Engineering topics

Freeman, Robert A.

Publications and source records attributed to Freeman, Robert A..

6 records

NASA NTRS1992

Synthesis of actively adjustable springs by antagonistic redundant actuation

A methodology for active spring generation is presented based on antagonistic redundant actuation. Antagonistic properties are characterized using an effective system stiffness. 'Antagonistic stiffness' is generated by preloading a closed-chain (parallel) linkage system. Internal load distribution is investigated along with the necessary conditions for spring synthesis. The performance and stability of a proposed active spring are shown by simulation, and applications are discussed.

Yi, Byung-Ju↗

NASA NTRS1989

Modeling and simulation of a Stewart platform type parallel structure robot

The kinematics and dynamics of a Stewart Platform type parallel structure robot (NASA's Dynamic Docking Test System) were modeled using the method of kinematic influence coefficients (KIC) and isomorphic transformations of system dependence from one set of generalized coordinates to another. By specifying the end-effector (platform) time trajectory, the required generalized input forces which would theoretically yield the desired motion were determined. It was found that the relationship between the platform motion and the actuators motion was nonlinear. In addition, the contribution to the total generalized forces, required at the actuators, from the acceleration related terms were found to be more significant than the velocity related terms. Hence, the curve representing the total required actuator force generally resembled the curve for the acceleration related force. Another observation revealed that the acceleration related effective inertia matrix I sub dd had the tendency to decouple, with the elements on the main diagonal of I sub dd being larger than the off-diagonal elements, while the velocity related inertia power array P sub ddd did not show such tendency. This tendency results in the acceleration related force curve of a given actuator resembling the acceleration profile of that particular actuator. Furthermore, it was indicated that the effective inertia matrix for the legs is more decoupled than that for the platform. These observations provide essential information for further research to develop an effective control strategy for real-time control of the Dynamic Docking Test System.

Lim, Gee Kwang↗

NASA NTRS1989

Distribution of dynamic loads for multiple cooperating robot manipulators

For the situation of multiple cooperating manipulators handling a single object, a formulation is presented which allows load distribution of the combined system to be made while taking manipulator dynamics into account. First, object dynamics are used to transform the motion task. An integrated procedure for modeling arm dynamics are used to transform the motion task. An integrated procedure for modeling arm dynamics is detailed. Then, a method is introduced which transforms the object load to the joint level. At this level, various methods of load distribution that allow subtask performance are proposed. These methods allow desired object motion while selecting loads desirable to alleviate manipulator dynamic loads.

Walker, Ian D.↗

NASA NTRS1989

Multiple cooperating manipulators: The case of kinematically redundant arms

Existing work concerning two or more manipulators simultaneously grasping and transferring a common load is continued and extended. Specifically considered is the case of one or more arms being kinematically redundant. Some existing results in the modeling and control of single redundant arms and multiple manipulators are reviewed. The cooperating situation is modeled in terms of a set of coordinates representing object motion and internal object squeezing. Nominal trajectories in these coordinates are produced via actuator load distribution algorithms introduced previously. A controller is developed to track these desired object trajectories while making use of the kinematic redundancy to additionally aid the cooperation and coordination of the system. It is shown how the existence of kinematic redundancy within the system may be used to enhance the degree of cooperation achievable.

Walker, Ian D.↗

NASA NTRS1989

Internal object loading for multiple cooperating robot manipulators

For an object being rigidly grasped and manipulated by multiple robotic mechanisms, the internal loading characteristics at a common coordinate set within the object are considered. It is demonstrated that representation of internal forces and moments in these common coordinates gives insight into force and load distribution schemes developed previously. In particular, it is shown how internal loads may be created in some end-effector force distribution schemes even when no component in the null-space of the grasp matrix is included. It is further shown that a particular pseudoinverse of the grasp matrix, which can be shown to be consistent with the kinematic constraints, may be used to eliminate this situation. The case of a two-arm system is used to illustrate the concepts introduced.

Walker, Ian D.↗

NASA NTRS1988

Dynamic task distribution for multiple cooperating robot manipulators

The issue of distributing the task among multiple robot arms while considering the manipulator dynamics is considered. The forces and moments required to move an object are distributed in such a way that extra degrees of freedom within the system may be used to satisfy or optimize criteria related to the manipulator dynamics. A method to perform such subtasks is introduced, and examples of possible criteria noted. It is expected that such techniques will produce trajectories which will be more desirable for the individual arms dynamically, since the dynamics are considered in the task distribution.

Walker, Ian D.↗