The vibration sensor measures the deformation of the structure it is attached to. It is a thin piezoelectric foil, transforming the vibration of the structure into an electrical signal.

WHY MONITOR VIBRATION?

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

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.

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 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.

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.

OTHER SENSOR TYPES

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.

CHOOSING AN INDUSTRIAL SENSOR

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. 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.

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 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.