Chapter-8 TRANSDUCERS

 Topics to be discussed: Mechanical, Electrical, Electronic Transducers its advantages & Disadvantages.

Introduction:

•In a measurement system, the measurand (the quantity under measurement) makes its first contact with system through a detector. The measurand is converted into an analogous form by the detector.

•The measurand or input signal is called information for the measurement system.

•The information may be in the form of a physical phenomenon or it may be an electrical signal.

•The function of the detector is to sense the information and convert it into a convenient form for acceptance by the later stages of the system. The process of detection & conversion of input signal from one form to another requires energy.

•This energy may be extracted from the signal, thereby, causing loading effects.

•If energy is extracted from the signal, it will not be reproduced after conversion leading to errors in measurement.

•Energy is supplied to the detector from external sources so that the input signal is converted into a usable output without drawing an undue amount of energy from the signal.

•The ideal conversion will be when absolutely no energy is extracted from the signal so that it is not distorted and the analogous output of the detector is a faithful representation of the input signal.

 TRANSDUCER

• A transducer is a device used to covert motional, thermal and optical signals into electrical quantities, that may be amplified, recorded and otherwise processed in the instrumentation system.

•Transducers are also known as prime sensors, gauges, pickups and signal generators.

TYPES OF TRANSDUCERS

With the fast developing technology various types of transducers have appeared for measurement of one particular quantity.

The following are the types of transducers and the quantities to be measured.

1.Capacitive Transducers – for the measurement of Displacement, Thickness, Velocity, Acceleration, Pressure, Flow, Humidity, Viscosity, Level, etc.

2.Electron tube Transducers – for the measurement of Displacement, Velocity, Acceleration & Pressure.

3.Inductive type of Transducers – for the measurement of Mass, Displacement, Thickness, Velocity, Acceleration, Force, Pressure, and Flow.

4.Magneto-Electric Transducers – for the measurement of Mass, Displacement, Velocity, Acceleration, Pressure and Flow.

5.Photo Electric Transducers – for the measurement of Displacement, Thickness, Velocity, Temperature, level and height.

6.Piezo Electric Transducers – for the measurement of mass, Displacement, Thickness, Velocity, Acceleration, Force, Pressure, Flow, Viscosity, Level, etc.

7.Radio active Transducers - for the measurement of mass, Displacement, Thickness, Velocity, Force, Pressure, Flow, Level, etc.

8.Resistance Transducers - for the measurement of mass, Displacement, Thickness, Velocity, Acceleration, Force, Pressure, Flow, Viscosity, Level, etc.

The basic function of first stage, or sensor-transducer stage, is to detect or sense the input signal and transfer it to the second stage. Thus, in measurements, the information required is picked up from the source by the first stage detectors.

Usually, the sensed information, i.e., the input is transduced into a convenient analogous form, like mechanical displacement into electrical voltage form. These devices which sense the input & convert them into an analogous form are called ‘detector-transducer’

Transfer Efficiency:

The sensed input Iin and the transduced analogous form, Iout can

be related as Iout = f (Iin) and transfer efficiency = Iout / Iin < 1

Transfer efficiency will always be less than unity, because of the losses in sensing and processing the input. In measurements, a transfer efficiency as high as possible is desired for accurate results.

PRIMARY & SECONDARY TRANSDUCERS:

The devices which sense the input, transform them to a convenient analogous form, and send the signal to the next stage are known as transducers. These are the basic elements of any measurement systems, which can be distinguished as primary and secondary transducers, based on the functions they perform. They act in combination one after the other.

 The primary transducer basically senses and sends the signal without any modification, like a displacement as a result of measurement of displacement, force, pressure, temperature, etc. This displacement is amplified in the second stage and used to indicate the reading in the third stage.

A secondary transducer, works subsequent to a primary transducer receiving the input from it. For example, the displacement of the primary transducer used in a Linear Variable Differential Transformer (LVDT) to produce an electrical output which can be amplified and used at the recorder.

