Autofix an expert CAD system for jigs and fixtures.pdf

int j mach tools manufa t vol 30 no 3 pp 403 411 1990 0890 6955 9053 00 00 printed in great britain pergamon press p c autofix an expert cad system for jigs and fixtures d t pham and a de sam lazarot received 4 august 1989 in final form 31 october 1989 abstract this paper describes autofix a fully automated cad package for configuring complex fixtures from a database of modular elements the program is also capable of designing special elements if standard ones do not exist in the database a feature of autofix is the use of finite element analysis to compute the deflection of the fixtured workpiece and determine the optimum position of supports examples of fixtures designed by autofix are presented introduction jigs and fixtures or fixturing systems are devices used in manufacturing processes such as machining welding bonding and assembly they locate and hold the workpiece in position and ensure that it is in a state of stable equilibrium and that dimensional accuracy is maintained during the manufacturing operation fixturing systems have traditionally been designed by experienced designers using empirical rules and formulae the decline in the number of experienced fixture design ers the long lead time for manual design progress in the field of computer aided design cad and the availability of modular tooling and knowledge based expert systems are factors which have motivated researchers to develop automated fixture design programs several systems with various degrees of automation have been developed in the last decade for a review of these systems see for instance 1 or 21 this paper describes autofix a fully automated jig and fixture design system a special feature of autofix that distinguishes it from the systems surveyed in 1 2 is the integration of knowledge based reasoning with a whole range of traditional cad techniques including solid modelling parametric design and finite element analysis this has been achieved through using an artificial intelligence programming language ops5 in conjunction with a modern suite of cad packages i deas from structural dynamics research corporation sdrc u s a in the following sections the main characteristics of ops5 and i deas will be highlighted first details of autofix will then be examined and the procedure to be followed when designing a fixture in autofix will be outlined finally examples of fixtures produced by autofix will be presented salient features of ops5 3 the rule based language ops5 uses data elements which are either vectors or objects with associated attribute value pairs for example to represent the assertion that location of a workpiece in a fixture should be undertaken prior to clamping ops5 would use the vector task order geometric location clamping intelligent systems research group school of electrical electronic and systems engineering university of wales cardiff u k tdepartment of mechanical and materials engineering washington state university pullman washington u s a 403 404 d t pham and a de sam lazaro similarly attributes are represented as statements with the distinguishing operator for example material steel 1018 hardened yes would represent hardened steel with ansi no 1018 matching of one or more attributes alone may be required for identification of an object rules have the format ai ai2 a k ci ci2 ci where aik is an antecedent of the rule and cij is a consequent which would turn true should all antecedents be matched with some knowledge from the knowledge base a consequent can also be matched to other antecedents and thus trigger other rules or programs routines typical actions which could be ordered by a consequent are make a new element in the working memory remove modify an existing element or write a line of text the inference engine interpreter performs forward chaining applying the following strategies in case more than one rule is satisfied a rules with the most recently created working memory elements are triggered first recency strategy b rules which are more specific are triggered in preference to more general ones specificity strategy c once a rule has been triggered it is prevented from looping on itself refraction strategy one of the strengths of ops5 is its generality making it easy to tailor the design of a knowledge based system to fit the characteristics of its domain the principal weaknesses reside in the lack of a front end facility for expanding or modifying the database also most numerical computations have to be carried out by external scientific languages like fortran and pascal communication with other programs can be achieved either by conversion of symbolic atoms which are ops5 entities into appropriate types or by using buffer data files in autofix the latter option was adopted i deas cad package i deas is an interactive cad package which has solid modelling and engineering analysis facilities artifacts or objects can be generated either in 2d or 3d in wire frame or solid models for autofix the 3d solid model option was selected as it enabled the system to detect interference between components during assembly objects are described interactively and their attributes stored in keystroke files which are records of the operator s inputs from the keyboard these files are later played back to generate the object it is possible to run them in batch mode with all dimensions and attributes assigned to variables the values of which are governed by a control program figure 1 is an example of a keystroke file for a cylindrical primitive several such files may be run in tandem to produce a workpiece or a fixture both fixture elements and workpiece primitives are stored as shells in a database dimensions as decided by autofix are then supplied by a batch control file a finite element analysis program is also available with the cad package this is used in autofix to determine workpiece deflection as will be described later because i deas has been designed to be run