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Toward the ideal mechanical engineering design support system
David G.Ullman
Abstract This paper details the progress toward the development oftheidealmechanical engineering design support system.It attempts to look at the gap between the needs of a mechanicalengineerandwhatis currently available onCAD systems Since the tern：CAD emphasizes that the computer is an aid to the human designer，this paper is designer centric it is based heavi~on the activities performed by designers and the types ofinformation developed by then：Seventeen goals for the ideal mechanical design support system are listed These are directed at the types ofinfor mation developed during the design process and the activ ities used to develop them For each of the seventeen，background information，the current state of the art，and opportunities for future development are itemized
Keywords Mechanical engineering design，CAD，Support system
1
Introduction
This paper summarizes the progress made toward the development of the ideal mechanical engineering design support system For nearly 30 years，computer aided de sign(CAD)systems have been touted by their developers as systems that support engineering designers developing products CAD systems have had a major impact on how design is accomplished in the workplace This being said，there is amazingly little ibrmal research on the effects of these systems on the designers and on the final products’This paper presents a structure for discussing these effects
In doing so、lt summarizes what is known and what needs co be studied Finally、it discusses how CAD systems have evoh,edto supportincreasingportions ofthe activitiesthat are used to develop products.
The term CAD emphasizes that the computer ls an aid to the human designer，so this paper is designer centric it is based heavily on the activities pertbrmed by designers and the types ofintbrmation developed by them in many ways，this is an update of two earlier papers，‘‘The importance of drawing in the mechanical design process”[29 J and“issues critical to the development of design history，design ratio nale and design intent systems”【311 The latter paper developed 13 outstanding issues that needed to be resolvedto realize the capture and query ofengineering design intormation as a potential lor lmprovmg the design process
and the reuse ofdesign Intormation.
The tbundation for the first paper was the study of the marks made on paper by nve mechanical design engineers of varying backgrounds and experience They were each given the initial specifications tbr one of two t~irly simple， yet realistic，mechanical design problems taken from professional practice The engineers were requested to think aloud as they soh,ed the problems Their verbal reports、drawings，and gestures were video and audio taped for aperiod of6 10 h The taped data were then transcribed to obtain a‘‘protocol”of the design session Each designer made numerous drawings during his or her solution of the problem A11 0fthesewere on paper CAD systemswere not usedinthe studgbecausenone ofthedesigners usedCADln their daily practice，and its use would have added another variable to an already complex experiment.
From the more than 40 h ofdata taken，15 sections were selected that represented typical conceptual，layout，detail and selection design for each subject The 15 sections of protocol data consisted of 174 rain of data The data were analyzed to explore the observations that drawings are
usedto：
1 Archive the geometric torm of the design
2 Communicate ldeas between designers and between the designers and manufacturing personnel
3 Act as an analysis tool Often，missing dimensions and tolerances are calculated on the drawing as it ls developed
4 Simulate the design
5 Serve as a completeness checker As sketches or other drawings are being made，the details left to be designed become apparent to the designer This，in ett)ct，helps establish an agenda of design tasks leL to accomplish
6 Act as an extension of the designer’s short term memory Designers often unconsciouslv make sketches to help them remember ldeas that they might otherwise torget
The 1990 paper refined and supported these observa tions AdditionalIv，although the subiects did not use CAD systems，the resuhs suggested that：
1 CAD systerns nl[ist allow tbr sketching lnput
2 CAD systerns nlust allow for a variety of Interfaces tbr the designer This does not mean more ways to define a circle、but an eftort to match the interface and the im age on the CAD system to that needed by the designer
3 CAD systems iiltlSt recognize domain dependent features and treat them as entities
4 CAD tools need to be able to manage constraints(even abstract and tkmctional constraints)and ensure their satistaction、as it is evident that human designers are cognitively limited in this ability
