Sensitivity Studies

Every design has a goal, most have many. We always want a design to be less expensive, usually smaller, lighter, easier to use etc... But each design project includes more variables than you could ever hope to change. Which ones have the most impact on your design and which are negligible? Formal sensitivity analysis lets us quickly and efficiently tests the sensitivity of a design to all sorts of variables.

As a simple example consider the cantilevered spring. Classical mechanics without sensitivity analysis tells us that the stiffness of that spring increases much more quickly by changing its thickness than by changing width. Now imagine complex molded plastic parts. In addition to wall thickness, you've got ribs inside. You could change the number of ribs, their size, shape, location etc... Trying to figure out which of these will give the largest gain for the least amount of plastic would be next to impossible by hand. Sensitivity analysis studies can be used to see which of these variables increases the stiffness most quickly.

Another application of sensitivity analysis studies is to help understand the impact of tolerances. A few years back we designed a handheld medical device that detected sub-cranial bleeding in head injury patients. The product works by shining near infrared laser light into the head and measuring how much came back. In this system the alignment of the optics for sending and receiving the light was critical. We were able to use a multi variable sensitivity analysis study to quantify the amount of light lost due to tolerance variation within the parts. Using this information from our sensitivity analysis we were able to design a better system of aligning the optics and reduced light loss to below 1%.

At Bresslergroup we are always "looking for the biggest knob to turn," doing analysis on the variables in a design that allow us to achieve our goals with the least amount of cost. Instead of wasting time changing design parameters that yield small returns we concentrate on those aspects of the design with the greatest ability to affect the outcome. This is where sensitivity analysis really pays for itself.

Although this sounds obvious, sorting the powerful variables from the mundane is rarely as straight forward as one might hope. Systematic sensitivity analysis helps bring these issues into focus supporting the most efficient design process possible.