Position sensor life in the weld cell environment

The welding cell environment is one of the toughest places to put a position sensor, Weld spatter, high temperatures, impact damage, and incorrect mounting all contribute to premature sensor failure

However, when all mechanisms in the cell perform correctly, welding cells can operate with remarkable speed and accuracy, producing consistently high quality parts and products.

To that end, sensors in welding cells are used to verify that parts to be joined are in the correct clamped or unclamped position and that components are aligned and seated properly before any welding takes place.

Sensors are also often used to Poke-Yoke or error proof particular aspects of customer sub assemblies, ensuring, for example, that specific parts or features such as nuts, threads, or studs are present.

When production steps are not consistently monitored and verified, missing welds, machine down time, and unnecessary maintenance can occur, not to mention parts re-work, sorting, and worst of all scrap.

If sensors are not properly mounted, sited, and protected from weld spatter and improper parts loading impact damage, they will not last as long as they should, causing unnecessary cost increases and lost production time.

The normal mean time before failure for an inductive proximity sensor is generally in excess of 100,000 hours when used correctly under manufacturers' stated specifications.

However, if sensors are misused in the weld cell environment, they rarely enjoy anywhere near that kind of life expectancy.

Consumption rates for misapplied or unprotected sensors can be significant.

It's common for large end-users of welding cells to spend GBP1000's per month on material replacement costs alone.

Much of this cost is preventable.

Moreover, many sensor suppliers compound the problem, not by attacking the core issues within the weld cell, but by increasing the flow of replacement sensors, and by placing spares stocks nearby.

This makes it easier for maintenance teams to replace destroyed sensors rather than fix the reasons for their premature failure.

Improving the supply chain process does not improve the core welding process.

It only puts up a smoke screen over the problem, increasing base costs, and masking the real path to increased welding cell productivity.

There are several categories of sensors commonly integrated into welding cells, with the most common by far being the inductive proximity sensor.

1) Inductive Proximity Sensors.

Originally these sensors were used predominantly to sense a mild steel target, mild steel facilitating maximum sensing range, now however, so called multi metal or Factor 1 sensors perform equally well on all metals and are becoming increasingly used.

In a welding cell environment and in the presence of strong electromagnetic fields emitted by a weld gun, it's imperative to use electronically weld field immune (WFI) inductive sensors to prevent false triggering or "chatter".

The sensor circuitry is designed to ignore electromagnetic fields.

Moreover, if the sensor is located in the presence of hot weld debris, flimsy plastic mounting brackets must be avoided.

Instead, the user should encapsulate sensors in metal mounting hardware and only use materials that repel weld spatter.

Heavy duty Teflon/ceramic coated sensors (face and housing) as well as coated mounting brackets lengthen maintenance intervals while allowing for mechanical removal of accumulated slag during scheduled maintenance periods.

2) Photoelectric Sensors.

Diffuse-reflective (with and without background suppression), retro-reflective (used with a dependable target, a reflector) and through-beam types (comprising emitter/receiver pair) are all found in many welding applications.

Regardless of the category, the same mounting and protective methods, should be incorporated as with inductive proximity types, but with a twist.

Just like a pair of glasses, if the optical lens becomes damaged or pitted from weld spatter, it becomes increasingly difficult to dependably sense a target.

Putting the right photocell in the right place for the right application requires special attention if we're to dependably and repeatedly detect part features in a weld cell.

Consider also physical protection with application specific brackets, deep seating the sensor into a protective cowl.

Fibre optics can also be found in weld cells, but their function is dependent on constant cleaning, routine maintenance alignment and most importantly, physical damage.

One speck of debris and the fibre optics function is generally rendered useless.

3) Pneumatic Cylinder Clamping Sensors.

Through-the-wall Reed and Hall Effect sensors are commonly found mounted to rod-style, profile, dovetail, slot or cylindrical styles of pneumatic clamping cylinders in the weld cell.

