Understanding the Piezoelectric Effect: A Key Aspect of Piezoelectric Accelerometers
One crucial aspect of piezoelectric accelerometers is the piezoelectric effect. In general, piezoelectric materials can generate electrical power when subjected to mechanical stress.
Conversely, applying an electric field to piezoelectric materials can cause them to deform and generate small mechanical forces. While most electronic engineers are familiar with the piezoelectric effect, sometimes the details of this intriguing phenomenon are not fully understood.
Gaining a deeper understanding of this effect can help us better comprehend the working principles of piezoelectric sensors. Figure 1 illustrates the influence of external mechanical forces on piezoelectric materials.
Figure 1: Piezoelectric materials with no mechanical stress (a) and with applied stress (b).
As you can see, some fixed charges appear on the surface of the piezoelectric material. The generated amount of charge is directly proportional to the applied force.
How Piezoelectric Materials Generate Current
Piezoelectric materials are a type of dielectric material. They are insulators or very poor conductors of electric current. However, by depositing two metal electrodes on the opposing surfaces of the piezoelectric material, we can utilize the electric field generated by the piezoelectric effect to generate electricity.
If we connect the two electrodes together with a wire, as shown in Figure 1(b), the free electrons in the conductor will flow towards the positively charged electrode, creating a current. This current accumulates free electrons at the positive pole and generates an electric field in the opposite direction of the original field produced by the piezoelectric effect.
This effect explains why the current generated by static forces only lasts for a short duration. The current persists until the electric field generated by the accumulation of free electrons counters the electric field produced by the piezoelectric effect.
Now, if we remove the external force, the material returns to its original shape, and the electric field generated by the piezoelectric effect disappears, meaning that the current flows through the wire in the opposite direction.
Structure of Piezoelectric Accelerometers
In a piezoelectric accelerometer, a piezoelectric element is used to connect a known mass (often referred to as the proof mass) to the accelerometer body.
When the sensor frame undergoes acceleration due to an external force, the proof mass tends to “move backward” due to its inertia and causes slight deformation in the piezoelectric element. This leads to the generation of a charge output by the piezoelectric element, and this charge output can be measured to determine the input acceleration.