bending loading) required by the embodiment phase. Once the design has been completed, defining the geometry of each component, a three-dimensional representation of the mechanism can be achieved
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A prototype expert system for embodiment design of mechanisms and articulated systems
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Fig. 2. Example of data structure. At the end of a single design iteration, the current solution is examined by the G D M and, if it is found to be unsatisfactory (or if some of the area modules failed to find a solution), subproblem specifications are modified and a new iteration performed. The process continues until the current design is accepted or cannot be further improved. In order to accept a solution, the fulfilment of the following conditions is required:
--explicit design requirements and limits (e.g. maximum lateral dimension or total weight not larger than given values) are satisfied; no incompatibility exists between distinct links or between distinct part of the same link as regards mechanism operation or manufacturability; - - the current solution does not conflict with implicit design requirements deriving from mechanism specifications and common design rules (e.g. if the mechanism will operate in an aggressive environment, no material having poor resistance to that environment is allowed).
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Information on the articulated system was structured as linked tree-structures (see Fig. 2), which, according to the PROLOG syntax, were coded as 'lists' in the EMBMEC program. Each link of the mechanism was represented as an object composed of several distinct parts (e.g. rods and end connections), whose geometry is defined by giving their general shape and a set of characteristic dimensions. Shapes are selected among the content of specific 'shape libraries', an example of which is reported in Fig. 3. These libraries were designed to allow an easy expansion of their content, in order to deal with a wider set of design problems.
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The fulfilment of another type of design specification, defining the required target (or optimality criteria) for the current problem (e.g. the requirement to minimize weight or cost), is achieved by a solution strategies ranking method. According to this approach, the KBES tries to apply first the technical solutions which are known to give better performances from a given point of view (this was mainly obtained by properly ordering rules and facts in the EMBMEC knowledge base). It is worth noting that this technique only allows one to achieve an approximate optimal design, similar to that obtained by an experienced human designer. Further improvements could be achieved, in many cases, by coupling the present method with numerical optimization techniques in order to refine the solution.
5 DESIGN SUPERVISOR M O D U L E The G D M module, whose flow-chart is depicted in Fig. 4, acts as a supervisor of the entire design process and manages the exchange of information with the external environment.
HOLLOW SOLID
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Leonardo Bertini FROM SYNTHESIS
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Fig. 4. Structure of the General Design Manager. It analyses the files containing the results of the mechanism synthesis (i.e. geometry and forces acting in each link as a function of mechanism spatial position) in order to evaluate some relevant parameters, such as the design loads for each rod and bearing, the relative rotation angle of adjacent links, etc. A few general design specifications are interactively required of the user. They include: general design criteria and requirements, according to the anticipated application field (whether it is required to minimize weight or cost, operating limits such as deformability, etc.); operating conditions (environment, presence of impact loads, etc.); - - technological requirements (type of production, availability of some technologies such as composite materials, manufacturing, etc.). -
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- - p r o b a b l e limiting conditions for the whole mechanism and for single member dimensioning (i.e. stiffness or strength). The specifications for the three types of subproblems are then defined, either in numerical (e.g. yield strength >400MPa) or in symbolic (e.g. good resistance to marine corrosion) form. If subproblems cannot be solved or if solutions lead to an unacceptable design, the GDM will try to modify subproblems specifications to overcome these difficulties. All of the above choices are based on the rules contained in the knowledge base and on the current problem data. These rules may be roughly classified as: general rules, containing general machine design knowledge (e.g. if the maximum permissible dimension normal to the plane of the mechanism is small as compared with its overall size, then rolling contact bearing should be preferred); - - domain specific rules, codifying criteria commonly applied in the design of this type of machines (e.g. if one of the end-connections of a member is male (female), then it is preferable the other to be male (female) also); design refinement rules, applied to keep design more adherent to actual technological shapes and to solve some types of conflict; - - control rules, employed to drive design modifications at each iteration (e.g. if it was not possible to satisfy dimension limits for some member then try to select an higher strength material). -
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The problem of conflict resolution and of design refinement, which is of the greatest relevance for improving the quality of design, will be dealt with in greater detail in one of the following sections.
