Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Patent
1994-05-09
1996-06-18
Cabeca, John W.
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
31856822, 901 3, 901 16, B25J 922
Patent
active
055281168
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a method for calibration of a number of movement axes of an industrial robot, which has a plurality of movement axes with a position transducer for each one of the axes which is adapted to supply an output signal which defines the current position of the axis, and a robot hand for supporting a tool.
The invention also relates to an industrial robot system with an industrial robot which has a plurality of movement axes with a position transducer for each one of the axes which is adapted to supply an output signal which defines the current position of the axis, a robot hand for supporting a tool, and a control system for controlling the position and orientation of the robot hand in accordance with a program and adapted to receive the output signals of the position transducers.
BACKGROUND ART
A typical industrial robot is previously known from U.S. Pat. No. 3,909,600 and U.S. Pat. No. 3,920,972. Such a robot has a stand which is rotatably arranged on a foot and which supports a first robot arm which is rotatable in relation to the stand. In the outer end of this arm, and rotatable in relation thereto, a second robot arm is arranged. In its outer end this second arm supports a hand which is provided with a tool attachment and which is rotatable in two or three degrees of freedom in relation to the other arm. The robot is provided with control equipment for controlling the position and orientation of the robot hand. For each one of the above-mentioned axes of rotation, servo equipment comprising a driving motor and a position transducer is provided, the position transducer delivering a signal which is a measure of the angle of rotation of the current axis in relation to a reference position. To the servo system of each axis there is supplied a desired value for the angle of rotation of the axis, and the driving motor of the axis causes the robot to move in the current axis until the angular position indicated by the position transducer of the axis corresponds to the desired value supplied to the servo system.
For the position and orientation of the robot hand to correspond to the desired position and orientation, the mechanical structure of the robot and the data which describe it must be known with a high accuracy. This means that it is not sufficient for the nominal robot model to be known but also the individual deviations from the nominal model must be known. These deviations may be: variations in arm lengths, deviations in the orientations of the axes of rotation of the joints, and lateral displacements (offset) of the axes. These deviations arise in the manufacture of the different mechanical components and in the assembly of these. To this is also to be added that the angle indicated by the position transducer of an axis must with great accuracy correspond to the actual angle of rotation of that mechanical part of the robot which is controlled with the aid of the axis in question.
Because of difficulties in carrying out the calibration in a manner which is economical and adapted to production, the currently normal method is only to consider the nominal structure of the robot for describing the geometry of the robot.
For determining the relationship between the position transducer signals of the robot axes and the actual angles of rotation of the arms of the robot, different forms of calibration methods are used.
In one such calibration method, the robot is caused to assume such a position that the actual angles of rotation in the different axes are known, whereupon the angles of rotation indicated by the position transducers are compared with the actual angles. The position transducers may thereafter be adjusted such that the angles indicated thereby correspond to the actual angles. Alternatively, the deviations between the indicated angles and the actual angles may be stored and then be used during operation for correction of the output signals from the position transducers.
According to a previously known calibration method, the different parts of the robot
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patent: 4639878 (1987-01-01), Day et al.
patent: 4670849 (1987-06-01), Okada et al.
patent: 4763276 (1988-08-01), Perreirra et al.
patent: 4815006 (1989-03-01), Anderson et al.
patent: 4965499 (1990-10-01), Taft et al.
Asea Brown Boveri AB
Cabeca John W.
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