A kinematically compatible framework for cooperative payload transport by nonholonomic mobile manipulators

In this paper, we examine the development of a kinematically compatible control framework for a modular system of wheeled mobile manipulators that can team up to cooperatively transport a common payload. Each individually autonomous mobile manipulator consists of a differentially-driven Wheeled Mobile Robot (WMR) with a mounted two degree-of-freedom (d.o.f) revolute-jointed, planar and passive manipulator arm. The composite wheeled vehicle, formed by placing a payload at the end-effectors of two (or more) such mobile manipulators, has the capability to accommodate, detect and correct both instantaneous and finite relative configuration errors. The kinematically-compatible motion-planning/control framework developed here is intended to facilitate maintenance of all kinematic (holonomic and nonholonomic) constraints within such systems. Given an arbitrary end-effector trajectory, each individual mobile-manipulator's bi-level hierarchical controller first generates a kinematically-feasible desired trajectory for the WMR base, which is then tracked by a suitable lower-level posture stabilizing controller. Two variants of system-level cooperative control schemes—leader-follower and decentralized control—are then created based on the individual mobile-manipulator control scheme. Both methods are evaluated within an implementation framework that emphasizes both virtual prototyping (VP) and hardware-in-the-loop (HIL) experimentation. Simulation and experimental results of an example of a two-module system are used to highlight the capabilities of a real-time local sensor-based controller for accommodation, detection and corection of relative formation errors.     

Synthesis on forward kinematics problem algebraic modeling for the planar parallel manipulator: displacement-based equation systems

Based on a proven exact method which solves the forward kinematics problem (FKP) this article investigates the FKP formulation specifically applied to planar parallel manipulators. It focuses on the displacement-based equation systems. The majority of planar tripods can modeled by the 3-RPR parallel manipulator, which is a tripod constituted by a fixed base and a triangular mobile platform attached to three kinematics chains with linear (prismatic) actuators located between two revolute joints. In order to implement the algebraic method, the parallel manipulator kinematics are formulated as polynomial equation systems where the number of equations is equal to or exceeds the number of unknowns. Three geometrical formulations are derived to model the difficult FKP. The selected proven algebraic method uses Gröbner bases from which it constructs an equivalent univariate system. Then, the real roots are isolated using this last system. Each real solution exactly corresponds to one manipulator assembly mode, which is also called a manipulator posture. The FKP resolution of the planar 3-RPR parallel manipulator outputs six complex solutions which become a proven real solution number upper bound. In several typical examples, the resolution performances (computation times and memory usage) are given. It is then possible to compare the models and to reject one. Moreover, a number of real solutions are obtained and the corresponding postures drawn. The algebraic method is exact and produces certified results.     

Aerial cooperative transporting and assembling control using multiple quadrotor–manipulator systems

In this paper, a fully distributed control scheme for aerial cooperative transporting and assembling is proposed using multiple quadrotor–manipulator systems with each quadrotor equipped with a robotic manipulator. First, the kinematic and dynamic models of a quadrotor with multi-Degree of Freedom (DOF) robotic manipulator are established together using Euler–Lagrange equations. Based on the aggregated dynamic model, the control scheme consisting of position controller, attitude controller and manipulator controller is presented. Regarding cooperative transporting and assembling, multiple quadrotor–manipulator systems should be able to form a desired formation without collision among quadrotors from any initial position. The desired formation is achieved by the distributed position controller and attitude controller, while the collision avoidance is guaranteed by an artificial potential function method. Then, the transporting and assembling tasks request the manipulators to reach the desired angles cooperatively, which is achieved by the distributed manipulator controller. The overall stability of the closed-loop system is proven by a Lyapunov method and Matrosov's theorem. In the end, the proposed control scheme is simplified for the real application and then validated by two formation flying missions of four quadrotors with 2-DOF manipulators.     

Dynamic Modeling Based on Real-Time Deflection Measurement and Compensation Control for Flexible Multi-Link Manipulators

The problem of tip trajectory tracking control is considered in this paper for flexible multi-link manipulators. An integrated optical laser sensor system is utilized to measure the tip deformations of the flexible links. The Lagrangian assumed-mode method incorporating the measured linear displacements and angular deflections of flexible links is used to derive the dynamic model of the flexible manipulator. To reduce as far as possible the tip tracking/positioning errors caused by the link flexibility, an error compensation approach is proposed. The additional compensation amounts of joint variables are calculated kinematically in terms of the measured deformations, and are added to the nominal commands generated by the computed torque controller. The simulation results demonstrate the effectiveness of the proposed approach.     

