Translational Parallel Manipulators: New Proposals

Translational parallel manipulators are parallel manipulators wherein the end‐effector performs only spatial translations. This paper presents a new family of translational parallel manipulators. The manipulators of this family are independent constraint manipulators. They have three limbs that are topologically identical and have no rotation singularity. The limbs of these manipulators feature five one‐degree‐of‐freedom kinematic pairs in series. Four joints are revolute pairs and the remaining one, called T‐pair, is a kinematic pair that can be manufactured in different ways. In each limb, three adjacent revolute pairs have parallel axes and the remaining revolute pair has an axis that is not parallel to the axes of the other revolute pairs. The mobility analysis of the manipulators of this new family is addressed by taking into account two different choices for the actuated pairs. One of the results of this analysis is that the geometry of a translational parallel manipulator free from singularities can be defined for a particular choice of the actuated pairs. © 2002 Wiley Periodicals, Inc.     

一种新型并联机床的驱动输入选择

基于螺旋理论,将并联机床全部分支中可作为驱动输入的运动副锁定后,可以得到作用在动平台上的 约束螺旋．选出与机构的自由度数目相同的约束螺旋与机构本身固有的结构约束螺旋组成约束矩阵,根据约束矩阵 的秩确定输入的合理性．将约束力／力矩矩阵的最小奇异值与最大奇异值之比定义为约束力／力矩各向同性度,并 以此为指标对各驱动输入组合的优劣性进行评价．驱动输入选择结果表明,该并联机床存在5 种合理输入,其中以 UPS 分支中的5 个P 副为驱动输入是最优组合．     

A new family of hybrid 4‐DOF parallel mechanisms with two platforms and its application to a footpad device

This paper proposes a new family of 4‐degrees‐of‐freedom (DOF) parallel mechanisms with two platforms and its application to a footpad device that can simulate the spatial motions of the human foot. The new mechanism consists of front and rear platforms, and three limbs. Two limbs with 6‐DOF serial joints (P ‐S‐P‐P) are attached to each platform and are perpendicular to the base plate, while the middle limb is attached to the revolute joint that connects the front and rear platforms. The middle limb is driven by the 2‐DOF driving mechanism that is equivalent to active serial prismatic and revolute joints (P_e ‐R_e ), or prismatic and prismatic joints (P_e ‐P_e ) with two base‐fixed prismatic actuators. Since the middle limb perpendicular to the base plate has 3‐DOF serial joints (P_e ‐R_e ‐R or P_e ‐P_e ‐R), two new 4‐DOF parallel mechanisms with two platforms can generate pitch motion of each platform, and roll and heave motions (1T‐3R) or pitch motion of each platform and two translational motions (2T‐2R) at both platforms, according to the type of the 2‐DOF driving mechanism. Kinematic analyses of the 1T‐3R mechanism were performed, including inverse and forward kinematics and velocity analysis. Based on the 1T‐3R mechanism, a footpad device was designed to generate foot trajectories for natural walking. © 2005 Wiley Periodicals, Inc.     

Kinematics of a Hybrid Manipulator by Means of Screw Theory

In this work the kinematics of a hybrid manipulator, namely a fully parallel-serial manipulator, with a particular topology is approached by means of the theory of screws. Given the length of the six independent limbs, the forward position analysis of the mechanism under study, indeed the computation of the resulting pose, position and orientation, of the end-platform with respect to the fixed platform, is carried out in closed-form solution. Therefore conveniently this initial analysis avoids the use of a numerical technique such as the Newton-Raphson method. Writing in screw form the reduced acceleration state of the translational platform, with respect to the fixed platform, a simple expression for the computation of the acceleration of the translational platform is derived by taking advantage of the properties of reciprocal screws, via the Klein form, a bilinear symmetric form of the Lie algebra e(3). Following a similar procedure, a simple expression for the computation of the angular acceleration of the end-platform, with respect to the translational platform, is easily derived. Naturally, as an intermediate step, this contribution also provides the forward and inverse velocity analyses of the chosen parallel-serial manipulator. Finally, in order to prove the versatility of the expressions obtained via screw theory for solving the kinematics, up to the acceleration analysis, of the proposed spatial mechanism, a numerical example is solved with the help of commercial computer codes.     

