From the printing press to the jet engine, mechanical machines with moving parts have been the basis of technology for several centuries. With the U.S. industry developing smaller mechanical systems, they experience greater challenges - microscopic parts tend to stick together and wear out when they come in touch with each other.
Researchers at the National Institute of Standards and Technology (NIST) are going back to basics by cautiously measuring how parts shift and interact in order to enhance the performance of microscopic mechanical (micromechanical) systems for advanced technologies.
The NIST researchers have measured the transfer of motion through the contacting parts of a microelectromechanical system at microradian and nanometer scales. Their test system was made up of a two-part linkage, with the motion of one link driving the other. The researchers resolved the motion with record precision and also analyzed its reliability and performance.
This research could influence the operation and fabrication of a wide variety of micromechanical systems, including manufacturing platforms, robotic insects and safety switches.
The motion of micromechanical systems is at times extremely small - displacements of just a few nanometers, or one billionth of a meter, with equally small rotations of a few microradians - for the current methods to resolve. One microradian is the angle corresponding to an arc’s length, which is about 10 meters along the earth’s circumference.
“There has been a gap between fabrication technology and motion metrology—the processes exist to manufacture complex mechanical systems with microscopic parts, but the performance and reliability of these systems depends on motion that has been difficult to measure. We are closing that gap,” said Samuel Stavis, a project leader at NIST.
Despite how simple this system appears, no one had measured how it moves at the length and angle scales that we investigated. Before commercial manufacturers can optimize the design of more complex systems such as microscopic switches or motors, it is helpful to understand how relatively simple systems operate under various conditions.
Craig Copeland, Researcher, NIST
In Microsystems & Nanoengineering, the researchers present measurements that rely on optical microscopy to trace surface features on the parts that move. It is possible for the manufacturer to build in the surface features during the fabrication process, allowing the system to be ready for measurement right out of the foundry. The researchers could also apply fluorescent nanoparticles to the system following the fabrication process in order to enhance precision.
This measurement method was introduced by the NIST researchers in an earlier study, and they also used corresponding methods to trace the motion and interaction of various other small systems. The researchers were able to study the details of the interaction as they were able to trace the motion of several parts in a micromechanical system.
As part of their experiment, the team analyzed the transfer of motion through a mechanical linkage, which is a system comprising of parts linked in order to monitor forces and movement in machines. The test system comprised of two links that connected and disconnected through a joint, which is considered to be the point at which the links apply forces to each other.
One link’s thermal expansion and electrical heating drove the rotation of the other link around a pivot. The team created a model to illustrate the movement of the system under ideal working conditions. This model was then used by the researchers to comprehend their measurements of how the system shifted under practical working conditions.
The researchers discovered that play in the joint existing between the links, which is essential to permit fabrication tolerances and prevent jamming of the parts, had a crucial role in the system’s motion. The amount of play was considered to be a significant factor in accurately determining how the links coupled and uncoupled, and also how repeatable this motion transfer could actually be.
The system functioned well as long as the electrical input driving the system was comparatively free of noise. The motion was transferred from one part to another part in a continuous manner for thousands of operating cycles.
It was perfectly repeatable within measurement uncertainty, and reasonably consistent with our ideal model.
Craig Copeland, Researcher, NIST
Copeland highlights that this is important as a few researchers expect that the friction existing between small parts would hinder the reliability and performance of such a system. A number of engineers have even given up the idea of developing micromechanical systems from moving parts that make contact, and instead they have now turned to micromechanical systems comprising of parts that shift by flexing in order to prevent the parts from coming in close contact with each other.
The findings suggest that micromechanical systems that transmit motion through contacting parts “may have underexplored applications,” said Stavis.
The team discovered that the addition of a regular amount of electrical noise to the driving mechanism resulted in the system becoming less reliable, and the system failed at times in sending motion from one link to the other. Moreover, the system’s exposure to atmospheric humidity for many weeks resulted in the parts sticking together even though it was possible for the team to break them loose and make them move again.
Currently, the researchers plan to enhance their measurements and focus on more complicated systems made up of several moving parts.
Micromechanical systems have a number of potential commercial applications. We think that innovative measurements will help to realize that potential.
Samuel Stavis, Project Leader, NIST
This project was a joint effort between researchers in NIST’s Center for Nanoscale Science and Technology (CNST), NIST’s Physical Measurement Laboratory (PML) and the University of Maryland as part of the NIST Innovations in Measurement Science program.
(Top) A microelectromechanical linkage converts translation (straight arrow) into rotation (curved arrow). The red box indicates the region of the rotating part that has fluorescent nanoparticles on it. (Bottom) Video showing the fluorescent nanoparticles on the rotating part of the linkage. Tracking the nanoparticles enables tests of the performance and reliability of the system. Credit NIST