Advanced Welding Processes: Technologies and Process Control

Monitoring and control of welding processes 189
∑ conventional meters;
∑ computer-based instrumentation;
∑ measurement of welding parameters;
∑ stability measurement;
∑ dynamic resistance measurement;
∑ deviation monitors;
∑ vision systems.
Conventional meters
Analogue and digital measurement. Both analogue and digital techniques
are applied in the measuring of welding parameters.
In analogue systems, the signal to be measured is converted into an indication
which changes continuously in response to variations of the input signal. In
analogue meters, the incoming voltage may be converted into the deflection
of an indicating needle electromagnetically. Alternatively, for rapidly changing
signals, the indicator may be a beam of electrons which is scanned across a
cathode ray tube screen by the deflecting coils of an oscilloscope. 4 Analogue
measuring techniques give a good visual indication of the rate of change of
parameters and are useful when assessing process stability. Analogue meters
also give a good indication of mean output levels when the signal is subject
to random variations (e.g. when measuring the current in MMA welding).
Quantitative measurements may be made by reading a calibrated scale. The
indication provided by this type of meter is continuous and the resolution
depends on the scale and the care with which the readings are taken. In order
to be accurate, the meter scale needs to be large, but, with conventional
systems, this implies a fairly heavy mechanical movement which may reduce
the rate of response of the instrument. To obtain a permanent record of the
indicated values it is necessary to record the movement of the indicator on
paper or film; this may be achieved using a strip of continuously moving
recording medium, which is marked by a pen attached to a moving
potentiometric carriage or a light beam which is deflected onto photosensitive
paper by a galvanometer.
In digital measuring systems, the analogue signal is converted to a number
on a predefined scale before it is displayed. The resolution of the instrument
will depend on the analogue-to-digital conversion (ADC) device used, an
eight-bit device will provide a resolution of 256 increments, whilst a 12-bit
device will give 4096 and a 16-bit device will allow 65536 increments to be
resolved.
4 Oscilloscopes give a clear indication of the amplitude and frequency of regular, periodically
varying signals but for transient or irregular signals single-shot instruments such as the
computer-based systems described below must be used.