Electricity: conductors and insulators – Boxes and housings – Hermetic sealed envelope type
Reexamination Certificate
2001-12-28
2004-05-18
Ngo, Hung V. (Department: 2831)
Electricity: conductors and insulators
Boxes and housings
Hermetic sealed envelope type
C073S493000, C073S514160
Reexamination Certificate
active
06737580
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to protective housings for control sensors used in harsh operating conditions. More particularly, the present invention relates to a control sensor housing having an exterior surface that is provided with a laminate that is resistant to weld flash, high temperatures, pitting, abrasion, acidic and caustic solutions, solvents, and the like.
Robotics and automation have been used in manufacturing for many years. A typical use for robotics and automation is in the automotive industry, and particularly in operations such as car body assembly. In such an operations, welding instead of bolting or riveting is the preferred method of joining car body parts together. This produces consistent, predictable results and the assembled car bodies are not only stronger and better able to resist vibrations. they are less likely to develop squeaks and rattles as they age. Car body fabrication generally involves moving a chassis or platform along an assembly line past a series of welding stations that join car body components such as door pillars, firewalls and floor pans together at predetermined weld locations on the chassis. Since the car body components are not all the same size and shape, it will be appreciated that different types of welds may be required. For instance, a door pillar may require one type of weld, a firewall may require another type of weld, and a floor pan may require yet another type of weld. And, since welding units are somewhat specialized, each welding station is often equipped with a plurality of different welding set-ups, so that a welding station may be equipped with stationary or mobile spot welding units that are used to join pieces of sheet metal together. Or, the welding station may include stationary or mobile units equipped for welding thick frame members together with a single bead.
It will be appreciated that it is not uncommon to fabricate different models or makes of vehicles at a single plant. That is, a particular car may be available as a coupe, a sedan, or a convertible. Or, a plant may be used to fabricate different makes of cars within a family of cars. In situations like this, welding requirements will vary between vehicle models and makes, and welding stations may be provided with extra welding units.
In such aforementioned welding units, welding operations are often partially controlled by sensors that detect the presence or absence of a work piece at a particular, predetermined location, or which monitor and control robotic units as they traverse along predetermined paths of motion. Such sensors are available in a wide variety of shapes and sizes and are usually referred to generically as proximity detectors. Because of the harsh work environment inherent to welding stations, such sensors are usually provided with protective housings. Usually, at least one wall of the protective housing is composed of a non-metallic material such as glass reinforced thermoplastics or thermoset plastics against which the working end of the sensor is positioned and through which the sensor may operate. Such housings are also typically provided with brackets and fittings that allow the housing and sensor to be operatively connected to supports and other electrical components, respectively. In use, these protected sensors are usually positioned near the welding electrodes or rods so that they are better able to determine when a work piece and/or welding electrode or rod is in the correct position for welding. As one might expect, the closer a sensor is positioned to a welding electrode or rod, the more apt it is to be exposed to weld flash and high temperatures.
Additionally, sensors may be subject to accidental impacts from a variety of sources, inadvertent contact with corrosive chemicals, or temperature extremes, all of which may shorten the operational life of the sensors. For example, a sensor may be impacted and scratched by machinery that has become broken, bent or misaligned. Or a sensor might become inoperable due to contact with highly reactive materials used during fabrication, or corrosive chemicals used during periodic cleaning. Or the sensor might be exposed to ambient temperatures in excess of its designed operational range.
Of the aforementioned operational conditions, weld flash is of the greatest concern because it cannot be easily ameliorated or eliminated. Weld flash occurs during the welding operation and comprises small bullet-like projectiles of molten weld material that are randomly ejected from the weld site by minute impurities in the weld material as they are consumed by the heat generated by the welding electrodes or rods. These hot projectiles can vary in size from 5 to over 200 mg, have speeds of over 11 meters per second, and have kinetic energies of over 2.3×10
−3
joules. Most of the projectiles are ejected radially from the weld site in a weld flash zone that is determined largely by the configuration of the parts being welded and the operational characteristics of the welding unit itself. Unfortunately, for optimum operation, the sensor(s) usually are required to be positioned within this weld flash zone. While the odds of a sensor being impacted by weld flash are fairly low compared to the total area of the weld flash zone, one has to remember that robotic welders will perform a particular weld or welds hundreds if not thousands of times a day; day after day. Thus, over time, even a sensor having a small surface area will be impacted by a significant amount of weld flash. Of equal importance is the fact that due to the configuration and arrangement of the welding units at any given welding station, it is not uncommon for weld zones of the weld units to overlap. Thus, a sensor could be subjected to hot projectiles from a plurality of different sources.
Weld flash is particularly troublesome because the destructive effect it has on the non-metallic sensing surfaces of the sensor housings. When weld flash material impacts a typical housing sensing surface comprised of glass reinforced thermoplastic or thermoset plastic, it may bounce off harmlessly, but more often than not it forms a pit or becomes embedded in the material. As one may appreciate, pitting and embedding form surface irregularities that increase the surface area of the housing sensing surface upon which successive bits of weld flash may more easily adhere. Over time, weld flash will often form an accumulation or accretion on the sensing surface of the housing. And, because this accretion is primarily metallic, it affects the operation of the sensor (which is usually designed to sense metallic objects). That is, the flash may accumulate to the extent where it becomes detectable and it combines with the material to be welded to trigger the sensor prematurely. Or, the flash may accumulate to the extent where it effectively operates as the material to be welded and the sensor is continuously triggered.
Thus, the sensors must be continually inspected and tested for the effects of weld flash, or alternatively, be periodically replaced according to a predetermined schedule. In either case, the fact remains that sensors used in the above-mentioned working conditions will ultimately require replacement—in as little as 500 weld cycles per welding unit. And each time a sensor has to be replaced, the assembly line must be shut down for servicing. One can appreciate the magnitude of the problem this creates when one considers that there may be a plurality of assembly lines, and each assembly line may have a plurality of welding stations, and each of these welding stations may have a plurality of sensors, and each sensor is subject to the effects of weld flash damage and high temperature. And as one may imagine, replacing such sensors can result in significant down time.
Initially, thermoplastic materials such as glass filled nylon 6 and glass filled nylon 6.6 were used for the non-metallic sensing surfaces of sensor housings. With this type of material, small bits of slow moving weld flash having low levels of k
Eaton William
Saba Roger J.
Moore, Hansen & Sumner
Ngo Hung V.
Turck Inc.
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