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.


For more information:

PCB Piezotronics Ltd.
Email :
ukinfo@pcb.com
Tel : +44 (0)1462 429710
Website :
www.pcb.com





  

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