- Email: [email protected]
Design Criteria
Copyright ~ :wAC Infonnation Control in Manufacturing, Nancy - Metz, France, 1998
Design Criteria Garro 0., Brissaud D, Blanco E.
Affiliation Lab 3S, University of Grenoble.
Communication address Domaine universitaire, BP 53, F-38041 Grenoble Cedex 9 E-Mail Tel.
Fax
[email protected] +33/0-4-76-82-52-78 +33/0-4-76-82-70-43
Abstract The main objective of this paper is to describe design criteria. We have noted that criteria have a fundamental importance during all the design of a product. Carrying out observations of products in industrial context, we have been hit by the manner. of which they are mobilised in the action. They are used in order to define the product as well as to validate and to enrich it. In this paper, we wish to define this important point. Copyright © 1998 IFAC
Keywords Concurrent Engineering, Design, Criteria According to the norm, a criteria exists in relation to a functional requirement. It can be specified for one or several functions. Several criteria are generally necessary to specify a precise function. A criteria is always definite by a level. This level is the one that is aimed in the action of design. More, it is valued by a class of flexibility. When the level must be reached of imperative manner, one will qualify it by FO. If it is given to indicative title, one will qualify it by F3. The intermediate classes (of Ft to F2) indicate the biggest adaptability of the level. This level can be associated to limits of acceptance (classes of flexibility) that are going to give domains of validity of the criteria also.
Introduction The design of a product is an activity which involves the representation and resolution of problems the understanding of which is still partial, and for which the tools and methods for assisting the designer remains as at today rudimentary . Design activity manifest itself in various diversified domains and could involve a lot of heterogeneous actors all along the processes of elaboration and realisation of the product or system to be produced. All products follow a certain iterative life cycle starting from its specification to its destruction. The complexity of this design process create a multitude of point of view on the product to be designed, based on different rationality. These points of view are translated by the actors into different design criteria in order to evaluate the performance of the product. The purpose of this paper is to present the concept of design criteria that seems to be fluently used by designers in order to get under control the uncertainty of a project.
An example of criteria related to functions can be found in [Salaii 95] (fig. 1). Ft.EXlBn..rrY I FUNCllON I Li.miloof CJUTEllON LEVEL ! I
C~.
;II
Il.>Ic of ;air cbancancnt
Ensure the air
chan2cmcnl
I
O..pollibility
Az volume per hour
31
B 'K oflllJlC
32
\:ocuptabon A"(O)
A"
I
B
Figure 1 : A function requirement and its criteria.
Criteria, in a normative approach of the desi&n.
This approach of criteria of design brings us to make several remarks:
The European norms on the value analysis give a precise definition of criteria [NFX 50-150].Their are qualified in these norms of acceptance criteria.
a) Criteria at first are used to define functional requirements. Then, they permit to validate choices of design. Criteria are "instruments" constructed at the beginning of the design to surround the domain better. These instruments serve to validate the ulterior work. Their construction is located in the beginning of the design and their use is made in end of design. They are not supposed to be used at
Definition: A criteria is a characteristic used to value the performance waited ofa product.
743
the middle stage of the design activity. But this fact is in contradiction with our observations of design that we led in the industrial surroundings [Blanco & al. 97]. Indeed, criteria are pennanently mobilised.
The result was a listing of 39 design criteria. The hypothesis of Genrikh Altschuller is based on the fact that all technical conflicts could be resume to only a few, universal technical conflicts. In other words, all possible technical conflicts in the world may be categorised into a limited number of "universal" conflicts, expressed in a "universal" parametric language. Altschuller and his associates completed the Herculean task of establishing the minimum number of design criteria as they are listed below: The 39 Standard Features
b) In fact, criteria, as they are presented, suppose a classical and sequential approach of design activity. There are related to three distinct phases. These phases understand the definition of the functional requirements, then the research of solutions and at the end, the choice of solutions. c) An interesting element, the class of flexibility, break this approach however a little too rigid. This concept opens the door to the negotiation. The possibility to negotiate the level of a criteria permits to come back behind in order to improve the proposed solutions. This flexibility pennits the evolution of understanding of requirements without changing it's formulation. Finally the notion of flexibility reinforces the role of the functional requirement sheet.
