Audio ICs

Improved design could be game-changer for MEMS actuators

04 September 2019
Mechanical engineering Prof. Ronald N. Miles, whose recent research has been focused on building actuators for microphones. Source: Binghamton University, State University of New York.

In a study that could have wide-reaching implications for common electronic devices, researchers at Binghamton University, State University of New York, have reported an improved method for using the actuators that control microelectromechanical systems (MEMS) — microscopic devices with moving parts that are often produced in the same way as electronics.

The Binghamton team combined two methods for electrostatic actuation, parallel-plate and levitation, to achieve a predictable linearity that neither system offered on its own. The researchers didn’t realize that the combination would provide such a desirable outcome — what mechanical engineering professor Ronald N. Miles, who served as co-principal investigator for the study, called “dumb luck.”

The findings could be revolutionary for the manufacture of microphones, allowing a signal to be boosted past the point of electronics noise interference. More than 2 billion microphones are made around the world each year — a number that is growing as more devices featuring vocal interaction are produced.

"The electronic noise is really hard to get rid of," said Miles, whose recent research has been focused on building actuators for microphones. "You hear this hiss in the background. When you make really small microphones — which is what we want to do — the noise is a bigger and bigger issue."

Miles explained that the nature of actuators makes it challenging to achieve predictability. "In a sensor, life is much easier if it moves one unit and the output voltage increases in one unit, or something in proportion as you go," he said. "In an actuator, you're trying to push things, so if you're giving it twice as much voltage, you want it to go twice as far and not four times as far.

"It's like if you had a ruler where the inches varied in length as you moved up. With capacitive sensors, you have these strange variations with sensitivity and output as you move up the scale. That's a massive headache."

Both Miles and Mark Pallay, a Ph.D. student, completed their investigation under the supervision of mechanical engineering professor Shahrzad Towfighian, who has studied MEMS extensively. Towfighian explained that actuators in the micro-devices are normally just two plates with a gap between; when a certain voltage is received, the plates close and the device activates. This type of actuator is difficult to fine-tune, but adding electrodes on either side of the plates creates a levitation effect that allows better control.

"(By) combining the two systems, we can get rid of nonlinearity," she said. "If you give it some voltage, it stands at some distance and maintains that over a large range of motion."

“The nonlinearities cancel each other out," added Miles. He explained that having both electrode configurations allows more control over the design. “It gives you more knobs to turn and more adjustments you can make with applying voltages,” he said.

In addition to opening the possibility of smaller, better and cheaper microphones, Towfighian noted that the new actuator design could be used in gyroscopes, accelerometers, pressure sensors and other kinds of switches. "We showed this concept at a basic level, but it has wide applications," she said. "It can improve the function of many devices, so the impact could be huge."

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