ELECTRICAL TRANSDUCERS:RESISTIVE TRANSDUCERS

MEHRAN UNIVERSITY OF ENGINEERING AND TECHNOLOGY, JAMSHORO SINDH, PAKISTAN

DEPARTMENT OF METALLURGY AND MATERIALS ENGINEERING

What is an Transducer? A transducer is a device which converts signals

from one form to another. The electrical measurements are used for

measurement of electrical quantities but its use in measurement of non- electrical quantities is growing. In the measurement of non- electrical quantities a detector is used which usually converts the physical quantity in displacement.The displacement actuates an electric transducer that gives an output which is electrical in nature.The resultant electrical output gives the magnitude of the physical quantity being measured.

The electrical signal could be a voltage, current or frequency.The production of these signals is based upon

the resistive, inductive or capacitive effects to produce different types of transducers.

RESISTIVE TRANSDUCERS

The resistive transducers or resistive sensors are also called as variable resistance transducers.These are one of the most commonly used types of transducers. They can be used for measuring various physical quantities, such as,

1. Temperature2. Pressure3. Displacement4. Force5. Vibrations

WORKING PRINCIPLEThe variable resistance transducer elements work on the principle that the resistance of the conductor is directly proportional to the length of the conductor and inversely proportional to the area of the conductor

R = ρ L /A

Working Principle (continued) Thus, if L is the length of the conductor (m) and A

is its area (m2) as shown in Fig.16.1, then its resistance R (ohms) is given by:

R = ρL/A Where ρ is called as resistivity of the material

measured in ohm-meter and it is constant for the given material.

Some of the popular variable resistance transducers that arebeing used for various applications are as below:

Sliding contact devices In the sliding contact type of variable resistance

transducers, there is a long conductor whose effective length is variable.

One end of the conductor is fixed, while the position of the other end is decided by the slider or the brush that can move along the whole length of the conductor.The slider is connected to the body whose displacement is to be measured. When the body moves the slider also moves along the conductor so its effective length changes, due to which it resistance also changes.

SLIDING CONTACT DEVICES ( CONTINUED)

The value of the resistance is calibrated against the input quantity, whose value can be measured directly.The effective resistance is measured as the resistance between the fixed position of the conductor and the position of the sliding contact as shown in Fig.

Fig. Sliding contact type of variable resistance element

One of most popular sliding contact type of variable resistance transducer is the potentiometer.These devices can be used to measured translational as well as angular displacement and are shown in Fig.

Fig. Rotational potentiometer

POTENTIOMETER

Strain GaugeA Strain gauge (sometimes referred to as a Strain gage) is a sensor whose resistance varies with applied force; It converts force, pressure, tension, weight, etc., into a change in electrical resistance which can then be measured.When external forces are applied to a stationary object, stress and strain are the result. Stress is defined as the object's internal resisting forces, and strain is defined as the displacement and deformation that occur. (When an electrical conductor is stretched within the limits of its elasticity such that it does not break or permanently deform, it will become narrower and longer, changes that increase its electrical resistance end-to-end. Conversely, when a conductor is compressed such that it does not buckle, it will broaden and shorten in size, the changes that decrease its electrical resistance end-to-end.

STRAIN GAUGE ( CONTINUED)A typical strain gauge arranges a long, thin conductive strip, as shown in Fig. 16.3 (a), made in a zigzag pattern of parallel lines such that a small amount of stress in the direction of the orientation of the parallel lines results in a larger strain over the effective length of the conductor Fig. 16.3 (b).The change in resistance of a strain gauge can be measured using a Wheatstone bridge.

Fig. Working principle of strain gauge

Gauge/Gage FactorA fundamental parameter of the strain gage is its sensitivity to strain. It is expressed quantitatively as the gauge factor (GF).Gauge factor is defined as the ratio of fractional change inelectrical resistance to the fractional change in length (strain).The gage factor for metallic strain gages is typically around 2.Where:

ΔR = change in resistance caused by strain RG = resistance of the un-deformed gauge ε = strain

ThermistorThermistor works on the principle that resistance of some materials changes with the change in their temperature.When the temperature of the material changes, its resistance changes and it can be measured easily and calibrated against the input quantity.Thermistor has high negative temperature correlation. The commonly used thermistors are made up of the ceramic like semiconducting materials such as oxides of manganese, nickel and cobalt.Thermistor can be used for the measurement of temperature, as electric power sensing devices and also as the controls for various processes.

