In-process relative robot workcell calibration

• US 6,812,665 B2
• Filed: 04/19/2002
• Issued: 11/02/2004
• Est. Priority Date: 04/19/2002
• Status: Expired due to Fees

• Current Assignee: ABB AB (ABB Limited)
• Original Assignee: ABB AB (ABB Limited)
• Inventors: Tang, Qing, Gan, Zhongxue, Sun, Yunquan
• Primary Examiner(s): Gibson, Randy W.
• Assistant Examiner(s): COLON SANTANA, EDUARDO
• Application Number:
  US10/126,988
• Publication Number:
  US 20030200042A1
• Time in Patent Office: 928 Days
• Field of Search: 318/567, 318/568.1, 318/568.11, 318/568.16, 318/568.21, 318/567.17, 318/567.19, 318/568.12, 700/186, 700/160, 700/245, 700/264, 738/665
• US Class Current: 318/568.11
• CPC Class Codes: B25J 9/1692 Calibration of manipulator
• CPC Class Codes: G05B 2219/39015 With different manipulator configurations, contact known sphere, ballbar
• CPC Class Codes: G05B 2219/39021 With probe, touch reference positions
• CPC Class Codes: G05B 2219/39044 Estimate error model from error at different attitudes and points

Abstract

A relative calibration system and method for robot workcell calibration is capable of correcting errors between the robot tool center point (TCP) and the work-object frame according to a relative reference, in that a precision path will be created based on this calibrated workcell.

Claims

1. In a robotic workcell having a robot, a sample workpiece, a calibration station and a tooling system, a method for off-line relative calibration comprising the steps of:

determining the tool center point of the robot;

creating a relative reference between the robot and the sample workpiece;

making relative measurements of the sample workpiece to calculate calibration parameters;

calculating a work-object coordinate compensation matrix; and

calibrating the tooling system, wherein calibrating the tooling system comprises measuring the relative difference between the fixed tool center point on the tooling system and the calibration station.

2. The method of claim 1 wherein determining the tool center point comprises the steps of:

selecting a calibration target having known geometric parameters;

touching the target from random direction, at least two different directions to measure the variation from the known geometric parameters; and

calculating the tool center point.

3. The method of claim 2 wherein the tool center point is calculated by non-linear least square optimization.

4. The method of claim 1 wherein creating the relative reference between the robot and the sample workpiece comprises the steps of:

grasping the sample workpiece;

measuring the sample workpiece in a plurality of locations; and

calculating the difference between a perfect model of the workpiece and the sample workpiece and adding all measured errors to the perfect model to construct a relative reference.

5. The method of claim 1 wherein generating the relative error map comprises the steps of:

measuring a two dimensional cross-section closed profile of the workpiece to determine errors in a X-Y plane for (Δ

x, Δ

y, Δ

θ

), where Δ

θ

represents the direction of roll;

measuring multiple two-dimensional cross-section closed profiles of the workpiece along a Z direction to determine errors in a Y-Z plane for (Δ

z, Δ

α

, Δ

β

), where (Δ

α

, Δ

β

) represent the direction of yaw and pitch.

6. The method of claim 1 wherein calculating a work-object coordinate compensation matrix comprises the steps of:

optimizing an offset and orientation along the workpiece in a first plane;

optimizing an offset and orientation along the workpiece in a second plane;

iteratively repeating the first and second steps until all relative errors are minimized.

7. The method of claim 1 wherein calibrating the tooling system comprises the steps of:

touching the robot to the tooling at a first location and recording the robot position;

touching the robot to the tooling at a second location and recording the robot position;

calculating the relative difference between the robot TCP and a parameter defined by the first and second locations.

8. In a robotic workcell having a robot, a sample workpiece, a calibration station and a tooling system, a method for off-line relative calibration comprising the steps of:

determining the tool center point of the robot;

creating a relative reference between the robot and the sample workpiece;

making relative measurements of the sample workpiece to calculate calibration parameters;

calculating a work-object coordinate compensation matrix; and

calibrating the tooling system, wherein calibrating the tooling system comprises

moving the robot to a first contact point between the robot and the tooling system;

recording the force between the robot and tooling system;

repeating the above procedure until only a substantially normal force is generated;

transferring and calculating the residual error between the robot and the tooling system by converting force error components into position variation; and

calculating the error-compensation matrix and adding into a tooling coordinate frame.

9. A method for calibration of a robotic workcell including a robot, a sample finished workpiece, a calibration station, and a tooling system, the method comprising:

making reference measurements of the sample finished workpiece in the calibration station;

making reference measurements of a raw unfinished workpiece in the calibration station;

performing a comparison of the reference measurements of the sample finished with the reference measurements of the raw unfinished workpiece; and

calculating a work-object error compensation matrix based upon the comparison.

10. A method for relative calibration of a robot work cell, the robot work cell comprising a robot, a sample workpiece, a tooling system, a force sensor and a calibration station including a displacement measurement sensor, the method comprising:

tool calibration;

workobject calibration comprising

programming calibration paths on a surface of the sample workpiece to define calibration movements for the measurement of workobject surface positions relative to the displacement measurement sensor,

defining a workobject geometry in a cartesian coordinate system with orthogonal x-, y-, and z-axes, in which the workobject can be translated dx, dy and dz and rotated dRx, dRy and dRz,

obtaining the calibration paths from a reference position of the workpiece relative to a robot mounting plate and from closed two-dimensional cross sectional workpiece profiles in parallel xy-planes,

calibrating dz by measuring a relative distance between the reference position of workpiece and the robot mounting plate,

after running calibration paths, utilizing one dimensional displacement measurements from the calibration station to calculate relative errors dx, dy and dRz of an actual workpiece profile relative to a corresponding ideal workpiece profile,

calculating errors in dRx and dRy based upon the distance between the workpiece profiles and at least two sets of dx, dy and dRz,

calculating dx, dy,dz,dRx,dRy, dRz for the workobject,

moving the robot to approach and touch the tool;

performing tool calibration using a force sensor mounted on arms of the robot; and

calculating a tool coordinate relative to the calibrated workobject.