Pugh Method: How to decide between different designs?

How can engineers decide systematically between different designs? How can engineers do a concept evaluation and selection?

One method, called Pugh method, helps engineers in design decisions by establishing a procedure to choose the best design from the considered designs.  This method is also known as Decision-Matrix Method or Pugh Concept Selection.  There are variations of the method however I’m going to explain here how I use it.

Step 1:  Make a list of the criteria that you want to compare between different designs.  Each criterion should be an objectively quantifiable measure.

Criteria
Criterion 1
Criterion 2
Criterion M

 

Step 2: Establish weights factors for each criterion.  A number between 1 and 10 can be chosen for each criteria, the bigger the scale the more experienced you should be to impose the weights.  Other approach can be to distribute a number of points (e.g. 100) between all criterions.  This step can be challenging for novice engineers, one way to overcome this is to just classify them in a 3-point scale where 1 is important, 2 is very important and 3 is extremely important.  The last option is to omit the use of the weights; this would mean that all criterions are equally important.  Whatever weight approach you choose I have to warn that the design selected is influenced by the selection of the weights.  The last issue before passing to the next step is that the order matters when you use this method, establish the weights factor before any analysis is made!  Otherwise you will be unconsciously biased toward one design and assign weights that benefit the strong criterions of that particular design.

Criteria Weights
Criterion 1

3

Criterion 2

2

Criterion M

3

 

Step 3:  Generation of different designs.  The designs can be generated with Brainstorming, or TRIZ just to mention two examples.  However the way to generate the designs is not the focus here.  The number of designs to evaluate will depend on the complexity of the product being designed.  That being said I would advice not to do a Pugh matrix for just 2 designs, in practice something between 3 to 7 designs could be compared.  At first generate as many designs as possible but then filter them to a manageable quantity.

Criteria Weights Design 1 Design 2 Design N
Criterion 1

3

Criterion 2

2

Criterion M

3

 

 

Step 4: Analysis of designs.  This is the step were the classical engineering takes place.  You will quantify mass, energy lost, stress, flow, etc.  All the criterions will need an analysis to quantify it, thus those numbers will have units.

Criteria Weights Design 1 Design 2 Design N
Criterion 1 Analysis

3

#.## [Kg]

#.## [Kg]

#.## [Kg]

Criterion 2Analysis

2

#.## %

#.## %

#.## %

Criterion MAnalysis

3

#.## [MPa]

#.## [MPa]

#.## [MPa]

 

Step 5: Fill the matrix.  Now for each design a number has to be calculated to fill its criterion cell.

Criteria Weights Design 1 Design 2 Design N
Criterion 1

3

?

?

?

Criterion 2

2

?

?

?

Criterion M

3

?

?

?

Again, there is more than one way to do this.  A common way is to establish one of the designs as the Datum design, and compare the other designs criterion analysis numbers (from Step 4) against the Datum design.  A scale is established beforehand, a common one goes from -3 to 3.  If the design is better than the Datum it will get a positive number and the magnitude of the number depends on how much better it is.

After using this approach, I started to modify it in order to have a minimal number of decisions based on the designer assessment of the analysis numbers.  So instead of choosing a number between -3 and 3, I calculated one.  The procedure starts by calculating the average across designs for the criterions.   Then that average is subtracted to each design criterion and that is the number that is input into the decision matrix.

Criteria Weights Design 1 Design 2 Design N
Criterion 1

3

±#.##

±#.##

±#.##

Criterion 2

2

±#.##

±#.##

±#.##

Criterion M

3

±#.##

±#.##

±#.##

 

Step 6: Calculate each design score.  This is done by multiplying each criterion weight by the design cell value (±#.##) and summing all the values for the design.  This procedure is repeated for all designs.  Then the design with the higher score is the best design and the decision was made taken into consideration all of the criterions and designs in an objective manner.