6 MATERIAL SELECTION MODULE
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Once the problem has been completely defined, the GDM performs some 'high level' design choices, among which the following may be cited: - - general characteristics of each link (type of end connection (male or female), single or double parallel bars, etc.); - - possible exclusion of one or more material classes, for technological or cost limitations;
The aim of this module is the choice of the proper material either for single members or for the whole articulated system. It makes use of a small material database (MDB), containing about 50 different materials, with their most important mechanical, physical and chemical properties. The MDB, likewise the other databases employed in the KBES, was codified by simply introducing in the knowledge base a PROLOG assertion (fact) for each material, defining its name and mechanical properties. Although this technique proved adequate for the scopes of the present research, interfacing with one or more external modern (e.g. relational) databases will surely be required in order to develop an actual design tool. Material selection is performed in two steps (Fig. 5), which make use of different search methods. Firstly, the following requirements:
A prototype expert system for embodiment design of mechanisms and articulated systems
The choice of the better section shape (T, hollow circular, rectangular, square, etc.), once given the internal force factors (i.e. normal force, shear and bending moment), must account for the relative importance of all the components, together with some geometrical characteristic of the beam (e.g. its length, affecting instability behaviour). In EMBMEC, a mixed (rule-based and algorithmic) approach is currently implemented. A few heuristic rules are employed to perform some general choice (e.g. the better height to width ratio for 'I' or rectangular shapes). A purposely designed algorithm (Fig. 6), based on practical experience and allowing a quantitative comparison of different section shapes, is then applied. This algorithm includes the following steps:
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calculations of the area required for each section shape and for each force factor acting separately; evaluation of the maximum area among those calculated for a single section shape under different force factors; - - s e l e c t i o n of the shape for which the above maximum area (or the maximum multiplied by a shape-dependent cost factor) assumes the minimum value.
are employed as selection criteria, with 'AND' Boolean logic, in order to single out a subset of acceptable materials. Materials contained in the subset are then sorted according to: general design criteria (e.g. relevance of weight and cost); - - structural criteria (e.g. relevance of strength or stiffness) -
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7 CONNECTING ROD GEOMETRY DESIGN MODULE The aim of this module is that of defining the geometry of the connecting rod of each member, which implies: selection of a section shape from the available library; definition of section dimensions. -
-
-
-
-
-
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Although not completely rigorous, this approach proved to be very effective in section shape comparison, particularly under mixed mode loading. Once the most suitable section shape has been selected, it can be dimensioned in order to satisfy strength or stiffness criteria. The module can access a small commercial rod database, in order to select standardized profile whenever possible.
and the most favourable one is selected for application.
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8 HINGED JOINT DESIGN MODULE The task of this module is to select, for each hinge of the
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Fig. 6. Connecting rod geometry selection procedure.
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Leonardo Bertini
mechanism, the beating type (e.g. rolling or sliding contact) and dimensions. Moreover, the module defines the geometry of the pin and of the end connections for the links attached to the joint. The selection of the proper bearing type and the correct proportioning of the other parts of the hinged joint were obtained by means of typical rules derived from machine design practice. These rules take account of several factors, such as the requirement of minimizing axial or radial joint dimensions or the presence of impact loads. Moreover, simple closed form relationships, derived from elementary machine design theory, were employed to evaluate the strength of the components. The HJDM can access a rolling contact bearing database and a simple end connections shape library.
9 DESIGN MODIFICATION AND CONFLICT RESOLUTION Two main types of difficulty may be encountered, within the present approach, during the solution of a specific design task: -
-
-
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an area module may fail to obtain a solution, with given subproblem specifications; a conflict arises between the solutions given by two area modules.
It is not possible, in the present paper, to analyse in detail the rather large gamut of different situations which can cause the onset of both types of difficulties. However, some relevant examples are discussed in the following, in order to give information about the techniques these problems are dealt with in the EMBMEC program. In any case, it is worthy of note that EMBMEC present capabilities in dealing with solution difficulties only allow it to operate in a limited set of design problems, as required for research aims. These capabilities should probably be significantly increased to allow the program to operate satisfactorily as an actual design tool. If the first type of difficuhy occurs, a specific error flag is returned to the GDM, which tries to overcome the difficulty itself by proper modifications of subproblem statement. For instance, the RGDM may be unable to define a section shape fulfilling both dimensional limits and strength criteria, based on current material selection by MSM. In this case, material minimum strength requirement is increased by GDM, taking account of the results obtained by the RGDM, and a new material selection is required for the MSM. As regards the second type of difficulty, conflicts between the area module solutions may pertain to two categories:
-
-
-
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conflicts between parts of the same link; conflicts between distinct links.
Two examples of the first type of conflict and of possible solutions are reported in Fig. 7. In case (a), a dimensional compatibility problem between the end connection and the rod body is shown. This problem can be solved by the GDM either lowering the maximum allowed in plane height of the rod (so forcing the RGDM to select a different section shape, usually less favourable as regards costs and weight) or requiring the HJDM to select an end connection of given (larger) diameter. The selection of the most appropriate strategy is performed by the GDM through a set of design rules, taking account of its feasibility, as compared to design limits such as allowed lateral dimension, and of the relative importance of the rod and of the joint for the total weight/cost of the link. In case (b), interference between adjacent hinges can be solved by reducing the hinge radius. This can be obtained either by changing the bearing type (for instance, rolling contact bearings can be replaced with sliding contact bearings) or by moving one of the bearings to the other link of the hinged joint. The two solution strategies are attempted in the given order by the GDM. As regards conflicts between adjacent links, these are usually related with interferences occurring during mechanism operation, which can be solved by the GDM by splitting one of the links into two parallel bars (Fig. 8).
