Steven Dubowsky

Time-optimal control of robotic manipulators along specified paths

The minimum-time manipulator control problem is solved for the case when the path is specified and the actuator torque limitations are known. The optimal open-loop torques are found, and a method is given for implementing these torques with a conventional linear feedback control system. The algorithm allows bounds on the torques that may be arbitrary functions of the joint angles and angular velocities. This method is valid for any path and orientation of the end- effector that is specified. The algorithm can be used for any manipulator that has rigid links, known dynamic equations of motion, and joint angles that can be determined at a given position on the path.

The kinematics, dynamics, and control of free-flying and free-floating space robotic systems

Some important dynamics and control problems unique to space robotic systems are discussed. Particular attention is paid to free-flying and free-floating space robots that might be used for such tasks as space station repair and construction. Advances in solving these problems are briefly reviewed. Three promising methods for planning and controlling the motion of space robotic systems are presented. It is suggested that a thorough understanding of the fundamental dynamics of these systems will result in effective solutions to their control problems. >

On the nature of control algorithms for free-floating space manipulators

It is suggested that nearly any control algorithm that can be used for fixed-based manipulators also can be employed in the control of free-floating space manipulator systems, with the additional conditions of estimating or measuring a spacecrafts orientation and of avoiding dynamic singularities. This result is based on the structural similarities between the kinematic and dynamic equations for the same manipulator but with a fixed base. Barycenters are used to formulate the kinematic and dynamic equations of free-floating space manipulators. A control algorithm for a space manipulator system is designed to demonstrate the value of the analysis. >

On the dynamics of manipulators in space using the virtual manipulator approach

Robotic manipulators carried by future spacecraft are expected to perform important tasks in space, like servicing satellites. Such applications will encounter problems due to the dynamic coupling between the manipulator and the spacecraft. A Virtual Manipulator (VM) concept has been developed recently for the modelling of manipulators working in space. This paper shows that the VM facilitates planning and control of the motions of manipulators mounted on spacecraft, that minimizes the degrading consequences of manipulator/vehicle dynamic interactions. The VM is a new theoretical approach for the design and development of future space manipulator systems.

On computing the global time-optimal motions of robotic manipulators in the presence of obstacles

A method for computing the time-optimal motions of robotic manipulators is presented that considers the nonlinear manipulator dynamics, actuator constraints, joint limits, and obstacles. The optimization problem is reduced to a search for the time-optimal path in the n-dimensional position space. A small set of near-optimal paths is first efficiently selected for a grid, using a branch and bound search and a series of lower bound estimates on the traveling time along a given path. These paths are further optimized with a local path optimization to yield the global optimal solution. Obstacles are considered by eliminating the collision points from the tessellated space and by adding a penalty function to the motion time in the local optimization. The computational efficiency of the method stems from the reduced dimensionality of the searched spaced and from combining the grid search with a local optimization. The method is demonstrated in several examples for two- and six-degree-of-freedom manipulators with obstacles. >

An Adaptive Shared Control System for an Intelligent Mobility Aid for the Elderly

The control system for a personal aid for mobility and health monitoring (PAMM) for the elderly is presented. PAMM is intended to assist the elderly living independently or in senior assisted living facilities. It provides physical support and guidance, as well as monitoring basic vital signs for users that may have both limited physical and cognitive capabilities. This paper presents the design of a bi-level control system for PAMM. The first level is an admittance-based mobility controller that provides a natural and intuitive human machine interface. The second level is an adaptive shared controller that allocates control between the user and the computer based on metrics of the users performance. Field trials at an eldercare facility show the effectiveness of the design.

PAMM - a robotic aid to the elderly for mobility assistance and monitoring: a "helping-hand" for the elderly

Meeting the needs of the elderly presents important technical challenges. In this research, a system concept for a robotic aid to provide mobility assistance and monitoring for the elderly and its enabling technologies are being developed. The system, called PAMM (personal aid for mobility and monitoring), is intended to assist the elderly living independently or in senior assisted living facilities. It provides physical support and guidance, and it monitors the users basic vital signs. An experimental test-bed used to evaluate the PAMM technology is described. This test-bed has a cane based configuration with a nonholonomic drive. Preliminary field trials at an Eldercare Facility are presented.

The kinematics and dynamics of space manipulators: the virtual manipulator approach

Future robotic manipulator systems will be required to per form complex tasks in space such as satellite repair. These robotic manipulators will encounter a number of kinematic, dynamic, and control problems caused by the dynamic coup ling between the manipulators and its spacecraft. This dy namic coupling also makes it difficult to analyze these sys tems. This paper introduces a new analytical modeling method for space manipulators called the Virtual Manipula tor (VM), which has a fixed base in inertial space at a point called a Virtual Ground. The kinematics and dynamics of the manipulator, spacecraft, and payload can be described relatively easily in terms of the VM. With its fixed base, the Virtual Manipulator is shown to have the potential to be an effective aid for the analysis, design, and development of future space manipulator systems.

Robot Path Planning with Obstacles, Actuator, Gripper, and Payload Constraints

A method is presented to obtain the time-optimal motions for robotic manipulators. It considers the full nonlinear dy namics of the manipulator, its actuator saturation limits, and gripper and payload constraints. It also accounts for both the presence of obstacles in the work space and restrictions on the motion of the manipulators joints. The method is com putationally practical and has been implemented for the optimal trajectory planning of general six degree-of-freedom manipulators. Examples are presented that demonstrate the substantial improvement in manipulator performance that can be achieved using this method.

On the Dynamics of Space Manipulators Using the Virtual Manipulator, with Applications to Path Planning

Robotic manipulators carried by future spacecraft are expected to perform important tasks in space, such as the servicing of satellites. However, the performance of these systems could be severely degraded by dynamic disturbances to the spacecraft caused by manipulator motions. This paper presents a method for representing the dynamics of space manipulator systems using the recently developed Virtual Manipulator (VM) concept. This representation is then applied to develop algorithms which can be used to plan manipulator motions that minimize disturbances of the spacecraft.