l.WEIGHT (of an object that can move, or is moving) 2.WEIGHT (of an object that is still or can't move) 3.DIMENSION (of an object that can move, or is moving) 4.DIMENSION (of an object that is still or can't move) 5.AREA (of an object that can move, or is moving) 6.AREA (of an object that is still or can't move) 7.VOLUME (of an object that can move, or is moving) 8.VOLUME (of an object that is still or can't move) 9.SPEED IO.FORCE II.PRESSURE, TENSION I2.SHAPE 13.STABILITY 14. STRENGTH I5.DURABILITY (of an object that can move, or is moving) I6.DURABILITY (of an object that is still or can't move) 17.TEMPERATURE I8.BRIGHTNESS I9.ENERGY (used by of an object that can move, or is moving) 20.ENERGY (used by of an object that is still or can't move) 21.POWER 22.ENERGY WASTED 23.MATIER WASTED 24.INFORMATION LOSS 25.TIME WASTED 26.QUANTITY OF MATIER 27.RELIABILITY 28.MEASUREMENT ACCURACY 29.MANUFACTURING ACCURACY 30.HARMFUL EFFECTS (on object) 31.HARMFUL SIDE EFFECTS 32.MANUFACTURABILITY 33.EASE OF USE 34.EASE OF REPAIR 35.ADAPTABILITY 36.SYSTEM COMPLEXITY 37.CONTROL COMPLEXITY 38.DEGREE OF AUTOMATION 39.PRODUCTIVITY
d) In this method the criteria that are developed are only external criteria. By external criteria, we mean criteria that carry on the relation of the product to its environment, that is to say of criteria carrying on the use. The internal criteria that are going to characterise the structure of the product are not developed here. It is for example the mechanical resistance of a part or its machining ability. d) From an operational point of view, this approach is very difficult and heavy to lead. Even though it only concerns a part of criteria (the external criteria), it is going to carry on an important number of parameters. Very often it is a little bit inapplicable in the case of innovative design where the difficulty is to find criteria without already basing on existing solutions. But it can be adapted to routine design in which criteria are already well known. In this case, they appear as parameters of the system that has to been realised.
A methodology of innovation based on criteria. The TRIZ methodology is based on the works of a Russian scientist, Genrikh Altschuller, and his research team [AltschuUer 94]. They postulate that inventive problems are those that are based upon criteria's conflicts. And all technical conflicts can be restated using a "universal language of conflicts." Over the period 1946 and into the 1970's, the Russians had been examining the global patent collection with several aims in mind, one of which was to "universalise" the language of engineering parameters and characteristics used to address the important attributes of a technical system."
744
What is interesting in this methodology for our purpose is that 39 criteria, called "the 39 standard features" are defined. These criteria are enough generic to be used in every design process. Their legitimacy is the result of the analysis of the whole patent collection. But nobody is able to know if a new criteria will appear in the future, depending of the progress of technology or of the evolution of global requirements due to the evolution of the society. For instance hox green design should be taking into account by these 39 criteria? An other aspect is that during design process, a great amount of criteria appear (more than 39). But this criteria seem to be hierarchically dependent of those of Altschuller. At this time, we are not sure that the criteria of Altschuller are not coming from a too high level of abstraction to be efficient in the design of a complex system.