Vision-Based Task-Level Control of a Flexible-Link Manipulator

This paper discusses a vision-based approach to implement task-level control in flexible-link manipulators. The proposed approach emphasizes the advantage of using vision in the control of flexible manipulators. It is pointed out that taking advantage of the inherent robustness, implementation of an image-based visual servo can be regarded as a synthetic solution to precise task-level control of flexible manipulators. This approach is implemented in a three-dimensional flexible-link manipulator. The implementation makes good use of filters in decoupling task-level control and vibration suppression control. Moreover, we point out that although the robustness of the approach can help to overcome the difficulty in control resulting from the complex measurement of the link's elastic deformation, it lacks in capability of tip trajectory specification. This problem is analyzed in this research and it leads to the proposal for the integration of the image interpolation technique. This technique makes the proposed approach adequate for tasks involved with complex tip trajectories. For flexible-link manipulators, the proposed approach with the remedy is the first vision-based synthetic solution that attempts to make a flexible manipulator usable for a practical task.     

Detection of needle puncture to blood vessel by using electric conductivity of blood for automatic blood sampling

Although blood sampling is frequently performed, a system to take blood sample automatically has not yet been developed. To achieve needle control for automatic blood sampling, the changes in puncture force and electric conduction by blood were evaluated. An injection needle with electric conduction electrode was set on a device for measuring puncture force by using a loadcell. When the needle punctured a blood vessel, blood flowed into the needle and caused a short circuit between electrodes attached on the skin surface and inside of the needle. Totally 18 of puncture experiments were carried out on vein of a rabbit's ear. The peak of puncture force caused by piercing the vessel wall and the electric conduction by the blood, were simultaneously observed in successful cases. When the needle pierced the vein out, no peak of puncture force but the electric conduction of the blood was obtained. This result shows that the electric conduction by blood can be applied to control needle insertion to the vein for automatic blood sampling.     

Posture measure and control for robot compliance tasks

Various measures have been proposed for evaluating the compatibility of manipulator postures with respect to task requirements from kinematic and dynamic standpoints. In most previous studies, the measures were used to determine optimal postures for manipulators in advance, and their effects on system performance were generally examined statically. When posture measures are applied to controlled dynamic systems, however, their effects are usually not evident, because deficiencies (merits) caused by bad (good) postures can be compensated for by the controller. On the other hand, postures determined according to proper measures can still alleviate the controller's load and be helpful in control strategy realization. In this paper, we propose that planned compliant motion trajectories should be accompanied by proper postures for compliance tasks. Thus, we analyze manipulator dynamic behavior by using postures specified according to various measures. And, because different postures are used in different phases of the compliance task, a posture selection and control scheme is also proposed to govern the sequence of postures selected according to task requirements and environments. Redundant robot manipulators were used for investigation because of their better manipulability. Simulations that demonstrate the effectiveness of the proposed scheme are described. © 1998 John Wiley & Sons, Inc.     

Visibility Enhancement using Autonomous Multicamera Controls with Situational Role Assignment for Teleoperated Work Machines

The aim of this study is to provide a machine operator with enhanced visibility and more adaptive visual information suited to the work situation, particularly advanced unmanned construction. Toward that end, we propose a method for autonomously controlling multiple environmental cameras. Situations in which the yaw, pitch, and zoom of cameras should be controlled are analyzed. Additionally, we define imaging objects, including the machine, manipulators, and end points; and imaging modes, including tracking, zoom, posture, and trajectory modes. To control each camera simply and effectively, four practical camera roles with different combinations of the imaging objects and modes were defined: overview machine, enlarge end point, posture‐manipulator, and trajectory‐manipulator. A real‐time role assignment system is described for assigning the four camera roles to four out of six cameras suitable for the work situation (e.g., reaching, grasping, transport, and releasing) on the basis of the assignment‐priority rules. To test this system, debris‐removal tasks were performed in a virtual reality simulation to compare performance among fixed camera, manual control camera, and autonomous control camera systems. The results showed that the autonomous system was the best of the three at decreasing the number of grasping misses and erroneous contacts and simultaneously increasing the subjective usability and time efficiency.     

Developing a method to plan robotic straight needle insertion using a probability-based assessment of puncture occurrence

Needle-based treatments for cancer require accurate placement of the needle tip into the target tissue. However, it is often difficult to insert a needle accurately because of the cancer displacement caused by organ deformation. Therefore, developing a planning method based on a numerical simulation that analyzes organ deformation is important for accurate needle insertion. However, predicting the puncture conditions, including the force applied to the needle is not trivial owing to marked variations in the experimental data. The purpose of this research is to develop a novel method for predicting a robust path for straight needle insertion with various puncture points. The method is based on the probabilities of the various puncture conditions and evaluates the expected accuracy of needle placement. First, a probability-based puncture condition was established and the expected accuracy of needle placement was defined. We also performed in vitro needle insertion experiments using a porcine liver. Significant variations in puncture force were observed. Accordingly, we established a probability distribution for the tissue stress caused by the puncture. An in vitro experiment was performed to measure needle placement accuracy using the optimized path. Experimental results show that the proposed method achieves a mean accuracy of 1.5?mm.     

