Protecting sensors increases weld cell productivity

Improving sensor survivability in weld cells is one of the easiest and fastest ways to reduce unplanned downtime and lower cost.

When it comes to sensors, cables and connectors in weld cells, weld cell management people are so used to the high cost of constant replacement, downtime and lost productivity that they begin to think it’s natural that weld cells are just that way. Sensors are constantly damaged by loading impact. Slag, weld debris and heat ruin not only the sensors, but their associated connectivity.

It gets to the point that most people involved with weld cells start thinking there’s not much you can do about the wastage but put in a vending machine or some kind of sensor dispensing system close at hand — as if having replacement parts nearby is a viable process improvement.

It’s time to break the high sensor wastage paradigm

It’s time to dispel the myth that maintaining weld cells equals high costs, constant replacement and frequent maintenance episodes.

The reason that many weld cells have such high sensor replacement costs is that the sensors used may not match the application, or they are incorrectly placed in the cell, insufficiently protected from heat, slag and impact. And it doesn’t end there. Often connectivity is supplied using the wrong cable jacketing material. Sensor mounts are often of the wrong design for weld cell service or they are manufactured from the wrong materials such as lightweight plastics that are vulnerable to weld hostilities.

Problem: Heat and slag

High ambient temperatures and weld debris, also known as weld slag or weld berries, attack sensor enclosures, faces, connections and flimsy plastic mounting brackets.

Solution 1: Choose the right sensor

Choose the right sensor for the right application in every cell location, taking into account the type of welding being accomplished. Sensors are rated devices and are application-specific. MIG, TIG, laser and resistance welding all have their own unique set of characteristics. Not every cell location can accept the same sensor type.

Coated sensors provide a thermal barrier and resist weld debris, slag accumulation and, to a degree, resist impact on the sensor face, while steel-faced sensors tend to be more robust and impact resistant.

Try to use only flush (shielded) sensors in weld cells. They can be surrounded or encapsulated in metal and there’s less potential of shearing off the exposed coil as with tubular non-flush types. Sensors with one-piece gun-drilled stainless steel housings stand up to major incidental impacts, and some have long-range characteristics which, combined with PTFE coatings, give them long-term survivability in tough weld cell applications.

Figure 1: Sensor with weld slag and connector damage.

Solution 2: Protect your sensors and connectivity

Protect your sensors by applying a total heat and slag solution to your sensors, cabling and connectors.

An application-specific coating applied to the face of a proximity sensor repels weld slag accumulation and protects it from damage even in the most severe welding environments. For the connectivity, start with a high durability TPE cable, and then cover the cable, sensor and its protective products with silicone tubing and weld wrap. This system guards the cable and secures the jacket in its proper location while sealing remaining connectivity components against harsh, hot weld spray.

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. PUR styles (polyurethane) offer a better degree of nick resistance, flex characteristics and resistance to welding debris, but TPE (thermoplastic elastomer) takes the positive aspects of PUR to a higher degree of performance.

While TPE cables have outperformed other cable materials such as PVC and PUR, there are additional steps that can be taken to protect connector cables. Tubular silicone jacketing, cut to length and applied to the cables back from the connector will protect the cabling from ambient temperatures of up to 260°C as well as prevent slag build-up on the cabling.

There is also silicone wrap, which is a self-bonding, non-adhesive silicone tape that once applied like tape on a hockey stick, will protect the sensor and connector from slag and heat just as well as silicone jacketing. Once applied, the wrap bonds to itself, becoming a solid barrier to heat and slag, protecting anything it is applied to.

Problem: Parts loading impact

Parts to be joined, or completed components that are loaded and unloaded either manually or by robot, are often dropped on exposed and vulnerable sensors, physically destroying the sensor or the entire sensing system. If an inductive proximity sensor located on a clamp is hit by metal to be joined, usually through loading impact, extensive sensor damage and premature failure may result.

Figure 2: A sensor damaged by impact.

Solution: Impact protection

Bunker and protect! Mechanical protection is central to the integration of any sensor in hostile manufacturing environments. Mechanical accessories provide a means of rapid change-out and act as a heatsink, while guarding against the heaviest of direct impact and weld debris.

The greater the protection, the longer the sensor life. Using a bunker block in conjunction with a quick-change prox mount protects the sensor body and face from debilitating physical damage. Prox mounts and bunker blocks are made of machined aluminium or steel and can be PTFE coated. PTFE coating significantly prolongs sensor life by providing a thermal barrier to protect against heat. It also retards the build-up of weld slag spatter and spray, and eases removal of surrounding deposits of weld debris during scheduled maintenance periods.

