These are generated once the brief has been understood and the customer requirements are defined. They do not specify a solution. They’re both important, but discreetly different. Recognise the difference between them.
Constraints – are limits on design freedom (Dieter and Schmidt 2009: 92). They form a ‘checklist’ (Dym et al 2009: 71). They define where the design should start and stop by either adhering to it or not; for example a ladder should have a step deflection of 1.25mm or not (Dym et al 2009: 50), either it meets the Constraint or not. They may be a specific size limit determined by factors beyond the control of design (consider the limits on aircraft baggage Figure 2). Constraints may be physical limits or standards such as British Standards, ISO or governmental requirements. In Figure 1, the design is guided down this ‘tunnel’ or boundary lines, the Constraints define the boundary line limits.
If the brief specifies;
A value or a number – it is a Constraint
The design must contain a certain requirement – it is a Constraint
The design should contain a certain requirement – it is a Constraint
Figure 1 – Constraints ‘Tunnel’ defining boundaries.
Figure 2 – Constraints imposed on aircraft baggage.
Criteria – are desirable characteristics that would be preferable in the final design. General requirements that are guidance rather than specific. These could be; colour, indicate an area within competitor’s price ranges, general use of materials, general safety. These are variables by which a successful design maybe identified (Cross 2008: 9) but allow a design to explore outside the limits of the Constraints (Dym et al 2009: 50). They are generally open-ended.
The brief for the Aluminium can crusher example contains a number of Constraints and Criteria. For example the requirement “Performance based on cans crushed per minute”, contains no significant information that could be measured against, therefore is a Criteria. The words highlighted signify the requirement is a Constraint.
“The device must be fully automatic (Constraint). It should crush the can (Constraint). The following are required;
• The device must have a continuous feeding mechanism. (Constraint)
• Cans should be in good condition without dents. (Constraint)
• Can must be crushed to 1/5th of it’s volume. (Constraint)
• Max dimensions of entire device should be < 450 x 450 x 300 mm. (Constraint)
• Performance based on cans crushed per minute. (Criteria)
• Device must be standalone. (Constraint)
• Should be operable by children aged 8+. (Constraint)
• Final design should retail <£200.” (Constraint)
Haik and Shahin (2011: 110)
| Constraints may also take the form of; Use of OEM parts and standard ‘off-shelf’ components. Legal governing standards (BS, ISO, TUV).Statement such as; – not weigh more than 1000N. – pass through 1m x 1m hatch. – cost less than £500. – conform to U.S. anti-pollution laws. |
Criteria may also take the form of; Indicate general, unmeasurable characteristics within desired market.Statement such as; – should be safe. – should be light. – should be comfortable to use. – should be cheap. |
Summary
The final product will be measured against the original Constraints, (Cross 2008: 8 ) should it not meet them, it will considered a failure and you will simply not get paid!
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