Module 1 Introduction. IIT, Bombay

Transcription

1 Module 1 Introduction

2 Lecture 2 Concept Generation and Evaluation

3 Instructional objectives The primary objective of this lecture is to outline the importance of concept generation and selection in decision making exercises in a product development. Introduction Concept generation and the final selection of a concept through proper evaluation are critical decision making steps in product development. The primary aim of concept generation and evaluation is to ensure that the product can perform all of the major functions. This may be done by simple calculations, sketches, circuit diagram, proof-of-concept models, or by a detailed written description of the concept. The stage of concept generation and evaluation should minimize the possibility of misrepresenting a solution, which may actually be effective, and consider different ramification of a final decision. For example, not considering the customer s need during the concept generation and evaluation phase may lead to the failure of the product in the market. Typical steps involved in concept generation and evaluation is shown below [Figure 1.2.1]. Figure Various stages involved during concept generation and evaluation

4 Creative Thinking Creative thinking is critical for concept generation for a product development. The process of creative thinking can be viewed as a step to move from an unstructured idea to a well-structured, from an implicit to an explicit design. Following steps are considered helpful in encouraging effective creative thinking in the process of concept generation. Develop a creative attitude It is very essential to develop confidence that one can provide a creative solution to a given problem. Unlock your imagination One should always ask questions like what or what if and discuss all possibilities. One should spend time on understanding the problem given and be able to realize various queries that may be associated with the problem given. Be persistent Most of the problems are never solved in their first or even initial few attempts. One should rather peruse the solution of a given problem with persistence to find out alternate solutions or designs. Develop an open mind One should always be receptive to ideas from any and all sources for newer concepts. Suspend your judgment The concept generation stage should preclude early judgments. Often the creative ideas develop slowly and require time to proceed in an explicit manner. Thus, the concept generation stage should not be hampered by critical judgment at the initial level. Set problem boundaries This is extremely important for concept generation. It is widely accepted that setting problem boundaries at the very initial stage does not limit creative design ideas but rather focuses it more. Conceptual Decomposition An effective way to solve a complex problem is to decompose it into smaller parts that are easier to manage and then recombine all the ideas or designs to arrive at the final solution. There are two main approaches towards conceptual decomposition.

5 Decomposition in Physical Domain In this stage, the product design is decomposed directly into a number of subassemblies and parts / components. This is the initial step. It should be possible to describe how these subassemblies and part/components work together to accomplish the required function of the product. The function of each of the parts and sub-assemblies, and the way there are connected and interact with each other should be realized explicitly. Each of these subassemblies may be further decomposed into smaller subassemblies and component. This continues till we reach the component level of all the subassemblies. The design information that is available at every level of decomposition is slightly different from its preceding level. However, the functions of the components and subassemblies down the line would possibly be quite different from the function of the final product. Following example [Figure 1.2.2] shows the decomposition of a typical bicycle in the physical domain for the purpose of product design. Figure Decomposition of a bicycle in physical domain for product design purpose Decomposition in Function Domain In functional decomposition, the system functions are descried as the transformation between an initial stage and the desired final state. The approach of concept generation by functional decompositions was originated in the German schools of design. The input and the output of the functional devices are usually described in terms of either energy flow, material flow or information flow. The functions associated with the flow of energy are classified both by the type of energy and by its action on the system. The types of energy are usually classified as mechanical, chemical, electrical, fluid and thermal energy. The actions on the system are envisaged as change, change back, enlarge, reduce, change in direction and so on. The material flow is classified as through-flow or material-conserving process in which the position or shape

6 of the material is changed, diverging flow in which the material is divided in two or more number of small parts, and converging flow, in which the materials are joined or assembled. The information flow is basically in the form of mechanical or electrical signal or software instruction. Hence, the process of functional decomposition describes the design problem in terms of flow of energy material and information. Table shows the decomposition of typical devices in functional domain. Table Functional domain decomposition of three common devices Device Input Function Other effect Output Gear Rotating mechanical energy Change speed of rotation Change direction of rotation Rotational mechanical energy Pencil Mechanical energy Transfers Graphite from pencil to paper Graphite deposit on paper Motor Electrical energy Converts electrical energy to mechanical energy Generate Thermal energy Rotational mechanical energy Generating Design Concepts The design concepts are necessary to build the functions of the product. In other words, the design concepts provide the answer how for the intended function of a product. Usually, a design team is formed in which every team member spends several hours working individually on a few subsets of the overall problem for example, how to identify the sub-functions, and so on, Next, the team members would assemble together to discuss and improve the concepts developed individually and in turn, a number of small design concepts would be generated. Morphological Chart The morphological chart is a method to arrange all the functions and sub-functions in a logical order. The morphological chart also enlists the possible how s for each sub-functions with an

