多臂空间机器人稳定抓取力分配和柔顺控制策略

针对影响多臂抓取稳定性的接触力不平衡和接触振动问题,提出多臂空间机器人力分配和柔顺控制策略.首先,分析满足多臂稳定抓取的力学条件,基于摩擦锥约束设计抓取力安全系数,并将其引入力优化模型进行抓取力分配,实现目标物体稳定抓取条件下受力最小;然后,分析抓取过渡过程的振动成因,设计基于动能消耗的末端输出力控制策略实现快速振动抑制和柔顺抓取;最后,设计机械臂末端控制律切换策略,一旦在抓取过渡过程中发生接触脱离可引导其快速返回物体表面.仿真结果表明,所提出方法提升了稳定抓取安全裕度,显著降低了机械臂末端的振动幅值、持续时间和接触力,提升了空间机器人多臂抓取目标操作的稳定性和柔顺性.     

Nonlinear feedback control of robot manipulator and compliant wrist

Many robotic applications require the direct contact of the end-effector with the environment. Passive compliance attached to the robot wrist, hand, or finger is desirable to produce smooth transitions between the free motion and contact, as well as to allow self-correction in order to accommodate geometric uncertainties in assembly and manufacturing. However, the use of passive compliance degenerates the positioning capability of the manipulator when the robot moves in free space. When the robot makes contact on workpiece, active adjustment of stiffness for various tasks in different directions is needed. We proposed to use passive compliance that is instrumented so that the system provides the necessary flexibility and also sensing to actively control the contact forces or to compensate the positioning error during motion and contact. In this article, the dynamic control of the manipulator with a compliant wrist is addressed. The measured deformation information of the instrumented compliant wrist is utilized in the feedback loop to increase the stiffness of the overall system in position control and to decrease the stiffness in force control. The dynamics model for both unconstrained and constrained cases is established. Applying nonlinear feedback control techniques, the dynamics of the manipulator-wrist system is linearized and decoupled, which allows the controller design to be carried out by using the linear system theory. Editor: J.M. Skowronski     

Grasp evaluation and contact points planning for polyhedral objects using a ray-shooting algorithm

Grasp evaluation and planning are two fundamental issues in robotic grasping and dexterous manipulation. Most traditional methods for grasp quality evaluation suffer from non-uniformity of the wrench space and a dependence on the scale and choice of the reference frame. To overcome these weaknesses, we present a grasp evaluation method based on disturbance force rejection under the assumption that the normal component of each individual contact force is less than one. The evaluation criterion is solved using an enhanced ray-shooting algorithm in which the geometry of the grasp wrench space is read by the support mapping. This evaluation procedure is very fast due to the efficiency of the ray-shooting algorithm without linearization of the friction cones. Based on a necessary condition for grasp quality improvement, a heuristic searching algorithm for polyhedral object regrasp is also proposed. It starts from an initial force-closure unit grasp configuration and iteratively improves the grasp quality to find the locally optimum contact points. The efficiency and effectiveness of the proposed algorithms are illustrated by a number of numerical examples.     

A self-adjusting active compliance controller for multiple robots handling an object

This paper presents the concept and experimental validation of a self-adjusting active compliance controller for n robots handling its compliant behaviour concerning partly unknown flexible object. The control strategy is based on the decomposition of the 6n-dimensional position/force space and includes a feedforward and feedback level. The feedforward level contains motion coordination, force distribution of external forces, creation of internal forces, and an additional loop adding the elastic displacements due to the applied forces to the planned robot positions. The feedback level is organized in the form of an active compliance control law. For adjusting the controller to the, in general, unknown flexible behaviour, which in practice is the main problem of the controller design, a quasi-static model of the system is derived for different contact cases of the object and a procedure is presented, which by use of this model is capable of determining the compliance of the considered system and therefore of adjusting the controller. Experiments with two puma-type robots have been conducted to show the applicability of the self-adjusting control strategy. The task has been to grasp and move an unconstrained object. It is shown, that the system can adjust the control parameters to the unknown system compliance and that the control performance is improved considerably.     