CLASSIFICATION OF FIRST STAGE DEVICES:

First Stage Devices can be broadly classified as follows –

1. First stage elements used as detectors only.

2. First stage elements used as detector and single transducer

3. First stage elements used as detector with two transducers

 The elements used as detectors may be very simple, like a mechanical spindle or a contact member that picks up and transmits the input to a secondary transducer. Generally first stage devices are a combination of detectors and transducers, since transducing is essential for further stages (e.g. Bourdon pressure gage-Bourdon tube with this linkage). Based on its two functions,

Primary Detector

Transducer

Secondary Transducer

first stage can be represented schematically as shown in figure 1.

IAS – Intermediate analogous signal

ADS - Analogous driving signal

 

ACTIVE AND PASSIVE TRANSDUCERS:

Based on the input circuitry adopted and method of operation, the Electrical detector transducers can also be classified as active type and passive type.

ACTIVE TRANSDUCERS:

Active transducers are those, which are self-powered, hence require no external source of energy. For example, a piezoelectric accelerometer does not require any external source of energy, because it works on its own energy. Main advantage of this type is that its circuit design is simpler.

PASSIVE TRANSDUCERS:

Passive transducers are those which require an external source of energy for their operation. For example, a simple bonded wire strain gage using resistance bridge requires an auxiliary source of energy. This type of transducer requires a complicated input design since it requires special arrangement to introduce the auxiliary energy.

TYPES OF TRANSDUCERS WITH THEIR FUNCTIONS:

The following table gives a list of some primary detector transducer elements generally used in measurements, along with their functions.

Element Operation

Mechanical :A. Contact spindle, pin or finger

 

Displacement to displacement 

B. Elastic member 1.      Helical spring 2.      Bourdon tube 3.      Diaphragm 4.      Liquid column

 

Force to displacement Pressure to displacementPressure to displacementPressure to displacement 

C. Mass 1.Spismic mass 2. Pendulum  3. Liquid column 

 

Force to displacementGravitational acceleration to frequency or periodPressure to displacement

D. Thermal 1. Thermo couple 2. Bimaterial 3. Thermistor 4. Chemicals

 

Temperature to electric currentTemperature to displacementTemperature to resistance changeTemperature to chemical phase 

E. Hydro-pneumatic 1.      Float 2.      Orifice 3.      Pitot tube 4.      Vanes

 

Fluid level to displacementFluid velocity to pressure changeFluid velocity to pressure changeVelocity to force 

Electrical: A. Resistance 1. Contact type 2. Variable length/area 3. Variable resistivity 4.Variable dimensions

Displacement to resistance changeDisplacement to resistance changeTemperature to resistance changeStrain to resistance change

A.      Inductance: 1. Variable dimensions 2. Variable air gap 3. Changing core position 4. Moving coil 5. Moving magnet 6. Moving core 

Displacement to change in inductionDisplacement to change in inductionDisplacement to change in inductionVelocity to change in inductionVelocity to change in inductionVelocity to change in induction

B.  Capacitance 1. Changing air gap 2. Changing plate area 3. Changing dielectric

 

Displacement to change in capacityDisplacement to change in capacityDisplacement to change in capacity 

C.  Piezoelectric Displacement to voltage or voltage to displacement 

D.  Photoelectric Light intensity to voltage or voltage to displacement 

It can be seen from the list that, most mechanical sensors transduce the inputs to displacement, while electrical sensors transduce displacement to voltage/current/resistance change. In practice, the mechanical elements are used as primary transducers, while electrical elements are used as secondary transducers.

Q] LIST THE ADVANTAGES OF ELECTRICAL TRANSDUCERS OVER MECHANICAL TRANSDUCERS

The electrical transducer elements have many advantages over mechanical elements, which are:

1. Amplification or attenuation is easier

2. Mass-inertia effects are minimum

3. Friction problems are negligible

4. Output power of desired magnitude is possible

5. Remote indication/recording is feasible

6.These are lighter and smaller compared to mechanical elements.

ELECTRICAL TRANSDUCER ELEMENTS:

These transducers converts mechanical displacement to voltage. The quantity of interest is first detected and transduced to displacement by some form of mechanical element; then the electric element serves as the secondary transducer, transforming the analogous displacement into an analog voltage or current. The basic electrical change may be resistive, inductive, Capacitive, etc., from which the voltage or current change results.

