DeltaShear® design
Three piezoelectric elements and three seismic masses are arranged in a triangular configuration around a triangular centre post. They are held in place using a high-tensile clamping ring. The DeltaShear design gives a high sensitivity-to-mass ratio compared to other designs, and has 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. A single-pole filter at the input of the built-in preamplifier extends the accelerometer’s usable frequency range to approximately 50% of the mounted resonance frequency. Special efforts have been made to minimise interference from RF (radio frequency) electromagnetic fields.
Characteristics
The accelerometers are supplied with individual calibration charts.
Uni-Gain Sensitivity
The Brüel & Kjær Uni-Gain design means that the accelerometer sensitivity is adjusted during manufacture to within 2% of either 1 or 10 mV/ms -2.
Frequency Response
The upper frequency limits given in the specifications are the frequencies where the deviation from the reference sensitivity is less than 10%. It is approximately 50% of the mounted resonance frequency. This assumes that the accelerometer is correctly mounted onto the test structure – a poor mounting can have a marked effect on the mounted resonance frequency.
The lower frequency limits and phase response are determined by the built-in preamplifiers. The lower frequency limits are given in the specifications for deviations from reference sensitivity of less than 10%.
Increased measurement accuracy can be achieved by dividing the actual measurement with the individual frequency response.
The frequency response curves given on the calibration chart are individually measured over most of the frequency range. At low frequencies, the curves given are typical.
The calibration chart also includes individual data that, together with a general formula, best fits the measured frequency response. The expression can be used for frequency response compensation in the specified frequency range. Transverse Sensitivity
All piezoelectric accelerometers are slightly sensitive to acceleration perpendicular to their main sensitivity axis. This transverse sensitivity is measured during factory calibration using 30 Hz and 100 ms -2 excitation, and is given as a percentage of the corresponding main axis sensitivity.
Transverse Resonance Frequency
Typical values for the transverse resonance frequency are obtained by mounting an accelerometer on the side of a steel cube attached to a Calibration Exciter Type 4290.
Dynamic Range
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 to which the accelerometer is subjected and is determined by the prestressing of the piezoelectric elements as well as by the mechanical strength of the element.
The acceleration ranges given in the specifications are determined by the measuring limits of the integral preamplifiers. For transporting and handling, the maximum non-destructive shock is given.
When short duration transient signals are measured, care must be taken to avoid ringing effects due to the high-frequency resonance of the accelerometer. As a general rule, the duration of a half sine shock pulse should be greater than 5 / fR for an amplitude error of less than 10%, where fR is the mounted resonance frequency of the accelerometer.
Lower Limit
The lower limit is imposed by the noise level of the integral preamplifier, which has been constructed to give very low noise levels, and by the environment in which the measurements are made.