Since that paper was written，CAD systerns have matured and have addressed、at least to some degree、all南ur of the conclusions iIowever，even the most recent systems are a ong way tbom the ideal mechanical engineering design support system In this paper，the ideal system will be described and progress toward this ldeal discussed
2
A model of design problem solving
it may someday be possible for a designer to put on a ‘‘thinking cap”that can take his or her thoughts and de elop a hardware reDresen【atlon Research on under standing cognitive processes，CAD，and rapid prototyping ls certainly moving ln that direction This ideal implies that we can tbrmtflate concepts Jn our heads that are sumciently well tormed to warrant hardware It also as sumes that CAD systems are suflicientlg developed to take our thoughts and manage the evolution of parts and as semblies CAD system development will require an tin derstanding of the cognitive workings of designers so that the transition tbom thought to representation is possible
To explore what ls known about this link、consider the relationship between the human problem soh,er and the external environment shown in Fig 1 This figure ls based on the model developed bv Newell and Simon『20l and ls called the information processing system(IPS)The figure is a simple‘‘map’’ofwhere information about the design is developed and stored The figure shows an internal，human problem solving environment(inside the mind of the designer)and an external environment(outside the mind of the designer)Within the designer，two locations correspond to the two different kinds of memory：short term memory(STM)and long term memory(LTM) External to the designer，there are many‘‘design storage locations”including graphical representation media such as pieces of paper and CAD tools，as well as other media such as textual notes，handbooks，and colleagues Each location has certain properties that affect how it can be
used in design
Detail on the charac Leris【ics of the STM and the LTM ls based on Newell and Simon’s model[20 J Extensions have been made to it for visual imagery【1 l，12，13 J and efforts
have been made to codil~~it【21 it must be realized that the contents of the model given here are not folly agreed to in the cognitive psychology community，but the),are cer tainly secure enough to provide a basis for discussing the role of CAD in mechanical design
2．1
Short-term memory Short tern：memorg ls verv fast and powerful The contents of the STM are the information we are aware of,that is，our conscious mind All design operations(。g，visual perception and drawing creation)arebased oninformation in the STM Unfortunately，the STM has limited capacity Studies show that it is limited to approximately seven cognitive units or chunks of information During design， these chunks are visualimages offorn：s，information about fonction，mentaln：odels offit，stepsto represent anideain a CAD system，or other discrete pieces of information Although limited in capacity，the STM is a fast processor with processing times on the order of 100 nls[41
2．2
Long。term memory The long term memory，on the other hand，has essentially infinite capacity，but access is slow Access to LTM is also not direct Memories rntlSt be triggered by some cue or
retrieval strategy based on intbrmation in STM During design，parts of the design are stored in LTM These are relatively easy to cue because，at any time，currently in： portant parts of the design are in the STM and can act as pointers for the knowledge in the LTM
2．3
ExternaI environment
in the experiments run in 1990[291，it was clear that many drawing actions were not used to document the results of the design activity but were part of the design process itselg if the subjects could bare performed these activities in their heads they would have done so without making the sketches，notes，and calculations on paper Thus，it is concluded that the external environment ls often used as an extension of the STM and LTM it ls critical that the media used in this environment support the designer’s cognition Itemizing the match or mismatch between the media and human cognition is one of the objectives of this paper
The approach taken in this paper is to first describe the types ot'information managed(Sect 3)and then discuss the activities performed by the external environment support ing the designer(Sect 4)The types of information and activities are developed in terms of the capabilities of an ideal system Each subsection begins with statements about what the ideal engineering design support system should do Supporting information follows these statements Next， there is a description of how paper and pencil，2 D CAD systems，solid modeling systems，parametric systems，and other support tools meet the ideal Each subsection concludes with 0DD0r【unl Lles for lmprovement
3
Information managed by an ideal mechanical engineering design suppo~system
Mechanical engineers manage a broad range of informa tion in this section，the X arious types of information will be itemized，beginning with the most basic and progress ing to the most demanding