The most commonly used principles of operation employed are

1. Variable resistance transducer elements.

2. Variable inductance transducer elements.

3. Variable capacitance transducer elements.

4. Piezoelectric transducers.

5. Photoelectric transducers.

ADVANTAGES OF ELECTRICAL TRANSDUCER ELEMENTS:

The following are the advantages of Electrical Transducer Elements

1. Amplification or attenuation may be easily obtained.

2. Mass-inertia effects are minimized.

3. The effects of friction are minimized.

4. An output with sufficient power for control may be provided.

5. Remote indication or recording is feasible.

6. The transducers are usually susceptible to miniaturization.

1. VARIABLE RESISTANCE TRANSDUCER:

The resistance of an electrical conductor varies according to the following relation.

R = L / A Where, R = Resistance in ohms,

L = length of conductor in cm

A = cross-sectional area of conductor, cm2

= resistivity of material, ohms-cm.

In variable resistance transducers, mechanical displacement is converted into electrical output, such as voltage or current. This is achieved by changing the value of L in the above equation.

A simple form of variable resistance transducer consists of a resistance element and sliding brush with guide [sliding contact resistive transducer].

SLIDING CONTACT RESISTIVE TRANSDUCER:

In a sliding contact resistive transducer as shown in figure [1], mechanical displacement is converted into electrical output, such as voltage or current. This is achieved by changing the value of L in the equation R = L / A

Where, R = Resistance in ohms,

L = length of conductor in cm

 

A = cross-sectional area of conductor, cm2

Fig-1 Variable resistance consisting of a wire and movable contractor or brush. This is often referred to as a slide wire.

SLIDER OR BUSH

GUIDE ROD

RESISTANCE WIRE

The brush always maintains contact with the resistance element. In operation, when mechanical displacement is sensed by the slides along the guide rod, which changes its contact position on the resistance wire, there by changing the effective length of the resistor. This in turn leads to a change in the electrical output, analogous to the displacement.

Resistance Potentiometer:

In practical applications, the resistance elements are wrapped around a bar, and the turns are insulated so as to prevent shorting. The contact brush slides from one turn to the next. Also, the arrangement can be either to obtain a rectilinear movement or an arc with angular movement [Fig-2]. The angular motion devices are commonly called as “resistance potentiometers” or simply “Pots.”

FIG-2 RESISTANCE POTENTIOMETER

When used as transducers, for a potentiometer to be very effective in measuring the input quantity, the two requirements are a high potentiometer resolution and linearity.

a) High Potentiometer Resolution: It is the smallest resistance variation that can be achieved by the given pot. For a uniformly wound pot, the resolution is equal to the ratio, one turn of the resistance wire to the total number of turns.

b) Potentiometer Linearity: A pot is said to be linear when the variation in resistance is directly proportional to the position of the contactor, i.e., the resistance measured is a direct function of the position of the contactor over the resistance element. However, in practice it is difficult to achieve a perfect linearity, hence, deviation limits are provided by calibration for better measurement results.

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER [LVDT]:

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

This is the most useful mutual inductance transducer which provides an a.c. voltage output proportional to the displacement of a core passing through the windings.

It is a mutual-inductance device having three coils mounted on a hollow concentric non-magnetic form. One winding, central one is primary and other two are secondary windings. The central coil is energized from an external a.c.power source and the two secondary coils are connected together in phase opposition and act as output side. The construction of an LVDT is as shown in figure 4-8b.

The centre coil (primary) is energised from an external a.c.supply and the two end coils are (secondary) connected together in opposite phases are used as pick up coils. The core passing through the primary and secondary coils provides a magnetic coupling.

In operation, an a.c.input is impressed on the primary coil, and an a.c.output voltage depending on the magnetic coupling between the core and the coils is obtained at the secondary coils. The magnetic coupling, hence the output depends upon the position of the core. Usually a linear range is specified for a differential transformer, and when the core is operated within this range, it is called as “Liner Variable Differential Transformer (LVDT)”.

