Supportive continuum robots (SCRs) enhance the load-bearing capability of an operative continuum robot by mechanically coupling it with a supportive arm. However, their passive stiffness is determined by the mechanical configuration and cannot be adjusted online for varying payloads or interaction forces. Active stiffness control is therefore needed to regulate the load response and maintain positioning accuracy. Meanwhile, the closed-chain structure introduces kinematic constraints that complicate task-space regulation and stiffness control. This paper presents an active task-space stiffness control framework for a tendon-driven SCR. An existing geometric variable strain model describes the closed-chain dynamics, which are projected onto the constraint-consistent motion subspace. A projected sliding mode controller regulates the operative arm tip while preserving the constraints, and closed-loop stability is established through Lyapunov analysis. After position regulation, active apparent stiffness is introduced through a virtual Cartesian spring based on position-error feedback to shape the force--displacement response. The framework is evaluated in simulation and experimentally validated under prescribed external loads and different desired configurations. Results show that increasing the commanded stiffness gain reduces load-induced tip deflection and increases apparent directional stiffness, thereby improving load resistance and positioning robustness under external loading.
Robotic systems operating in unstructured environments face a conflicting requirement: they must be manoeuvrable to easily navigate tight and unstructured environments, yet stiff enough to perform precise, heavy-duty tasks. Tendon-driven hyper-redundant manipulators offer a compelling solution due to their lightweight and slender designs. In this work, a novel stiffness-adjusting strategy for a tendon-driven manipulator is presented. It goes beyond complex antagonistic drives, joint-level added components, and passive high-pretension schemes. This approach utilises a single active tensioning system to modulate a global reference tension, providing on-demand stiffness adjustment on the robot. Through a systematic experimental campaign involving varying payloads and tension levels, the behaviour of a five-degree-of-freedom prototype is characterised using high-precision motion capture. The results demonstrate that this mechanically simple input can amplify tip stiffness by a factor of three to four. Furthermore, the presented compliance model validates the efficiency of the global tensioning strategy in modulating the system’s overall stiffness.
A. Poka, Federico Manara, D. Ludovico et al.· 2026 IEEE/ASME International...· 0 citations
Passive stiffness modulation is essential for compliant robotic systems in dynamic and uncertain environments, where rigid actuators or constant stiffness designs are often insufficient to ensure safety and adaptability. This paper presents the design, modeling, optimization, and experimental validation of a Load-Dependent Contact-Aided Compliant Joint (LCCJ) that passively modulates stiffness in response to external torque. An integrated design pipeline is established, which bridges a high-fidelity analytical framework with a physically-motivated optimization strategy. The framework combines a chained pseudo-rigid-body model (CPRBM) with Karush-Kuhn-Tucker (KKT) conditions to describe the complex beam-boundary interactions. Using the distinct deformation regimes of the mechanism, the pipeline employs a two-stage optimization strategy to precisely map the desired stiffness modulation back to the physical geometric parameters. Simulation results demonstrate that, across the benchmark and optimized-design validation cases, the model predictions agree closely with finite element analysis (FEA), with maximum relative errors in the torque–deformation response of 4.02% and 3.18% in the pre- and post-contact regions, respectively. Experimental validation confirms the effectiveness of the proposed design; compared to the FEA predictions, the experimental results exhibit relative errors below 2.6% in the pre-contact region and 5.22%–6.21% in the post-contact region. The LCCJ offers a compact and monolithic solution for passive stiffness modulation in compliant joint applications.
Zhenjie Shi, Yanjun Liu, Yiran Wang et al.· Journal of Mechanisms and Ro...· 0 citations
Low-tolerance assembly tasks in physical human–robot interaction (pHRI) often involve intermittent contact with stiff environments, which can induce instability in admittance-controlled manipulators. Increasing virtual damping improves stability but reduces responsiveness and increases operator effort. This paper introduces a series nonlinear compliance mechanism that reshapes interaction dynamics through adjustable stiffness at the robot end-effector. The mechanism provides low stiffness during fine manipulation and higher stiffness at larger displacements to enhance stability during contact-rich assembly. Experimental evaluation in a collaborative peg-in-hole task demonstrates reduced contact forces, improved insertion stability, and faster task completion. In an ultra-low-clearance scenario (0.05 mm), insertion fails under conventional rigid configurations but succeeds consistently with the proposed mechanism. Task completion time is reduced by up to 30% without increasing admittance damping. These results demonstrate that passive nonlinear compliance can improve stability without sacrificing responsiveness in contact-rich pHRI tasks.
