Delbert Tesar

A control structure for fault-tolerant operation of robotic manipulators

Failure of any robotic system component during operation is a matter of concern. Internal shock phenomena due to the failure of joint actuation, and a recovery algorithm for both serial and parallel mechanisms under such circumstances are investigated. A control algorithm is studied that consists of a model reference algorithm and a simple proportional integral derivative (PID) controller in the feedback process. Simulation results illustrate the effectiveness of this recovery algorithm which attempts to reduce the internal shock when failure occurs, and accomplish the continued tracking of the given end-effector trajectory.<<ETX>>

A reusable software architecture for manual controller integration

This paper examines the commonalities of manual controllers used in robotics for teleoperation. These include devices ranging from simple joysticks to force-reflecting controllers. The similarities in functionality and behaviour of these controllers is further exploited to develop a reusable software architecture for manual controller interfacing. The development of this architecture is based on object-oriented design. The application of this design philosophy led to the development of a hierarchy of software components that are manual controller independent and also have a standardized interface. Reusability of these components is supported through generality and extensibility. The key design requirements for this architecture were: open-system, reusable, application independent, extensive error-handling and safety checking, applicability to real-time control and simulation, and reduction in program development time. This paper discusses the software analysis and design issues that were faced to meet the architecture requirements. Further, this architecture is demonstrated using four different manual controllers and a teleoperated dual-arm robotic manipulator.

A criteria-based approach to grasp synthesis

This study introduces a criteria-based methodology for grasp synthesis. The method allows the use of multiple performance criteria both at the finger and the hand levels, which are used to generate a preliminary grasp and an optimum grasp, respectively. The approach offers reduced complexity by decomposing grasp synthesis into manageable phases. A case study using a physical hand demonstrates the effectiveness of the method.

Network-based infrastructure for distributed remote operations and robotics research

The establishment of a unique infrastructure for distributed robotics and remote operations research within an educational environment is reported. The distributed laboratory consists of sites at four universities and NASAs Johnson Space Center. The distributed laboratory configuration provides the opportunity to quantitatively study the effects of various system components and technologies on overall telerobotic task performance. The ability to execute representative inspection and manipulation tasks with multiple control, robot, and performance/workload monitoring sites simultaneously connected has been demonstrated. Operations are carried out on a routine basis. During the process, needs for hardware and software standards development have been identified. The current implementation provides a basis for linking government, industrial, and university facilities to realize a truly collaborative research and development environment, enabling graduate students to experience educational opportunities that would otherwise not be possible. >

Open-loop stiffness control of overconstrained mechanisms/robotic linkage systems

A novel approach for the control of task-space stiffness characteristics in systems consisting of a superabundance of kinematically dependent inputs is proposed. When there are more input actuations than operational degrees of freedom, internal preloads can be generated that produce effective restoring forces in the face of displacement or disturbances imposed on the system. Examples of this excessive actuation can be found in certain modes of structurally overconstrained parallel manipulators and in antagonistically structured serial manipulators. The input loads are synthesized offline (prior to operation) and entered as a feedforward component so that the desired effective loads on objects are obtained, and (simultaneously) significant disturbances at the task level can be largely rejected in an open-loop fashion. This reduces the burden and shortcomings of standard feedback schemes. Moreover, a layered feedback scheme is used to compensate for small perturbations and unmodeled dynamics. The open-loop task-based stiffness control scheme as applied to structurally parallel mechanism/robotic linkage systems is investigated. The schemes applicability as a programmable active compliance device is also discussed.<<ETX>>

Geometric stability in force control

Previous implementations of robot force control seldom produced satisfactory results, and researchers in the past have experienced significant instability problems associated with their force controllers. When a manipulator is constrained to an environment (force-controlled), geometric stability due to the manipulator configuration and the force-controlled direction is shown to be a significant factor in overall system stability. This exploratory study points out a rather intuitive, geometrically based stability and analyzes the phenomenon both analytically and graphically. Sequential joint self-motion algorithms for kinematically redundant manipulators are suggested for reduced transitional impact and greater stability in the ensuing force-controlled operation.<<ETX>>

Parametric identification for industrial manipulators using experimental modal analysis

Establishing the complete dynamic model of a manipulator arm requires precise knowledge of the system mass and stiffness properties. An experimental procedure based on the modal analysis concept to identify the dynamic parameters of industrial manipulator arms is presented. The mass and stiffness content of an industrial robot are extracted from the modal behavior of the arm utilizing a general mathematical model for serial manipulators. The validity of the parameters is verified by comparison of the measured vibration behavior of the arm with the behavior of the proposed model. Some suggestions to improve the performance and operating speed of the robot by increasing the structural integrity of the system are presented. >

Deflection prediction for serial manipulators

A method of deflection analysis for a general serial manipulator which considers end-effector, gravity, and inertial loads is described. The conceptualized manipulator can have many degrees of freedom and be of the most general geometry. The analysis returns end-effector error based on a quasistatic approach. Thus, small oscillations (due to vibrations) are ignored and the inertial loads are considered applied loads. A kinematic analysis based on geometric influence coefficients is used to obtaining resultant joint loads. A flexibility analysis sufficient to describe general link deformation is performed to yield the local link deformations. These local values are transformed to end-effector deflections through the use of the kinematic influence coefficients as a first order approximation. End-effector deflection due to flexibilities in the joint actuators is addressed. A description of the generalized end-effector spring is presented.<<ETX>>

Architectures for fault-tolerant mechanical systems

Fault tolerance is an increasingly important specification for advanced mechanical and electronic systems. In this paper we consider the architectural features of a technology that can realize this stringent specification in the domain of mechanical systems. We examine elements of a fault-tolerant mechanical architecture and a failure-responsive control system architecture that operates over it from the perspective of robotic manipulator systems.<<ETX>>

Accessibility and controllability of flexible robotic manipulators

Although serial manipulator arms modeled with rigid links show full system controllability in the joint space, this condition does not necessarily hold for flexible robotic systems. In particular, in certain robot configurations, called inaccessible robot positions, one or more of the flexibilities may not be accessed directly by the actuators. This condition may significantly deteriorate system performance as reported earlier by the authors (Tosunoglu et al., 1988, 1989). The present study addresses the relationship between the accessibility and controllability concepts and establishes accessibility as a distinct concept from controllability. Although the theoretical framework is developed for general n -link, spatial manipulators modeled with m oscillation components, example case studies demonstrate the concepts on one- and two-link arms for brevity. Specifically, it is shown that although inaccessibility and uncontrollability may coincide in certain instances (as shown on a one-link arm), counter examples may be found where an arm in an inaccessible position can simultaneously demonstrate full system controllability (as shown on a two-link arm).