Steve Ulrich

Modeling and Direct Adaptive Control of a Flexible- Joint Manipulator

DOI: 10.2514/1.54083 This paper addresses the problem of adaptive trajectory control of space manipulators that exhibit elastic vibrations in their joints and that are subject to parametric uncertainties and modeling errors. First, it presents a comprehensive study of rigid and linear flexible-joint stiffness models, to propose a dynamic formulation that includes nonlinear effects such as soft-windup and time-varying joint stiffness. Second, it develops an adaptive composite control scheme for tracking the end effector of a two-link flexible-joint manipulator. The control scheme consists of a direct model reference adaptive system designed to stabilize the rigid dynamics and a linear correction termtoimprovedampingofvibrationsatthejoints.Numericalsimulationscomparetheperformanceoftheadaptive controllerwithitsnonadaptiveversioninthecontextofa12:6 12:6 msquaretrajectorytracking.Resultsobtained withtheadaptivecontrolstrategyshowanincreasedrobustnesstomodelingerrorsanduncertaintiesinjointstiffness coefficients, and greatly improved tracking performance, compared with the nonadaptive strategy.

Nonlinear Adaptive Output Feedback Control of Flexible-Joint Space Manipulators with Joint Stiffness Uncertainties

Growing research interest in space robotic systems capable of accurately performing autonomous manipulation tasks within an acceptable execution time has led to an increased demand for lightweight materials and mechanisms. As a result, joint flexibility effects become important and represent the main limitation to achieving satisfactory trajectory-tracking performance. This paper addresses the nonlinear adaptive output feedback control problem for flexible-joint space manipulators. Composite control schemes in which decentralized simple adaptive control-based adaptation mechanisms to control the quasi-steady-state robot model are added to a linear correction term to stabilize the boundary-layer model are proposed. An almost strictly passivity-based approach is adopted to guarantee closed-loop stability of the quasi-steady-state model. Simulation results are included to highlight the performance and robustness of the proposed adaptive composite control methodologies to parametric and dynamics modeling unce...

Modified Simple Adaptive Control for a two-link space robot

An adaptive control scheme is proposed for tracking a 12.6 × 12.6 m square trajectory by the endpoint of a two-link rigid joint space robot. The adaptive controller is based on the classical Transpose Jacobian control law where the controller gains are adapted using a modified version of the Simple Adaptive Control (SAC) adaptation law. The formal Lyapunov proof of stability for the adaptive control system is derived. Simulation results show that the proposed adaptive control methodology is a promising concept when applied to space robots and yields improved tracking results compared to a nonadaptive control strategy.

Flight Results of Vision-Based Navigation for Autonomous Spacecraft Inspection of Unknown Objects

This paper describes a vision-based relative navigation and control strategy for inspecting an unknown, noncooperative, and possibly spinning object in space using a visual–inertial system that is designed to minimize the computational requirements while maintaining a safe relative distance. The proposed spacecraft inspection system relies solely on a calibrated stereo camera and a three-axis gyroscope to maintain a safe inspection distance while following a circular trajectory around the object. The navigation system is based on image processing algorithms, which extract the relative position and velocity between the inspector and the object, and a simple control approach is used to ensure that the desired range and bearing are maintained throughout the inspection maneuver. The hardware implementation details of the system are provided. Computer simulation results and experiments conducted aboard the International Space Station during Expedition 34 are reported to demonstrate the performance and applicab...

Autonomous atmospheric entry on mars: Performance improvement using a novel adaptive control algorithm

Upcoming landing missions to Mars will require on-board guidance and control systems in order to meet the scientific requirement of landing safely within hundreds of meters to the target of interest. More specifically, in the longitudinal plane, the first objective of the entry guidance and control system is to bring the vehicle to its specified velocity at the specified altitude (as required for safe parachute deployment), while the second objective is to reach the target position in the longitudinal plane. This paper proposes an improvement to the robustness of the constant flight path angle guidance law for achieving the first objective. The improvement consists of combining this guidance law with a novel adaptive control scheme, derived from the so-called Simple Adaptive Control (SAC) technique. Monte-Carlo simulation results are shown to demonstrate the accuracy and the robustness of the proposed guidance and adaptive control system.