10 DESIGN REFINEMENT Design refinement appeared as a key stage in order to obtain, for the mechanism components, a shape very close to (or even coincident with) that achievable by an
a)
u SLIDING CONTACT BEARINGS
b) ROLLING CONTACT BEARINGS
BEARING MOVED TO ADJACENT LINK
Fig. 7. Conflict between distinct parts of the same link.
A prototype expert system for embodiment design of mechanisms and articulated systems
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from the user were as follows:
Fig. 8. Conflict between distinct links.
experienced designer. AI techniques appear as a very attractive tool for dealing with this type of problem, whose solution usually requires a large amount of heuristic knowledge. To date, EMBMEC embodies a set of rules for design refinement, which try to account for technological and manufacturing economy requirements. These rules were usually defined in order to solve some of the problems encountered when running the system and actually constitute only a first step towards an efficient design improvement. As a consequence, new rules surely need to be added to the knowledge base in the future. Two application examples for the design refinement rules are reported in Fig. 9. In case (a), a slightly tapering connecting rod pertaining to a single or small lot production (mainly based on milling or turning operations) was converted to constant section in order to avoid unuseful costly machining. In case (b), the short length of the connecting bar led one to prefer a flat link shape.
11 APPLICATION EXAMPLE A simple example of an EMBMEC work session will be discussed herein, in order to provide a demonstration of its capabilities. The KBES was required to design a three bar linkage, whose scheme is reported in Fig. 10. Input data required
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Fig. 9. Examples of design improvements.
EMBMEC: Enter the name of the file containing mechanism geometry and load analysis: User: M E C H I D A T Max. allowed dimension orthogonal to the mechanism plane (ram)? 70 Is the possibility to inspect easily the bearings required? Yes Is it required to minimize in-plane dimensions? Yes Are impact loads anticipated? Yes Is it important to minimize servicing work? No Operating environment contains dust? No Operating environment for the mechanism: 1) Dry air 2) Moist air 3) Marine atmosphere 4) Tap water 5) Sea water Please, select an option: 1
Is it allowed to employ composite materials? No Should the mechanism be constructed as: 1) single or small-lot production 2) large-lot or mass production Please, select an option: 2 Is it required to minimize: 1) weight 2) cost Please, select an option: 1
Name of the ftle containing maximum allowed hinged joints displacements: MECHIDIS The results obtained are shown in Fig. 11. First of all it is worth noting that relatively large in-plane joint displacements (1 mm in all directions) were allowed. As
I Link Length [
(ram)
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15° < a < 85 ° Fig. 10. Scheme for EMBMEC application.
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Leonardo Bertini lellt section shape
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Circular section shape
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Material: A1 alloy ~
Fig. !1. Results of the EMBMEC application, represented as an ANSYS solid mesh (one half of the mechanism, which was cut along its middle plane, is depicted). a consequence, the mechanism analysis was dominated by strength criteria. EMBMEC selected T shaped sections for links 1 and 3, where bending stresses are important, and a circular section for link 2, which is always subjected to pure tension. This link was subdivided into two parallel bars to avoid interference with links 1 and 2 during motion. Sliding contact bearings were employed in the hinged joints, in order to reduce in-plane dimensions. Moreover, the bearings were placed in the external (female), rather than in the internal part of the joints to facilitate inspection and servicing. The material chosen by the program was an aluminium alloy, which allowed reduction of the overall weight of the linkage, as required in the problem general specifications. In addition to the previous discussion, it can be interesting to analyse the answer of EMBMEC to some modifications in input data. For instance, changing the sign of the applied force Fy resulted in changing to an T shape for link 2 also, in order to prevent the onset of axial instability problems. The requirement of minimum cost, rather than minimum weight, design made the KBES select a different material (i.e. a C steel, AISI 1020). Finally, a reduction in the allowed dimension normal to the mechanism plane (from 70 to 30 mm) led the system to use rolling contact, rather than sliding contact, bearings. As a concluding remark, it can be noted that the design of the mechanism is clearly far from being complete (e.g. geometrical details, such as fillets, have not been included yet) and actually constitutes only a first step towards the automation of the embodiment phase. However, results appeared rather encouraging as regards the use of KBES approach to embodiment design.
12 CONCLUSIONS The main features of a prototype KBES for mechanical
design were reported. The KBES operates in the field of embodiment design of mechanisms and it's capabilities are presently limited to 2D linkages with rotational joints. The model of the design process which was implemented in the KBES is characterized by a 'topdown' structure with iterative modifications. The main problem is subdivided into three main subproblems (i.e. link geometry design, hinge design and material selection), which are separately solved. Solutions are then analysed and subproblem specifications modified until a satisfactory design is achieved. As a final step, specific design refinement rules are applied in order to keep the component geometry more adherent to technological and economical requirements. The KBES includes both 'heuristic' and 'analytical' knowledge and makes use of simple shape libraries in order to define the geometry of the links. Preliminary applications gave encoura~ng results, apparently indicating that AI techniques can be successfully applied to the embodiment design stage. Future developments must surely include an enlargement of the databases, an extension of component shape libraries and the link with numerical shape optimization techniques.
