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).
