Design and Experimental Characterization of Hydraulically-actuated Revolute Joint based on Multimaterial Additive Manufacturing

Design of fluidic actuators remain challenging in specific contexts such as the medical field, when solutions have for instance to be compatible with the stringent requirements of magnetic resonance imaging. In this paper, an innovative design of hydraulically-actuated revolute joint is introduced. The design originality is linked to the use of multimaterial additive manufacturing for its production. Hydraulic actuation and polymer manufacturing are selected to have compatibility with the medical context. A design taking advantage of the process capabilities is proposed. The proposed component associates a large stroke compliant revolute joint and miniature pistons. An helical rack-and-pinion mechanism is integrated to the compliant joint to control the joint rotation. A specific gear geometry is elaborated to minimize the joint size. It is experimentally characterized in terms of range of motion, stiffness and available torque, to discuss the suitability of the component as a fluidic actuator. The component offers an interesting compactness, range of motion and the process is shown to be adequate for the design of functional systems.

Folding responses of origami-inspired structures connected by groove compliant joints

Folding responses of a set of notch-type compliant joint candidates are first numerically explored before the victorious one is implemented in actuating the deployment of Miura origami-inspired plate structure. The considered notch-type compliant joints are groove, elliptical holes, rectangular holes, and outside LET types. The exploration and examination of the kinematic and dynamic characteristics of these joints include performance indicators such as stress contour, load-deformation, moment-angle, and stiffness-angle relationships for different geometric parameters, with a specific interest in their hysteretic behaviors. Considering various performance features, the groove joints have been identified as the most suitable to be employed as the Miura origami-inspired hinge. The Miura origami-inspired plate folding behaviors are further explored considering various numbers and placements of groove joints. It has been found that the Miura plate performs better with the groove joint compared to that without one and that the single and double groove joint modes are inter-correlated. The study offers a comprehensive understanding of the effects of geometrical variation of numerous compliant joints on the folding behaviors as well as the further implementation of the victorious one in actuating the deployment of the Miura origami-inspired plate structure in accordance with the number and location of the joint.

Approximating Hinges With Multimaterial Compliant Joints

This paper investigates the use of multimaterial compliant joints produced through additive manufacturing in order to approximate a revolute joint. Compliant joints benefit from low friction and reduced wear, but at the cost of increased joint stiffness, reduced range of motion, and a reduced ability to resist loading. In addition, they might also provide a poor approximation of the revolute joints they intend to replace. In this paper, we experiment with three multimaterial compliant joint configurations. The first joint emphasizes accurate kinematics, the second joint aims to reduce axis-aligned stiffness, and the third joint compromises between the two. Samples were fabricated on a desktop 3D printer using PLA (polylactic acid) as the rigid material and TPU (thermoplastic polyurethane) for its flexibility. Samples were measured for tensile stiffness, torsional stiffness, range of motion, and approximation of a hinge motion. Our results indicate design trade offs where joints that measure most ideal for one property will be least ideal for another. The most novel design in this paper straddles this trade off. In the end, the suitability of each joint design is determined by the loading, accuracy, and range of motion requirements posed by a given application.

An Open-Source ROS-Gazebo Toolbox for Simulating Robots With Compliant Actuators

To enable the design of planning and control strategies in simulated environments before their direct application to the real robot, exploiting the Sim2Real practice, powerful and realistic dynamic simulation tools have been proposed, e.g., the ROS-Gazebo framework. However, the majority of such simulators do not account for some of the properties of recently developed advanced systems, e.g., dynamic elastic behaviors shown by all those robots that purposely incorporate compliant elements into their actuators, the so-called Articulated Soft Robots ASRs. This paper presents an open-source ROS-Gazebo toolbox for simulating ASRs equipped with the aforementioned types of compliant actuators. To achieve this result, the toolbox consists of two ROS-Gazebo modules: a plugin that implements the custom compliant characteristics of a given actuator and simulates the internal motor dynamics, and a Robotic Operation System (ROS) manager node used to organize and simplify the overall toolbox usage. The toolbox can implement different compliant joint structures to perform realistic and representative simulations of ASRs, also when they interact with the environment. The simulated ASRs can be also used to retrieve information about the physical behavior of the real system from its simulation, and to develop control policies that can be transferred back to the real world, leveraging the Sim2Real practice. To assess the versatility of the proposed plugin, we report simulations of different compliant actuators. Then, to show the reliability of the simulated results, we present experiments executed on two ASRs and compare the performance of the real hardware with the simulations. Finally, to validate the toolbox effectiveness for Sim2Real control design, we learn a control policy in simulation, then feed it to the real system in feed-forward comparing the results.