On Clamping Planning in Workpiece-Fixture Systems

Deformations of contacts between the workpiece and locators/clamps resulting from large contact forces cause overall workpiece displacement, and affect the localization accuracy of the workpiece. An important characteristic of a workpiece-fixture system is that locators are passive elements and can only react to clamping forces and external loads, whereas clamps are active elements and apply a predetermined normal load to the surface of workpiece to prevent it from losing contact with the locators. Clamping forces play an important role in determining the final workpiece quality. This paper presents a general method for determining the optimal clamping forces including their magnitudes and positions. First, we derive a set of “compatibility” equations that describe the relationship between the displacement of the workpiece and the deformations at contacts. Further, we develop a locally elastic contact model to characterize the nonlinear coupling between the contact force and elastic deformation at the individual contact. We define the minimum norm of the elastic deformations at contacts as the objective function, then formulate the problem of determining the optimal clamping forces as a constrained nonlinear programming problem which guarantees that the fixturing of the workpiece is force closure. Using the exterior penalty function method, we transform the constrained nonlinear programming into an unconstrained nonlinear programming which is, in fact, the nonlinear least square. Consequently, the optimal magnitudes and positions of clamping forces are obtained by using the Levenberg–Marquardt method which is globally convergent. The proposed planning method of optimal clamping forces, which may also have an application to other passive, indeterminate problems such as power grasps in robotics, is illustrated with numerical example.      

Modeling of Viscoelastic Contacts and Evolution of Limit Surface for Robotic Contact Interface

Viscoelastic contact is a type of contact which includes, in addition to linear or nonlinear elastic response, time-dependent response due to relaxation or creep phenomena that govern the contact behavior. The characteristics of the time-dependent relaxation of such a viscoelastic contact are typically exponentially decaying functions, and exponentially growing functions for creep, respectively. Such contacts can be found in anthropomorphic robotic fingers, soft materials, viscoelastic skin with rigid core, and human fingers and feet. In this paper, the nature of viscoelastic contacts is investigated, and the evolution of their friction limit surfaces and of the pressure distributions at the contact interface are studied. Two cases commonly found in robotic grasping and manipulation are discussed. Based on the modeling formulation, it is found that the two important parameters of analysis and modeling for such contacts, i.e., the radius of contact area and the profile of pressure distribution, can be chosen using proposed coupling equations as the viscoelastic contact interface evolves with time. The new contribution of this paper includes a proposal of coupling equations between the two important parameters to describe the viscoelastic contact interface, and a study of the evolution of limit surfaces for viscoelastic contact interface due to temporal dependency, and the implication on grasp stability. It is found from the evolution of limit surfaces that when normal force is applied with typical viscoelastic contacts, grasp becomes more stable as time elapses. The modeling can be applied to the design of fingertips and the analysis of robotic grasping and manipulation involving viscoelastic fingers     

Force/position tracking for a robotic manipulator in compliant contact with a surface using neuro-adaptive control

The problem of force/position tracking for a robotic manipulator in compliant contact with a surface under non-parametric uncertainties is considered. In particular, structural uncertainties are assumed to characterize the compliance and surface friction models, as well as the robot dynamic model. A novel neuro-adaptive controller is proposed, that exploits the approximation capabilities of the linear in the weights neural networks, guaranteeing the uniform ultimate boundedness of force and position error with respect to arbitrarily small sets, plus the boundedness of all signals in the closed loop. Simulations highlight the approach.     

Prediction of part orientation error tolerance of a robotic gripper

This paper presents a model which predicts the part orientation error tolerance of a three-fingered robotic gripper. The concept of “self-alignment” is introduced, where the gripper uses the grasping process to bring the workpiece into its final state of orientation. The gripper and part are represented mathematically, and initial contact locations upon grasp closure determined. This information is used to solve for the contact forces present, and criteria are developed to determine if beneficial part motion resulting in self-alignment is expected. The results are visualized via a boundary projected on a reference plane below the part. The model is validated experimentally with a number of part configurations with favorable results. This method presents a useful tool by which the mechanical designer can quantitatively predict the performance of an intuitively designed gripping system.     

空间机器人抓捕目标后基于任务相容性的消旋策略

针对双臂空间机器人抓捕自旋目标后的镇定操作,在考虑机器人系统输入约束的条件下,提出了一种基于任务相容性的消旋规划与控制方法。首先,给出空间机器人抓捕目标后的组合系统的动力学模型,作为规划与控制的基础。然后,根据动力学可操作度和任务相容性设计了目标的快速消旋策略,其期望加速度的方向和大小分别取作速度的反方向和机器人系统输入约束允许的最大值。最后,基于所推导的运动学和动力学模型,通过对目标和机械臂末端分别建立柔顺度等式,提出了一种跟踪期望运动轨迹同时对末端接触力进行调节的柔顺控制方法。通过双臂7自由度空间机器人消除目标自旋运动的仿真结果,验证了所提方法的有效性。     

Graspit! A versatile simulator for robotic grasping

A robotic grasping simulator, called Graspit!, is presented as versatile tool for the grasping community. The focus of the grasp analysis has been on force-closure grasps, which are useful for pick-and-place type tasks. This work discusses the different types of world elements and the general robot definition, and presented the robot library. The paper also describes the user interface of Graspit! and present the collision detection and contact determination system. The grasp analysis and visualization method were also presented that allow a user to evaluate a grasp and compute optimal grasping forces. A brief overview of the dynamic simulation system was provided.     