S. Jujjavarapu, E. Esfahani· 2026 IEEE/ASME International...· 0 citations
This paper develops a contact-control scheme for multi-degree-of-freedom (DOF) robot arms that relies on dynamic impedance to jointly improve precision, stability, and robustness. Classical impedance controllers keep stiffness, damping, and inertia fixed, which limits how well they adapt as operating conditions change. Here an impedance model is built specifically around the end-effector, and position control is refined by adding force-based corrections to the desired position along the force- controlled axes. Dynamic impedance is realized by tying the impedance parameters to the joint angles and angular velocities, using the final value theorem together with quadratic-form transient-response analysis. The resulting method is validated in MATLAB/Simulink through wall-wiping simulations performed with a 6-DOF RPY-type robot.
Kaisei Hosoyama, Qingjiu Huang· 2026 IEEE International Conf...· 0 citations
ABSTRACT Enhancing contact compliance and task flexibility is essential for expanding the real‐world use of robotic systems. This paper presents a modular collaborative robot system that combines antagonistic actuation with hybrid soft–rigid variable‐stiffness components, including a modular variable‐stiffness bending joint (mvsBJ), a modular variable‐stiffness rotational joint (mvsRJ), and a modular variable‐stiffness link (mvsL), supported by an integrated control framework. Experimental characterization shows that the mvsBJ achieves a 55.96° bending range and nearly threefold stiffness variation, from 9.16 to 24.72 Nm/rad, through fluidic pressure adjustment. The mvsRJ achieves 145° bidirectional rotation. The platform is benchmarked across repeated trials in healthcare assistance and industrial support scenarios. In an assistive feeding task, a 3‐DoF RBBL configuration operates in a low‐stiffness mode, maintaining bounded force‐tracking variation with mean error drift below 0.16 N. Collision tests across 40–60 mm/s show peak interaction forces increasing predictably from 3.93 to 5.38 N. In an industrial chamfering task, the 2‐DoF RB configuration achieves pressurized rigidity comparable to a conventional mechanical workbench. These results demonstrate plug‐and‐play reconfigurability with reliable adjustable compliance, providing a novel architecture for diverse physical human–robot collaboration applications.
Wenlong Gaozhang, Yue Li, Jialei Shi et al.· Advancement of science· 0 citations
Pneumatic soft actuators are widely used in soft robotic systems because of their inherent compliance and smooth deformation. However, their practical application is often limited by low structural stiffness, restricted load-bearing capability, and insufficient tip output force. These limitations become more pronounced in tasks that require stable force transmission or precise interaction with the environment. In response to these limitations, this study proposes a stiffness-enhanced pneumatic soft actuator based on a modified multilayer structural configuration. The actuator integrates chamber layers, a constraint layer, and periodically distributed rigid reinforcement elements. This structural arrangement improves the way internal pressure is converted into bending deformation and external force output, while avoiding excessive local expansion of the chambers. Based on this actuator, a coupled theoretical model is developed to describe the relationship between internal pressure, bending angle, and tip force. The model considers both the hyperelastic behavior of the silicone material and the geometric constraints introduced by chamber deformation. Finite element simulations are performed to examine the actuator’s mechanical response under different pressure inputs. This effect becomes evident at higher pressure levels. Both free-bending behavior and tip contact force generation are analyzed. The simulation results follow the same trends as the theoretical predictions, and the overall deviation remains below 10% across the investigated pressure range. The agreement shows that the model reflects the main mechanical response. Compared with a conventional pneumatic soft actuator, the proposed design achieves higher stiffness and larger tip output force, while maintaining compliant motion and smooth bending behavior. The actuator structure and model may serve as a useful basis for pneumatic actuator design in force-demanding tasks.
Wenjing Zhou, Rui Ma, Mingyue Lu et al.· SAE technical paper series· 0 citations