These are also used to indicate "clamp" or "un-clamped" position.

Generally, failure rates in this environment with these two technologies are significant.

Damage to lightweight mechanical mounting systems occurs regularly.

Reed switches are generally inexpensive to replace, but these mechanical devices are failure-prone.

Hall Effect sensors are solid-state devices, but generally possess their own set of issues regarding drift (movement away from normal, dependable electronic functions due to temperature, board-level degradation etc over time).

Hall Effect sensors also are generally not short circuit-protected or reverse polarity-protected.

There are successful alternative technologies available, such as magneto-resistive magnetic field sensors.

This category of cylinder sensor eliminates many of the undesirable characteristics found in Reed and Hall sensors.

4) Power Clamp Sensors.

Newer generations of power clamps from a wide range of manufacturers grip parts to be joined, sensors in these devices are used to detect "clamp" or "un-clamped" position.

These inductive proximity types (generally a pair that are joined into a common connector) sense mechanically actuated components inside the power clamp to indicate extended or retracted position of the cylinder powering the clamp jaws.

These are generally protected from welding hostilities, as they are hidden well inside the clamp body.

Improving the weld cell sensor system.

Coated metal mounting brackets with a positive stop protect inductive sensors and photoelectric sensors from hostility, act as a heat sink, resist weld debris and enable quick replacement.

The positive stop ensuring that the sensor is always installed at correct target distance.

In areas where parts-loading in the cell cause impact damage to sensors the use of heavy solid aluminium cubic brackets, again with positive stop and quick change facility, eliminates damage to tubular style sensors.

With block-style sensors, heavy shielding resists loading impact.

Spring return cushion clamps, with up to 15mm of over-travel, can be used to minimise sensor damage in instances where target impact on sensor face is unavoidable.

The tiny ceramic particles suspended in heat-resistant industrial epoxies used in weld sensor coatings provide a thermal barrier, protect sensor faces and allow for slag removal.

PTFE coatings on enclosures resist weld spatter accumulation.

Shielding photoelectric sensors increases performance ensures alignment and protects sensors bodies.

New high excess gain photoelectric sensors can sense through dense weld smoke, metal housings resist impact.

Robust magneto-resistive cylinder position sensors with heavy mounting hardware can be installed on essentially any pneumatic cylinder style regardless of magnet orientation or gauss strength.

These have the benefit of being available with Weld Field Immune circuitry and a quick replacement connector.

By utilising a number of specifically designed brackets it is also possible to use one common sensor across a variety of differing cylinders.

Cable and Connector Protection.

It's also important to note that all of these sensors types are generally plug and play connection to M12 DC Micro or M8 Nano-style connectors.

One of the largest problems with sensors in weld cells revolves around the issue of cable /connector burn through.

Standard PVC jacket material on connectors should never be used in a weld environment.

PVC burns through quickly and can become extremely brittle in a short period of time.

SJTO rated connectors, with a protective metal braid, have been used successfully for many years and provide a robust solution to weld spatter.

Where exaggerated movement occurs sacrificial strategically positioned, and hence quickly replaceable cables, can be used.

Irradiated PUR styles (polyurethane) offers a better degree of knick resistance and flex characteristics, but a new generation of thermoplastic elastomer (TPE) takes the positive aspects of PUR to a higher degree of positive-performance.

Splitter boxes which marshal sensor signals also contribute to quick-change connection via the double ended moulded connector.

How do you get started towards a more efficient weld cell production process: - Well, the first step is to get a weld cell audit.

If you're experiencing what you believe to be heavy consumption of sensors used in your day-to-day welding process, or you believe maintenance time is above what it should be, an audit of each individual sensor in every weld cell location may be warranted.

In almost every instance, it's possible-even highly probable - that you'll dramatically increase production, reduce machine down time, reduce material and maintenance costs, and increase profitability by integration of even a few of these recommended weld cell improvement methods.

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