LIBRARY ı6ıul 0) - Cranfield University

3.2.2 Programming error correction
Considering that these errors are mainly caused by model mismatch, the best
approach to dealing with them is to ensure that the computer models mirror exactly
the robot behaviour. The robot kinematic parameters
identified in the static calibration
procedure could be used to correct the robot model in the computer. The workcell
model can be corrected with the positions measured during the workcell calibration.
In order to match the robot behaviour in both simulation and real cell, the
programmer must choose the orientation representation that provides the same
movements as that of the robot.
3.2.3 Component error compensation
The component errors are regarded as more complicated to accommodate
than the other errors, since they depend on individual component variances.
In the literature, several different approaches have been used to deal with the
component errors. Two main strategies can be identified: a) the setting of the
manufacturing tolerances to the levels required by an automated welding system; and
b) the use of sensors and adaptive control.
The first approach can sometimes represent a large increase in the
manufacturing costs, which may be unacceptable; while the second approach provides
compensation for the discrepancies, resulting in consistent welds. However,
depending on the type of sensor (e. g. laser systems), the initial investment can be high.
The best approach for compensating the variation in joint positioning due to
component errors is to implement pre-weld joint searching (to determine the weld
start position), on-line seam tracking and on-line contact tip-to-workpiece distance
control, to ensure that the weld bead is deposited in the right place and to keep the
torch-to-workpiece relative distance constant. This approach was adopted in this
work and will be described in detail in the next chapter.
3.2.4 Welding parameters
Setting the right combination of welding parameters is of major importance for
any welding process. Particularly, in gas metal arc welding of thin sheet steel, the
welding parameters must be set such that a stable and robust process is obtained and
the risk of defects is minimised, yielding the required weld quality. The more stable
the process is, the more robust it is to external disturbances.
Therefore, the best way to deal with process errors is to ensure that the
welding parameters are adequate for the quality requirements. It is also necessary to
implement on-line monitoring and control of the process, such that deterioration
trends in the process stability and weld quality caused by unexpected process
disturbances can be detected and corrected before they compromise the quality of the
whole weld.
The control strategy proposed is based on procedural (off-line) control and
on-line control methods. The procedural control is based on the off-line optimisation
of welding parameters, based on previously established [ref. 51] welding regression
models, such that the welding parameters are selected from a list of predicted welding
parameters which are expected to produce the required quality. The on-line control
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