Link Robot Research Articles

Vibration reduction of a flexible robot link using a frictional damper

One of the most important factors reducing flexible manipulator efficiency is the residual vibration occurrence. In this research, vibration reduction of flexible manipulators is investigated using an internal frictional damper. At first, the vibration equation of a manipulator is obtained using the finite element method with the Euler–Bernoulli beam element to study its vibrations in a reciprocal motion. In addition, an analytical model is developed to investigate the effect of the frictional damper on robot link vibrations. Using particle swam optimization, ICA, NSGA-II, and GWO methods, the optimal structure for the damper is obtained to maximize its effect. The optimally damped link is fabricated, and its dynamic characteristics are extracted from a modal test experiment. The modal test results show a considerable improvement in the damping ratio of the damped link in comparison with a simple link. The fabricated link samples are then tested in a realistic situation. The experimental results are in coincidence with the simulation results, certifying the performance of the proposed plan in vibration reduction of a robot link.

Electromagnetic and Thermal Analysis of a Permanent Magnet Motor Considering the Effect of Articulated Robot Link

This paper presents the electromagnetic and thermal characteristics of a permanent magnet synchronous motor (PMSM) in a joint actuator which is used for articulated robot application. In an attempt to design a compact PMSM for the articulated robot, robot link should be taken into consideration during the motor design process as it can reduce the temperature distribution on motor, thus reducing the volume of the motor. A lumped-parameter thermal model of PMSM with and without a link is proposed considering the core loss, copper loss, and mechanical loss as heat sources. The electromagnetic and thermal analysis results are well confirmed by the experiment in a 400 W 20-pole/24-slot PMSM. The experiment results show that the robot link helps to reduce the motor end-winding temperature by about 40%, and this leads to an increase in power density of the motor.

Fuzzy Digital Filter Device Study for the Robot Manipulator

Рurpose of research. Situations that reduce the accuracy of links positioning often arise due to the drab of contacts during the operation of robot manipulator. One of this problem solutions is signal filtering. Modern digital filtering algorithms have a rather complex mathematical structure. Data processing in such filters takes long time. So the purpose of this work is to develop fast-acting fuzzy digital filter device capable to eliminate the problem of contact scrapping when positioning the links of robot manipulator.Methods. Generalized mathematical model was used in the development of digital filter device. This model is implemented by using fuzzy logic theory. Generalized mathematical model of the fuzzy digital filter includes a model determining a support of fuzzy output variable, a model for determining coefficients of fuzzy digital filter and a model for converting the output voltage into a rotation angle of robot manipulator servo drive. This mathematical model makes it possible to recalculate the voltage supplied to the input of fuzzy digital filter using two control coefficients thereby improving the positioning accuracy of robot manipulator links.Results. During experimental studies, comparative analysis of developed fast-acting device of fuzzy digital filter with Kalman filter was carried out. The RMSE standard error for two filters was calculated. The average RMSE value of the fuzzy digital filter was 0.0185 of Kallman's filter was 0.0193.Conclusion. Generalized mathematical model of fuzzy digital filter device consisting of three models is described. Device implementing determination of caliper of fuzzy output variable, experimental model of robot manipulator, experimental research and proposed mathematical model are presented.

Wire rod linear actuator seals inspired by earthworm: tube seal

This Letter presents a flexible wire-driven linear actuator using a bio-inspired sealing method that allows to replace a rigid rod by a flexible one and reduce the size of a robot link containing pneumatic or hydraulic linear actuators. Conventional pneumatic or hydraulic linear actuator sealing hinders the use of flexible wires as actuator rods. In contrast, the proposed tube sealing resembles a long balloon wrapping around the wire and stretching or contracting with the wire rod motion. Therefore, the wire is separated from the chamber and prevents fluid leakage through the flexible wire rod. This sealing method is inspired by earthworm which has slippery skin and stretching or contracting motion. The authors fabricated a prototype wire rod pneumatic linear actuator using the proposed tube seal and conducted leakage tests. In addition, the authors evaluated linearity between the input pressure and output force to determine the linear actuator performance.

