Sensing applications are one of the main uses of “soft” piezoelectric ceramic materials. Piezoelectric materials are categorized as a “soft” material or a “hard” material based on the physical and piezoelectric properties that they exhibit.
Sensing ceramics are characterized by having larger electromechanical coupling factors, larger piezoelectric constants, higher permittivity, larger dielectric constants, slightly larger dielectric losses and lower mechanical quality factors. Sensing ceramics produce larger displacements, relative to hard ceramics, but some exhibit greater hysteresis, and are more susceptible to depolarization from heating under high duty cycles and/or stress. Generally, higher dissipation factors limit the use of sensing ceramics in applications that require high electric fields.
APC offers two piezoelectric materials for use in sensing applications: APC 850 and APC 855. APC 850 is the material of choice for most sensing applications. It has the best overall combination of properties and should be the first material to try for sensing. APC 855 offers higher sensitivity, with a few limitations. It has a lower use temperate, higher dielectric constant and higher dissipations factor. If these limitations are acceptable, then APC 855 may be the material to use.
APC’s sensing materials can be used in a variety of applications. The applications can be broadly divided into two general use categories:
Ceramics used in active sensing applications measure the time of flight of a pulse echo response or between a transmitter and receiver. Piezoelectric ceramics used as the transmitter typically operate at their resonance frequency while sensors used as a receiver typically operate in an anti-resonant mode. Examples of piezoelectric ceramics used in an active sensing application include:
Ceramics used in passive sensing applications operate below their resonance frequency resulting in a broader band response. This allows the ceramic to receive a signal across a wide frequency range. Examples of piezoelectric ceramics used in a passive sensing application include:
Typical piezoelectric sensors will generate a signal only when it experiences a change in the applied force or pressure. Under a static input, free charge carriers in the ceramic element migrate toward the dipoles, neutralizing the charges on the dipoles and thus effectively electrically discharging the element. The charge will drain across the input resistance of the device used to measure the signal from the sensor. A stress upsets the balanced state and restores an electric charge, but if the stress is maintained the charge will drain again. In practice, systems for measuring low-frequency signals – input frequencies far below the resonance frequency of the system – are conveniently described by the time constant. The time constant of the system is the product of the capacitance of the ceramic element and the input resistance of the electronic circuit. As a rule of thumb, the time constant should be 1/10th the period time of the input signal. For example, to measure a signal with a frequency of 10 Hz, the time constant must be less than one second. There are three alternatives that keep the input resistance acceptably low, while enabling low frequency inputs to be measured: constructing the sensor from multiple parallel-connected layers, incorporating a charge amplifier in the system, or incorporating a capacitor in the system, in parallel with the sensor.
For more information about APC International’s piezoelectric materials and piezoelectric sensors please consult the following:
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