Weld-Immune Proximity Sensors: Surviving a Welding Line

A resistance welding line is one of the few places where a proximity sensor can fail for two completely unrelated reasons at the same time, and where fixing one does nothing for the other. If you are replacing sensors on a weld cell more often than anywhere else in the plant, this is usually why.

Problem one: the magnetic field

Resistance welding passes very large currents for very short times. That current creates a strong magnetic field around the weld gun, the cables and the transformer, and an inductive proximity sensor works by sensing changes in an electromagnetic field.

The result is false switching. The sensor reports a target when the gun fires, or drops out when it should not. The characteristic symptom is a fault that only happens during welding and cannot be reproduced when the cell is jogged by hand, which is exactly the fault profile that gets blamed on the PLC for a year.

What actually helps:

• Distance from the weld cables. The field falls off quickly. Routing sensor cable away from weld cable, and moving the sensor itself where possible, does more than any specification change.
• Screened cable, earthed at one end only. Earthing both ends creates a loop that makes things worse.
• Never run sensor cable parallel to weld cable in the same tray or conduit. Cross at right angles if they must meet.
• Weld field immune construction, where the sensor electronics are designed to reject the interference rather than respond to it. This is the specification to ask for where the geometry cannot be changed.

Problem two: the spatter

Molten metal thrown from the weld lands on the sensor face, sticks, and builds up. Two things then happen. The sensing face accumulates ferrous material, which an inductive sensor reads as a permanent target. And the build-up physically damages the face.

What actually helps:

• A PTFE coating on the sensing face and barrel. Spatter does not adhere well to PTFE, so most of it falls away instead of sticking. This is the single most effective measure and it is the usual meaning of a spatter-resistant sensor.
• A ceramic or spatter-resistant face where the thermal exposure is severe.
• A mechanical deflector positioned so the sensor is out of the direct line of the spray. Cheap and frequently overlooked.
• Mounting the face downward or vertically wherever the geometry allows, so material falls off rather than collecting.

Note that these are two separate specifications. A sensor with excellent weld field immunity and a bare steel face will still fail on spatter, and a PTFE-coated sensor with ordinary electronics will still false trigger. On a weld cell you generally need both.

Problem three, which is really the cable

As in most hostile installations, the cable fails before the sensor does. On a weld cell it is exposed to spatter, to heat, and usually to movement if the sensor is on a moving fixture. A cable that is chafed, cooked or crushed produces exactly the same intermittent symptoms as weld field interference, so check it before changing the sensor specification.

Specify the cable length you actually need so the first joint is outside the cell, use a protective sleeve through the spatter zone, and support the cable so the gland carries no weight.

Where this applies

Automotive body shops and tier-one component lines are the obvious case, with sensors on weld fixtures, clamp position, part present detection and gun position. But the same two problems appear on general fabrication cells, on wire mesh and rebar welding, on seam welders in drum and cylinder manufacture, and anywhere a resistance welder sits close to automated handling.

Diagnosing which problem you have

Useful before buying anything:

• Fault only during welding, sensor face clean points to the magnetic field. Look at cable routing first.
• Sensor stuck on, visible build-up on the face is spatter. A PTFE face and a deflector will do more than better electronics.
• Fault that comes and goes with fixture movement is usually the cable.
• Sensor dead altogether is often the output destroyed by a short in a damaged cable, which is an argument for always specifying short-circuit protected versions.

Specifying one

Tell us the distance from the weld gun and from the weld cables, whether the sensor is in the direct spatter path, the target material and sensing gap, how the cable is routed, and whether the sensor moves with a fixture. A photograph of the cell with the sensor position visible answers most of it.

We manufacture proximity switches in Mumbai and build specials including coated faces and specified cable lengths, which on a weld line is usually the right route rather than adapting a catalogue part.