How can you choose, use, and trust a piezoelectric accelerometer?

Dynamic vibration measurements are critical in many applications, from military to automotive/trucks and machinery, as well as critical manufacturing sites, such as semiconductor wafer fabrication (fabs), and even to detect/monitor seismic events. Piezoelectric accelerometers are one of the sensor types frequently used for these measurements.

Parameters for specifying piezoelectric accelerometers include response type (AC or DC), sensitivity, output type (charge or voltage), frequency range, amplitude (g-level), broadband resolution, spectral noise, transverse sensitivity, integrated electronics/signal conditioning, mounting type (stud, adhesive, or through-hole, and more)), connector type, and environmental conditions (including operating temperature range, overload limit, and temperature coefficient of sensitivity).

One of the more recent (1980) specifications is for Velocity Criteria (VC) curves for a variety of applications.  Figure 1show the rms velocity (in µm/s) plotted vs. the 1/3 octave band frequency (in Hz) for the most recent VC levels. The first four vibration levels show typical readings/levels for four different building environment effects on people, per the International Organization for Standardization (ISO).  VC limits are widely used to assess and manage the impact of vibration on sensitive medical and scientific equipment and other seismic-sensitive situations.

Note: curves VC-B and up use metric values, and lower curves use imperial units. Only the lower-level limits are specified for VC-C through VC-G. VC-H through VC-M are not recommended for use as a design criterion, only for the evaluation of extremely quiet research spaces.

Choosing

The choice of the right piezoelectric accelerometer depends on meeting the requirements for key parameters for a specific application. Industrial organizations such as the American Society of Heating, Refrigeration, and Air-conditioning (ASHRAE), the Institute for Environmental Sciences and Technology (IEST), the American Institute of Steel Construction (AISC), and the U.S. National Institute of Standards (NIST) provide requirements/criteria for many vibration parameters.

Perhaps the most significant aspect of a supplier’s data sheet is the fact that it conforms to a specific level required for the application. If this is not indicated, users should contact the supplier to confirm the desired capability of a specific accelerometer. For example, Figure 2 compares the noise floor of an ultra-high-sensitivity accelerometer against VC-G and NIST-A levels. This accelerometer can evaluate VC-G compliance across the full frequency range, and NIST-A compliance at frequencies down to approximately 0.5 Hz.

Using

In addition to selecting the right sensor based on range/bandwidth and other parameters, it may be necessary to calibrate for offsets and scale, as well as calibrate/observe other application criteria.

Since the range of frequencies and amplitudes of vibrations being measured depends on the speed and noise level of the accelerometer, it may be necessary to filter out noise using low-pass filters or shock mounts.

For best results, the accelerometer should have a data rate at least 4x faster than the fastest vibration frequency being measured. This often requires sensor fusion, such as a Kalman filter, for accurate 3D orientation.

The frequency response of most industrial accelerometers is specified at their ±3dB point, so the mounting method dramatically changes the calibrated range of the accelerometer system (sensor and mount), and a firm mount must be ensured. Also, the noise floor of the sensor must be significantly smaller than the signal being measured.

Accelerometer suppliers typically have application recommendations and tips on their website that often include mounting and operating instructions. For example, How Sensor Mounting Affects Measurements points out several aspects of sensor mounting that can significantly impact the repeatability and reliability of the measurements.

Trusting

Three single-axis accelerometers mounted to cubic blocks of metal are often used to simulate tri-axial measurements. If the accelerometers are not mounted properly or their relative directions are not perfectly orthogonal (90 degrees from each other), measurements can be significantly biased, especially when estimating rotational stiffnesses.

One key performance specification for tri-axial accelerometers is crosstalk, or the maximum amount of signal that can occur in an off-axis direction to an applied force. For example, crosstalk occurs with the measurement of false vibration in the Y direction when the system is driven in the X direction. Specification sheets list transverse sensitivity, which quantifies this effect. To be within these specifications, tri-axial accelerometers are carefully assembled to minimize crosstalk.

Cable installation can affect both the measurement (reliability and accuracy) and the cable’s life. Again, suppliers’ data sheets often provide application recommendations to minimize problems.

A guaranteed lifetime warranty is perhaps the final part of trusting the accelerometer that has been chosen.

References

Selecting the Correct High Sensitivity Piezoelectric Accelerometers for Infrastructure Monitoring (SHM)
Understanding and Meeting Vibration Specifications with Piezoelectric Accelerometers
Vibration Criterion (VC) Curves-Charts | Minus K Vibration Isolation Technology
Vibration Criterion Curves – Part 1: Evolution and Interpretation
Model 393C31 Installation and Operating Manual
Accelerometer Guide
How Sensor Mounting Affects Measurements

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Accelerometer Guide