Stiffness Of Robot Research Articles

Collaborative Human–Robot Interaction Interface: Development for a Spinal Surgery Robotic Assistant

The growing introduction of robotics in non-industrial applications where the environment is unstructured and changing, has led to the need of development of safer and more intuitive, human–robot interfaces. In such environments, the use of collaborative robots has potential benefits, due to the combination of user experience, knowledge and flexibility with the robot’s accuracy, stiffness and repeatability. Nevertheless, in order to guarantee a functional collaboration in these environments, the interaction between user and robot must be intuitive, natural, fast and easy to use. On one hand, commercial collaborative robots are less accurate and less stiff than the traditional industrial ones, on the other hand, the later have not intuitive interaction interfaces. There are tasks in which the stiffness of industrial robots and the intuitive interaction interfaces of collaborative commercial robots, are desirable. This is the case of some robotic assisted surgical procedures, such as robotic assisted spine surgery, with high accuracy demands and with the need of intuitive surgeon–robot interaction. This paper presents a hand guiding methodology for functional human–robot collaboration and the introduction of novel algorithms to enhance its behavior. Also its implementation on a robotic surgical assistant for spine procedures is presented. It is emphasized how a traditional industrial robot can be used as a collaborative one when the available commercial collaborative robots do not have the required accuracy and stiffness for the task.

Investigation of the relationship between gait pattern and spinal stiffness of a four-legged robot equipped with tensegrity spine.

Investigation of the relationship between gait pattern and spinal stiffness of a four-legged robot equipped with tensegrity spine.

Configuration optimization through redundancy angle and tool posture by force induced error index in robot ball-end milling

Industrial robots are widely applied in manufacture industry recently because of their great flexibility and large working space. Due to the posture-dependent stiffness of industrial robots, a force-induced error index (FEI), which simultaneously takes milling force and robot tool end stiffness in consideration, is proposed to describe the error degree with different redundant angle and tool posture. Based on the FEI, a robot ball-end milling configuration optimization model considering the limits of FEI, movement smoothness and configuration singularity is proposed, and the shortest path searching algorithm is applied to generate robot trajectory. Corresponding experiments were carried out, and experimental results validated the effectiveness of the proposed FEI and the robot trajectory optimization method.

Emotional Expression by Controlling Perceived Stiffness of Social Robots

Emotional Expression by Controlling Perceived Stiffness of Social Robots

A study on body stiffness and walking stability of a quadruped robot

A study on body stiffness and walking stability of a quadruped robot

Modeling and Analysis of the Stiffness Distribution of Host–Parasite Robots

The stiffness distribution (SD) of robot has a great influence on the robot pose accuracy, but the calculation efficiency and accuracy of stiffness distribution are still low. This study presents a finite element fitting method with an extremely small number of computational cells. It was developed based on experimental results of robot stiffness. This method can be employed to establish single- and multi-source fitted SD (FSD) (S-FSD and M-FSD) models for host–parasite (H–P) robots. The computational efficiency and correctness of the FSD models were verified by case studies. The configurations of six evolutionary mechanisms of an H–P robot were subjected to an SD analysis. A comparison of the six configurations shows that adding parasitic branched chains can improve the SD of the H–P robot to varying degrees. In particular, the most notable improvement was for H–P mechanism. Specifically, by averaging the stiffness of all positions, the average-stiffnesses of H–P mechanism in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> -, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula> -, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$z$ </tex-math></inline-formula> -directions were 104.10%, 1427.78%, and 1101.62% of those of the host mechanism, respectively. In the SD diagram, the medium- and high-stiffness regions of mechanism F are large and distributed in a banded pattern between the highest pose point and the furthest pose point, whereas its low-stiffness region is small and concentrated near the nearest pose point.

End-effector Cartesian stiffness shaping - sequential least squares programming approach

Control of robot end-effector (EE) Cartesian stiffness matrix (or the whole mechanical impedance) is still a challenging open issue in physical humanrobot interaction (pHRI). This paper presents an optimization approach for shaping the robot EE Cartesian stiffness. This research targets collaborative robots with intrinsic compliance - serial elastic actuators (SEAs). Although robots with SEAs have constant joint stiffness, task redundancy (null-space) for a specific task could be used for robot reconfiguration and shaping the stiffness matrix while still keeping the EE position unchanged. The method proposed in this paper to investigate null-space reconfiguration's influence on Cartesian robot stiffness is based on the Sequential Least Squares Programming (SLSQP) algorithm, which presents an expansion of the quadratic programming algorithm for nonlinear functions with constraints. The method is tested in simulations for 4 DOF planar robot. Results are presented for control of the EE Cartesian stiffness initially along one axis, and then control of stiffness along both planar axis - shaping the main diagonal of the EE stiffness matrix.

