End-effector Constraints Research Articles

A novel planner framework compatible with various end-effector constraints

Purpose Aiming at the problem of insufficient adaptability of robot motion planners under the diversity of end-effector constraints, this paper proposes Transformation Cross-sampling Framework (TC-Framework) that enables the planner to adapt to different end-effector constraints. Design/methodology/approach This work presents a standard constraint methodology for representing end-effector constraints as a collection of constraint primitives. The constraint primitives are merged sequentially into the planner, and a unified constraint input interface and constraint module are added to the standard sampling-based planner framework. This approach enables the realization of a generic planner framework that avoids the need to build separate planners for different end-effector constraints. Findings Simulation tests have demonstrated that the planner based on TC-framework can adapt to various end-effector constraints. Physical experiments have also confirmed that the framework can be used in real robotic systems to perform autonomous operational tasks. The framework’s strong compatibility with constraints allows for generalization to other tasks without modifying the scheduler, significantly reducing the difficulty of robot deployment in task-diverse scenarios. Originality/value This paper proposes a unified constraint method based on constraint primitives to enhance the sampling-based planner. The planner can now adapt to different end effector constraints by opening up the input interface for constraints. A series of simulation tests were conducted to evaluate the TC-Framework-based planner, which demonstrated its ability to adapt to various end-effector constraints. Tests on a physical experimental system show that the framework allows the robot to perform various operational tasks without requiring modifications to the planner. This enhances the value of robots for applications in fields with diverse tasks.

Whole-Body Dynamics for Humanoid Robot Fall Protection Trajectory Generation with Wall Support.

When humanoid robots work in human environments, they are prone to falling. However, when there are objects around that can be utilized, humanoid robots can leverage them to achieve balance. To address this issue, this paper established the state equation of a robot using a variable height-inverted pendulum model and implemented online trajectory optimization using model predictive control. For the arms' optimal joint angles during movement, this paper took the distributed polygon method to calculate the arm postures. To ensure that the robot reached the target position smoothly and rapidly during its motion, this paper adopts a whole-body motion control approach, establishing a cost function for multi-objective constraints on the robot's movement. These constraints include whole-body dynamics, center of mass constraints, arm's end effector constraints, friction constraints, and center of pressure constraints. In the simulation, four sets of methods were compared, and the experimental results indicate that compared to free fall motion, adopting the method proposed in this paper reduces the maximum acceleration of the robot when it touches the wall to 69.1 m/s2, effectively reducing the impact force upon landing. Finally, in the actual experiment, we positioned the robot 0.85 m away from the wall and applied a forward pushing force. We observed that the robot could stably land on the wall, and the impact force was within the range acceptable to the robot, confirming the practical effectiveness of the proposed method.

An efficient constraint method for solving planning problems under end-effector constraints

Purpose In response to the challenge of reduced efficiency or failure of robot motion planning algorithms when faced with end-effector constraints, this study aims to propose a new constraint method to improve the performance of the sampling-based planner. Design/methodology/approach In this work, a constraint method (TC method) based on the idea of cross-sampling is proposed. This method uses the tangent space in the workspace to approximate the constrained manifold pattern and projects the entire sampling process into the workspace for constraint correction. This method avoids the need for extensive computational work involving multiple iterations of the Jacobi inverse matrix in the configuration space and retains the sampling properties of the sampling-based algorithm. Findings Simulation results demonstrate that the performance of the planner when using the TC method under the end-effector constraint surpasses that of other methods. Physical experiments further confirm that the TC-Planner does not cause excessive constraint errors that might lead to task failure. Moreover, field tests conducted on robots underscore the effectiveness of the TC-Planner, and its excellent performance, thereby advancing the autonomy of robots in power-line connection tasks. Originality/value This paper proposes a new constraint method combined with the rapid-exploring random trees algorithm to generate collision-free trajectories that satisfy the constraints for a high-dimensional robotic system under end-effector constraints. In a series of simulation and experimental tests, the planner using the TC method under end-effector constraints efficiently performs. Tests on a power distribution live-line operation robot also show that the TC method can greatly aid the robot in completing operation tasks with end-effector constraints. This helps robots to perform tasks with complex end-effector constraints such as grinding and welding more efficiently and autonomously.

