John E. McInroy

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.

Modeling and design of flexure jointed Stewart platforms for control purposes

A number of researchers have been investigating the use of Stewart platforms (or hexapods) for precision applications including machining, vibration isolation and precise pointing. To avoid friction and backlash, these hexapods often employ flexure joints. This does eliminate nonlinear friction and backlash, but adds linear spring/damper dynamics. In addition, since the motion is so accurate, base and/or payload vibrations become significant disturbances to suppress. This paper develops guidelines for designing the flexure joints to facilitate closed-loop control. In addition, since base accelerations are typically the dominant disturbance, their effect is derived. Unlike most prior hexapod dynamic formulations, the model is experimentally verified.

Optimal, fault-tolerant mappings to achieve secondary goals without compromising primary performance

In many applications, the manipulations require only part of the degrees of freedom (DOFs) of the end-effector, or some DOFs are more important than the rest. We name these applications prioritized manipulations. The end-effectors DOFs are divided into those which are critical and must be controlled as precisely as possible, and those which have loose specifications, so their tracking performance can be traded off to achieve other needs. In this paper, for the class of general constrained rigid multibody systems (including passive joints and multiple closed kinematic loops), we derive a formulation for partitioning the task space into major and secondary task directions, and finding the velocity and static force mappings that precisely accomplish the major task and optimize some secondary goals such as reliability enhancement, obstacle and singularity avoidance, fault tolerance, or joint limit avoidance. The major task and secondary goals need to be specified in term of velocities/forces. In addition, a framework is developed to handle two kinds of common actuator failures, torque failure and position failure, by reconfiguring the differential kinematics and static force models. The techniques are tested on a 6-DOF parallel robot. Experimental results illustrate that the approach is practical and yields good performance.

Fault tolerance of parallel manipulators using task space and kinematic redundancy

When a parallel manipulator suffers from failures, its performance can be significantly affected. Thus, fault tolerance is essential for task-critical applications or applications in which maintenance is hard to implement. In this paper, we consider three types of common strut failures corresponding to stuck joints, unactuated actuators, or the complete loss of struts, respectively. The impacts of different failures on the kinematics of a manipulator are examined, and the task space redundancy and kinematic redundancies are used to help overcome these failures. In addition, local measures of fault tolerance and their properties are analyzed. These measures can be helpful in architecture design and path planning

Decoupled control of flexure-jointed hexapods using estimated joint-space mass-inertia matrix

By exploiting properties of the joint-space mass-inertia matrix of flexure-jointed hexapods (or Stewart platforms), a new decoupling method is proposed. The new decoupling method, through a static input-output mapping, transforms the highly coupled six-input six-output dynamics into six independent single-input single-output (SISO) channels. Controls for these SISO channels are far simpler than their multiple-input multiple-output (MIMO) counterparts. Prior decoupling control methods imposed severe constraints on the allowable geometry and payload. The new method loosens and removes these constraints, thus greatly expanding the applications. Based on the new decoupling method, identification algorithms using the constrained least squares (CLS) and the symmetric positive definite estimation (SPDE) methods are introduced to estimate the joint-space mass-inertia matrix using payload accelerations and base forces. These identification algorithms can be used for precision payload calibration, thus improving performance and removing the labor required to design the control for different payloads. The new decoupling method, together with the identification algorithms, is experimentally compared with earlier techniques. These experimental results indicate that the new approach is practical and improves performance.

Orthogonal Gough-Stewart platforms for micromanipulation

Development of methods to design optimal Gough-Stewart platform geometries capable of meeting desired specifications is of high interest. Computationally intensive methods have been used to treat this problem in various settings. This paper uses analytic methods to characterize all orthogonal Gough-Stewart platforms (OGSPs) and to study their properties over a small workspace. This characterization is used to design optimal OGSPs for precision applications that achieve a desired hyperellipsoid of velocities. Some examples demonstrating the versatility of this theory are discussed.

Finding symmetric orthogonal Gough-Stewart platforms

This paper develops new, analytical methods to find a large class of orthogonal Gough-Stewart platforms (OGSPs) having desired properties at their home position. In contrast, prior methods have been computationally intensive, relying on numerical search techniques. By exploiting symmetry, 27 equations are reduced to only two. The new techniques are directly applicable to clean-sheet design of micro-manipulators, vibration isolators, and Cartesian stiffness matrices. In addition, straightforward methods for retro-fitting existing OGSPs are illustrated. Because the new theory greatly simplifies OGSP formulas about a single point, it is expected that these results will also prove to be very useful when numerically designing gross motion platforms

Trajectory Tracking with Parallel Robots Using Low Chattering, Fuzzy Sliding Mode Controller

This paper proposes a trajectory tracking scheme which belongs to the sliding mode control (SMC) for the 4-degree-of-freedom (DOF) parallel robots. Two fuzzy logic systems (FLS) are first put forward to replace the constant switching control gain and the width of the boundary layer. The fuzzy adaptive supervisory controller (FASC) is combined with the fuzzy sliding mode control (FSMC) to further reduce the chattering. The design is simple and less fuzzy rules are required. The simulation results demonstrate that the chattering of the SMC is reduced greatly and the parallel robot realizes the trajectory tracking with very good robustness to the parameter uncertainties and external disturbances.

Generating classes of locally orthogonal Gough-Stewart platforms

This paper develops methods for generating classes of orthogonal Gough-Stewart platforms (OGSPs). First, a new, two-parameter class of six-strut OGSPs which leads to isotropic manipulators are found. Next, this class is extended to include redundant Gough-Stewart platforms (GSPs). For an even number of struts, the same algorithm used to generate the six-strut case can be employed. For an odd number of struts, similar essential concepts are used to derive seven-strut and nine-strut OGSPs. Maximization of fault tolerance is implemented for a nine-strut isotropic OGSP. By exploiting invariant properties of the inverse Jacobian, new methods for favorably altering the center of gravity, strut attachment surface, and strut spatial distribution are developed.

Using Redundancy to Optimize Manipulability of Stewart Platforms

For any robotic system, fault tolerance is a desirable property. This paper uses a comparative approach to investigate fault tolerance and the associated problem of reduced manipulability of robots. It is shown that for a certain class of manipulators, the mean squared relative manipulability over all possible cases of a given number of actuator failures is always constant irrespective of the geometry of the manipulator. In this context, optimal fault tolerant manipulability is quantified. The theory is applied to a special class of parallel manipulators called orthogonal Gough-Stewart platforms (orthogonal GSPs or OGSPs). A class of symmetric OGSPs that inherently provide for optimal fault tolerant manipulability under a single failure is developed.