How Piezo Electric Crystal works? What are the different axes of crystal?

The Piezo Electric Crystal is used to measure pressure. Piezo is an Italian word meaning pressure. It is very accurate in measurement. A piezo crystal is a transducer which can convert pressure into electrical signal. It is an active transducer. It is found that Rochelle salt and Tourmaline substances.

It can work as a dual type of transducer. It can covert dynamic pressure into proportional electric signals and it can also convert high frequency AC voltage connected to it into proportional mechanical vibrations. These are known as Piezo Electric Effect and Inverse Piezo Electric Effect.

Piezoelectric Effect: When a mechanical stress is applied to the mechanical axis with respect to optical axis, a potential difference is developed across the electrical axis with respect to optic axis.

Inverse Piezoelectric Effect: When an alternating electric field is applied to electrical axis with respect to optical axis, expansion or contraction takes place in the mechanical axis with respect to optical axis.

Axes of Piezo Electric Crystal

As shown below, piezo electric crystal has three axes, electrical axis (x-x’), mechanical axis (y-y’) and optical axis (z-z’). It is cut in to different shapes of slices for different applications. In electronics electronic oscillator circuits the z-cut crystal slice is widely used.

1. The x-cut crystal slice is used to produce longitudinal ultrasonic waves.
2. The y-cut crystal slice is used to produce transverse ultrasonic waves.
3. The z-cut crystal slice is also used to produce ultrasonic waves. But this type of slice of piezo electric crystal is mainly used to convert dynamic mechanical pressure into proportional electric signal.

Working: When the crystal is subjected to mechanical pressure, along y-axis, it converts it into proportional electric voltage. This effect of pressure (piezo) is called as piezo-electric effect. It is an active transducer, which does not require external battery voltage.

Similarly, when high frequency AC voltage (greater than 20kHz) is applied across its x-axis, it vibrates along y-axis. This happens due to mechanical deformation along mechanical axis. So we can produce high frequency sound waves like ultrasonic waves.

Since it can convert mechanical energy into electrical energy and electric energy into mechanical energy, it is called dual type of transducer.

Measurement of Air Pressure using Piezo Electric Crystal

The piezoelectric transducer works on the principle that when dynamic pressure is applied along mechanical axis of a crystal slice, it produces proportional e.m.f. along its electrical axis.

The setup of piezoelectric transducer circuit is given above. A z-cut crystal slice is placed between a rigid support and the force summing elements. When external pressure is applied through pressure port, it acts on top of the crystal.

So a proportional e.m.f. is produced across electrical axis (x-x’). This voltage is equal to the magnitude of applied pressure. It is also found that such e.m.f. is produced when either strain (tension) or stress (pressure) is applied on the crystal. However, this transducer cannot be used to measure static conditions of pressure or tension. Only dynamic conditions can be measured. Read more about piezo electric crystal on this link.

Applications

1. Used in crystal microphone.
2. It is used in electronic weighing machines.
3. It is used in generation of high frequency ultrasonic waves in oscillator circuits.
4. It is used in old phonogram pickup cartridge.

Advantages

1. It is very small in size.
2. It has stable temperature characteristics.
3. High frequency response.
4. It is active transducer. So external power supply is not necessary.

Disadvantages

1. It cannot measure static conditions of pressure.
2. It produces very small output voltage during conversion.
3. It is brittle.