Automatic calibration assures robot welding accuracy

Citation

(2000), "Automatic calibration assures robot welding accuracy", Industrial Robot, Vol. 27 No. 6. https://doi.org/10.1108/ir.2000.04927faf.007

Automatic calibration assures robot welding accuracy

Keywords: Calibration, Robots, Welding

A novel method for verifying the absolute position of a robot-mounted arc welding torch has been developed by Plymouth-based Autotech Robotics and Meta Vision Systems, Eynsham, UK (see Plate 5). The calibration system uses laser seam tracking equipment manufactured by Meta to automate the procedure of establishing the X, Y and Z co-ordinates of the tip of the weld wire to an accuracy previously unheard of in a production welding environment.

There can be many problems in ensuring that robot welding systems continue to produce high quality fabrications. Such problems are often traced to out-of-position welds caused by the peripheral welding equipment rather than the robot. Wear of the contact tip, weld torch damage and wire feed problems can necessitate frequent and significant adjustment of the robot path to ensure that welds are placed correctly. Some larger users of robots have checking fixtures so that damaged torches can be bent back into shape. All these errors may now be corrected quickly and automatically.

Plate 5 Autotech Robotics has harnessed Meta Vision Systems' laser seam tracking equipment to establish automatically and very accurately the tool centre point in robot welding systems

A Meta Vision laser scanning unit, which normally moves with the torch on the robot wrist to track the weld seam as it is being laid, has found a new role in this application whereby it is mounted in a fixed position within a cabinet adjacent to the torch cleaning unit. When spatter has been removed from the torch after the previous weld, the tip is presented into the station where its X and Y positions are logged by the laser beam. The Z-axis co-ordinate is measured by traversing the tip of the wire at a known speed vertically through the beam.

Results are displayed on a readout within the cabinet to a resolution of 0.1mm. At the same time, analogue signals are sent to the robot controller where they are compared with the originally taught co-ordinates. If the tip position is outside a preset tolerance band, offsets are automatically incorporated into the robot program to bring the torch tip back into correct alignment for the next weld. If three successive out-of-tolerance results are received, the system automatically shuts down to allow manual investigation of the problem.

The tip calibration sequence clearly has the potential to increase the welding cycle time and hence adversely affect production output. However, the objective is to complete the torch clean and tip measurement functions within the pallet index time, in which case there is no loss of productivity.

The technique is applicable to tungsten inert gas (TIG) as well as metal inert gas (MIG) welding and may be retrofitted to existing installations. The system can also be used to ensure correct location when mechanical positioning cannot achieve the necessary tolerances. It has additionally been shown to provide useful verification of the accuracy of robot laser welding systems during commissioning, using a mechanical element attached to the head to represent the position of the focal point of the laser.

Contact: Dr Andrew Pryce, Sales and Marketing Director, Meta Vision Systems Ltd, Oakfield House, Oakfield Industrial Estate, Eynsham, Oxfordshire OX81TH. Tel: +44 (0) 1865 887900; Fax: +44 (0) 1865 887901.