Data processing: generic control systems or specific application – Specific application – apparatus or process – Robot control
Reexamination Certificate
1998-06-09
2001-05-08
Grant, William (Department: 2121)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Robot control
C700S262000
Reexamination Certificate
active
06230079
ABSTRACT:
TECHNICAL FIELD
The invention relates to a method for control of an industrial robot, which has a plurality of movement axes with a position sensor for each one of the axes which is adapted to deliver a signal which defines the current position of the robot, and a control system for controlling the axes of the robot, the output signals from the position sensors being supplied to the control system.
BACKGROUND ART
The service life of an industrial robot is determined by how its mechanical structure and its components are loaded during the movements carried out by the robot during its service life. The mechanics of robots of today is dimensioned such that the robot is to be able to carry out the severest movement pattern conceivable during the whole of its specified service life. Only a small number of robots are run with such movement programs, which are unfavorable for the mechanical structure, and therefore almost all robots of today are mechanically oversized in relation to their specified service life.
A typical industrial robot comprises a number of robot arms, which are rotatable in relation to each other, and a hand provided with a tool attachment. The robot hand is rotatable in two or three degrees of freedom relative to the arm supporting the hand. The robot is provided with a control system which controls the position and the orientation of the robot hand. For each one of the movement axes of the robot, servo equipment comprising a driving motor and a position sensor is provided. The position sensor delivers a signal which is a measure of the angle of rotation of the axis in question. The servo system of each axis is supplied with a reference value for the angle of rotation of the axis, and the driving motor of the axis brings the robot to move in the axis in question until the axis position indicated by the position sensor of the axis coincides with the reference value supplied to the servo system.
To prevent the loads on the mechanical components of the robot, for example bearings, shafts, stays, motor housing and arm attachments, from becoming too high, limits to the maximum permissible torques and speed for each one of the axes of the robots are set. These limits are set prior to delivery of the robot and limit the performance of the robot, that is, the maximum speeds and maximum torques of the axes, during the whole service life of the robot. The limits to the maximum permissible axis torque and axis speed are calculated based on the guaranteed service life of the robot and fatigue diagrams for the mechanical structure. The calculation starts from a worst conceivable case with abnormal movement patterns and with an abnormal number of cycles per unit of time.
The mechanical load on a mechanical component at a certain time depends on several different factors, for example the speed, acceleration, configuration, and load of the robot. This means that if the robot has an advantageous configuration or a small load, the limits set to the maximum permissible axis torque and axis speed may be exceeded without the load of the component becoming too high.
SUMMARY OF THE INVENTION
The object of the invention is to increase the degree of utilization of the robot so that its mechanics are utilized to their maximum.
The current load at at least one mechanically critical point is calculated continuously on the basis of the output signals from the position sensors and a mathematical model for the robot. At regular intervals, a utilization factor for the critical point is calculated based on the load spectrum of the point during a predetermined period of observation. On the basis of the utilization factor and the service life of the robot, the maximally permissible load at the critical point is calculated. The service life may either be predetermined or optional. The maximum permissible load is calculated continuously during the robot cycles in question for each one of the critical points. The current load and the maximum permissible load are compared continuously and if the current load exceeds the maximum permissible load, axis speeds and axis torques are limited such that the load at the critical point is reduced.
By taking into account the magnitude of the loads to which certain mechanical structures are subjected, axis speeds and axis torques may be increased such that their mechanics are utilized to their maximum. In this way, a robot which carries out movement patterns which are favorable to the mechanics will have a higher performance than a robot which is run with unfavourable movement patterns. Likewise, a robot which handles small tool and arm loads will have a higher performance.
In one embodiment of the invention, the maximum permissible load is calculated in view of the desired service life of the robot. The maximum permissible load is then calculated on the basis of the whole prehistory of the robot. At any time, the user may be informed by the control system of the time remaining of the service life of the robot. If the user is not satisfied with the remaining service life, he may change the value of the remaining service life, after which the control system determines a new maximum permissible load on the basis of the new value of the service life. In this way, the user may himself choose between higher performance and shorter service life or lower performance and longer service life of his robot.
In another embodiment, a fixed service life is determined on installation of the robot. The control system then calculates during the observation periods the maximum permissible loads in relation to the fixed service life, whereby axis speeds and axis torques are adjusted such that the maximum permissible loads are not exceeded. In this case, no data about the prehistory of the robot need be stored, which is an advantage when replacing the control cabinet or in case of memory failures.
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Brogardh Torgny
Dahlquist Hakan
Elfving Staffan
Henriksson Tord
Asea Brown Boveri AB
Cabrera Zoila
Connolly Bove & Lodge & Hutz LLP
Grant William
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