Piezoelectric Sensor Technology

The functional properties of piezoelectric materials were discovered in the 1880s by brothers Pierre and Jacques Curie.

The discovery came from experiments that measured an electrical charge on a layer of crystals (e.g., tourmaline, quartz, topaz, and cane sugar) after subjecting these crystals to mechanical stress. This led to an understanding of the concept of piezoelectricity, or the accumulation of an electrical charge in crystals from applied stress.

Accumulation of an electrical charge as a result of a change in the state of pressure is a type of electromechanical interaction. With a change in pressure being the main stimulus to creating a charge in the crystal material, the piezoelectric effect can be reversed as soon as the pressure is lifted.

When applying an electrical force to the crystal structure, the cells in this lattice change dimensions. The video below animates the movement of a piezoelectric disk that can generate a voltage.

Functional Principle

A piezoelectric effect is detected by a sensor and from this detection, measurements can be made on changes in pressure, acceleration, and force – displayed in the form of an electrical charge. Piezoelectric crystals such as barium titanate and ceramic materials such as lead zirconate titanate are considered smart materials for the purpose of generating a piezoelectric effect and are built-in piezoelectric sensor technology.

There are two types of pressure sensors (mechanical and electrical) that are typically used to measure the piezoelectric effect. Mechanical sensors are made of a sensing element known as a diaphragm, a rod, a three-terminal resistor that acts as a voltage divider, and a direct current voltage source. During the application of pressure, the diaphragm component to the piezoelectric sensor displaces the connecting rod. This then activates the potentiometer component to the sensor, which generates an output voltage (Fig. 1a).

a) Mechanical sensor illustrating the piezoelectric effect.


b) Pressure sensor illustrating the piezoelectric effect without a transducing element.
 

The second type of pressure sensor works by a slightly different principle. The diaphragm to this type of piezoelectric sensor is made up of polymer polyvinylidene difluoride (PVDF) and works as sensing and transducing element. Upon application of pressure to the diaphragm, the pressure is directly converted into a voltage, instead of using a potentiometer (Fig. 1b).

It is important to understand the behavior of the piezoelectric crystals when determining the piezoelectric effect. The solid crystal material in the piezoelectric sensor will display an electric dipole moment. This polarization, measured in Cm/m3, can be determined by measuring the movement of dipoles as a fraction of the unit cell in the crystalline lattice.

Thus, any change in polarization due to mechanical stress comes from a reconfiguration of the electric dipole moment in the solid structure, and this results in the manifestation of piezoelectricity.

Applications

The fact that an electrical power source is not necessarily required to generate a piezoelectric effect has opened the path to the engineering of energy-efficient technology for pedestrian lighting.

For example, technology company Pavegen Systems have introduced a novel pavement tile whereby renewable energy is harvested from the footstep. The basic principle of this type of technology utilizes the piezoelectric effect and involves the conversion of kinetic energy into electricity. The Pavegen tile is engineered with a central light fixture that glows as a pedestrian applies mechanical pressure to the tile. The electrical energy that is created from this piezoelectric stimulus is collectively used to power pedestrian lighting and wayfinding solutions.

The composite of the tile is made up of 100% recycled materials and is applied to outdoor locations. The material composite of the tile is waterproof, making it ideal for application in outdoor sites. It is also interesting to know that, while the power source generated by a piezoelectric device is insignificant as a single unit, energy from thousands of piezoelectric devices as a result of pressure from highways could be powerful enough to collect a significant amount of energy.

The following video demonstrates the generation of energy from footsteps that are then stored and used to power lighting.

Triboelectric Nanogenerators

Research on improving the functional capacity of piezoelectric material is starting to evolve. Professor Zhong Lin Wang and a team of researchers at the Georgia Institute of Technology have focused on looking at the use of triboelectric nanogenerators to enforce the mechanical motion.

The triboelectric nanogenerator is made of flexible polymeric materials. By applying force to this material, it is possible to generate an electrical current. This technology involves a sheet of polyester that makes contact with a layer of polydimethylsiloxane (PDMS) resulting in the exchange of electrons. When parting both layers, a voltage decreases and generates a current. The current is generated by applying external pressure to the zinc oxide nanowires that make up the generator and deforming these wires to force the rubbing motion between the two layers (Fig. 2).

The aim of this research is to design generators that are going to be inexpensive and acceptable for mass production. The harvesting of energy from using such technology could be applied to the manufacturing of touch-sensitive devices and, again, work with footsteps to generate a collective amount of energy to power display and signage.

Piezoelectric technology clearly demonstrates an advantage in the sourcing of renewable energy. This technology is slowly evolving to adapt better techniques and materials to generate energy.

Sources and Further Reading

• GeorgiaTech/Research News
• John, V., Reghu, A.  Introduction to Sensors. (2011). Broken Sound Parkway, NW: Taylor and Francis Group.
• Nuffer, J., Bein, T. Application of Piezoelectric Materials in Transportation Industry. Fraunhofer Institute for Structural Durability and System Reliability, LBF. Germany: Global Symposium on Innovative Solutions for the Advancement of the Transport Industry, 4–6. October 2006, San Sebastian, Spain.  
• Piezo Systems, Inc.
• Pavegen Systems. - http://www.pavegen.com/

This article was updated on the 4^th October, 2019.