Advanced Welding Processes: Technologies and Process Control

194
Advanced welding processes
and may need to be amplified; in addition, corrections need to be applied for
the cold junction temperature and the current must be converted into a
temperature reading. These functions are most easily performed electronically
and, since most temperature signals change relatively slowly, digital meters
often provide a convenient output display.
Non-contact measurements may be made by detecting the infrared radiation
from the material and thermal imaging cameras which give an indication of
the temperature profile over the area of interest are available. These devices
are, however, relatively costly and their use is at present restricted to research
and automatic sensing systems.
Current. Instantaneous current values may be displayed on an oscilloscope
and the overall shape of both AC and DC waveforms may be examined.
However, the input of the oscilloscope amplifier will usually be calibrated in
V cm –1 and the maximum level of input will often be around 5 V cm –1 . In
order to convert the normal high currents used in welding into a low-voltage
signal, one of the following devices is normally used: current shunt; current
transformer or Hall-effect probe.
The current shunt is a high-power, low-value resistance, which is placed
in series with the circuit through which the current to be measured is flowing.
It produces a low-voltage signal (e.g. 50–200 mV) proportional to the current
passing through it. Whilst this is suitable for measuring relatively slowly
changing waveforms, most shunts do have some inherent inductance, which
limits the rate of change of the signal and distorts the observed waveform.
Resistive heating of the shunt can also lead to inaccuracies. For improved
response and accuracy, water-cooled non-inductive shunts have been devised,
but these are usually costly and less convenient to use.
For AC waveforms, current transformers may be used to reduce the output
to a suitable level. These usually take the form of a toroid or coil placed
around the conductor carrying the current to be measured, although low-cost
devices that employ a clamp-type construction are available. Again, these
devices may produce some distortion of rapidly varying waveforms.
Hall-effect probes are based on semiconductor elements which respond to
the magnetic field produced when current passes through a conductor. They
are capable of detecting and indicating DC and AC current and have excellent
frequency response which enables them to be used to detect transient
phenomena in rapidly changing waveforms.
The use of oscilloscopes is, however, generally restricted to research and
the servicing of welding equipment. For production purposes, a quantitative,
single-value measure of current is normally required. Steady DC current
may be measured with analogue, moving-coil and digital meters. For DC
currents, the value indicated is the steady DC value or, in the case of fluctuating
current waveforms, the mean value. Analogue moving-iron meters indicate