Our Sites

Keeping error-proofing sensors working in harsh welding environments

Protect your sensors … they’re sensitive

Eaten Alive. This is what happened to a 40-mm by 40-mm cube switch plastic body sensor in a very hot weld environment with little or no protection from weld spatter. Weld spatter sticks to the sensors, rendering them useless.

Failures in automated welding systems will bring production to a halt. Ironically, the robot, machine bed, pneumatic or hydraulic cylinders, bearings, and other heavy mechanics in the systems don’t fail as frequently as the more vulnerable peripheral sensing devices. Some metal fabrication companies are forced to change these devices out daily—even hourly in some cases.

Problems related to sensored error-proofing systems encountered during automated welding usually can be attributed to weld spatter, connectivity damage, heat, electrical noise, or physical contact or impact—although problems are not typically isolated to only one.

Weld spatter is the largest driver of downtime from malfunctioning sensors in gas metal arc or resistance welding. It sticks to the sensors and destroys plastic mounts and sensor faces. Quality problems will occur if weld spatter is overlooked (see lead photo).

Physical contact or impact is the second-highest driver of downtime from sensors (see Figure 1). One hit—or regular contact over time—can take out a traditional proximity sensor. Plastic sensing faces and thin metal housings fail quickly. Specialized steel-face sensors should be used when mechanical impact is likely.

Connectivity damage can cause intermittent nuisance stops. Cables may exceed their bend radii, or weld spatter accumulation may be the culprit. Connectivity damage is hard to diagnose, and electronics can take significant time to replace. Following proper exit geometry can take stress and strain out of welding sensor connectivity. A “straight exit” cable should be replaced with a right angle exit format to reduce strain (see Figure 2).

Heat causes significant and extreme temperature swings that negatively affect electronics (see Figure 3) and related connectivity. Heat can seem difficult to avoid in a welding environment, but placement of the sensors as far as possible from the weld will help, because proximity to the weld increases heat exposure. Also, proper application-specific sensor selection and mounting protection will go far in extending sensor system life.

Electrical noise can cause intermittent nuisance stops, affecting the sensors and input/output (see Figure 4). Electromotive force interference causes false triggering of the sensor and I/O. Grounding problems can be difficult to troubleshoot. Application-specific inductive weld sensors generally carry strong weld field immunity properties.

Because of the problems that can occur, selecting the right sensor for the technology, location, and environment is paramount. Application specificity is the name of the game—putting the right sensors in the right place to do the job. The weld environment must be taken into account.

  • Is weld debris on the cable?
  • Is the sensor subject to heat?
  • Is physical contact probable?
  • Where is the best mounting location?
  • Is the bend radius too sharp?
  • Is the cable physically damaged?
  • Is an alternative welding technology called for?

First, look at possible simple solutions. Is more space available to help avoid contact? Can you change the tooling?

One trick of the trade is to use sacrificial cables to extend sensor life (see Figure 5). In automated systems, devices must be connected. Typically, a home run cable runs from the host control to a sensor. If one part is damaged, the entire line must be replaced.

Figure 1
A very sharp blow has breached the plastic face of a newly installed nesting indication/part presence inductive proximity sensor in a welding cell.

By inserting a sacrificial cable between the sensor’s pigtail or the quick-connect and the home run cable, you can distance the home run cable away from the hostile environment. This modularizes the system so if any one component gets damaged, that’s all that must be replaced—the sensor or the sacrifice cable; the home run cable then will probably never need to be replaced. A whole family of application-specific, highly durable cables can be used (see Figure 6).

Protecting Sensors in Harsh Environments

Sensor protection makes sense. A sensor will perform only as well as it is mounted, bunkered, and guarded. The type of protection it needs depends on its location, function, and the malfunction cause.

Some of the following everyday, off-the-shelf devices can help protect photoelectric and proximity sensors.

Photoelectric Sensors. Steel housings can be mounted on arms and fixturing (see Figure 7).

Photoelectric sensors’ lenses can be vulnerable to flying debris in weld environments (see Figure 8). Specialized outer glass lenses protect optical sensor lenses from weld debris and heat.

Blowoff shields are really useful where there is a lot of slag, lube, or dust (see Figure 9). By inserting a pneumatic line into the front of them, you can build up air to blow off dust and debris from lenses.

