Accelerometers For Vibration Measurement

An accelerometer uses a spring‐mass system to generate a force proportional to the vibration responding to a wide range of research and industry test applications. Brüel & Kjær charge and CCLD sensors feature ICP©, DeltaTron™, and TEDS technology for easy setup and consistent measurements. With single-axis and three-axis (triaxial) configurations, the adaptability is extensive. Get in touch with your local Brüel & Kjær sales representative, or learn more about our sensor types and specifications below.

Accelerometer Types

SENSOR SPECIFICATIONS

Hz
C
mV or pC/ms^2
g
gram

OVER 70 YEARS OF INNOVATION

In 1943, Per V. Brüel, developed the world's first commercial piezoelectric accelerometer from Rochelle salt crystals.

In fact, many of the piezoelectric sensors available on the market today, are based on the technology invented by Brüel & Kjær.

LEARN MORE:

Accelerometer history

HISTORY OF THE ACCELEROMETER 

Accelerometer Types

What is an Accelerometer?

An accelerometer is an electromechanical transducer that produces at its output terminals an e.m.f., proportional to the acceleration to which the transducer 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. When the accelerometer is subjected to vibration, the combined seismic mass exerts a variable force on the piezoelectric element. Due to the piezoelectric effect, this force produces a corresponding electrical charge.

This is illustrated as such for our various sensor designs:

Planar shear

Accelerometer Designs and Types

Brüel & Kjær delivers a spectrum of transducer and sensor types, responding to varying needs and applications. Measuring acceleration, displacement, and velocity, we provide a range of high-quality accelerometers, designed for specific environments, tasks, and operating conditions.

Charge accelerometer

Charge Accelerometers

A charge‐type piezoelectric accelerometer is robust and designed specifically for high‐temperature vibration measurement. The unique design of our charge sensors delivers a high dynamic range, long‐term stability, and ruggedness.

Available in single axial and triaxial configurations.

CCLD Accelerometer

CCLD Accelerometers

A CCLD accelerometer is designed to make vibration measurement easy because the preamplifier is built into the accelerometer. It features low impedance output enabling the use of an inexpensive cable and can drive long cables.

Available in single axial and triaxial configurations.

Industrial Accelerometer

Industrial Accelerometers

An industrial accelerometer - with its rugged design - is both robust and reliable and covers a wide range of permanent vibration monitoring applications including operations in wet, dusty, and potentially explosive areas.

Charge and CCLD types are both available.

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:

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. Its sensitivity is defined as the ratio of its output to the acceleration it is subjected to, and 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 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 each 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.

 


Frequency Range and Frequency Response

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.

 


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 given in the Specifications are measuring limits. For transportation and handling, the maximum non-destructive shock is specified. The maximum shock and continuous vibration limits are specified for vibration in any direction and 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 by back-to-back comparison with a primary reference standard accelerometer calibrated at the Danish Primary Laboratory of Acoustics (DPLA), 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. Numerical details of the calibration are reported on the calibration chart supplied with each transducer.

 


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.