Reconfigurability and unified kinematics modeling of a 3rTPS metamorphic parallel mechanism with perpendicular constraint screws

This paper investigates reconfigurability and unified analytical kinematics analysis of a new 3rTPS metamorphic parallel mechanism consisting of three reconfigurable rTPS limbs in perpendicular base planes. Constraint screw systems show that in one phase the rTPS limb has no constraint to the platform and in the other phase obtained by altering the reconfigurable Hooke joint, it provides a constraint force. The two phases of the limb qualify the 3rTPS metamorphic parallel mechanism to have four topologies with ability of mobility change among 3R (three rotations), 3R1T (three rotations one translation), 3R2T and mobility 6. By considering the difference of the two phases of the limb, a unified kinematics modeling is proposed based on the actuation scheme analysis by taking one phase as a special case of the other. Following this, a unified kinematics modeling of the 3rTPS metamorphic parallel mechanism is obtained by covering all its four topologies. Both inverse and forward kinematics analysis are solved analytically and numerical examples confirm these theoretical results.     

Handling uncertainties of robot manipulators and active vision by constraint propagation

Joint errors are inevitable in robot manipulation. These uncertainties propagate to give rise to translational and orientational errors in the position and orientation of the robot end‐effector. The displacement of the active vision head mounted on the robot end‐effector produces distortion of the projected object on the image. Upon active visual inspection, the observed dimension of a mechanical part is given dimension by the measurement on the projected edge segment on the image. The difference between the observed dimension and the actual dimension is the displacement error in active vision. For different motion of the active vision head, the resulting displacement errors are different. Given the uncertainties of the robot manipulator's joint errors, constraint propagation can be employed to assign the motion of the active sensor in order to satisfy the tolerance of the displacement errors for inspection. In this article, we define the constraint consistency and network satisfaction in the constraint network for the problem of displacement errors in active vision. A constraint network is a network where the nodes represent variables, or constraints, and the arcs represent the relationships between the output variables and the input variables of the constraints. In the displacement errors problem, the tolerance of the displacement errors and the translational and orientational errors of robot manipulators have interval values while the sensor motion has real values. Constraint propagation is developed to propagate the tolerance of displacement errors in the hierarchical interval constraint network in order to find the feasible robot motion. © 2002 Wiley Periodicals, Inc.     

Structural design of a novel family of 2-DOF translational parallel robots to enhance the normal-direction stiffness using passive limbs

Many planar 2-DOF translational parallel robots which were invented for the simple industrial tasks are easy to suffer the problem of the poor intrinsic stiffness along the normal direction to the plane of motion. To solve this problem, the passive limbs can be introduced into the design of parallel mechanisms to increase the stiffness and stability of the robots. Besides the capability of stiffness increasing, the passive limbs can also provide constraints, generate decoupled configuration, bear full or partial weights and/or payloads, liberate constraints from the actuations, and even decrease the required actuating forces of active limbs. However, there is still no systematic study on the utilization of passive limbs to date. In this paper, the stiffness–robust 2-DOF translation parallel robots with passive limbs are investigated in terms of type synthesis. Based on the distribution of wrench system among the active limbs and passive limbs, a full-scale criterion is developed for effectively and efficiently synthesizing all kinds of 2-DOF translational parallel mechanisms with one or more passive limbs. All 14 types of the 2-DOF translational parallel mechanisms with passive limbs are synthesized and exemplified through kinematic diagrams. A qualitative stiffness index is purposed to evaluate the stiffness performance of all of the synthesized configurations directly and rapidly. Finally, an optimized configuration of stiffness-enhanced 2T PM is derived and exhibits the best stiffness performance.     

Immersive singularity‐free full‐body interactions with reduced marker set

Despite the large success of games grounded on movement‐based interactions the current state of full‐body motion capture technologies still prevents the exploitation of precise interactions with complex environments. The first key requirement in the line of work we present here is to ensure a precise spatial correspondence between the user and the Avatar. For that purpose, we build upon our past effort in human postural control with a prioritized inverse kinematics (PIK) framework. One of its key advantages is to ease the dynamic‐ and priority‐based combination of multiple conflicting constraints such as ensuring the balance and reaching a goal. However, its reliance on a linearized approximation of the problem makes it vulnerable to the well‐known full extension singularity of the limbs. We address this issue by presenting a new type of 1D analytic constraint that smoothly integrates within the PIK framework under the name of FLEXT constraint (for FLexion‐EXTension constraint). We further ease the full‐body interaction by combining this new constraint with a recently introduced motion constraint to exploit the data‐based synergy of full‐body reach motions. The combination of both techniques allows immersive full‐body interactions with a small set of active optical marker. Copyright © 2010 John Wiley & Sons, Ltd.     

Parasitic motion comparison of 3-PRS parallel mechanism with different limb arrangements

The 3-PRS parallel mechanism (PM) is an important category of lower-mobility parallel mechanisms. Because it can undergo one translational degree of freedom (DOF) and two rotational DOFs, the 3-PRS PM has great potential in practical application and has received extensive attention. Parasitic motion occurring in the constrained DOFs of a 3-PRS PM is a key issue affecting its application. First, the 3-PRS PM is classified into seven subcategories based on the geometrical arrangements of limbs. Then, parasitic motion of each subcategory is discussed in detail and case studies are presented. It is illustrated that the complexity of parasitic motion of the 3-PRS PM is determined by the limb arrangement. The architecture of the 3-PRS PM without parasitic motion is also identified.     

