Browse Prior Art Database

Robot Wrist Calibration Method and Fixture

IP.com Disclosure Number: IPCOM000037249D
Original Publication Date: 1989-Dec-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 4 page(s) / 89K

Publishing Venue

IBM

Related People

Mahlbacher, JC: AUTHOR [+3]

Abstract

This article describes a calibration technique and fixture which are used to increase the accuracy of robot wrists by correcting for joint offsets caused by wear, drift or maintenance actions. (Image Omitted)

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Robot Wrist Calibration Method and Fixture

This article describes a calibration technique and fixture which are used to increase the accuracy of robot wrists by correcting for joint offsets caused by wear, drift or maintenance actions.

(Image Omitted)

Calibration provides a means by which inaccuracies in robotic positioning can be corrected through software instead of reteaching application points. The electric drive robot (EDR) is a six degree of freedom manipulator with a modular wrist assembly. The wrist can be moved for maintenance and replaced on the robot in a repeatable fashion. Since the EDR is primarily used in clean room environments, the expedient disclosed herein allows wrist maintenance and calibration to be performed outside of the clean room.

The EDR wrist utilizes incremental digital encoders and harmonic drive gear systems to achieve high positioning resolution. The home position of each wrist joint (roll, pitch, and yaw) is defined at the encoder index pulse found by an optical switch tripped on the output drive. Because of inconsistencies in assembly, the home position will not necessarily be the same as the zero position of the wrist as illustrated in Figs. 1A and 1B. The zero position is used as a reference to maintain orientation during robot operation.

The fixture described herein provides a means of finding the joint offsets between the home and the zero reference position of the EDR wrist. The offsets can be used to correct the joint position with the following relationship.

(Image Omitted)

rcor = ract + roff + e

Wherercor = Corrected joint position ract = Actual joint position

roff = Joint offset

e = Measurement error

Fig. 2 is a top view of the calibration fixture. Figs. 3 and 4 are front and side views, respectively. It is an anodized aluminum fixture with a machined mounting surface. Eight hardened jig buttons 2 are ground with respect to the mounting surface to provide reference surfaces. A calibration end effector 3 is doweled and bolted to the EDR wrist 4. It has two stainless steel tooling balls 5. One is centered on the last axis of the wrist and the other is offset 4 inches radially from the last wrist axis. The wrist is doweled and bolted to the fixture on a mounting sur...