Accelerometers For Vibration Measurement

CONTENTS

1. What Is An Accelerometer?
2. Triaxial Accelerometers
3. Signal Conditioning / Preamplifiers
4. Selecting Your Accelerometer
5. Accelerometer Specifications

What Is An Accelerometer?

An accelerometer is an electromechanical transducer that produces at its output terminals an electrical output proportional to the acceleration to which it is subjected. The output signal can be electronically processed and read on a meter or some other suitable indicating device.

The active element of Brüel & Kjær accelerometers consists of piezoelectric discs or slices loaded by seismic masses and held in position by a clamping arrangement.

The piezoelectric elements produce a charge proportional to the applied force. When the accelerometer is subjected to vibration, the combined seismic mass exerts a variable force on the piezoelectric element, proportional to the acceleration of the seismic masses.

This is illustrated as such for our various sensor designs:

Accelerometer Preamplifier

With the exception of DeltaTron accelerometers that have built-in preamplifiers, the outputs from Brüel & Kjær charge accelerometers need to be fed through a preamplifier.

Charge amplifiers are recommended, and Brüel & Kjær produce a wide selection of high-performance preamplifiers for this purpose.  With charge preamplifiers, you are able to use very long connection cables without altering the specified sensitivity of the accelerometer and preamplifier combination.

Since ease of calibration and measurement are usually just as important as overall gain and frequency range, most Brüel & Kjær accelerometer preamplifiers have one or more of the following signal conditioning aids:

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SIGNAL CONDITIONING

• Sensitivity Conditioning Networks : Allow direct dial-in of transducer sensitivity on the preamplifier, giving unified system sensitivities.
• Integration Networks : Automatically convert measured acceleration to a velocity and displacement proportional signal.
• High- and Low-pass Filters : Permit selection of different lower and upper frequency limits on the preamplifier to exclude unwanted signals and the influence of the accelerometer resonance from measurements.

Single- and Triaxial Accelerometers

Triaxial accelerometers ( also referred to as 3-axial accelerometers ) are used for vibration measurements in three mutually perpendicular directions (axes) simultaneously. As a rule of thumb triaxial accelerometers provide more measurement information than regular single axis sensors.

Normally, triaxial accelerometers consist of three individual accelerometers mounted in a single housing 90 degrees apart. However, this limits the minimum size of the accelerometer and means that each axis has a different point of reference.

Brüel & Kjær's accelerometers use a common seismic mass and piezoelectric element. This design results in a very compact triaxial accelerometer where all the axes have the same point of reference. This design also ensures accurate and consistent measurements, even when the accelerometer is exposed to complex vibration patterns.

Selecting Your Accelerometer

When selecting your accelerometer, these are the specifications you need to be aware of to achieve the best results for your test application and environment:

SPECIFICATIONS

> Uni-gain and "V"
> Sensitivity
> Freq. Response
> Dynamic Range
> Calibration

Uni-Gain and "V"

We list both “V” types as well as Uni-Gain sensor types:

The “V” Types: The sensors without Uni-Gain types are recognized by the “V” suffix in the type name. The difference between these two types is that all the specifications on the calibration chart for “V” types (except the sensitivity), are typical.

In contrast, the sensitivity and other parameters for the Uni-Gain accelerometers are guaranteed within tight tolerances for easy interchangeability without recalibration.

Uni-Gain Sensitivity: This designation indicates that the measured accelerometer sensitivity has been adjusted during manufacture to within 2% of a convenient value, for example (in 10 dB steps), 1, 3.16, or 10 pC/ms ^–2 .

Charge and Voltage Sensitivity

A piezoelectric accelerometer may be treated as a charge or voltage source. Sensor sensitivity is defined as the ratio of its output to the acceleration it is subjected to. This is expressed in terms of charge per unit acceleration (e.g. pC/ms ^–2 ) or in terms of voltage per unit acceleration (e.g., mV/ms ^–2 ).

