Date of Award

Spring 1993

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Electrical & Computer Engineering

Program/Concentration

Control Engineering

Committee Director

Oscar R. Gonzalez

Committee Member

John R. James

Committee Member

Derya A. Jacobs

Committee Member

Griffith J. McRee

Call Number for Print

Special Collections; LD4331.E55D64

Abstract

The most important property of an automated mechanical system is reliable execution of a specified task. Automated mechanical systems work best in precisely structured environments where all operations are repeatable. Typically, these systems are difficult to modify and expensive to install. Sensors can be used to detect disturbances, thus reducing hardware cost by offsetting the need for precise rigid construction components. Sensors, however, increase the complexity and cost of the control software. Efficient methods of reacting to sensor information during automated system operation are required to reduce software complexity and expense.

This thesis analyzes a robotic construction task and describes a new rule based system which could be used to coordinate the construction. Within NASA Langley's Automated Structures Assembly Laboratory a tetrahedral truss structure, representative of expected space structures, is constructed using an industrial robot equipped with a special purpose end-effector. The completed structure is eight meters in diameter and composed of 102 truss members. The robot is used to extract truss members from a storage canister and move the truss members to a position where they may be installed into the structure by the end-effector. Currently the robot motion is based on stored paths. These paths were developed in a trial and error process within the NASA facility. Successful construction relies on the repeatability of these paths which are specific to a particular robot and structure. An automated path planner is being developed to overcome these limitations.

This system uses a geometric model of the robot and truss structure to determine a suitable path. Thus, different geometry's may be constructed by changing the graphic model. Unfortunately, differences between the graphic model and the actual hardware prevent successful execution of the paths in an open loop fashion. Sensor information must be used to augment the path from the path planner. Currently the executive program supervising the construction proceeds in a serial fashion. Integration of automated path planning and sensor information will require an event driven system which can respond to asynchronous sensor information. In this thesis, a new rule based system will be proposed and a demonstration system developed that is capable of directing the robot during the construction operation. In addition, this thesis will detail the development of a force torque control strategy for a six degree of freedom robot. Force torque information from a wrist mounted sensor will be used to provide active compliance. Compliance is required during contact operations to minimize contact forces and prevent mechanical jamming. Two Cartesian force torque schemes, inverse kinematics and resolved rate, are analyzed in depth. The resolved rate scheme is shown to be more efficient when the entire control loop is analyzed. This is a new result which may be specific to robots with the kinematic configuration of the American Cimflex Merlin robot used within the Automated Structures Assembly Laboratory.

The thesis begins with a description of the assembly system, followed by development of a force velocity control strategy for the robot, and then the design of a rule based system to switch between the different control strategies used during construction. Key elements to the rule base design are discussed, including problem partitioning, the ability to respond to asynchronous events, and sensor information verification.

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DOI

10.25777/9y85-q518

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