On the grasping stability and optimality under external perturbations

There are two types of grasping analysis in robotics research: find the grasping force distributions among the grasping fingers when given the contact points and find a good set of the contact points when given the shape of the object. Each kind of problem is associated with optimality and stability analysis. In this article, we investigate the grasping stability and optimality issues under the influence of external perturbations. A rotation‐displacement geometry model is used in computing the changes of grasping forces under external perturbations. Using these results, we present the concept of perturbation closure, which plays the central role in our analysis. A method for finding the local minimal perturbation resisting forces required for non‐slip contacts is developed based on this concept. A grasp so determined is guaranteed to be stable if the external perturbations do not exceed the threshold. Based on this property, we develop a quantitative measurement that can be used to evaluate the performance of different grasping configurations. One can use this measurement to determine the best grasping configuration from a set of perturbation resisting grasps. This actually gives a method which enables the optimal grasping configuration to be found. Both two‐dimensional and three‐dimensional cases are discussed in detail for determining the perturbation closure, the local minimal perturbation resisting force, and the perturbation resisting grasp. Examples are given at the end of the article to illustrate our idea. ©1999 John Wiley & Sons, Inc.     

Intelligent rotational direction control of passive robotic joint with embedded sensors

Passive compliant joints with springs and dampers ensure a smooth contact with the surroundings, especially if robots are in contact with humans, but the passive compliant joints cannot determine precisely the position of the members of the joint or direction of the collision force. In this paper was proposed the structure of a passive compliant robotic joint with conductive silicone rubber elements as internal embedded sensors. The sensors can operate as absorbers of excessive external collision force instead of springs and dampers and can be used for some measurements. Therefore, this joint presents one type of safe robotic mechanisms with an internally measuring system. The sensors were made by press-curing from carbon-black filled silicone rubber which is an electro active material. Various compression tests of the sensors were done. The main task of this study is to investigate the application of a control algorithm for detecting the direction of the robotic joint angular rotation when subjected to an external collision force. Soft computing methodology, adaptive neuro fuzzy inference strategy (ANFIS), was used for the controller development. The simulation results presented in this paper show the effectiveness of the developed method.     

Contact sensing and grasping performance of compliant hands

Limitations in modern sensing technologies result in large errors in sensed target object geometry and location in unstructured environments. As a result, positioning a robotic end-effector includes inherent error that will often lead to unsuccessful grasps. In previous work, we demonstrated that optimized configuration, compliance, viscosity, and adaptability in the mechanical structure of a robot hand facilitates reliable grasping in unstructured environments, even with purely feedforward control of the hand. In this paper we describe the addition of a simple contact sensor to the fingerpads of the SDM Hand (Shape Deposition Manufactured Hand), which, along with a basic control algorithm, significantly expands the grasp space of the hand and reduces contact forces during the acquisition phase of the grasp. The combination of the passive mechanics of the SDM Hand along with this basic sensor suite enables positioning errors of over 5 cm in any direction. In the context of mobile manipulation, the performance demonstrated here may reduce the need for much of the complex array of sensing currently utilized on mobile platforms, greatly increase reliability, and speed task execution, which can often be prohibitively slow.     

Computation of force-closure grasps: an iterative algorithm

Computation of grasps with form/force closure is one of the fundamental problems in the study of multifingered grasping and dextrous manipulation. Based on the geometric condition of the closure property, this paper presents a numerical test to quantify how far a grasp is from losing form/force closure. With the polyhedral approximation of the friction cone, the proposed numerical test can be formulated as a single linear program. An iterative algorithm for computing optimal force-closure grasps, which is implemented by minimizing the proposed numerical test in the grasp configuration space, is also developed. The algorithm is computationally efficient and generally applicable. It can be used for computing form/force-closure grasps on 3-D objects with curved surfaces, and with any number of contact points. Simulation examples are given to show the effectiveness and computational efficiency of the proposed algorithm.     