Continuous fuzzy nonsingular terminal sliding mode control of flexible joints robot manipulators based on nonlinear finite time observer in the presence of matched and mismatched uncertainties

In the present study, a continuous fuzzy nonsingular terminal sliding mode control (CFNTSMC) was proposed based on the nonlinear finite-time observer (NFTO), in a voltage-based manner, to control the position of a serial-chain n-link flexible-joint electrically-driven robot manipulator in presence of matched and mismatched uncertainties with the electrical and mechanical equations. In order to design the controller sliding surface, a continuous function was proposed, which could make the concerned sliding surface zero at the beginning of the control process so that the proposed control had only the sliding phase. Afterward, an NFTO was adopted to facilitate access to the information of the state variables and matched and mismatched uncertainties. According to the Lyapunov stability theorems, the closed-loop control system had finite-time global asymptotically stability in presence of NFTO and existing uncertainties. Finally, a fuzzy-based control law was proposed to avoid chattering phenomena in the control input, whose computational complexity was extremely low. In order to evaluate the efficiency of the proposed control, multi-stage simulations were conducted for a 2-link flexible-joint serial robot manipulator. The mathematical evidence and simulation results confirmed the desired performance of the proposed CFNTSMC.

Toward Tradeoff Between Impact Force Reduction and Maximum Safe Speed: Dynamic Parameter Optimization of Variable Stiffness Robots

Abstract Variable stiffness robots may provide an effective way of trading-off between safety and speed during physical human–robot interaction. In such a compromise, the impact force reduction capability and maximum safe speed are two key performance measures. To quantitatively study how dynamic parameters such as mass, inertia, and stiffness affect these two performance measures, performance indices for impact force reduction capability and maximum speed of variable stiffness robots are proposed based on the impact ellipsoid in this paper. The proposed performance indices consider different impact directions and kinematic configurations in the large. Combining the two performance indices, the global performance of variable stiffness robots is defined. A two-step optimization method is designed to achieve this global performance. A two-link variable stiffness link robot example is provided to show the efficacy of the proposed method.

Development of a New Multi-cavity Pneumatic-driven Earthworm-like Soft Robot

SUMMARYThis paper presents a soft robot which can imitate the crawling locomotion of an earthworm. Locomotion of the robot can be achieved by expanding and contracting the body that is made of flexible material. A link of the earthworm-like robot is combined with three modules, and a multi-cavity earthworm-like soft robot is combined with multiple links. The multiple links of the earthworm-like soft robot are fabricated by silicone in the three-dimensional printed customized molds. Experiments on a single module, two-links, and three-links show that the soft robot can move and bend on condition of modules extension and contraction in a specified gait. The development of the earthworm-like soft robot shows a great prospect in many complicated environments such as pipeline detection.

Inverse kinematic analysis of 4 DOF pick and place arm robot manipulator using fuzzy logic controller

The arm robot manipulator is suitable for substituting humans working in tomato plantation to ensure tomatoes are handled efficiently. The best design for this robot is four links with robust flexibility in x, y, and z-coordinates axis. Inverse kinematics and fuzzy logic controller (FLC) application are for precise and smooth motion. Inverse kinematics designs the most efficient position and motion of the arm robot by adjusting mechanical parameters. The FLC utilizes data input from the sensors to set the right position and motion of the end-effector. The predicted parameters are compared with experimental results to show the effectiveness of the proposed design and method. The position errors (in x, y, and z-axis) are 0.1%, 0.1%, and 0.04%. The rotation errors of each robot links (θ1, θ2, and θ3) are 0%, 0.7% and 0.3%. The FLC provides the suitable angle of the servo motor (θ4) responsible in gripper motion, and the experimental results correspond to FLC’s rules-based as the input to the gripper motion system. This setup is essential to avoid excessive force or miss-placed position that can damage tomatoes. The arm robot manipulator discussed in this study is a pick and place robot to move the harvested tomatoes to a packing system.

Performance based design optimization of an intrinsically compliant 6-dof parallel robot

Parallel robots are preferred over serial robots owing to their enhanced accuracy and rigidity which comes from their higher stiffness. However, there are applications both in industry and in healthcare where higher accuracy is required alongside high compliance (reduced stiffness). Accuracy and compliance being conflicting to each other are difficult to achieve simultaneously. To address this issue, an intrinsically compliant 6-dof parallel robot is proposed in this work. Kinematic and analytical modeling is performed for its conceptual design to obtain the Jacobian matrix and thereby map the joint and Cartesian spaces. Robot’s structure design is analyzed, and the wrench analysis is also carried out to estimate the link forces and stiffness. It is shown that by small changes in the proposed robot design; its compliance can be altered making it suitable for a range of applications. It is also shown mathematically that the robot design can be optimized to maintain higher accuracy together with higher compliance. To carry out design optimization, three important performance criteria, namely; global condition number (for higher accuracies), norm of link forces (to reduce actuator power requirement) and robot compliance (for response to an external wrench) are mathematically formulated. Later, a multi-criteria optimization is performed using an evolutionary algorithm to simultaneously optimize these performance criteria. From the final robot design selected, it is evident that a higher robot compliance with optimal condition number and link forces can be achieved.

