LIBRARY ı6ıul 0) - Cranfield University

from three previously estimated values. This resulted in a stand-off control cycle of
approximately 750 milliseconds with maximum correction of ±1mm per cycle. This
limit in the table movement was imposed to prevent any undesirable arc instability due
to an excessive sudden change in stand-off. This control strategy gave a good and
robust performance.
4.2.5 Communications delay
An important factor to be considered in the control loop is the delay resulting
from the transfer of control instructions and their full implementation by the welding
power source and the table controller. These delays were compensated for by
introducing time delays between the control cycles.
In a control loop, the control instructions are the result of applying the control
rules to a measured process state. If during the measurement the process state is still
changing as a result of a previous control instruction, an unreliable measurement will
be obtained. If the controller uses this measurement to generate new control
instructions to compensate for deviations, this may result in oscillations in the control
loop and in process instability. Therefore, by introducing suitable time delays in the
control loop, these unstable situations can be prevented.
In the case of stand-off control, the use of a third order filter guaranteed that
the table controller was given sufficient time to adjust the stand-off without incurring
instability. However, since a correction signal could be issued every monitoring cycle,
in the case of the welding process control a time delay should be introduced. In order
to have a good estimate of this time delay, some-voltage step input tests were carried
out using two different ways of communicating with the welding power sources: a)
via direct serial communication with the power source controller and b) via analogue
signals, using a special interface tailored for robots. The results of the tests can be
found in section 6.3. The power source was found to have a response time of
approximately 50 ms for direct serial communications and 200 ms for analogue input
via the robot interface. Since the serial communications protocol for the power source
used was still being developed, some limitations were found in terms of available
power source commands. This led the author to opt for the robot interface, thus in
effect having a time delay of 200 ms. This time delay was introduced in the control
cycle in such a way that the control instructions to the power source would be issued
every other monitoring cycle. Consequently a voltage control cycle of approximately
500 milliseconds was obtained, that is, two control cycles would occur every second.
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