What is Piezoelectric Effect?
There are certain materials that generate electric potential or voltage when mechanical strain is applied to them or conversely when the voltage is applied to them, they tend to change the dimensions along certain plane. This effect is called as the piezoelectric effect. This effect was discovered in the year 1880 by Pierre and Jacques Curie. Some of the materials that exhibit piezoelectric effect are quartz, Rochelle salt, polarized barium titanate, ammonium dihydrogen, ordinary sugar etc.
The piezoelectric transducers work on the principle of piezoelectric effect. When mechanical stress or forces are applied to some materials along certain planes, they produce electric voltage. This electric voltage can be measured easily by the voltage measuring instruments, which can be used to measure the stress or force.
The physical quantities like stress and force cannot be measured directly. In such cases the material exhibiting piezoelectric transducers can be used. The stress or the force that has to be measured is applied along certain planes to these materials. The voltage output obtained from these materials due to piezoelectric effect is proportional to the applied stress or force. The output voltage can be calibrated against the applied stress or the force so that the measured value of the output voltage directly gives the value of the applied stress or force. In fact the scale can be marked directly in terms of stress or force to give the values directly.
The voltage output obtained from the materials due to piezoelectric effect is very small and it has high impedance. To measure the output some amplifiers, auxiliary circuit and the connecting cables are required.
Materials used for the Piezoelectric Transducers
There are various materials that exhibit piezoelectric effect as mentioned above. The materials used for the measurement purpose should posses desirable properties like stability, high output, insensitive to the extreme temperature and humidity and ability to be formed or machined into any shape. But none of the materials exhibiting piezoelectric effect possesses all the properties. Quartz, which is a natural crystal, is highly stable but the output obtained from it is very small. It also offers the advantage of measuring very slowly varying parameter as they have very low leakage when they are used with high input impedance amplifiers.
Due to its stability, quartz is used commonly in the piezoelectric transducers. It is usually cut into rectangular or square plate shape and held between two electrodes. The crystal is connected to the appropriate electronic circuit to obtain sufficient output.
Rochelle salt, a synthetic crystal, gives the highest output amongst all the materials exhibiting piezoelectric effect. However, it has to be protected from the moisture and cannot be used at temperature above 115 degree F. Overall the synthetic crystals are more sensitive and give greater output than the natural crystals.
Advantages of Piezoelectric Transducers
Every devise has certain advantages and limitations. The piezoelectric transducers offer several advantages as mentioned below:
1) High frequency response: They offer very high frequency response that means the parameter changing at very high speeds can be sensed easily.
2) High transient response: The piezoelectric transducers can detect the events of microseconds and also give the linear output.
3) High output: They offer high output that be measured in the electronic circuit.
4) The piezoelectric transducers are small in size and have rugged construction.
Limitations of Piezoelectric Transducers
Some of the limitations of piezoelectric transducers are:
1) Output is low: The output obtained from the piezoelectric transducers is low, so external electronic circuit has to be connected.
2) High impedance: The piezoelectric crystals have high impedance so they have to be connected to the amplifier and the auxiliary circuit, which have the potential to cause errors in measurement. To reduce these errors amplifiers high input impedance and long cables should be used.
3) Forming into shape: It is very difficult to give the desired shape to the crystals with sufficient strength.
Applications of the Piezoelectric Transducers
1) The piezoelectric transducers are more useful for the dynamic measurements, i.e. the parameters that are changing at the fast rate. This is because the potential developed under the static conditions is not held by the instrument. Thus piezoelectric crystals are primarily used measurement of quantities like surface roughness, and also in accelerometers and vibration pickups.
2) For the same reasons they can be used for studying high speed phenomenon like explosions and blast waves. They are also used in aerodynamic shock tube work and seismograph (used for measurement of acceleration and vibration in rockets).
3) Many times the piezo sensors or transducers are used along with the strain gauges for measurement of force, stress, vibrations, etc.
4) The automotive companies used piezoelectric transducers to detect detonations in the engine blocks.
5) Piezoelectric transducers are used in medical treatment, sonochemistry and industrial processing equipments for monitoring the power.
1) Book: Mechanical and Industrial Measurements by R. K. Jain, Khanna Publishers
2) Mechanical Measurements by Thomas G. Beckwith and N. Lewis Buck