48610 Chap.6 Concept Generation, Scoring and Selection
Concept Generation, Scoring and Selection
Week 5 fills in the gaps the function structure opened. For each sub function you generate several alternative partial solutions, then combine them into whole designs.
The tool is the morphological table, with sub functions down the side and partial solutions across each row; you pre evaluate the cells and discard the infeasible ones, then build alternative overall solutions by taking exactly one solution from every row and joining them with a coloured line, using a different colour for each.
The unused cells are not waste: they are documented fallbacks for when the preferred solution turns out not to work.
Selection then uses two tools at two levels. A pro and con analysis compares partial solutions, which is the right instrument when there are only a few and they differ in some respects.
A weighted concept scoring matrix compares whole configurations: criteria in the rows, an importance weight against each, a rating for how well each solution meets it, and the sum of rating times weight as the score.
The project brief fixes the criteria and their weights, so what you choose and must justify are the ratings, and the guide is explicit that scores should be backed by engineering calculations rather than guesswork. Two checks follow: a sensitivity analysis asking how much the result moves when a weight is adjusted, and a plausibility analysis asking whether the outcome is reasonable.
If the totals are too close, the guide says you cannot make a sound decision and must revise the assessment.
What this chapter covers
- 01
Partial solutions, one row per sub function
- 02
Pre evaluating and excluding infeasible cells
- 03
Building a configuration as a path through the table
- 04
Why conceptual work stays as hand sketches at this stage
- 05
Pro and con analysis for partial solutions
- 06
The concept scoring matrix for whole configurations
- 07
Criteria, weights, ratings and weighted scores
- 08
The five published project criteria and their weights
- 09
Sensitivity analysis: re-running with a changed weight
- 10
Plausibility analysis: is the winner reasonable
- 11
The too close rule and what to do about it
Scoring three launch concepts, then trying to break the result
- +1Check the unweighted totals first, because they are informative. A adds to 32, B adds to 32 and C adds to 32. All three tie, so a group ranking these by adding ratings would be arguing about nothing.
- +1Apply the published weights of 5, 5, 4, 3 and 4. A gives 40 + 20 + 28 + 24 + 20 = 132. B gives 35 + 45 + 20 + 12 + 28 = 140. C gives 25 + 30 + 32 + 27 + 16 = 130. Rank: B first, A second, C third.
- +1Run the sensitivity analysis. Adjustability is the weight most open to argument, so re-run it at 3 instead of 5: A becomes 124, B becomes 122 and C becomes 118. The ranking flips and A now leads, so the result is not robust.
- +1Run the plausibility analysis. The outcome is reasonable in itself: B scores 9 on adjustability because a torsion spring can be preloaded to a repeatable angle, and the brief weights adjustability at 5 because accuracy matters as much as range.
- +1Apply the too close rule. B and A are six per cent apart and one defensible weight change reverses them, so the group cannot make a sound decision on this matrix. The honest next move is to improve the evidence behind the adjustability ratings by estimating the achievable range for each concept.
Key terms
- Partial solution
- One way of achieving a single sub function, such as a torsion spring or a raised mass for storing energy. Several are generated per sub function before any whole design is assembled.
- Configuration
- A complete design formed by selecting exactly one partial solution from every row of the morphological table. The project brief asks for three, each described and hand drawn.
- Pro and con analysis
- A two column comparison of the strengths and weaknesses of alternatives, used for partial solutions where a handful of options differ in only some respects and a number would add nothing.
- Concept scoring matrix
- A weighted comparison of whole configurations, in which each criterion carries an importance weight and each solution a rating, and the sum of the products ranks the alternatives.
- Criterion weight
- A number expressing how important a selection criterion is. For the prototyping project these are published in the brief and are not yours to choose; the ratings are.
- Sensitivity analysis
- Re-running a weighted score with one weight changed, to find out how robust the ranking is. A ranking that reverses under a defensible change is not yet a decision.
- Plausibility analysis
- Asking whether a computed result is reasonable or whether you would have expected something different and why. It catches a matrix that is arithmetically correct and physically absurd.
Concept Generation, Scoring and Selection FAQ
How many cells should the morphological table have?
The conceptual design criteria put 2.5 of the 10 marks on the table including the problem sub structuring, and the top band asks for at least five sub problems or sub functions with at least five partial conceptual solutions each, every one explained with a clear sketch or dot points. That is twenty five populated cells.
The bottom band is described as as few as one or two partial solutions against each sub problem, barely described, so the count is doing real work in the marking.
How do I fill a row when I run out of ideas?
Vary the physical principle rather than the detail. For storing energy, elastic strain, a torsion spring, a raised mass and compressed air are four genuinely different answers, whereas four sizes of rubber band are one answer written four times.
It also helps to leave the obviously poor ideas in the table: a cell that is plainly infeasible still shows it was considered, and the pre evaluation step is where you say why it was excluded.
Why not model the concepts properly before choosing?
Because the point of the conceptual stage is to discard alternatives cheaply, and the cost of producing an alternative sets how many you will consider. A hand sketch takes ninety seconds and can be thrown away without regret; the same mechanism modelled properly takes an afternoon, after which nobody wants to hear that it will not fit in the box.
The subject reflects this in the assessment: conceptual design is assessed with hand drawings and only the detailed design with computer models.
What do I do when two configurations score almost the same?
Not pick one. The guide's instruction is that totals that sit too close together do not support a sound decision and the assessment has to be revised. Revising means improving the evidence behind the ratings on the criteria that separate the two, usually by doing the calculation you had been estimating.
It does not mean re-scoring until a winner appears, and a marker can often tell the difference, because a reverse engineered matrix has the favourite winning on every single criterion, which real alternatives almost never do.
Assessment move
Generate before you evaluate, and keep the two activities in separate sittings. Fill the morphological table as a group with no criticism allowed, aiming for the twenty five cells, then come back the next day to pre evaluate and exclude.
Build the scoring matrix in a spreadsheet so that the sensitivity analysis is one cell edit rather than a re-calculation, and record both the original and the re-run in your submission, because showing that you tested your own result is the difference between the top and middle bands. Keep every sketch, including the rejected ones; they are the evidence that the selection was a choice.
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