UNIT-III

TRANSDUCERS

BY

Dr.L.RAJA MOHAN REDDY

Lecturer in Physics

GDC, Rajampeta

COMMOSSIONERATE OF COLLEGIATE EDUCATION

Contents:

• Introduction
• Classification of transducers
• Selection of transducers
• Displacement transducer
• Resistive transducer
• Capacitive transducer
• inductive transducer
• LVDT
• Piezoelectric transducer

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INTRODUCTION:

A transducer is an element which converts one form of energy into another. The input measurement quantities for most instrumentation systems are non-electrical. The non-electrical quantity is converted to an electrical signal by a device called transducer and is used for measurement and control. In general, the energy transmission by a transducer may be mechanical, electrical, magnetic, thermal, radiant or chemical. An instrumentation system generally consists of three components, namely, an input device, a signal conditioning device and an output device as shown in Fig.1. The input device with transducer in it, senses the quantity under measurement and changes it to a proportional electrical signal. In the signal conditioning device, the signal is amplified, filtered or otherwise modified to a level accepted to the output device. The output device may be an indicator or a storage medium.

Fig. .1

Classification of Transducers:

There are two basic types of transducers, namely, active transducers and passive transducers

Transducers:

Active Transducer

Passive Transducer

Thermocouple

Moving coil generator

Photovoltaic cells

Piezo electric pickup

Potentiometer

Resistance thermometer

Strain gauge

Photo conductive cell

Thermistor

Capacitor transducer

Dielectric transducer

Magnetic transducer

Photo emissive cells

Types of Active Transducers

1. Thermocouple: In a thermocouple an emf is generated across the junction of two dissimilar metals, when the junction is heated. Typical applications are: measurement of temperature, heat flow and radiation..

2. Moving coil generator: Here voltage is generated when a coil is rotated in a magnetic field. This is used in measuring velocity and vibration.

3. Photovoltaic cells: In this case an emf is generated in a semiconductor junction device, when radiant energy stimulates the cell. Typical applications are in light meters and solar cells..

4. Piezo electric pickup: Here an emf is generated when an external force is applied across a crystalline material. Typical applications are vibration, acceleration and sound measurement devices.

Types of Passive Transducers

1. Potentiometer: In the potentiometer the contact position of the slide wire is varied by an external force causing the change in resistance. These are applied in measuring pressure, displacement, etc.

2. Resistance thermometer: Here the resistance of the metal varies with the temperature. It is used in temperature and radiant heat measurements.

3. Strain gauge: In this type, resistance of a coil wire changes when external stress is applied. They are used in measuring instruments of stress, strain, force, torque, displacement.

4. Photo conductive cell: Here the resistance of the cell varies with incident light. They are used in photosensitive relays.

5. Thermistor: A thermistor is a non-metallic resistance (semi-conductor of the ceramic material) having a negative coefficient which varies with temperature. This is used in temperature measurement.

6. Resistance hygrometer: In this case, the resistance of the conductive strip changes. with moisture content. It is used in measuring the relative humidity.

7. Capacitor transducer: Here, variation of capacitance is produced by varying the distance between the plates by an external force. It is used in measuring pressure, displacement, etc.

8. Capacitor microphone: In the capacitor microphone, the capacitance between the fixed plates and movable diaphragm varies due to the sound pressure. It is used in recognizing noise and speech.

9. Dielectric transducer: Here variations in dielectric changes the capacitance. It is used in measuring the liquid level and thickness of a material.

10. Differential transformer: Here the differential voltage of the two secondary winding of a transformer is varied by varying the position of the magnetic core by an external force. This is also used in measuring the pressure, displacement, etc.

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11. Reluctance pickup: Here, reluctance of the magnetic circuit is varied by changing the position of the iron core of a coil. It is used in measuring displacement, position etc.

12. Magnetic transducer: Here, inductance of the alternating-current excited coil is varied by changing the magnetic circuit. It is also used in measuring the displacement.

