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Grades 9–12 · Machines

How can an electric field move a piezoelectric actuator?

A lens or probe can be positioned by an element that changes shape when an electric field is applied. This small, controllable movement is the converse piezoelectric effect. The direct effect goes the other way: mechanical loading produces an electrical response, linking the material's electrical and mechanical behavior.

The connection comes from the material’s internal charge arrangement. An electric field exerts forces on positive and negative charges in opposite directions. In a suitable piezoelectric structure, their small relative shifts couple to deformation rather than canceling completely through symmetry. In polarized piezoelectric ceramics, changes in domain orientation can contribute as well. Material structure and direction therefore matter.

Strain means fractional length change: S = ΔL/L. A small unloaded response along one chosen direction can be approximated by S = dE, where E is field strength and d is an effective coupling coefficient. This equation summarizes the material response; it does not say that every dimension changes equally or that all piezoelectric materials behave alike.

Field E acts along the element's Length L toward its Free end. The invented coefficient and 25 kV/m field give five microstrain and 50 nm extension, whose drawn size is greatly exaggerated.

M27 visual explanationWith the invented constant d = 200 pm/V, field 25 kV/m gives 5 microstrain. A 10 mm unloaded element extends 50 nanometres. Motion is exaggerated; real hysteresis, load and creep are excluded.Applied field changes a piezo element’s lengthField E25 kV/mFree endLength L10 mmMotion exaggerated • unloaded linear modeld = 200 picometres per volt (invented)Strain 5 µstrain; extension 50 nm

In the diagramApplied field changes a piezo element’s lengthField E25 kV/mFree endLength L10 mmMotion exaggerated • unloaded linear modeld = 200 picometres per volt (invented)Strain 5 µstrain; extension 50 nm

An applied field produces five microstrain under the invented linear coefficient. The unloaded 10-millimeter element extends by 50 nanometres; the drawing greatly exaggerates that motion.

The illustration supplies d = 200 picometres per volt. A field of 25 kilovolts per meter gives a strain of 0.000005, or five microstrain. A 10-millimeter element then extends by 50 nanometres. The drawing greatly enlarges this change. A longer element gives more displacement at the same fractional strain.

Electrical input supplies energy for the motion, and an external load changes the free displacement shown here. Real devices can also show hysteresis, depending on earlier driving, and creep during held input. The linear snapshot isolates field-to-strain coupling without predicting force output or operating limits; its enlarged motion makes a nanoscale extension visible.

About this explanation

Starting knowledge: Use scientific unit conversions and distinguish a length change from fractional strain.

Sources

  • Cambridge University Press, Inderjit Chopra and Jayant Sirohi — Piezoelectric Actuators and Sensors
  • Physik Instrumente — Displacement Behavior
  • Physik Instrumente — Fundamentals of Piezo Technology
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