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Understanding piezoelectric accelerometer charge sensitivity degradation over time

Question

Will the charge sensitivity of my piezoelectric accelerometer decrease over time?

Answer

This is a question that we often receive in the Applications Engineering department. Unfortunately, there is not an exact answer. This brief article will cover general guidelines specific to Endevco® piezoelectric accelerometers, and expressly those incorporating the use of ferroelectric ceramic sensing elements.

Using vertically integrated proprietary in-house techniques, Meggitt designs and manufactures its own custom ferroelectric ceramic crystal sensing elements. Most Endevco® piezoelectric accelerometer lines incorporate the use of these sensing elements as primary. The main advantage presented by ferroelectric ceramics is their design flexibility, such as the ability to generate tight tolerances and complex shapes, while still allowing for effective orientation control over the sensitive axis. In addition, ferroelectric ceramic sensing properties may be custom tailored to specific requirements through compositional and process engineering methods.

By virtue of their inherent material properties, ferroelectric ceramics exhibit a slight decay in sensitivity over time, a phenomenon commonly referred to as "aging rate." Aging rate is consistent over time and typically expressed as a percentage of change per year. To minimize the effects of aging rate on accelerometer charge sensitivity performance, all Endevco® accelerometers are subject to an extensive stabilization process following polarization. This process accelerates the natural aging of the ferroelectric ceramic material to a point where sensitivity loss is virtually undetectable.

A general "rule of thumb" is that an Endevco® piezoelectric accelerometer design which incorporates the use of ferroelectric crystal sensing elements will likely experience a sensitivity loss of less than 0.5% per year. Most transducer sensitivity changes are not discernible. This conclusion is based on collected data from units returned to Meggitt for recalibration over an extended timeframe. Thus, the aging rate of ferroelectric ceramic sensing elements has a negligible effect on overall transducer performance.

An example of the overall reliability and performance of Meggitt piezoelectric accelerometers was shown in 2008, when longtime customer, The Charles Stark Draper Laboratory, submitted an Endevco® model 2224C for evaluation. The unit, which incorporates the use of ferroelectric ceramic sensing elements, was originally purchased and factory calibrated in July 1962. When performance tested again in 2007, nearly 50 years later, data showed rather remarkable original vs. current frequency deviations of +2.0% and +1.19% at 20 Hz; 0% and +0.02% at 50 Hz; 0% and -0.76% at 100 Hz; and 0% and -1.46% at 200 Hz, all demonstrative of piezoelectric accelerometer performance stability over time.

There are, of course, exceptions to the aforementioned "rule-of-thumb." For example, if the piezoelectric accelerometer was exposed to certain external factors, such as excessive heat or high shock levels, further sensitivity reductions are possible. In addition, both environmental conditions and user mishandling may cause both sensitivity reduction and permanent sensor damage.

It is important to again bear in mind that this stated rule applies expressly to ferroelectric ceramic crystal sensing elements. Quartz, another commonly used material, may exhibit better stability over time, though its limitations in size, shape and direction of polarization make it generally limited to highly specialized applications. Statements here are also expressly applicable to Endevco® ferroelectric ceramic materials, as design, process methods, material composition and quality standards will differ between manufacturers. To ensure accurate measurements, annual accelerometer calibration is always recommended, regardless of crystal material type or manufacturer.