Dynamic tracking line: feasible tracking region of a robot inconveyor systems

The concept of dynamic tracking line is proposed as the feasible tracking region for a robot in a robot-conveyor system, which takes the conveyor speed into consideration. This paper presents an effective method to find the dynamic tracking line in a robotic workcell. The maximum permissible line-speed which is a quantitative measure of the robot capability for conveyor tracking, is defined on the basis of the relation between the end effector speed and the bounds on the joint velocities, accelerations, and torques. This measure is derived in an analytic form using the parameterized dynamics and kinematics of the manipulator, and some of its properties are established mathematically. The problem of finding the dynamic tracking line is then formulated as a root-solving problem for a single-variable equation, and solved by the use of a simple numerical technique. Finally, numerical examples are presented to demonstrate the methodology and its applications in workspace specification.     

Introducing Radius of Torsure and Cylindroid of Torsure

Robotic path planning can involve the specification of the position and orientation of an end‐effector to achieve a desired task (e.g., deburring, welding, or surface metrology). Under such scenarios the end‐effector is instantaneously rotating and translating about a unique axis; the ISA. Alternately, the performance of direct contact mechanisms (viz., cam systems and gear pairs) are dependent on the surface geometry between two surfaces in direct contact. Determination of this geometry can entail the parametrization of a family of geodesics curves embedded within each surface. This parametrization is tantamount to an end‐effector rotating and translating about an ISA. In both scenarios there is a unique ISA for each geodesic embedded in a surface. Here, curvature and torsion of a spatial curve are coupled together to define an alternative definition for the radius‐of‐curvature of a spatial curve. This new definition is identified as radius of torsure to distinguish it from the classical definition for radius‐of‐curvature. Further, it is shown that the family of twists that corresponds to the pencil of geodesics coincident with a point on a surface defines a cylindroid: the cylindroid of torsure. An illustrative example is provided to demonstrate this difference. © 2003 Wiley Periodicals, Inc.     

The constant‐Jacobian method for kinematics of a three‐DOF planar micro‐motion stage

This paper concerns the development of a class of devices that generate end‐effector motion in the range of less than 100 μm and with sub‐nanometer resolution; in particular, a parallel manipulator configuration that generates a planar x‐y‐γ motion is considered. The parallel manipulator is implemented as a compliant mechanism. A problem with parallel manipulators is that the forward kinematics is usually too complex, which can hinder the implementation of advanced control algorithms. The contribution of this paper is that a simple method, called the constant‐Jacobian (CJ) method, is developed based on the pseudo‐rigid body (PRB) approach to compliant mechanisms. The experiment validates the CJ method. © 2002 John Wiley & Sons, Inc.     

Learning Control of Robot Manipulators in Task Space

Two important properties of industrial tasks performed by robot manipulators, namely, periodicity (i.e., repetitive nature) of the task and the need for the task to be performed by the end‐effector, motivated this work. Not being able to utilize the robot manipulator dynamics due to uncertainties complicated the control design. In a seemingly novel departure from the existing works in the literature, the tracking problem is formulated in the task space and the control input torque is aimed to decrease the task space tracking error directly without making use of inverse kinematics at the position level. A repetitive learning controller is designed which “learns” the overall uncertainties in the robot manipulator dynamics. The stability of the closed‐loop system and asymptotic end‐effector tracking of a periodic desired trajectory are guaranteed via Lyapunov based analysis methods. Experiments performed on an in‐house developed robot manipulator are presented to illustrate the performance and viability of the proposed controller.     

Motion Control of a Nonholonomic Mobile Manipulator in Task Space

In this paper, the motion control of a mobile manipulator subjected to nonholonomic constraints is investigated. The control objective is to design a computed‐torque controller based on the coupled dynamics of the mobile manipulator. The proposed controller achieves the capability of simultaneous tracking of a reference velocity for the mobile base and a reference trajectory for the end‐effector. The aforementioned reference velocity and trajectory are defined in the task space, such task setting imitates the actual working conditions of a mobile manipulator and thus makes the control problem practical. To solve this tracking problem, a steering velocity is firstly designed based on the first‐order kinematic model of the nonholonomic mobile base via dynamic feedback linearization. The main merit of the proposed steering velocity design is that it directly utilizes the reference velocity set in the task space without requiring the knowledge of a reference orientation. A torque controller is subsequently developed based on a proposed Lyapunov function which explicitly considers the coupled dynamics of the mobile manipulator to ensure the mobile base and end‐effector track the reference velocity and trajectory respectively. This proposed computed‐torque controller is able to realize asymptotic stability of both the base velocity tracking error and the end‐effector motion tracking error. Simulations are conducted to demonstrate the effectiveness of the proposed controller.     

