Consider the sentence "The car must be light, safe, and fuel-efficient." Here, three PCs are given for the product's weight, safety, and efficiency.
PCs describe what a product ought to be, but not what the product ought to do. As such, PCs can almost always be described using adjectives and adverbs. PCs are never phrased as verbs (that what a functional requirement is for).
At first glance, PCs might seem almost vacuous, contributing almost nothing to the study of the design problem. However, this is not really the case. A common source of error in design is that the designers will forget to consider the impact of one or more PCs in their work. It is human nature: a designer will tend to focus on issues that are of particular importance to him, and expect someone else to take care of other issues. Sometimes, some issues will "fall through the cracks" resulting in a poor design.
Determining and cataloguing the PCs early in a design process will help the designers to be mindful of the various issues and the interplay between them.
A PC of a product is not necessarily a PC of every part of the product. For example, a car battery is a relatively safe product, but the acid inside the battery is quite dangerous.
Some examples of good PC statements include the following.
- The elevator must be safe.
- The blue car must be durable.
- The lawnmower must be quiet.
- The lawnmower must be assembled easily.
Here are some examples of bad PCs.
- The car must be less than 1000 kg in weight.
This is really a constraint because it sets a hard limit on the product.
- The elevator must be able to carry passengers.
This is really a FR because it actually describes something the elevator must do, not be — even though the sentence includes the phrase "...must be...." Indeed, this is clumsy english; it should have been phrased as The elevator must carry passengers. A useful rule for this is: Avoid generic verbs like ...be able to.... Try to rewrite the statement as a Functional requirement. Choose the way that rings the most true.
- The robot must be a model XYZ gantry robot.
This is a bad PC because it explicitly states the structure of the product.
Most designers quickly learn that there are certain PCs that are always relevant. In fact, one can create a list of PCs and check off which PCs are needed for a particular problem. This is a very useful tool for designers because it helps ensure that no important PCs have been forgotten.
A product characteristics list is available. These are the most general PCs, which apply to nearly every product. In a practical case, one would begin with the PCs in this list, but then refine them. For example, one might start with "The antenna must be durable." How can an antenna be durable? One way is by being stiff; another way is by being properly stowed when not in use. Both these are PCs that explain the PC of durability.
Every PC will impact on every basic property of a product. The basic properties of a product (from the point of view of mechanical engineering) are:
- size,
- shape,
- mass & inertia,
- material, and
- surface finish (including colour).
For any PC, you should ask yourself, how will the basic properties of the product let the product exhibit this characteristic? Of course, you may not know the answer yet (e.g. how can you know how a PC affects a product's shape if you have not yet designed the product?). Still, it is important to settle in your mind what the issues are surrounding the PCs of a product because it will quicken your response time to specific design tasks in later stages of product development.