Motion Planning With Pose Constraints Based on Direct Projection for Anthropomorphic Manipulators

This paper presents an efficient planning algorithm, called Direct-Projection Rapidly-exploring Random Tree (DP-RRT), to address the motion planning with end-effector pose constraints for anthropomorphic manipulators. The key of this planning problem is to find constraint-satisfying configurations on the constraint manifolds and connect them to generate a collision-free and smooth path. In the previous works, the configurations that satisfy pose constraints are generally calculated by the numerical iteration methods based on Jacobian projection techniques. However, such approaches have many technical challenges, such as joint limits and singularity, many numerical iterations and much computing time. In this work, we propose a Direct Projection method based on the analytic inverse kinematics (IK) that can directly project configurations onto the constraint manifolds instead of the numerical iteration methods. The proposed DP-RRT algorithm combines the Direct Projection method with the Rapidly-exploring Random Tree (RRT) algorithm, where the RRT algorithm is employed to explore the ambient space by growing tree branches, and the Direct Projection method is used to project the tree branches onto the constraint manifolds for constructing a constraint-satisfying path. As the analytic IK solver is used to calculate the constraint-satisfying configurations, the DP-RRT algorithm is characterized by high efficiency and no numerical iteration. Besides, avoiding joint limits and singularity, as well as the smoothness of the end-effector and the joints trajectory are also considered. The effectiveness of the proposed algorithm is demonstrated on the Willow Garage’s PR2 simulation platform in a wide range of pose-constrained cases.

Constrained Whole Body Motion Planning in Task Configuration and Time

Our goal in this research was to develop an efficient motion planning algorithm for a humanoid to enable it to perform complex tasks. To perform the tasks, the humanoid sometimes must be able to generate whole body motion trajectory under the end effector constraints as well as collisions; self-collision and unexpected collision with obstacles in the environment. Also, more complex constraints such as joint limit, joint speed limit would make planning whole body motion more difficult. This constrained redundancy resolution makes the motion planning for humanoid difficult. Although the previous motion planning algorithms are still feasible to find solution, they are typically planning in the high dimensional joint configuration space then finally require high computing. This paper proposes that planning on the specified whole body task space such as pelvis height, chest orientation is able to find feasible solution while satisfying constraints and collision free. We also expect this approach that can be applied on various humanoids so we demonstrated on Atlas and Valkyrie robot. In order to demonstrate the feasibility of the proposed approach, we applied our motion planning algorithm humanoid performing tasks that are proposed by DARPA challenge such as opening a door, closing valve, cutting wall, pulling emergency switch.

An integrated control scheme for space robot after capturing non-cooperative target

How to identify the mass properties and eliminate the unknown angular momentum of space robotic system after capturing a non-cooperative target is of great challenge. This paper focuses on designing an integrated control framework which includes detumbling strategy, coordination control and parameter identification. Firstly, inverted and forward chain approaches are synthesized for space robot to obtain dynamic equation in operational space. Secondly, a detumbling strategy is introduced using elementary functions with normalized time, while the imposed end-effector constraints are considered. Next, a coordination control scheme for stabilizing both base and end-effector based on impedance control is implemented with the target's parameter uncertainty. With the measurements of the forces and torques exerted on the target, its mass properties are estimated during the detumbling process accordingly. Simulation results are presented using a 7 degree-of-freedom kinematically redundant space manipulator, which verifies the performance and effectiveness of the proposed method.

Task difficulty and inertial properties of hand-held tools: An assessment of their concurrent effects on precision aiming

Aiming hand-held tools at targets in space entails adjustments in the dynamical organization of aiming patterns according to the required precision. We asked whether and how these adjustments are modified by the tool’s mass distribution. Twelve participants performed reciprocal aiming movements with a 50-cm long wooden probe. Kinematic patterns of probe movements were used as a window into the behavioral dynamic underlying performance of a reciprocal aiming task. We crossed three levels of task difficulty (IDs 2.8, 4.5 and 6.1) with two types of probe varying in their mass distribution (proximal vs distal loading). Movement duration was affected by task difficulty and probe loading (shorter for larger targets and proximal probe loading). Progressive deviations from a sinusoidal movement pattern were observed as task difficulty increased. Such deviations were more pronounced with proximal probe loading. Results point to a higher degree of non-linearity in aiming dynamics when the probe was loaded proximally, which might reflect employment of additional perceptual-motor processes to control the position of its less stable tip at the vicinity of the targets. More generally, the effects of probe loading on aiming pattern and dynamics suggest that perceptual-motor processes responding to task level constraints are sensitive to, and not independent from, biomechanical, end-effector constraints.

