A Family of Symmetrical Lower‐Mobility Parallel Mechanisms with Spherical and Parallel Subchains

This paper presents a new family of symmetrical lower‐mobility parallel mechanisms (PMs) with spherical and parallel subchains, which consists of two 5‐DOF (degrees of freedom) PMs, one 4‐DOF PM and five 3‐DOF PMs. The basic feature of this family is that each limb consists of five revolute pairs and can be constructed with two subchains, a 2R pointing subchain and a 3R parallel subchain, or a 3R spherical subchain and a 2R parallel subchain. Different geometrical arrangements of the limbs lead to different degrees of freedom. All the PMs of this family can be modularized easily due to the simple structure of the subchains. © 2003 Wiley Periodicals, Inc.     

空间冗余机械臂路径规划方法研究

针对空间站遥操作7DOF冗余机械臂路径规划的安全性、可靠性问题,提出了基于臂型角逆运动学的优化A*路径规划算法.本文根据臂型角参数化完善了逆运动学方法,得到了32组完备逆解集,增加了路径规划时逆解选择的灵活性;通过臂型角搜索和最小奇异值优化A*路径规划算法,提高机械臂避障、避奇异能力,机械臂操作的灵活性和路径的安全可靠性;同时根据路径优化策略,有效平滑了路径,减少了机械臂的磨损.仿真结果说明了该方法的有效性.     

Experimental two-axis vibration suppression and control of a flexible robot arm

This article focuses on the implementation of a dual-mode controller for the maneuver of a two-axis flexible robotic arm. The joint angle trajectory tracking is accomplished by proportional and derivative and feedforward controllers. Based on the pole placement technique, a linear stabilizer is designed for elastic mode stabilization in the plane perpendicular to each joint axis. The stabilizer is switched on when the trajectory reaches the vicinity of the terminal state. The effect of switching time of the stabilizer and varying payload on arm vibration are investigated. With the proposed control system, accurate joint angle tracking and elastic mode stabilization can be accomplished. © 1993 John Wiley & Sons, Inc.     

Forward kinematic singularity avoiding design of a Schönflies motion generator by asymmetric attachment of subchains

In most of the previous studies on parallel mechanisms (PMs), architectural design mainly relying on symmetric geometry was investigated without in-depth analysis of its performance. This work demonstrates that such a symmetric geometry of multiple subchains sometimes induces a forward kinematic singularity which degrades the overall kinematic performance of PMs within the desired workspace and claims that an asymmetric attachment of those subchains on a moving platform can effectively resolve such a singularity problem. A 4-Degree-of-Freedom (DOF) PM exhibiting Schönflies motions is examined as an example device. First, its mobility analysis and kinematic modeling via screw theory are conducted. Then a singularity analysis based on Grassmann line geometric conditions is carried out, and the forward kinematic singularities of the mechanism are identified and verified by simulations. Based on these analysis and simulations, a forward kinematic singularity-free design is suggested. To show the high potential of the device in practical applications, its output stiffness and dynamic motion capability are examined. Then a prototype is built and its motions capability is verified through experiments.     

空间机器人目标捕获过程中的载体姿态扰动优化

针对自由飘浮空间机器人在捕获目标的过程中会对其载体姿态产生扰动的问题,提出了一种新的笛卡儿轨迹参数化方法.建立了反映载体姿态变化的日标函数,该函数既可以限制机器人关节角的运动范围,也可以避免动力学奇异的影响.利用遗传算法进行目标函数的优化,有效降低了空间机器人末端执行器在跟踪笛卡儿轨迹时对载体姿态的影响.仿真结果验证了...     

The assessment of material handling strategies in dealing with sudden loading: The effects of load handling position on trunk biomechanics

Back injury caused by sudden loading is a significant risk among workers that perform manual handling tasks. The present study investigated the effects of load handling position on trunk biomechanics (flexion angle, L5/S1 joint moment and compression force) during sudden loading. Eleven subjects were exposed to a 6.8 kg sudden loading while standing upright, facing forward and holding load at three different vertical heights in the sagittal plane or 45° left to the sagittal plane (created by arm rotation). Results showed that the increase of load holding height significantly elevated the peak L5/S1 joint compression force and reduced the magnitude of trunk flexion. Further, experiencing sudden loading from an asymmetric direction resulted in significantly smaller peak L5/S1 joint compression force, trunk flexion angle and L5/S1 joint moment than a symmetric posture. These findings suggest that handling loads in a lower position could work as a protective strategy during sudden loading.     