Best achievable performance: non-switching compensation for multiple models

This paper presents a solution to the best achievable performance problem for a family of process models that are simultaneously stabilized by a non-switching LTI compensator. Specifically, a method is presented that can quantify the best dynamic performance achievable by a dynamic feedback compensator, for a finite family of process models. Closed-loop dynamic performance is quantified through a new performance measure that guarantees performance with respect to all allowable disturbances and allows for closed-loop response shaping with respect to fixed disturbances. The simultaneous performance problem is then formulated as a quadratically constrained minimax optimization problem that is non-differentiable and infinite dimensional. It is shown that the simultaneous performance problem can be solved through the iterative solution of appropriately constructed finite-dimensional nonlinear programming problems. A method that identifies ϵ-globally optimal solutions to this type of problems is presented. Finally, the proposed approach is demonstrated through an illustrative numerical example problem. Copyright © 1999 John Wiley & Sons, Ltd.     

Dynamic tracking line: feasible tracking region of a robot inconveyor systems

The concept of dynamic tracking line is proposed as the feasible tracking region for a robot in a robot-conveyor system, which takes the conveyor speed into consideration. This paper presents an effective method to find the dynamic tracking line in a robotic workcell. The maximum permissible line-speed which is a quantitative measure of the robot capability for conveyor tracking, is defined on the basis of the relation between the end-effector speed and the bounds on the joint velocities, accelerations, and torques. This measure is derived in an analytic form using the parameterized dynamics and kinematics of the manipulator, and some of its properties are established mathematically. The problem of finding the dynamic tracking line is then formulated as a root-solving problem for a single-variable equation, and solved by the use of a simple numerical technique. Finally, numerical examples are presented to demonstrate the methodology and its applications in workspace specification.     

An output feedback nonlinear decentralized controller for unmanned vehicle co‐ordination

This paper presents vision‐based control strategies for decentralized stabilization of unmanned vehicle formations. Three leader–follower formation control algorithms, which ensure asymptotic co‐ordinated motion, are described and compared. The first algorithm is a full state feedback nonlinear controller that requires full knowledge of the leader's velocities and accelerations. The second algorithm is a robust state feedback nonlinear controller that requires knowledge of the rate of change of the relative position error. Finally, the third algorithm is an output feedback approach that uses a high‐gain observer to estimate the derivative of the unmanned vehicles' relative position. Thus, this algorithm only requires knowledge of the leader–follower relative distance and bearing angle. Both data are computed using measurements from a single camera, eliminating sensitivity to information flow between vehicles. Lyapunov's stability theory‐based analysis and numerical simulations in a realistic 3D environment show the stability properties of the control methodologies. Copyright © 2007 John Wiley & Sons, Ltd.     

Inverse Velocity and Singularity Analysis of Low‐DOF Serial Manipulators

This paper presents a method for exact inverse velocity analysis of low‐DOF (degrees of freedom n<6) serial manipulators. For a low‐DOF serial manipulator, the number of independently controllable variables in the Cartesian space is equal to the number of joint variables in the joint space, and the remaining 6?n variables are linearly dependent on these independent variables. This paper employs the theory of reciprocal screws to determine a mapping between the independent velocity components in the Cartesian space and the joint rates in the joint space. It is shown that singular conditions of a low‐DOF manipulator depend on choice of independent variables. A 5‐DOF and a 4‐DOF manipulator are analyzed, and a numerical example in which the end effector of a 4‐DOF manipulator is commanded to follow a straight line is used to demonstrate the methodology. © 2003 Wiley Periodicals, Inc.     