Therefore we obtain a hierarchy of criteria with at least three levels. The high level, represented by the three previous criteria, depends on the strategy of the project. The low level is composed of all the criteria that emerge during the process. Their are operational criteria. And we have an intermediate level with the criteria of Altschuller which could be a tactical level.
d) A network of criteria. Establish a hierarchy, don't give an opinion on the importance of criteria for such or such product, but more a notion of active ordering from a global point of view to a more operational aspect. We noted that in design projects, criteria were mobilised not according to their level in this hierarchy, but rather according to their relevance. On the other hand, criteria are very often linked by relations of conflict, as noted by Altschuller, or on the contrary by relations of cumulative effect. There is relation of conflict between two criteria when the improvement of a criteria damages another. There is cumulative effect when the improvement of a criteria improves another. Criteria are bound therefore at a time by a hierarchy and by an effect of interaction. During the design process, the actors construct permanently this network while trying to find solutions that optimize criteria.
A typology of criteria
a)Internal-external We already noticed that criteria could be either internal or external for the product. The external criteria depend on performances waited of the product. There are criteria that can be found by the value analysis methodology. These criteria acquire their full sense only when the design is finished and the product is in use. The internal criteriaare bound to functions that the most often result from choices of designers. These criteriaare fundamental during the design process. In oposition with the external one's, these criteria are mainly local.
· . . .Process criteria
Product criteria
QUALITY
ExreJ 1
b) Product - project Another way to classify the criteria should be to consider their position in relation to the product or to the project. Usually in design one speaks of three very global criteria witch are the cost, the quality and the delay. The quality is a criterion bound to the product solely. Delays are bound to the time of the project. Roughly costs should be divided in development cost (bound to the project) and cost of the product.. Of course interactions are strong between project and product and therefore between these three criteria. All internal and external criteria, as described below are bound to the product.
cntena
:
c~\s~: · ..
. DELAY· .
· . Development costs. .
Internal criteria Product cost
Figure 2 : a typology of criteria
In the reality: the dynamics of criteria In the different projects of design that we followed, as well as in design experiments that we achieved, we noted a particular dynamics, own to criteria [Garro 95, Blanco 97]. On the following figure (fig 3),· we tried to represent the process in which appears criteria. Of course, this representation is simplified because we have separated the process in four different phases whereas all is extremely mixed. The context is the design of a part of a mechanism of a small robot. It is about a work meeting in which solutions for this mechanism must be found.
c) A hierarchy of criteria The evocation of the three precedent criteria brings us to make a hierarchy in criteria. Cost, quality and delays are three very general type of criteria. For the product, the quality can be divided in a multitude of under-criteria as those presented by Altschuller. These criteria could be either internal or external.
745
(1) in a first time an actor achieves a drawing. This drawing carries a potential solution. (2) the group mobilises himself around the drawing and gives out critiques that are discussed then. During this interaction, the group seizes of the solution. (3) positive and negative critiques produced by the group are made by the way of particular criteria. Criteria appear therefore in relation to a solution. These Permit to define the problem and then to evaluate the solution. (4) then the drawing is let as and it is reinforced. The respect of criteria is judged sufficient. The solution is adopted. (4 ') either criteria are not enough respected. The drawing is modified then directly by one of actors. It is improved. (4 ") if the gap between the solution and the problem is perceived by the group as too important, that is to say if the criteria is not enough respected, then a new solution appears, materialised by a new drawing. A new drawing makes often appear new criteria. The old solutions and the new are submitted to the test of all criteria. In this process, the problem is constructed progressively in relation to a whole of criteria that emerge. An other aspect is that at the beginning of a design, it is very rare that a criteria can be quantified. But the different solution are compared two to two with the help of criteria.