Design and experimental characterization of a high performance hydrostatic transmission for robot actuation

This paper introduces a hydrostatic torque-transmission for robotic applications based on rolling diaphragm cylinders. The proposed design solution, that relies on a novel floating-cylinder architecture, can be effectively employed for the implementation of robots with actuators that are remotely located with respect to their kinematic structure, without sacrificing performance and controllability. A prototype of proposed system is designed and implemented to comply with requirements for the actuation of a robotic arm exoskeleton. Such a system brings together a set of interesting attributes such as an extremely low level of friction ($$ 140^\circ$$), simple design, ease in assembling, low-cost, reduced encumbrance (compatible with the integration with robotic links) and lightweight. Additionally, we demonstrate that the presented system allows to integrate a very accurate pressure-based output torque estimation which can be employed to further improve the system performance without the use of strain-based load-cells.

Modal Control for Active Vibration Damping of Cable-Driven Parallel Robots

Abstract Cable-driven parallel robots are well suited for applications that require a very large workspace. Thanks to their lightweight moving parts, they can achieve high dynamics while remaining pretty safe for nearby human workers. Furthermore, their size depends only on the length of the cables; thus, their scale is almost totally decoupled from their cost. However, due to the cables, the stiffness is very low with respect to rigid link robots, inducing slowly damped oscillations of the end effector. Previous works have shown that those vibrations can be effectively damped by the winch actuators thanks to active vibration damping techniques. In this paper, a gain scheduling approach is proposed based on a linearized model of the robot dynamics. This model is projected in the modal space yielding six decoupled transfer functions for six degrees-of-freedom (DoFs) of a cable-driven parallel robot using thin cables. The stability of the proposed control law is analyzed for a static and a moving end effector. The proposed control algorithm is validated experimentally on an eight-cable suspended robot prototype.

Optimization-Based Posture Generation for Whole-Body Contact Motion by Contact Point Search on the Body Surface

Whole-body contact is an effective strategy for improving the stability and efficiency of the motion of robots. For robots to automatically perform such motions, we propose a posture generation method that employs all available surfaces of the robot links. By representing the contact point on the body surface by two-dimensional configuration variables, the joint positions and contact points are simultaneously determined through a gradient-based optimization. By generating motions with the proposed method, we present experiments in which robots manipulate objects effectively utilizing whole-body contact.

Test Bench for Evaluation of a Soft Robotic Link.

In this paper we describe the control approaches tested in the improved version of an existing soft robotic neck with two Degrees Of Freedom (DOF), able to achieve flexion, extension, and lateral bending movements similar to those of a human neck. The design is based on a cable-driven mechanism consisting of a spring acting as a cervical spine and three servomotor actuated tendons that let the neck to reach all desired postures. The prototype was manufactured using a 3D printer. Two control approaches are proposed and tested experimentally: a motor position approach using encoder feedback and a tip position approach using Inertial Measurement Unit (IMU) feedback, both applying fractional-order controllers. The platform operation is tested for different load configurations so that the robustness of the system can be checked.

A Neural Network Technique of Compensating for an Inertia Model Error in a Time-delayed Controller for Robot Manipulators

A time-delayed control (TDC) method is known as a simple, robust and non model-based control scheme that requires the fast sampling time, the accurate measurement of joint acceleration signals, and the accuracy of the inertia model of a robot manipulator. Among them, sampling time and acceleration signals are hardware dependent and can be solved. Then a user specified inertia model becomes a key role for the performance of TDC. When the selection of the diagonal element of the inertia matrix of a robot manipulator is used, the ill selection of the constant inertia matrix may lead to the poor tracking performance as well as instability. In addition, an appropriate selection of an inertia matrix for different tasks of the robot is not easy. Therefore, in this paper, an intelligent way of using a neural network is proposed to compensate for the deviation of the constant inertia matrix of a robot manipulator. The role of the neural network is to improve the tracking performance of a robot manipulator by compensating for the deviated error of the inertia matrix while satisfying the stability bound. Simulation studies of a three link robot are presented to confirm the proposal.

