New Continuum Robot Enables Decoupled Motion for Single-Port Surgery
In a significant leap forward for minimally invasive surgical robotics, researchers from the Chinese Academy of Sciences and Beijing Friendship Hospital have unveiled a novel continuum robot specifically engineered for single-port surgery. This innovation addresses long-standing challenges in surgical robotics—namely, the complex coupling between motion segments and the difficulty of achieving precise, independent control over position and orientation. By introducing a unique topologically decoupled architecture, the team has not only simplified the robot’s kinematic model but also enabled direct analytical solutions for inverse kinematics—a rarity in the field of soft and continuum robotics.
The newly developed robot features a three-segment design: a proximal actuated segment (S1), a passive intermediate linkage segment (S2), and a distal actuated segment (S3). The key breakthrough lies in the intermediate segment, which mirrors the bending motion of the proximal segment in the opposite direction. This mirroring ensures that the distal segment’s pose remains unaffected by movements in the proximal region, effectively decoupling the robot’s position and orientation. As a result, the end-effector’s orientation is determined solely by the distal segment, while its position is a composite of all segments—yet independently controllable.
This design philosophy draws inspiration from classical rigid-link manipulators that satisfy the Pieper criterion, where the last three joint axes intersect at a single point, enabling closed-form inverse kinematic solutions. However, applying such principles to flexible, tendon-driven continuum structures has historically been impractical due to their inherent compliance and strong inter-segment coupling. The research team overcame this by rethinking the mechanical topology rather than relying on software-based decoupling or iterative numerical methods.
The robot’s backbone is constructed from a series of spatial cross-curved disks fabricated via metal 3D printing using stainless steel. Each disk features orthogonal curved surfaces that allow smooth, large-angle bending in four directions. Twelve evenly distributed channels guide the actuation tendons—made of nickel-titanium alloy—through the structure. Crucially, the disks are assembled in alternating pairs rotated by 90 degrees, creating a continuous, stable contact interface that supports omnidirectional flexibility without buckling.
One of the most compelling aspects of this architecture is its variable stiffness profile: the backbone becomes progressively stiffer from distal to proximal segments. This gradient aligns with the natural mechanical demands of surgical tools, where bending moments increase toward the base due to distal loads. The design thus inherently accommodates the physical realities of in vivo manipulation, enhancing both stability and load-bearing capacity without additional mechanisms.
From a control standpoint, the decoupled structure eliminates the need for computationally intensive iterative solvers—such as those based on Jacobian matrices—that are prone to instability, slow convergence, and inaccuracies near workspace boundaries. Instead, the team derived a direct analytical solution for inverse kinematics. Given a target end-effector pose, the algorithm first computes the distal segment’s parameters (bending angle θ₂ and plane angle α₂) directly from the orientation components of the pose matrix. Once these are known, the position components are used to solve for the proximal segment’s parameters, with only a single transcendental equation requiring numerical resolution via Newton’s method—a far more efficient and robust approach than full iterative schemes.
Experimental validation confirmed the efficacy of this design. In actuation decoupling tests, when the proximal segment was bent while the distal tendons remained fixed, the intermediate segment mirrored the proximal curvature with an average angular error of just 2.39 degrees. Meanwhile, the distal segment’s orientation deviated by only 0.15 degrees on average—demonstrating exceptional isolation. Conversely, when the distal segment was actuated under various fixed proximal configurations, the proximal and intermediate segments showed minimal unintended motion, with average angular changes below 0.2 degrees in most cases.
Trajectory tracking experiments further underscored the system’s precision and robustness. The robot was tasked with tracing a 30 mm × 30 mm square path at constant orientation, under both no-load and 100-gram loading conditions, and at speeds of 20, 30, and 40 mm/s. At 20 mm/s with no load, the average positional tracking error was a mere 1.46 mm, with a standard deviation of 0.78 mm. Notably, even with a 100-gram load—a significant burden for a 7.5 mm diameter continuum robot—the tracking error increased only marginally, indicating strong mechanical integrity and effective model fidelity.