A New Approach to the Consideration and Analysis of Critical Factors in Robotic Machining

The relative low stiffness of industrial robots is a major limitation on the development of flexible and reconfigurable systems in applications in which process forces and vibration lead into significant tool path deviations with respect to the programmed path as in the case of robotic machining. This paper presents a novel factorial procedure that allows for the preliminary study of the main conditions in robotic machining operations and it determines the critical factors that are affecting the machining path of any robotic cell in order to obtain the process conditions with lower path deviations. In this procedure the most influential robotic machining constraints were identified and classified, the factorial design of experiments was used to enable the execution of the experimental tests and the machining tool path deviation predictive methodology (PREMET) was used to determine the cutting tool path deviation between the programmed and the experimental path as a function of the process variables. Experimental trials have been carried out in order to determine the main factors that affect the robotic machining and influence the main constraints of the process, showing a reduction greater than a 36% of the cutting tool path deviation in groove milling of aluminum. The critical factors identified in order of importance are: hardness of the material, location of the workpiece, orientation of milling head relative to working direction and cutting conditions. This procedure can be extended to future factorial studies to improve the precision of robotic machining (in operations such as face milling, contouring, pocketing) and to establish design criteria for machining robotic cells.

로봇 중재 시술을 위한 하이퍼-리던던트 로봇의 강성 및 형상 변형에 관한 연구

로봇 중재 시술을 위한 하이퍼-리던던트 로봇의 강성 및 형상 변형에 관한 연구

Dynamic analysis and decoupled control of a heavy-duty walking robot with flexible feet based on super twisting algorithm

In this paper, the dynamics and control strategies of a biped robot with 6-DOF parallel leg mechanism are studied. Firstly, the multi-body kinematic model and dynamic model of the robot are established. Secondly, the insufficient stiffness of robot’s feet and the coupling effect between the kinematic chains are considered in dynamics modeling, and the rigid-flexible coupling model is established by using ADAMS and finite element method. Finally, aiming at the position deviation and system vibration caused by the flexible foot, a strategy based on the combination of a computed torque controller (CTC) and a second-order sliding-mode super twisting algorithm (STA) is designed. At the same time, the control strategy based on CTC and PID and the control strategy based on CTC and sliding mode control (SMC) are designed to compare with CTC-STA. The results show that CTC-STA has faster regulation ability and stronger robustness than CTC-SMC and CTC-PID.

Experimental Analysis of the Dynamic Stiffness in Industrial Robots

Industrial robots are regularly being employed for machining tasks. As a key machining parameter, dynamic stiffness has a significant influence on robotic machining performance. This paper examines two experimental approaches for obtaining the dynamic stiffness of an industrial robot. One approach is based on experimental modal analysis. The other approach is a direct calculation with the acquisition of excitation forces and vibration displacement. Different excitation frequencies are planned to stimulate an industrial robot. The dynamic stiffnesses obtained by the two approaches are thoroughly evaluated. Large deviation appears between the dynamic stiffness measured by two approaches. The factors that result in variations in the measurements are discussed.

Positioning Error Compensation for Industrial Robots Based on Stiffness Modelling

Insufficient stiffness of industrial robots is a significant factor which affects its positioning accuracy. To improve the positioning accuracy, a novel positioning error compensation method based on the stiffness modelling is proposed in this paper. First, the positioning errors considering the end load and gravity of industrial robots due to stiffness are analyzed. Based on the results of analysis, it is found that the positioning errors can be described by two kinds of deformation errors at joints: the axial deformation error and the radial deformation error. Then, the axial deformation error is modelled by the differential relationship of kinematics equations. The model of radial deformation error is deduced through the recurrence method and rotation transformation between joints. Finally, these two models are transformed into a Cartesian coordinate system, and a positioning error compensation method based on these two models is presented. Simulations based on the finite element analysis are implemented to verify the positioning error compensation method. The results show that the suggested method can efficiently predict the positioning error according to the gravity and loads, so that the positioning accuracy of industrial robots can be improved with the proposed method.

Auditory Feedback Effectiveness for Enabling Safe Sclera Force in Robot-Assisted Vitreoretinal Surgery: a Multi-User Study.

