active element of an accelerometer is a piezoelectric material. Figure
1 illustrates the piezoelectric effect with the help of a compression
disk. A compression disk looks like a capacitor with the piezoceramic
material sandwiched between two electrodes. A force applied
perpendicular to the disk causes a charge production and a voltage at
Figure 1: Piezoelectric effect, basic calculations
The sensing element of a piezoelectric accelerometer consists of two
- Piezoceramic material
- Seismic mass
One side of the piezoelectric material is connected to a rigid
the sensor base. The so -called seismic mass is attached to the other
side. When the accelerometer is subjected to vibration, a force is
generated which acts on the piezoelectric element (compare Figure 2).
According to Newton’s Law this force is equal to the product of the
acceleration and the seismic mass. By the piezoelectric effect a charge
output proportional to the applied force is generated. Since the
seismic mass is constant the charge output signal is proportional to
the acceleration of the mass.
Figure 2: Principle of a piezoelectric accelerometer
Over a wide frequency range both sensor base and seismic mass
have the same acceleration magnitude. Hence, the sensor measures the
acceleration of the test object.
The piezoelectric element is connected to the sensor socket
via a pair of electrodes. Some accelerometers feature an integrated
electronic circuit which converts the high impedance charge output into
a low impedance voltage signal.
Within the useful operating frequency range the sensitivity is
independent of frequency, apart from the later mentioned limitations.
A piezoelectric accelerometer can be regarded as a mechanical
low-pass with resonance peak. The seismic mass and the piezoceramics
(plus other "flexible" components) form a spring mass system. It shows
the typical resonance behavior and defines the upper frequency limit of
an accelerometer. In order to achieve a wider operating frequency range
the resonance frequency should be increased. This is usually done by
reducing the seismic mass. However, the lower the seismic mass, the
lower the sensitivity. Therefore, an accelerometer with high resonance
frequency, for example a shock accelerometer, will be less sensitive
whereas a seismic accelerometer with high sensitivity has a low
Figure 3 shows a typical frequency response curve of an
accelerometer when it is excited by a constant acceleration.
Figure 3: Frequency response
Several useful frequency ranges can be derived from this curve:
- At approximately 1/5 the resonance frequency the response
of the sensor
is 1.05. This means that the measured error compared to lower
frequencies is 5 %.
- At approximately 1/3 the resonance frequency the error is
For this reason the "linear" frequency range should be considered
limited to 1/3 the resonance frequency.
- The 3 dB limit with approximately 30 % error is
obtained at approximately one half times the resonance frequency.
The lower frequency limit mainly depends on the chosen
Often it can be adjusted. With voltage amplifiers, the low frequency
limit is a function of the RC time constant formed by accelerometer,
cable, and amplifier input capacitance together with the amplifier
Proceed to chapter Accelerometer