含摩擦滑移铰及驱动约束多刚体系统数值算法

研究了具有驱动约束及非光滑滑移铰多体系统动力学方程的建模与数值计算方法．将驱动约束视为非定常约束，非光滑滑移铰视为双边定常约束，滑移铰的摩擦模型采用库仑摩擦模型；应用第一类Lagrange方程建立系统的动力学方程，应用距离函数建立滑移铰的约束方程；将线性互补方法和Baumgarte约束稳定化方法引入，以解决滑移铰法向约束力的计算以及约束方程违约问题．最后应用曲柄摇杆机构作为算例，说明该方法的有效性．     

Dynamic optimization of N‐joint robotic limb deployments

We describe an approach using a stochastic optimization framework (SOF) for operating complex mobile systems with several degrees of freedom (DOFs), such as robotic limbs with N joints, in environments that can contain obstacles. As part of the SOF, we have employed an efficient simulated annealing algorithm normally used in computationally highly expensive optimization and search problems such as the traveling salesman problem and protein design. This algorithm is particularly suited to run onboard industrial robots, robots in telemedicine, remote spacecraft, planetary landers, and rovers, i.e., robotic platforms with limited computational capabilities. The robotic limb deployment optimization approach presented here offers an alternative to time‐intensive robotic arm deployment path planning algorithms in general and in particular for robotic limb systems in which closed‐form solutions do not exist. Application examples for a (N = 4)‐DOF arm on a planetary exploration rover are presented. © 2009 Wiley Periodicals, Inc.     

康复机器人患肢参数的在线辨识

根据患肢训练时力和位置等信息反馈,提出用解超定方程组的方法在线辨识患肢的动力学参数,实现患肢动力学模型的在线辨识,为远程康复训练机器人系统的实时控制提供较为准确的依据.仿真实验验证,该方法能较好地改善系统的动态性能,使系统具有较好的稳定性和鲁棒性.     

基于输出反馈的欠驱动TORA系统的有界输入控制

针对欠驱动TORA(Translational oscillations with a rotational actuator)系统,设计了一种具有执行器饱和约束的输出反馈等[7]特别地,由Rand学者提出的TORA系统,最初控制器.与其他现有方法相比,本文方法不仅考虑了执行器饱和约束和作为双自旋航天器的简化模型用于研究自振现象.后来由于速度信号不可测情形,而且考虑了旋转小球可能存在的循环行为.具体而TORA系统具有强耦合、高度非线性、欠驱动等特性,而被言,首先根据TORA系统模型分析了TORA系统的控制目标;随后,作为一种非线性基准系统主要用于非线性控制器设计、验证构造了一种新颖的能量函数,在此基础上设计了一种考虑执行器饱和约非线性控制算法的控制性能或教学研究.目前国内外已有多束的输出反馈控制器,并通过严格的数学分析证明了闭环系统关于平衡点的稳定性;最后,借助数值仿真测试检验了所提控制器的控制性能,并所高校和研究机构针对TORA系统的控制问题展开研究.对与已有方法进行了对比.仿真测试结果表明本文所提方法具有更好的控法制性能.     

Tri-pyramid Robot: Design and kinematic analysis of a 3-DOF translational parallel manipulator

3-DOF translational parallel manipulators have been developed in many different forms, but they still have respective disadvantages in different applications. To overcome their disadvantages, the structure and constraint design of a 3-DOF translational parallel manipulator is presented and named the Tri-pyramid Robot. In the constraint design of the presented manipulator, a conical displacement subset is defined based on displacement group theory. A triangular pyramidal constraint is presented and applied in the constraint designs between the manipulator?s subchains. The structural properties including the decoupled motions, overconstraint elimination, singularity free workspace, fixed actuators and isotropic configuration are analyzed and compared to existing structures. The Tri-pyramid Robot is constrained and realized by a minimal number of 1-DOF joints. The kinematic position solutions, workspace with variation of structural parameters, Jacobian matrix, isotropic and dexterity analysis are performed and evaluated in the numerical simulations.     

