WHY MONITOR VIBRATION?
Global competition and pressure on corporate performance
makes productivity a primary concern for any business in the
90's. Machinery vibration monitoring programs are effective
in reducing overall operating costs of industrial plants.
Vibrations produced by industrial machinery are vital
indicators of machinery health. Machinery monitoring
programs record a machine's vibration history. Monitoring
vibration levels over time allows the plant engineer to predict
problems before serious damage occurs. Machinery damage
and costly production delays caused by unforeseen
machinery failure can be prevented. When pending
problems are discovered early, the plant engineer has the
opportunity to schedule maintenance and reduce downtime
in a cost effective manner. Vibration analysis is used as a
tool to determine machine condition and the specific cause
and location of machinery problems. This expedites repairs
and minimizes costs.
COMMON VIBRATION SENSORS
Critical to vibration monitoring and analysis is the machine
mounted sensor. Three parameters representing motion
detected by vibration monitors are displacement, velocity,
and acceleration. These parameters are mathematically
related and can be derived from a variety of motion sensors.
Selection of a sensor proportional to displacement, velocity
or acceleration depends on the frequencies of interest and the
signal levels involved. Figure 1 shows the relationship
between velocity and displacement to constant acceleration.
Sensor selection and installation is often the determining
factor in accurate diagnoses of machinery condition.
Displacement Sensors
Displacement sensors are used to measure shaft motion and
internal clearances. Monitors have used non-contact
proximity sensors such as eddy probes to sense shaft
vibration relative to bearings or some other support structure.
These sensors are best suited for measuring low frequency
and low amplitude displacements typically found in sleeve
bearing machine designs. Piezoelectric displacement
transducers (doubly integrated accelerometers) have been
developed to overcome problems associated with mounting
non-contact probes, and are more suitable for rolling element
bearing machine designs. Piezoelectric sensors yield an
output proportional to the absolute motion of a structure,
rather than relative motion between the proximity sensor
mounting point and target surface, such as a shaft.
Velocity Sensors
Velocity sensors are used for low to medium frequency
measurements. They are useful for vibration monitoring and
balancing operations on rotating machinery. As compared to
accelerometers, velocity sensors have lower sensitivity to
high frequency vibrations. Thus, they are less susceptible to
amplifier overloads. Overloads can compromise the fidelity
of low amplitude, low frequency signals. Traditional velocity
sensors use an electromagnetic (coil and magnet) system to
generate the velocity signal. Now, hardier piezoelectric
velocity sensors (internally integrated accelerometers) are
gaining in popularity due to their improved capabilities. A
comparison between the traditional coil and magnetic
velocity sensor and the modern piezoelectric velocity sensor
is shown below in Table 1.
Accelerometers
Accelerometers are the preferred motion sensors for most
vibration monitoring applications. They are useful for
measuring low to very high frequencies and are available in
a wide variety of general purpose and application specific
designs. The piezoelectric accelerometer is unmatched for
frequency and amplitude range. The piezoelectric sensor is
versatile, reliable and the most popular vibration sensor for
machinery monitoring.
PIEZOELECTRIC SENSORS
The rugged, solid-state construction of industrial
piezoelectric sensors enables them to operate under most
harsh environmental conditions. They are unaffected by dirt,
oil, and most chemical atmospheres. They perform well over
a wide temperature range and resist damage due to severe
shocks and vibrations. Most piezoelectric sensors used in
vibration monitoring today contain internal amplifiers.
The piezoelectric element in the sensor produces a signal
proportional to acceleration. This small acceleration signal
can be amplified for acceleration measurements or converted
(electronically integrated) within the sensor into a velocity or
displacement signal. The piezoelectric velocity sensor is
more rugged than a coil and magnet sensor, has a wider
frequency range, and can perform accurate phase
measurements.
Piezoelectric Materials
The two basic piezoelectric materials used in vibration
sensors today are synthetic piezoelectric ceramics and quartz.
