Piezoelectric transducers | Construction | Diagram | Switches
Here we learn about piezoelectric transducers, with diagram and construction with working.
Piezoelectric transducers :
— Certain materials, when mechanically strained generate within them an electric charge. lhe effect is given the name of piezoelectric effect (pressure electricity).
This effect is reversible, that is, if a charge is applied, the material deforms mechanically.
Such materials, when subjected to alternating electric field, will expand and contract alternatively.
— These materials fall into three main groups :
Natural (quartz, Rochelle salt) and
Synthetic (lithium sulphate, ammonium dihydrogen phosphate crystals)
Polarized ferroelectric ceramics (barium titanate).
— The desirable properties of piezoelectric materials, materials are stability, high output and ability to function under
high temperature and humidity conditions, Quartz has high stability but low output. Rochelle salt has high output but requires protection from moisture. Barium titanate is commonly used in ultrasonic generators.
~ Their main applications are for force, acceleration and surface roughness measurements and for making ultrasonic devices.
~ The use of piezoelectric transducers, however, is limited to dynamic measurements as, the charge is not held (leaks) under static conditions. Also, their output impedance is high of the order of 10000 mΩ
~ Charge sensitivity and voltage sensitivity are two important characteristics of piezoelectric materials.
Important relations are,
Q = kF (charge developed is proportional to force) ...(i)
V = Q/C (voltage charge relation) ..(ii)
where
f = force applied
Q = charge developed
K = charge sensitivity
V = voltage developed
C = capacitance
t = thickness
A = area
E = elastic constant of material
e = dielectric constant of crystal material
The main features of piezo electric sensors are high output, high frequency response and negligible phase shift.
Switches
~ A switch is a mechanical device used to connect and disconnect a circuit as and when required. A variety of switches are available from subminiature up to industrial plant switching megawatts of power on high voltage distribution lines.
~ In applications as where multiple switching is required to be done (e.g., a telephone service), electronic switching devices which can be automated and intelligently controlled are preferred over mechanical switches.
~ In the simplest possible design, a switch has two pieces of metal called contacts that touch to make a circuit, and separate to break the circuit.
~ The contact material is chosen on the basis of their resistance to corrosion, because most metals when exposed to air form insulating oxides that would prevent the switch from working.
~ Contact materials are also chosen on the basis of the characteristics such as electrical conductivity, hardness, mechanical strength, low cost and low toxicity.
~ Sometimes the contacts are plated with noble metals. They may be designed t0 wipe against each other to clean off any contamination. Nonmetallic conductors, such as conductive plastic, are sometimes used.
~ The moving-part that applies the operating force t0 the contacts is called the actuator, and may be a toggle or dolly, a rocker, a push-button or any type of mechanical linkage as shown in Fig. 13.5.1 .
~ A pair 0f contacts is said to be 'closed' when there is no space between them. This would allow current to flow from one contact to the other. When the contacts are separated by an air gap, they are said to be open, and no electric current can flow at normal operating voltages.
Piezoelectric transducers :
— Certain materials, when mechanically strained generate within them an electric charge. lhe effect is given the name of piezoelectric effect (pressure electricity).
This effect is reversible, that is, if a charge is applied, the material deforms mechanically.
Such materials, when subjected to alternating electric field, will expand and contract alternatively.
— These materials fall into three main groups :
Natural (quartz, Rochelle salt) and
Synthetic (lithium sulphate, ammonium dihydrogen phosphate crystals)
Polarized ferroelectric ceramics (barium titanate).
— The desirable properties of piezoelectric materials, materials are stability, high output and ability to function under
high temperature and humidity conditions, Quartz has high stability but low output. Rochelle salt has high output but requires protection from moisture. Barium titanate is commonly used in ultrasonic generators.
~ Their main applications are for force, acceleration and surface roughness measurements and for making ultrasonic devices.
~ The use of piezoelectric transducers, however, is limited to dynamic measurements as, the charge is not held (leaks) under static conditions. Also, their output impedance is high of the order of 10000 mΩ
~ Charge sensitivity and voltage sensitivity are two important characteristics of piezoelectric materials.
Important relations are,
Q = kF (charge developed is proportional to force) ...(i)
V = Q/C (voltage charge relation) ..(ii)
where
f = force applied
Q = charge developed
K = charge sensitivity
V = voltage developed
C = capacitance
t = thickness
A = area
E = elastic constant of material
e = dielectric constant of crystal material
The main features of piezo electric sensors are high output, high frequency response and negligible phase shift.
Switches
~ A switch is a mechanical device used to connect and disconnect a circuit as and when required. A variety of switches are available from subminiature up to industrial plant switching megawatts of power on high voltage distribution lines.
~ In applications as where multiple switching is required to be done (e.g., a telephone service), electronic switching devices which can be automated and intelligently controlled are preferred over mechanical switches.
~ In the simplest possible design, a switch has two pieces of metal called contacts that touch to make a circuit, and separate to break the circuit.
~ The contact material is chosen on the basis of their resistance to corrosion, because most metals when exposed to air form insulating oxides that would prevent the switch from working.
~ Contact materials are also chosen on the basis of the characteristics such as electrical conductivity, hardness, mechanical strength, low cost and low toxicity.
~ Sometimes the contacts are plated with noble metals. They may be designed t0 wipe against each other to clean off any contamination. Nonmetallic conductors, such as conductive plastic, are sometimes used.
~ The moving-part that applies the operating force t0 the contacts is called the actuator, and may be a toggle or dolly, a rocker, a push-button or any type of mechanical linkage as shown in Fig. 13.5.1 .
~ A pair 0f contacts is said to be 'closed' when there is no space between them. This would allow current to flow from one contact to the other. When the contacts are separated by an air gap, they are said to be open, and no electric current can flow at normal operating voltages.
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