The researchers attribute residual errors to several practical factors: minor slippage between disk interfaces due to point contact, clearance between tendon channels and the 0.7 mm diameter NiTi wires (0.1 mm larger to ensure smooth sliding), and deviations from the assumed constant-curvature bending model caused by internal friction. Future iterations may incorporate active actuation in the intermediate segment to enable real-time force compensation, further enhancing decoupling fidelity.
This work represents more than a technical refinement—it redefines how continuum robots can be architected for clinical use. Single-port surgery, which accesses the abdominal cavity through a single small incision (often via the umbilicus), demands instruments with high dexterity in confined spaces. Traditional laparoscopic tools suffer from limited degrees of freedom and “sword-fighting” interference when multiple instruments are inserted through the same port. Robotic solutions have emerged, but many remain bulky, complex, or reliant on external mechanisms that compromise sterility or maneuverability.
The new continuum robot, by contrast, integrates all necessary functionality within a slender 7.5 mm shaft—compatible with standard single-port access platforms. Its six degrees of freedom (including a distal wrist joint) enable full positioning and orienting capability, essential for suturing, dissection, and other delicate tasks. Moreover, the decoupled control paradigm simplifies surgeon interaction: orientation adjustments no longer inadvertently shift the tool tip’s location, reducing cognitive load and improving procedural safety.
The implications extend beyond single-port laparoscopy. The same decoupling principle could be adapted for natural orifice transluminal endoscopic surgery (NOTES), bronchoscopy, neurosurgery, or any application requiring precise navigation through tortuous anatomy. The modular design also lends itself to customization—varying segment lengths, stiffness profiles, or actuation schemes for specific clinical scenarios.
Critically, the robot’s control architecture is computationally lightweight, making it suitable for real-time implementation on embedded systems. This is vital for teleoperation or autonomous assistance, where latency and reliability are paramount. The avoidance of iterative solvers not only boosts speed but also enhances determinism—a key requirement for medical device certification.
The research team, led by Yuanyuan Zhou and Hao Liu from the State Key Laboratory of Robotics at the Shenyang Institute of Automation, Chinese Academy of Sciences, collaborated closely with surgeons from Beijing Friendship Hospital, ensuring clinical relevance throughout the design process. This clinician-engineer partnership exemplifies the transdisciplinary approach needed to translate robotic innovations from bench to bedside.
Published in the July 2021 issue of Robot, a leading Chinese journal in robotics research, the paper has already sparked interest in both academic and industrial circles. The journal, known for its rigorous peer review and focus on applied robotics, provides a credible platform for disseminating high-impact engineering solutions with real-world potential.
Looking ahead, the team plans to integrate force sensing at the distal tip for haptic feedback, develop sterilizable packaging for clinical trials, and explore machine learning-enhanced control to compensate for model inaccuracies in real time. They also aim to miniaturize the drive system—currently based on lead screws and Maxon brushless motors—to create a fully integrated handheld or bedside console.
In an era where surgical robotics is rapidly evolving from large, fixed-base systems like the da Vinci to compact, flexible, and patient-centered platforms, this continuum robot stands out as a thoughtful fusion of mechanical ingenuity, mathematical elegance, and clinical pragmatism. By solving the decoupling problem at the architectural level, the researchers have not only improved performance but also opened a new design pathway for the next generation of surgical robots.
As minimally invasive techniques continue to dominate modern surgery, tools that combine dexterity, precision, and simplicity will be indispensable. This work demonstrates that sometimes, the most powerful innovations come not from adding complexity, but from reimagining the fundamentals.
Authors: Yuanyuan Zhou, Zhenxing Wang, Chongyang Wang, Dingjia Li, Cheng Zhang, Wei Guo, Zhongtao Zhang, Hao Liu
Affiliations: State Key Laboratory of Robotics, Shenyang Institute of Automation, Chinese Academy of Sciences; Institutes for Robotics and Intelligent Manufacturing, Chinese Academy of Sciences; Liaoning Province Key Laboratory of Minimally Invasive Surgical Robot; University of Chinese Academy of Sciences; Beijing Friendship Hospital, Capital Medical University
Published in: Robot, Vol. 43, No. 4, July 2021
DOI: 10.13973/j.cnki.robot.200550