Measuring and testing – Instrument proving or calibrating – Angle – direction – or inclination
Reexamination Certificate
2001-04-17
2002-07-16
Raevis, Robert (Department: 2856)
Measuring and testing
Instrument proving or calibrating
Angle, direction, or inclination
Reexamination Certificate
active
06418774
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a device for calibration of an industrial robot, which calibration device is adapted for being in contact during the calibration with at least one plane of reference arranged on the robot, the device comprising an angle measuring member arranged for measuring an angle relative to the vertical line.
The present invention further relates to an industrial robot comprising at least two links moveably attached to each other by a joint, at least one plane of reference arranged on any of the links, and a calibration device adapted for being in contact with the plane of reference during the calibration.
The present invention also relates to a method for calibration of an industrial robot.
PRIOR ART
An industrial robot can be viewed as a chain of stiff links. Two adjacent links are joined with each other so that they either are rotatable relative to each other around a rotational axis or linearly displaceable relative to each other. The first link in the chain is the base of the robot and the last link usually constitutes a tool attachment. To be able to determine the position of the robot, each joint usually is provided with an angle measuring device in the form of an encoder or a resolver indicating the position of the joint relative to a zero position. Before an industrial robot can be used it must be calibrated, which means that each of the angle measuring devices is calibrated with reference to the zero position. The robot is calibrated in the production plant before it is delivered and sometimes on site before being set to work. Thereafter, the robot is calibrated after larger reparations such as engine or arm changes.
In the patent document U.S. Pat. No. 5,239,855 a known method of calibration is shown, in which an inclinometer or some other type of instrument for measuring the inclination is used to calibrate the angle measuring devices. An inclinometer measures the angle between an object and the vertical line and can for example be an electronic spirit level. The inclinometer is placed on a plane of reference on one of the links, and generates a signal, which is a measure of the angle between the plane of reference of the link and the vertical line. Thereafter, the joint is moved in dependence of the generated signal until it has a predetermined angle relative to the vertical line. The other links are calibrated in the same way.
The placement of the planes of reference, against which the calibration device is to be attached, is predetermined and is formed by accurately machined surfaces in order to obtain a high degree of flatness. When the robot is to be calibrated, the robot is moved to a predetermined calibration configuration. In this configuration, usually at least one of the links is oriented in a direction which departs from the directions of the other links. Usually, the angles between the length axes of the links are approximately 90°. Thus, the planes of reference may have different directions depending on which link to be calibrated. The planes of reference are usually either horizontal or vertical during the calibration.
A problem with said method of calibration is that the inclinometer must be mounted on the planes of reference of the robot with a very high precision. Today the inclinometer is first put on an inclinometer plate, which is then mounted on an adapter plate. The adapter plate is then in turn attached to on the plane of reference. Depending on the direction of the plane of reference to be calibrated, different adapter plates are used today. Usually, one type of adapter plate is used for horizontal planes of reference and another type of adapter plate is used for vertical planes. The adapter plate is attached to the robot with screws. This way of attaching the inclinometer to the robot includes a large number of sources of error. Examples of sources of error are mounting errors between the adapter plate and the plane of reference of the robot, mounting errors between the inclinometer plate and the adapter plate, and errors of tolerance of the adapter plate. The fact that several different adapter plates are used also contributes to increasing the mounting error.
A common type of industrial robot comprises a base adapted for resting on a horizontal foundation and a first arm link, which is rotationally arranged relative to the base around a vertical axis. Since the axis being calibrated is essentially parallel with the vertical line, it cannot be calibrated with an inclinometer attached to the first arm link. Accordingly, a second problem with the method of calibration described above is that it cannot calibrate rotational axes parallel to the vertical line.
DESCRIPTION OF THE INVENTION
The object of the invention is to achieve a device for calibration of an industrial robot reducing the number of sources of error during the mounting of the calibration device on the plane of reference.
This object is achieved with the initially defined device characterised in that the device further comprises two contact elements each having a curved surface, and the contact elements are arranged for being in contact with the plane of reference. According to the invention the inclinometer plate is replaced by at least two contact elements. The contact area between a curved surface and a plane surface is considerably smaller than the contact area between two plane surfaces. By decreasing the contact area between the calibration device and the robot, the mounting error decreases as well. Since a measurement of an angle relative to the vertical line is carried out in relation to a straight line, the number of contact elements should be at least two for full accuracy.
In one embodiment of the invention, the curved surfaces are mainly spherical. The spherical surfaces may comprise the whole or a part of a sphere. A spherical surface is advantageous in a manufacturing point of view. For instance, balls from ordinary ball bearings may be used.
In one preferred embodiment of the invention, the number of contact elements is three. With three contact elements one has at least three contact points between the calibration device and the plane of reference of the robot, which means that the angle measuring member and the robot are fixed relative to each other with at least three degrees of freedom. A stable attachment of the calibration device against the robot is thus obtained. The total number of degrees of freedom available for the calibration device and the robot to move in relation to each other is six, (in three directions and around three axes of rotation). The more degrees of freedom the calibration device and the robot are fixed in relative to each other, the more well-defined is the measurement and the smaller becomes the measurement error.
In another embodiment of the invention, the calibration device comprises a first wall element, and the contact elements are arranged for being in contact with the first wall element and the plane of reference. The wall element, which advantageously is firmly attached to the angle measuring member, has a function corresponding to the function of the adapter plate in the prior art.
In another embodiment of the invention, the device comprises a second wall element arranged in an angle relative to the first wall element, wherein said angle essentially corresponds to the angle between two planes of reference of the robot when it stands in a calibration configuration. With such a second wall element, the same calibration device can be used for calibration of planes of reference being arranged in two different directions. Accordingly, there is no need for using different calibration devices or different adapter plates, such as in the prior art, for measuring two planes of reference with different angles relative to the vertical line. The consequence of this is that one source of error, difficult to master during calibration today, directly disappears. The second wall element is preferably arranged essentially perpendicular to the first wall element. If the wall elements are arranged perp
Brogårdh Torgny
Jerregård Henrik
Robertson Alec
Slocum Alexander
Willoughby Patrick
ABB AB
Raevis Robert
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