LIBRARY ı6ıul 0) - Cranfield University

of error was classified into this group due to its unpredictability, a characteristic that
is also found on the other component error sources.
3.1.2 Welding process errors
The process errors are related to the stability of the process (see section 2.1.5)
and to the adequacy of the welding parameters to produce the required weld quality.
Process errors are mainly characterised by the presence of defects such as undercut,
lack of penetration, porosity and burn-through, and/or the failure to achieve some
required quality specifications (such as the minimum leg length and minimum fusion
penetration), as a result of inadequate choice of welding parameters for the specific
requirements.
It should be noted that positioning errors may induce process errors. For
example, variation in contact tip-to-workpiece distance would result in change in
welding current and fusion penetration while excessive gap in the joint could lead to
overpenetration and burn-through.
3.2 Error compensation and proposed corrective measures
In order to compensate for the errors more effectively, each source of error
should initially be evaluated separately.
3.2.1 Robot error correction
The best way to deal with lack of positioning accuracy of a robot is to perform
a kinematic calibration (see section 2.4.2). In robotic gas metal arc welding, the
velocities during the process are normally low, if compared to spot welding or
machine loading and unloading. Therefore, a static calibration would suffice in this
case. Considering that off-line programming is used and that it is based on a robot
model, a strategy for controlling the robot errors in a robotic arc welding cell is
proposed. This consists of three main actions, namely:
1. To perform an initial static forward calibration in the robot arm after the
installation of the cell;
2. To calibrate the working cell: this includes workpiece positioner and can be
accomplished by using the calibrated robot as a measuring tool. A new
calibration of the work cell is necessary every time its layout is changed;
3. To perform periodic calibration checks in order to correct minor deviations
due to drift error and to detect when a new static calibration is necessary.
Points within the cell could be equipped with measuring devices which would
be responsible for detecting the drift error. These three actions should be repeated for
all robots involved in the production line, thus reducing or eliminating this source of
error.
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