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SENSOR OVERLOADING A common problem in industrial vibrations monitoring application is sensor overload. As discussedelsewhere for a given sensor sensitivity and powering there is a limit to the total amount of vibration(usually 50g to 80g for a 100 mV/g sensor) that can be measured. When the total vibration limit isexceeded, erroneous data will be produced. As shown in Figure 5, the time wave data will exhibit a clipped signal and the frequency data will show excessive low frequency vibration commonly known as a “ski-slope” effect. Thiserroneous data will confuse data analysis and possibly lead to improper fault diagnosis. To test for sensor overload,a lower sensitivity sensor could be substituted. Use of a 10mV/g sensor versus a 100mV/g unit will provide 10 times the vibration range (500 g’s vs. 50 g’s). The additional range will allow the user to accurately measure thetotal vibration present and select the most suitable sensitivity for the application. Another test method is to use a“mechanical filter,” such as a piece of rubber or even a business card which can be temporarily placed between the sensor and machine. This type of mechanism will filter out much of the high frequency vibration which may beoverloading the sensor. If the clippings and ski-slope are no longer present, the original sensor was most likelyoverloaded. Using an accelerometer with a lower sensitivity will provide enough range for the application. Beware, however, that a lower sensitivity accelerometer will give less output for a given vibration level (and lower signal tonoise ratio) which may impair acquisition of low amplitude data. Also note that sensor overload is one of manycauses of ski slope data.
SupplyVoltage–20VBOVTime0 Clipped Signal Clipped Signal –18V Voltage –2V Figure 5: Signal Overload, ClippingFigure 6: Low Frequency Distortion, Ski-Slope ENVIRONMENTAL CONSIDERATIONS Whether an industrial sensor is permanently mounted to amachine or used with a portable analyzer, it must be suitable for the intended environment. Furthermore, the environment mustbe considered when choosing cables and industrial connectors. Refer toConnectors & Cables for more information.Case material and sealing are critical considerations to insurelong-term sensor survival. The case material which has provento be the most durable in industrial applications is 316L stainlesssteel. Other materials such as plastic or aluminum do not provide adequate protection against chemicals, radiation andheat found on paper machines, cooling towers, refineries or evenseemingly benign outdoor installations.Proper sealing is also critical for long sensor life. Besides heat,contamination is the biggest enemy of the electronics withinpiezoelectric sensors. A quality sensor is built with extraattention to circuit cleanliness and leaves the factory only after all contamination has been removed from within the sensor. Inadequate sealing of the sensor will allow contaminants suchas moisture and chemicals to enter the transducer during itsuse and will eventually lead to sensor failure. The best type of sealing is hermetic – metal to metal welds or metal to glassfusion. Other seals such as epoxy or mechanical couplingsare not appropriate for sensors in harsh industrial installations.If the case of the accelerometer is made of rugged 316Lstainless steel and has a qualified hermetic industrial seal, the mostsignificant environmental threat to sensor longevity isexposure to elevated temperatures. It is widely known that high temperatures and electronics are generally notcompatible. The circuitry of internally amplified sensors is nodifferent. While nearly all Wilcoxon industrial sensors are rated to 250ºF (120ºC), exposures to higher temperatures willlead to amplifier failures. A sensor which fails due toexcessive heat will generally have a drop in bias output voltage. Wilcoxon’s Fire FET
® sensors (793-6 and 797-6) andcharge mode sensors are designed to operate at temperaturesbeyond 250ºF. (Refer to the High Temperature sensor section)
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