Six-DOF Impedance Control of Dual-Arm Cooperative Manipulators

In this paper, the problem of impedance control of dual-arm cooperative manipulators is studied. A general impedance control scheme is adopted, which encompasses a centralized impedance control strategy, aimed at conferring a compliant behavior at the object level, and a decentralized impedance control, enforced at the end-effector level, aimed at avoiding large internal loading of the object. Remarkably, the mechanical impedance behavior is defined in terms of geometrically consistent stiffness. The overall control scheme is based on a two-loop arrangement, where a simple proportional integral derivative inner motion loop is adopted for each manipulator, while an outer loop, using force and moment measurements at the robots wrists, is aimed at imposing the desired impedance behaviors. The developed control scheme is experimentally tested on a dual-arm setup composed of two 6-DOF industrial manipulators carrying a common object. The experimental investigation concerns the four different controller configurations that can be achieved by activating/deactivating the single impedance controllers.     

An adaptive multi-objective motion distribution framework for wheeled mobile manipulators via null-space exploration

Wheeled mobile manipulators (WMMs) are primarily composed of a manipulator and a mobile base, which lead to their agility, maneuverability, and mobility. Despite impressive progress in recent years, there remains some substantial work in improving WMMs’ motion precision, owing to the current limitations in unpredictable wheel slippage, high system redundancy, and absence of external perception information. One intuition is to distribute more of a given set of end-effector motion requirements to the manipulator adaptively — to reduce the motion error introduced by wheel locomotion. With the ultimate goal of improving a WMM’s motion accuracy without external perception information, here we present a novel adaptive multi-objective motion distribution framework (AMoMDiF) for a redundant WMM, which is drawn inspiration from null-space exploration. The framework adopts a hierarchical structure to explore the WMM’s null space iteratively to achieve three tasks, which are end-effector motion achievement (primary objective), adaptive motion distribution (secondary objective), and manipulability enhancement to avoid singularity (tertiary objective). The secondary objective strives to assign more of an end-effector motion to the manipulator when possible. The tertiary goal will enhance the manipulator’s capability to stay away from singularities via the WMM’s remaining redundancy after the primary and secondary tasks are completed. Experiment results on a physical platform show that, compared with the traditional motion planning method, the proposed AMoMDiF significantly improves the WMM’s motion accuracy through the achievements of the three objectives.     

Disturbance-observer-based motion control of redundant manipulatorsusing inertially decoupled dynamics

In this paper, a robust motion control method for kinematically redundant manipulators is addressed to control the motion of the end-effector, as well as the null-space motion. To reduce the disturbance effects on the system performance, a new disturbance observer is proposed to improve the performance of the conventional structure. A minimal parametrization of the null space is performed to visualize the null-space motion explicitly using the weighted decomposition of joint space. Augmenting this null motion with conventional velocity relations, an extended task space formulation is obtained. The proposed disturbance observer is adopted in the extended task space formulation and a motion control method is devised based on the passivity concept. The performance of the proposed controller is verified through experiments with a three-link planar direct-drive manipulator     

Experimental Evaluation of Dynamic Redundancy Resolution in a Nonholonomic Wheeled Mobile Manipulator

Mobile manipulators derive significant novel capabilities for enhanced interactions with the world by merging mobility with manipulation. However, a careful resolution of the redundancy and active control of the reconfigurability, created by the surplus articulated DOFs and actuation, are the keys to unlocking this potential. Nonholonomic wheeled mobile manipulators, formed by mounting manipulator arms on disc-wheeled mobile bases, are a small but important subclass of mobile manipulators. The primary control challenges arise due to the dynamic-level coupling of the nonholonomy of the wheeled mobile bases with the inherent kinematic and actuation redundancy within the articulated chain. The solution approach in this paper builds upon a dynamically consistent and decoupled partitioning of the articulated system dynamics between the external (task) space and internal (null) space. The independent controllers, developed within each decoupled space, facilitate active internal reconfiguration, in addition to resolving redundancy at the dynamic level. Specifically, two variants of null-space controllers are implemented to improve disturbance rejection and active reconfiguration during performance of end-effector tasks by a primary end-effector impedance mode controller. These algorithms are evaluated within an implementation framework that emphasizes both virtual prototyping and hardware-in-the-loop testing with representative case studies.     

