Sensor Selection Guide


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.

Figure 1: the relationship of Velocity and Displacement to Acceleration

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.

Table 1. Electromagnetic Velocity Sensors vs...

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 100C, 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 1400F. Currently, high temperature sensors are not internally amplified above 177C (350F). 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 180C. 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.