The 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 the electrodes.
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 post
at 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
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
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 resonance frequency.
Figure 3 shows a typical frequency response curve of an accelerometer
when it is excited by a constant acceleration.
Figure 3: Frequency response curve
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 10
%. 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 preamplifier.
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 input
Proceed to chapter Accelerometer