Page 118 - Instrumentation Reference Book 3E

Sensor practice 103

The constant a depends on many factors PZT material itself contributes only of the
including the geometry of the crystal, position of order of 0.03 of critical damping. If no additional
electrodes, and material used. Typical materials damping is added, PZT transducers must not be
now used (natural quartz is less sensitive and, used too close to their resonant frequency.
therefore, less applicable) include barium titanate Mounting arrangements within the case will also
with controlled impurities, lead zirconate, lead add some additional damping. Some designs
niobate, and many that are trade secrets. The make use of an additional spring element; some
material is made from loose powder that, after use an additional spring to precompress the PZT
shaping, is fired at very high temperature. Whilst element so that it remains biased in compression
cooling, the blocks are subjected to an electric under all working amplitudes: this makes for
field that polarizes the substance. more linear operation.
The sensitivity of these so-called “PZT mater- Typical sensor sensitivities range from 0.003
iais” is temperature-dependent through the charge pC/m s-~ up to 1000pC/m s-~, implying that the
sensitivity and the capacitance value, both of following pre-amplifier units will also need to vary
which alter with temperature. These changes do considerably.
not follow simple linear laws. Such materials have
a critical temperature, called the “Curie point.” 6.3.4.4 Amplifiers for piezoelectric sensol’s
They must never be taken above it. The Curie
point varies from 120°C for the simpler barium An amplifier for reading out the state of the PZT
titanate Corms ranging up to values close to sensor is one that has very high input impedance,
600°C. For the interested reader more explan- an adequate frequency response, and low output
ation is to be found in Bruel and Kjaer (1976): impedance. Adjustment of gain and filtering
Endevco (1980), Harris and Crede (1961), Klaasen action and integration to yield velocity and dis-
(1978): and Trampe-Broch (1980), and in the placement are usually also needed to provide easy
detailed information provided by the makers of use for a variety of applications. Figure 6.19 is a
PZT materials. typical system incorporating most features that
To read the charge of a PZT sensor an electro- might be needed.
nic amplifier that converts charge magnitude to The amplifier could be designed to see the sen-
a voltage equivalent is used. The nature of the sor either as a voltage source or as a charge
system provides no true DC response. source. The latter is preferred for, using modern
In practice the PZT sensors used to measure electronic-feedback operational amplifier tech-
acceleration can be operated down to around niques, the effect of cable, sensor, and amplifier
0.1 Hz, dependent on the amplitude to be meas- capacitances can be made negligible (which, in
ured. With natural resonant frequency that can be the voltage-reading method, is not the case).
made relatively very high (up to 100,000 Hz in some Cable length is, therefore, of no consequence.
designs), PZT sensors provide a useful frequency This is justified as follows.
range that can cover most vibration needs. The Figure 6.20 is the relevant equivalent circuit for
system response, however, relies not only on the a PZT accelerometer that is connected to an
sensor but upon the cables and the pre-amplifier operational amplifier (the pre-amplifier) via a
used with the PZT unit. cable. It includes the dominant capacitances that
The PZT material can be used in pure compres- occur.
sion; shear, or bending, to produce the charge. It can be shown (see Trampe-Broch (1980) for
Figure 6.12 gives some examples of commercially example) that the use of feedback in this way and
available PZT accelerometers. The sensor design a very high amplifier gain A gives
is amenable to the combination of three units
giving the three translation components of
vibration.

Figure 6.19 Blockdiagram of vibration measuring system showing functions that may be required. Courtesy, Bruel & Kjaer.