Transducers Overview and Classification

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Questions and Answers

What determines the voltage induced in the secondary coils of a Linear Variable Differential Transformer (LVDT)?

The resistance of the primary coil

The frequency of the excitation signal

The length of the coils

The position of the core (correct)

What happens to the output at NULL position in an LVDT?

Both secondary coils have different voltages

The output voltage is negative

Both secondary coils have the same voltage (correct)

The output is maximized

Which of the following statements about the advantages of LVDT is true?

There is a risk of wear and tear due to physical contact.

It has high range and can measure small displacements. (correct)

It cannot operate in corrosive environments.

It can only measure displacements up to 1mm.

How is displacement measured in an LVDT?

By the magnitude of differential voltage Signup and view all the answers

What is a primary characteristic of the output of an LVDT at small displacements?

It is linear Signup and view all the answers

What is the formula for the gauge factor Gf?

$Gf = 1 + 2,\nu + \frac{\Delta \rho}{\rho}$ Signup and view all the answers

Which of the following is NOT a key requirement for resistance temperature detectors (RTDs)?

Minimal change in resistance with temperature Signup and view all the answers

How does Poisson’s ratio ν relate lateral strain to longitudinal strain?

ν = -\frac{\partial D / D}{\partial L / L} Signup and view all the answers

In the equation for resistance thermometers, what does the term $R_0$ refer to?

The resistance at zero degrees Celsius Signup and view all the answers

Which statement correctly describes thermistors?

Most thermistors have negative temperature coefficients. Signup and view all the answers

What occurs when the relation between cross-section area A and diameter D is used in resistance calculations?

A change in D results in a quadratic change in A. Signup and view all the answers

What principle is NOT commonly utilized in inductance transducers?

Change of material properties Signup and view all the answers

In the relationship $rac{dR}{ds} = \frac{1}{R} \left(-\frac{\partial L}{\partial s} + \frac{1}{A} \frac{\partial A}{\partial s} + \frac{1}{\rho} \frac{\partial \rho}{\partial s}\right)$, what does the term $rac{1}{\rho} \frac{\partial \rho}{\partial s}$ indicate?

Contribution of resistivity to the rate of change in resistance Signup and view all the answers

What is the relationship between capacitance and distance between plates in a capacitor?

Capacitance is inversely proportional to distance Signup and view all the answers

What factor affects the sensitivity of a cylindrical capacitor?

The charge per unit length Signup and view all the answers

In the context of capacitive transducers, how is displacement typically measured?

Through overlapping area and distance between plates Signup and view all the answers

Which formula represents the sensitivity of a parallel plate capacitor with respect to distance (d)?

S = ϵ Signup and view all the answers

How does the sensitivity of a capacitive pickup to rotation change with respect to angle (θ)?

It is constant despite changes in angle Signup and view all the answers

What property of capacitors is impacted by the breakdown voltage?

The maximum allowable distance between plates Signup and view all the answers

What is true about the capacitance versus distance (C vs d) relationship?

It follows a hyperbolic relationship but can be linear over a small range Signup and view all the answers

Which of the following factors increases the sensitivity of a capacitive transducer?

Decreasing distance between plates Signup and view all the answers

How is the area between two semi-circular plates determined?

A = θr^2/2 Signup and view all the answers

What is the capacitance formula for a cylindrical capacitor?

C = 2πϵx/ln(D2/D1) Signup and view all the answers

What is one of the primary characteristics of piezoelectric materials?

They produce a charge when subjected to stress. Signup and view all the answers

Which material is known for providing the highest output among piezoelectric materials?

Rochelle salt Signup and view all the answers

Which of the following operations would best describe the phenomenon of piezoelectricity?

It deforms when a potential is applied, creating charge. Signup and view all the answers

What is the significance of the Young's modulus in the context of piezoelectric materials?

It indicates the material's resistance to deformation under stress. Signup and view all the answers

In piezoelectric materials, what does the variable $D$ represent?

The charge produced per unit force. Signup and view all the answers

Which of the following statements about the stability of piezoelectric materials is true?

High stability is desired in piezoelectric materials. Signup and view all the answers

What type of applications commonly utilize piezoelectric materials?

Dynamic measurements such as acceleration and vibration. Signup and view all the answers

How does humidity affect Rochelle salt as a piezoelectric material?

It decreases its output and stability. Signup and view all the answers

What is a significant disadvantage of using a Hall effect sensor?

It is sensitive to temperature variations. Signup and view all the answers

Which statement accurately describes the measurement capabilities of Hall effect sensors?

They are capable of measuring currents up to thousands of amperes. Signup and view all the answers

What is the main breakthrough in photovoltaic technology attributed to Albert Einstein?

Examination of the photoelectric effect. Signup and view all the answers

Which type of transducer provides output that can indicate bidirectional motion?

Incremental transducers. Signup and view all the answers

What feature of photovoltaic cells allows them to be utilized in applications like punched card readers?

Their logarithmic output. Signup and view all the answers

What application is unsuitable for tachometer transducers?

Capturing data from bi-directional movement. Signup and view all the answers

What is the main operational principle of photovoltaic transducers when they output data?

Logarithmic nature of output dependent on light intensity. Signup and view all the answers

What was a significant development for photovoltaic technology in the 1950s?

Creation of the first photovoltaic module. Signup and view all the answers

What is the purpose of the movable plate in the differential arrangement?

To measure linear displacement Signup and view all the answers

In a differential arrangement, what is the condition when the differential voltage ∆E equals zero?

When E1 equals E2 Signup and view all the answers

How is the sensitivity S of the differential voltage calculated?

S = ∆E / x Signup and view all the answers

What does a change in capacitance ∆C depend on in the context of dielectric materials?

The relative change in dielectric constant Signup and view all the answers

What measurement technique is used for measuring liquid levels?

With two concentric cylinders Signup and view all the answers

What characteristic of piezo-electric transducers allows them to generate electric potential?

Surface deformation under stress Signup and view all the answers

Which formula correctly describes the relationship between the capacitance and the lengths of the dielectric material?

C = ϵ0 w (l1 - x + (l2 + x)ϵr) Signup and view all the answers

What creates a linear relationship between change in capacitance and displacement when using dielectric materials?

The relative change in dielectric constant being constant Signup and view all the answers

Study Notes

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.

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Description

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.