REFERENCES I. Valliere, D. & Lee, J., Expert systems that really work. Computers in Mech. Eng., 1988 (July-Aug.), 14-16. 2. Sell, P.S., Expert Systems - - A Practical Introduction. Macmillan Computer Science Series, London, 1984, chpts I-3, pp. 1-20. 3. Ellsworth, R,, Parkinson, A. & Cain, F., The complementary roles of knowledge based systems and numerical optimization in engineering design software. ASME J. Mech. Trans. and Automation in Design, 1989. 111, 100-3. 4. Allen, R.H. & Bosen, A., ACOLADE: A hybrid knowledge based system for preliminary composite laminate design. In Proc. ASME Int. Computers in Eng. Conf. and Exhib., New York, Eds. R. Raghaven & T.J. Cokonis. ASME, New York, 1987, Vol. I, pp. 51-8. 5. Gitchel, K.R., Computerized experts refine gear design. Machine Design, 1987, (Feb.), 87-9.
A prototype expert system for embodiment design of mechanisms and articulated systems 6. Forde, B.W.R. & Stiemer, S.F., Knowledge based control for finite element analysis. Eng. with Computers, 1989, 5, 195-204. 7. Ramirez, M.R. & Belytschko, T., An expert system for setting time steps in dynamic finite element programs. Eng. with Computers, 1989, 5, 205-19. 8. Weiss, V. & Greene, K.J., Expert Systems for Material Selection. Syracuse University Rep., Syracuse, NY, 1988. 9. Dixon, J.R., Howe, A., Cohen, P.R., Simmons, M.K. & Dominic, I. Progress towards domain independence in design by iterative redesign. In Proc. ASME Int. Computers in Eng. Conf. and Exhib., Chicago, Ed. G. Gupta. ASME, New York, 1986, Vol I, pp. 199-206.
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10. Chieng, W. & Hoeltzel, D.A., Interactive hybrid (symbolic-numeric) system approach to near optimal design of mechanical components. Eng. with Computers, 1987, 2, I I 1-23.
11. Ramachandran, N., Shah, A. & Langrana, N.A., Expert systems approach in design of mechanical components. Eng. with Computers, 1988, 4, 185-95. 12. Thompson, T.R., Riley, D.R. & Erdman, A.G., An Expert Systems Approach to Type Synthesis of Mechanisms. In Proc. ASME Int. Computers in Eng, Cot~ and Exhib., Boston. ASME, New York, 1985, Vol. 1, pp. 71-5. 13. Ansys PC-Linear 4.3 Reference Manual, Swanson Analysis System, Houston, TX, 1988.
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A prototype expert system for embodiment design of mechanisms and articulated systems Leonardo Bertini Dipartimento di Construzioni Meccaniche e Nucleari, University of Pisa, Via Diotisalvi 2, 56126 PISA, Italy
(Received 17 May 1991; revised version received 17 November 1992; accepted 19 November 1992) The embodiment phase plays a significant role in mechanical design, particularly in the case of mechanisms and articulated systems. In order to investigate the capabilities offered by artificial intelligence techniques for the automation of this design stage, a prototype expert system (ES), called EMBMEC, was developed. The ES possesses a 'hybrid' structure, making use of both symbolic inference and numerical calculation techniques. At present, it can operate with plane mechanisms, selecting materials, link geometry and bearings. In the paper, the most relevant features of the system are discussed and a simple application example is reported. Key words: expert systems, mechanical design, embodiment design, mechanisms
required to make an integrated use of two distinct types of knowledge:
1 INTRODUCTION The application of knowledge based expert systems (KBESs) in the area of design (particularly mechanical design) are significantly less frequent than in other engineering fields (e.g. fault diagnosis, process planning, monitoring and real time control), which can be regarded as natural extensions of the earlier applications to the medical area. ~'2 KBESs for mechanical design were generally developed to operate in single highly specialized fields, such as valve, 3 composite laminates 4 or gear 5 design, finite element analysis 6'7 and material selection, s In addition to these single-domain applications, usually operating more as area consultants than as designers, there were attempts 9-~1 to develop KBESs based on general purpose design models. These systems, although still at the research stage, could, at least in principle, be applied to a rather large variety of engineering domains. Most of them try to define frameworks to solve the whole design problem, from the conceptual stage to detailing, so emulating the capabilities of actual human designers (or design staff). The experience gained with the development of such KBESs led to the recognition that, during the resolution of a mechanical design problem in contrast to classical artificial intelligence (AI) applications, it is usually
- - ' S y m b o l i c ' knowledge, i.e. facts, relationships among them and rules represented in symbolic form; this type of knowledge usually also includes empirical rules, expertise and practical criteria applied by the experienced designer in a given application domain (heuristic knowledge). - - ' A n a l y t i c a l ' knowledge, i.e. the knowledge on how to solve problems, which are stated as mathematical models, by means of 'algorithmic' techniques. KBESs employing both types of knowledge (i.e. making use of both symbolic inference and numerical calculation techniques) in order to solve problems are usually called 'hybrid', and actually constitute the great majority of modern AI applications to mechanical engineering design. The present paper illustrates a prototype 'hybrid' expert system (ES), which was developed for a relatively new application field, i.e. the 'embodiment' design of articulated systems. The general features, data structure and design model are discussed, emphasizing the main difficulties encountered during the implementation. 2 P R O B L E M STATEMENT The design process of mechanisms or articulated systems involves two main distinct phases:
Artificial Intelligence in Engineering 0954-1810/93/$06.00 © 1993 Elsevier Science Publishers Ltd.