Robust servosystem design with two degrees of freedom and itsapplication to novel motion control of robot manipulators

A novel robust servosystem design method based on the two-degree-of-freedom (TDOF) controller and its application to advanced motion control for a robot manipulator is proposed. This servosystem is derived from the simple parametrization. The command input response and the closed loop characteristics can be specified independently by using two parameters which belong to the ring of stable and proper rational functions. The sensitivity and the complementary sensitivity functions can be determined straightforwardly through the optimization of the two design parameters. The control performance of the servosystem has been demonstrated. A completely decentralized joint control system for multiaxis robut manipulators has been realized. In particular, various kinds of robot motion controls, such as compliance, force, and hybrid controls, are realized in a unified way based on the robust position control. This servosystem has been implemented using DSP     

Automatic control for a miniature manipulator based on 3D vision servo of soft objects

This paper presents a new strategy for automating three-dimensional miniaturized manipulators using a visual feedback. This automation scheme can be used to handle the challenges associated with manipulation of minute soft components such as biological entities. The developed vision control system combines a depth and planar motion control using a single camera and an auto focus algorithm. This visual process was conducted by applying image segmentation and probe recognition along with depth alignment. To demonstrate the feasibility, reliability and robustness of this newly proposed strategy, extensive computer simulations were performed on images of a Zyvex® nanomanipulator obtained using a Scanning Electron Microscope (SEM). Also, a prototype manipulator with a visual feedback and an automatic controller was constructed and tested using soft elements. The results showed good accuracy and robustness of the end-effector recognition, positioning and aligning without occlusion difficulties.     

A cooperative optimization strategy for distributed multi-robot manipulation with obstacle avoidance and internal performance maximization

Reactive multi-robot manipulation can be of great assistance in many applications. In this article a distributed cooperation strategy for object manipulation with multiple robots in un-modelled environments is proposed to tackle workpiece transportation and dynamic obstacle avoidance simultaneously. A decentralised optimisation process will run first to generate robot pose references and then a joint-level control is responsible for tracking. Since in multi-arm manipulation systems the available workspace for each robot is more restricted, necessary internal performance optimisation is also included in the planning phase. A velocity-level optimum will be solved and then transformed to a position-level control reference so that it can be combined with many compliant interactive controllers such as impedance control. Although no torque or force sensor is required and each robot only has access to local information, when avoiding obstacles the coordination strategy can generate synchronised group motion so that internal forces are avoided. Only task-space variables will be communicated, therefore it is easy to team up different types of robots, making the system more flexible. Simulation and experiments with two redundant manipulators have been done to validate the proposed design, and data show that the online computation speed is fast.     

Calibration of a multi-mobile coil magnetic manipulation system utilizing a control-oriented magnetic model

In this paper, we address the calibration of a family of magnetic manipulation systems composed of several coils that are moved around by serial robot manipulators. We show in this paper that the calibration of the whole system ultimately results in calibrating the manipulator and coil separately up to an unknown rigid transformation. For calibration of the coil, we propose to use a model that has not been used so far in the literature; a control-oriented model which is sufficiently accurate and computes the magnetic field in real time. A protocol for calibrating the magnetic manipulation system using the Nelder–Mead algorithm to estimate the model parameters is presented. Calibration was performed through simulations and validated experimentally on a physical system. It was observed that the root mean square error was reduced by 37% after calibration of the physical system, indicating an improvement in accurately estimating the magnetic model.     

