Advanced Welding Processes: Technologies and Process Control

Data
RAM
ADC
I/O
device
Monitoring and control of welding processes 191
Microprocessor
I/O
1
Multiplexer
Program
ROM
I/O
2
Store Display Print
Signal
conditioning
10.4 Principle of computer-based instrumentation.
Sensors
Welding
process
The digital output from the ADC is sampled at a rate determined by the
clock rate of the microprocessor and the control program. A range of programs
and operating parameters may be stored in the program memory (usually an
EPROM) and the appropriate sampling conditions may be chosen for a specific
application. The microprocessor also determines where the digitized values
are stored, how they are processed and whether they are displayed on some
in-built indicator or transferred to an external device for storage or display.
Systems of this type are capable of reading the instantaneous value of the
input level every 100 ms or even faster. It is possible to store these instantaneous
values in the RAM and they may be displayed so as to reproduce the waveform
of the incoming signal in much the same way as an oscilloscope, the difference
being that the values are stored and may be examined repeatedly. The data
may also be transferred to non-volatile memory (e.g. floppy disc, batterybacked
RAM, tape) and hard copy of the waveform may be obtained with the
aid of a printer or plotter. In more sophisticated systems, the data may be
exported via an Ethernet link, the internet, or factory bus to remote locations.
In order to avoid inaccurate representation of the waveform, known as
‘aliasing’, the sampling frequency should be at least ten times that of the
waveform being measured; with inter-sample times of 100 ms, the sampling
frequency is 10 kHz and signal frequencies lower than 1 kHz are accurately
represented; this response is perfectly adequate for general assessment of the
common welding waveforms found in pulsed GTAW and GMAW although
higher sampling speeds are sometimes required when investigating highspeed
transient events.