Efficient Plant June 2022 - 25

machine health
Specify the
Accelerometer
That Fits the Need
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BASIC SENSOR DESIGN
Piezoelectric accelerometers are available in flexural, compression, and
shear design configurations. Each design structure contains a sensing
element (ceramic or quartz), amplifier, base, and housing.
Shear and compression designs are better suited than flexural designs for
industrial machinery monitoring. Flexural designs have lower resonant
frequency. Because of their very high output (to 100 V/g), they excel in
low-level, low-frequency seismic applications.
Compression designs are generally good for high amplitude and force
measurements because of their high resonance and durability. They should
only be used on thick-walled structures because they can be susceptible to
base strain and thermal transient sensitivities.
The shear design is the most common and versatile. The sensing element
and seismic mass are secured to a centered, upright base, producing
a stiff structure with good frequency response and greater mechanical
integrity. Since the sensitive axis is not in-line with the mounting surface,
adverse environmental conditions do not produce false signals.
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Properly specified accelerometers will provide the data you need to
monitor and optimize asset performance.
ACCELEROMETERS, IN GENERAL, are the most common
sensor type for measuring industrial vibrations, but you might wonder
why there are so many options. A short and unsatisfying answer
is that accelerometers are designed for precise measurements, and
the design must account for a lot of variables to ensure reliability.
When specifying an accelerometer, it's important to look at the
measurements you hope to make. What is the amplitude range and
frequency of the vibration? How harsh is the environment where
the sensor will be mounted? With these key factors in mind, and
the following overview of structure and specifications, you can
demystify the spec sheet and find the accelerometer that's right for
your application.
CRITICAL SPECIFICATIONS
Frequency range and frequency response are two of the most important
parameters in accelerometer selection. Use these specifications to compare
against the frequency of the vibration you measure.
Compare the amplitude of your intended vibration or shock against
the sensor's amplitude range. For small vibrations, a higher sensitivity
is desirable to provide a cleaner signal (higher signal-to-noise ratio).
When trying to measure higher amplitudes for shock events, use a lower
sensitivity.
Consider the sensor's lowest measurable vibration level. This is specified
as either the noise floor or the sensor resolution. The resolution of a
piezoelectric sensor is determined by internal amplifier electrical noise and
mechanical gain of the mass/piezoelectric system.
Other critical specifications include temperature coefficient, transverse
sensitivity, base strain, and non-linearity.
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ENVIRONMENTAL CONSIDERATIONS
Many industrial applications require that sensors be protected from
elements such as dust and debris, moisture, and intense temperatures. In
these cases, the materials used in a sensor become critical.
In the presence of corrosive chemicals, 316L stainless-steel cases are
required over anodized aluminum cases to ensure sensor survivability.
Their nonmagnetic properties make them ideal for use around large
motors.
For more information about accelerometer selection and mounting techniques,
visit the IMI Sensors, a division of PCB Piezotronics, Depew, NY, website
at pcb.com.
JUNE 2022
If contaminants are present, hermetically sealed or welded connectors are
recommended. Non-hermetic connectors introduce the risk of contaminants
reaching the sensing element. EP
EFFICIENTPLANTMAG.COM | 25
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Efficient Plant June 2022

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