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ACCELEROMETERS


Back to Basics on Accelerometer Selection


HBM provides advice on the basics of selecting appropriate accelerometers for vibration testing.


T


he most common type of sensor used for measuring vibration is the piezoelectric accelerometer consisting in its most basic form of a piezoelectric crystal mounted inside a case under


a fixed mass. During vibration, the mass exerts tensile and compressive forces on the crystal, producing a charge which is conditioned and amplified for display on a measuring instrument. According to Guy Rickard of HBM, piezoelectric accelerometers have the advantages of being small, lightweight, convenient to mount on test specimens and have a wide frequency bandwidth.


ACCELEROMETER TYPES Whilst such accelerometers have wide application across a broad spectrum of vibration measurement tasks, they’re not suitable for all types of use and there are a number of variations that should be considered for specific tasks: Piezoresistive & Variable Capacitance: These are more suitable for measuring static events, such as the constant acceleration of a centrifuge or for certain dynamic events including long duration shock events. They suffer from having a smaller temperature resistance and can only be used up to 125°C due to the construction of the device, which includes internal electronics. These are absent in piezoelectric accelerometers, which can withstand much higher temperature ranges. IEPE (Integral Electronics Piezoelectric Accelerometer): HBM explains that these are now gaining more popularity in the market. They have the advantage of a low impedance voltage output in mV per m/sec2


rather than the pV per m/sec2 output of


standard accelerometers. This means that the output signal doesn’t need conditioning and can be fed directly into most frequency analysers or data loggers. The output is a standard signal meaning that in most cases, it can be regarded as plug-and-play. However, IEPE devices need to be powered by an


external constant current 4mA power source so can’t be operated entirely independently. Construction variations: A wide range of different construction configurations have been developed


10 /// Climatic & Vibration Testing Vol 2 No. 1


❱ ❱ A few key facts can help in selecting the right accelerometer even for the most complex of applications


❱ ❱ Environment and cabling should be


considered when choosing which


accelerometer to use and how to position it


and this is one of the distinguishing factors between suppliers and levels of precision and quality. The two basic construction types are compression and shear accelerometers. The differences largely centre on mounting convenience, mechanical isolation to prevent spurious readings resulting from base bending and thermal isolation to prevent test sample temperature from affecting the results.


• Frequency – What is the range of frequencies that needs to be measured and does the accelerometer


• Sensitivity – What are the maximum and minimum acceleration levels that need to be


have sufficient dynamic response to cover this range?


• Linearity – Is the accelerometer output linear across the full expected range. All accelerometers


• Temperature – Different accelerometer types have different temperature ranges under which they can


should be supplied with a calibration certificate that demonstrates this.


perform. Test conditions


• Environment – Temperature and humidity of the environment where the accelerometer will be used will influence the need to seal the unit and its cable interface.


• Mounting – What mounting options are available and what additional equipment is needed to fix the


• Size & Mass – Will the mass affect the test results and is the accelerometer the correct size for


accelerometer to the test piece?


• Cabling – Distance from the accelerometer to the measuring instrument should be considered as well


mounting on difficult or small specimens?


as the availability of signal conditioning equipment if necessary.


detected and is the accelerometer sensitive enough to detect them?


SELECTION CRITERIA A number of factors need to be considered when selecting which accelerometer to use including the measurement parameters and the conditions in which it will be used: Measurement parameters


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