地面和空间机械臂的动力学等效条件与控制相似律

针对如何在地面机械臂上有效验证空间机械臂控制律的问题,本文研究空间机械臂和地面机械臂系统间的动力学等效条件和控制相似律.首先,基于量纲分析研究地面机械臂和空间机械臂之间的动力学等效条件,并基于等效条件设计地面机械臂系统.其次,基于量纲分析建立空间和地面机械臂系统间的控制相似律,设计的空间机械臂控制律通过控制相似律将可以转化为地面机械臂相应的控制律.最后,考虑地面机械臂基座往往无法和空间机械臂的基座航天器一样进行全6自由度运动,以及地面机械臂运动时受到重力影响,使得地面机械臂不再满足动力学等效条件,基于反馈线性化技术设计一种地面机械臂的动力学误差补偿策略,使得地面机械臂和空间机械臂具有相似的闭环动力学行为.这样,空间机械臂的控制律可以在设计的地面机械臂上进行验证,仿真中以在地面机械臂上验证空间机械臂的PID控制器为例说明所提方法的有效性.     

Model of Mobile Manipulator Performance Measurement using SysML

Test methods for measuring safety and performance of mobile manipulators have yet to be developed. Therefore, potential mobile manipulator users cannot compare one system to another. Systems Modeling Language (SysML) is a general-purpose modeling language for systems engineering applications that supports the specification, analysis, design, verification, and validation of simple through complex systems, such as mobile manipulators. As test methods are developed to allow performance comparison of the varied mobile manipulators, so to should be the case of allowing comparison of most any mobile manipulator configuration and control strategy during performance measurements. Additionally, mobile manipulator manufacturers and users can then compare these systems to tasks using various methods. This paper uses SysML to describe two measurement methods (optical tracking and artifacts) and the performance measurement of mobile manipulators performing assembly tasks. The SysML models are verified through systems review, referenced experimentation and summarize with uncertainty propagation models of the mobile manipulator.     

Adaptive tracking control of rigid manipulators using only position measurements

This article presents a new approach to trajectory tracking control of uncertain rigid manipulators using only position measurements. The proposed control strategy is an adaptive scheme that is very general and computationally efficient, requires virtually no information regarding the manipulator dynamic model, and is implementable without calculation of the robot inverse dynamics or inverse kinematic transformations. It is shown that the controller ensures semiglobal uniform boundedness of all signals in the presence of bounded disturbances, and that the ultimate size of the tracking errors can be made arbitrarily small. Additionally, it is demonstrated that the proposed strategy can be used as the basis for developing controllers for “cascaded” robotic systems, such as manipulators with significant actuator dynamics or joint flexibility. The efficacy of this approach to manipulator control is illustrated through both computer simulations and hardware experiments. © 1997 John Wiley & Sons, Inc.     

Integrated Environment for Modelling, Simulation and Control Design for Robotic Manipulators

In this paper an integrated environment for the design of robotic controllers implemented on a PC is described. It is based on the Planar Manipulators Toolbox for dynamic simulation of redundant planar manipulators. The tools are fully integrated in the MATLAB/SIMULINK and hence, a lot of standard tools are available for the analysis and control design. Using the real-time simulation it is possible to apply the developed controllers to a real robot manipulator, which can be included in the system via corresponding interfaces, without any additional coding. The main advantage is the flexibility in fast prototyping of different algorithms in the field of control of robotic systems, especially for redundant manipulators.     

机器人的鲁棒预测控制

本文提出基于误差预测的机器人鲁棒控制器。考虑到机器人的动力学建模误差影响其控制性能,本文建立机器人的误差模型,给出预测建模误差对运动轨迹偏差的作用的有效方法,并提出建模误差的鲁棒性补偿。本文分别在关节空间和直角空间针对冗余机器人和非冗余机器人提出鲁棒预测控制器设计,其有效性由仿真例子检验。     

空间机械臂在轨维修的视觉伺服操控策略

首先分析了空间机械臂/机械手系统在轨旋拧螺钉任务的尺寸链误差．为修正微重力环境和机械臂、模拟维修单机在轨安装导致的位姿偏差,特别是消除机械手抓取电动工具导致的随机误差影响,提出了航天员在轨标定电动工具位姿/全局相机测量电动工具位姿并引入机械臂进行视觉伺服的控制策略．设计了视觉伺服控制器,给出了该控制算法的收敛证明和稳定性分析．通过在轨实施,该策略实现了机械臂/机械手系统拧松螺钉时位姿误差不超过3 mm/2°的任务要求．     

