THE PIEZOELECTRIC
ACCELEROMETER
(Shear Mode version)
How do shear
mode accelerometers work ?
Shear mode accelerometer (vibration sensor) designs feature sensing
crystals attached between a center post and a seismic mass.
A compression ring or stud applies a pre-load force to the element
assembly to insure a rigid structure and linear behavior.
Under acceleration,
the mass causes a shear stress to be applied to the sensing crystals.
This stress results in a proportional electrical output by the
piezoelectric material.
The output is collected by electrodes and transmitted by lightweight
lead wires to either the built-in signal conditioning circuitry
of ICP* sensors, or directly to the electrical connector for
charge mode types.
By having the
sensing crystals isolated from the base and housing. shear mode
accelerometers excel in rejecting thermal transient and base-bending
effects. Also, the shear geometry lends itself to small size,
which promotes high frequency
response while minimizing mass loading effects on the test structure.
With this combination
of ideal characteristics, shear mode accelerometers offer optimum
performance.
* ICP is a registered
trademark of PCB Piezotronics Inc.
How
to choose an accelerometer
Selecting
the best accelerometer for specific predictive maintenance application
can be a daunting task, even for the most seasoned of engineers.
Typically, the process can be filtered down to a series of qualifying
questions. By discovering the answers to these, as it applies
to a particular application, the best vibration monitoring solution
can be identified.
QUESTION 1:
WHAT IS BEING MEASURED?
This might seem obvious at first, but stop for a second. What
is actually being measured? In other words, what are the goals?
What is expected? What is going to be done with
', the data? Vibration can be monitored with accelerometers that
provide raw vibration data or transmitters that provide the calculated
overall root mean square (RMS) vibration.
Analysts find raw vibration readings to be useful because they
contain all the information in the vibration signal. the true
peak amplitudes and vibration frequencies.
The overall RMS or peak values are useful in control systems
such as PLC, DCS. SCADA and PI because of their continuous 4-20mA
signal. Some applications use both. By determining which signal
variety is required for the application, it is possible to significantly
narrow the search. Also, is vibration being measured in terms
of acceleration, velocity or displacement? Consider too that
some industrial sensors can output temperature along with vibration.
Finally, some applications, such as vertical pumps, are best
monitored
in more than one vibration axis in which case does the application
require single, biaxial or triaxial measurement?
QUESTION
2:
PRECISION OR LOW-COST
There are two main differences between low-cost and precision
accelerometers. First, precision units typically receive a full
calibration, that is, the sensitivity response is plotted with
respect to the usable frequency range. Low cost accelerometers
receive a single-point calibration and the sensitivity is shown
only at a single frequency. Second, precision accelerometers
have tighter tolerances on some specifications such as sensitivity
and frequency range.
(figure 1)
For example, a precision accelerometer might have a nominal sensitivity
of 100mV/g ± 5% (95 to 105mV/g) (see Figure 1) while a
low-cost accelerometer might have a sensitivity of 100mV/g ±
10°% (90 to 110mV/g). Customers with data acquisition systems
will often normalise the inputs with respect to the calibrated
sensitivity. This allows a group of lowcost sensors to provide
accurate, repeatable data. Regarding frequency, a precision accelerometer
typically has frequency ranges in which the maximum deviation
is 5% while low-cost sensors might offer a 3dB frequency band.
Even so, a lowcost sensor might offer excellent frequency response.
QUESTION
3:
WHAT IS THE VIBRATION AMPLITUDE?
The maximum amplitude or range of the vibration being measured
determines the sensor range that can be used. Typical accelerometer
sensitivities are 100mV/g for a standard application (50g range)
and 500mV/g for a low-frequency or low-amplitude application
(10g range). General industrial applications with 4-20mA transmitters
commonly use a range of 0-25mm; s or 0-50mm/s.
QUESTION
4: WHAT 1S THE VIBRATION FREQUENCY?
Physical structures and dynamic systems respond differently to
varying excitation frequencies. A vibration sensor is no different.
Piezoelectric materials, by nature, act as high pass filters
and as a result, even the best piezoelectric sensor will have
a low-frequency limit near 0.2Hz. A sensor that acts as a dynamic
system with one degree of freedom exhibits natural frequencies.
The signal is greatly amplified at the natural frequency, leading
to significant change in sensitivity and possible saturation.
Most industrial accelerometers have single or double-pole RC
filters to combat saturation excitation at the resonant frequency.
Thus it is critical to select a sensor with a usable frequency
range that includes every frequency of interest.
QUESTION
5:
WHAT IS THE TEMPERATURE OF THE ENVIRONMENT?
Applications with extremely high temperatures can j pose a threat
to the electronics built into accelerometers and 4-20mA transmitters,
Charge-mode accelerometers are available for use in very high
temperature applications. These have no built-in electronics,
but instead have remote charge amplifiers. Charge-mode accelerometers
with integral hard
line line cable are available for applications hotter than 260°C,
such as gas turbine vibration monitoring.
QUESTION
6:
WILL THE ACCELEROMETER BE IMMERSED IN LIQUID?
Industrial accelerometers with integral polyurethane cable can
be completely immersed in liquid for permanent installation.
For high-pressure applications, it is a good idea to test the
sensors at pressure for one hour. An integral cable is also normally
required if the application is sprayed rather than being completely
immersed, such as cutting fluid on machine tools.
QUESTION
7:
WILL IT BE EXPOSED TO POTENTIALLY HARMFUL CHEMICALS OR DEBRIS?
Industrial accelerometers can be constructed with corrosion and
chemical resistant stainless steel bodies.
Consider using PTFE cable with corrosion resistant boot connectors
if the application is in an environment with harmful chemicals.
Consulting a chemical compatibility chart is strongly recommended
for any suspect chemicals. Integral armour-jacketed cables offer
excellent protection for cables that might come into contact
with debris such as cutting chips or workers' tools.
QUESTION
8:
IS A TOP EXIT, SIDE EXIT OR A LOW PROFILE CONNECTION REQUIRED?
Ultimately, the sensor will need to be installed on equipment
in convenient position. However, sensor geometry has little effect
on its performance, but factors such as the
space available and positioning that ensures that a maintenance
engineer can gain safe access, do need to be taken into account.
QUESTION
9:
WHAT ABOUT SPECIAL APPROVALS?
Accelerometers and 4-2OmA transmitters are both available with
CSA and ATEX approvals for use in hazardous areas. Compare the
type of approval needed with the sensor's published approvals
to ensure it meets requirements.
QUESTION
10:
WHAT ABOUT VIBRATION SENSOR TECHNOLOGY?
It is also worth considering whether to specify a shear or compression
technology sensor. This question could command an article all
of its own but in essence, the argument boils down to the proven
reliability, accuracy and repeatable performance delivered by
shear designs against the earlier compression technique that
can be sensitive to base bending and thermal transient effects
causing measurement errors.
The answers to these questions can greatly narrow searches for
the best solution in a specific application. Keep in mind, some
combination of answers might be mutually exclusive, ie a solution
meeting every criterion does not exist. For example, a particular
model might not carry the proper ATEX certification for use in
hazardous area applications. Additionally, specialised applications
might have other considerations.
