WHEN SHOULD I USE A VELOCITY OUTPUT ACCELEROMETER?
Velocity output accelerometers are usually used in condition monitoring applications where velocity is a much better parameter for judging the health of a machine. Doubling of velocity vibration equates to a doubling of the deterioration of the health of the machine. Velocity can also be used in lower frequency applications where the acceleration amplitude of vibration is too small to measure and the velocity vibration maybe of a higher and more meaningful value. Velocity vibration accelerometers are only really effective if the frequency of vibration is higher than 2Hz but more ideally 5 Hz.
WHAT SIGNAL CONDITIONING DO I NEED FOR MY ACCELEROMETER?
All internally amplified accelerometers need a power supply be it a constant current IEPE supply, a 4-20mA loop, a 10V bridge excitation or a bipolar +/- 15V supply for a three wire accelerometer. The output of the accelerometer is now conditioned to an AC voltage whose amplitude is proportional to the amplitude of vibration with a frequency the same as the frequency of the vibration. An AC voltage signal needs further signal conditioning to retrieve any useful data.
This signal conditioning takes three main forms:
a) Overall voltage levels in either RMS or peak to peak
b) Spectral content analysis
c) Snap shot time domain analysis
WHAT ARE GROUND ISOLATED ACCELEROMETERS?
Ground loops can be a significant problem to all type of sensors where the signal is un-amplified or the signal levels are low. Ground loops occur when different parts of the structure lab or building have different electrical grounds. These grounds may only differ by a few millivolts or less. When areas with different grounds are connected by sensor cables then unless measures are taken to prevent it a ground loop are set up in the cable that can be significant when compared to low level voltage signals that come from the sensor.
WHAT IS AN ISOLATED STUD?
An accelerometer isolated stud is used in application where the possibility for ground loops exists which can corrupt the output of the sensor. Isolated studs do reduce the frequency response of the accelerometer somewhat so caution should be taken if high frequency data needs to be measured.
HOW DO I INSTALL A CHARGE AMPLIFIER?
Charge output accelerometers are used in applications where:
High temperatures environments are encountered
High radiation environments are encountered
Very high frequency accelerometers are used where no room exists for internal electronics
Charge output accelerometers are self-generating and so no excitation is required but a local charge amplifier is used to convert the charge output to a voltage. The charge output accelerometers do however have high output impedance. This high output impedance makes them susceptible to noise, cable movement (tribe-electric effect) and low insulation resistance. To minimize these effects it is important to have; a charge amplifier-impedance converter mounted as close to the accelerometer as possible, prevent cable movement, use low noise co-axial cable and ensure all surfaces are kept clean and dry.
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WHAT IS THE TRIBO-ELECTRIC EFFECT?
Tribo-electric effect is when a spurious signal is generated by a charge output accelerometer by the movement of the co-axial cable. To prevent the turboelectric effect the low noise cable needs to be clamped down as close to the accelerometer as possible. See how do I install a charge amplifier?
WHAT IS THE OUTPUT OF AN IEPE ACCELEROMETER?
An IEPE accelerometer is a two-wire sensor that requires a constant current supply and outputs an AC voltage output on a DC voltage bias. The DC bias is often removed by the use of a decoupling capacitor.
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