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Robot Arm Positioning With Light Feedback Closed-Loop

IP.com Disclosure Number: IPCOM000047958D
Original Publication Date: 1983-Dec-01
Included in the Prior Art Database: 2005-Feb-08
Document File: 5 page(s) / 54K

Publishing Venue

IBM

Related People

Green, PE: AUTHOR

Abstract

Placing a number of interferometer receivers at known locations about the workspace of a robot, connecting all the interferometer receivers to a laser light source at reference frequency by fiber optics, connecting the same laser source to the robot arm to constitute a fiber-optic lamp, and correlating the signal from the fiber-optic lamp against the reference frequency signals, permits calculation of the exact position of the robot arm. This permits closed-loop positioning of the robot arm, even though some of the fiber-optic lamps may be obscured. THE PROBLEM There are a number of situations in research and manufacturing in which the ability to determine accurately the position of a point on some object being moved about within some workspace is important.

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Robot Arm Positioning With Light Feedback Closed-Loop

Placing a number of interferometer receivers at known locations about the workspace of a robot, connecting all the interferometer receivers to a laser light source at reference frequency by fiber optics, connecting the same laser source to the robot arm to constitute a fiber-optic lamp, and correlating the signal from the fiber-optic lamp against the reference frequency signals, permits calculation of the exact position of the robot arm. This permits closed-loop positioning of the robot arm, even though some of the fiber-optic lamps may be obscured. THE PROBLEM There are a number of situations in research and manufacturing in which the ability to determine accurately the position of a point on some object being moved about within some workspace is important. In this disclosure, we focus on a specific situation in the area of industrial robots.

The robots available today gain their accuracy in positioning the end of the arm by using stiff, heavy, precisely machined minimum-backlash parts, to form the linkage between the frame and the arm endpoint.

These systems run open-loop using expensive linear actuators. By "open-loop" is meant that there is no servo control mechanism acting end-to-end to reduce to zero the difference between actual position to which the endpoint is driven and the intended endpoint position;
i.e., there is no servo loop closed about the whole linkage all the way out to the endpoint (although there may be loops closed about the estimated lengths or angles of individual linkage components). If a means were available to measure the endpoint's position accurately, then the difference between this and the intended position could be used as the input to the servo controller. BASIC APPROACH The proposed structure (three-dimensional interferometer (TDI)) is intended for sensing to great accuracy the actual position of the endpoint anywhere in the WORKSPACE, the volume in which the endpoint may be expected to move. The difference between this OBSERVED endpoint position and the TARGET endpoint position then constitutes the input to the controller of the desired closed-loop servo system.

This controller contains all the integration, differentiation, minor feedback loops, nonlinearities, etc., required to give the servo system the desired dynamic characteristics. The use of this technique allows industrial robots with a given combination of precision and payload to be built much more cheaply since they can use considerably lighter, less precise parts. Alternatively, robots built with today's mechanical technology should be able to handle increased payloads, or they can be made much more precise under present payloads. The TDI approach to position location can be applied to other situations where great precision of position location in a three-dimensional space is required over a volume some tens of centimeters to one or two meters on a side. Examples include step-and-re...