Design of a novel tendon-driven manipulator structure based on monolithic compliant rolling-contact joint for minimally invasive surgery

Purpose Compliant mechanisms are commonly used in the design of manipulator and surgical robotic tools for minimally invasive surgery (MIS) thanks to their compactness, ability of miniaturization and lower part count. However, conventional compliant joint has higher internal stiffness, which limits the bending radius. To overcome this problem, a novel tendon-driven manipulator structure based on monolithic compliant rolling-contact joint (CRCJ) is proposed. Methods The proposed rolling-contact mechanism is used to prevent cable slack during actuation, which occurs in conventional compliant joint design. By means of selective laser sintering (SLS) technique, the CRCJ can be fabricated in a monolithic structure, thus granting the CRCJ both the advantages of compliant joints and rolling-contact mechanism. Simulations with nonlinear finite element analysis (FEA) and experiments were conducted to evaluate and compare the mechanical properties of the proposed CRCJ with conventional leaf-type compliant joint including the bending and compliant motion. Results Experimental results showed that the CRCJ has lower bending stiffness, higher maximum bending angle (over $$180^{\circ }$$ 180 ∘ ) and a higher compliance compared to conventional compliant hinges, which allows a larger workspace and reduces the possibility of tissue injury. Agreement was also found between the nonlinear FEA and experiments regarding the relation between actuation force and bending angle. A primary prototype of a 3-DOF handheld laparoscopic manipulator with a diameter of 7 mm was further developed. Conclusion A dexterous tendon-driven monolithic manipulator structure based on CRCJ for MIS is proposed. A preliminary prototype of a handheld laparoscopic manipulator demonstrates the capability of the CRCJ for steerable medical devices. However, design improvements based on FEA and application-orientated prototypes considering anatomical requirements still show room for improvements.

Joint Equivalence Design and Analysis of a Tensegrity Joint

We propose a method to design a tensegrity joint, making its elastic deformation an accurate joint-like motion, such as a rotation around the designed rotational center. The tensegrity joint can be a three rotational degree-of-freedom (DOF) joint through this method. Axis drift is presented as a design criterion to describe the rotational center's deviation degree concerning the compliance center since the rotational center is not fixed to one point for different positions of the tensegrity joint. The axis drift is designed to be in a prescribed range so that the tensegrity joint is approximately equivalent to a rigid joint. In other words, the tensegrity joint's elastic response under external torque and force becomes precise rigid-joint-like kinematics and can replace rigid joints to transfer motion, force, and energy. A large-size tensegrity joint is developed to verify the joint equivalence experimentally. The experimental results show that the tensegrity joint achieved maximum dimensionless axis drift less than 2%, and indicate an excellent joint equivalence. The tensegrity joints' ability to replace rigid joints as modular joints to construct a hyper redundant serial structure is demonstrated using a tensegrity robotic arm. The proposed tensegrity compliant joint has notable benefits of tensegrity structure such as high mechanical efficiency, modularity, and scalability, and can be extended to many robotic applications, such as large-size serial robotic arms and snake-like robots.

Cartesian Aerial Manipulator with Compliant Arm

This paper presents an aerial manipulation robot consisting of a hexa-rotor equipped with a 2-DOF (degree of freedom) Cartesian base (XY–axes) that supports a 1-DOF compliant joint arm that integrates a gripper and an elastic linear force sensor. The proposed kinematic configuration improves the positioning accuracy of the end effector with respect to robotic arms with revolute joints, where each coordinate of the Cartesian position depends on all the joint angles. The Cartesian base reduces the inertia of the manipulator and the energy consumption since it does not need to lift its own weight. Consequently, the required torque is lower and, thus, the weight of the actuators. The linear and angular deflection sensors of the arm allow the estimation, monitoring and control of the interaction wrenches exerted in two axes (XZ) at the end effector. The kinematic and dynamic models are derived and compared with respect to a revolute-joint arm, proposing a force-position control scheme for the aerial robot. A battery counterweight mechanism is also incorporated in the X–axis linear guide to partially compensate for the motion of the manipulator. Experimental results indoors and outdoors show the performance of the robot, including object grasping and retrieval, contact force control, and force monitoring in grabbing situations.

Preliminary design of a revolute to prismatic morphing compliant joint

The reduction of weight and size of mechanisms are important and difficult challenges considering portability, energy efficiency, and simplicity of fabrication. One of the solutions to address these issues consists of mechanisms with variable topology for which the mobility of the output is a succession of several simpler elementary motions. This change of mobility allows for achieving complex motions without necessitating a complicated design where many actuators or types of mechanical transmissions are required. Indeed, these variable topology mechanisms, also referred to as morphing mechanisms, have the ability to change their output motion throughout their workspace. Hand tools, medical devices, and aerospace robotic end-effectors are potential applications of this technology. In this paper, conceptual designs of such a revolute to prismatic morphing joint and its implementation using compliant hinges are proposed. Additionally, performance indexes pertaining to the desired output motion are proposed. First, a pseudo-rigid body model of a design candidate is presented, and simulations of this model are compared with finite element analyses to ensure accuracy. Then, several design features are quantitatively evaluated to propose improvements for future versions of the design. Finally, an early prototype illustrates the potential and feasibility of the proposed design as well as a possible application. Video