基于强化学习的煤矸石分拣机械臂智能控制算法研究

针对传统煤矸石分拣机械臂控制算法如抓取函数法、基于费拉里法的动态目标抓取算法等依赖于精确的环境模型、且控制过程缺乏自适应性,传统深度确定性策略梯度(DDPG)等智能控制算法存在输出动作过大及稀疏奖励容易被淹没等问题,对传统DDPG算法中的神经网络结构和奖励函数进行了改进,提出了一种适合处理六自由度煤矸石分拣机械臂的基于强化学习的改进DDPG算法。煤矸石进入机械臂工作空间后,改进DDPG算法可根据相应传感器返回的煤矸石位置及机械臂状态进行决策,并向相应运动控制器输出一组关节角状态控制量,根据煤矸石位置及关节角状态控制量控制机械臂运动,使机械臂运动到煤矸石附近,实现煤矸石分拣。仿真实验结果表明:改进DDPG算法相较于传统DDPG算法具有无模型通用性强及在与环境交互中可自适应学习抓取姿态的优势,可率先收敛于探索过程中所遇的最大奖励值,利用改进DDPG算法控制的机械臂所学策略泛化性更好、输出的关节角状态控制量更小、煤矸石分拣效率更高。     

Reaching with multi-referential dynamical systems

We study a reaching movement controller for a redundant serial arm manipulator, based on two principles believed to be central to biological motion control: multi-referential control and dynamical system control. The resulting controller is based on two concurrent dynamical systems acting on different, yet redundant variables. The first dynamical system acts on the end-effector location variables and the second one acts on the joint angle variables. Coherence constraints are enforced between those two redundant representations of the movement and can be used to modulate the relative influence of each dynamical system. We illustrate the advantages of such a redundant representation of the movement regarding singularities and joint angle avoidance.     

A partitioned redundancy management scheme for an eight‐joint revolute manipulator

A partitioned redundancy management approach is developed for a class of eight‐joint manipulators with four‐axis intersecting wrists. Segmenting the excess degrees of freedom at the wrist simplifies the inverse kinematics problem substantially and allows different and often competing schemes to be used for controlling the motion of the arm. In the upper arm, the roll angle of the shoulder–elbow–wrist plane is controlled directly. In the wrist, the spare joint is used to minimize the instantaneous joint velocities. In addition, the partitioning scheme substantially reduces the number of singularities that can be encountered by the arm and thus actively avoided. A new four‐axis “skew” wrist design is also introduced which doubles the workspace size of the traditional orthogonal design while retaining the ability to avoid singularities. Simulations are used to illustrate the redundancy management approach on the dexterous manipulator for a submersible robotic vehicle. © 2000 John Wiley & Sons, Inc.     

A Geometric Theory for Analysis and Synthesis of Sub-6 DoF Parallel Manipulators

Mechanism synthesis is mostly dependent on the designer's experience and intuition and is difficult to automate. This paper aims to develop a rigorous and precise geometric theory for analysis and synthesis of sub-6 DoF (or lower mobility) parallel manipulators. Using Lie subgroups and submanifolds of the special Euclidean group SE(3), we first develop a unified framework for modelling commonly used primitive joints and task spaces. We provide a mathematically rigorous definition of the notion of motion type using conjugacy classes. Then, we introduce a new structure for subchains of parallel manipulators using the product of two subgroups of SE(3) and discuss its realization in terms of the primitive joints. We propose the notion of quotient manipulators that substantially enriches the topologies of serial manipulators. Finally, we present a general procedure for specifying the subchain structures given the desired motion type of a parallel manipulator. The parallel mechanism synthesis problem is thus solved using the realization techniques developed for serial manipulators. Generality of the theory is demonstrated by systematically generating a large class of feasible topologies for (parallel or serial) mechanisms with a desired motion type of either a Lie subgroup or a submanifold.     