The robotic interception of moving objects in industrial settings:strategy development and experiment

A novel active prediction, planning, and execution (APPE) system is presented for the robotic interception of moving objects. An APPE system's objective is simply to move the robot to the earliest pregrasping location. A fine-motion tracking algorithm can take over the motion control at that point, utilizing proximity sensors mounted on the robot's end-effector. This approach eliminates the necessity of tracking the motion of the object, as required by conventional tracking-based techniques, where the distance between the robot's end-effector and the object is reduced continuously. In this paper, the proposed APPE system is first briefly introduced, and its individual modules are then discussed in detail. Simulation and experimental results are presented in support of the developed optimal-interception strategy     

Multi-hierarchy interaction control of a redundant robot using impedance learning

The control of robots with a compliant joint motion is important for reducing collision forces and improving safety during human robot interactions. In this paper, a multi-hierarchy control framework is proposed for the redundant robot to enable the robot end-effector to physically interact with the unknown environment, while providing compliance to the joint space motion. To this end, an impedance learning method is designed to iteratively update the stiffness and damping parameters of the end-effector with desired performance. In addition, based on a null space projection technique, an extra low stiffness impedance controller is included to improve compliant joint motion behaviour when interaction forces are acted on the robot body. With an adaptive disturbance observer, the proposed controller can achieve satisfactory performance of the end-effector control even with the external disturbances in the joint space. Experimental studies on a 7 DOF Sawyer robot show that the learning framework can not only update the target impedance model according to a given cost function, but also enhance the task performance when interaction forces are applied on the robot body.     

时变质量梁系统的振动主动控制研究

为了保证机械臂运动的高精度、准定位,采用主动控制技术降低结构的干扰振动是一项迫切而紧急的任务。该文以含变质量构件的弹性梁系统为对象,开展变质量弹性结构的振动特性研究和振动主动控制研究。使用模态叠加法推导了粘贴有压电片的变质量-柔性梁组合结构的控制方程,通过仿真获得变质量弹性梁系统的振动特性和运动规律。然后设计反馈控制器对质量增大系统和质量减小系统进行了振动主动控制,并采用时频分析技术分析了控制效果。仿真结果表明,变化的质量在引起系统振动频率改变的同时,还引起了一个附加的阻尼。在变质量时变系统的共振频率区间,通过反馈控制可有效抑制结构的振动。     

Force feedback control of robot manipulator by the accelerationtracing orientation method

The authors propose a novel approach to force and compliance control of multi-degree-of-freedom (DOF) robot manipulators. The acceleration tracing orientation method (ATOM) is applied to both controllers. The control law is described in the Cartesian space; however, the final command is the acceleration in the joint space. The interactive terms in each joint disturb and deteriorate the joint motion. The disturbance observer cancels out the total sum of these terms and enables each joint to trace the acceleration command. As a result, a robust control is possible in the force task. The testing of the proposed system in a three-DOF robot manipulator is discussed     

一类非线性互联系统的H∞模型参考跟踪分散输出反馈模糊控制

本文对一类不确定状态不可测非线性互联系统,给出了一种基于观测器的H_∞模型参考跟踪分散输出反馈模糊控制方法.设计中,首先采用模糊不确定T-S模型对非线性互联系统进行模糊建模,在此基础上,给出模糊分散观测器的H_∞设计和基于观测器的模型参考跟踪分散模糊控制的设计.应用李亚普诺夫和线性矩阵不等式方法给出了模糊分散系统稳定的充分条件.仿真结果进一步验证了所提出的模糊分散控制方法的有效性.     

An efficient inter-unit interference reduction technique for a MSTTD system

A multiple space-time transmit diversity (MSTTD) detection scheme on quasi-static Rayleigh flat-fading channels is proposed. The proposed detection algorithm first determines the candidates for M units each using a QR decomposition, and computes a reference metric for a full-length vector consisting of the first candidates at each unit. Then it performs a more likely searching simply by comparing the reference metric with other candidates such that an inter-unit interference can be effectively eliminated. Through these procedures it can approach the ML performance with less complexity even compared to an improved Schnorr-Euchner (ISE) scheme.     