While both are adequate for successful vibration sensor
design, differences in their properties allow for design
flexibility. For example, natural piezoelectric quartz has
lower charge sensitivity and exhibits a higher noise floor
when compared to the modern "tailored" piezoceramic
materials. Most vibration sensor manufacturers now use
piezoceramic materials developed specifically for sensor
applications. Special formulations yield optimized
characteristics to provide accurate data in extreme operating
environments. The exceptionally high output sensitivity of
piezoceramic material allows the design of sensors with
increased frequency response when compared to quartz.
Much has been said of the thermal response of quartz versus
piezoceramics. Both quartz and piezoceramics exhibit an
output during a temperature change (pyroelectric effect)
when the material is not mounted within a sensor housing.
Although this effect is much lower in quartz than in
piezoceramics, when properly mounted within a sensor
housing the elements are isolated from fast thermal
transients. The difference in materials then becomes
insignificant. The dominant thermal signals are caused by
metal case expansion strains reaching the base of the crystal.
These erroneous signals are then a function of the
mechanical design rather than sensing material (quartz or
piezoceramic). Proper sensor designs isolate strains and
minimize thermally induced signals. (See "Temperature
Range" section)
High quality piezoceramic sensors undergo artificial aging
during the production process. This ensures stable and
repeatable output characteristics for long term vibration
monitoring programs. Theoretical stability advantages of
quartz versus ceramic designs are eliminated as a practical
concern. Development of advanced piezoceramics with
higher sensitivities and capability to operate at higher
temperatures is anticipated.
CHOOSING AN INDUSTRIAL SENSOR
When selecting a piezoelectric industrial vibration sensor
(acceleration, velocity, or displacement), many factors
should be considered so that the best sensor is chosen for the
application. The user who addresses application specific
questions will become more familiar with sensor
requirements.
Typical questions include:
- What is the vibration level?
- What is the frequency range of interest?
- What is the temperature range required?
- Are any corrosive chemicals present?
- Is the atmosphere combustible?
- Are intense acoustic or electromagnetic fields present?
- Is there significant electrostatic discharge (ESD) present in the area?
- Is the machinery grounded?
- Are there sensor size and weight constraints?
Other questions must be answered about the connector, cable, and associated electronics:
- What cable lengths are required?
- Is armored cable required?
- To what temperatures will the cable be exposed?
- Does the sensor require a splash-proof connector?
- What other instrumentation will be used?
- What are the power supply requirements?
PRIMARY SENSOR CONSIDERATIONS
Two of the main parameters of a piezoelectric sensor are the
sensitivity and the frequency range. In general, most high
frequency sensors have low sensitivities, and conversely,
most high sensitivity sensors have low frequency ranges. It
is therefore necessary to compromise between the sensitivity
and the frequency response.
The Sensitivity Range
The sensitivity of industrial accelerometers typically range
between 10 and 100 mV/g; higher and lower sensitivities are
also available. To choose the correct sensitivity for an
application, it is necessary to understand the range of
vibration amplitude levels to which the sensor will be
exposed during measurements.
As a rule of thumb, if the machine produces high amplitude
vibrations (greater than 10 g rms) at the measurement point,
a low sensitivity (10 mV/g) sensor is preferable. If the
vibration is less than 10 g rms, a 100 mV/g sensor should
generally be used. In no case should the peak g level exceed
the acceleration range of the sensor. This would result in
amplifier overload and signal distortion; therefore generating
erroneous data. Higher sensitivity accelerometers are
available for special applications, such as low frequency/low
amplitude measurements. In general, higher sensitivity
accelerometers have limited high frequency operating
ranges. One of the excellent properties of the piezoelectric
sensor is its wide operating range. It is important that
anticipated amplitudes of the application fall reasonably
within the operating range of the sensor. Velocity sensors
with sensitivities from 20 mV/in/sec up to 500 mV/in/sec are
available. For most applications, a sensitivity of 100
mV/in/sec is satisfactory..