Criteria Weights Design 1 Design 2 Design N
Criterion 1

3

±#.##

±#.##

±#.##

Criterion 2

2

±#.##

±#.##

±#.##

Criterion M

3

±#.##

±#.##

±#.##

Total:

#.##

#.##

#.##

 

 

Now that the steps are explained, we can go over a specific example.  Since a previous post already discussed Baja and Formula SAE Frame Design we are going to use a frame / chassis as the example for the Pugh Method (decision-matrix method).

 

Step 1: Make a list of the criteria that you want to compare between different designs.

  • Torsional Stiffness
  • Torsional Stiffness to Weight ratio
  • Frontal Impact (Max Stress)
  • Roll Over (Max Stress)
  • CG height
  • Weight

Step 2: Establish weight factors for each criterion.  In this case choose a number between 1 and 10.

Criteria Weight (1-10)
Torsional Stiffness 9
Torsional Stiffness to weight ratio 10
Frontal Impact 7
Roll Over 8
CG height 8

 

Step 3: Generate Different Designs.

 

Step 4: Analysis of designs.

Criteria Design 1 Design 2 Design 3 Design 4 Design 5 Design 6
Torsional Stiffness [lbf-deg] 857.81 1057.3 1128.5 1444.9 1009.26 1430.8
Torsional Stiffness to weight ratio 14.767 17.595 18.761 32.293 16.877 23.141
Frontal Impact [psi] 53,011 47,775 38,961 24,444 36,791 26,238
Roll Over [psi] 33,929 28,835 30,995 28,174 36,176 32,705
CG height [in.] 9.64 9.47 9.94 9.78 9.77 9.60

 

Step 5: Fill in the matrix.  In this case each criteria was averaged across designs.  Then each criteria average was subtracted from each design criterion.  This is known as to center the values.  See the example below.

Criteria Design 1 Design 2 Design 3 Design 4 Design 5 Design 6 Average
Torsional Stiffness [lbf-deg]

857.81

1,057.3

1,128.5

1,444.9

1,009.26

1,430.8

1,154.76 (Average of all designs TS)

= Criterion-Average

857.81-1,154.76 =

 -296.95

 

Then the procedure is repeated for the whole table.

Criteria Design 1 Design 2 Design 3 Design 4 Design 5 Design 6 Average
Torsional Stiffness [lbf-deg]

8,57.81

1,057.3

1,128.5

1,444.9

1,009.26

1,430.8

1,154.76

= Criterion-Average

-296.95

-97.46

-26.26

290.13

-145.50

276.04

Torsional Stiffness to weight ratio

14.767

17.595

18.761

32.293

16.877

23.141

20.57

= Criterion-Average

-5.80

-2.98

-1.81

11.72

-3.69

2.56

Frontal Impact [psi]

53,011

47,775

38,961

24,444

36,791

26238

37,870

= Criterion-Average

15,141

9,905

1,091

-13,426

-1,079

-11632

Roll Over [psi]

33,929

28,835

30,995

28,174

36,176

32705

31,802.33

= Criterion-Average

2,127

-2,967

-807

-3,628

4,374

902.67

CG height [in.]

9.64

9.47

9.94

9.78

9.77

9.6

9.7

= Criterion-Average

-0.06

-0.23

0.24

0.08

0.07

-0.1

The only problem now is that each criterion is on different scales, we want to  have all in the same scale.  This can be accomplished by dividing each centered value by the biggest value for that criterion.  The resulting table should look like this:

Criteria Weight Design 1 Design 2 Design 3 Design 4 Design 5 Design 6
Torsional Stiffness [lbf-deg]

9

-1.0234

-0.3359

-0.0905

1

-0.5014

0.9514

Torsional Stiffness to weight ratio

10

-0.4953

-0.2540

-0.1545

1

-0.3152

0.2191

Frontal Impact [psi]

7

1

0.6541

0.0720

-0.8867

-0.0712

-0.7682

Roll Over [psi]

8

0.4862

-0.6784

-0.1845

-0.8295

1

0.2063

CG height [in.]

8

-0.25

-0.9583

1

0.3333

0.2916

-0.4166

 

Step 6: Calculate each design score. See the example for Design 1

Criteria Weight Design 1
Torsional Stiffness [lbf-deg]

9

-1.0234

Torsional Stiffness to weight ratio

10

-0.4953

Frontal Impact [psi]

7

1

Roll Over [psi]

8

0.4862

CG height [in.]

