(1) A more advanced seam tracking system will decrease resources needed for programming. In the general case, this will allow the sensor to guide the robot from start to end with only a small number of intermediate poses in the case of complex weld joints. Naturally, the sensor guidance must include orientation as well as this is an important parameter to obtain defined quality and productivity.
(2) In cases where the sensor detects a change of orientation of the weld joint, the welding torch must be oriented at a specific angle relative the joint to produce the weld in accordance with the welding specification procedure. This procedure defines the conditions to comply with quality requirements and the torch orientation is as important as many other parameters to control.
Moreover, joint limits and singularities are not only a problem related to sensors. A common case in robot arc welding is to produce long welds. When the weld gets longer, it will be more difficult to select a proper starting configuration of the robot so it can produce the weld move without ending up at joint limits. When this happens the robot stops or changes configuration, which usually means that it rotates the wrist axes so it can proceed with the motion. There are however solutions to this problem: as the welding torch is symmetrical along the torch direction a rotation around this axis can be done without disturbing the quality. This may seem good but in the same way that an orientation may produce side effects such as joint limits and singularities, it may also produce unwanted movements of the other parts of the robot. During welding the robot moves close to the work piece and even very small movements may produce collisions.
As indicated, single problems in arc welding can be tackled with today's technology and what is needed it to apply a holistic point of view in integrating all tools available to get an integrated system for fulfilling the task defined by the product and the welding procedure specification. This integration needs to raise the abstraction level to a world model as can be shown in simulation systems for robots. Access to the world model gives the system the possibility of addressing all issues described above and during real time react and control the robot in an error recovery mode to fulfill the task.
The majority of industrial robots have 6 degrees of freedom. Many typical robot applications on the other hand require only 5 degrees of freedom, including arc and spotwelding. The extra degree of freedom available can be used to choose an infinite number of kinematic robot configurations that will all fulfill the given task; the system is redundant. This is also the case when positioners and gantries are used. The robot controllers of today do not take advantage of this and the robot operator is responsible for choosing one of the configurations which he/ she thinks is the best suited for the given situation. The robot controller (or the general simulation system during off-line programming) does not give any feedback regarding whether or not this is the optimal one. The operator has to trust his/her intuition or experience with the system to choose the best one. The non-linear structure of a robot arm with revolute joints makes it difficult to predict possible problems with singularities, exceeded joint limits and tangled cables. Especially when sensor control is utilized and the state of the system during execution can vary.
It is hard to fully utilize the potential of robot simulation system when sensors are used. The condition of the system is not fully known in advance and the sensors are used to handle deviations. The complexity of the system makes it difficult to predict all the problems that could arise during execution in the physical world. Industry copes with this problem basically by avoiding sensors, which has the consequence of lower system flexibility, although the complexity of the system is reduced at the same time. The approach to make full use of simulation systems and sensors in robotic welding could be twofold:
* During off-line preparation of the task, software tools should be available to perform a sensitivity analysis. Possible problems like near collisions, joint-limits or kinematic singularity should be paid attention to and suggestions for improvements should be proposed to increase the robustness. Models of the process, robot and sensors are used to accurately resemble the complexity of the real world.
* During on-line execution continuous supervision of the system is performed. Requests to models of the process, robot and other active devices in the workcell are made in real-time in order to optimize quality and handle limitations. A continuous update of a world model enables a correct action to any emerged situation to be undertaken. Problems like near collisions, joint-limits or kinematic singularity can be handled on-line but only if an updated world model is accessible.
Typically answering such questions as whether or not a change of torch orientation in order to avoid a singularity would cause collisions with any other object in the workcell.
The object model of the virtual world should enable easy programming, reuse and maintenance of the robot tasks. The possibility of linking any attributes and methods to the different objects increases the knowledge base of the virtual world to more than a geometrical description.
by Bolmsjo, GunnarView Profile; Olsson, Magnus; Cederberg, Per. The Industrial Robot29.2(2002)
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