Engineers need to wrestle with a wide variety of analytical and computational tools to solve complex engineering problems, such as designing a turbine blade that can withstand turbulent hot fluids, and designing a control system for an autonomous robot that can tread through a forest. But to solve most of the simpler engineering problems, a good understanding of basic principles of engineering and good judgment prove to be more useful than any sophisticated problem-solving tools.
There are many basic principles and maxims that engineers can use to solve simple problems. One such trick is to ask at least two questions. First, what is necessary to satisfy the requirement? Second, what is sufficient to satisfy the requirement? You will have a good, acceptable design if you incorporate answers to these two questions into your design. The questioning not only helps in designing but also in setting the specifications.
A necessary condition is the mandatory condition that the design should satisfy to meet the requirement. Examples of necessary conditions include: the rivet must support a shear stress of at least 150 MPa; the bottle must withstand a pressure of at least 80 psi; and the resistor must have a power rating of at least 1W.
A sufficient condition is the level of safety that's sufficient for the design. Examples of sufficient conditions include: the bridge design that can carry 100 tons of load must have a sufficient safety level; the container design that can withstand a 200C temperature must have a sufficient safety level; and the resistor with a 5W power rating must have a sufficient safety level.
If you observe closely, the relationship between necessary and sufficient conditions is "factor of safety," which is discussed in most engineering books. Necessary condition is obtained by applying basic engineering principles, and sufficient condition is obtained by multiplying the necessary condition with a factor of safety. It's often the engineer's experience and judgment that plays a key role in setting the numerical value for the factor of safety.
The questioning is also useful in setting the specifications for a system. For example, it's necessary for an information system to respond to a request for an approval of a credit transaction in three seconds and it's sufficient if it can respond in 0.5 seconds. The three-second necessary condition is based on the maximum time a customer will wait for the approval before losing his patience. The 0.5 second sufficient condition is based on the fastest computing system that can be used for the application without breaking the bank. Note that in this example, the sufficient condition is obtained by dividing (instead of multiplying) the necessary condition by the factor of safety.
The same principle can be applied in your personal life. Before going on a road trip for a long weekend, ask yourself: How much cash do I need to cover all the tolls, tips, and parking meters, and how much cash is needed to provide a level of safety that's sufficient?
ó Raghavendra Angara, PhD, is a senior mechatronics R&D engineer. He belongs to the ASME, ISA, IEEE, and ASQ.