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mounting fixture. The voltage signal was divided by 10 and filtered in the conditioning
board before being converted to the digital form.
Current sensor
The welding current was measured by using a Hall effect probe (RS Catalogue
No. 256-174) with specification as follows:
Rating
Calibration accuracy at 23 °C
Zero offset
(23 °C)
Zero offset (0-60 °C)
dUdt (transient) following
Output:
500 A
± 1% of range
± 0.1% of range
± 1% of range
> 200 Alµs
0-10Vdc
This signal was also filtered before being converted to digital form.
5.5 Touch sensor
In order to provide touch sensing capabilities to the robot/table system, a
touch sensor was devised. It was based on the fact that when the wire touches the
workpiece, the resistance between the voltage sensor leads drops to zero.
Measurements of resistance across the terminals of the power source, whether it is on
or off, indicate that there is a constant resistance of 330 ohms in its internal circuit.
This resistance appears in parallel with the one between the electrode and the
workpiece. A simple electronic circuit was built to detect the drop in resultant
resistance caused by the touch of the wire on the workpiece. This was accomplished
by a resistive Wheatstone bridge whose output was amplified and connected to a
photo-isolated relay, which would close or open another inductive relay, resulting in
an optically isolated digital output. The sensor circuit could be connected to (or
disconnected from) the power source terminals by closing (opening) a switch (digital
input) which was linked to a digital output from the robot. In the event of accidentally
leaving the touch sensor connected to the power source when attempting to strike an
arc, a protection circuit, based on TransZorbs and common fuses, was
implemented. This was tested successfully. The touch sensor circuit diagram is shown
in Figure 5.1.
The output of the sensor was connected to a digital input of the table
controller in order to provide it with information on when it should stop the table
during a search routine.
A list of the electronic components used in the circuit can be found in the
Appendix F.
5.6 sliding table and controller
Since the robot used did not allow the user to modify its path during program
run-time a moving table was used to correct the position of the workpiece relative to
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