IMPROVE WELDING ACCURACY IN COMPONENT ASSEMBLY APPLICATIONS

Using sensors to error-proof welding operations can prove costly and challenging without the correct equipment. From extreme temperatures and high currents up to 35,000 amps to frequent weld flashes, these hostile welding environments can cause traditional sensors to fail within a short period of time ? leading to high replacement costs.

Even higher are the costs of sending a rejected part through the welding process or incorrectly welding two parts together, either yielding a faulty product or causing significant downtime to remove the reject from the production line. By replacing traditional devices with magnetic-inductive sensors in these applications, operators can ensure reliable sensing and long-lasting operation, despite the challenging surroundings.

Traditionally, optical or vision sensors have been employed for error-proofing sheet-metal processing in the automotive industry. In this application, sensors are used to facilitate continuous control of supply parts ? such as spacer sleeves and weld nuts ? commonly used to assemble vehicle elements such as frames, U-shaped carriers and fuel tanks. Sensors are strategically mounted to detect part-in-place prior to welding: optical sensors simply detect whether or not a part is present, while vision sensors use a camera and controller to visually analyze the part's position against predetermined conditions. Once the sensor confirms that all the components are properly placed, the welding robots are signaled to begin welding the sleeves or nuts onto the sheet metal.

When automotive manufacturers use conventional proximity or photoelectric sensors to detect weld nuts, sleeves and spacers in these applications, oftentimes the sensors are difficult to mount properly within the application environment. A standard inductive sensor must be placed close to the target objects in order to assure reliable detection, but this placement may result in the sensor being hit and subsequently damaged by these parts. Also, in many cases a suitable mounting area is not available in close proximity to the target objects ? so the sensor's accuracy may suffer.

Plus, sensors placed directly in the welding environment are extremely susceptible to weld flashes and the resulting weld slag and splatter that, once it accumulates, render a conventional sensor useless. The high costs of purchasing and installing replacement sensors, as well as the expensive downtime that ensues when a sensor fails on the production line, make this a very costly solution.

As an alternative, some manufacturers have selected vision sensors to identify small components during chassis construction. A vision system is a more expensive solution and may require extensive, PC-based configuration. Also, consistent lighting is a key component in the success of a vision system, as the correct lighting will create sufficient contrast between the component and the sheet metal ? allowing the sensor to reliably identify its target object. However, welding zones frequently experience sparks and weld flashes, so lighting conditions in these environments constantly change. Also, vision sensors can still malfunction if dirt and weld splatter obstruct the sensor's view of the process. Once again, as this residue accumulates, the sensor becomes irreparably damaged and must be replaced. This results in production downtime and further replacement costs, including installation and configuration time.

To solve these applications effectively and cost-efficiently, the magnetic-inductive weld nut sensor was specifically developed with a rugged IP67 design and simple, dependable operation ? reliably detecting nuts, sleeves and spacers on plates, brackets and panels in assembly and subassembly applications. This sensor uses signal attenuation to verify that ferromagnetic components are present and correctly placed before they are welded onto the sheet metal, with no software or supplementary electronics required. Once the sensor has been programmed, it is mounted through a hole in the sheet metal, over which the weld nut will be placed. When a nut is not present, the sensor defaults to its "no output" condition. As soon as a weld nut is placed, the sensor detects its presence and sends an output to initiate the welding process. The output then ceases when the nut is no longer present, and welding stops until the next component is in place.

Programming the weld nut sensor to differentiate between the nuts, bushings, or sleeves and the sheet metal is easily accomplished by shorting the leads or by using a teach adapter. With a simple push of a button, the sensor "learns" the status of the weld nut/sheet metal combination, as well as the status of the sheet metal alone, delivering accurate component detection. Because it can be taught to sense ferromagnetic components of various sizes and shapes, the sensor accommodates a broad range of applications, reducing sensor inventories. While alternative sensing methods often require a separate remote amplifier for programming ? which can be easily affected by accidental operator contact ? the weld nut sensor is self-contained. This makes set-up much easier and more efficient, and it helps ensure reliable, repeatable detection.

With its compact form factor, the weld nut sensor easily integrates into existing production lines, replacing traditional location bolts. It also offers several design features that make it exceptionally resistant to harsh welding conditions, including an IP67 chrome-plated brass housing, as well as a more durable stainless steel housing with Teflon® coating or TIN (Titanium Nitride) coating to protect the sensor's internal components. Temperature compensation is another key sensor feature, since welding environments experience dramatic thermal changes and escalated temperatures up to 1200 deg F.

For protection against physical damage from environmental residue, the weld nut sensor incorporates a new mounting technique that allows it to be mounted through the actual nut, sleeve or spacer. With this design, the sensor does not come into contact with the welding head or robotic clamping mechanism and is further distanced from weld flashes, slag and splatter. Because the sensors are less susceptible to these environmental elements, they are less likely to be damaged ? requiring fewer replacements and optimizing production uptime. Along with saving replacement and installation costs, as well as reducing downtime, the weld nut sensor assures quality control by not only detecting ferromagnetic components, but also outputting this data to a designated control unit to be viewed in real time. The sensor additionally features bright LED status indicators ? appearing green when the weld nut is absent and yellow when it is present ? to immediately signal to plant floor operators if any weld nuts or spacers are missing.

The combination of precise sensing, simple operation and rugged construction makes the magnetic-inductive weld nut sensor particularly well suited for controlling part and assembly quality in automotive as well as many other industries. With its ability to effectively monitor welding processes while resisting harsh plant environments, the sensor delivers a cost-effective, dependable error-proofing solution for optimizing welding operations.

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Brian Tarbox is the business development manager for TURCK, www.turck.com