From surgical robots and rescue devices to wearable systems and dexterous robotic hands, many robots must transmit motion through long, flexible pathways rather than through rigid linkages. Tendon-sheath mechanisms are attractive for these tasks because they allow motors to stay away from confined working areas while still driving distal end-effectors.
But that flexibility comes at a cost. As the tendon slides inside the sheath, friction causes tendon elongation and backlash-like hysteresis, meaning the motion commanded at the input does not perfectly match the motion delivered at the output. The problem becomes even harder when the transmission route bends arbitrarily or changes over time.
Researchers have proposed several ways to address this challenge, including adaptive controllers that rely on distal feedback and feedforward methods based on pre-identified hysteresis models. Yet distal sensing is often difficult or impossible to install in compact robotic end-effectors, and pre-identified models can lose accuracy once the transmission route changes. These limitations raise a practical question: can a control strategy remain accurate without distal sensing while still adapting to arbitrary routing?
Answering that question requires route sensing. Existing options such as optical tracking, electromagnetic tracking, and fiber Bragg grating sensing can capture rich route information, but they are often costly, sensitive to environmental interference, or dependent on bulky signal-processing units. For mobile or space-constrained robotic systems, these requirements can be a serious barrier to deployment.
Scientists therefore asked whether a simpler signal might be enough. Instead of trying to reconstruct the full route distribution everywhere along a tendon-sheath mechanism, they investigated whether the accumulated curve angle—a more compact and easier-to-measure quantity—could capture the most important route-dependent effects for position compensation.
To explore this idea, the researchers systematically reviewed existing position transmission control strategies and route-sensing techniques, and then carried out targeted experiments on arbitrarily routed tendon-sheath configurations. Their study, published in SmartBot , also used these findings to guide the design of an integrated control framework aimed at real-world robotic deployment.
Using a dedicated tendon-elongation measurement platform, the team compared elongation estimation based on full route information from an FBG sensor with estimation based only on accumulated curve angle across four different route configurations. They found that although the richer FBG measurements delivered the best accuracy, the accumulated curve angle alone still achieved practically comparable performance, with only limited loss in estimation quality. “Our results suggest that, for position transmission control, friction-induced elongation is governed much more strongly by the accumulated curve angle than by the detailed route distribution,” says Prof Gao.
Based on that insight, the scientists proposed a comprehensive feedforward control framework that combines route sensing, online input tension measurement, input displacement measurement, backlash or phase identification, and tendon-elongation compensation. They also outlined a more integration-friendly sensing scheme, including wireless signal transmission, a compact cable displacement sensor, and the replacement of the traditional sensing tendon with a Ni-Ti rod to improve measurement reliability under arbitrary bends.
Together, these advances point to a practical strategy for robots that must remain both compact and adaptable, including flexible surgical robots, rescue robots, wearable devices, musculoskeletal robots, and other systems that operate in narrow or tortuous environments. By reducing reliance on distal sensors and bulky route-sensing hardware, the approach could make accurate position transmission control more feasible outside highly specialized laboratory settings.
The authors believe the work provides both a technical roadmap and a new way of thinking about route-dependent control in tendon-sheath mechanisms. Rather than insisting on complete route reconstruction in every application, future systems may be able to rely on carefully chosen route-related signals that are easier to sense and integrate. “We hope this study helps move tendon-sheath control from elegant theory toward robust, deployable robotic systems,” says Prof Gao.
Abou t Nanjing University of Aeronautics and Astronautics, China
Nanjing University of Aeronautics and Astronautics is one of China’s leading research universities, committed to advancing science and technology while serving national strategic needs through education, innovation, and interdisciplinary research. Established in 1952, the university has developed into a key institution with strong strengths in aerospace engineering, automation, mechanical engineering, information science, and related fields. NAUAA integrates fundamental research with engineering applications, supporting cutting-edge developments in areas such as aerospace systems, intelligent manufacturing, robotics, and advanced materials. With a comprehensive academic structure spanning engineering, natural sciences, management, and the humanities, the university fosters collaboration across disciplines and research platforms. Through its dedication to academic excellence, technological innovation, and international cooperation, Nanjing University of Aeronautics and Astronautics continues to contribute to scientific progress and the cultivation of high-level talent.
Website: http://www.nuaa.edu.cn
About Johns Hopkins University, USA
As one of the world’s leading research universities, Johns Hopkins University is dedicated to advancing knowledge and addressing society’s most pressing challenges through innovation, discovery, and education. Founded in 1876 as the first research university in the United States, Johns Hopkins integrates rigorous scholarship with real-world impact across a broad spectrum of disciplines. The university offers comprehensive academic programs spanning medicine, public health, engineering, international studies, the natural and social sciences, and the humanities. Its interdisciplinary structure fosters collaboration among its schools and research centers, enabling groundbreaking discoveries and transformative educational experiences. By combining academic excellence with a strong commitment to service and global engagement, Johns Hopkins University continues to shape the future through research, leadership, and innovation.
Website: https://www.jhu.edu/
About Jiaqi Li from Nanjing University of Aeronautics and Astronautics, China
Jiaqi Li received his bachelor’s degree in Electrical Engineering and Automation from Nanjing Normal University in 2023, and was also awarded a bachelor’s degree in Electrical Engineering and Automation from the University of Northumbria at Newcastle, UK. He is currently a master’s student (Class of 2024) in the program of Weapon Science and Technology at the College of Automation Engineering, Nanjing University of Aeronautics and Astronautics. His research interests include tendon-sheath–driven exoskeleton robots and aerospace vehicle system design.
About Prof. Qian Gao from Johns Hopkins University, USA
Prof. Gao received the Ph.D. degree in Computer and Information Engineering from The Chinese University of Hong Kong, Shenzhen, China, in 2024. In 2019, he was a Visiting Scholar with the Department of Mechanical and Automation Engineering at The Chinese University of Hong Kong, Hong Kong, where he conducted research on the da Vinci surgical system. He is currently a Postdoctoral Fellow with the Laboratory for Computational Sensing and Robotics (LCSR), Johns Hopkins University, Baltimore, MD, USA. His postdoctoral research focuses on evaluation experiments in optical coherence tomography (OCT)-guided, robot-assisted eye surgery. He is advised by Prof. Iulian I. Iordachita and Prof. Russell H. Taylor. Dr. Gao's research interests include surgical robotics, medical mechatronics, and robotic systems for OCT-guided ophthalmic microsurgery. He has published several first-author papers in leading robotics journals, including IEEE Transactions on Robotics, Biomimetic Intelligence and Robotics, and IEEE Transactions on Medical Robotics and Bionics. He also serves as a reviewer for IEEE Transactions on Robotics, IEEE Transactions on Medical Robotics and Bionics, and the International Journal of Medical Robotics and Computer Assisted Surgery.
SmartBot
Literature review
Position Transmission Control in Tendon-Sheath Mechanisms: A Critical Review and a Promising Solution
5-Mar-2026