Off-line programming of twelve axes

Automotive engineering Underbody protection

In a system for welding very large sub-assemblies, the programs generated off-line lead to deviations.

Off-line programming of twelve axes

Cell description

The cell consists of an IRB 2400 from ABB that is mounted on an XYZ portal in an inverted position. The portal has a travel path of approx. 8 m x 3 m x 3 m. Very large sub-assemblies – such as excavator shovels or crane parts – are welded in a MIG/MAG welding procedure on a 3-axis positioner consisting of a rotating/swivel unit combined with a lifting unit.

Task definition

The customer sometimes handles lot sizes of just a single piece in the cell. This means the robot programs must be generated individually for each part. With a normal teach-in process this would mean that production in the cell stops during this period.

Moreover, it is extremely difficult and time-consuming to teach-in a cell with twelve axes. The optimal paths for the welding process can be ascertained far more effectively and quickly on a CAD system than is possible in a teach-in procedure.

The customer's problem was that the programs generated off-line led to deviations of up to 30 mm on the real system. This was partly due to the fact that the robot and portal system lacked absolute accuracy, and partly because the geometric relationship between portal and positioner could only be established imprecisely with the standard methods of the robot manufacturer.

Implementation

A base plate was attached to a pillar of the portal and the LaserLAB flange-mounted onto it. A measuring rod with measuring ball was screwed onto the welding torch instead of the contact tip. The measuring rod was configured so that the central point of the measuring ball corresponded to the working point of the welding wire (TCP). With the LaserLAB and the loop:in software, all the robot axes and the axes of the XYZ portal were now controlled automatically so that the measuring ball always remained in the centre of the LaserLAB. This provided about 30 measurements from which the optimal values for the fine calibration of the robot were obtained.

The second step was to determine the geometric relationship between the additional axes (positioner) and the portal. For this purpose the LaserLAB was flange-mounted to the rotation axis of the positioner eccentrically. The position of the LaserLAB was now measured at twelve different positions with the absolutely accurate system consisting of portal, robot and measuring ball. It was important to move the additional axes to positions that were as different as possible.

The measurement is carried out by the kir:in software using a semi-automatic procedure. The result of the calibration is saved by kir:in in the moc.cfg file that is uploaded to the robot.

Customer benefit

It was possible to improve substantially the accuracy of the overall system with 12 axes, consisting of portal robot, tool and 3-axis positioner. Testing with a program generated off-line revealed an improvement in the overall accuracy by several centimetres to 1.5 mm.

Measuring with LaserLAB and loop:in takes less than 5 minutes and can be repeated by the customer at any time. As a result of the calibration, the robot and the portal axes are placed by loop:in in the positions for fine calibration. The customer simply runs a fine calibration of all axes at this position.

If the alignment is lost, the customer can calibrate all the axes automatically. Previously he had to use an extra lifting platform to raise the portal axes to the alignment positions at a height of about 3 metres.