In this work, a novel planar parallel continuum robot (PCR) is introduced, consisting of three kinematic chains that are coupled at a triangular end-effector platform and include tendon-actuated continuum segments. The kinematics of the resulting structure are derived by adapting the descriptions for conventional planar parallel manipulators to include constant curvature bending of the utilized continuous segments. To account for friction and non-linear material effects, a data-driven model is used to relate tendon displacements and curvature of the utilized continuum segments. A calibration of the derived kinematic model is conducted to specifically represent the constructed prototype. This includes the calibration of geometric parameters for each kinematic chain and for the end-effector platform.During evaluation, positioning repeatability of 1.0% in relation to one continuum segment length of the robot, and positioning accuracy of 1.4%, are achieved. These results are comparable to commonly used kineto-static modeling approaches for PCR. The presented model achieves high path accuracies regarding the robot's end-effector pose in an open-loop control scenario.
In complex and unpredictable environments or in situations of human-robot interaction, a soft and flexible robot performs more safely and impact resistant compared to a traditional rigid robot. To enable the robot with the bionic features (flexibility, compliance and variable stiffness) similar to human joints, structures involving suspended tubercle tensegrity is researched. The suspended tubercle gives the joint compliance and flexibility by isolating two moving parts. The variable stiffness capacity is achieved by changing the internal stress of tensegrity through the simultaneous contraction or relaxation of the driving tendons. A wrist-inspired tensegrity based bionic joint is proposed as a case study. It has variable stiffness and two rotations with a total of three degrees of freedom. Through theoretical derivation and simulation calculation in NTRTsim, the range of motion, stiffness adjustable capacity, and their interaction are studied. A prototype is built and tested under a motion capture system. The experimental result agrees well with the theoretical simulation. Our experiments show that the suspended tubercle type tensegrity is flexible, stiffness adjustable and easy to control and has great potential for bionic joints.
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