57
58
Leonardo Bertini
'synthesis': in this phase, given the required space and time motion law, the overall geometry of the mechanism, inclusive of the number and type of degrees of freedom and links, is defined; --'embodiment': in this phase, all the actual components of the articulated system are defined, as regards both geometry and materials. -
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Although several applications of AI techniques to mechanism synthesis have been developed in recent years, some of which gave appreciable results, 12 nearly no attempt was made, with the author's knowledge, to apply such techniques to the 'embodiment' phase. A research program was then set up at the University of Pisa, in order to analyse the capabilities offered by AI tools in this second design stage. As a result of this research work, a prototype KBES for the automatic embodiment of plane mechanism was developed, which was called EMBMEC ('embodiment of mechanisms'). Its capabilities are presently limited to the treatment of rather simple problems, i.e. articulated systems composed of an arbitrary number of links, each carrying two or three aligned hinged joints. Nevertheless, the results obtained can give useful indications for the development of KBES aimed at the automation of this design phase, both for mechanisms and for other types of mechanical systems. 3 KNOWLEDGE ACQUISITION AND SOFTWARE TOOLS The 'analytical' knowledge used by the ES was easily derived from machine design or stress analysis manuals, or directly included through the use of external analysis programs. As regards 'symbolic' knowledge, it can be classified as:
making use of the graphics capabilities of the ANSYS Finite Element program. ~3 To this end, FORTRAN code was developed which, based on the results of the EMBMEC system (which are made available as ASCII files), generates a command input file for the ANSYS code, so allowing the automatic creation of a solid model of the mechanism. Besides being useful for presentation purposes, the interface with the ANSYS program is actually thought to be the first step toward an automated finite element analysis of the links, which could be employed, in the future, to extend the capabilities of the KBES.
4 STRUCTURE OF THE KBES AND MODEL OF THE DESIGN PROCESS The EMBMEC system, whose general structure is shown in Fig. i, embodies a typical 'top-down' design process model with iterative modifications. It includes a supervisor module, called General Design Manager (GDM), and three area modules for the solution of specific subproblems, i.e. material selection module (MSM), rod geometry design module (RGDM) and hinged joint design module (HJDM). As a first step, general specifications and requirements for the current design problem are processed by the GDM, which defines the specifications for subproblems. These are solved, in the sequence given in Fig. 1, by the area modules, which embodies both heuristic and analytical specific knowledge. Subproblem solution is based both on the specifications given by the GDM and on the results previously obtained by other area modules (e.g. mechanical properties of the material selected by the MSM are employed by the RGDM to define rod dimension).
general (or domain-independent) knowledge, which includes commonly available facts and rules related to mechanical design and was mainly derived from machine design manuals; specific (domain-dependent) knowledge, which includes heuristic rules and facts typical of the particular field considered and was derived partly from experienced designers and partly by a critical examination of the system responses, as compared with known correct solutions. The EMBMEC system was developed, on an IBM personal computer, making use of the PROLOG-2 language. This allowed one to codify facts and rules easily, according to the usual PROLOG syntax, and also to perform simple calculations (e.g. the evaluation of the stress in a rod under bending loading) required by the embodiment phase. Once the design has been completed, defining the geometry of each component, a three-dimensional representation of the mechanism can be achieved
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A prototype expert system for embodiment design of mechanisms and articulated systems
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Fig. 2. Example of data structure. At the end of a single design iteration, the current solution is examined by the G D M and, if it is found to be unsatisfactory (or if some of the area modules failed to find a solution), subproblem specifications are modified and a new iteration performed. The process continues until the current design is accepted or cannot be further improved. In order to accept a solution, the fulfilment of the following conditions is required:
--explicit design requirements and limits (e.g. maximum lateral dimension or total weight not larger than given values) are satisfied; no incompatibility exists between distinct links or between distinct part of the same link as regards mechanism operation or manufacturability; - - the current solution does not conflict with implicit design requirements deriving from mechanism specifications and common design rules (e.g. if the mechanism will operate in an aggressive environment, no material having poor resistance to that environment is allowed).
