Page 111 - Welding Robots Technology, System Issues, and Applications

Sensors for Welding Robots

All parameters except the electrode extension can be measured directly, but the
electrode extension can be estimated from the weld current and the weld voltage. 97

The analog signal conditioning unit carries out low-pass filtering of the analog
signals from the sensors in order to prevent aliasing after the A/D converter. As the
environment around the measurements of the weld process can be highly disturbed
by electrical noise, the filter has the additional task of suppressing such noise. The
bandwidth required for the monitoring system is, however, mainly determined by
the short-circuit time, which is in the order of milliseconds, or about 500 Hz. The
rise time of the whole system should not exceed 10 % of the short-circuit time and
thus, the bandwidth needed should be in the order of 5 kHz. This makes it possible
to extract features like peak pulse time, background pulse time and short- circuit
time with adequate accuracy. Furthermore, high voltage transients caused by
spatter or short-circuitings can also be measured.

computable
signal Analog signal signal
Process Sensors conditioning
and A/D

Test
Feature quantity
feature Filtering S/N Level
extraction
algorithm enhancement detector

Figure 3.13. Block diagram for a monitoring system

Most modern commercial monitoring systems for GMAW divide the process into
time segments or windows and the sample mean for each segment is estimated.
These features are filtered and compared with a predefined threshold. As a result of
the mean value calculation, information about higher frequencies is lost since the
mean value of the signal has gone through a low-pass filtering process.

An appropriate feature which retains information about higher frequencies is the
variance. This is due to the fact that the variance (or the area below the curve of the
power spectral density) is an AC power estimate. A change in variance due to
disturbance is an indication of changes in the power spectral density.

Another feature which may be relevant for monitoring purposes is the peak of the
weld voltage spectrum. When welding in either the spray arc or short-circuiting