8

-0.25

Totals

9 x (-1.02) +10 x ( -0.49) + 7 x 1 +8 x 0.48 + 8 x (-0.25) = -5.2744

 

This is the final Pugh Decision Matrix

Criteria Weight Design 1 Design 2 Design 3 Design 4 Design 5 Design 6
Torsional Stiffness [lbf-deg]

9

-1.0234

-0.3359

-0.0905

1

-0.5014

0.9514

Torsional Stiffness to weight ratio

10

-0.4953

-0.2540

-0.1545

1

-0.3152

0.2191

Frontal Impact [psi]

7

1

0.6541

0.0720

-0.8867

-0.0712

-0.7682

Roll Over [psi]

8

0.4862

-0.6784

-0.1845

-0.8295

1

0.2063

CG height [in.]

8

-0.25

-0.9583

1

0.3333

0.2916

-0.4166

Totals

-5.2744

-14.0784

4.6676

8.8228

2.1682

3.6942

 

Design 4 is the design that the decision matrix chose based in the analysis and weight factors.  With the specific procedure carried here, once the designer establish the criterion weights, all other numbers are calculated without need of the designer to interpret or assign ratings to the designs.

As was mentioned in the description of the general steps there are many variations to the Pugh method.  This is the version that I ended up using, after using it over the years for Formula SAE design decision-making.

Baja and Formula SAE Frame Design

The purpose of this post is to give an idea of how to design a tubular space frame for the Baja or Formula SAE competitions. This is the procedure that I have come up with after being involved in the design, construction and testing of frames for Formula SAE vehicles.

First, what is a frame? What is its role in the vehicle? The frame is a bracket that holds many systems of the car together. The frame also transmits the loads of the suspension! These two are the two most important general roles of the frame.

Where do you start? I have experienced myself through the years all the possible combinations: define suspension points and engine first, then design the frame and adapt systems to the frame design, to the other end where you let all your systems floating in space and design a frame around the systems. My conclusion so far is that you should try to design everything at once and iterate as much as possible. This is because the frame is another system of the car!

Where to start? Pencil and paper, with a sketch, many sketches. The idea at this stage is to generate as many designs as possible. In your sketch of the frame try to also incorporate other systems (e.g. engine).  When sketching first just draw the required rules members and then add the rest.  Also have in mind the manufacturability of the design (angles of notches and diameter of tube bends).  Once the sketches are generated look at them and start to combine the good parts of the sketches and leave the parts you don’t like. At the end choose at least 4 designs but no more than 8. Then decide what are going to be the metrics by which you will judge the design (e.g. weight, cg, torsional stiffness).

This leaves us with the task to model the frame in CAD software. It does not matter what software you are using these steps are generalized:

1. Make a hand drawn sketch with front, side and top view.

2. Identify all the nodes of the sketch and number them.

3. Make a table with the coordinates of all the nodes (at this point these will be rough numbers but the idea is to start, they can be changed later).

4. Now open your prefer CAD software.

5. Create all the points from the table in step 3.

6. Draw lines between points (for curve sections a center point of the arc is needed most of the time).

7. Then almost all software packages have a piping, frame or beam toolbox where you can select the beam cross section and apply it to the line.  This step can vary greatly between different CAD software, but the idea underneath is the same.

8. Most likely the beams are crossing each other at the nodes, thus usually the same toolbar where the cross sections were applied to the lines will have a mating or coupling section where you can specify the connectivity between them (which tube goes first and which one is notched).

9. Save.

Once you have the model go into assembly mode and start adding all the components even if they are not completely designed.  At this point the integration between systems starts an iteration process.  At the same time, the metrics by which the design of the chassis was going to be chosen now can be calculated.

Steps 4 through 8 are shown using PTC CREO 2.0


These post will always be evolving and if you have any suggestions to improve it feel free to comment below or send me an email JLugo{at}ND.edu. Thanks to Bob Kobayashi and Oliver Chmell for their suggestions.