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Information on the articulated system was structured as linked tree-structures (see Fig. 2), which, according to the PROLOG syntax, were coded as 'lists' in the EMBMEC program. Each link of the mechanism was represented as an object composed of several distinct parts (e.g. rods and end connections), whose geometry is defined by giving their general shape and a set of characteristic dimensions. Shapes are selected among the content of specific 'shape libraries', an example of which is reported in Fig. 3. These libraries were designed to allow an easy expansion of their content, in order to deal with a wider set of design problems.
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The fulfilment of another type of design specification, defining the required target (or optimality criteria) for the current problem (e.g. the requirement to minimize weight or cost), is achieved by a solution strategies ranking method. According to this approach, the KBES tries to apply first the technical solutions which are known to give better performances from a given point of view (this was mainly obtained by properly ordering rules and facts in the EMBMEC knowledge base). It is worth noting that this technique only allows one to achieve an approximate optimal design, similar to that obtained by an experienced human designer. Further improvements could be achieved, in many cases, by coupling the present method with numerical optimization techniques in order to refine the solution.
5 DESIGN SUPERVISOR M O D U L E The G D M module, whose flow-chart is depicted in Fig. 4, acts as a supervisor of the entire design process and manages the exchange of information with the external environment.
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60
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Fig. 4. Structure of the General Design Manager. It analyses the files containing the results of the mechanism synthesis (i.e. geometry and forces acting in each link as a function of mechanism spatial position) in order to evaluate some relevant parameters, such as the design loads for each rod and bearing, the relative rotation angle of adjacent links, etc. A few general design specifications are interactively required of the user. They include: general design criteria and requirements, according to the anticipated application field (whether it is required to minimize weight or cost, operating limits such as deformability, etc.); operating conditions (environment, presence of impact loads, etc.); - - technological requirements (type of production, availability of some technologies such as composite materials, manufacturing, etc.). -
-
- - p r o b a b l e limiting conditions for the whole mechanism and for single member dimensioning (i.e. stiffness or strength). The specifications for the three types of subproblems are then defined, either in numerical (e.g. yield strength >400MPa) or in symbolic (e.g. good resistance to marine corrosion) form. If subproblems cannot be solved or if solutions lead to an unacceptable design, the GDM will try to modify subproblems specifications to overcome these difficulties. All of the above choices are based on the rules contained in the knowledge base and on the current problem data. These rules may be roughly classified as: general rules, containing general machine design knowledge (e.g. if the maximum permissible dimension normal to the plane of the mechanism is small as compared with its overall size, then rolling contact bearing should be preferred); - - domain specific rules, codifying criteria commonly applied in the design of this type of machines (e.g. if one of the end-connections of a member is male (female), then it is preferable the other to be male (female) also); design refinement rules, applied to keep design more adherent to actual technological shapes and to solve some types of conflict; - - control rules, employed to drive design modifications at each iteration (e.g. if it was not possible to satisfy dimension limits for some member then try to select an higher strength material). -
-
-
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The problem of conflict resolution and of design refinement, which is of the greatest relevance for improving the quality of design, will be dealt with in greater detail in one of the following sections.
6 MATERIAL SELECTION MODULE
-
-
Once the problem has been completely defined, the GDM performs some 'high level' design choices, among which the following may be cited: - - general characteristics of each link (type of end connection (male or female), single or double parallel bars, etc.); - - possible exclusion of one or more material classes, for technological or cost limitations;
The aim of this module is the choice of the proper material either for single members or for the whole articulated system. It makes use of a small material database (MDB), containing about 50 different materials, with their most important mechanical, physical and chemical properties. The MDB, likewise the other databases employed in the KBES, was codified by simply introducing in the knowledge base a PROLOG assertion (fact) for each material, defining its name and mechanical properties. Although this technique proved adequate for the scopes of the present research, interfacing with one or more external modern (e.g. relational) databases will surely be required in order to develop an actual design tool. Material selection is performed in two steps (Fig. 5), which make use of different search methods. Firstly, the following requirements:
A prototype expert system for embodiment design of mechanisms and articulated systems
The choice of the better section shape (T, hollow circular, rectangular, square, etc.), once given the internal force factors (i.e. normal force, shear and bending moment), must account for the relative importance of all the components, together with some geometrical characteristic of the beam (e.g. its length, affecting instability behaviour). In EMBMEC, a mixed (rule-based and algorithmic) approach is currently implemented. A few heuristic rules are employed to perform some general choice (e.g. the better height to width ratio for 'I' or rectangular shapes). A purposely designed algorithm (Fig. 6), based on practical experience and allowing a quantitative comparison of different section shapes, is then applied. This algorithm includes the following steps:
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calculations of the area required for each section shape and for each force factor acting separately; evaluation of the maximum area among those calculated for a single section shape under different force factors; - - s e l e c t i o n of the shape for which the above maximum area (or the maximum multiplied by a shape-dependent cost factor) assumes the minimum value.
