Transducers Overview and Classification

Transducers Outline

• Transducers are devices that convert one form of energy into another.
• Transducers can be classified based on the type of transduction, as primary and secondary transducers.
• Transducers can also be classified as passive and active transducers.
• Transducers can be classified as transducers and inverse transducers.

Classification of Transducers

• Transducers can be categorized based on the form of transduction, primary and secondary transducers, passive and active transducers, and transducers and inverse transducers.

Resistance Transducers

• Resistive transducers are a type of transducer that uses resistance variation to measure various physical parameters.
• They are preferred for both AC and DC.
• Resistance of a metal conductor: Resistance = (ρ * L) / A -ρ = resistivity -L = length -A = cross-sectional area
• Resistive transducers use variation in one parameter, such as translational or rotational displacement.
• Strain gauges measure change in resistance from being strained.
• Resistance thermometers and thermistors use resistivity changes with temperature to measure temperature.

Potentiometers

• Potentiometers, also called POTs, use a sliding contact called a wiper to measure translational and rotational displacement.
• Translational pots have a stroke of 2mm to 0.5m.
• Rotational pots can have full-scale angular displacement as small as 10° and up to 3500°.
• Helipots can measure up to 3500°.
• Loading effect occurs due to meter resistance (Rm).

Strain Gauge

• Strain gauges measure changes in resistance due to changes in the dimensions of an elastic material under stress.
• Resistance = (ρ * L) / A(where ρ is resistivity, L is length and A is cross-sectional area).
• Gauge factor (Gf) relates the change in resistance to the change in length.

Resistance Thermometers and Thermistors

• Resistance thermometers measure temperature by the change in resistance of a material.
• The resistance of a conductor with respect to temperature change is calculated as R= Ro(1 + a₁T+a₂T²....), where a₁ and a₂ denote temperature coefficients.
• Resistive temperature detectors (RTDs) should have high resistivity.
• RTDs should have a continuous relation between resistance and temperature.
• Thermistors consist of semiconductor materials; most have negative temperature coefficients.

Inductance Transducers

• Inductance transducers use the principle of self or mutual inductance, or eddy currents to measure physical quantities.
• A coil's inductance (L) is given as L = N²/R, where N is the number of turns in the coil and R is the reluctance.
• Reluctance (R) of a coil is given by R = l/(µA), where l is the length and A is cross-sectional area and µ is the permeability of the material.

Differential Output

• Differential output in inductance transducers, is the difference in outputs to increase sensitivity and accuracy.
• External magnetic fields have less effect on the output.

Change of Mutual Inductance

• Mutual inductance (M) between two coils is given by M = K√(L₁L₂), where L₁ and L₂ are self-inductances of the coils and, K is the coefficient of coupling.

Production of Eddy Currents

• Eddy currents, generated in a conducting plate near a coil, affect the flux and inductance of the coil.
• The plate behaves as a short-circuited secondary winding, producing a reverse magnetic field.

Linear Variable Differential Transformer (LVDT)

• An LVDT consists of a primary coil and two secondary coils.
• Displacement is measured through the difference in voltages across the secondary coils.
• Direction is determined by the phase of the output voltage.
• The output is linear as long as the displacement is small.

Advantages of LVDT

• LVDTs measure a wide range of displacements.
• LVDTs are highly sensitive with 0.25% full scale linearity.
• LVDTs have high immunity to external effects.

Disadvantages of LVDT

• LVDTs have a large displacement required.
• LVDTs are sensitive to magnetic fields.
• Performance is affected by vibrations.

Applications of LVDT

• LVDTs are used to measure small displacements.
• LVDTs measure displacement as a primary transducer.
• LVDTs are used for force, weight, pressure measurements.

Rotary Variable Differential Transformer (RVDT)

• RVDT is a variation of the LVDT that measures rotation.
• The core is cam shaped and symmetrical.
• Differential voltage of the secondary coils is zero at null position.

Capacitive Transducers

• Capacitive transducers measure changes in capacitance to determine a range of physical variables.
• The capacitance of parallel plate capacitor is given as C = (εA)/d -(ε) is the medium permittivity -(A) is the area of the plates -(d) is the distance between plates.
• A change occurs in the effective area of overlapping (A), distance between plates (d), or dielectric constant will change the capacitance.

Sensitivity of a Capacitor

• Sensitivity (S) for a parallel-plate capacitor is given by S = (dC/dx).
• Sensitivity (S) for a cylindrical capacitor is given by S = (εd²)/(2πε).

Capacitance Pickup with Distance

• Capacitance is inversely proportional to the distance between plates.
• Sensitivity of a capacitor is constant in relation to displacement.

Differential Arrangement

• Differential arrangement uses a movable plate between two fixed plates to measure displacements accurately.

Variation of Dielectric Constant

• A change in dielectric material produces a change in capacitance that is proportional to the displacement.
• The relationship between the dielectric constant and capacitance is linear.

Piezoelectric Transducers

• Piezoelectric materials generate voltage when deformed.
• The effect is reversible; deformation occurs when voltage is applied.
• Examples include Rochelle salt, quartz, ceramics A and B, etc.

Desired Properties of Piezo-electric Materials

• Stability, high output, and sensitivity to temperature and humidity changes.

Synchro Transmitter and Receiver

• Structure similar to alternators.
• Uses laminated steel stators with balanced 3-phase coils.
• Rotor is dumb-bell shaped for transmitter and cylindrical for the receiver.
• AC voltage applied to rotor.

Resolvers

• Resolvers convert angular positions into Cartesian coordinates.

Fiber Optic Cables

• Fiber optic cables transmit light, which can be used for sensing and measuring physical properties.
• Snell's Law and Total Internal Reflection are key principles.
• Numerical Aperture (NA) defines the cone of incidence.

Photo Optic/ Fiber Optic Sensors

• Fiber optic sensors use guided light transmission for high accuracy and resistance.
• Refractive indexes of the fiber change with temperature, resulting in change in the critical angle of the fiber.

Hall Effect Sensor

• Current-carrying strip in a magnetic field generates an emf.
• Magnitude depends on current, flux density, and Hall effect coefficient.
• Useful for measuring current, displacement, magnetic field strength.

Photovoltaic Cells

• Photovoltaic effect converts light directly into electrical energy.
• Semiconductors are used in construction, and advanced technology lowers costs.

Photovoltaic Transducers

• Photovoltaic cell outputs are logarithmic.
• The output is amplified with op-amps.
• Application in punched-card readers, width measurements.

Digital Transducers

• Digital transducers present data as discrete pulses, easily compatible with digital systems.
• Types include tachometer, incremental, and absolute encoders.

Shaft Encoders

• Shaft encoders provide precise angular position measurements, using various types of coding, such as gray codes.

This quiz covers the fundamentals of transducers, including their classifications into primary, secondary, active, passive, and inverse types. It also delves into resistance transducers, exploring their principles of operation and measurement techniques. Test your knowledge on the key concepts and applications of transducers.