How the Piezoelectric Effect is Used in Sensors
This series provides a look at how different physical phenomena can be converted into an electrical signal. In this article, the third in our series, we’ll discuss pressure.
How Is Voltage Generated From Pressure?
Piezoelectricity is the generation of voltage by the application of pressure. The piezoelectric effect is the electrical polarization of some materials when mechanically strained. The voltage potential produced by the pressure on a crystal is directly proportional to the amount of strain applied.
If a crystal is stretched, the voltage potential across the crystal changes polarity. For example, points on a crystal that are positive when pressure is applied change to negative when the crystal is stretched. See Figure 1.
Figure 1. Pressure on some crystals generates a surface charge on the crystal. The amount of charge is proportional to the amount of pressure.
Image Courtesy of the University of Queensland
Electrons flow in one direction in a circuit connected to a crystal when the crystal is compressed and in the opposite direction when the crystal is decompressed. Also, the electron flow within the crystal in both cases is from the positive terminal to the negative terminal.
The positive terminal has a deficiency of electrons; the negative terminal has an excess of electrons. If either the compression or decompression force is held constant, electron flow continues until these two charges are equalized, and then it ceases.
Pressure can be applied to a crystal by compression, stretching, twisting, or bending. The method of applying pressure depends on the crystal material. Some materials respond best to bending pressure, while others respond best to twisting pressure. The force of the pressure acts on the atoms of the crystal to force the electrons out of their orbits.
In 1880, Pierre and Jacques Curie demonstrated the connection between the application of pressure on certain crystals (tourmaline, quartz, topaz, cane sugar, and Rochelle salt) and the generation of a surface charge across the crystals.
The findings of Pierre and Jacques Curie regarding piezoelectricity were remarkable in that their discovery was made using only tinfoil, glue, wire, magnets, and a jeweler’s saw used to cut the crystals.
Piezoelectric Device Applications
A transducer is a device that is actuated by energy from one system and supplies energy in the same or different form to a second system. A piezoelectric transducer is a transducer that operates based on the interaction between the deformation of certain materials and the rearrangement of the material’s electric charge.
Piezoelectric transducers are used in microphones, watch timing circuits, radio and television station transmitters and receivers, ultrasound equipment, force and pressure measurement devices, piezoelectric translators (PZTs), and piezoelectric motors. Ultrasonic transducers are used in medical equipment. In addition, crystals make excellent filters. Specific frequencies can either be passed to a load or rejected.
A piezoelectric (crystal) microphone converts the pressure of sound waves into electric signals. The pressure from the sound waves causes the crystal material within the microphone to compress and decompress, producing an equivalent electric signal (see Figure 2). If an audio electric signal is applied to the crystal, the microphone becomes an earphone. In the earphone application, the applied electric signal causes the crystal to compress and decompress, creating a sound wave.
Figure 2. A piezoelectric (crystal) microphone converts the pressure of sound waves into electric signals
Image Courtesy: ElectricalAcademia
Measuring Force and Pressure
Piezoelectricity is also used for force and pressure measurement. These piezoelectric sensors consist of very fine electrical wires attached to the crystal.
When the crystal is compressed or stretched, the potential generated is proportional to the force and can be measured with a voltmeter. These devices can be used to measure the vibration of machinery and structures.
During the 1980s, piezoelectric translators (PZTs) and piezoelectric motors were developed. Both the translators and motors can make micropositioning adjustments due to the small movement of their crystals when voltage is applied.
Piezoelectric motors are lightweight, are low-speed, and have high torque (turning force). Because of the piezoelectric effect, these motors operate free of vibration and are used in applications such as precisely positioning a telescope. Their disadvantage is that they have a short life span.
Although the piezoelectric effect was known for many years, it was the mid-1930s before any practical applications were developed. Research, development, and the perfection of large single quartz crystals were encouraged during World War II due to the need for communications devices such as transmitters and receivers.