7 aim to realize the combinations of ideas comprising several design concepts. Following is the typical procedure to develop a morphological chart. (1) Establish the functions that the design product must perform (2) List the functions, one per row, in a chart. (3) For each function (row), list a wide range of sub-solutions, one per column. (4) Select an acceptable set of sub-solutions, one for each function. Table shows an example of a morphological chart for the packing of parts like nuts and screws, etc. In the chart, some of the alternatives along a row may be combined to give a single solution, e.g. for picking up the parts, a vacuum arm could be used and for orienting parts, step feeder can be used. If every solution on each row is compatible with all the solutions on the other rows the number of the possible solutions to the system is a multiple of all the ideas on the rows, the possibilities would be enormous. Table Schematic presentation of a morphological chart of packing parts Functions Option1 Option1 Option1 Picking parts Mechanical Arm Vacuum Arm Pneumatic arm Orienting parts Step feeder Bowl feeder Centrifugal feeder Storing parts Rack system Shelves and Bins Drawer Storage Transporting parts Industrial manual Industrial trucks Automatic Guided trolleys (powered) vehicles Combining Concepts This is the step when many fragmented small design concepts are combined to yield a final design concept. Number of possible combinations may be many and all should equally be evaluated or checked for viability. The next step is to combine the concepts to arrive at a set of ultimate design concepts.

8 Evaluation Method Once all the design concepts are more or less selected based on feasibility, these concepts are to be evaluated before the final finished design. Evaluation of these concepts involves various comparisons either in an absolute or in a relative manner among several possible design concepts. Absolute Comparison of Design Concepts It consists of comparing the concepts to a series of absolute filters. [1] Evaluation based on judgment of feasibility of the design: This is the first screening and involves an assessment of the feasibility of the design concepts by the experts. [2] Evaluation based on assessment of technology readiness: This is the second screening and involves an assessment of the readiness of a product manufacturer to produce the designed product without additional research efforts. [3] Evaluation based on go-no-go screening of the customers requirements: This step involves an evaluation whether the design has undertaken the customers requirements or feedbacks. Each customer requirement should be transformed in to a question and should be answerable as either yes (go), maybe(go) or no (no-go). This should help to eliminate any design concept that cannot address an important customer requirement. Relative Comparison of Design Concepts Relative comparisons help to work out the importance of a number of options relative to each other especially when there are no objective data available to set the standard for comparison relative comparison really helps to remove any sort of ambiguity while selecting the most appropriate candidate. There are various ways methods for comparing concept designs by relative comparisons. Some of the most important ones are Pugh's Concept selection Method Weighted Decision Matrix Analytical hierarchy process.

9 Pugh's Concept Selection Method This is a widely accepted method for comparing concepts that are not refined enough for direct comparison with the engineering requirements. Following are the steps involved in this method which are explained with the help of an example. In the example comparison of the effectiveness between three different types of hinges that are used in cabinets is being done. The three options for the hinge are butt hinge, flush hinge and barrel hinge as shown in the Figure Choose or develop the criteria for comparison: The criteria can be identified by examining the customer requirements and generating a corresponding a set of engineering requirements and targets. In our example the criterias are cost of the part, durability, time to production of the part and reliability. 2. Select the alternatives to be compared: The alternatives refer to the alternate ideas developed during concept generation. All concepts should be compared at the same level of generalization and in similar language. In the following example, the barrel hinge is taken as the datum and the other two hinge are compared with this datum level 3. Generate Scores: Designers should pick one of the design concepts that they think is the most appropriate and call it the datum. Now all the other being compared to the datum concept as measured by each of the customer requirements. For each comparison the product should be evaluated as being better (+), the same (S), or worse (-).If it is impossible to make a comparison, more information should be developed. The scores are shown in Table for our example. 4. Compute the total score: Four scores will be generated, the number of plus scores, minus scores, the overall total and the weighted total. The overall total is the number of plus scores minus the number of minus scores. The weighted total is the scores times their respective weighting factors, added up. The totals should not be treated as absolute in the decision making process but as the guidance only. If the two top scores are very close or very similar, then they should be examined more closely to make a more informed decision. For our example, we can clearly see that the overall score for the flush hinge is 1 and that for the butt hinge is -1.