Inverse kinodynamics: Editing and constraining kinematic approximations of dynamic motion

We present inverse kinodynamics (IKD), an animator friendly kinematic work flow that both encapsulates short-lived dynamics and allows precise space–time constraints. Kinodynamics (KD), defines the system state at any given time as the result of a kinematic state in the recent past, physically simulated over a short time window to the present. KD is a well suited kinematic approximation to animated characters and other dynamic systems with dominant kinematic motion and short-lived dynamics. Given a dynamic system, we first choose an appropriate kinodynamic window size based on accelerations in the kinematic trajectory and the physical properties of the system. We then present an inverse kinodynamics (IKD) algorithm, where a kinodynamic system can precisely attain a set of animator constraints at specified times. Our approach solves the IKD problem iteratively, and is able to handle full pose or end effector constraints at both position and velocity level, as well as multiple constraints in close temporal proximity. Our approach can also be used to solve position and velocity constraints on passive systems attached to kinematically driven bodies. We describe both manual and automatic procedures for selecting the kinodynamic window size necessary to approximate the dynamic trajectory to a given accuracy. We demonstrate the convergence properties of our IKD approach, and give details of a typical work flow example that an animator would use to create an animation with our system. We show IKD to be a compelling approach to the direct kinematic control of character, with secondary dynamics via examples of skeletal dynamics and facial animation.

Path planning with general end-effector constraints

In this paper, we address the path planning problem with general end-effector constraints (PPGEC) for robot manipulators. Two approaches are proposed. The first approach is the Adapted-RGD method, which is adapted from an existing randomized gradient descent (RGD) method for closed-chain robots. The second approach is radically different. We call it ATACE, Alternate Task-space And Configuration-space Exploration. Unlike the first approach which searches purely in C-space, ATACE works in both task space and C-space. It explores the task space for end-effector paths satisfying given constraints, and utilizes trajectory tracking technique(s) as a local planner(s) to track these paths in the configuration space. We have implemented both approaches and compared their relative performances in different scenarios. ATACE outperforms Adapted-RGD in the majority (but not all) of the scenarios. We outline intuitive explanations for the relative performances of these two approaches.

A unified geometric approach to modeling and control of constrained mechanical systems

Dynamic control of constrained mechanical systems, such as robotic manipulators under end-effector constraints, parallel manipulators, and multifingered robotic hands under closure constraints have been classic problems in robotics research. In this paper, we provide a unified geometric framework for modeling, analysis, and control of constrained mechanical systems. Starting with the constraint, we define two canonical subspaces, namely the subspace of constraint forces and the tangent space of the constraint manifold for holonomic constraint. Using the kinetic energy metric, we define the remaining subspaces and show explicitly the relations among these subspaces. We project the Euler-Lagrange equation of a constrained mechanical system into two orthogonal components and give geometric and physical interpretations of the projected equations. Based on the projected equations, a unified and asymptotically stable hybrid position/force-control algorithm is proposed, along with experimental results for several practical examples. In the case of nonholonomic constraints, we show that the equations can be projected to the distribution/codistribution associated with the constraints and the control law reduces to hybrid velocity/force control.

Constrained robot dynamics II: Parallel machines

Part I of this two-part article considered the dynamics of serially connected robot arms with holonomic end-effector constraints. Here, the work is extended to parallel linkages by first finding the dynamic equations for general tree and star structured mechanisms. Parallel linkages are then formed by applying constraints to the terminal links of the tree or star. The formalism is illustrated with two examples: bipedal walking machines and the Stewart platform.

Constrained robot dynamics I: Serial robots with end-effector constraints

We examine the rigid-body dynamics of serial robots subject to time-invariant holonomic constraints on their end-effectors. We begin by looking at several examples of constraints and the subspaces they determine in the group of rigid-body displacements. This preparation allows us to give a general description of the dynamics of kinematic chains with constrained terminal links. Two examples are given to illustrate the main ideas: a planar four-bar mechanism and a 3-joint robot with a point on the final link constrained to lie on a line. ©1999 John Wiley & Sons, Inc.

ワークピースの幾何モデルを必要としないマニピュレータ・ポジショナーシステムの拘束運動制御

Coordinated compliant motion of two robotic devices, i.e., a manipulator-positioner system, can introduce greater flexibility and dexterity into manufacturing and improve the productivity of their processes. This paper addresses the coordinated compliant motion control of a manipulator-positioner system without using a geometric model of a workpiece. Dynamics and control of manipulators under time-dependent holonomic constraints are studied by formulating coordinated compliant motion of manipulator-positioner systems as constrained motion of a manipulator under geometric end-effector constraints imposed by a positioner. A virtual constraint plane is introduced to prescribe a desired position trajectory of an end-effector of a manipulator. A compliant motion control scheme without using a geometric model of a workpiece is realized by fusing the measurements of the articular displacements and velocities of a manipulator and a positioner and the constraint force of an end-effector of a manipulator. A numerical simulation and an experiment for coordinated contour-following tasks are conducted to demonstrate the feasibility of the proposed compliant motion control scheme without using a workpiece geometric model.

