Fully automatic gun offset: Robot guns ELMA VISION CR VR
Author: Prof. Dr.-Ing. Peter Puschner,
Welding expert and consultant of ELMA-Tech GmbH
Task and equipment
The ELMATECH robot guns are movably mounted on the robot between the fixture case, which is equipped with five planes of flanges. In the case of C guns, the gun body is equipped via a linear guide with the fixture case in the pin-type direction. V guns are connected to the attachment cage via a swivel joint, which allows mobility in the axis of the electrodes (the resulting degree of freedom follows an arc of large radius and can approximatively be assumed to be linear).
Due to the described mobility in the range of approx. 20 mm, it is achieved that no forces occur deforming the component during the closing of the gun and during the welding with forces up to 5 kN between the electrode caps.
As a result, both component tolerances and shifts of the TCP (Tool Center Point) after milling operations with a change in the electrode geometry remain without any detrimental influence due to unwanted forces acting on the construction itself.
In order to perform this so-called gun offset, all ELMATECH robot guns are equipped with a two-chamber pneumatic cylinder (compensating cylinder, controlled via servo-proportional valves) between fixture case and gun body which operate in the differential pressure principle. Thus, the high force required for the joining operation acts only on the plates themselves and never on the construction.
The arm deflections which are unavoidable in the case of larger gun opening also do not lead to an undesirable force input on the component in the described gun offset..
If no welding is carried out and the robot moves the gun in the space, the gun is held and fixed in its idle position with maximum force relative to the fixture case by means of the compensating cylinder. In preparation for welding, the gun is guided to the point to be welded in such a way, that the fixed electrode is located approx. 1 to 2 mm away from the surface of the sheet metal. This is done at each welding spot to be approached, the point coordinates are stored in the robot controller. It is thus already ensured that components or device tolerances can not contribute to a force effect in the subsequent positioning of the gun by the robot from point to point.
As soon as the robot reports, that it has positioned the gun in the point position, it gives the control authority to the ELMATECH welding control. The control unit determines in the ms range the gravitational component which is effective depending on the spatial position of the gun and applies - via fast acting servoproportional valves -the force to the compensating cylinder with exactly the force, which balances the gun in the direction of the electrode axis thus providing the so-called float operation.
At the same time, the movable electrode is moved to the component via the main cylinder and hits the ground. In order to realize the preliminary force, the gun with the fixed electrode is now also applied practically without force on the opposite side to the component. Finally, the prestressing force is built up without the formation of force components which have a detrimental effect on the construction of the component.
This completes the preparations for the start of the welding process. After the welding has been completed, the gun is first opened via the master cylinder. The gun offset is deactivated only after reaching or passing a minimum distance of the movable electrode from the component (stored in the gun control) and thus the gun body is driven into the rest position and clamped, so that the robot, after reaching the opening position (via the servo-pneumatics of the main cylinder) can be moved out of the component or to the next point. This process ensures that no force is applied to the component also during the opening phase.
Advantages of the ELMA-Tech gun offset
Since the compensation force for the float operation is fully automatically determined and set up, any programming via the robot control is dispensed according to the respective spatial position of the gun, which can change from point to point, and which has therefore hitherto been required.
Due to the multiple milling processes of the electrodes up to an electrode change, the distance between the fixed electrode and the component originally programmed by the teaching process changes after each milling operation. The distance between the fixed electrode and the component increases. This can be compensated by a shift of the TCP in the robot controller, in which case the new cap geometry must be taken into account, for example, by a calibration process. The automatic ELMATECH gun offset makes this completely dispensable.
The acting compensating force for the float operation, respectively determined via the acceleration sensor and derived therefrom, can be provided with an offset. This programmable, minimum-acting force in the range of 50 to 100 N relative to the fixed electrode in the direction of the component, causes the gun to move onto the component until it is placed on, so that the gun has not to be pulled through the preceding electrode by the main cylinder.
- Due to the fully automatic ELMA-Tech gun offset no programming effort is required in the robot control.
- The required balancing force for the float operation, depending on the spatial position of the welding gun, is determined by the ELMA-Tech process control for each point.
- Possible component tolerances do not create any damaging forces on the design.
- After milling operations with change of the cap geometry (removal of the cap surfaces), no TCP offset is required.
- An arm deflection, which can not be avoided under great welding forces and in case of large gun openings, does not generate any damaging forces on the construction, even if it is greater than 1 mm in case of light-weight guns for example.