10 Butt hinge Flush hinge Barrel hinge Figure Schematic presentation of three different types of hinges [Source: Table Evaluation on the basis of Pugh's Concept Selection Method Criterion Butt Hinge Flush hinge Barrel hinge Cost of part - + Durability + S D Time to produce S - A Reliability - + T Σ + Σ - Σ S U M Weighted Decision Matrix A decision matrix is used to evaluate the competing design concepts by ranking them with weighting factors and scoring the degree to which each design concept meets the criteria. It is a simple tool that can be very useful in making complex decisions, especially in cases where there are many alternatives and many criteria to be considered. Thus it makes as a qualitative tool to evaluate the alternatives. The procedure for weighted decision matrix is explained below with the help of the above example [Figure 1.2.3]. The above example considers three different types of hinges (1) butt hinge, (2) flush hinge, and (3) barrel hinge that are used in a cabinet. These hinges are required to be produced in bulk. Following is usually the procedure to evaluate the concept based on a weighted decision matrix.

11 1. Identify the Criteria: The more specific the criteria are, the better will be the results of the evaluation. While it is also desirable to have the criteria that are independent of one another, it is rarely possible. For our current example the criteria are the cost of the part, the time to production of the part, and durability and reliability of the parts. 2. Rank and Weigh the Criteria: Some criteria are probably more important than the others. The relative ranking of the criteria will off course affect the evaluation. It is therefore preferable to find out a a way of assigning weights to the criteria so that their relative importance (e.g., reliability may be more important than cost of the part) can be quantified. We can consider the following criteria and the respective weights within parenthesis. (a) Cost of the part (0.50); (b) Durability (0.30) (c) Time to production of the part (0.10) (d) Reliability (0.10) 3. Choose a Ranking Scale: In order to evaluate each design concept option, we need to confirm which one is better (with respect to each criterion). An often-used scale for this is a linear, symmetrical scale as shown below in Table Table Evaluation scale for design objective 11-point scale Description 11-point scale Description 0 Totally useless solution 6 Good solution but a few drawbacks 1 Very inadequate solution 7 Good solution 2 Weak solution 8 Very good solution 3 Poor solution 9 Excellent 4 Tolerable solution 10 Ideal solution 5 Satisfactory solution 4. Calculating the weighting factor for each criterion: This is achieved by multiplying the weightage of the criteria by the score of the criteria for each of the design concept. The calculation for the above example is shown below [Table 1.2.5].

12 Table Weight decision matrix for selection of a hinge Design Criteria Weight Butt Hinge Flush hinge Barrel hinge Factor Score Rating Score Rating Score Rating Cost of part Durability Time to produce Reliability Total Overall score rating: This is the sum of the weighted factors of all the criteria for a particular design concept in step 4. For example, the overall rating for the butt hinge is = The one with the highest score is the best design concept which is the Barrel Hinge in Table Analytical Hierarchy Process Analytical Hierarchy Process (AHP) is designed to solve multi-criteria decision problems. Several alternatives are compared in AHP on the basis of the same set of attributes. The typical steps involved in performing the AHP: (a) make pairwise comparisons, (b) synthesize judgments, and (c) check for consistency. A typical AHP based evaluation process is explained in detail in Figure considering the selection of the most suitable hinge as in the previous method.

13 Selecting the best hinge Criteria Cost Durability Time to produce Butt hinge Butt hinge Butt hinge Alternatives Flush hinge Flush hinge Flush hinge Barrel hinge Barrel hinge Barrel hinge Figure Evaluation of different types of hinges through AHP method 1. Make pairwise comparisons: Pairwise comparison is widely found to be effective with the assignment of relative weights. We compare here each alternative with another in a pairwise manner for each criterion. Following [Table 1.2.6] is the scale that is used for pairwise comparison [in Table 1.2.7, Table 1.2.8, Table and Table ]. Table Scale / Rating used for selection of a hinge Verbal Judgment of Preferences Numerical Rating Extremely preferred 9 Very strongly to extremely 8 Very strongly preferred 7 Strongly to very strongly 6 Strongly preferred 5 Moderately to strongly 4 Moderately preferred 3 Equally to moderately 2 Equally preferred 1