are employed as selection criteria, with 'AND' Boolean logic, in order to single out a subset of acceptable materials. Materials contained in the subset are then sorted according to: general design criteria (e.g. relevance of weight and cost); - - structural criteria (e.g. relevance of strength or stiffness) -
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7 CONNECTING ROD GEOMETRY DESIGN MODULE The aim of this module is that of defining the geometry of the connecting rod of each member, which implies: selection of a section shape from the available library; definition of section dimensions. -
-
-
-
-
-
-
Although not completely rigorous, this approach proved to be very effective in section shape comparison, particularly under mixed mode loading. Once the most suitable section shape has been selected, it can be dimensioned in order to satisfy strength or stiffness criteria. The module can access a small commercial rod database, in order to select standardized profile whenever possible.
and the most favourable one is selected for application.
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8 HINGED JOINT DESIGN MODULE The task of this module is to select, for each hinge of the
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62
Leonardo Bertini
mechanism, the beating type (e.g. rolling or sliding contact) and dimensions. Moreover, the module defines the geometry of the pin and of the end connections for the links attached to the joint. The selection of the proper bearing type and the correct proportioning of the other parts of the hinged joint were obtained by means of typical rules derived from machine design practice. These rules take account of several factors, such as the requirement of minimizing axial or radial joint dimensions or the presence of impact loads. Moreover, simple closed form relationships, derived from elementary machine design theory, were employed to evaluate the strength of the components. The HJDM can access a rolling contact bearing database and a simple end connections shape library.
9 DESIGN MODIFICATION AND CONFLICT RESOLUTION Two main types of difficulty may be encountered, within the present approach, during the solution of a specific design task: -
-
-
-
an area module may fail to obtain a solution, with given subproblem specifications; a conflict arises between the solutions given by two area modules.
It is not possible, in the present paper, to analyse in detail the rather large gamut of different situations which can cause the onset of both types of difficulties. However, some relevant examples are discussed in the following, in order to give information about the techniques these problems are dealt with in the EMBMEC program. In any case, it is worthy of note that EMBMEC present capabilities in dealing with solution difficulties only allow it to operate in a limited set of design problems, as required for research aims. These capabilities should probably be significantly increased to allow the program to operate satisfactorily as an actual design tool. If the first type of difficuhy occurs, a specific error flag is returned to the GDM, which tries to overcome the difficulty itself by proper modifications of subproblem statement. For instance, the RGDM may be unable to define a section shape fulfilling both dimensional limits and strength criteria, based on current material selection by MSM. In this case, material minimum strength requirement is increased by GDM, taking account of the results obtained by the RGDM, and a new material selection is required for the MSM. As regards the second type of difficulty, conflicts between the area module solutions may pertain to two categories:
-
-
-
-
conflicts between parts of the same link; conflicts between distinct links.
Two examples of the first type of conflict and of possible solutions are reported in Fig. 7. In case (a), a dimensional compatibility problem between the end connection and the rod body is shown. This problem can be solved by the GDM either lowering the maximum allowed in plane height of the rod (so forcing the RGDM to select a different section shape, usually less favourable as regards costs and weight) or requiring the HJDM to select an end connection of given (larger) diameter. The selection of the most appropriate strategy is performed by the GDM through a set of design rules, taking account of its feasibility, as compared to design limits such as allowed lateral dimension, and of the relative importance of the rod and of the joint for the total weight/cost of the link. In case (b), interference between adjacent hinges can be solved by reducing the hinge radius. This can be obtained either by changing the bearing type (for instance, rolling contact bearings can be replaced with sliding contact bearings) or by moving one of the bearings to the other link of the hinged joint. The two solution strategies are attempted in the given order by the GDM. As regards conflicts between adjacent links, these are usually related with interferences occurring during mechanism operation, which can be solved by the GDM by splitting one of the links into two parallel bars (Fig. 8).
10 DESIGN REFINEMENT Design refinement appeared as a key stage in order to obtain, for the mechanism components, a shape very close to (or even coincident with) that achievable by an
a)
u SLIDING CONTACT BEARINGS
b) ROLLING CONTACT BEARINGS
BEARING MOVED TO ADJACENT LINK
Fig. 7. Conflict between distinct parts of the same link.
A prototype expert system for embodiment design of mechanisms and articulated systems
63
from the user were as follows:
Fig. 8. Conflict between distinct links.
experienced designer. AI techniques appear as a very attractive tool for dealing with this type of problem, whose solution usually requires a large amount of heuristic knowledge. To date, EMBMEC embodies a set of rules for design refinement, which try to account for technological and manufacturing economy requirements. These rules were usually defined in order to solve some of the problems encountered when running the system and actually constitute only a first step towards an efficient design improvement. As a consequence, new rules surely need to be added to the knowledge base in the future. Two application examples for the design refinement rules are reported in Fig. 9. In case (a), a slightly tapering connecting rod pertaining to a single or small lot production (mainly based on milling or turning operations) was converted to constant section in order to avoid unuseful costly machining. In case (b), the short length of the connecting bar led one to prefer a flat link shape.