interactively and is highly compartmenta lized communication is necessary to permit the process of design to advance automati cally from one stage to another a communication interface was written to enable the system to execute commands and transfer data between modules autofix 405 k k mf k pr k color 4 k n l k d1 20 k d2 0 k d3 10 k xcoord 10 k ycoord 0 5 k zcoord 5 k orientx 0 k orienty 0 k orientz 0 k r k cylinder k k c k sp k dl k 16 k color k or k tr k od20 k k sto k 550 k k c k o k od20 k 000 k dl k no k k b k j k k 550 k or k r k k indicates that the command on the line indicated is a keystroke the sign is to identify variables and their values run the file name cylinder this section is a generic description of a file to create a cylinder accessing values from those declared above k orientx orienty orientz k tr k xcoord ycoord zcoord k k ma k de k 550 k y fl 1 keystroke file for a cylindrical primitive details of autofix based on the program structure autofix can be divided into five sections namely the input stage the knowledge base satellite programs the output stage and the finite element analysis program a block diagram of the system is given in fig 2 input stage the input to the system is served by two distinct programs the part geometry is extracted from the cad system and converted into information which is readable by the knowledge based system this portion of the input stage also determines the overall dimensions of the workpiece the thickest and thinnest points and the availability of flat surfaces the second program ascertains from the operator information which cannot be extracted from the cad package data such as batch size workpiece finish machine configuration type s of operation and so on are facts known only to the operator this information is global in nature and would be required for any type of machining operation additional information would be called for as the knowledge based system solves specific design problems questions such as do you propose to carry out gang drilling or are multiple milling cutters being used for this operation are asked in order to define the particular problem more clearly very rudimentary knowledge is expected of the operator and the input stage is mainly menu driven 406 d t pham and a de sam lazaro ops5 program input processing phase knowledge base planning and design i cad decoding program t user interface program facts part dimension program meta rules clamping position program knowledge rules l location points for flanges assembly rules fixturing component l database search program output fixturing component stage 1 database search non symmetrical ports i i cad encoding cad presentation i program fir 2 block diagram of autofix s o t e l l i t e p r o g r a m s the knowledge base the most important section of the program is the knowledge base it consists of facts meta rules knowledge rules for design and assembly and organizational rules in addition there are rules introduced mainly for communication and file handling facts are either built into the system or are acquired from the operator during the input phase this information does not change and may be used anywhere in the program a built in fact for example is that fixtures can be classified into fixed shuttle swivel tilt indexing and universal this would appear in the program as fixture type fixed shuttle swivel tilt indexing universal where fixture would represent the class of object and type its attribute the value of which is one of the atoms from within the double angle brackets facts ascertained from the operator would generally be process and machine data and these would be bound to the attributes of certain objects they would normally have only one value meta rules are production rules governing the groups of rules which will be used under a given set of conditions in practice they generate working memory elements which trigger groups of rules required for a particular configuration of facts for example if the workpiece is prismatic the batch size is not small and in the planning stage it has been decided that a fixture is required all facts ascertained during the input and planning stages then the rules for location of prismatic shapes should be examined this would appear as the production rule shown in fig 3 the working memory element location apply 3 2 l rule will be used to trigger another group of rules from among the knowledge rules the bulk of the rules in the knowledge based system are knowledge rules two rules with comments are shown in fig 4 it will be noticed that both these rules require the presence of the element location apply 3 2 l rule in the working memory to trigger them depending on various other conditions such as type of material size and shape of workpiece type of chips being produced and so on the final configuration of autofix 407 p cube fixture workpiece shape rectangular production level value small fixture required make location apply 3 2 1 rule fl 3 a meta rule the following rule caters for rectangular workpleces which need the 3 2 1 rule p prismatic workpiecesl location apply 3 2 1 rule multlple 2art hol e workpiece dl chuck chips type discontinuous process type milling machine cutters single etc 7 make part partneme iocationpin dimension compute 0 i make part partname cube for cnc workholding dimension compute 0 85 remove the next rule is for determining the position of the three pins for planar location p prismatic workpieces2 location apply 3 2 1 rule workpiece dl 2 d3 machine cutter single etc 7 call dim program to determine largest side make part partname location pin dimension compute 0 085 x y z orientz remove note the variables x y and z are the coordinates of the centre of the locator and each locator will have one set of such coordinates fig 4 examples of knowlege rules the fixture is worked out these rules and those used to derive information necessary to arrive at a decision fixture configuration form the bulk of the knowledge rules for design