•FIG: LINEAR VARIABLE DIFFERENTIAL TRANSFORMERFIG: LINEAR VARIABLE DIFFERENTIAL TRANSFORMER

Theoretically, there should be a core position for which the voltage induced in the secondary coils will be the same as that of the input and the resulting output should be zero. This position is called as “null position”, and is difficult to obtain in practice.

CHARACTERISTICS OF LVDT:

Typical differential-transformer characteristics are illustrated in Figure, which shows output versus core movement. Within limits, on either side of the null position, core displacement results in a proportional output. In general, the linear range is primarily dependent on the length of the secondary coils. While the output voltage magnitudes are ideally the same for equal core displacements on null balance, the phase relation existing between power source and output changes 180 through null. It is therefore possible, through phase determination or by use of a phase-sensitive circuit arrangement, to distinguish between outputs resulting from displacements on each side of null.

FIG: CHARACTERISTICS OF LVDT

ADVANTAGES OF LVDT:

1. It serves as a primary detector transducers, it converts mechanical displacement into a proportional electrical voltage, without the assistance of any elastic member.

2. It cannot be overloaded, since the core is completely separable from the device.

3.  It is insensitive to temperature changes.

4. It gives high output without any intermediate amplification

5. It is an economical device.

LIMITATIONS OF LVDT:

In the field of dynamic measurements LVDT has some limitations. They are:

1. When used with a strain gage, the core is of appreciable mass, compared to the bonded strain gage.

2.  It is designed for a specific input frequency.

3.  The device becomes complicated, if the direction from null is also required.

VARIABLE RELUCTANCE TRANSDUCERS:

If is a inductance transducer having a permanent magnet as the core material around which a coil is wound. Any variation of the permeance of the magnetic circuit causes a change in the flux. As the flux field expands or collapses, a voltage is developed in the coil, which is the output. Generally, such transducers are limited to dynamic applications.

PIEZOELECTRIC TRANSDUCERS:

Certain materials are capable of generating an electrical potential when they are subjected to mechanical deformations. Such materials also undergo the reverse process. i.e, they change their dimensions when an electrical potential is applied across. This phenomenon of generating electrical potential due to change in dimensions is known as ‘piezoelectric effect’. Examples of such materials are quartz, rochelle salt, properly polarized barium titanate, ordinary sugar, etc.

CHARACTERISTICS OF PIEZOELECTRIC MATERIALS:

None of the piezoelectric materials possesses all the desirable properties, such as stability, high output, insensitivity to temperature variations and humidity, and easy formability. The characters of important piezoelectric materials are discussed below.

The Element is heated to a temperature above the curie point of 120 degree centigrade and a high DC voltage is applied across

the faces of the element. The magnitude of the voltage depends on the thickness of the element. It is of the order of 10,000 V/cm.

The element is then cooled and with the voltage applied and results in the element exhibiting Piezo effect

1. ROCHELLE SALT: Among the available materials, Rochelle salt gives the highest output but needs protection from moisture in the air and limited to use up to 450 C only.

2.  QUARTZ: Quartz is the most stable material of all, but has a very low output. Because of its high stability, quartz is commonly used for stabilizing the electronic oscillators. Usually quartz is shaped into a thin disk, and silvered an either faces for attaching the electrodes. The disk thickness is so maintained that it provides a mechanically resonant frequency corresponding to the desired electrical frequency.

3. BARIUM TITANATE: While Rochelle salt and quartz are monocrystalline, there are some polycrystalline materials that are used to obtain piezoelectric effect. An    example is the barium titanate. The advantage of such a polycrystalline material is that it can be formed into any suitable size and shape.

PHOTOELECTRIC TRANSDUCERS:

The Photoelectric transducers use the light beam as the input source and convert the light beam into an analogous electric signal. Since they sense the light beam these are also called as Photosensors or Photocells. Based on their method of operation the photoelectric transducers can be classified into three types.