Toward specific tools based on criteria for design process. This notion of criteria, permits to define tools in order to help the designer. Two ways are possible. The first consists to define criteria for a specific profession that help designers. In our work about locating and clamping of part for machining, criteria upon the product have been defined. We present two of them here [Paris 95, Brissaud 97]. The criteria for revolving represents the risk for the part for revolving around its supports during the machining process. It is defined in order to give an account of the force distribution on every support point. At this time, it can be use from a complete definition of the part geometry. It is a feasibility criteria: the workpiece will revolve or will not, and so, a solution can be forbidden. But it is more particularly an criteria which acts upon the current design. Active because it assists for choosing the best solution for fixturing if several solution criteria are compared while the process plan elaboration. Active too because it points the parameters, notably their way and their variation, able to improve the current solution. In our example, for a better solution for the revolving point of view, either the distance between the locating feature and the machining feature has to be reduced, either the dimension of the locating feature has to grow up. The criteria for the locating quality represents the minimal quality necessary for the locating features to achieve the positioning tolerance required by the designer on the machining features. While the criterion of revolving was leaning on mechanical principles, this criterion integrates the knowledge and know-how of the machining job. Quality of used machine-tools and toolings, settings difficulty have to be known and controlled in order to qualify the difficulty for achieving the necessary locating quality. The action upon current design of this criteria goes more than that as it allows the designer to require the adapted locating feature which affirms the required positioning quality. Therefore, the designer is able to build the geometry which replies to the request.
Criteria have an important role therefore in the choice of a solution. They are found, sometimes by simple association of idea, discussed then and used finally on solutions. The discussion on criteria permits to define the problem better. Their use permits to define solutions of the problem better. A definition of criteria A criteria is therefore an important element of the design of a product. On the first hand it Permits the emergence of the solution while on the second hand it participate to the definition of the requirement (problem). But in the same time criteria are very heterogeneous and various. So we will adopt the following definition of a criteria: The criteria is a pragmatic entity that represents the product in its future life cycle (manufacture, use, maintenance or destruction... ) in order to evaluate it. In this definition what is important is that the criteria, whatever is its form, is able to help the designer to take a decision or a choice. The more often a criteria cannot be quantified. But a comparison between two solution, based on this criteria is possible. Then the designer can choose the solution or propose a new alternative.
The second way consists to propose to the designer a list of criteria in order to shorten the time that it is going to spend to find them. A second effect of this gait is to increase the quality of the design while avoiding to the designer to forget some important criteria. To be efficient this checklist must be composed of criteria of low level (oPerational level). A non exhaustive list of criteria upon the product is presented in table 1. Some of them are finalised in different design methodologies (for instance the DFA methodology is based on pecular design criteria which are related
746
experimentale. Colloque de Genie Industriel, Albi sept 97. [Boothroyd 92] G. Boothroyd an L. Alting Design For Assembly and disassembly, annals of the CIRP, voI4112/1992. [Garro 97] Garro 0., Contribution a la modelisation de la conception de systeme mecanique, HDR, Grenoble 97. [NFX 50-150... ] Norme AFNOR relative a l'analyse de la valeur n° 150, 151, 152 et 153. [Paris 95] Paris H., Contribution a la conception automatique de gamme d'usinage, These de l'UJF Grenoble, 1995. [Salaii 95] Salau I., La conception distribuee: theorie et methodologie, These universite Nancy 1, 1995.
with assembly knowledge, [Boothroyd 92]); others have to be created. An other important potentiality of criteria consist in their importance in the decision making. Then they could be used to construct a memory of the process. This need a specific tool that stock all the criteria and their consquences during the design process.
Bibliography [Altschuller 94] G. Altschuller, And suddently the inventor appeared. Ed. TIC 94. [Brissaud 97] D. Brissaud, Contribution a la conception de gamme d'usinage, HDR de Grenoble, 97. [Blanco 97] Eric Blanco, Olivier Garro, Alain Jeantet, La conception distribuee, approche
747
4"- New draft
4- Choice of a solution
Figure 3 : the product ion of the solution by a group of three designe rs
CRITE RION Design Product ivity Functio nal Classifi cation Machin ing Feature Cost Machin ability
RDM FEM
N° Existence
Global /local
Exact /Fuzzy
Comple te Incomp lete
Actor
Use Evaluat e to redesign possibly Classify the function s Evaluat e the cost of the product Possible or not
1
Y
G
E
C
Functio nal
2
Y
G
F
I
Functio nal
3
y
L
E
C
4
Y
G
E
C
5 6
Y y
L L
E-F
C C
Manufa cturi ng Manufa cturi ng Structur al Structur al
E
Table 1 : Differen t criteria and their domain of validity
748
OK or not OK or not
Design Criteria Garro 0., Brissaud D, Blanco E.
