Scott M. Babcock

An efficient gradient projection optimization scheme for a seven-degree-of-freedom redundant robot with spherical wrist

A computationally effective kinematic control scheme is presented for a seven-degree-of-freedom redundant robot with spherical wrist. This scheme uses a gradient projection optimization method, which eliminates the need to determine the generalized inverse of the Jacobian when solving for joint velocities for given Cartesian end-effector velocities. Closed-form solutions are obtained for joint velocities using this approach. The application of this scheme to the seven-degree-of-freedom manipulator at the Center for Engineering Systems Advanced Research (CESAR) is described.<<ETX>>

Real-time implementation of an optimization scheme for seven-degree-of-freedom redundant manipulators

A computationally efficient kinematic optimal control scheme for seven-degree-of-freedom (7-DOF) manipulators is presented. This scheme uses the gradient projection optimization method in the framework of resolved motion rate control and does not require calculation of the pseudoinverse of the Jacobian. An efficient formulation for determining joint velocities for given Cartesian components of linear and angular end-effector velocities is obtained. This control scheme is well suited for real-time implementation, which is essential if the end-effector trajectory is continuously modified based on sensory feedback. Implementation of this scheme on a Motorola 6820 VME bus-based controller of the 7-DOF manipulator is described. A comparison of computational complexity with previously available schemes is presented. >

Inverse dynamics position control of a compliant manipulator

The cycle time of typical industrial manipulators is limited by vibrational characteristics associated with drive train compliance. In this paper the effects of drive train compliance and actuator dynamics in a two-degree-of-freedom manipulator are modeled and a position controller based on inverse dynamics is developed. Simulation of various controllers is performed utilizing the Advanced Continuous Simulation Language. It is shown that the closed loop frequency response of the manipulator can be increased beyond typical industrial practices by considering the drive train compliance in the design and providing suitable feedback measurements. Simulations suggest that satisfactory performance can be obtained as high as the limit imposed by the manipulators unmodeled higher vibration modes. The controller based on inverse dynamics is shown to effectively decouple the manipulators nonlinearly coupled degrees of freedom and perform better than a linear independent joint controller which also considers drive train compliance.

Self motion determination based on actuator velocity bounds for redundant manipulators

The movement of redundant manipulator joints that does not cause any end-effector motion is referred to as its self motion. Control schemes for redundant manipulators utilize its self motion to optimize a performance criterion. Thus, commanded joint motion at each sampling step is the sum of the minimum joint motion required for the desired end-effector motion and the self motion. However, the amount of self motion is limited by the bounds on actuator velocities, which are limited by the actuator torque bounds. A scheme is presented to determine the magnitude of self motion, the direction of which is determined by a gradient projection scheme. Implementation of this scheme on a Motorola 68020 VMEbus-based controller of a seven-degree-of-freedom manipulator is described.

A comparison of two real-time control schemes for redundant manipulators with bounded joint velocities

A comparison of two real-time control schemes for redundant manipulators is presented. Both schemes are developed in the framework of resolved rate control, and were presented by the authors in earlier articles (R.V. Dubey et al., 1988, J.A. Euler et al., 1988, and S.M. Babcock, 1988). The first scheme is the gradient projection scheme, which determines only the direction of the self-motion for the optimization of a scalar performance criterion. The second scheme determines the direction as well as the magnitude of the self motion. However, when the magnitude of the self-motion is limited by the hardware bounds on joint velocities, the results obtained using the two schemes are remarkably similar when the computational frequency is high (above 100 Hz). Therefore, the gradient projection scheme, which is computationally more efficient, is recommended. An explanation for the similarity of results is presented. Scheme 1 was implemented in real-time on a Motorola 68020 VME bus-based controller of the seven-degree-of-freedom manipulator at the Center for Engineering Systems Advanced Research.<<ETX>>