A parallel-guided compliant mechanism with variable stiffness based on layer jamming

This article introduces a parallel-guided compliant mechanism, which can achieve a high stiffness ratio (maximum over minimum) of 75 times through pneumatic actuated layer jamming. The compliant mechanism is composed of two flexible beams sandwiched with thin plastic friction layers. With a novel beam cross-section, the beams have large thickness, but still retain high flexibility. The effect of layer jamming is augmented by this large thickness due to the increased leverage of friction force. The beams in parallel-guided configuration have higher vertical and torsional stability compared to a single beam setup. The functionalities of the compliant mechanism have been validated experimentally: stiffness is measured as a function of the applied vacuum pressure. This paper describes the design concept, FEA validation of the design concept, prototyping, experiment results, analytical model, and analysis of vertical and torsional stability. The proposed concept of the compliant mechanism provides a potential solution for design of variable stiffness robotic links for addressing safety concerns in physical human robot interaction. The analytical model for identifying critical design parameters for maximum stiffness-variation effect provides a guideline for high stiffness-variation design on similar structures.

Experimental Validation of Hybrid Polymer Composite Material Robotic Single Link Flexible Manipulator

The prime aim of this research is to conduct an experimental validation for the assessment of behaviour of a hybrid composite material. The hybrid lightweight composite material is used as a robotic link for replacement of rigid and giant manipulators. The combination of fibre material methodically processed and technically merged with epoxy, resulting in hybrid composite material which is used as robotic link movement application and experimentally validated with respect to its functional behavior and cost-effectiveness. In this experimental investigation, composite material is taken as a flexible link for vibration amplitude control analysis and flexible deflection determination using modern control system with various joint stiffness coefficient. The numerical evaluations are conducted for lightweight composite material as an alternate of rigid and solid link. The modelling of composite flexible link is carried out for precision and accuracy on the basis of Lagrange’s equations of motion. The vibration investigation of the system is carried out and reduction of vibration is evaluated using model-based controller in the experiment.

Enhanced D-H: an improved convention for establishing a robot link coordinate system fixed on the joint

PurposeRobot kinematic modeling needs to be based on clear physical concepts. The widely used Denavit–Hartenberg (D–H) convention requires the coordinate system to be established on an extension of the axis. This leads to non-trivial problems which this study seeks to address by developing an improved convention.Design/methodology/approachFirst, the problems associated with the traditional D–H convention are systematically analyzed. Then, pursuant of solving these problems, an enhanced Denavit–Hartenberg (ED–H) convention is proposed, and a procedure is delineated for establishing the coordinate frame and obtaining the associated parameters. The transformation equations are derived based on a homogeneous matrix. The characteristics of traditional D–H and ED–H with regard to kinematics and dynamics are comprehensively compared. Finally, an application of dynamics for lead-through programming and collision protection is undertaken to validate the proposed ED–H method. Simulations and experiments are carried out using the Tiansui-One cooperative robot platform with the aim of exploring the merits of the proposed convention.FindingsThe proposed convention is compatible with traditional methods and can solve the problems inherent in these methods. The main characteristic of ED–H is that the coordinate system is fixed on the joint, which is a general modeling method.Originality/valueAn enhanced D–H convention is proposed to establish a unified, intuitive and accurate link model that exhibits stronger adaptability than traditional D–H and can be used effectively in kinematic and dynamic modeling of mechanical arms.

Experimentnal Approach to the Design of the Links of a Delta Robot

Experimentnal Approach to the Design of the Links of a Delta Robot

Nonlinear Swing down Control of the Acrobot

In this paper, we concern swing down control of the Acrobot which is a 2-link planar robot with a single actuator driving the second joint, whose control objective is to stabilize the Acrobot to the downward equilibrium point with the two links in the downward position for all initial states of the Acrobot with the exception of a set of Lebesgue measure zero. To achieve this control objective, we design a nonlinear controller by using nonnegative linear feedback of the sine function of the angle of the second joint in addition to the negative linear feedback of its angular velocity. By analyzing globally the solution of the closed-loop system consisting of the Acrobot and the presented controller and focusing on the equilibrium points of the closed-loop system and their stability, we prove that the control objective is achieved provided that some conditions on two control gains are satisfied. We design the two control gains such that the real parts of the dominant poles of the linearized model of the closed-loop system around the downward equilibrium point are minimized. We provide simulation results for two Acrobots to show the effectiveness of the presented controller.

Calibration control to maintain US focal point accuracy and its application to parallel link robot

Calibration control to maintain US focal point accuracy and its application to parallel link robot