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.
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:
With the exception of DeltaTron accelerometer s 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.
Using charge preamplifiers, very long connec tion cables can be used without altering the specified sensitivity of the acceler ometer and preamplifier combination.
Since ease of calibration and measurement are usually just as important as overall gain and frequency range, most Brüel & Kjær preamp lifiers have one or more of the following signal-conditioning aids:
- Sensitivity Conditioning Networks: Allow direct dial-in of tr ansducer sensitivity on the preamplifier, giving unified system sensitivities.
- Integration Networks: Automatically convert measured acceleration to a velocity and/ or displacement proportional signal.
- High- and Low-pass Filters: Permit selection of differ ent lower and upper frequency limits on the preamplifier to exclude un wanted signals and the influence of the accelerometer resonance from measurements.
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.
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.
Available in single axial and triaxial configurations.
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:Additional specifications:
> Uni-gain and "V"
> Charge / Voltage Sensitivity
> Frequency Response
> Dynamic Range
> Calibration and Stability
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.
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.
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 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.
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.
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.
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.