13. Hall effect pickup: In this type, voltage is generated across a semiconductor plate, when magnetic flux interacts with an applied current. It is used to measure the magnetic flux and current.

14. Photo emissive cells: Here, electron emission takes place due to radiations on photosensitive surface. It is used in measuring the radiation and light.

BASIC REQUIREMENTS OF TRANSDUCERS

1. Range: The transducer must be operable over the minimum to maximum values of the parameters to be measured.

2. Accuracy: It is the conformity of an indicated value to an accepted value. A

transducer should be accurate.

3. Repeatability: The closeness of the output value, among a number of consecutive measurements for the same value of the input is called repeatability. A transducer should have better repeatability.

4. Sensitivity: The ratio of change in output to the change in input should be high enough for the transducer for better-resolution of the system.

5. Environmental effects: The influence of temperature, acceleration, shock, vibration and corrosion should not affect the performance of a transducer.

6. Noise: The output signal from the transducer should not be affected by noise.

7. Loading effect: It is determined by the mass exterior size, geometric configuration of the transducer. All transducers absorb some energy because of these factors. So the loading effect should be less.

8. Output impedance: This shuld be matched with the rest of the measuring systems.

9. Power requirements: Proper voltage should be applied for the externally excited transducers.

10. Frequency response: The transducer system must have an accurate response to the maximum rate of change of the system. The change of output with input fre- quency should be flat over the measurement range.

11. Linearity: The input and output characteristics of a transducer should be linear.

12. Ruggedness: The transducer should be rugged enough to withstand the over- loads.

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DISPLACEMENT TRANSDUCERS

The displacement transducers make use of both primary and secondary transducers. Here the primary transducers are also called force summing devices. They convert the mechanical force into displacement with the help of mechanical elements like diaphragm, bellows, bourdon tube and single or double suspension cantilever etc., as shown in Fig. 2.

Bollowo

​

Force

�

​

Force

Pressure bourdon tube

Cantilovor

Mass

Mass Cantilovor

Diaphragm

Fig. 2.

The displacement activated by the force-summing devices is converted into electrical signals, using secondary transducers like resistivetransducer, potentiometer, resistance strain gauge, capacitive transducers, LVDT, piezo-electric and photoelectric etc.

Thus, the above-mentioned transducers fall under the category of displacement transducers.

Resistive transducer

Here a resistive element is used to measure several non-electrical quantities. The resistance of a material is given by

where R is the resistance in ohms

p is the specific resistance in ohm-metre

l is the length of the material in metres

A is the area of cross section in square metres.

By causing change in displacement (linear or angular), pressure (strain) and temperature, we can change the values 'l', 'A' and 'p' respectively. Potentiometer, resistance strain gauge, resistance thermometer and thermistor are some of the examples of resistance transducers.

• Thermistor is a resistive transducer whose resistivity depends upon surrounding temperature. For this reason it can be used as Temperature sensor.
• The term Thermistor is a combination of “thermal” and “resistor”
• It is made up of semiconductor material. Thermistor devices are generally made from oxides of certain metals like Manganese, Cobalt & Nickel etc.
• There are two types of thermistors: Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC).

Thermistor:

• NTC Type:

In this type when temperature increases, resistance decreases. Similarly, when temperature decreases, resistance increases. This type of thermistor is used the most.

• PTC Type:

In this type when temperature increases, the resistance increases, and when temperature decreases, resistance decreases.

• Working Principle:
• As the temperature of a thermistor increases its resistance decreases exponentially.
• The mathematical expression for the relationship between resistance of thermistor and temperature is

Where,

𝑅_𝑇1= resistance of the thermistor at temperature T1

𝑅_𝑇2 = resistance of the thermistor at given temperature T2

β = constant, its value depends upon the material used in the construction of thermistor, typically its value ranges from 3500 to 4500.

This above equation is known as characteristic equation of Thermistor

Advantages

• They are compact and inexpensive.
• They have good stability and high sensitivity.
• Their response is very fast.
• They are not affected by stray magnetic and electric fields.
• Due to all these advantages, thermistors are preferred over other temperature detecting devices like RTDs and thermocouples.