The Frequency Range
In order to select the frequency range of a piezoelectric
sensor, it is necessary to determine the frequency
requirements of the application. The required frequency
range is often already known from vibration data collected
from similar systems or applications. The plant engineer may
have enough information on the machinery to calculate the
frequencies of interest. Sometimes the best method to
determine the frequency content of a machine is to place a
test sensor at various locations on the machine and evaluate
the data collected.
The high frequency range of the sensor is constrained by its
increase in sensitivity as it approaches resonance. The low
frequency range is constrained by the amplifier roll-off filter,
as shown in Figure 2. Many sensor amplifiers also filter the
high end of the frequency range in order to attenuate the
resonance amplitude. This extends the operating range and
reduces electronic distortion.
Most vibrations of industrial machinery contain frequencies
below 1000 Hz (60,000 rpm), but signal components of
interest often exist at higher frequencies. For example, if the
running speed of a rotating shaft is known, the highest
frequency of interest may be a harmonic of the product of the
running speed and the number of bearings supporting the
shaft. The user should determine the high frequency
requirement of the application and choose a sensor with an
adequate frequency range while also meeting sensitivity and
amplitude range requirements. Note: Sensors with lower
frequency ranges tend to have lower electronic noise floors.
Lower noise floors increase the sensor's dynamic range and
may be more important to the application than the high
frequency measurements.
ENVIRONMENTAL REQUIREMENTS
Temperature Range
Sensors must be able to survive temperature extremes of the
application environment. The sensitivity variation versus
temperature must be acceptable to the measurement
requirement. Temperature transients (hot air or oil splash)
can cause metal case expansion resulting in erroneous output
during low frequency measurements (<5Hz). A thermal
isolating sleeve should be used to eliminate these errors.
Humidity
All Wilcoxon Research vibration sensors are sealed to
prevent the entry of high humidity and moisture. In addition,
cable connectors and jackets are available to withstand high
humidity or wet environments.
High Amplitude Vibration Signals
The sensor operating environment must be evaluated to
ensure that the sensor's signal range not only covers the
vibration amplitude of interest, but also the highest vibration
levels that are present at that measurement point. Exceeding
the sensor's amplitude range can cause signal distortion
throughout the entire operating frequency range of the
sensor.
Hazardous Environments-Gas, Dust, etc.
Vibration sensors certified as being Intrinsically Safe should
be used in areas subjected to hazardous concentrations of
flammable gas, vapor, mist, or combustible dust in
suspension. Intrinsic Safety requirements for electrical
equipment limit the electrical and thermal energy to levels
that are insufficient to ignite an explosive atmosphere under
normal or abnormal conditions. Even if the fuel-to-air
mixture in a hazardous environment is in its most volatile
concentration, Intrinsically Safe vibration sensors are
incapable of causing ignition. This greatly reduces the risk
of explosions in environments where vibration sensors are
needed. Many industrial vibration sensors are now certified
Intrinsically Safe by certifying agencies, such as Factory
Mutual (FM), Canadian Standards Association (CSA),
EECS, and CENELEC. Please consult Wilcoxon Research
for more information on Intrinsic Safety.
ELECTRICAL POWERING REQUIREMENTS
Most internally amplified vibration sensors require a
constant current DC power source. Generally, the power
supply contains an 18 to 30 volt source with a 2 to 10 mA
constant current diode (CCD) (see Figure 3). When other
powering schemes are used, consultation with the sensor
manufacturer is recommended. A more thorough discussion
of powering requirements follows.
AC Coupling and the DC Bias Voltage
The sensor output is an AC signal proportional to the
vibration of the structure at the mounting point of the sensor.
This AC signal is superimposed on a DC bias voltage (also
referred to as Bias Output Voltage or Rest Voltage). The DC
component of the signal is blocked by a capacitor. This
capacitor, however, passes the AC output signal to the
monitor. Most monitors and sensor power supply units
contain an internal blocking capacitor for AC coupling. If
not included, a blocking capacitor must be field installed.
