Working Out the Bugs
Fri, 05/08/2009 - 12:19pm
Among the most important factor in medical device design is timely launch to market of a quality product. Aiding designers is an extremely useful processDesign Failure Mode Effects Analysis (DFMEA). DFMEA enables design teams to review their products, down to the component level, to ensure that all elements are working properly and within necessary parameters.
Paul Nickelsberg, president and CTO of Orchid Technologies Engineering and Consulting, Inc., has over 20 years experience in electronic product design and development. He can be reached at firstname.lastname@example.org.
Medical products must meet a high standard of efficacy and safety. One of the most effective tools in the designer's bag of tricks is the Design Failure Mode Effects Analysis (DFMEA). This type of analysis was first introduced into the design of military equipment over sixty years ago. Since then, it has been effectively applied in commercial automotive design, chemical process control design, and medical equipment product design.
Overview of DFMEA
During the design of a forced hot air patient warming medical productintended to keep unconscious patients warm during surgerythe team used the DFMEA to identify industrial, mechanical, electrical, software, and documentation design risks.
In the implementation of DFMEA, procedures are a team effort. Product marketing, clinical testing, development engineering, and quality assurance departments, as well as legal and technical regulations, provide input into the process. DFMEA is a meeting activity; it evolves with the product. Numerous revisions are required to obtain its full benefit. Copious revisions are required to reach the full DFMEA process. The DFMEA should include all Bill of Materials (BOM), components, subsystems, and systems in the product design. Components and samples supplied by the customer, as well as customer provided drawings, simulations, and animations are part of the input.
Where to Begin
For example, for a patient warming device, one might identify the possibility of burning a patient's skin as a risk. That risk could be assigned a high-level severity. Another risk that the team might identify is the lack of therapysomething might prevent the product from working. Depending upon the known usage model for the product, one might imagine that "lack of therapy" carries a lower risk severity than risk of burns. The team then assigns a less severe risk value to the risk category. In this way, all of an organization's departments that have product responsibility are represented and have input into the risk severity definition process. Strong team leadership is essential to keep the process on track.
The ProcessThe DFMEA should be performed down to the component levela rigorous task. Every resistor, capacitor, and semiconductor device is analyzed for its contribution to product safety. Components, including transistors and diodes, are reviewed for open, short, and wrong values. Digital semiconductor components are reviewed on a pin-by-pin basis for stuck at 1, stuck at 0, open, and short conditions. More rigorous DFMEA might add other factors to the component failure matrix. Less rigorous DFMEA might subtract factors considering failure in a more general manner.
With risks identified and severity assigned, the development team can proceed with the DFMEA process. Failure modes may now be identified. Potential failure modes are the consequences of the failure cause. One failure mode may cause a number of risks. Often the development team selects the highest risk severity for the failure mode identified.
One must review possible product failures. One such failure might be in the heating elements. For example, a design team may identify heating element failure and assign a numerical probability value. All such identified failure types are then assigned probability values. A design team may now understand its RPN for each failure-made: risk severity pair.
The last factor to be taken into consideration by the team is failure risk probability. The team considers if a particular failure may be detected. Again assign numbers.
ConclusionAs one can imagine, a component level DFMEA results in a very large matrix. Using a spreadsheet program, this large matrix can be managed and shared between all concerned parties. A single component level DFMEA can have thousands of individual risk entries. As the analysis proceeds, patterns emerge. Product designers look for these patterns. The DFMEA can be used to ferret out design weaknesses.
Early in the electronic product design cycle, the DFMEA often picks up a number of design weaknesses. These deficiencies are recorded and the design is iterated. Another DFMEA is performed on the iterated design. Fewer deficiencies are identified and improving detectability reduces others. After a few design updates and DFMEA iterations, the design team reaches a high degree of confidence that the particular electronic design has a failure mode effect, which is ALARP. It is a good feeling to know the team's design has gone through a rigorous development process.
OnlineFor additional information on the technologies and products discussed in this article, see MDT online at www.mdtmag.com or Orchid Technologies at www.orchid-tech.com.