基于鲁棒滑模观测器的两关节柔性机械手控制

柔性机械手系统为非最小相位系统, 当控制有界时, 该特性阻碍其端点位移渐近跟踪期望轨迹. 本文首先重新定义柔性机械手系统的输出, 通过输入输出线性化, 将系统分解为输入输出子系统和零动态子系统; 然后提出一种用于观测柔性模态导数的鲁棒滑模观测器, 使状态估计达到预期的指标, 解决了柔性模态导数难以获得的问题; 设计积分滑模控制策略, 使输入输出子系统在有限时间收敛到零; 选择适当的控制器参数, 使零动态子系统在 平衡点附近渐近稳定, 从而保证整个系统的渐近稳定. 本文提出的方法设计过程简单, 易于实现. 仿真结果证明了设计的有效性.     

Robust computationally efficient control of cooperative closed-chain manipulators with uncertain dynamics

This article presents a decentralized control scheme for the complex problem of simultaneous position and internal force control in cooperative multiple manipulator systems. The proposed controller is composed of a sliding mode control term and a force robustifying term to simultaneously control the payload's position/orientation as well as the internal forces induced in the system. This is accomplished independently of the manipulators dynamics. Unlike most controllers that do not require prior knowledge of the manipulators dynamics, the suggested controller does not use fuzzy logic inferencing and is computationally inexpensive. Using a Lyapunov stability approach, the controller is proven to be robust in the face of varying system's dynamics. The payload's position/orientation and the internal force errors are also shown to asymptotically converge to zero under such conditions.     

Adaptive Control of a Two-Link Robot Using Batch Least-Square Identifier

We design a regulation-triggered adaptive controller for robot manipulators to efficiently estimate unknown parameters and to achieve asymptotic stability in the presence of coupled uncertainties. Robot manipulators are widely used in telemanipulation systems where they are subject to model and environmental uncertainties. Using conventional control algorithms on such systems can cause not only poor control performance, but also expensive computational costs and catastrophic instabilities. Therefore, system uncertainties need to be estimated through designing a computationally efficient adaptive control law. We focus on robot manipulators as an example of a highly nonlinear system. As a case study, a 2-DOF manipulator subject to four parametric uncertainties is investigated. First, the dynamic equations of the manipulator are derived, and the corresponding regressor matrix is constructed for the unknown parameters. For a general nonlinear system, a theorem is presented to guarantee the asymptotic stability of the system and the convergence of parameters’ estimations. Finally, simulation results are discussed for a two-link manipulator, and the performance of the proposed scheme is thoroughly evaluated.      

Dynamic modeling and vibration characteristics analysis of flexible-link and flexible-joint space manipulator

With the development of space technology, lighter and larger space manipulators will be born, of which flexible characteristics are more obvious. The manipulator vibration caused by the flexibility not only reduces the efficiency of the manipulator but also affects the accuracy of the operation. The flexibility of space manipulator mainly comes from structural flexibility of links and transmission flexibility of harmonic gear reducer in joints. The vibrations generated by these two kinds of flexibility are coupled and transformed mutually, making the dynamics characteristics of space manipulator system complicated. Therefore it is difficult to assess respective effects of these flexibilities on vibrations of the manipulator tip. And the characteristics of integrated vibration of manipulator tip with different link and joint stiffnesses are not very clear. In this paper, the dynamic equations of multi-link multi-DOF flexible manipulator are established. Then, vibration responses of the tip under different elastic modulus, damping and joint stiffness were studied, and vibration characteristics of the tip with both link and joint were also analyzed. Moreover, the effects of motion planning on the vibration of the tip were analyzed. Finally, the vibration characteristics of the manipulator with flexible joints and links are verified by a two-degree-of-freedom manipulator experimental system. Dynamics analysis results presented some useful rules for the path planning and control to suppress the vibration of the flexible space manipulator.     

Position synchronised control of multiple robotic manipulators based on integral sliding mode

In this study, a new position synchronised control algorithm is developed for multiple robotic manipulator systems. In the merit of system synchronisation and integral sliding mode control, the proposed approach can stabilise position tracking of each robotic manipulator while coordinating its motion with the other manipulators. With the integral sliding mode, the proposed approach has insensitiveness against the lumped system uncertainty within the entire process of operation. Further, a perturbation estimator is proposed to reduce chattering effect. The corresponding stability analysis is presented to lay a foundation for theoretical understanding to the underlying issues as well as safely operating real systems. An illustrative example is bench tested to validate the effectiveness of the proposed approach.