John F. O'Brien

Precise, fault-tolerant pointing using a Stewart platform

Presents a precision pointing strategy. The principal contribution is the development of a fault-tolerant control which allows active pointing to continue despite multiple failures. A six-axes active platform is utilized to reject disturbances from a vibrating base to a precision payload. A decentralized controller is proposed which converts desired rotations into corresponding strut lengths via a decoupling transformation. The decoupling approach allows for simple single-input-single-output compensator design and for the incorporation of fault-tolerant strategies. The proposed strategy was evaluating on the microprecision interferometer testbed (a full-scale model of a future spaceborne optical interferometer) at the Jet Propulsion Laboratory, Pasadena, CA. Experimental pointing results demonstrate 50 dB of disturbance rejection at low frequency. In the laboratory ambient disturbance environment, this corresponds to a 1-/spl mu/rad rms pointing error.

Singularities in three-legged platform-type parallel mechanisms

Parallel mechanisms frequently contain an unstable type of singularity that has no counterpart in serial mechanisms. When the mechanism is at or near this type of singularity, it loses the ability to counteract external forces in certain directions. The determination of unstable singular configurations in parallel robots is challenging in general, and is usually tackled via an exhaustive search of the workspace using an accurate analytical model of the mechanism kinematics. This paper considers the singularity determination problem from a geometric perspective for planar and spatial three-legged parallel mechanisms. By using the constraints on the passive joint velocities, we derive a necessary condition for the unstable singularities. Using this condition, certain singularities can be found for certain type of platforms. As an example, new singular poses are discovered using this approach for a six-degree-of-freedom machining center.

A robotic approach to fault-tolerant, precision pointing

Future spaceborne optical interferometers and laser satellite communication systems are two space applications that require a precision pointing function in order to meet mission goals. Spaceborne interferometers provide a promising means to discover Earth-like planets in other solar-like systems. The laser communication systems provide a low-power, low-cost, lightweight means of data relay between ground and space and for deep-space communications to interplanetary probes. Both applications share the need to acquire and track a target. The interferometry application requires pointing errors to be submicroradian while the laser communication application requires microradian-level errors. In order to meet the precision pointing requirements found in these applications, a precision pointing strategy has been developed. The strategy employs a hexapod as the pointing platform to reject vibrations from a noisy spacecraft bus over all frequencies: at low frequency using 2- or 3-axis pointing and at high frequency using 6-axis vibration isolation. The benefits include broadband pointing stability without a high-bandwidth pointing sensor or destabilizing excitation of high-frequency structural modes, as well as tolerance to failures. This article outlines this approach to pointing.

Determination of unstable singularities in parallel robots with N arms

Parallel mechanisms frequently possess an unstable type of singularity that has no counterpart in serial mechanisms. When the mechanism is at or near this type of singularity, it loses the ability to counteract external forces in certain directions. The determination of unstable singular configurations in parallel robots is challenging, and in the past, has been tackled by exhaustive numerical searches of the mechanism workspace using an accurate analytical model of the mechanism kinematics. This paper considers the singularity-determination problem from a geometric perspective for n-legged spatial parallel mechanisms. By using the constraints on the passive joint velocities, a necessary condition for an unstable singularity is derived.

A Case Study of Planar 3-RPR Parallel Robot Singularity Free Workspace Design

A method to design a singularity-free planar 3-RPR planar parallel mechanism is presented. All the singular configurations of planar 3-RPR planar parallel mechanism are identified and categorized for design purposes. The location of the third base joint is taken as the design variable. A sequential design procedure is presented. And a solution set for the design variable is found.

Locally decoupled micromanipulation using an even number of parallel force actuators

New methods are found for arranging force actuators around a rigid body so that the system has locally decoupled and optimal manipulation characteristics. The closed-form solution leads directly to simple analytic formulas for the singular values of the manipulator Jacobian in terms of geometric design parameters. This makes it possible to easily design the local kinematics so that they meet desired specifications. Explicit formulas for designing the wrench/twist capabilities, achieving isotropy, maximizing the volume of achievable motions, and maximizing the minimum singular values are derived. Applications include design of generalized Gough-Stewart platforms (GSPs) and other parallel machines. To illustrate the power of the theory, the new methods are used to redesign an actual manipulator currently in use on the International Space Station (ISS). Unlike the existing manipulator, the new design is kinematically decoupled, isotropic, and fault tolerant - all highly desirable properties, especially in aerospace applications.

Designing Micromanipulation Systems for Decoupled Dynamics and Control

A new, seven parameter class of micromanipulators is found with decoupled dynamics. The resonant frequencies and damping ratios are found to be simple functions of the parameters, making it possible to design manipulators to meet resonant frequency or damping ratio specifications. Methods for designing the dynamics so that they are amenable to control are derived, which leads to significant increase in the closed-loop performance and robustness. To illustrate the new theory, a manipulator currently used on the International Space Station (ISS) is redesigned to greatly enhance its fault tolerance and closed-loop performance. Even though both manipulators control the same payload over the same bandwidth with exactly the same struts, the H∞ controller for the new manipulator is five times less sensitive to worst case disturbances than the H∞ controller for the existing manipulator. Moreover, the decoupling facilitates the design of Nyquist stable controllers with nonlinear dynamic compensators which give the new decoupled manipulator 100 times higher performance at low frequencies. This greatly increased performance and robustness requires implementation of only the six compensators along the diagonal of the six degree-of-freedom system, versus implementation of all 36 compensators for the ISS manipulator. This performance improvement is achieved with no increase in system cost, mass, or power budget; it is exclusively the result of the new theory presented in this paper.

Development of a voice coil-actuated limited-degree-of-freedom parallel mechanism for vibration suppression

Abstract The development of a prototype limited-degree-of-freedom parallel mechanism for vibration suppression is presented. The mechanism is novel in that voice-coil actuators capable of delivering large forces are used to provide high-bandwidth suppression for heavy payloads. A summary of general parallel mechanism kinematic theory is provided, and the singular conditions for the prototype mechanism are explained. A two-axis vibration suppression control system is described, and experimental evidence of closed-loop performance is provided. The prototype mechanism delivers a maximum of 27 dB of disturbance rejection over the target frequency interval 1–10 Hz.

Finding Unmanipulable Singularities in Parallel Mechanisms Using Jacobian Decomposition

A geometric method is presented to determine the unmanipulable singular configurations of a general class of parallel mechanisms. In unmanipulable singular configurations, the composite Jacobian matrix that maps active joints velocities to end-effector velocity loses rank, indicating the loss of a task degree-of-freedom. Finding unmanipulable configurations is difficult due to the complexity of the Jacobian matrix. The problem is greatly simplified by a novel decomposition of the matrix presented in this paper. The method is used to find singularities in several example parallel machines.

Loop Shaping of a Wide-Area Damping Controller Using HVDC

Insufficiently damped inter-area oscillatory behavior in large power systems may be mitigated by the application of a feedback control system. Thorough investigation of the resonant features unique to the western North American Power System (wNAPS) suggests that the modulation of active power using high voltage DC (HVDC) has great impact on retaining system stability when actuation is provided by the geographically expansive Pacific DC Intertie (PDCI) transmission line. The controller is limited in bandwidth by the influences of time delay, sensor and actuator dynamics, and specific plant characteristics. While proportional feedback of the scaled difference of two disparate bus frequencies has shown to be a valid method for controlling this particular system in previous work, the authors submit that a shaped loop transmission function provides performance and noise response improvement with guaranteed stability in saturation. Results of simulations using a high fidelity model of the system show the efficacy of the approach.