Adjustable mounting brackets work like the old headlights with a sealed beam that had to be adjusted with a target to make sure they were positioned properly (see Figure 10). Once the bracket is mounted down, they are adjusted with three tension springs. These are very important when using precision sensing like lasers provide.

Inductive Proximity Sensors. It is easy to forget the role of inductive proximity sensors. They are short-range sensors and they’re application-specific.

Prox mounts have a bevel at the end that gives it a positive stop at the sending end (see Figure 11). Once the sensor is gapped and mounted on the machine, you can take full advantage of the sensing range. When a sensor is inserted into a prox mount, the mount stops it and protects the most vulnerable end of an inductive prox switch right where the face meets the barrel. The prox mounts resist weld slag accumulation and also act as a heat sink for sensor electronics in welding. Once there is a breach, the switch ends start degrading rapidly.

In high-impact environments, very aggressive bunkering must be used. This is done with bunker blocks. An aluminum block with a positive stop can block weld spatter (see Figure 12). Weld beads don’t like to stick to aluminum, so these blocks are appropriate for those applications and for general applications as well.

Figure 2
Connectivity damage is hard to diagnose, and replacing electronics is time-consuming. Impact and weld spatter has damaged the front of this tubular diffuse reflective photoelectric sensor.

Copper-clad steel blocks can take high impact and resist weld beads (see Figure 13).

Very large sensors, such as the limit switch-style unit sensors and shorter, compact sensors, have great range but still can get a lot of abuse. Large hoods can go over them to provide substantial protection (see Figure 14).

A steel-faced plate with a silicone coating attached to a silicone cable protects sensors from welding spatter and impact (see Figure 15).

The pneumatic cylinders that clamp all the parts together during welding are vulnerable because they get peppered with constant weld debris, as do read switches. Weld repel wrap can provide blanket protection. It bonds to itself and silicone tubing, sealing components from liquids and welding debris.

Clear silicone tubing can be used to cover bundles of wire tightly while permitting sensor status and power LEDs to shine through for monitoring.

Ceramic caps simply screw on the face of a prox switch. They protect the sensor when nothing else can.

Planning, Protecting Prevent Heartburn

Considering how much time it can take to swap out a sensor when one fails, planned maintenance intervals, and especially unplanned machine downtime costs, making provisions for proper bunkering and protection of sensing devices is truly smart manufacturing.

By anticipating the “hostilities” that sensors will encounter in the early phase of design and planning with application-specific sensors and sensor protection, manufacturers can avoid time-consuming and costly downtime.

Bunkered Sensors Accelerate Automotive Productivity

Australian organization Futuris is a provider of interior trim components and seating to automotive manufacturers, including GM, Ford, Tesla, Toyota, Chery, JAC, SAIC, and Brilliance.

Late in summer 2016, the company’s North American manufacturing plant in Newark, Calif., embarked on a time-critical continuous improvement project to improve welding cell durability. The company needed to increase output in its welding cell to meet unexpected demand growth from one of its high-profile automotive customers.

Figure 3
Sensors should be placed as far as possible from the weld. This inductive parts presence-detecting sensor sits right on top of a weld and gets cooked in a matter of hours or a day.

The manufacturer rapidly assembled a dedicated “wrenching team,” which conducted an immediate sensor durability analysis of “hot spots” that were occurring with increasing frequency in the seat frame welding operation. In fact, the entire cell had become a hot spot.

It was determined that the root cause was that many non-application-specific M12 and M18 inductive proximity sensors in the welding cells were failing prematurely because they weren’t engineered to operate under the extreme ramped-up cell cycle time demands necessary to deliver JIT delivery.

The team acquired a pallet of Balluff weld field-immune sensors with specially coated cable protection devices and bunkered proximity sensors. In all, they replaced about 120 individual sensors over a weekend.

The installation of the welding application-specific sensors and guarding devices solved the problem. No significant failures have occurred since that upgrade in roughly six months of operation (see photos).

About the Author

Dave Bird

Sensor-based Error Proofing Specialist

Metals Fabrication Process Improvement

1061 Colina Drive

Villa Hills, KY 41017

513-207-5681