Affiliation Lab 3S, University of Grenoble.
Communication address Domaine universitaire, BP 53, F-38041 Grenoble Cedex 9 E-Mail Tel.
Fax
[email protected] +33/0-4-76-82-52-78 +33/0-4-76-82-70-43
Abstract The main objective of this paper is to describe design criteria. We have noted that criteria have a fundamental importance during all the design of a product. Carrying out observations of products in industrial context, we have been hit by the manner. of which they are mobilised in the action. They are used in order to define the product as well as to validate and to enrich it. In this paper, we wish to define this important point. Copyright © 1998 IFAC
Keywords Concurrent Engineering, Design, Criteria According to the norm, a criteria exists in relation to a functional requirement. It can be specified for one or several functions. Several criteria are generally necessary to specify a precise function. A criteria is always definite by a level. This level is the one that is aimed in the action of design. More, it is valued by a class of flexibility. When the level must be reached of imperative manner, one will qualify it by FO. If it is given to indicative title, one will qualify it by F3. The intermediate classes (of Ft to F2) indicate the biggest adaptability of the level. This level can be associated to limits of acceptance (classes of flexibility) that are going to give domains of validity of the criteria also.
Introduction The design of a product is an activity which involves the representation and resolution of problems the understanding of which is still partial, and for which the tools and methods for assisting the designer remains as at today rudimentary . Design activity manifest itself in various diversified domains and could involve a lot of heterogeneous actors all along the processes of elaboration and realisation of the product or system to be produced. All products follow a certain iterative life cycle starting from its specification to its destruction. The complexity of this design process create a multitude of point of view on the product to be designed, based on different rationality. These points of view are translated by the actors into different design criteria in order to evaluate the performance of the product. The purpose of this paper is to present the concept of design criteria that seems to be fluently used by designers in order to get under control the uncertainty of a project.
An example of criteria related to functions can be found in [Salaii 95] (fig. 1). Ft.EXlBn..rrY I FUNCllON I Li.miloof CJUTEllON LEVEL ! I
C~.
;II
Il.>Ic of ;air cbancancnt
Ensure the air
chan2cmcnl
I
O..pollibility
Az volume per hour
31
B 'K oflllJlC
32
\:ocuptabon A"(O)
A"
I
B
Figure 1 : A function requirement and its criteria.
Criteria, in a normative approach of the desi&n.
This approach of criteria of design brings us to make several remarks:
The European norms on the value analysis give a precise definition of criteria [NFX 50-150].Their are qualified in these norms of acceptance criteria.
a) Criteria at first are used to define functional requirements. Then, they permit to validate choices of design. Criteria are "instruments" constructed at the beginning of the design to surround the domain better. These instruments serve to validate the ulterior work. Their construction is located in the beginning of the design and their use is made in end of design. They are not supposed to be used at
Definition: A criteria is a characteristic used to value the performance waited ofa product.
743
the middle stage of the design activity. But this fact is in contradiction with our observations of design that we led in the industrial surroundings [Blanco & al. 97]. Indeed, criteria are pennanently mobilised.
The result was a listing of 39 design criteria. The hypothesis of Genrikh Altschuller is based on the fact that all technical conflicts could be resume to only a few, universal technical conflicts. In other words, all possible technical conflicts in the world may be categorised into a limited number of "universal" conflicts, expressed in a "universal" parametric language. Altschuller and his associates completed the Herculean task of establishing the minimum number of design criteria as they are listed below: The 39 Standard Features
b) In fact, criteria, as they are presented, suppose a classical and sequential approach of design activity. There are related to three distinct phases. These phases understand the definition of the functional requirements, then the research of solutions and at the end, the choice of solutions. c) An interesting element, the class of flexibility, break this approach however a little too rigid. This concept opens the door to the negotiation. The possibility to negotiate the level of a criteria permits to come back behind in order to improve the proposed solutions. This flexibility pennits the evolution of understanding of requirements without changing it's formulation. Finally the notion of flexibility reinforces the role of the functional requirement sheet.