Potentiometer (POT):

It is a passive transducer with a thin wire of platinum or nickel alloy of 0.01mm diameter carefully wound on an insulated former. The resistive element can be energised by either d.c or a.c. voltage. It has a sliding contact called wiper whose motion is either linear or rotary. The POT for linear motion and for the rotating motion is as shown in the Fig. 3

(a) Linear POT

(b) Rotary POT

Input voltage in volts

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Output voltage in volts

Total length in metres

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Displacement of wiper in metres

q= Angular displacement in degrees.

Resistance thermometer:

It is a primary electrical transducer used for measuring temperature changes in terms of resistance changes. A metal or an alloy is used as a resistive element. The specific resistance of the metal increases with increase in temperature. As the temperature changes, the length and specific resistance changes and hence the change in resistance. Platinum, nickel, copper and tungsten are the commonly used metals for the wire which are wound on mica or ceramic former as shown in the Fig. 4.

The fluid whose temperature is to be measured is brought in direct contact with open element. The drawback with this type is that (l) if the fluid is in motion, it will cause error in measurement. (2) If the fluid is corrosive it will spoil the element. This could be avoided by enclosing the elements in a protective tube of pyrex glass or porcelain or quartz or nickel.

R_T = R₀ (1 +αT)

If R₀ is the resistance at 0 °C, then the resistance R_T at T °C is

Where, α = temperature coefficient of resistance of a particular material.

Capacitive transducer

The transducer whose capacitance changes with respect to change in input parameter is known as capacitance transducer.

• The working principle of a capacitive transducer is variable capacitance. It consists of two parallel metal pates which are separated by dielectric medium (such as air).

• The capacitance of the variable capacitor can be measured by this formula.

C = capacitance of the variable capacitor , 𝝐_𝟎 = permittivity of free space =8.854 x 10^-12 farads/metre, 𝝐_𝒓 = relative permittivity , 𝝐 =𝜖₀𝜖_𝑟

A = overlapping area between the two plates, d = distance between the two plates

By varying the parameters like A, d & 𝝐_𝑟 of the variable capacitor the capacitance can

be changed.

So the capacitive transducer is of three types:

• Variable Area(A) Capacitive Transducer
• Variable distance between two plates (d) type capacitive Transducer
• Variable dielectric constant (𝝐 ) type capacitive Transducers

Variable Area Capacitive Transducer:

• In this type capacitive transducer the overlapping area (A) between the two plates changes due to the application of Displacement, Force or Pressure.
• Since parameter ‘A’ changes, the capacitance ‘C’ also changes, as ‘C’ is directly proportional to ‘A’.

𝑪

=

𝝐

(

−

wx

𝒅

A

)

Where, ‘x’ is the displacement of the plate and ‘w’ is the width of the plate

• It can be used as Displacement, Force or Pressure sensors.

Variable distance between two plates type capacitive Transducers

• In this type capacitive transducer the distance (d) / separation between the two plates changes due to the application of Displacement, Force or Pressure.

• Since parameter ‘d’ changes, the capacitance ‘C’ also changes, as ‘C’ is inversely proportional to ‘d’.

Where, ‘x’ is the displacement of the plate

It can be used as Displacement, Force or Pressure sensors.

Variable dielectric capacitive transducer

Here a solid dielectric cylinder connected to the actuator is moved between the plates of a co-axial cylindrical capacitor as shown in Fig.

Actuator

Solid dielectric cylinder

Plates of coaxial capacitor

Inductive transducer:

The transducer whose inductance changes with respect to change in input parameter is known as inductive transducer. It is used to measure quantities such as pressure, acceleration, force and displacement or position. The inductance of a coil depends on the magnetic flux linking with it. A change in the inductance proportional to the quantity to be measured is obtained by changing the flux linkage. A bridge or an oscillatory circuit can be used to measure the change in inductance. The variation of inductance (flux linkages) can be obtained by the movement of an armature as shown in Fig.