Simple Adaptive Control for Spacecraft Proximity Operations

This paper addresses the problem of adaptive output feedback control for spacecraft proximity operations under parametric uncertainties and unknown disturbances. Control laws using the simple adaptive control theory, which is based on the so-called model reference adaptive control approach, are derived. In the first control scheme, a position feedback adaptive control law employing a parallel feedforward configuration to satisfy sufficient conditions guaranteeing closed-loop stability is developed. Then, it is shown how the performance of this adaptive controller can be significantly improved by using a position-plus-velocity feedback adaptive control strategy. Simulations compare the performance of the adaptive controllers with a fixed gain proportional-derivative controller. Obtained results demonstrate that both simple adaptive control methodologies yield improved performance, regardless of an uncertainty in the spacecraft mass and an unknown external perturbation, when compared to the linear-time invariant benchmark controller. In addition, the position-plus-velocity adaptive feedback methodology is shown to greatly reduce the required control input force, making its implementation onboard nanosatellites feasible. Finally, experiments conducted at the Massachusetts Institute of Technology’s Synchronized Position Hold Engage Reorient Experimental Satellites research facility are reported and discussed.

Attitude Stabilization of an Uncooperative Spacecraft in an Orbital Environment using Visco-Elastic Tethers

Active removal of large de-commissioned satellites is critical to the continued use of many of Earth’s orbits, as predicted by Donald J. Kessler in 1978. One solution to this problem is to use a tethered spacecraft system to capture and tow the highest risk debris to a disposal orbit. A signicant technical challenge lies with the capture, and subsequent stabilization, of a large and possibly tumbling debris. This paper addresses the target attitude stabilization aspect of the capture process. The two systems analyzed consist of 1) the currently accepted tethered spacecraft system where a single tether joins the target and the active chaser spacecraft, and 2) a newly proposed tethered spacecraft conguration that consists of a single tether attached to the active chaser spacecraft, which branches into four sub-tethers attached to the debris. An orbital environment is simulated, including gravity gradient torques. Incorporating the thrust ability of the chaser and exploiting the visco-elastic properties of the tethers, it is shown through numerical simulations that the proposed novel tethered spacecraft conguration provides an improved means of controlling the attitude of an uncooperative debris.

Direct Fuzzy Adaptive Control of a Manipulator with Elastic Joints

The use of harmonic drive gear boxes in space robotic applications offer several attractive properties, such as high reduction ratio, compact size, low mass, and coaxial assembly. However, the flexibility effects of this type of gear mechanism in the joints of robotic manipulators are significant enough to make real-time control challenging. This paper addresses the problem of fuzzy adaptive trajectory control of space manipulators that exhibit elastic vibrations in their joints and that are subject to parametric uncertainties and dynamics modeling errors. The developed control scheme uses a fuzzy adaptation mechanism that varies in real-time the gains of a slow control term designed to stabilize the rigid dynamics, and a linear correction term to further reduce the elastic vibrations at the joints. The proposed controller is validated in numerical simulations in a trajectory tracking scenario by a flexible-joint manipulator. Simulation results suggest that the controller is robust to uncertainties in joint stiffness coefficients and to modeling errors, and yields improved tracking performance compared with conventional flexible-joint control strategies.

Extended Kalman filtering for flexible joint space robot control

In this paper, an extended Kalman filter (EKF) strategy to estimate state variables from noisy measurements in flexible joint space manipulators is presented. First, an EKF that estimates the link and motor positions/velocities using only measurements from motor sensors is developed for space robots modeled with a classical linear joint dynamics model. Second, an extension for a novel nonlinear joint dynamics formulation is provided. The state estimates are coupled to a flexible joint adaptive controller in order to provide a complete closed-loop solution for real-time estimation and control. In numerical simulations, the EKF-adaptive controller combination demonstrates, for both dynamics representations, good performance when tracking a 12.6 × 12.6 m square trajectory.

Direct Model Reference Adaptive Control of a Flexible Joint Robot

Flexible effects in the joints of large space robots make their real-time operation a challenging task, especially when accurate endpoint positioning is required. The problem is further aggravated when the flexible joint stiffness matrix is not well known. This paper discusses the application of a model reference adaptive control (MRAC) composite system for tracking the endpoint of a flexible joint space robotic manipulator. The composite control scheme consists in a flexible control term designed to damp the joint vibrations plus a Transpose Jacobian rigid control term for which the control gains are adapted using a novel direct MRAC adaptation law. Numerical simulations show that the adaptive composite controller can maintain stability and good tracking performance despite significant uncertainties in the joint stiffness coefficients. Nomenclature ) , ( q q C & r = rigid centrifugal and Coriolis matrix x e , y e = model reference endpoint position error, ref x – x and ref y – y i I = inertia of link i , n i , , 1 K = m J = motor inertia matrix i m J = inertia of motor i , n