14 Table Pairwise comparison of cost, durability and time to produce Cost Durability Time to produce Cost Durability 1/5 1 1/3 Time to produce 1/6 3 1 Table Pairwise comparison of three different hinges for cost Butt hinge Flush hinge Barrel hinge Butt hinge 1 4 1/3 Flush hinge ¼ 1 1/7 Barrel hinge Table Pairwise comparison of three different hinges for durability Butt hinge Flush hinge Barrel hinge Butt hinge 1 1/6 1 Flush hinge Barrel hinge 1 1/6 1 Table Pairwise comparison of three different hinges for time to produce Butt hinge Flush hinge Barrel hinge Butt hinge Flush hinge 1/5 1 3 Barrel hinge 1/8 1/ Synthesis The priority of each criterion in terms of its contribution to the overall goal of achieving your goal is computed in this step. It involves the following step. [a] Sum values in each column of pairwise comparison matrix [b] Divide each element by its column total (gives normalized pairwise comparison matrix) [c] Compute average in each row (gives estimate of relative priorities of elements being compared) by dividing each element by the column total [Tables to ]

15 Table Assign priority rating of each criterion for pairwise comparison of cost Butt hinge Flush hinge Barrel hinge Butt hinge 1 4 1/3 Flush hinge 1/4 1 1/7 Barrel hinge SUM 17/ /21 Table Compute average priority of each criterion for pairwise comparison of cost Butt hinge Flush hinge Barrel hinge Average Butt hinge 1 4 1/ Flush hinge 1/4 1 1/ Barrel hinge Relative Priority Normalized pairwise comparison matrix These relative priority means that with respect to Cost, the barrel hinge will be preferred first (65%), followed by butt hinge (27%) and flush hinge (8%). We can do similar calculations for durability and time to produce. Table Assign priority rating of each criterion for pairwise comparison of durability Butt hinge Flush hinge Barrel hinge Butt hinge 1 1/6 1 Flush hinge Barrel hinge 1 1/6 1 SUM 8 8/6 8

16 Table Compute average priority of each criterion for pairwise comparison of durability Butt hinge Flush hinge Barrel hinge Average Butt hinge Flush hinge Barrel hinge Relative Priority Normalized pairwise comparison matrix Table Assign priority rating of each criterion for pairwise comparison of time to produce Butt hinge Flush hinge Barrel hinge Butt hinge Flush hinge 1/5 1 3 Barrel hinge 1/8 1/3 1 SUM 53/40 19/3 12 Table Compute average priority of each criterion for pairwise comparison of time to produce Butt hinge Flush hinge Barrel hinge Average Butt hinge Flush hinge Barrel hinge Relative Priority Normalized pairwise comparison matrix We will follow the same process for calculating the relative priority for the criteria as follows [Tables ] Table Assign rating of each criterion for pairwise comparison of relative priority Cost Durability Time to produce Cost Durability 1/5 1 1/3 Time to produce 1/6 3 1 SUM 41/ /3

17 Table Compute average rating for pairwise comparison of relative priority Cost Durability Time to produce Average Cost Durability Time to produce Relative Priority Normalized pairwise comparison matrix To calculate the overall ranking of the alternatives we now multiply the relative priority of each criteria with each of the attributes o and add them up [Table ]. Table Overall ranking of the three hinge alternatives Cost (0.7) Durability (0.1) Time to produce (0.2) Final Score Butt hinge 0.266*0.7= *0.1= *0.2= Flush hinge 0.08*0.7= *0.1= *0.2= Barrel hinge 0.654*0.7= *0.1= *0.2= So we can see that the barrel hinge depicts as the best option followed by butt hinge and then the flush hinge 3. Check for Consistency: A key step in the making of several pairwise comparisons is considering the consistency of the pairwise judgments. Example: If A compared to B = 3 and B compared to C = 2 then A compared to C should be 6 (3 x 2). Otherwise, an inconsistency will occur.

18 Exercise 1. Create a functional decomposition of a refrigerator References 1. G Dieter, Engineering Design - a materials and processing approach, McGraw Hill, NY, 2000.