11 APPLICATION EXAMPLE A simple example of an EMBMEC work session will be discussed herein, in order to provide a demonstration of its capabilities. The KBES was required to design a three bar linkage, whose scheme is reported in Fig. 10. Input data required
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Fig. 9. Examples of design improvements.
EMBMEC: Enter the name of the file containing mechanism geometry and load analysis: User: M E C H I D A T Max. allowed dimension orthogonal to the mechanism plane (ram)? 70 Is the possibility to inspect easily the bearings required? Yes Is it required to minimize in-plane dimensions? Yes Are impact loads anticipated? Yes Is it important to minimize servicing work? No Operating environment contains dust? No Operating environment for the mechanism: 1) Dry air 2) Moist air 3) Marine atmosphere 4) Tap water 5) Sea water Please, select an option: 1
Is it allowed to employ composite materials? No Should the mechanism be constructed as: 1) single or small-lot production 2) large-lot or mass production Please, select an option: 2 Is it required to minimize: 1) weight 2) cost Please, select an option: 1
Name of the ftle containing maximum allowed hinged joints displacements: MECHIDIS The results obtained are shown in Fig. 11. First of all it is worth noting that relatively large in-plane joint displacements (1 mm in all directions) were allowed. As
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64
Leonardo Bertini lellt section shape
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Fig. !1. Results of the EMBMEC application, represented as an ANSYS solid mesh (one half of the mechanism, which was cut along its middle plane, is depicted). a consequence, the mechanism analysis was dominated by strength criteria. EMBMEC selected T shaped sections for links 1 and 3, where bending stresses are important, and a circular section for link 2, which is always subjected to pure tension. This link was subdivided into two parallel bars to avoid interference with links 1 and 2 during motion. Sliding contact bearings were employed in the hinged joints, in order to reduce in-plane dimensions. Moreover, the bearings were placed in the external (female), rather than in the internal part of the joints to facilitate inspection and servicing. The material chosen by the program was an aluminium alloy, which allowed reduction of the overall weight of the linkage, as required in the problem general specifications. In addition to the previous discussion, it can be interesting to analyse the answer of EMBMEC to some modifications in input data. For instance, changing the sign of the applied force Fy resulted in changing to an T shape for link 2 also, in order to prevent the onset of axial instability problems. The requirement of minimum cost, rather than minimum weight, design made the KBES select a different material (i.e. a C steel, AISI 1020). Finally, a reduction in the allowed dimension normal to the mechanism plane (from 70 to 30 mm) led the system to use rolling contact, rather than sliding contact, bearings. As a concluding remark, it can be noted that the design of the mechanism is clearly far from being complete (e.g. geometrical details, such as fillets, have not been included yet) and actually constitutes only a first step towards the automation of the embodiment phase. However, results appeared rather encouraging as regards the use of KBES approach to embodiment design.
12 CONCLUSIONS The main features of a prototype KBES for mechanical
design were reported. The KBES operates in the field of embodiment design of mechanisms and it's capabilities are presently limited to 2D linkages with rotational joints. The model of the design process which was implemented in the KBES is characterized by a 'topdown' structure with iterative modifications. The main problem is subdivided into three main subproblems (i.e. link geometry design, hinge design and material selection), which are separately solved. Solutions are then analysed and subproblem specifications modified until a satisfactory design is achieved. As a final step, specific design refinement rules are applied in order to keep the component geometry more adherent to technological and economical requirements. The KBES includes both 'heuristic' and 'analytical' knowledge and makes use of simple shape libraries in order to define the geometry of the links. Preliminary applications gave encoura~ng results, apparently indicating that AI techniques can be successfully applied to the embodiment design stage. Future developments must surely include an enlargement of the databases, an extension of component shape libraries and the link with numerical shape optimization techniques.
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10. Chieng, W. & Hoeltzel, D.A., Interactive hybrid (symbolic-numeric) system approach to near optimal design of mechanical components. Eng. with Computers, 1987, 2, I I 1-23.
11. Ramachandran, N., Shah, A. & Langrana, N.A., Expert systems approach in design of mechanical components. Eng. with Computers, 1988, 4, 185-95. 12. Thompson, T.R., Riley, D.R. & Erdman, A.G., An Expert Systems Approach to Type Synthesis of Mechanisms. In Proc. ASME Int. Computers in Eng, Cot~ and Exhib., Boston. ASME, New York, 1985, Vol. 1, pp. 71-5. 13. Ansys PC-Linear 4.3 Reference Manual, Swanson Analysis System, Houston, TX, 1988.