IMI Industrial Vibration Sensors Katalog Seite 166

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There will usually be several accelerometer models that will meet the required measurement parameters so the question naturally arises which should be used This section provides detailed explanations for the questions on the Accelerometer Selection Worksheet on page 178 Use the information provided here to help answer the questions on the Worksheet as accurately as possible This will result in a set of key specifications required for the accelerometer For detailed specifications on these sensors refer to the Products by Technology Section pages 81 176 1 Measurement Range Sensitivity Determine the maximum peak vibration amplitude that will be measured and select a sensor with an appropriate measurement range For a typical accelerometer the maximum measurement range is equal to 5 volts divided by the sensitivity For example if the sensitivity is 100 mV g then the measurement range is 5 V 0 1 V g 50 g Allow some overhead in case the vibration is a little higher than expected 2 Frequency Range Determine the lowest and highest frequencies to be analyzed If you are not sure what the upper frequency range should be use the following table showing Recommended Frequency Spans as a guideline Recommended Frequency Spans Upper Frequency Shaft Vibration 10 x RPM Gearbox 3 x GMF Rolling Element Bearings 10 x BPFI Pumps 3 x VP Motors Generators 3 x 2 x LF Fans 3 x BP Sleeve Bearings 10 x RPM RPM Revolutions Per Minute GMF Gear Mesh Frequency BPFI Ball Pass Frequency Inner race VP Vane Pass frequency LF Line Frequency 60 Hz in USA BP Blade Pass frequency The above table was taken from Eshleman Ronald L Basic Machinery Vibrations An Introduction to Machine Testing Analysis and Monitoring VIPress Incorporated 1999 p 2 4 Select an accelerometer that has a frequency range that encompasses both the low and high frequencies of interest In some cases it may not be possible to measure the entire range of interest with a single accelerometer In such a case select the sensor that comes the closest to what is needed High Frequency Caution Many machines such as pumps compressors and some spindles generate high frequencies beyond the measurement range of interest Even though these vibrations are out of the range of interest the accelerometer is still excited by them Since high frequencies are usually accompanied by high accelerations they will often drive higher sensitivity accelerometers 100 and 500 mV g models into saturation causing erroneous readings If a significant high frequency vibration is suspected or if saturation occurs a lower sensitivity typically 10 or 50 mV g accelerometer should be used For some applications IMI offers higher sensitivity accelerometers with built in low pass filters These sensors filter out the unwanted high frequency signals and thus provide better amplitude resolution at the frequencies of interest Contact an IMI Application Specialist for assistance if you experience this problem To determine if you have a condition that will overdrive saturate the accelerometer look at the raw vibration signal in the time domain on a data collector spectrum analyzer or oscilloscope Set the analyzer for a range greater than the maximum rated output of the accelerometer If the amplitude exceeds the maximum rated measurement range of the accelerometer typically 5 volts or 50 g for a 100 mV g unit then a lower sensitivity sensor should be selected If the higher sensitivity sensor is used clipping of the signal and saturation of the electronics is likely to occur This will result in false harmonics ski slope as well as many other serious measurement errors 3 Broadband Resolution Noise Determine the amplitude resolution that is required This will be the smaller of either the lowest vibration level or the smallest change in amplitude that must be measured Select a sensor that has a broadband resolution value equal to or less than this value For example if measuring a precision spindle with 0 0001 g minimum amplitude choose an accelerometer with 100 µg or better resolution If the known vibration levels are in velocity in s or displacement mils convert the amplitudes to acceleration g at the primary frequencies Note The lower there solution value the better the resolution is Generally ceramic sensing elements have better resolutions less noise than do quartz Accelerometer Selection Guidelines Typical Accelerometer Frequency Response Plot for a Filtered Sensor Technical Information visit us online at www imi sensors com 1 716 684 0003 163