Ultrasonics – Transducers – Piezoelectric Effect

Today, the vast majority of transducers used for ultrasonic cleaning applications utilize the “piezoelectric” effect to transform electrical energy to mechanical motion.  These devices are sometimes called “piezos” because they are driven by piezoelectric elements which are integral to the transducer.  Piezoelectricity was discovered by Maria and Pierre Curie who also experimented with radioactivity and several other phenomenon.  They initially discovered that Rochelle salt (technically Potassium sodium tartrate) produced an electrical charge when stressed.  This property was initially used in phonograph pickups and stress measuring devices.  It was also discovered that the material would change dimension or create a force if an electrical charge was applied to it.  This is the principle utilized in driving ultrasonic transducers.



Illustration showing the piezoelectric effect
If an electrical charge is applied to a piece of piezoelectric material it changes dimension as shown above. The displacement depends on the polarization of the piezoelectric material and the polarity of the electrical charge. This phenomenon also works in reverse – if a force is applied to the piezoelectric material, an electrical charge results.



Note – We use the piezoelectric effect in our daily lives.  The igniter on most gas grills, for example, consists of a piece of a piezoelectric material.  This piezoelectric element when it is stressed by the impact of a small hammer which is cocked and released by pushing the button, creates an electrical spark across two electrodes placed near the gas burner.  Piezoelectric devices are used in sonar to both produce (convert electrical energy to motion) and receive (convert a reflected sound wave into an electrical signal) to detect objects under water.  Medical sonograms are an extension of sonar technology.  Many motion detectors used to control lights and detect intruders use flexible piezoelectric films as both senders and receivers.

Rochelle salt was the first known piezoelectric material and was used in several early applications including as transducers for the very first ultrasonic cleaners.  However, as a crystalline salt, Rochelle Salt was susceptible to moisture and temperature effects and had to be used under very controlled conditions.  It also fractured easily and was not suitable for many applications requiring durability.

Note – Our occasional reference to piezoelectric drivers as “crystals” stems from the early use of crystaline materials as piezoelectric devices.  Although use of the nomenclature persists, is no longer technically correct as today’s piezoelectric materials are really ceramics.

Of course, once the piezoelectric effect was identified, a search was on for more stable and robust piezoelectric materials.  As time passed, and the world was at war, this work was supported by the United States Navy as the application of piezoelectric devices to sonar technology was of critical importance to the nation’s defense.  As a result of this work, ceramics including Lead Zirconate Titanate (which is the predominantly used piezoelectric material today) were developed and perfected.  Ceramic materials have the advantage of being stronger than the early crystalline materials and can be manufactured in nearly any shape and dimension.  Electrical properties of ceramic piezoelectric materials can be precisely controlled in the manufacturing process and are customized for each application.

In some early ultrasonic cleaners (and even in some very inexpensive, low power ultrasonic cleaners today), piezoelectric elements were attached directly to the cleaning tank by adhesive technology.  For ultrasonic cleaning applications requiring higher levels of ultrasonic power, however, a piezoelectric device by itself is not able to produce sufficient amplitude at the ultrasonic frequency as piezoelectric displacements are relatively small.  The composite Langevin transducer which will be described in an upcoming blog utilizes the principle of resonance to produce higher ultrasonic amplitude using ceramic piezoelectric elements as drivers.

–  FJF  –

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