Trajectory optimization for kinematically redundant arms

A review of local optimization methods for resolving joint configurations in underconstrained manipulation tasks is conducted. A new approach is developed for observing joint limits and avoiding obstacles during the trajectory planning. The methodology is used in a four-link arm example to avoid a workspace singularity and is compared with results using the extended Moore-Penrose technique. An alternative measure of arm “manipulability” based directly on the rank of the Jacobian is also introduced.     

Keyswitch orientation can reduce finger joint torques during tapping on a computer keyswitch

OBJECTIVE: To examine the effects of keyswitch orientation on joint torques. BACKGROUND: The fingertip produces primarily vertical forces during single-finger tapping on a computer keyswitch. However, horizontal force components within the sagittal plane of the finger could reduce net joint torques. METHOD: Eleven participants tapped on a keyswitch oriented in three directions: vertical, tilted 30 degrees such that when pressed it moved away from the user (similar to a positive-tilt keyboard), and tilted 30 degrees such that when pressed it moved toward the user (similar to a negative-tilt keyboard). Participants also tapped on a prototype cantilever keyswitch design in which the key cap moves along the arc of a bending beam gradually away from the user. Miniature electro-optical goniometers measured the finger posture, and a two-axis force sensor measured fingertip forces. RESULTS: Tapping on a keyswitch oriented such that it moves away from the user when pressed reduced net joint torques by 47% relative to tapping on a vertically orientated keyswitch and by 56% relative to tapping on a keyswitch oriented toward the user, whereas the cantilever design resulted in 14% decreases in net joint torque relative to the vertical orientation. CONCLUSION: Reductions of torques resulted from decreasing the moment arm of the fingertip force about the joints. APPLICATION: Keyboard design should incorporate keyswitch mechanism angles along with other postural and geometric constraints to reduce exposure of the finger joints and muscles to force during typing.     

Effects of sitting and standing, reach distance, and arm orientation on isokinetic pull strengths in the horizontal plane

In recent years, isokinetic strengths (dynamic strength exertions at constant speed) have almost replaced isometric (static) strengths in laboratory studies as measures of a person's strength exertion capabilities. Many industries are also showing a keen interest in replacing static strength usage with dynamic strength usage. The increasing acceptance of isokinetic strengths as a more valid and accurate measure of people's strength exertion capability has necessitated the development of isokinetic strength databases. This paper presents one-arm isokinetic pull strength profiles of males, engaged in infrequent exertion in a horizontal plane, as a function of posture (sitting and standing), reach distance (25, 40, and 55 cm for the sitting posture; 45, 65, and 85 cm for the standing posture), and angle of the preferred (stronger) arm from the frontal plane (0—frontal plane, 30, 60, 90, 120, and 150 deg). Twenty-five males participated in the study. The results indicated that more strength is exerted while standing. The strength also increases with the reach distance. The strength exertion becomes stronger as the angle of the arm increases to 90° from the frontal plane (i.e., the arm moves to the sagittal plane) and then weakens.     

Trajectory control of a PUMA robot arm by inversion and servocompensation

Based on invertibility and functional reproducibility, an approach to the control of the PUMA arm with six rotational degrees of freedom is presented. The control system has two important features: (i) the ability for fast, precise trajectory tracking; and (ii) robustness to uncertainty in the system, such as variable payload, inertia, etc. The control law derived using invertibility gives decoupled, independently controlled responses in each joint angle using torquers. For robustness, a first-order servocompensator is designed for each decoupled inner loop driven by the joint angle error. For smooth joint angle responses, a command generator is constructed that produces the reference trajectories to be tracked. Simulation results are presented to show that rapid, accurate trajectory following is achieved in the closed-loop system in spite of large payload uncertainty.     