Linear variable differential transformer (LVDT):

• The Linear Variable Inductive Transformer converts the linear displacement into an electrical signal.
• It works on the principle of mutual induction, i.e., the flux of the primary winding is induced to the secondary winding. The output of the transformer is obtained because of the difference of the secondary voltages, and hence it is called a differential transformer.

Construction of LVDT:

• The basic construction of the LVDT is shown above in the figure. LVDT consist of a primary winding and two secondary windings S₁ and S₂. The secondary winding is wound on the cylindrical former.
• The secondary windings have an equal number of turns, and it is placed identically on both the side of the primary winding.
• The output voltage of the secondary winding S₁ is ES₁ and that of the S₂ is ES₂.
• The secondary voltage signal is converted into an electrical signal by connecting the secondary winding in series opposition as shown in the figure above.
• The output voltage of the transducer is determined by subtracting the voltage of the secondary windings.

Output voltage (E₀) = ES₁ - ES₂

Working:

The change in output voltage is directly proportional to the displacement of the core. Any displacement will increase the flux of one of the secondary winding and on the other hand, reduces the other which develops a differential voltage at the output. There could be three possible conditions which are described below:

Condition-I:

• When the soft core moved towards left, the flux linked in S₁ is more as compared to S₂.
• The output voltage of the winding S₁ is more than the S₂.
• Since ES₁ >ES₂, E₀ is positive. So E₀ is in phase with the primary voltage.

Condition-II:

• When the soft iron core move towards right the magnitude of the flux linked S₂ is more than S₁.
• The output voltage of the winding S₁ is less than the S₂.
• Since ES₁ <ES₂, E₀ is negative. The output voltage E₀ is 180º out of phase with the primary winding.

Condition-III:

• When the soft iron core is at the centre of S₁ and S_2, the flux linked in S₁ and S₂ are same.
• The output voltage of the winding S₁ is equal to S₂.
• So E₀= ES₁ -ES₂ =0 V

Fig. LVDT Characteristics

The curve between the output voltage and the input displacement is shown in the figure.

The curve is linear for small displacement between A & B.

• Uses of LVDT:
• It is used for measuring the displacement having a range from few mm to cm. The LVDT directly converts the displacement into an electrical signal.
• The LVDT is used as a device for measuring the force, weight and pressure. Some of the LVDT used for measuring the load and pressure.

Piezoelectric Transducers

• A piezoelectric transducer is a device which can convert mechanical energy like Force or Pressure into an electrical energy.
• It uses piezoelectric effect for the generation of electric charge.
• It is an active transducer.

Construction and Working:

• Piezoelectric materials like Quartz, Rochelle salt, ammonium dihydrogen phosphate, dipotassium tartarate, potassium dihydrogen phosphate, lithium sulphate etc can be used to make the transducer.

• The faces of piezoelectric material, usual quartz, is coated with a thin layer of conducting material such as silver known as Electrode.
• When stress is applied, the ions in the material move towards one of the conducting surface while moving away from the other. This results in the generation of charge.
• This charge is used for calibration of stress. The polarity of the produced charge depends upon the direction of the applied stress/ force.
• If F is the applied force and Q is the charge developed due to it then Q ∝ F.

⇒Q = d x F

Where, d is known as piezoelectric coefficient of the material.

• Due to the charge Q, potential difference V_o developed between the electrodes which can be taken as output.
• This transducer is used as Force, Pressure or Stress sensor.

Touch Sensors

Touch sensors are also called as tactile sensors and are sensitive to touch, force or pressure. They are one of the simplest and useful sensors. The working of a touch sensor is similar to that of a simple switch. When there is contact with the surface of the touch sensor, the circuit is closed inside the sensor and there is a flow of current. When the contact is released, the circuit is opened and no current flows.

The pictorial representation of working of a touch sensor is shown below.