A unified dynamic model and control synthesis for robotic manipulators with geometric end-effector constraints

A compact dynamic model and a hybrid position/force controller for a constrained robot manipulator, subject to a set of holonomic (integrable) constraints have been developed in this study. The joint-space dynamics (DAEs) has been transformed into the constraint-space model in which the system dynamics can be readily decomposed into two orthogonal subsystems; the motion-controlled subsystem is specified in the direction tangential to the known constraint surfaces, and the force-controlled subsystem is regulated in the orthogonal direction. Also utilizing the transformed dynamics, we have presented a hybrid adaptive control law to simultaneously manipulate the end-effector position and the contact force. Further, by a Lyapunov theory, it has been shown that the corresponding closed-loop system is globally stable under the parametric uncertainties.

Optimal load of robotic manipulator with joint elasticity using accuracy and actuator constraints

A computational technique for obtaining the maximum load-carrying capacity for a robotic manipulator with joint elasticity, subject to accuracy and actuator constraints, is described. Full load motions and increased productivity are linked in the industrial applications of many robotic manipulators; the maximum load carrying capacity which can be achieved by a manipulator during a given trajectory is limited by a number of factors. The dynamic properties of a manipulator, its actuator limitations, and joint elasticity (transmissions, reducers, and servo drive system) are probably the most important factors. This paper presents a strategy for determining dynamic load carrying capacity (DLCC), subject to both accuracy and actuator constraints, where a series of cubical bounds centred at the desired trajectory is used in the end-effector oscillation constraint while a typical d.c. motor speed-torque characteristics curve is used in the actuator constraint. The technique which considers the full nonlinear manipulator dynamics, actuator constraints, and accuracy constraints permits the manipulator user to specify the trajectory completely. Finally, a numerical example involving a two-link manipulator with joint flexibility using this method is presented and the results are discussed.

A parallel algorithm and architecture for the control of kinematically redundant manipulators

Kinematically redundant manipulators are inherently capable of more dextrous manipulation due to their additional degrees of freedom. To achieve this dexterity, however, one must be able to efficiently calculate the most desirable configuration from the infinite number of possible configurations that satisfy the end-effector constraint. It has been previously shown that the singular value decomposition (SVD) plays a crucial role in doing such calculations. In this work, a parallel algorithm for calculating the SVD is incorporated into a computational scheme for solving the equations of motion for kinematically redundant systems. This algorithm, which generalizes the damped least squares formulation to include solutions that utilize null-space projections and task prioritization as well as augmented or extended Jacobians, is then implemented on a simple linear array of processing elements. By taking advantage of the error bounds on the perturbation of the SVD, it is shown that an array of only four AT&T DSP chips can result in control cycle times of less than 3 ms for a seven degree-of-freedom manipulator.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dynamic hybrid position/force control of robot manipulators-controller design and experiment

An approach to designing controllers for dynamic hybrid position/force control of robot manipulators is presented, and preliminary experimental results are given. Dynamic hybrid control is an extension of an approach proposed by M.H. Raibert and J.J. Craig (1981) to the case where the full manipulator dynamics is taken into consideration and the end-effector constraint is explicitly given by the constraint hypersurfaces. This design method consists of two steps. The first step is the linearization of the manipulator dynamics by nonlinear state feedback. Formulation of the constraint by the constraint hypersurfaces plays an essential role in establishing the linearizing law. The second step is the design of position and force controllers for the linearized model using the concept of two-degrees-of-freedom servocontroller. The merit of this servocontroller is that it can take account of both the command response and the robustness of the controllers to modeling errors and disturbances. Preliminary experiments using a SCARA robot show the validity of the approach. >

ロボットアームの位置と力の動的ハイブリッド制御 制御系設計と実験

本論文は, ロボットアームのための位置と力の動的ハイブリッド制御系の一つの設計法を与え, その基礎的な実験結果を示したものである.動的ハイブリッド制御とはRaibertとCraigの提案したハイブリッド制御の考え方をアームの動特性を考慮にいれた場合にまで拡張したものである.本手法は二つのステップから成る.第一のステップは非線形フィードバックによるアームの動特性の線形化である.第二のステップは線形化されたモデルに対して, i) 目標値に対するシステムの応答特性と, ii) モデル化誤差や外乱に対する制御系のロバスト性を共に考慮にいれた, 位置・力制御系の設計である.本手法の妥当性を示すためにスカラ型ロボットを用いた基礎的な実験の結果を与えている.