How mighty MEMS move and break

Editorial

Rebecca Pool

Tuesday, December 4, 2018 - 12:30
Image: Still from an incredible video that tracks exactly how MEMS move (see video below).
 
Researchers at the National Institute of Standards and Technology have unveiled an incredible video of a moving microelectromechanical motor rotating a gear at nanometre and microradian, and millisecond scales.
 
Captured using a custom particle-tracking method, the video forms part of NIST's investigations to understand how microelectromechanical systems - MEMS - operate, wear and break.
 
By using such microscopic failure analysis, researchers hope to improve the reliability of MEMS components used in miniature drones, forceps for eye surgery, sensors and more.
 
Over the past decade, NIST researchers have measured the motion and interactions between MEMS components with the latest measurements being made on the scale of thousandths of a second.
 
Operating at these blistering fast time scales allowed Samuel Stavis, Craig Copeland, and colleagues, to resolve fine details of the transient and erratic motions that can take place before and during MEMS failure.
 
What's more, the measurements also allowed repetitive testing, necessary for assessing the durability of the miniature mechanical systems, to be conducted more quickly. 
 
As part of the system set-up, the researchers labelled the MEMS torsion frame with fluorescent particles to track motion.
 
Using brightfield optical microscopy and CMOS cameras to view and image the light-emitting particles, the researchers tracked displacements as small as a few billionths of a metre and rotations as tiny as several millionths of a radian. 
 
"If you cannot measure how the components of a MEMS move at the relevant length and time scales, then it is difficult to understand how they work and how to improve them," highlights Copeland.
 
This video shows real images of a microscopic gear and actuator in a MEMS (microelectromechanical system) device. A tiny actuator moves back and forth in a ratcheting motion that drives the rotation of a microscopic ring gear. [Jennifer Lauren Lee/NIST. Music credit: Kevin MacLeod].
 
According to the researchers, they calibrated their optical microscope to ensure accurate localization, eliminating errors due to non-uniform magnification.
 
They also corrected errors due to rotation of the torsion frame out of the imaging plane.
 
As Copeland points out, they also traded-off spatial range for temporal resolution by decreasing the readout height of their CMOS camera to 128 pixels, and increasing the readout rate to 1 kHz.
 
In their test system, the researchers tested part of a microelectromechanical motor, with the test part snapping back and forth, rotating a gear through a ratchet mechanism.
 
The team found that the jostling of contacting parts in the system, and wear of the contacting surfaces, could all play a key role in the durability of MEMS.
 
"We tested a torsional ratcheting actuator and observed dynamic behaviour ranging from nearly perfect repeatability, to transient feedback and stiction, to terminal failure," says Stavis. "This
new measurement capability will help to understand and improve MEMS motion."
 
"Our tracking method is broadly applicable to study the motion of microsystems, and we continue to advance it," he adds.
 
 
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