Two-dimensional broadband and wide angle scanning multi-beam antenna based on leaky-wave metasurface

In this article, a novel two-dimensional multi-beam antenna with a broad band and a wide angle scanning range is proposed. It is composed a leaky-wave metasurface (MTS) and a four-port feeding network with high isolation. The leaky-wave MTS formed by T-shaped slots is displayed as radiator and divided into four angular sectors, each one devoted to the formation of a beam in a given elevation plane. At every fixed frequency, the antenna can radiate multi beam in azimuth plane through exciting different ports. Also, multi-beam radiation with a broad band and a wide angle scanning range in the elevation plane is realized when fixed port is excited at different frequency. The antenna with overall size of 207 mm by 207 mm by 2.0 mm is fabricated on FR-4 substrate. The measured and simulated results show that the ?10 dB relative bandwidth is 30% (from 9.44 to 12.77 GHz). When different ports are excited at the same frequency, the azimuth of radiation beam is steered to 0°, 90°, 180°, and 270°. In addition, the beam-scanning range of the prototyped antenna is from 29° to 75° when the frequency sweeps in the range of 9.5–12.0 GHz. Also, the maximum radiation efficiency reaches to 31.1% and the measured peak gain within the scanning range is 12.29 dBi.     

一种基于指数积的移动机械臂联合标定方法

提出一种基于指数积的移动机械臂联合标定方法,以实现移动平台和机械臂两者间位姿标定与机械臂运动学参数标定模型的统一.机械臂运动学参数标定使用最多的是基于D-H参数法,但D-H参数法无法克服相邻关节平行或接近平行时的奇异性问题,以及建模过程复杂、建模后的模型通用性差等问题.基于指数积的移动机械臂联合标定方法建模时不会因为关节轴平行而出现奇异性问题,建模过程简单.通过对整个系统的运动学方程进行微分运算,获得末端位姿误差和移动机械臂零位状态旋量误差及关节旋量误差的线性化模型.利用伴随矩阵方式建立关节旋量理论值与关节旋量实际值的关系,并通过改变伴随矩阵实现基于最小二乘法的参数辨识计算过程中参数更新.使用高精度激光跟踪仪作为测量工具,通过实验验证所提出方法的有效性.     

Reconfigurable main‐arm for assembly of all revolute‐only kinematically simple branches

Within previous work by Podhorodeski and Pittens,a class of kinematically simple (KS) joint layouts was defined for spatial serial joint‐assemblies consisting of revolute joints only. The identified KS class of layouts was comprised of main‐arms with three successively parallel and/or perpendicular revolute joints and end located spherical joint groups. Arguments of degeneracy and kinematic equivalency can be used to demonstrate that only five unique main‐arm layouts belong to the KS class. Within the current work, the design of a reconfigurable main‐arm (RMA) allowing the construction of all of the KS branches is examined. Sufficient joints are included within the RMA to allow, by the locking of certain joints and the freeing (actuation) of others, the kinematic attributes of each of the unique KS main‐arm layouts to be acquired. It is concluded that five successively perpendicular revolute joints are required within the RMA to allow such reconfiguration. Inverse kinematic solutions are presented for each of the five layouts. Potential RMA applications, use of the RMA for kinematically redundant layouts, and an RMA prototype are discussed. ©2000 John Wiley & Sons, Inc.     

Determination of comfortable position for tractor Driver's hands based on dynamic load

Disorders in the musculoskeletal system are currently the most common illnesses reported by physical workers. This paper presents the possibility of improving safety conditions for vehicle and machine operators by the adjustment of a steering panel to their individual anthropometric features. These results can be used in the workstation-designing phase. When driving a tractor for a long time, the position of the human body influences the working comfort and can lead to disorders. To assess the working comfort on a mobile workstation as well as a farm tractor, surface electromyography was used. Based on the given parameter maximum voluntary contraction (MVC), the lowest workload for the forearm, arm and wrist was determined, considering the position of the steering column and the angle between arm and forearm (elbow angle). Experimental tests showed that the most comfortable position for a driver's hands was recorded when the tested elbow angle was 100°, as well as when the inclination angle of the steering column against the horizontal plane was approximately 50°. In this study, the MVC parameter was determined on the basis of muscle tension amplitude related to the maximum muscle isometric contraction. Based on this parameter, the percentage of developed force of tested muscle in relation to its maximum force-generating capabilities was described. This approach helped to define a relationship between human body position and generated load in human muscles.     

On the use of accelerometers in iterative learning control of a flexible robot arm

Iterative learning control (ILC) is applied to a robot arm with joint flexibility. The ILC algorithm uses an estimate of the arm angle, where the estimate is computed using measurements of the motor angle and the arm angular acceleration. The design of the ILC algorithm is evaluated experimentally on a laboratory scale robot arm with good results.