Showing papers by "N.H. McClamroch published in 1993"

Position and force control for constrained manipulator motion: Lyapunov's direct method

Abstract: A design procedure for simultaneous position and force control is developed, using Lyapunov's direct method, for manipulators in contact with a rigid environment that can be described by holonomic constraints. Many manipulators that interact with their environment require taking into account the effects of these constraints in the control design. The forces of constraint play a critical role in constrained motion and are, along with displacements and velocities, to be regulated at specified values. Lyapunov's direct method is used to develop a class of position and force feedback controllers. The conditions for gain selection demonstrate the importance of the constraints. Force feedback has been shown not to be mandatory for closed-loop stabilization, but it is useful in improving certain closed-loop robustness properties. >

Automatic spacecraft docking using computer vision-based guidance and control techniques

Abstract: An innovative approach to automatic spacecraft docking using a computer vision-based control system is introduced. Precision control of the relative spacecraft velocity is required to achieve "soft" docking with a docking platform on a space station or on another spacecraft. We propose use of a computer vision system as a position and orientation sensor for obtaining feedback information used by guidance and control loops. A camera, fixed to the spacecraft, tracks a standard rhombus mark fixed on the docking platform. Discrete-time position and orientation estimates of the spacecraft, relative to a coordinate frame fixed to the docking platform, are obtained by solving a constrained nonlinear least-squares problem and are used by the spacecraft feedback control loops. The accuracy of the computer vision estimates improves as the relative range decreases, thereby providing improved feedback information when it is most critical. Feedback control loops for the spacecraft, using three pairs of gas jet thrusters, are suggested that keep the camera always pointed at the rhombus mark and that perform precise control of the spacecraft range to achieve soft docking. The interactions between the vision system and the control systems are emphasized. Computer simulations of an integrated docking system verify the practical feasibility of this proposed automatic docking approach.

An asymptotic output tracking problem for a nonlinear control system with fewer outputs than inputs

Abstract: An asymptotic output tracking problem is studied for a class of nonlinear control systems with fewer outputs than inputs. The development is based on a standard relative degree assumption. The most interesting feature of the development is that the extra control degrees of freedom can be used to influence the closed loop characteristics, including the zero dynamics. Conditions are developed for when this approach is successful. >

Control and stabilization of a general class of nonholonomic dynamic systems

Abstract: This paper extends results in Bloch, Reyhanogln, and McClamroch (1992) to a more general class of nonholonomic dynamic systems that are important in applications. These nonlinear control systems are referred to as nonholonomic control systems due to certain nonintegrability assumptions which are made. The class of systems considered in this paper is characterized by general nonlinear base space dynamics that are input-output decouplable. Controllability and stabilizability results for this class of nonholonomic control systems are presented. >

Adaptive Processing for Vision-Based Ranging

Abstract: Consider a camera that moves through a stationary environment. The time-varying range to features in the camera's field of view may be estimated recursively using a sequence of images. We study processing these images adaptively; our goal is accurate range estimation and increased range map density in the presence of constrained computational resources. The problem of autonomous nap-of-the-earth rotorcraft flight motivates our work. An onboard camera and inertial measurement unit provide sensing for the construction of local range maps for an obstacle avoidance function. To provide more features yielding accurate range estimates, we assume a high resolution, high data rate camera. The resulting explosion in the image measurement data rate can be handled by selecting which measurements to process, and by making the best use of onboard computation in processing these measurements. Computer vision research provides many ideas on selecting which measurements to process. We study how to process the measurements. Specifically, for a single feature, we explore the idea of adaptively trading spatial resolution and image rate to obtain accurate range estimation while simultaneously satisfying an upper limit on computation rate. Simulations are presented to explore the effectiveness of this idea. Our work is applicable to other vision-based ranging problems.