多机协同作战任务决策方法多智能体结构框架

多机协同攻击能够充分利用各个飞机的作战资源和空间占位,是未来空战的主要模式。为了描述多架飞机协同作战过程中的信息组织形式和传递流程,基于多智能体的行为特征和构造方式,从飞机编队和飞机平台两个层面建立了多智能体结构框架。由编队中的各架飞机为智能体组成第一层多智能体,由每架飞机内部的各个功能模块为智能体组成第二层多智能体结构。以编队层多智能体和长机内部的决策功能智能体为例,介绍了多智能体数学模型的设计细节。依据所建立的多智能体结构框架和数学模型,以编队层为例,描述了多机协同作战任务决策方法的信息流程。研究表明,以飞机编队和飞机平台两个层面多智能体结构框架描述多机协同作战过程是恰当的。     

A fractional approach for the motion planning of redundant and hyper-redundant manipulators

The trajectory planning of redundant robots through the pseudoinverse control leads to undesirable drift in the joint space. This paper presents a new technique to solve the inverse kinematics problem of redundant manipulators, which uses a fractional differential of order α to control the joint positions. Two performance measures are defined to examine the strength and weakness of the proposed method. The positional error index measures the precision of the manipulator's end-effector at the target position. The repeatability performance index is adopted to evaluate if the joint positions are repetitive when the manipulator execute repetitive trajectories in the operational workspace. Redundant and hyper-redundant planar manipulators reveal that it is possible to choose in a large range of possible values of α in order to get repetitive trajectories in the joint space.     

Contact control concepts in manipulation robotics-an overview

This paper presents the state of the art in the control of robotic manipulators in constrained motion tasks. Contact control concepts are classified using different criteria, and their main characteristics are analyzed. For each of the presented contact control concepts, the essential characteristics are stated. The paper covers some early ideas and their later improvements, as well as trends in this field. The advantages and drawbacks of the various control schemes are outlined, and they are compared from the standpoint of their implementation issues. In the paper, all characteristic results in the stability analysis of robotic manipulators in the constrained motion tasks are briefly reported. A new approach to the correct solution of contact tasks control is mentioned as well     

Robust Fuzzy Neural Network Sliding-Mode Control for Two-Axis Motion Control System

A robust fuzzy neural network (RFNN) sliding-mode control based on computed torque control design for a two-axis motion control system is proposed in this paper. The two-axis motion control system is an$x-y$table composed of two permanent-magnet linear synchronous motors. First, a single-axis motion dynamics with the introduction of a lumped uncertainty including cross-coupled interference between the two-axis mechanism is derived. Then, to improve the control performance in reference contours tracking, the RFNN sliding-mode control system is proposed to effectively approximate the equivalent control of the sliding-mode control method. Moreover, the motions at$x$-axis and$y$-axis are controlled separately. Using the proposed control, the motion tracking performance is significantly improved, and robustness to parameter variations, external disturbances, cross-coupled interference, and friction force can be obtained as well. Furthermore, the proposed control algorithms are implemented in a TMS320C32 DSP-based control computer. From the simulated and experimental results due to circle and four leaves reference contours, the dynamic behaviors of the proposed control systems are robust with regard to uncertainties.     

An optimization-based approach for elasticity-aware trajectory planning of link-elastic manipulators

Elastic manipulators often result from lightweight construction or safety requirements in human–robot-collaboration scenarios. In many cases, vibration-damping becomes necessary to enable stable and precise operation, which imposes high demands on controllers. A fundamental challenge for link-elastic manipulators with general kinematics and no dedicated damping actuators is the potential inability of a joint to control the vibration of a link depending on the manipulator’s configuration. Numerous control concepts assume a sufficiently high ability to control vibrations by, for example, dedicated actuators or special kinematic structures. This work presents an online, optimization-based motion planning approach with three methods for maximizing this ability for link-elastic manipulators. The planning algorithm utilizes modified cost functions to incorporate the controllability of vibrations by prioritization and adaptive activation of competing objectives. The effectiveness of the approach is demonstrated on a real manipulator with 3 actuated DoFs and two elastic links in different disturbance scenarios. The results show that the amplitudes of vibrations caused by disturbances decay noticeably faster than with conventional motion planning.     

光子相关谱仪试样台设计

光于相关谱仪是激光散射技术领域的重要仪器，本文提出了其核心部件试验台的设计方案，并设计实现了光子相关谱仪的运动控制及温度检测单元．控制系统的核心是步进电机细分技术和步距角细分检测系统，其软件、硬件电路具有通用性和可扩展性，能够满足不同细分要求。完成的试验台设计可为国内光子相关谱仪的开发提供参考。