Piezoelectric Sensing Elements - Quartz vs Ceramic
Benefits and Applications
Over the years there has been a great deal of discussion over the merits of selecting either quartz or ceramic as the sensing element for a piezoelectric accelerometer. This month we’ll review the benefits of each, along with citing the certain applications where either one or the other excels....pun?
First off, Quartz has long been recognized for its benefit of being naturally piezoelectric. As such, it exhibits the best long term stability of any piezoelectric sensing material. Quartz exhibits no pyro-electric output and has a small stable thermal coefficient. The low capacitance of quartz gives it a high voltage output (V=Q/C). This high output is, in fact, why most dynamic quartz force sensors are actually attenuated from their open circuit sensitivity (over a 1.3 volt/lb force).
Most often quartz is not run at higher temperatures because there are other piezoelectric ceramic materials that are more appropriate due to their higher charge output. Sometime there is also a slightly higher noise floor in voltage operation due to the MosFET amplifier used for internal impedance conversion having a higher noise floor than the typical internal charge converting J-FET amplifier. The noise can also be slightly higher due the fact that the resistors used to set time constant are normally larger than in charge amplified systems. Final limitations in quartz are centered about limited cuts and geometry for natural polarization, ie no annular shear quartz.
Quartz sensing elements find outstanding applications in the following:
Dynamic Calibration reference standards due to outstanding long term stability
Many thermal active and cycled environments like Cryogenic, as well Environmental Stress Screening (ESS) / Highly Accelerated Life Test (HALT) / Highly Accelerated Stress Screen (HASS)
Dynamic Force sensors of single and triaxial configuration
Dynamic Pressure in active environments like turbine combustion instability and airflow in wind tunnels over airfoils.
When it comes to ceramic piezoelectric sensing materials the key benefits are in the controllable polarization process, (ie polycrystalline elements can be made into a variety of shapes/geometries) and the high charge amplified output. Coupling the piezo-ceramics with low noise J-FET internal amplifiers provides outstanding resolution with noise floors as low as those found on charge accelerometers coupled to typical cable lengths connected to a laboratory grade charge amplifier. Everything being equal, the noise floor of charge sensors with built-in electronics will be lower than with an external charge amplifier. Noise is based on total capacitance divided by feedback capacitance. The total capacitance of a system with an external charge amplifier will increase because of the capacitance of the cable between the sensor and the external charge amp. More recently, specialty ceramics have been introduced for high temperature operating environments like those found in turbine engine operation.
For limitations, there is slightly less long term stability and a higher thermal coefficient with ceramics than quartz (though some high temperature ceramics approach quartz). Additionally, the pyroelectric sensitivity of ceramics limits its use in certain blast/shock applications. Since the elements are polarized as part of the manufacturing process it is imperative that the sensor vendor be reviewed and trusted for quality and reliability.
Due to the combination of low noise, light weight and flexible geometry, ceramic based sensing elements find outstanding applications in:
Low cost Modal/Automotive NVH/Aircraft GVT array accelerometers where low signal levels with long cable runs are the standard operation
Specialty aerospace applications operating at high temperature (up to 900 deg F)
While these are a few of the guidelines for selection, in the end, there are always subtle specifics to your dynamic applications and the various sensing types. Consultation with an experienced field application engineer or a factory application specialist is the best way to ensure that you are selecting the right sensor for your needs.