another group of knowledge rules are used for assembly these generally concentrate on factors such as optimum clamping positions type and size of baseplate clamps and other structures and sequence of assembly of components if components need to be designed specifically for the workpiece and operation under consideration then this is achieved by a set of design rules the positioning of the components is implemented using the assembly rules organizational or control rules are necessary to guide the course of the program according to design requirements these rules ensure that all input parameters are entered first all input and output files are open at the appropriate times data is made available for design and assembly is started after the entire design process has been completed an illustration of how organizational rules start and partition the program is given in fig 5 satellite programs as mentioned earlier symbolic languages like ops5 are not designed to carry out numerical computation other than simple arithmetic in order to solve more complex 408 d t pham and a de snm lnzaro startup watch 2 sets the level of information reported back disable halt enables use of a command file openfile infilel opsln txt in opens input file make start1 makes the first working memory element which starts the first section of the program make input status active activates the input phase run there now follows a series of productions for input of information and computing the part geometry the next rule activates the design stage of the program p checkpointl input astatus terminated stopl make design status active make stopl fig 5 organizational rules mathematical problems it is necessary to use a high level language in this case fortran has been adopted the external programs developed with this system have the features of being autonomous and at the same time are able to communicate with the main program this gives them the status of satellite programs nine such satellites communicate with the main program via buffer data files this serves two useful purposes a it enables the user to examine the data that is fed into the system in order to ascertain validity of the information it is also possible to halt the progress of the design at any stage and to examine and perhaps alter certain parameters to suit a particular design b it permits alterations to be made to the satellite programs without changes to the main program this structure makes parallel processing possible thereby potentially reducing the time for design at present the system runs on a single processor but with simple modifications it is capable of being executed in a multi processing environment the output stage this is the final stage in the process of design namely the presentation of the results the fixture components selected by the system are now brought up on the screen of the cad workstation using a driver program this program creates a keystroke file to call various units from the cad database dimension them and position them relative to the global system of axes it uses the output information from the ops5 program to create this file and format it as shown in fig 1 since the amount of data that could be output from the ops5 program would vary from one unit in case of a chuck to over fifteen in a more complicated fixture the output program had to be made as flexible as possible also the nature of the data varies with some components having only two or three parameters to define their geometry and others requiring ten the output program allows for up to fifteen parameters and automatically regulates the size of each block of data depending on the number of parameters finite element program in the course of the design autofix generally positions one or two supports near the centroid of the workpiece this is to reduce deflections due to clamping and cutting forces in the case of thin workpieces or if the cutting forces are high these deflections could be appreciable leading to deviations from dimensions or even damage to the component in either case this would translate into rejected parts autofix therefore first carries out a finite element analysis to determine the point of maximum deflection the engineering analysis package of i deas has been utilized for this task a communi cations interface has been designed to automate most aspects of the finite element study a block diagram of the fe section of autofix is given in fig 6 autofix 409 sdrc i deaa i autofix l i 1 j autofix 1 11 dsas i dea8 i deas i trigger driver pre model post process solution proeesl j autofix i object modelling link data decoding i program j a f support repositioning program outpu report file fig 6 blocl diagram of autofix fe control software execution of autofix the workpiece blank which is to be fixtured is drawn on the cad package in terms of primitives the prototype of autofix permits up to 6 primitives but this number could be extended the operator then types in a single command namely fix and then answers a series of questions from a menu as already mentioned these relate to characteristics of the machine and workpiece and other information which is not available from i deas the geometric and spatial attributes of the workpiece are automatically accessed from the cad package and autofix proceeds to design the fixture with no assistance from the operator in the course of program execution certain other questions may arise and these generally require yes no answers there are four stages in the design process planning preliminary design final design and construction assembly over 500 design rules are examined coded in approximately 1750 productions autofix accesses a database of modular components and attempts to design the fixture using the components from the database if no appropriate component is available the system will design one to suit the workpiece once the fixture has been designed autofix generates a batch file which contains the parameters for the work piece and the