The equivalent circuit diagram for accelerometers without built-in preamplifier

The sensitivities given in the individual calibration charts have been measured at 160 Hz with an acceleration of 100 ms ^–2 . For a 99.9% confidence level, the accuracy of the factory calibration is ± 2% and includes the influence of the connecting cable supplied with the accelerometer. 

Transverse Sensitivity

Accelerometers are slightly sensitive to acceleration normal to their main sensitivity axis. This transverse sensitivity is measured during the factory calibration process using a 30 Hz and 100 ms ^–2 excitation and is given as a percentage of the corresponding main axis sensitivity.

The relative response of an accelerometer to main axis and transverse axis vibrations

Frequency Response and Range

The upper-frequency limits give n in the specifications are calculated as 30% and 22% of the mounted resonance frequency to give errors of less than 10% and 5% respectively. These calculations assume that the accelerometer is properly fixed to the test specimen, as poor mounting can have a marked effect on the mounted resonance frequency.

The low-frequency response of an accelerometer depends primarily on the type of preamplifier used in the measurement setup.

Transverse Resonance Frequency: Typical values for the transverse resonance frequency are obtained by vibrating the accelerometers mounted on the side of a steel or beryllium cube using Calibration Exciter Type 4294.

Phase Response and Damping:  The low damping of Brüel & Kjær accelerometers leads to the single, well-defined resonance peak plotted on the individual frequency-response curves.

Accelerometer transverse sensitivity in any direction (when the maximum transverse sensitivity is known).

Dynamic Range: Upper and Lower Limit

The dynamic range defines the range over which its electrical output is directly proportional to the acceleration applied to its base.

Upper Limit:  In general, the smaller the accelerometer, the higher the vibration level at which it can be used. The upper limit depends on the type of vibration, and is determined by the pre-stressing of the piezoelectric element as well as by the mechanical strength of the element.

For accelerometers with built-in preamplifiers, the maximum shock and continuous vibration limits listed in the Specifications are measuring limits. For transportation and handling, the maximum non-destructive shock is specified and the maximum shock and continuous vibration limits are specified for any direction for frequencies of up to one third of the mounted resonance frequency.

When measuring short duration transient signals, care must be taken to avoid ringing effects due to the high-frequency resonance of the accelerometer. A general rule of thumb for a half sine shock pulse to obtain amplitude errors of less than 5% is to ensure that the duration of the pulse exceeds 10/ /f/R, where /f/R is the mounted resonance frequency of the accelerometer.

Lower Limit: Theoretically, the output of a piezoelectric accelerometer is linear down to the acceleration of the seismic mass due to the thermal noise, but a practical lower limit is imposed by the noise level of the measurement system and by the environment in which measurements are made.

Calibration and Stability

Brüel & Kjær accelerometers are thoroughly checked and examined at all stages of manufacture and assembly. Each accelerometer undergoes an extensive calibration procedure and artificial ageing process so as to ensure completely predictable performance and stable operation.

Calibration of Brüel & Kjær Piezoelectric Accelerometers is performed by back-to-back comparison with a primary reference standard accelerometer, calibrated at the Danish Primary Laboratory of Acoustics (DPLA), and checked by the American National Institute of Science and Technology (NIST), and the German Physikalisch–Technische Bundesanstalt (PTB) for traceability.

The overall accuracy of the back-to-back comparison is 2% with a 99.9% confidence level (1.6% for a 99% confidence level), while for the interferometry method the accuracy is better than ± 0.6% with a 99% confidence level.

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TRANSDUCER SMART SETUP

To simplify multi-channel setups our  Transducer Smart Setup App  provides the ideal solution. The app works in combination with data matrix codes (similar to QR codes™) placed on the sensors, providing instant access to specifications and  calibration.

Get in Touch!

For further details or questions in regard to specific sensor types, specifications and price, please get in touch with your local Brüel & Kjær sales representative.