Thermistor Resistance Temperature Detector (RTD),

commonly referred to generally as resistance thermometers, is a temperature sensor which measures temperature using the principle that the resistance of a metal changes with temperature. RTDs work on a basic correlation between metals and temperature. As the temperature of a metal increases, it’s resistance to the flow of electricity increases.An electrical current is transmitted through a piece of metal, that is, the RTD element or resistor located in proximity to the area where temperature is to be measured.The resistance value of the RTD element is then measured by an instrument.This resistance value is then correlated to temperature based upon the known resistance -temperature characteristics of the RTD element.Thermistors have been discussed in detail in Lesson 20.

ELECTRICAL TRANSDUCERS:CAPACITIVE

TRANSDUCERS

CAPACITANCE A capacitor consists of two conductors (plates) that are

electrically isolated from one another by a nonconductor (dielectric).When the two conductors are at different potentials (voltages), the system is capable of storing an electric charge.The storage capability of a capacitor is measured in farads.The principle of operation of capacitive transducers is based upon the equation for capacitance of a parallel plate capacitor as shown in Fig.17.1

Where,

A = Overlapping area of plates; m2, d = Distance between two plates; m,e = Permittivity (dielectric constant); F/m.

WORKING PRINCIPLE

The capacitive transducers work on the principle of change in capacitance of the capacitor.This change in capacitance could be caused by change in overlapping area ‘A’ of the plates, change in the distance

‘d’ between the plates and change in dielectric constant ’e’.

In most of the cases the above changes are caused by the physical variables, such as, displacement, force or pressure.Variation in capacitance is also there when the dielectric medium between the plates changes, as in the case of measurement of liquid or gas levels.

WORKING PRINCIPLE (CONTINUED)

• i) Change in capacitance due to change in overlapping area of plates.

• Change in capacitance due to change in distance between• the two plates.

Therefore, the capacitive transducers are commonly used for measurement of linear displacement, by employing the following effects as shown in Fig

ii) iii) Change in capacitance due to change

in dielectric between the two plates

WORKING PRINCIPLE

Fig. 17.2 Variable capacitive transducer varies; (a) area of overlap,(b) distance between plates, (c) amount of dielectric between plates

ELECTRICAL TRANSDUCERS:INDUCTIVETRANSDUCERS

INDUCTANCE Inductance is the ability of an inductor to store

energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit.

E.m.f induced in a coil The emf induced in a coil can be given by

the following expression=

Where;N= Number of turns in the coilΦ = BA = magnetic fluxB = external magnetic

field A = area of coil

WORKING PRINCIPLE

INDUCTIVE TRANSDUCERS

The inductive transducers work on the principle of the electromagnetic induction. Just as the resistance of the electric conductor depends on number of factors, the induction of the magnetic material depends on a number of variables like the number of turns of the coil on the material, the size of the magnetic material, and the permeability of the flux path. In the inductive transducers the magnetic materials are used in the flux path and there are one or more air gaps.The change in the air gap also results in change in the inductance of the circuit and in most of the inductive transducers it is used for the working of the instrument.

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER(LVDT)

The linear variable differential transformer (LVDT) is the most widely used inductive transducer to translate linear motion into electrical signal.

Construction & Working principle A differential transformer consists of a primary

winding and two secondary windings.The windings are arranged concentrically (Having a common centre) and next to each other.They are wound over a hollow non-magnetic and insulating material.A ferro-magnetic core (armature) in the shape of a rod of cylinder is attached to the transducer’s sensing shaft.The core slides freely within the hollow portion of the bobbin.The linear variable displacementtransformer LVDT is shown in

Fig.

LVDT(CONTINUED)

Fig. Linear variable differential transformer

LVDT(CONTINUED)

There is one primary and two secondary windings.The secondaries are connected so their outputs are

opposite. If an AC excitation is applied across the primary winding then voltages are induced in the secondaries.A movable core varies the coupling between it and the two secondary windings.When the core is in the centre position, the coupling to the secondary coils is equal.As the core moves away from the centre position, the coupling to one secondary becomes more and hence its output voltage increases, while the coupling and the output voltage of the other secondary decreases.

LVDT (MERITS AND DEMERITS)

The LVDT has infinite resolution and a high sensitivity. It is simple, light in weight, and easy to align and maintain.

These transducers can usually tolerate a high degree of shock and vibration without any adverse effects. In addition to this they have low hysteresis and hence repeatability is excellent under all conditions.

The disadvantages of LVDT include their relatively large displacements are requirement for appreciable differential output.They are sensitive to stray magnetic fields but this can be overcome by providing appropriate shielding.

Temperature affects the performance of the transducer.