Modeling and simulation of the nonsmooth planar rigid multibody systems with frictional translational joints

The main purpose of this paper is to present a modeling and simulation method for the rigid multibody system with frictional translational joints. The small clearance between a slider and guide is considered. The geometric constraints of the translational joints are treated as bilateral constraints and the impacts between sliders and guides are neglected when the clearance sizes of the translational joints are very small. The contact situations of the normal forces acting on the sliders are described by inequalities and complementarity conditions, while the frictional contacts are characterized by a set-valued force law of the type of Coulomb’s law for dry friction. The dynamic equations of the multibody systems with normal and tangential contact forces are written on the acceleration-force level using the Lagrange multiplier technique. The problem of the transitions of the contact situation of the normal forces acting on sliders and the transitions of the stick-slip of the sliders in the system is formulated as a horizontal linear complementarity problem (HLCP), which is solved by event-driven method. Baumgarte’s stabilization method is used to decrease the constraint drift. Finally, two typical mechanisms are considered as demonstrative application examples. The numerical results obtained show some dynamical behaviors of the systems with frictional translational joints and constraint stabilization effect.     

六自由度并联机器人的支链选取

提出了六自由度并联机器人的支链选取的系统方法． 并联机器人中有多种类型的支链可以用来实现末端执行器的6自由度运动， 每个支链的驱动关节应当在基座上或靠近基座的地方， 由支链所构成的整个并联机构的自由度应当为6．利用这些条件对并联机器人的支链形式进行筛选可以获得可行的支链形式． 通过这种方法可以去除很多不可行的支链形式, 得到可行的支链形式供后续设计使用．     

Position,Jacobian and workspace analysis of a 3-PSP spatial parallel manipulator

This paper investigates the problems of kinematics, Jacobian, singularity and workspace analysis of a spatial type of 3-PSP parallel manipulator. First, structure and motion variables of the robot are addressed. Two operational modes, non-pure translational and coupled mixed-type are considered. Two inverse kinematics solutions, an analytical and a numerical, for the two operational modes are presented. The direct kinematics of the robot is also solved utilizing a new geometrical approach. It is shown, unlike most parallel robots, the direct kinematics problem of this robot has a unique solution. Next, analytical expressions for the velocity and acceleration relations are derived in invariant form. Auxiliary vectors are introduced to eliminate passive velocity and acceleration vectors. The three types of conventional singularities are analyzed. The notion of non-pure rotational and non-pure translational Jacobian matrices is introduced. The non-pure rotational and non-pure translational Jacobian matrices are combined to form the Jacobian of constraint matrix which is then used to obtain the constraint singularity. Finally, two methods, a discretization method and one based on direct kinematics are presented and robot non-pure translation and coupled mixed-type reachable workspaces are obtained. The influence of tool length on workspace is also studied.     

Review: Don't Cross‐Thread the Screw!*

The screw is a geometrical element. As Sir Robert Ball stated a century or more ago, the screw joins the company of point, plane, and line. In mechanics, especially instantaneous kinematics and statics of rigid bodies, angular and translational velocity of a body combine as a twist on a screw; precisely analogously a force and a couple combine as awrench on a screw. In three dimensions screw theory provides essential and simple insight into the behavior of systems of jointed bodies. And screw theory can also play a leading role in the systematic selection of suitable geometrical patterns that should relate to such jointed systems. Yet the mechanics of robots—the prime example today of systems of jointed bodies—has developed with little respect for the screw, even at the most elementary level. Here I focus mainly on robot performance, the study of which has, to an excessive extent, been proceeding blindly in territory where there is an abundance of screws and systems of screws, all, as it were, shouting to be taken on board to show the way. Failure to recognize the screw, and an unwillingness to use it properly, has led to the screw's being violently abused—“cross‐threaded.” There are far too many examples of “cross‐threading” that have, regrettably, appeared in recent years in several well‐known journals. Offenses have been perpetrated and yet left unchallenged. Here I try to remove any mystery that some may see surrounding, and even concealing, the screw. I then venture to deal critically with a small selection of the many instances of cross‐threading. However, this symposium has as a main objective the celebration of a Centenary. So it betokens an element of light‐heartedness that I resort to at times within the pages that follow. © 2003 Wiley Periodicals, Inc.     

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.     

基于上下文语义分割信息的缺销螺丝部件识别

针对目前图像视觉领域对输电线路缺销螺丝部件研究较少,且在传统图像处理方法上,螺丝的识别精度不高等问题。文章采用一种基于上下文语义分割信息的缺销螺丝识别方法,在Deeplab v3+网络的基础上,对输电线路数据集进行图像裁剪分块和自适应Gamma校正增强预处理,将缺销螺丝识别的mIoU提升了17%左右;对于普通螺丝误识别,提出了结合上下文语义分割信息的方法,将分割出缺销螺丝区域分别和周围若干部件区域进行拓扑关系分析,根据拓扑关系类别排除误识别到的普通螺丝。通过多组实验结果表明,采用预处理和结合上下文语义信息的缺销螺丝识别方法要优于Deeplab v3+算法。