You've been assigned to a project to develop your company's latest wizbang widget, and one of your key responsibilities is to select the right sensors for the job. If you have experience with sensors, you probably feel comfortable with the product brochures—at least enough to know that there are devices available to sense the parameters required to make the widget work properly.
The Designer's Dilemma
Complications arise, however, when you have to make design trade-offs to accommodate the form factor of a component or limitations that weren't defined in the product literature. If you don't navigate this part of the development process carefully, your plan can unravel, derailed by delays and cost overruns.
Part of the difficulties stem from the fact that data sheets provide only part of the information you require to identify the most appropriate sensors for the application. These publications are designed to promote the product's features and benefits, and that allows a quick comparison with the competition. Nearly every sensor, however, has limitations, caveats, and implementation guidelines that complete the overall picture of the product. Unfortunately, these are not typically mentioned in the sales literature. So the question is: When do you bring the sensor vendor into the design process? To avoid some of the pitfalls that inevitably pop up, the best time is when you take your first look at the sensor requirements.
As you proceed, remember there is typically more than one technology option for sensing a physical parameter. For example, say you need to sense the position of a component rotating inside your widget. The space for the sensor is relatively small, which rules out laser triangulation sensors. Their electronics circuitry is incorporated into the sensor head, and there just is not enough room in the widget. Inductive and capacitive sensors both have probes connected to the electronics via small diameter cables, so as far as space is concerned, they would work. The target is metallic, again good for both technologies. The inductive sensor is a bit less expensive than the capacitive sensor. The target is ferrous steel, however, and it's rotating in front of the sensor. Without discussing your application with engineers from the inductive sensor company, you might not be aware that errors can arise with ferrous targets when the sensed surface translates in front of the inductive sensor. The error may be small, but without knowing what it is and including it in your error budget, your widget may not perform as specified.
Problems, Solutions, and Trade-offs
So what do you do? You call the capacitive sensor company and talk with them about your application. It looks like the sensor will work—that is until you tell them that your widget needs to work in an environment that routinely changes from low to high humidity levels. Now you're stuck with a trade-off. Do you redesign to make the target out of nonferrous metal and use the inductive sensor, or do you add a sensor to detect the change in humidity to correct the capacitance sensor's output. Either way, you lose precious time in your schedule, and as most project managers have learned the hard way, on schedule is the biggest contributor to on budget.
There is no sensor technology available that is perfect, no sensor that is totally error free. Knowing the sensor's operating guidelines, customization options, and associated costs up front will help avoid the cost and embarrassment of redesigning yourself out of a corner. There is always the possibility that, by spending time with the sensor vendor up front, you can find a sensor that results in a better-performing widget that comes in on budget and on schedule.
ABOUT THE AUTHOR
Dan Spohn is an applications engineer at Kaman Precision Products, a Division of Kaman Aerospace Corp. He can be reached at [email protected] or 719 635-6957.