3．1
Form，fit，and fun~ion
The ideal engineering design support system should：
1 Allow designers to work I?om des￡red Junction to the other types oj lnlornlation
2 Allow designers to flexibly work on the architecture, shape,^f，and function of parts and(zss0川blies
The mechanical design community has traditionally thought in terms of form，fit，and function Figure 2 shows the interplay among these basic types of information that describe the product being designed Generally，the reason for the obiect being designed is to fulfill some desired functions The form of the parts and assemblies，and the fit between them，depend on the desired function of the product Thus，the ideal system should allow the designer to work ffonl funotion to fornl and fit
The term‘‘form’’actually implies both the architecture and the shape of parts and assemblies(Fig 3)The term ‘a(chǎn)rchitecture’’has come to mean the skeletal structure that maps the function to the form Architecture is the‘‘stick figure”that can be easily manipulated and changed before the shape is refined Shape implies the geometry that adds hodV and detalt to tne arcnltecture utten deslgners i~l-st develop the general architecture of the object being de si~dned then add detaits about shape an(1 fit Where wP n憎today Engineers gener~dly work from the ftnlction of a systel~、to the architecture of an assembly，to the shape of parts iaunction occurs primarily at the con nections or fits between the parts ln an asselnbly ill other wor(1s mnction ls devdopetl ln assemb[ies lhis belng said，CAD systems have primali Ly supported the forln or geometry devetopnlent ot parts
Paper and penci[attm~s easy sketching of architecture with stick fi~ures and their evo[ution to conlponents Paper and pencil atso supports[imited function nlodeling through sketchinl4 actions that show lnotion or flow ln assemblies『9、14]
Both 2 D CA／3 systems and paper and pencil are fimited to simple input of line seglnents to represent the edges of COlnponents Solid naode[ing systelns are stilt Colnponent oriented even though they support the representation of edges，surfl~ces，and sotids Parametric systems gl eafly
lniprove(1 the lllode[illl4 of fornl with the Iimlted ability to modeI nt…d as…blles
Future systel~S need to help the designer visualize fllnction before geometry is fully defined Computer sys tenls are allowing better representation of fllnction，。g， kinelnatic，dynamic，fluid flow，and virtmd reality systems With the continued development 0f COlnputer support tools the ability to work fr0111 fllnction to form will …tlnl_e呲v0Lve
CA／3 systems to date have been part driven Parts are developed and then fittecl to~ether to make an assembly Ihe contributions oftheIayOtlt dlawinK have not beenwe[1 supported Paralnetric systems have begun to l~OVe to a nlore natural flow，but parametric nlode[ing requires the designer to PIan ahead of time the geometric constraint rekltionships that define the part,Mally parametric sys tells refer to the orderlng of the constraints as the design intent Ihis methodo[oD"，while lnoving to~ard the ide~d， does not well support the designer as the p[allllillg needed adcls burden and the orderin~nlay not be known initi~flly and lnay change during the deve[oplnent Further，“design intent”as used ill parametric systems is too[imiting(see discusslon of deslfan Intent belowl
OpportunI Extend CAD systems to~d[ow the designer to develop the architecture of parts and assemb[ies to mliltl neede【l mnotion fhey nltlst atlow the designer to work fr0111 the architecture to the shaDe an(1 fit of the conlpo nents themselves lhis wi[1 require workinl4 wlth abstrac tions of parts an(1 asselnbIies as we[1 as buildlng the geometry of objects from their architecture and interfaces with other objects
3．2
Material and manufacturing
The ideal engineering design support system should： 3 Integratetheman@zcturing and assembl),practices and (oltlltl01,1 material usage oy the compan),or its vendors
One of the cornerstones of concurrent engineering is the integration of the development of the product and the processes that support the product Key among these processes are those used to manufacture the parts and assemble them These activities also depend on the selec tion or development of the best materials for the product Thus，as shown in Fig 4，the basic tbrm(architecture and shape)，fit，and fonction need to be tied to materials，manufacturing，and assembly
Where we nre todi9,Currently、there are systems that aid in the development of injection molds and sheet metal parts flowever，for most manuthcturing and assembly methods，only text notes have supported this nongeo metric lnformation
OpportuMt),Extend CAD systems to provide the designer with intbrmation about anticipated material and manu
l；acturing methods This needs to be personalized as each
company and vendor has certain materials and manutac
turing and assembly methods that are standard and well
known Knowledge about these should be easily available