Amplitude Range and the Supply Voltage
The sensor manufacturer usually sets the bias voltage
halfway between the lower and upper cutoff voltages
(typically 2V above ground and 2V below the minimum
supply voltage). The difference between the bias and cutoff
voltages determines the voltage swing available at the output
of the sensor. The output voltage swing determines the peak
vibration amplitude range. (See Figure 4.) Thus, an
accelerometer with a sensitivity of 100 mV/g and a peak
output swing of 5 volts will have an amplitude range of 50 g
peak.
Note: If a higher supply voltage is used (22 to 30 VDC), the
amplitude range can be extended to 100 g peak. If a voltage
source lower than 18 volts is used, the amplitude range will
be lowered accordingly. Custom bias voltages are available
for lower or higher voltage supply applications.
Constant Current Diodes
Constant current diodes (CCD) are required for two wire
internally amplified sensors. In most cases, they are
included in the companion power unit or monitor supplied.
Generally, battery powered supplies contain a 2 mA CCD to
ensure long battery life. Line powered supplies (where
power consumption is not a concern) should contain 6 to 10
mA CCDs when driving long cables. For operation above
100¿C, where amplifier heat dissipation is a factor, limit the
current to less than 6 mA.
If the power supply does not contain a CCD for sensors
powering, one should be placed in series with the voltage
output of the supply. Note: Ensure that proper diode
polarity is observed! CCDs are available from Motorola and
Siliconix (4 mA Part # 1N5312 and J510 respectively).
OTHER SENSOR TYPES
High Temperature Piezoelectric Vibration Sensors
High temperature industrial sensors are available for
applications up to 1400¿F. Currently, high temperature
sensors are not internally amplified above 177¿C (350¿F).
Above this temperature, sensors are unamplified (charge
mode). Charge mode sensors usually require a charge
amplifier. The sensitivity of unamplified sensors should be
chosen to match the amplitude range of the amplifier
selected. The unit of sensitivity for charge mode
accelerometers is expressed in picocoulombs/g. It is
necessary to use special low-noise, high temperature cables
to avoid picking up erroneous signals caused by cable
motion.
It is recommended that a special thermal isolation mount be
used with amplified sensors for applications where the
frequency of interest is less than 5 kHz and the temperature
is below 180¿C. Research is underway to extend the
operating temperature of amplified transducers.
Triaxial Sensors
Many industrial customers are using triaxial vibration
sensors for multi-directional machine monitoring and
balancing. These devices contain three mutually
perpendicular sensors which give the user more information
concerning machinery health than conventional single-axis
units. Triaxial sensors are also easier to mount than three
individual sensors.
Handprobes
Handprobes are handheld vibration sensors used to measure
vibrations. Requiring no mounting, they are quick, easy to
use, and provide a good introduction to machine health
monitoring. Though their frequency response is limited
compared to stud mounted sensors, the information they
provide can be very useful. Handprobes, used with portable
dataloggers, are highly versatile instruments for vibration
analysis and trend monitoring.
SUMMARY
Vibration sensors are the initial source of machinery
information upon which productivity, product quality and
personnel safety decisions are based. It is crucial that
sensors be properly selected to ensure reliable signal
information. This technical note outlines some of the critical
parameters that must be condsidered when choosing
industrial vibration sensors. Following this process will
increase the effectiveness of your vibration monitoring
program and improve productivity of plant personnel and
equipment. The attached checklist may be used to aid in the
process of sensor selection.
Once industrial vibration senors have been selected, they
must be mounted on plant machinery. With a firm
understanding of the sensor requirements, capabilities, and
limitations the vibration analyst should have evaluated and
determined the mounting location of the individual sensors
based on the specific machine and vibration source to be
monitored. Refer to Wilcoxon Technical Note, Mounting
Considerations for Vibration Sensors (TN21) for assistance
with proper sensor mounting.
After the sensors have been properly mounted, installation
wiring can be accomplished. Refer to Wilcoxon Technical
Note, Vibration Sensor Cabling and Wiring (TN17) for
assistance with proper sensor wiring.
After wiring installation, verification of operation and
troubleshooting the installation may be necessary to
complete the process. Refer to the Wilcoxon Technical Note,
Trouble Shooting Industrial Accelerometer Installations
(TN14) for assistance.
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