Evangelos Papadopoulos

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. >

A new measure of tipover stability margin for mobile manipulators

Mobile manipulators operating in field environments will be required to perform tasks on uneven terrain which may cause the system to approach, or achieve, a dangerous tipover instability. To avoid tipover in an automatic system, or to provide a human operator with an indication of proximity to tipover, it is necessary to define a measure of stability margin. This work presents a new tipover stability measure (the force-angle stability measure) which is easily computed and sensitive to topheaviness. The proposed metric is applicable to systems subject to inertial and external forces, operating over even or uneven terrains. Performance of the measure is demonstrated using a forestry vehicle simulation.

Dynamic Singularities in Free-floating Space Manipulators

Dynamic Singularities are shown for free-floating space manipulator systems where the spacecraft moves in response to manipulator motions without compensation from its attitude control system. At a dynamic singularity the manipulator is unable to move its end-effector in some inertial direction; thus dynamic singularities must be considered in the design, planning, and control of free-floating space manipulator systems. The existence and location of dynamic singularities cannot be predicted solely from the manipulator kinematic structure because they are functions of the dynamic properties of the system, unlike the singularities for fixed-base manipulators. Also analyzed are the implications of dynamic singularities to the nature of the system’s workspace.

Free-flying robots in space: an overview of dynamics modeling, planning and control

Free-flying space manipulator systems, in which robotic manipulators are mounted on a free-flying spacecraft, are envisioned for assembling, maintenance, repair, and contingency operations in space. Nevertheless, even for fixed-base systems, control of mechanical manipulators is a challenging task. This is due to strong nonlinearities in the equations of motion, and consequently different algorithms have been suggested to control end-effector motion or force, since the early research in robotic systems. In this paper, first a brief review of basic concepts of various algorithms in controlling robotic manipulators is introduced. Then, specific problems related to application of such systems in space and a microgravity environment is highlighted. Basic issues of kinematics and dynamics modeling of such systems, trajectory planning and control strategies, cooperation of multiple arm space free-flying robots, and finally, experimental studies and technological aspects of such systems with their specific limitations are discussed.

THE FORCE-ANGLE MEASURE OF TIPOVER STABILITY MARGIN FOR MOBILE MANIPULATORS.

Mobile manipulators operating in field environments will be required to apply large forces, or manipulate large loads, and to perform such tasks on uneven terrain which may cause the system to approach, or reach, a dangerous tipover instability. To avoid tipover in an automatic system, or to provide a human operator with an indication of proximity to tipover, it is necessary to define a measure of available stability margin. This work presents a new tipover stability measure (the Force-Angle stability measure) which has a simple geometric interpretation, is easily computed, and is sensitive to changes in Center of Mass height. The proposed metric is applicable to systems subject to inertial and external forces, operating over even or uneven terrains. Requirements for computation and implementation of the measure are described, and several different categories of application of the measure are presented along with useful normalizations. Performance of the Force-Angle measure is demonstrated and compared with that of other stability margin measures using a forestry vehicle simulation. Results show the importance of considering both center-of-mass height and system heaviness, and confirm the effectiveness of the Force-Angle measure in monitoring the tipover stability margin.

Explicit dynamics of space free-flyers with multiple manipulators via SPACEMAPLE

This paper focuses on the dynamics of a multiple manipulator space free-flying robot (SFFR) with rigid links and issues relevant to the development of appropriate control algorithms. To develop an explicit dynamics model of such complex systems, the Lagrangian formulation is applied. First, the system kinetic energy is derived based on a developed kinematics approach. Then, through vigorous mathematical analyses, three formats are obtained which describe the contribution of each term of kinetic energy to the equations of motion. Next, explicit derivations of a systems mass matrix, and of the vectors of non-linear velocity terms and generalized forces are introduced for the first time. The obtained dynamics model is very useful for dynamics analyses, design and development of control algorithms for such complex systems. The explicit SFFR dynamics can be implemented either numerically or symbolically. Following the latter approach, the developed symbolic code for dynamics modeling, i.e. SPACEMAPLE, and its verification procedure are described, and issues relevant to the development and computation of dynamics models in control algorithms are briefly discussed. Specific dynamic characteristics of SFFRs compared to fixed-base manipulators are pointed out.

Coordinated manipulator/spacecraft motion control for space robotic systems

The coordinated control of space manipulators and their spacecraft is investigated. The dynamics of free-flying space robotic systems are written compactly as functions of the system barycentric vectors. A control technique is developed that includes requirements on a spacecrafts position and orientation as well as on its manipulator. This control scheme has the double advantage of allowing a systems motion to be planned to avoid impacts with is environment, and of maintaining a favorable manipulator configuration during the end-effectors motion. In addition, since a systems spacecraft can be moved, the workspace of its manipulator becomes unlimited. A transposed-Jacobian type controller with inertial feedback is developed, and an example is used to demonstrate this technique.<<ETX>>

Multiple Impedance Control for Space Free-Flying Robots

To increase the mobility of on-orbit robotic systems, space free-flying robots (SFFR), in which one or more manipulators are mounted on a thruster-equipped base, have been proposed. Unlike fixed-based manipulators, the robotic arms of SFFR are dynamically coupled with each other and the free-flying base; hence, the control problem becomes more challenging. The multiple impedance control (MIC) is developed to manipulate space objects by multiple arms of SFFR. The MIC law is based on the concept of designated impedances and enforces them at various system levels, that is, the free-flying base, all cooperating manipulators, and the manipulated object itself. The object can include an internal angular momentum source, as is the case in most satellite manipulation tasks. The disturbance rejection characteristic of this algorithm is also studied. The result of this analysis reveals that the effect of disturbances substantially reduces through appropriate tuning of the controller mass matrix gain. A system of three manipulators mounted on a free-flying base is simulated in which force and torque disturbances are exerted at several points. The system dynamics is developed symbolically, and the controlled system is simulated. The simulation results reveal the merits of the MIC algorithm in terms of smooth performance, that is, negligible small tracking errors in the presence of impacts as a result of contact with the obstacles and significant disturbances.

On the nature of control algorithms for space manipulators

A study of the characteristics of control algorithms that can be applied to the motion control of space manipulators is reported. The results obtained show that nearly any control algorithm that can be applied to conventional terrestrial fixed-base manipulators, with a few additional conditions, can be directly applied to free-floating space manipulators. Barycenters are used to formulate efficiently 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.<<ETX>>