OPTIMAL FEEDBACK CONTROL OF SISO SATURATION SYSTEMS OVER A CLASS OF NONLINEAR STABILIZING CONTROLLERS

This paper focuses on the feedback control of a linear single-input single-output process in the presence of input saturation constraints. First, closed-loop stability is guaranteed through the use of a parametrization of all nonlinear stabilizing controllers in terms of an S-stable parameter Q. Then, a performance measure is proposed that quantifies closed-loop performance with respect to a finite number of disturbances. Subsequently, the best achievable performance is quantified over a class of nonlinear stabilizing controllers parametrized in terms of absolutely summable second order Volterra operators. The resulting computational procedure involves the solution of a sequence of MILP's. An illustrative example is employed to demonstrate the proposed procedure. This example establishes that, for this performance measure, there may exist a value of the saturation bound for which the best achievable control system performance of the saturating system is strictly better than the corresponding best achievable linear performance. The same example also demonstrates that nonlinear control can perform strictly better than linear control for a performance measure that employs a finite number of disturbances. © 1997 by John Wiley & Sons, Ltd. Int. J. Robust Nonlinear Control, Vol. 7, 449–474 (1997).     

Robot perception of impedance

This paper proposes an algorithm for robot perception of impedance, which can be used as a fundamental technique for real‐time and qualitative perception of physical constraints imposed on the robot's end‐effector. This algorithm (1) estimates the impedance that represents the motion‐force relation of the end‐effector; (2) calculates the uncertainties of the estimates; and (3) detects discontinuous changes in the impedance. The estimated impedance can be used to recognize local dynamic properties of the environment and temporary constraint conditions imposed on the end‐effector. The detected discontinuities can be used to segment the manipulated tasks and to recognize the geometric structure of the environment. This method can be implemented in both autonomous and remote‐controlled robots because it is designed separately from control methodologies. Results of preliminary experiments are presented. © 2005 Wiley Periodicals, Inc.     

A Scalable Approach to Interactive Global Illumination

The addition of global illumination can dramatically increase the realism achievable when rendering virtual environments.In particular with interactive applications we expect the environment to reflect changes in the scenedue to global lighting effects instead of it being just a static backdrop. However, a sufficiently fast and accuratecomputation of global illumination at interactive rates has been difficult even with recent approaches based onrealtime ray tracing. In this paper we present a highly scalable approach to interactive global illumination. It fully recomputes a high‐qualitysolution for each frame and thus offers immediate feedback even for dynamic scenes, achieving more than20 fps for simple scenes. Compared to previous systems we increased the raw performance by a factor of up toeight and removed the bottlenecks that were limiting scalability. The system now scales linearly in quality andavailable computing resources, tested with up to 48 CPUs in a commodity PC‐cluster. Due to its logarithmicscaling property with respect to scene complexity it even supports lighting simulation in complex scenes with morethan 50 million triangles. This scalability allows applications to perform flexible performance trade‐offs. We alsoargue that the realism achievable through interactive global illumination will make it a standard feature of future3D graphics systems once the required computing resources are readily available.     

H∞ Control with Transients for Singular Systems

In this paper, the problem of a generalized type of H_∞ control is investigated for continuous‐time singular systems, which treats a mixed attenuation of exogenous inputs and initial conditions. First, a performance measure that is essentially the worst‐case norm of the regulated outputs over all exogenous inputs and initial conditions is introduced. A necessary and sufficient condition is obtained to ensure the singular system to be admissible and the performance measure to be less than a prescribed scalar. Based on the criterion a sufficient condition for the existence of a state‐feedback controller is established in terms of linear matrix inequalities. Moreover, the relationship between the performance measure and the standard H_∞ norm of the system is provided. Two numerical examples are given to demonstrate the properties of the obtained results.     

Decorating tokens to facilitate recognition of ambiguous language constructs

Software tools are fundamental to the comprehension, analysis, testing and debugging of application systems. A necessary first step in the development of many tools is the construction of a parser front‐end that can recognize the implementation language of the system under development. In this paper, we describe our use of token decoration to facilitate recognition of ambiguous language constructs. We apply our approach to the C++ language since its grammar is replete with ambiguous derivations such as the declaration/expression and template‐declaration/expression ambiguity. We describe our implementation of a parser front‐end for C++, keystone, and we describe our results in decorating tokens for our test suite including the examples from Clause Three of the C++ standard. We are currently exploiting the keystone front‐end to develop a taxonomy for implementation‐based class testing and to reverse‐engineer Unified Modeling Language (UML) class diagrams. Copyright © 2002 John Wiley & Sons, Ltd.     