l.WEIGHT (of an object that can move, or is moving) 2.WEIGHT (of an object that is still or can't move) 3.DIMENSION (of an object that can move, or is moving) 4.DIMENSION (of an object that is still or can't move) 5.AREA (of an object that can move, or is moving) 6.AREA (of an object that is still or can't move) 7.VOLUME (of an object that can move, or is moving) 8.VOLUME (of an object that is still or can't move) 9.SPEED IO.FORCE II.PRESSURE, TENSION I2.SHAPE 13.STABILITY 14. STRENGTH I5.DURABILITY (of an object that can move, or is moving) I6.DURABILITY (of an object that is still or can't move) 17.TEMPERATURE I8.BRIGHTNESS I9.ENERGY (used by of an object that can move, or is moving) 20.ENERGY (used by of an object that is still or can't move) 21.POWER 22.ENERGY WASTED 23.MATIER WASTED 24.INFORMATION LOSS 25.TIME WASTED 26.QUANTITY OF MATIER 27.RELIABILITY 28.MEASUREMENT ACCURACY 29.MANUFACTURING ACCURACY 30.HARMFUL EFFECTS (on object) 31.HARMFUL SIDE EFFECTS 32.MANUFACTURABILITY 33.EASE OF USE 34.EASE OF REPAIR 35.ADAPTABILITY 36.SYSTEM COMPLEXITY 37.CONTROL COMPLEXITY 38.DEGREE OF AUTOMATION 39.PRODUCTIVITY
d) In this method the criteria that are developed are only external criteria. By external criteria, we mean criteria that carry on the relation of the product to its environment, that is to say of criteria carrying on the use. The internal criteria that are going to characterise the structure of the product are not developed here. It is for example the mechanical resistance of a part or its machining ability. d) From an operational point of view, this approach is very difficult and heavy to lead. Even though it only concerns a part of criteria (the external criteria), it is going to carry on an important number of parameters. Very often it is a little bit inapplicable in the case of innovative design where the difficulty is to find criteria without already basing on existing solutions. But it can be adapted to routine design in which criteria are already well known. In this case, they appear as parameters of the system that has to been realised.
A methodology of innovation based on criteria. The TRIZ methodology is based on the works of a Russian scientist, Genrikh Altschuller, and his research team [AltschuUer 94]. They postulate that inventive problems are those that are based upon criteria's conflicts. And all technical conflicts can be restated using a "universal language of conflicts." Over the period 1946 and into the 1970's, the Russians had been examining the global patent collection with several aims in mind, one of which was to "universalise" the language of engineering parameters and characteristics used to address the important attributes of a technical system."
744
What is interesting in this methodology for our purpose is that 39 criteria, called "the 39 standard features" are defined. These criteria are enough generic to be used in every design process. Their legitimacy is the result of the analysis of the whole patent collection. But nobody is able to know if a new criteria will appear in the future, depending of the progress of technology or of the evolution of global requirements due to the evolution of the society. For instance hox green design should be taking into account by these 39 criteria? An other aspect is that during design process, a great amount of criteria appear (more than 39). But this criteria seem to be hierarchically dependent of those of Altschuller. At this time, we are not sure that the criteria of Altschuller are not coming from a too high level of abstraction to be efficient in the design of a complex system.
Therefore we obtain a hierarchy of criteria with at least three levels. The high level, represented by the three previous criteria, depends on the strategy of the project. The low level is composed of all the criteria that emerge during the process. Their are operational criteria. And we have an intermediate level with the criteria of Altschuller which could be a tactical level.
d) A network of criteria. Establish a hierarchy, don't give an opinion on the importance of criteria for such or such product, but more a notion of active ordering from a global point of view to a more operational aspect. We noted that in design projects, criteria were mobilised not according to their level in this hierarchy, but rather according to their relevance. On the other hand, criteria are very often linked by relations of conflict, as noted by Altschuller, or on the contrary by relations of cumulative effect. There is relation of conflict between two criteria when the improvement of a criteria damages another. There is cumulative effect when the improvement of a criteria improves another. Criteria are bound therefore at a time by a hierarchy and by an effect of interaction. During the design process, the actors construct permanently this network while trying to find solutions that optimize criteria.