to the designer to aid in the development of parts and
assemblies
3．3
Cost
The ideal engineering design support system should：
4 Support the engineer so she or he is ctlv(~re。l the e‰ect
。f each Jblture change 01,1 costⅢit is generated
The cost to make the object being designed is not a part 0f its description，yet it is a major factor in all design considerations It is shown in Fig 5 as closely tied to the material used and the manufacturing method and through these indirectb to the ftinction and fornl Often there is a disconnection during the design process between drawing
what is being designed，but the limitations and targets on it As such，it is critical information As shown in Fig 6，there are requirements on at[the other types of infor mation previously discussed Traditionally，engineers have done a poor]ob at developing requirements tbr products Wher~w。(It6"tod~9,One of the best practices current[}, used to deve[op requirements in industry is quality func tion dep[oyment(QFD)[7，321 Many companies use the results of this method to direct[},feed requirements to the development of components and assemb[ies Admittedly， many of the requirements deve[oped with the QFD are for funotion；however，there are a[ways many constraints on both function and geometry that drive the development of parts and assemb[ies To date，this is not we[1 integrated
with CAD systems
Stauffer【26 J showed that as the design process n~oves from conceptual through[ayout to detail design，the source of constraints moves from those imposed from outside the control of the designer to those based on previous design decisions This imp[ies that not only shou[d requirements like those deve[oped using QFD type methods be integrated，but the reasoning behind earlier decisions also needs to be supported This wit[be further
discussed in the seclion on design intent fSect 3 61
Opportunity CAD systems need to integrate requirements and constraints into the deve[opment of parts and as semblies
3．5
Issues and plans
The ideal engineering design support system should：
6 Support the development，Jbllowing,and updating plans
7 Support the lmtrlagemenl issues not phtnned Jot．
Whereas at[the types of information described so ihr represent the artifacts being designed and the require ments on them，the lot[owing types of information repre sent the process through which the artifacts are developed
The importance of the process has been a concern ln in dustry since the earh 1980s and an area of research since the mid 1980s The tie between product and process is a maior part of concurrent engineering in the late 1990s， this concern became prominent with the development of interest in integrated product and process development (IPPD)，the successor to concurrent engineering
Traditionally，the product design community addresses the design process ln terms of the tasks to be completed in dexeloping a new product These tasks are focused on specific design lssues that can be planned tbr in the de velopment of the product However、many issues arise duringthe design ofa productthat can notbe plannedtbr This ls especially true during the development of new
products or during the use of new technologies Figure 7 shows that lSStles and plans address all types of require merits and product information issues or tasks in product design require the designer to develop new lnformation 0ne of the first experiments aimed at unders【anding human Intbrmation processing during design tasks【27 J showed that over two thirds ofthe strategies used by the design engineers during the dexelopment of new products were searches through design space Searches imply that there is a range of potential sohltions to every issue and that the designer must look at several of these ahernative solutions to develop one that ls satisfactory Search strategies are well studied by the artificial intelligence community Three types of strategies defined by computer scientists and identified in the cognitive study were‘‘generate and test”， ‘‘generate and improve”、and‘‘means ends analysis”In each search type，the designer develops and refines the alternatives and compares them to the requirements until son2e satisfactory choice has been made ln the time vailable Based on these findings、in order to support designers，systems must not only track the completion of planned work，but must also support the development
and management of multiple ahernatives for all issues addressed
Where we nre todi9,Project planning and change n2an agement has always been a large part of engineering management Product data management(PDM)systems have made large strides toward integrating the actual de sign work with what was planned These systerns are still maturing
Opportunity Computer support tools need to continue to evoh,e to assist the engineer in developing the product and the process in an integrated fashion
3．6
Intent
The ideal engineering design support system should manage all the previously defined types ofinformat