Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Reexamination Certificate
2003-06-11
2004-11-23
Leykin, Rita (Department: 2837)
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
C318S568110, C318S568130, C318S568150, C901S003000
Reexamination Certificate
active
06822412
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a method for programming of an industrial robot having a robot coordinate system and particularly to an application that is programmed by means of a position-measuring unit adapted for measuring positions relative a measuring coordinate system. The invention is useful for applications comprising a tool having a tool coordinate system and a work object to be processed by the tool. The invention is particularly useful in applications that demand high accuracy, such as different types of machining applications, for example fettling, debarring, milling, sawing, grinding and drilling. The application is also useful in applications such as arc welding, water jet cutting, laser cutting, gluing and assembly.
PRIOR ART
A robot program comprises a number of program instructions controlling the movements of the robot. Generation of a robot program comprises a first step wherein positions and orientations of a path, to be followed by the robot during execution of the program, are defined, and a next step wherein program instructions are generated based on the defined positions and orientations of the path. The positions and orientations defined during the programming comprise the positions and orientations that a tool is expected to obtain in relation to a work object when running the robot program.
A robot application is an application in which an industrial robot is used for performing a work. Robots are often used in applications involving processing of the surface of a work object. Existing practice for programming a robot involves on-line teaching the robot a sequence of positions and orientations. The positions and orientations define a robot path, which the robot shall follow during processing of the object. The robot is taught how to perform a task by being leading the tool through the various positions and orientations along the desired operating path during the programming. The robot program is then generated, based on the specified positions. However programming a robot by teaching can be time-consuming, especially if the work object has a complex geometry.
The accuracy of an application is limited mainly by the repeatability of the robot mechanics and the accuracy with which it is possible to move and rotate the tool to the desired position and orientation. In many high accuracy applications like machining, laser cutting, laser welding etc. the robot repeatability is good enough but it is very difficult for a human eye to find the correct position and orientation of the tool. Moreover, even if a robot programmer with long programming experience manages to obtain the accuracy needed, it will take a lot of time. The problem of long programming time makes it also very expensive to duplicate programs to other robots and to reprogram a robot cell after geometrical changes in the cell or after an exchange of the robot.
The utilization of high-level computer programming language and CAD/CAM has made off-line robot programming more feasible for more complex applications. In order to make off-line programming feasible it is necessary to know the cell geometry and the robot exactly. The work object coordinate system and the tool coordinate system have to be known with high accuracy relative to the robot base coordinate system. Thus, cell, object, tool and robot calibration have been the key issues to use robot off-line programming.
The accuracy of a general-purpose robot is between 5 and 15 mm, which can be improved to between 0.5 and 1 mm by the identification of the kinematic errors of the robot and compensation of these errors by software in the robot controller. However, this robot calibration technique is expensive and very difficult to support in a robot cell, and still the work object and tool coordinate systems must be calibrated. Moreover, this kinematic error compensation method will not be good enough for high accuracy applications.
OBJECTS AND SUMMARY OF THE INVENTION
The object of the present invention is to provide a method for fast and easy robot programming providing a high accuracy in a robot application.
This object is achieved by a method comprising: selecting an object reference structure on or in a fixed relation to the work object, the object reference structure having at least one surface, defining a mathematical model for the object reference structure, defining an object coordinate system in a fixed relation to the object reference structure, providing measurements on said at least one surface of the object reference structure, the measurements being performed by the position-measuring unit and are provided relative to the measuring coordinate system, determining the object coordinate system in relation to the measuring coordinate system, by means of best fit between said measurements performed by the position-measuring unit and said mathematical model of the object reference structure, measuring a plurality of positions on a desired robot path on the object by means of the position-measuring unit, determining the positions of the robot path in the object coordinate system, based on said measured positions of the robot path and said determined object coordinate system, determining the relation between the object coordinate system and the robot coordinate system, determining the relation between the tool coordinate system and the robot coordinate system, and programming the robot path based on said positions of the robot path in the object coordinate system, said relation between the object coordinate system and the robot coordinate system, and said relation between the tool coordinate system and the robot coordinate system. Programming the robot path means generating program instructions for moving the robot so that the tool obtains the positions and orientations in relation to the work object when running the robot program. The robot coordinate system could be any coordinate system defined in relation to the robot. Normally, a robot coordinate system attached to the base of the robot is used as a reference coordinate system for the robot. Thus, said base coordinate system is preferably used as the robot coordinate system.
The method according to the invention is a method for off-line programming of a robot by means of a position-measuring unit, for example a digitizer. The digitizer is used to specify the positions and orientations of the robot path on the work object in relation to an object coordinate system. Off-line programming of a robot is both faster and easier than on-line programming. A problem with off-line programming by a digitizer is that there may be deviations between the tool positions and orientations taught with the digitizer and run with the robot.
To overcome this problem the programming method according to the invention comprises a calibration step utilizing the position-measuring unit for performing measurements on an object reference structure having an object coordinate system defined in a fixed relation to it. If the work object contains well defined surfaces, the reference structure could be selected on the work object, otherwise a reference structure having well defined surfaces is mounted on the work object.
The method according to the invention combines the advantages of off-line programming with the accuracy of lead through programming. Moreover it solves the problems with the limited human accuracy with respect to tool manipulation and makes the programming totally safe since the robot does not need to work during programming and calibration. To make this possible, the position-measuring unit is used both for programming and calibration in such a way that the robot accuracy will not depend on the total kinematic and geometrical errors of the cell, but on just the differential kinematic and geometrical errors. In principle, position differences are used instead of absolute positions for the calibration and programming.
According to an embodiment of the invention said object reference structure is three-dimensional and has at least three non-parallel surfaces, and measurem
Brantmark Håkan
Brogårdh Torgny
Gan Zhongxue
Li Xiongzi
Rossano Gregory
Franklin Eric J.
Leykin Rita
Venable LLP
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