Friction-free Precision, From Space To Watchmaking

In traditional joints, two or more elements meet to enable some type of mechanical movement. However, movable parts in contact with each other generate friction. "Friction is a big issue, and you cannot prevent it," explains Gilles Feusier, lecturer for EPFL space technology education programme and head of technology and science at EPFL Space Center. Feusier is co-chairing the ESMATS conference on Space Mechanisms and Tribology, organized by EPFL, CSEM and Almatech and held at the Swiss Convention Center from the 24^th to the 26^th of September.

Over time, wear creates debris and leads the mechanism to malfunction. This is why lubricant or replacement parts are essential. But this maintenance becomes a challenge when these mechanisms have to survive for more than a decade under extreme conditions in a remote location such as space.

Here is where compliant mechanisms, also known as flexible or flexure mechanisms, play a significant role in industries such as space or watchmaking. Their elastic properties enable high-precision motion while avoiding joints, hence removing friction. "Compliant mechanisms do not need lubricant and have no wear. This is a big advantage," says Feusier.

Despite their high precision, flexible mechanisms have limits: they cannot perform perfect straight lines or circles, nor produce unlimited rotations as their traditional counterparts can. "Flexures are limited to strokes of about 40 degrees before reaching their elastic limit. It's a challenge to go beyond that," says Simon Henein, associate professor at EPFL's Micromechanical and horological design laboratory (Instant-Lab). On the other hand, this limitation ensures longevity. As long as the rotation angle stays below a certain threshold, these mechanisms do not suffer from material fatigue, thus preserving internal material structure - theoretically allowing an infinite number of cycles. "These mechanisms are extremely reliable", says Henein.

Improving performance and broadening their use

A team of researchers led by Simon Henein and Florent Cosandier design new types of flexures that withstands higher loads and larger strokes. They also find ways to compensate for the restoring force these elements experience when displaced (the natural "snap-back" effect of elastic materials). "Unlike ideal joints, flexures tend to come back to their natural position. So we need to invent new ways to overcome this force," says Henein.

One approach leverages parasitic motion generated by the flexures' complex trajectories. This novel technique enables the application of high-precision compliant mechanisms to robotics and aerospace systems, such as the pointing mechanism for future telescopes and the positioning of medical gantries. The scientists also developed a new flexure-based oscillator with two degrees of freedom that shows a high robustness against external linear and angular accelerations, making it suitable for satellites in orbit. In addition, EPFL participation is key in big science experiments such as the Einstein telescope, a powerful European gravitational wave detector, currently under development. "The Einstein telescope will function at cryogenic temperatures and require extremely high motion resolutions. A perfect environment for compliant mechanisms," comments Henein.

Some of these advancements are often translated into patents that EPFL offers through different licensing opportunities. "We carry out our research independently, but when we have good ideas, we patent them," declares Henein. The EPFL's Technology Transfer Office (TTO) provides access to these patents, and eleven of them have already generated in the Instant-Lab for a total income of more than one million Swiss francs, with distributions to the inventors, the lab and the institution.

Building flexures

Flexible mechanisms can be made of a diversity of materials, including steel, titanium, and aluminum. Additive processes became a powerful technique to generate complex components without the need for assembly and complicated manufacturing. "Assembling parts means adding material and weight, which is always an issue in space. Additive manufacturing is an advantage because you can reduce the total payload mass," says Henein.

CSEM closely collaborates with EPFL in the development of new compliant mechanisms. There, engineers use several additives, manufacturing processes, including Laser Bed Powder Fusion (LPBF). In this technique, a layer of powder is deposited over a surface, and a laser heats and melts a given that will form the final metallic piece. "CSEM has extensive expertise in additive manufacturing of complex mechanisms of high precision, thanks to the multiple machines and parameter optimization strategies. The facilities have allowed the printing of watch mechanisms of accurate dimensions and surface finish, as well as high precision mechanisms for satellites", says Emmanuel Onillon, Business Leader Instrumentation at CSEM. "Some of these mechanisms need to be around 100 microns, as thin as a human hair. Achieving this thinness is still challenging. We have requests from the European Space Agency that require adapting our procedures."

From orbit to watches and medicine

Whether for scanning, pointing, calibrating, sampling, or stabilizing satellite instruments and payloads such as cameras and sensors, compliant mechanisms are ideally suited to the harsh conditions of space. Their intrinsic advantages include resistance to extreme temperature variations, robustness in dusty environments, and an "infinite life" design capability thanks to the absence of wear. Moreover, they can function as effective damping systems, absorbing vibrations in devices that demand extreme precision, such as inter-satellite laser communication systems or the mirrors of telescopes. "If we want to achieve very precise pointing, we must correct or filter out perturbations generated by rotating equipment inside the satellite," explains Fabrice Rottmeier, EPFL alumnus and CTO of Almatech, a Swiss company specialized in delivering critical hardware and engineering services for the space sector.

Yet the applications of compliant mechanisms extend far beyond space. "In the biomedical field, their main advantage is their monolithic design, which makes sterilization simpler and more efficient. This feature is equally beneficial in planetary exploration, where contamination must be strictly avoided," Rottmeier notes. In horology, the same technology is valued for its precision and long-term reliability, echoing its unique ability to move on from our wrists to cutting-edge instruments that reveal the secrets of our universe.