Problem: Sensor connection failure

Sensor cable connections are a major point of failure. Connectors need to be designed to withstand the hostile weld cell environment. If a sensor cable’s connection has too much stress from slag build-up or if it has the wrong angle, tension and pressure on the connection will cause premature failure. Heat, slag accumulation and flexing of the cable cause connectors to break at the most vulnerable location.

Most sensor types are generally hard-wired 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 and connector burn through.

Solution: Connector protection

Use only the highest grade of connectors available. TPE exhibits excellent chemical, lubricant, flex, heat, nick, coolant and pinch resistance. There are several models that can function with every sensor found in the typical weld facility, facilitating standardisation and transparency in the organisation. Follow proper cable exit geometry to avoid creating stress on the cable connection, especially in the presence of heat and other weld hostilities.

Once again, seal your entire sensor+connector+mounting system with self-fusing, self-bonding silicone wrap that’s rated to 260°C. It is clear so LEDs can be observed, and it guards connections against fine weld spray, while eliminating the need for hose clamps (which attract weld berries) and vulnerable zip ties for attachment.

Figure 3: Silicone wrap protects the cable and connector.

Problem: Inadequate mounting brackets

Plastic mounting brackets deteriorate rapidly in welding environments. This contributes to false sensing, no sensing or increased vulnerability of the sensor itself. Moreover, with these brackets, sensor bodies are usually not encapsulated, exposing them to high heat, weld debris spray and impact.

Solution: Protective mounting

Don’t use plastic mounts in the weld cell environment. Instead, use bunkered aluminium or steel mounting solutions. Bunkering protects the sensors from impact, and heat. Then, once the sensor and the bunker are protected with silicone wrap, slag build-up will be drastically impeded, increasing the intervals between maintenance as well as decreasing downtime during maintenance.

Problem: Incorrectly applied photoelectric sensors

Photoelectric sensors require attention to perform well in welding environments. Plastic-body photoelectric sensors must be protected from parts loading impact. In addition, just as with a pair of glasses, if the optical lens becomes excessively occluded, photoelectric sensors cannot perform their function.

Solution: Choose the right photosensors and protection

Choose only robust photoelectric sensors with heat and mar-resistant lenses. Choose devices with high excess gain properties that can sense through dense weld smoke and debris. Use lens blow-off shields or air knives to create a positive air pressure in front of the sensor, lengthening the time it takes to fog over and reduce frequent maintenance wipedowns. Bunker all photoelectric sensors as you would any inductive proximity type. Avoid fibre optics. Both glass and plastic fibre-optic bundles are frequently broken in welding cells, while one speck of debris and the fibre lens is usually rendered useless.

Figure 4: Sensor protection elements. For a larger image click here.

Vending machines: Detours on the road to productivity

Vending and other dispensing solutions may offer increased convenience, but they do nothing to lower operational costs — in fact, they tend to do the opposite. Vending machines make it easier to sacrifice sensors to a replacement process that actually may be out of control, with little tracking of sensors as to where, why and how often they are being installed. Before even more dispensing machines are installed, get to the root cause analysis of failure and fix the problems first. Worry about supply-chain management after sensor wastage problems have been fixed.

Streamline your storeroom MRO inventory. After you’ve gotten your arms around sensor-related problems, consolidate the number and types of sensors in stores, and weed out what you don’t need or will never use again. How many electrical stores carry totally obsolete sensors and connectors? How many times has the wrong device been installed causing another downtime issue?

How to get started towards a more efficient weld cell production process

Arrange for a thorough 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 out of the ordinary, an audit of each individual sensor in every weld cell location may be warranted. In almost every instance, you will dramatically increase production, reduce machine downtime, reduce material and maintenance costs, and increase profitability by integration of even a few of these recommended weld cell improvement methods.

Understand through a documented weld cell audit, how every sensor in every location on the plant floor is performing, where recurrent problems occur and why, and where maintenance people are constantly replacing sensors. Get a handle on the problems and regain control of your processes. Remember, the definition of insanity is doing the same thing over and over again and expecting a different result.

The bottom line

Once you’ve got your sensor problems straightened out, your stores will be current and organised. Resolve to never get into this situation again: on your next weld cell order, be certain that you meet with your most able sensor manufacturer representative and review sensor designs with that individual. Spend a little extra money on the front end for the best bunkering and protection, rapid change-out mounts, application-specific sensors and connectivity systems you can find and you will ensure sensor longevity.

Top image: ©stock.adobe.com/au/taaee