SeaWiFS discrimination of harmful algal bloom evolution

The discrimination of harmful algal blooms (HABs) from space would benefit both the capability of early warning systems and the study of environmental factors affecting the initiation of blooms. Unfortunately, there are no published techniques using global monitoring satellite sensors to distinguish the resulting subtle changes in ocean colour, so in situ sampling is needed to identify the species in any observed bloom. This paper investigates multivariate classification as an objective means to discriminate harmful and harmless algae and monitor their dynamics using ocean colour data derived from satellite sensors. The classifier is trained and tested using Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) data, though the method is designed to be generic for other sensors. Time‐series results are presented using the new HAB likelihood index and suggest that SeaWiFS has some capability for observing the dynamic evolution of harmful blooms of Karenia mikimotoi, Chattonella verruculosa and cyanobacteria. Further, a multi‐band spatial subtraction algorithm is described to automate the identification of bloom regions and improve the accuracy in discriminating HABs.     

Stability Robustness of Closed‐Loop Systems in Angular Metrics

H_∞‐norm is widely used in the analysis and synthesis of robust control, a field which continues to flourish and develop. However, H_∞‐norm can only be used to measure the distance between two stable systems, not unstable systems. Sometimes, it is not appropriate to measure the gap between two systems. In this paper, a new metric, angular metric, defined in linear spaces of real rational matrices, is used to measure the distance of two systems with different dimensions. It is also used to measure the uncertainties and describe the performance specifications of the robust control system. In the framework of this metric, the robust stability margin is proposed to characterize the stability robustness of the closed‐loop system. When both the plant and the controller have uncertainties simultaneously, we introduce structural robust stability and prove the necessary and sufficient conditions of the robust stability of the feedback control system.     

EXTRACTING SECONDARY BIO‐EVENT ARGUMENTS WITH EXTRACTION CONSTRAINTS

This paper describes our bio‐event extraction system developed for the BioNLP 2009 Shared Task 2, with focus on its capability of extracting secondary biological event arguments from literature. Shared Task 2 is particularly interesting because when browsing literature, biologists often need to understand conditions surrounding biological events, which are usually expressed by secondary event arguments (e.g., binding sites). To achieve our goal, we take an approach that extracts n‐ary relations from text using event extraction constraints automatically generated from a training corpus. Event constraints consist of sequences of trigger words and semantic roles which we automatically identify using Conditional Random Fields (CRFs). Unlike most other systems participating in this shared task, our system is light‐weight and relies on neither external resources (e.g., Ontologies and dictionaries) nor natural language processing software (e.g., POS taggers and parsers). The official test results show that our approach performed well on extracting secondary arguments in Task 2, yielding the highest precision at 76.62% and the second highest F‐measure at 43.22%.     

Nonlinear Control of a Robot Manipulator with a Nonholonomic Jerk Constraint

We study the control of a prismatic‐prismatic‐revolute (PPR) robot manipulator subject to a nonholonomic jerk constraint, i.e., a third‐order nonintegrable design constraint. The mathematical model is obtained using the method of Lagrange multipliers. The control inputs are two forces and a torque applied to the prismatic joints and the revolute joint, respectively. The control objective is to control the robot end‐effector movement while keeping the transverse jerk component as zero. The main result of the paper is the construction of a feedback control algorithm that transfers the manipulator from any initial equilibrium configuration to the zero equilibrium configuration in finite time. The effectiveness of the algorithm is illustrated through a simulation example.     

Robustness improvement of a nonlinear H∞ controller for robot manipulators via saturation functions

In this paper, previous works on nonlinear H_∞ control for robot manipulators are extended. In particular, integral terms are considered to cope with persistent disturbances, such as constant load at the end‐effector. The extended controller may be understood as a computed‐torque control with an external PID, whose gain matrices vary with the position and velocity of the robot joints. In addition, in order to increase the controller robustness, an extension of the algorithms with saturation functions has been carried out. This extension deals with the resulting nonlinear equation of the closed‐loop error. A modified expression for the required increment in the control signal is provided, and the local closed‐loop stability of this approach is discussed. Finally, simulation results for a two‐link robot and experimental results for an industrial robot are presented. The results obtained with this technique have been compared with those attained with the original controllers to show the improvements achieved by means of the proposed method. © 2005 Wiley Periodicals, Inc.