A typology of criteria
a)Internal-external We already noticed that criteria could be either internal or external for the product. The external criteria depend on performances waited of the product. There are criteria that can be found by the value analysis methodology. These criteria acquire their full sense only when the design is finished and the product is in use. The internal criteriaare bound to functions that the most often result from choices of designers. These criteriaare fundamental during the design process. In oposition with the external one's, these criteria are mainly local.
· . . .Process criteria
Product criteria
QUALITY
ExreJ 1
b) Product - project Another way to classify the criteria should be to consider their position in relation to the product or to the project. Usually in design one speaks of three very global criteria witch are the cost, the quality and the delay. The quality is a criterion bound to the product solely. Delays are bound to the time of the project. Roughly costs should be divided in development cost (bound to the project) and cost of the product.. Of course interactions are strong between project and product and therefore between these three criteria. All internal and external criteria, as described below are bound to the product.
cntena
:
c~\s~: · ..
. DELAY· .
· . Development costs. .
Internal criteria Product cost
Figure 2 : a typology of criteria
In the reality: the dynamics of criteria In the different projects of design that we followed, as well as in design experiments that we achieved, we noted a particular dynamics, own to criteria [Garro 95, Blanco 97]. On the following figure (fig 3),· we tried to represent the process in which appears criteria. Of course, this representation is simplified because we have separated the process in four different phases whereas all is extremely mixed. The context is the design of a part of a mechanism of a small robot. It is about a work meeting in which solutions for this mechanism must be found.
c) A hierarchy of criteria The evocation of the three precedent criteria brings us to make a hierarchy in criteria. Cost, quality and delays are three very general type of criteria. For the product, the quality can be divided in a multitude of under-criteria as those presented by Altschuller. These criteria could be either internal or external.
745
(1) in a first time an actor achieves a drawing. This drawing carries a potential solution. (2) the group mobilises himself around the drawing and gives out critiques that are discussed then. During this interaction, the group seizes of the solution. (3) positive and negative critiques produced by the group are made by the way of particular criteria. Criteria appear therefore in relation to a solution. These Permit to define the problem and then to evaluate the solution. (4) then the drawing is let as and it is reinforced. The respect of criteria is judged sufficient. The solution is adopted. (4 ') either criteria are not enough respected. The drawing is modified then directly by one of actors. It is improved. (4 ") if the gap between the solution and the problem is perceived by the group as too important, that is to say if the criteria is not enough respected, then a new solution appears, materialised by a new drawing. A new drawing makes often appear new criteria. The old solutions and the new are submitted to the test of all criteria. In this process, the problem is constructed progressively in relation to a whole of criteria that emerge. An other aspect is that at the beginning of a design, it is very rare that a criteria can be quantified. But the different solution are compared two to two with the help of criteria.
Toward specific tools based on criteria for design process. This notion of criteria, permits to define tools in order to help the designer. Two ways are possible. The first consists to define criteria for a specific profession that help designers. In our work about locating and clamping of part for machining, criteria upon the product have been defined. We present two of them here [Paris 95, Brissaud 97]. The criteria for revolving represents the risk for the part for revolving around its supports during the machining process. It is defined in order to give an account of the force distribution on every support point. At this time, it can be use from a complete definition of the part geometry. It is a feasibility criteria: the workpiece will revolve or will not, and so, a solution can be forbidden. But it is more particularly an criteria which acts upon the current design. Active because it assists for choosing the best solution for fixturing if several solution criteria are compared while the process plan elaboration. Active too because it points the parameters, notably their way and their variation, able to improve the current solution. In our example, for a better solution for the revolving point of view, either the distance between the locating feature and the machining feature has to be reduced, either the dimension of the locating feature has to grow up. The criteria for the locating quality represents the minimal quality necessary for the locating features to achieve the positioning tolerance required by the designer on the machining features. While the criterion of revolving was leaning on mechanical principles, this criterion integrates the knowledge and know-how of the machining job. Quality of used machine-tools and toolings, settings difficulty have to be known and controlled in order to qualify the difficulty for achieving the necessary locating quality. The action upon current design of this criteria goes more than that as it allows the designer to require the adapted locating feature which affirms the required positioning quality. Therefore, the designer is able to build the geometry which replies to the request.
