Selecting a Reference Accelerometer for Calibration

A successful accelerometer calibration program relies upon

• selection of the best components of the calibration system
• a disciplined approach
• outreach to the Metrology community for technical standards and interlaboratory comparison

The last two items in this list are ongoing and reflect proper maintenance of the calibration program. The first one is a decision that is made when selecting components, and commits the metrologist to a hardware design. This makes it a decision that is much more expensive to correct if a mistake is made. The following article informs the decision of choosing a reference accelerometer, a key system component. Of all the individual components of a vibration calibration system, the selection of the reference accelerometer, along with the shaker design, is the most fundamental. This month, we’ll review the various options for reference accelerometer design, and how to optimize the choices of sensing element, mechanical structure, and signal conditioning.

The sections that follow offer a brief analysis of these choices.

Sensing Element

The piezoelectric sensing element is the core of the vibration measurement. The sensing element is a crystal structure that transforms mechanical energy (vibration) into electrical energy (by collecting electrical charge on opposite sides of the crystal). There are two main categories of piezoelectric crystal: quartz and piezoceramic. The differences are shown below.

The advantages of piezoceramic (high sensitivity and extended operating temperature range) are not highly valued in a calibration situation, since calibration is performed in a high signal-to-noise environment and at room temperature. However, the advantageous characteristics of quartz (sensitivity stability and low sensitivity temperature coefficient) are directly applicable to designing a reference accelerometer. As such, quartz is the preferred choice over ceramic for a reference accelerometer. The stability of quartz is rooted in the fact that it is a naturally occurring compound. The man-made piezoceramic, on the other hand, consistently loses sensitivity over time.

Mechanical Structure

Now that we have selected quartz as the sensing element, we have the choice of how to build the structure of the crystal. First, we’ll clarify how mechanical energy is input to the crystal. While it is true that vibration creates the mechanical energy, the fundamental input to the crystal is mechanical strain. The mechanical strain is created when a seismic mass is exposed to vibration. The mechanical strain may be due to normal (compression/tension) or shear stresses. A reference accelerometer can be designed such that the crystal is strained in either mode. A comparison of the two designs can be seen below and a more detailed design comparison may be found here.

Again, the advantages and disadvantages of each design may be weighted according to their applicability to the calibration environment. The advantage of compression mode (higher resonant frequency) is not very applicable to the calibration environment where testing is done at relatively low frequencies with low distortion actuators. On the other hand, we know that technicians physically touch the reference accelerometer when mounting and dismounting sensors, introducing a thermal transient to the sensor. All things considered, the shear mode design is the best choice for a reference accelerometer.

Signal Conditioning

Two types of signal conditioning are available for piezoelectric reference accelerometers, ICP® and charge mode. A more thorough comparison of the two designs can be viewed here. This comparison is summarized in part below.

As in the case when we select the sensing element, we pause to consider if temperature range is relevant to the situation. As decided before, it is not, since calibrations are performed at room temperature. We can also acknowledge, as the linked article does, that ICP’s advantage of simplicity may be considered a disadvantage when casting it in terms of ‘flexibility’. We make the choice to consider simplicity in light of the need for consistency in calibration and reducing the opportunity for operator mistakes. Also, charge mode accelerometers’ susceptibility to electromagnetic noise is relevant when the sensor is placed in close proximity to the calibration shaker. The low impedance circuit design of ICP resists the shaker-induced noise, and is preferred. With this analysis, it becomes apparent that ICP is the preferred choice for a reference accelerometer.

Summary

It’s not often such a clear cut choice is apparent when selecting instrumentation, but with the brief review presented above, we hope this helps you with your decision. As brief as this article is, you may have additional questions about selecting a reference accelerometer, or even about selecting other components, such as a shaker. Feel free to email us or call.