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.
For frequencies from DC up to approximately one third of the resonance frequency of the accelerometer assembly, the acceleration of the seismic mass is equal to the acceleration of the whole transducer. Consequently, the charge produced by the piezoelectric element is proportional to the acceleration to which the transducer is subjected.
The electrical signal output from Brüel & Kjær accelerometers is self-generated, though the types with built-in preamplifiers require an external power supply for this signal to be measured.
All the piezoelectric accelerometer types described in this Product Data sheet are supplied with an individual calibration chart and, in most cases, an individually measured frequency-response curve. Data from these charts are summarised in the Specifications.
Design and Construction
All accelerometers, except Types 4321, 4321 V and 4326, measure uniaxial acceleration. These types measure acceleration in three mutually perpendicular directions.
With the exception of Triaxial Accelerometer Type 4326, Miniature Accelerometer Type 4374, Standard Reference Accelerometer Type 8305 and Shock Accelerometer Type 8309, all piezoelectric accelerometers in this data sheet use the DeltaShear® design. Type 4374 uses the planar shear design, Type 8305 uses the inverted centre-mounted compression design and Type 8309 uses the centre-mounted compression design.
The piezoelectric elements of most of the accelerometers are PZ 23 lead zirconate titanate elements. The Shock Accelerometer Type 8309 has a specially formulated ferroelectric ceramic PZ 45. Miniature Accelerometer Type 4374 has a lead zirconate titanate element PZ 27.
The housing material of all the accelerometers is the same as the base material (given in the Specifications) except Type 4374, which has a nickel-chromium alloy housing.
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 may be 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. With the exception of Triaxial Accelerometers Types 4321, 4321 V, 4326 A and 4326 A – 001, the direction of main axis sensitivity for these accelerometers is perpendicular to the base plane of the accelerometers. Types 4321, 4321 V, 4326 A and 4326 A – 001 have three mutually perpendicular axes of sensitivity.
DeltaShear® Accelerometers
The Delta design involves three piezoelectric elements and three masses arranged in a triangular configuration around a triangular centre post, as illustrated in Fig. 1. The Delta Shear design gives a high sensitivity-to-mass ratio compared to other designs, a relatively high resonance frequency and high isolation from base strains and temperature transients. The excellent overall characteristics of this design make it ideal for both general purpose accelerometers and more specialised types.
DeltaTron® Accelerometers
DeltaTron® accelerometers operate on a constant-current power supply and give output signals in the form of voltage modulation on the power supply line. Types 4394 has an insulated base. All DeltaTron® accelerometers are individually calibrated Uni-Gain types.
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.
Most Uni-Gain DeltaShear types have an indication of the angle of minimum transverse sensitivity.
Frequency Response
The upper frequency limits given 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. A detailed discussion of the effects of the measuring system on the low-frequency response of an accelerometer is given in the Brüel & Kjær ®Piezoelectric Accelerometers and Vibration Preamplifiers Handbook”.
All the standard piezoelectric accelerometer types are supplied with an individual calibration chart. With the exception of Types 4374, 4326 A, 4326 A – 001 and all V - types, all types have individually measured frequency response curves.
DeltaTron types are supplied with individual frequency curves from 5 to 10 000 Hz as well as typical curves below this range.
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 4290.
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. Brüel & Kjær accelerometers can be used at frequencies up to 30% of their mounted resonance frequency without noticeable phase distortion being introduced. The phase response up to this frequency is 0°± 1°.
Dynamic Range
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. Details concerning the selection of a suitable preamplifier, together with a discussion of environmental influences, can be found in the Brüel & Kjær Piezoelectric Accelerometers and Vibration Preamplifiers” handbook.