Criteria have an important role therefore in the choice of a solution. They are found, sometimes by simple association of idea, discussed then and used finally on solutions. The discussion on criteria permits to define the problem better. Their use permits to define solutions of the problem better. A definition of criteria A criteria is therefore an important element of the design of a product. On the first hand it Permits the emergence of the solution while on the second hand it participate to the definition of the requirement (problem). But in the same time criteria are very heterogeneous and various. So we will adopt the following definition of a criteria: The criteria is a pragmatic entity that represents the product in its future life cycle (manufacture, use, maintenance or destruction... ) in order to evaluate it. In this definition what is important is that the criteria, whatever is its form, is able to help the designer to take a decision or a choice. The more often a criteria cannot be quantified. But a comparison between two solution, based on this criteria is possible. Then the designer can choose the solution or propose a new alternative.
The second way consists to propose to the designer a list of criteria in order to shorten the time that it is going to spend to find them. A second effect of this gait is to increase the quality of the design while avoiding to the designer to forget some important criteria. To be efficient this checklist must be composed of criteria of low level (oPerational level). A non exhaustive list of criteria upon the product is presented in table 1. Some of them are finalised in different design methodologies (for instance the DFA methodology is based on pecular design criteria which are related
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experimentale. Colloque de Genie Industriel, Albi sept 97. [Boothroyd 92] G. Boothroyd an L. Alting Design For Assembly and disassembly, annals of the CIRP, voI4112/1992. [Garro 97] Garro 0., Contribution a la modelisation de la conception de systeme mecanique, HDR, Grenoble 97. [NFX 50-150... ] Norme AFNOR relative a l'analyse de la valeur n° 150, 151, 152 et 153. [Paris 95] Paris H., Contribution a la conception automatique de gamme d'usinage, These de l'UJF Grenoble, 1995. [Salaii 95] Salau I., La conception distribuee: theorie et methodologie, These universite Nancy 1, 1995.
with assembly knowledge, [Boothroyd 92]); others have to be created. An other important potentiality of criteria consist in their importance in the decision making. Then they could be used to construct a memory of the process. This need a specific tool that stock all the criteria and their consquences during the design process.
Bibliography [Altschuller 94] G. Altschuller, And suddently the inventor appeared. Ed. TIC 94. [Brissaud 97] D. Brissaud, Contribution a la conception de gamme d'usinage, HDR de Grenoble, 97. [Blanco 97] Eric Blanco, Olivier Garro, Alain Jeantet, La conception distribuee, approche
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4"- New draft
4- Choice of a solution
Figure 3 : the product ion of the solution by a group of three designe rs
CRITE RION Design Product ivity Functio nal Classifi cation Machin ing Feature Cost Machin ability
RDM FEM
N° Existence
Global /local
Exact /Fuzzy
Comple te Incomp lete
Actor
Use Evaluat e to redesign possibly Classify the function s Evaluat e the cost of the product Possible or not
1
Y
G
E
C
Functio nal
2
Y
G
F
I
Functio nal
3
y
L
E
C
4
Y
G
E
C
5 6
Y y
L L
E-F
C C
Manufa cturi ng Manufa cturi ng Structur al Structur al
E
Table 1 : Differen t criteria and their domain of validity
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OK or not OK or not