Robot-assisted retinal surgery has become increasingly prevalent in recent years in part due to the potential for robots to help surgeons improve the safety of an immensely delicate and difficult set of tasks. The integration of robots into retinal surgery has resulted in diminished surgeon perception of tool-to-tissue interaction forces due to robot's stiffness. The tactile perception of these interaction forces (sclera force) has long been a crucial source of feedback for surgeons who rely on them to guide surgical maneuvers and to prevent damaging forces from being applied to the eye. This problem is exacerbated when there are unfavorable sclera forces originating from patient movements (dynamic eyeball manipulation) during surgery which may cause the sclera forces to increase even drastically. In this study we aim at evaluating the efficacy of providing warning auditory feedback based on the level of sclera force measured by force sensing instruments. The intent is to enhance safety during dynamic eye manipulations in robot-assisted retinal surgery. The disturbances caused by lateral movement of patient's head are simulated using a piezo-actuated linear stage. The Johns Hopkins Steady-Hand Eye Robot (SHER), is then used in a multi-user experiment. Twelve participants are asked to perform a mock retinal surgery by following painted vessels inside an eye phantom using a force sensing instrument while auditory feedback is provided. The results indicate that the users are able to handle the eye motion disturbances while maintaining the sclera forces within safe boundaries when audio feedback is provided.

Stiffness-oriented performance indices defined on two-dimensional manifold for 6-DOF industrial robot

In this paper, the modeling of the stiffness of a 6-DOF industrial robot is introduced, and two different stiffness-oriented performance indices are compared to improve the two-dimensional manifold. In previous study, the two-dimensional manifold is applied to simplify the workspace for error compensation to avoid the curse of dimensionality. Stiffness is one of the most concerned factors in robotic machining to reduce the deflection at the end of robot arm, which makes a stiffness-oriented manifold is quite essential in the actual application. An improved two-dimensional index is presented based on the Frobenius-condition-number of translational compliance matrix to avoid the ill-conditions of manipulator. The superiority of the improved index over the previous one is theoretically proved with several propositions in matrix theory. The simulations performed with sample points on a panel surface imply that the ill-conditions of robot can be effectively avoided. The results of contrast experiments conducted on KUKA robot show that the improved still ensure the accuracy of positioning in robotic machining.

On the motion/stiffness decoupling property of articulated soft robots with application to model-free torque iterative learning control

This article tackles the problem of controlling articulated soft robots (ASRs), i.e., robots with either fixed or variable elasticity lumped at the joints. Classic control schemes rely on high-authority feedback actions, which have the drawback of altering the desired robot softness. The problem of accurate control of ASRs, without altering their inherent stiffness, is particularly challenging because of their complex and hard-to-model nonlinear dynamics. Leveraging a learned anticipatory action, iterative learning control (ILC) strategies do not suffer from these issues. Recently, ILC was adopted to perform position control of ASRs. However, the limitation of position-based ILC in controlling variable stiffness robots is that whenever the robot stiffness profile is changed, a different input action has to be learned. Our first contribution is to identify a wide class of ASRs, whose motion and stiffness adjusting dynamics can be proved to be decoupled. This class is described by two properties that we define: strong elastic coupling, relative to motors and links of the system and their connections; and homogeneity, relative to the characteristics of the motors. Furthermore, we design a torque-based ILC scheme that, starting from a rough estimation of the system parameters, refines the torque needed for the joint positions tracking. The resulting control scheme requires minimum knowledge of the system. Experiments on variable stiffness robots prove that the method effectively generalizes the iterative procedure with respect to the desired stiffness profile and allows good tracking performance. Finally, potential restrictions of the method, e.g., caused by friction phenomena, are discussed.

Hybrid compliance compensation for path accuracy enhancement in robot machining

Robot machining processes with high material removal rates lack of high path accuracy mainly due to the low stiffness of industrial robots. The low stiffness leads to process forces caused deviations of the tool center point (TCP) from the planned position of more than 1 mm in industrial applications. To enhance the path accuracy a novel hybrid compliance compensation is developed. It combines a force sensor and model based online compensation with forces of an offline simulation to instantly react to predictable high force changes e.g. at a milling cutter exit from the work piece. The method is applied to a KUKA KR 300 robot. A compliance model based on a forward kinematic with virtual joints is implemented on an external controller. Cartesian or axis specific compensation values are calculated and transferred to the robot via a control circuit. A compliance measurement method is developed and a force torque sensor is mounted to the flange of the robot. The system is validated in with Cartesian and axis specific compensation values as well as with and without pilot control.

