In brief

With their ability to twist, rotate and work at different joint stiffness settings, new variable stiffness actuators promise to bring man-robot collaborations into a safer reality.

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Safety first when man meets robot

14 Oct 2021

A fresh design approach to high-performing variable-stiffness rotating joints could enable humans to work safely alongside robots in industrial manufacturing environments.

Robotic automation offers pronounced advantages over human manpower in industrial settings. After all, robots can work tirelessly and perform difficult, repetitive tasks faster and more accurately. Nonetheless, it’s unlikely that robots will ever completely replace humans. Instead, optimal productivity would require man and machine to work synergistically in shared workspaces.

For now, true human-robot collaborations remain out of reach, with safety being a major concern. In industrial robots, the key components for achieving high-velocity and high-payload performance are the motors called actuators that move their joints. However, these actuators are often rigid and unable to adapt to sudden environmental changes; one wrong step, and they could easily collide with or crush a human co-worker.

Variable stiffness actuators (VSAs) offer a potential solution to this safety concern. VSAs contain elastic elements that enable a robot to work in a ‘safety mode’ when alongside humans, and switch to higher stiffness modes when performing more demanding robot-only applications.

“Without sacrificing payload and precision, VSAs can balance flexibility and rigidity to suit different application requirements,” explained Wei Lin, a Senior Scientist at A*STAR’s Singapore Institute of Manufacturing Technology (SIMTech). To push the limits of human-robot cooperation, Lin, his SIMTech colleague Haiyue Zhu and a team of robotics experts set out to create the next generation of VSA systems capable of more complex maneuvers and featuring enhanced safety profiles.

In their study, the team explored novel approaches to creating VSA-based robot joints that could twist and rotate, with the flexibility of working at different joint-stiffness settings. They adopted an internal spring mechanism that uses thin, flexible parts called rotary flexure hinges to connect the input shaft—the central rod that delivers power to the device—to the output frame.

With a variable stiffness mechanism based on four rotary flexure hinges at opposite orientations, the robotic joint’s stiffness can be adjusted with low inertia and friction.

© A*STAR Research

Until now, conventional single-spring mechanisms have been notoriously unreliable, with unbalanced twists between the input shaft and output frame during joint rotation leading to unpredictable positioning errors. To overcome this challenge in their new design, the researchers first modeled changes in actuator stiffness as the flexure hinge rotates. Based on these data, they compared the performance of six distinct flexure hinge configurations.

The optimal joint design, containing four flexure hinges at opposite orientations, could rotate freely about its axis without displaying any unwanted contortions. A prototype robot built by the team was found to be dynamic and adaptable, outperforming current VSAs by continually and rapidly modifying its joint stiffness. According to Zhu, their prototype reached to adjust from the lowest stiffness to the maximum stiffness in just 0.83 seconds, with a stiffness range that is designable and could easily be adapted to suit different industrial applications.

These results form a stepping stone in the researchers’ pursuit of next-generation precision collaborative robots. Musing on a future where such man-machine partnerships become a reality, Zhu said: “As a result, humans and robots will be able to work together and share a workspace to improve the adaptability, flexibility and efficiency for adaptive manufacturing.”

The A*STAR-affiliated researchers contributing to this research are from the Singapore Institute of Manufacturing Technology (SIMTech).

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Li, X., Zhu, H., Lin, W., Chen, W., Low, K.H. Structure-controlled variable stiffness robotic joint based on multiple rotary flexure hinges. IEEE Transactions on Industrial Electronics (2021) | article

About the Researchers

Wei Lin received a B.Sc. (Eng.) degree from University College London, U.K., followed by M.Sc. and Ph.D. degrees in mechanical engineering from the University of Florida, USA. He is a Senior Scientist in the Adaptive Robotics and Mechatronics Group at A*STAR’s Singapore Institute of Manufacturing Technology (SIMTech), and an Adjunct Research Associate Professor in the Department of Mechanical Engineering at the National University of Singapore. He is also currently the Academic Chair of the Singapore Mirror Committee of ISO/TC 299 on Robotics. His research focuses on robotics technologies for enabling the automation of challenging manufacturing processes.
Haiyue Zhu received a B.Eng. degree in automation from the School of Electrical Engineering and Automation and a B.Mgt. degree in business administration from the College of Management and Economics, Tianjin University, China. He subsequently received M.Sc. and Ph.D. degrees in electrical engineering from the National University of Singapore. He is currently a Scientist with the Adaptive Robotics and Mechatronics Group at A*STAR’s Singapore Institute of Manufacturing Technology (SIMTech). His current research interests include intelligent mechatronic and robotic systems, computer vision, robot grasping and manipulation.

This article was made for A*STAR Research by Wildtype Media Group