Sensor to focus a robot on a welding line. (PLC robotics)
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Figure 1. Edge detection using inductive or optical sensors.

If they are overlapped as shown in Figure 1 then it may be possible to trace out the edge using the robot with a couple of sensors in much the same way as is done on line-following robot toys / projects. The main programming challenge would be to accommodate the offset between the sensors and the following cutter. Also, be aware that proximity sensors can interfere with each other and for close proximity mounting certain types would be required. This might be enough to make optical a better solution (although dust may be a problem with these).



Figure 2. Simple overlay.

If the weld is on the edges the approach of Figure 1 would work again (if ever) but machine vision would be much simpler with lighting being less critical.