Wrench transformation technique for robot stiffness evaluation: Direct sum based solution and application to kinetostatic indices

A new method is developed to define the stiffness values of an elastically supported system defined through the dimensionally inhomogeneous robot stiffness matrix (RSM). The RSM in its initial form exhaustively describes the robot equilibrium conditions, but its eigenvalues are undefined. Regular formal operations on equilibrium equations transform the initial wrench and reduce the RSM to a unique direct sum of two submatrices, separately describing the linear and angular stiffness features under mutual force-torque action. Eigenvalues of the direct sum exist and either directly present kinetostatic indices (KSI) or are used for KSI synthesis. Physical nature of the considered problem results in the coupled translation-rotation form of the KSI. Simulated application examples demonstrate the validity of the developed method for both the serial-kinematics and the parallel-kinematics robots.

Design of an Accurate and Stiff Wooden Industrial Robot: First Steps Toward Robot Eco-sustainable Mechanical Design

Abstract This article investigates the feasibility of replacing metal robot links by wooden bodies for eco-sustainable design’s purpose. Wood is a material with low environmental impact and a good mass-to-stiffness ratio. However, it has significant dimensional and mechanical variabilities. This is an issue for industrial robots that must be accurate and stiff. To guarantee stiffness and accuracy performance of a wooden robot, we propose an integrated design process combining (i) proper wood selection, (ii) adequate sensor-based control strategies to ensure robot accuracy, and (iii) a robust design approach dealing with wood uncertainties. Based on the use of this integrated design process, a prototype of a wooden five-bar mechanism is designed and manufactured. Experimental results show that it is realistic to design a wooden robot with performance compatible with industry requirements in terms of stiffness (deformations lower than 400 μm for 20 N loads) and accuracy (repeatability lower than 60 μm), guaranteed in a workspace of 800 mm × 200 mm. This study provides a first step toward the eco-sustainable mechanical design of robots.

Design, Kinematic, Dynamic and Stiffness Analysis of a 5-DOF Single-incision Laparoscopic Surgery Robot

In order to further reduce the incisions of laparoscopic surgery and the possibility of infection, the organic combination of single-incision laparoscopic surgery (SILS) and robotics has made the degree of minimally invasive surgery further improved. A new 5-DOF single-incision laparoscopic surgery robot was designed based on Axiomatic Design Theory, whose structure consisting of the movement mechanism, the endoscope and the position and pose adjustment mechanism. The robot parts are connected in series and parallel, allowing a pivotal motion of the endoscope in the center of the robot for realizing the incision. In order to achieve a performance optimization and a dynamic control of the single-incision laparoscopic surgical robot, the kinematics and dynamic modeling and dynamic stiffness analysis of the robot are especially important. The forward kinematics equation, inverse kinematics equation and Jacobian matrix of the SILS robot are derived based on D-H method and geometric method, and the kinematics numerical simulation is carried out by Matlab. The dynamic equation of the robot is derived by Kane method. Subsequently, a numerical simulation of the robot dynamics equation is performed, with its virtual prototype utilized to set the motion plan of the robot mechanism. After robot’s dynamic simulation, the numerical changes of the driving force and torque for each robot’s moving mechanism are obtained. The performed simulation results further verify the correctness of the established robot’s dynamic model. Finally, utilizing the above methods, the dynamic stiffness model and evaluation index of the robot are determined, and the dynamic stiffness of the robot is analyzed and evaluated. The results of the kinematics, dynamic and stiffness analysis of the SILS robot further validate that the 5-DOF SILS robot has a reasonable structure, motion and sufficient stability to meet the needs of single-incision laparoscopic surgery.

FSW robot system dimensional optimization and trajectory planning based on soft stiffness indices

Abstract Serial robots provide an ideal alternative for friction stir welding of large-scale spatial curve welding seam due to their high flexibility and large workspace. However, the low stiffness of a serial robot will definitely affect its welding accuracy. In order to enhance the stiffness of the serial robot in completing friction stir welding tasks, this paper proposes a method of constructing the hybrid stiffness index. This method uses the sigmoid function to process the dexterity or joint limit index and applies it to the stiffness index as a weight coefficient. Then the soft stiffness index with dexterity constraint or joint limit constraint is constructed respectively, and these two hybrid stiffness indices have the same dimension as the stiffness index. Subsequently, the dimension synthesis of the ZK-500 serial robot is completed by maximizing the global soft stiffness index with dexterity constraint. Finally, based on the soft stiffness index with joint limit constraint, a joint trajectory planning algorithm for the ZK-500 robot and positioner system is proposed. The result of simulations shows that the soft stiffness index with a joint limit constraint can not only ensure the stiffness performance but also improve the smoothness of the joint trajectory in the robot trajectory planning task.