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Enhancing Proximity Sensor Performance

June 17, 2008 By: Craig Brockman, Rockwell Automation


Craig Brockman

The harsh conditions that typify welding environments in automotive assembly lines play havoc with inductive proximity sensors, drastically shortening their operational life. To reduce replacement and maintenance costs, automotive manufacturers have demanded more rugged sensing devices. Sensor providers have turned to innovative use of alternative materials and coatings to produce the desired results.

A Need for Enhancements

Some of the harshest environments for sensors in the automotive industry are created by the automated welding of frame and body assemblies. The high currents used in these processes create strong electromagnetic fields, which interfere with the operation of standard inductive sensors. Impacts between the sensor and the target being detected can damage the sensor face. And all equipment near the workpiece is showered by flying weld slag—a byproduct of the welding operation—which can cause a sensor to fail after just a few hours of use.

The introduction of weld-field immunity tremendously improved inductive sensor performance in weld cells. With the help of specialized coil windings and control circuitry, sensors become impervious to the magnetic fields typically found in automotive applications. Extended sensing-range technology has also reduced the risk of impact by allowing the sensor to be mounted farther from the workpiece. These enhancements have proven to be so effective that sensor design engineers are now turning their attention to the development of mechanical improvements that address the two leading causes of sensor failure: impact and weld-slag accumulation.

The Effects of Weld Slag

Weld slag is made up of small, high-temperature metallic particles, which are released at high velocities during resistance welding. Because inductive proximity sensors are used extensively to position metal automobile components accurately for welding, they are often in the direct path of the expelled slag.

Many conventional sensor faces are made of thermoplastic or thermoset, which can't withstand conditions found in welding environments. The high temperature of weld-slag particles melts thermoplastic and allows the particles to become embedded in, or even burn through, the sensor face. The continuous abrasive impact of high-velocity particles causes thermoset to quickly become pitted. This results in the micro-mechanical retention of particles, which only serves to attract more weld slag.

Prolonged exposure of either material to these conditions results in the formation of a layer of slag on the sensor face. The sensor detects the metallic content of the slag, so its output changes state and locks on. In other words, the sensor acts as if a metal target is present, even though the desired target is not nearby.

Inductive proximity sensors that lock on are a constant irritant and require maintenance personnel to either replace sensors frequently or manually scrape the slag buildup from the sensor face. The latter is a temporary solution that lasts only until the sensor face becomes coated again. The scraping process increases the probability that the sensor will be damaged.

Automobile production suffers as a result of the sensor's inability to operate reliably for long periods of time in the harsh welding environment, and the costs incurred through frequent component replacement have driven the automotive industry to demand improved sensor performance.

Two ways of preventing impact damage and weld slag buildup are the use of alternative construction materials and protective coatings.

Alternative Materials

Inductive sensors in weld cells generally have a cylindrical or cube shape. Historically, cylindrical sensors were housed either in nickel-plated brass, with a plastic face, or in an all–stainless steel package. Cube-shaped sensors were typically housed in plastic. Unfortunately, metal housings are susceptible to slag adherence, and standard plastic is easily melted and burned through. The poor performance of these "standard" sensors in automotive welding applications led to the investigation and introduction of alternative materials.

The operating principles of inductive sensing technology severely limit the kinds of material used for the sensor face. Ceramics, sapphire, and thermoset emerged as viable options that perform better than standard thermoplastics. But each of these materials has tradeoffs. Ceramics offer superior heat resistance but are brittle and susceptible to impact, and sapphire is costly. Thermoset proved to be the best all-around solution and is the most widely used. Its effectiveness, however, dwindles over time because it is prone to pitting from slag-particle impacts. Today, most sensors in weld cells have either thermoset faces or—in the case of cube- and limit switch-style sensors—complete thermoset housings.

While slag on the sensor barrel doesn't cause the sensor to lock on, eventually slag accumulates on the sensor face and creates a lock-on condition. As an alternative to nickel-plated brass and stainless steel, copper housings have been introduced because of copper's ability to dissipate heat rapidly. This enables the sensor to resist the burn-in and adhesion of weld expulsion.

Protective Coatings

The greatest advances in prolonging sensor life in welding applications have been in the area of protective coatings. The most popular method has been the application of a thin polytetrafluoroethylene (PTFE) coating, also known as Teflon. This cost-effective approach resists slag well, but PTFE is susceptible to abrasion. Eventually, flying weld expulsion erodes the coating, and slag sticks to the metal or plastic underneath. Repeated removal of the slag usually erases the coating, exposing the metal housing to slag.

The shortcomings of PTFE coating have led to the development of composite coatings, consisting of substrate materials reinforced with particles. The combination exhibits enhanced material properties that the constituent materials cannot achieve by themselves. The coatings designed specifically for use on sensors in weld-cell applications optimize heat, abrasion, and weld-slag resistance. They also offer improved impact resistance without the brittleness of ceramics. And in the event that slag must be removed from the sensor, the composite will remain, allowing repeated removal of slag without requiring sensor replacement.

Summary

Although the earliest improvements in weld-cell sensing technology were based in electrical design, future enhancements will depend on continued development of advanced materials for sensor housings, faces, and connection points. The result will be a complete weld-cell sensing solution that not only enhances sensor life and performance but also simplifies product replacement and reduces down time.






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