Table 1: Comparison of Piezoelectric Materials
Many different sizes and shapes of piezoelectric materials can be used in
piezoelectric sensors. Acting as true precision springs, the different element
configurations shown in Figure 2 offer various advantages and disadvantages.
(The red represents the piezoelectric crystals, while the arrows indicate how
the material is stressed. Accelerometers typically have a seismic mass, which
is represented by the gray color. A more complete description of sensor
structures is given in the next section.) The compression design features high
rigidity, making it useful for implementation in high frequency pressure and
force sensors. Its disadvantage is that it is somewhat sensitive to thermal
transients. The simplicity of the flexural design is offset by its narrow
frequency range and low overshock survivability. The shear configuration is
typically used in accelerometers as it offers a well balanced blend of wide
frequency range, low off axis sensitivity, low sensitivity to base strain and
low sensitivity to thermal inputs.
Figure 2: Material Configurations
With stiffness values on the order of 15E6 psi (104E9 N/m2), which is similar to
that of many metals, piezoelectric materials produce a high output with very
little strain. In other words, piezoelectric sensing elements have essentially
no deflection and are often referred to as solid-state devices. It is for this
reason that piezoelectric sensors are so rugged and feature excellent linearity
over a wide amplitude range. In fact, when coupled with properly designed
signal conditioners, piezoelectric sensors typically have a dynamic amplitude
range (ie: maximum measurement range to noise ratio) on the order of 120 dB.
This means that a single accelerometer can measure acceleration levels as low
as 0.0001 g's to as high as 100 g's!
A final important note about piezoelectric materials is that they can only
measure dynamic or changing events. Piezoelectric sensors are not able to
measure a continuous static event as would be the case with inertial guidance,
barometric pressure or weight measurements. While static events will cause an
initial output, this signal will slowly decay (or drain away) based on the
piezoelectric material or attached electronics time constant. This time
constant corresponds with a first order high pass filter and is based on the
capacitance and resistance of the device. This high pass filter ultimately
determines the low frequency cut-off or measuring limit of the device.
A representation of a typical force, pressure and acceleration sensor is shown
in Figure 3. (The gray color represents the test structure. The blue color
corresponds to the sensor housing. The piezoelectric crystals are colored red.
The black electrode is where the charge from the crystals accumulates before it
is conditioned by the yellow, micro-circuit. The acclerometer also incorporates
a mass which is shown by the green color.) Note that they differ very little in
internal configuration. In accelerometers, which measure motion, the invariant
seismic mass, 'M', is forced by the crystals to follow the motion of the base
and structure to which it is attached. The resulting force on the crystals is
easily calculated using Newton's Second Law of Motion: F=MA. Pressure and force
sensors are nearly identical and rely on an external force to strain the
crystals. The major difference being that the pressure sensors utilize a
diaphragm to collect pressure, which is simply force applied over an area.
Figure 3: Sensor Construction
Because of their similarity, sensors designed to measure one specific parameter
are also somewhat sensitive to other inputs. By minimizing their sensitivity to
unwanted events, sensors can more accurately measure their intended parameter.
For instance, sophisticated pressure sensors often utilize a compensation
element to reduce its sensitivity to acceleration. Other sensors employ thermal
compensating amplifiers to reduce the sensors overall thermal coefficient.
Finally, accelerometers utilize alternative shear-structured sensing elements
to reduce the affects of thermal transients, transverse motion and base strain.
After the sensing element produces a presumably desirable output, this signal
must be conditioned prior to being analzyed by the oscilloscope, analyzer,
recorder or other readout device. As shown in Figure 4, this signal processing
can be accomplished by two different methods: (1) internal to the sensor by a
microelectronic circuit; or, (2) external to the sensor in a "black box". (PCB
uses the registered trademark ICP® to denote sensors which include built-in
microelectronics. Sensors without electronics are typically referred to as
charge mode sensors.)
Figure 4: Sensor Systems
These analog processing circuits serve the same general functions which include:
(1) conversion to a useful, low impedance, voltage signal; (2) signal
amplification / attenuation; and (3) filtering. However, it is important to
note that the location of the circuit may be critical to the proper operation
of the sensing system. A more detailed description of each method follows.
The ICP® sensor will be discussed first. This concept has experienced a large
degree of technical improvements since its advancement in 1967. That is, the
circuits have become smaller, the component prices have dropped and the signal
processing capabilities have increased as a result of miniature integrated
circuits and mirco hi-meg resistors. Even with these improvements, the original
intent of the idea remains unchanged...simplicity and ease of use. This
two-wire system uses a common conductor for power / signal and an additional
conductor for the signal ground. The built-in circuits are miniature charge or
voltage amplifiers depending on the sensing element type. Power to these
components typically comes from an 18 to 30 VDC, 2 mA constant current supply.
(Aside from price, convenience and/or features, there is no technical advantage
from having a constant current power source which is external or built-in to
the readout device.) A detailed system schematic is shown in Figure 5.
Figure 5: ICP® Sensor System
The characteristics of this system include: (1) built-in microelectronics
produce a low impedance, voltage signal compatible with most readout equipment;
(2) requires only a simple, easy to use constant current signal conditioner
which results in a lower per channel cost; (3) signal is capable of being
transmitted over long cables through harsh environments with no loss in signal
quality; (4) operating temperature of circuit typically limited to 250 F (121
C) or sometimes 325 F (154 C); (5) functions with ordinary two-conductor
coaxial or twisted pair cables; and (6) characteristics of sensor (sensitivity
& frequency range) are fixed within the sensor and are independent of supply
Charge mode sensors utilze the same mechanical sensing structure as do ICP®
sensors, however, the signal processing electronics are placed externally.
Since integrated, micro-circuits had not yet been developed, the first
piezoelectric sensors, which were developed in the 1950's, operated under this
principal. These charge systems were often difficult to operate properly and
were traditionally expensive as a result of the sophisticated external charge
amplifier. (Alternative, lower cost in-line devices are becoming more popular.)
Today, charge mode sensors are typically only used in environments where the
temperature prohibits the use of sensors with built-in electronics.
As would one might expect, charge mode systems offer various advantages and
disadvantages which include: (1) sensor outputs a high impedance signal which
requires conditioning prior to being analyzed; (2) requires external signal
conditioner (laboratory charge amplifier, in-line source follower, etc...); (3)
high impedance signal has the potential to be contaminated by environmental
influences such as cable movement, electro-magnetic signals and radio frequency
interference; (4) since electronics are external, certain models are capable of
operation up to 1000 F (540 C); (5) requires special low-noise cabling; and (6)
characteristics of sensor (sensitivity & frequency range) are variable and can
be ranged by switching components in the external signal conditioner.
Piezoelectric sensors offer unique capabilities which are typically not found in
other sensing technologies. As discussed, there are certain advantages (such as
wide frequency and amplitude range) and disadvantages (no static measuring
capability) depending on the particular application. Therefore, when choosing a
specific sensor or sensor technology, it is important to pay close attention to
the performance specifications.