1.    Photoconductive

2.    Photovoltaic

3.    Photoemissive

1. PHOTOCONDUCTIVE TRANSDUCERS:

These use solid state semiconductor materials like selenium, or germanium or some metallic sulphides. The construction and principle of operation of a photoconductive transducer is shown in Figure. In this, a thin film of the semiconductor material is coated between electrodes on a glass plate. A d.c.potential is applied across the electrodes. This cell acts as a light beam falls on it, thereby increasing the current in the circuit. This increase is proportional to the intensity of the light beam and forms the output from the cell.

FIG:PHOTO CONDUCTIVE TRANSDUCER

1.    2.  PHOTOVOLTAIC TRANSDUCERS:

Photogenerative or photovoltaic transducer consists of a sandwich of unlike materials like a metal base, a semiconductor material and a transparent metal layer (such as a thin layer of iron selenide). When this cell is exposed to a light beam an electric potential is developed across the section [Fig-3]. The potential developed is proportional to the intensity of the light beam and forms the output of the cell.

1.    3.Photoemissive Transducers:

This is an electronic tube transducer. It consists of a cathode-anode combination housed in a glass or quartz tube. The tube is either evacuated or filled with an inert gas. In operation, a dc potential of 100-200 V is applied across the electrodes. When a light beam falls on the cathode, electrons flow towards the anode and complete the circuit thereby provide a small current in the external circuit (Figure-4). This current can be used as the output proportional to the intensity of the light beam.

Photoelactric transducers are used mainly for the measurement of light intensity, radiation of various wavelengths, counting, etc.

 

ELECTRONIC TRANSDUCERS:

These transducers convert mechanical displacement into an electric current. It is basically an electronic tube with certain moveable elements. An electronic transducer is shown in Figure.

ELECTRONIC TRANSDUCER

It consists of two plates mounted on a moveable arm which is extended through a flexible diaphragm at the end of the tube as shown in figure. In operation, a mechanical displacement applied at the external end of the arm causes the movement of the plates inside the tube. The plates which form a part of the electrode of the tube, when move, affect the characteristics of the tube, thereby providing an output. These transducers are used for the measurement of surface roughness, acceleration, force, pressure, etc.

 

IONIZATION TRANSDUCERS:

An Ionisation transducers consists of tow electrodes housed in a glass envelope, filled with a gas or gases under reduced pressure, an R-F generator and capacitor circuit.

A schematic arrangement of an ionisation transducer is shown in Figure. Here the radio-frequency (R-F) generator is used to ionize the gas in the tube by the filed from the two external electrodes. A space charge is created, furnishing a d.c. output signal (this depends on the configuration and the potential of the electrodes). A variation in either of the capacitance C1 and C2 of the circuit

tends to change the balance of the electric field, and thus produce an output.

In this circuit the capacitors serve as the secondary transducers. However, in the same circuit, if the tube is moved relative to the external electrodes, then also a d.c. potential results proportional to the displacement of the tube. This can be used as a primary transducer. Such transducers are used as pickups for the measurement of pressure, acceleration, displacement, humidity, etc.

FIG: IONIZATION TRANSDUCERS

EELCTROKINETIC TRANSDUCERS

The electrokinetic or the streaming potential phenomenon occurs when a polar liquid such as water, methanol, or acetonitrite is forced through a porous disk. When the liquid flows through the pores, a voltage is generated in phase with and proportional tot he differential pressure across the faces of the disk. When the direction of the flow is reversed, the polarity of the electrical signal is also reversed. The basic construction and principle of operation of such a transducer is illustrated in Figure. A typical cell, shown in Figure, consists of a porcelain disk glazed into the centre of an impermeable porcelain ring. Diaphragms tightly sealed on each side of the ring retain the polar liquid, which fills the space between the diaphragms. A wire mesh electrode is mounted on each side of the porous disk, with electric connections brought out via aluminium strings. The cell assembly is then clamped within a suitable housing

FIG: ELECTROKINETIC TRANSDUCER

The electrokinetic transducer is suitable only for measurement of small dynamic displacement, pressure and accelerations. The limitation of this type of transducers is that they are not suitable for static measurements.