Force and position control of grasp in multiple robotic mechanisms

One of the approaches to increase the dexterity of a robot manipulating system is a design philosophy that consists of multiple robotic mechanisms. Applications of such a collection of manipulators can be in the design of a dextrous end-effector, a reconfigurable fixture to locate and grip various sized objects, or cooperative robotic arms which through their coordinated motions are able to accomplish a given task. Although the applications of such a design philosophy are endless, many problems still remain to be addressed. One of these problems is the control of the contact forces (grasping forces) between the mechanisms and the position of the grasped object. This article addresses this problem. First, a model of the mechanisms in contact with the grasped object is postulated; second, the problem of controlling the grasping forces and the position of the grasped object is formulated in the linear multi-input/multi-output system, and, finally, a centralized optimal controller is proposed for controlling the desired variables. The results of this article are demonstrated using two examples. One of the main advantages of the proposed controller is that it also shapes the transient response of the grasping force, which is an important consideration in cases when grasping fragile objects. © 1996 John Wiley & Sons, Inc.     

PASA-GB Hand: a Novel Parallel and Self-adaptive Robot Hand with Gear-belt Mechanisms

This paper introduces a novel underactuated hand, the PASA-GB hand, which has a hybrid grasping mode. The hybrid grasping mode is a combination of parallel pinching (PA) grasp and self-adaptive enveloping (SA) grasp. In order to estimate the performance of grasping objects, the potential energy method is used to analyze the grasping poses and stabilities of the PASA-GB hand. The calculation of force distribution shows the influence of the size and position of objects and provides a method to optimize the force distribution. The switch condition between pinching and enveloping grasp is analyzed in detail. Experimental results verify wide adaptability and high practicability of the PASA-GB hand.     

On the manipulability ellipsoids of underactuated robotic hands with compliance

Underactuation in robotic hands is currently attracting a lot of interest from researchers. The challenging idea of underactuation in grasping is that hands, with reduced number of actuators, supported by suitable design and control, may not suffer from reduced performances. This trend is also strengthened by recent neuroscience studies which demonstrates that also humans use sensorimotor synergies to control the hand in performing grasping tasks. In this paper, we focus on the kinematic and force manipulability analyses of underactuated robotic hands. The performances of such hands, regarded as mechanical transformers of inputs as forces and speed into outputs as object wrench and displacements, are assessed by suitably defined manipulation indices. The whole analysis is not limited by rigid-body motion assumptions, but encompasses elastic motions and statically indeterminate configurations by introducing generalized compliance at contacts and actuation. Two examples show the validity of the proposed approach to evaluate underactuated hand performances.     

Multidirectional compliance and constraint for improved robotic deburring. Part 2: improved bracing

This two-part paper presents a method for both improving the positioning capability and increasing the effective stiffness (bracing) of a robotic manipulator through multidirectional compliance and constraint. Improved positioning and improved bracing are attained through the effective use of multiple unilateral kinematic constraints in different directions. The companion paper identified how to specify the compliant characteristics of a manipulator so contact forces lead to deflections that eliminate positional misalignments and result in improved relative positioning through force guidance. In this part, we show that the characteristics beneficial to force guidance are the same characteristics that provide improved bracing when partially constrained by contact. Improved bracing is demonstrated in the context of workpart edge deburring.     

Human-inspired force compliant grasping primitives

We address the problem of grasping everyday objects that are small relative to an anthropomorphic hand, such as pens, screwdrivers, cellphones, and hammers from their natural poses on a support surface, e.g., a table top. In such conditions, state of the art grasp generation techniques fail to provide robust, achievable solutions due to either ignoring or trying to avoid contact with the support surface. In contrast, when people grasp small objects, they often make use of substantial contact with the support surface. In this paper we give results of human subjects grasping studies which show the extent and characteristics of environment contact under different task conditions. We develop a simple closed-loop hybrid grasping controller that mimics this interactive, contact-rich strategy by a position-force, pre-grasp and landing strategy for finger placement. The approach uses a compliant control of the hand during the grasp and release of objects in order to preserve safety. We conducted extensive robotic grasping experiments on a variety of small objects with similar shape and size. The results demonstrate that our approach is robust to localization uncertainties and applies to many everyday objects.     

灵巧手抓持规划的虚拟仿真方法

本文介绍在虚拟环境中,通过仿真分析的手段来研究机器人灵巧手抓持规划方案的方法。研究中以人的经验为指导,根据手、物的形状及尺寸等相对关系初步给出定性的抓持方案,以此为基础在虚拟环境中对机器人灵巧手的抓持过程进行仿真分析,判定所给出的抓持规划是否能实现在虚拟环境中的稳定抓持。然后在可行方案的基础上进一步对灵巧手的抓持点位置及抓持姿态进行优化,最终可得到机器人灵巧手对于特定被抓持物的较令人满意的抓持规划方案。