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Tiny switches could lead to better battery life

BYU professor eliminates “stick” to make switches quick

Suppose you were told to keep a light switch from sticking. No problem, right? A little lubricant in the right spot and you’d be good to go.

But what if that switch was roughly the size of a human hair and had to move back and forth more than 1,000 times a second?

The solution, of course, isn’t fast, tiny fingers; it’s the engineering know-how of Brigham Young University professor Brian Jensen, whose work with tiny electrical switches (categorized as micro-electro mechanical systems, or MEMS) may one day extend the battery life in devices like your cell phone and laptop computer.

“A popular question right now is, ‘Are we at the frontier of battery technology?’” said Jensen, an assistant professor of mechanical engineering. “Battery life is what it is at the moment, and if we can find ways for these devices to lose less power, we can make them last longer.”

Jensen’s research, published in the June 15 issue of the “Journal of Applied Physics,” focuses on overcoming the limitations of MEMS switches, bringing us one step closer to better battery performance. MEMS switches are made of two miniscule pieces of metal, each roughly the size of a human hair, that are pushed together, allowing a signal to flow between them.

“They would mostly be used in communications devices, like a cell phone, where you’d want them to be able to switch a signal back and forth between different parts of a circuit very rapidly,” said Jensen, explaining that cell phones use one antenna to both transmit and receive signals. “A system that used MEMS switches could feasibly route those signals with lower power loss and reduce the number of electrical components needed to make the phone work.”

Not only do the tiny switches hold the promise of smaller cell phones with longer battery life, but they could also help improve communications on military aircraft.

“A reconfigurable antenna, a single antenna that can be made to operate in multiple frequency bands, is a relatively new idea that’s only possible using switches,” said Jensen, who earned his doctorate from the University of Michigan and worked as a micro mechanism designer at Sandia National Laboratories.

Where a cell phone operates at one frequency band, a fighter jet has to be able to communicate on multiple bands. MEMS switches could turn on and off different parts of an antenna, allowing it to send and receive at multiple frequencies. Fewer antennas would save money, weight and space on the aircraft, said Jensen. The improved antenna would also help to keep communications secure. If one frequency is being jammed, the antenna can be converted to operate at a different frequency, keeping communication lines open.

Of course, to be able to turn signals on and off while still retaining vital information, switches must move incredibly fast.

“Sometimes when the switches close and current runs through them, they stick,” said Jensen. “Some we’ve tested take almost a full second to open, and that’s just not good enough.”

Enter experiments by Jensen and researchers at the University of Michigan to characterize what affects the opening and closing time of MEMS switches with the goal of designing them to always operate quickly.

The researchers quantified just how hard switches need to be pulled apart to keep them from sticking. Additionally, they produced data showing that the smaller the contact area between the metal pieces that comprise the switch, the less it sticks.

“Common sense would tell you to make that area as big as you can to get more signal across it,” said Jensen. “But the smaller that area, the less the switch sticks. It also turns out that the size of the contact area doesn’t have any effect on signal resistance.”

Finally, the team developed a mathematical model to predict a switch’s “sweet spot” – that point at which electrical resistance is optimized to allow maximum signal flow and minimal stick.

Lior Kogut, a senior engineer with Qualcomm MEMS Technologies said, “The recent study by Dr. Jensen tackles one of the most critical issues in the emerging area of miniaturized electrical switches. In his novel study, Dr. Jensen developed a neat approach to measure the response time of microswitches. This is done in a very accurate, inexpensive and non-destructive means. Furthermore, Dr. Jensen developed an advanced model to interpret his experimental results and to provide insight into this complicated multi-physics problem.”

Joining Jensen on the study are Kuangwei Huang, Linda Chow and Katsuo Kurabayashi of the Mechanical Engineering Department at the University of Michigan. The research was supported by the National Science Foundation, a National Defense Science and Engineering Fellowship and the Central Intelligence Agency.

Photo of a microswitch - one piece of metal overlaps and crosses another. A small dimple in the top piece makes contact with the lower piece allowing current to flow.

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