Capacitive Touch Sensor

Capacitive touch sensors are widely used in most of the portable devices like mobile phones and MP3 players. Capacitive touch sensors can be found even in home appliances, automotive and industrial applications. The reasons for this development are durability, robustness, attractive product design and cost.

The principle of a capacitive touch sensor:

The simplest form of capacitor can be made with two conductors separated by an insulator. Metal plates can be considered as conductors. The formula of capacitance is shown below.

C = ε₀ * ε_r * A / d

Where

ε₀ is the permittivity of free space

ε_r is relative permittivity or dielectric constant

A is area of the plates and d is the distance between them.

Capacitance is directly proportional to the area and inversely proportional to the distance.

In capacitive touch sensors, the electrode represents one of the plates of the capacitor. The second plate is represented by two objects: one is the environment of the sensor electrode which forms parasitic capacitor C₀ and the other is a conductive object like human finger which forms touch capacitor C_T. The sensor electrode is connected to a measurement circuit and the capacitance is measured periodically. The output capacitance will increase if a conductive object touches or approaches the sensor electrode. The measurement circuit will detect the change in the capacitance and converts it into a trigger signal.

The working of a capacitive touch sensor is shown in below figure.

If the area of the sensor electrode is bigger and the thickness of the cover material is less, the touch capacitance C_T is also big. As a result, the capacitance difference between touching pad and untouched sensor pad is also big. This means that the size of the sensor electrode and covering material will influence the sensitivity of the sensor.

The measurement of capacitance is used in many applications like determining distance, pressure, acceleration, amplitude modulation, frequency modulation, time delay measurement, duty cycle, etc. The method of measurement of capacitance in touch sensors requires a reference plane located near by the sensing pad. In capacitive touch sensors, a finger trip forms the capacitance between the sensing electrode and reference plane. The skin oils or sweat from human body may cause a false trigger.

There are two types of capacitive touch sensors: surface capacitive sensing and projected capacitive sensing.

In surface capacitive sensing, an insulator is applied with a conductive coating on one side of its surface. On top of this conductive coating, a thin layer of insulator is applied. Current is applied to all the corners of the conductive coating. When an external conductor like a human finger comes in contact with the surface, a capacitance is formed between them and draws more current from the corners. The current at each corner is measured and their ratio will determine the position of the touch on the surface.

In projected capacitive sensing, the whole surface is not charged, but an X – Y grid of conductive material is placed between two insulating materials. The grid is often made of Copper or Gold on a PCB or Indium Tin Oxide on glass. An IC is used to charge and monitor the grid.

When a charge is pulled by external conducting object like a finger(s) from an area on the grid, the IC calculates the location of the finger on the touch surface. Touch sensors, made of projective capacitive technology can be used to sense a finger that is not touching its surface. They act as near proximity sensors.

Capacitive screens are typically used in most consumer products like tablets, laptops, and smartphones.

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Advantages of Capacitive touch screen Technology:

• Multi touch can be implemented in this capacitive touchscreen.
• Capacitive touch screen technology is moderately durable.
• Capacitive touch screens are easy to clean.
• Faster typing speeds can be achieved using this capacitive touch screen technology.
• The capacitive touch screen transmits about 90% of light from the display monitor.
• Capacitive touch screen is responsive than the other resistive touch.
• In capacitive touch screen, we can easily touch the screen without much pressure

Disadvantages of Capacitive touch screen technology:

• Capacitive touch screen are less accurate than the others.
• The capacitive touch screen does not respond to any other objects instead of human finger or skin. So we can’t work with a gloved hand or a stylus
• Capacitive touch screen are more expensive than others.
• Capasitive touch screen can be easily used without any pointed objects.

ResistiveTouch Sensor

The resistive touch screen resides of mainly two layers - a flexible top layer and a rigid bottom layer. The flexible upper layer is made with Polyethylene and the second or bottom layer is with glass. A conductive coating made up of Indium Tin Oxide (ITO) is provided for both the layers and also spacers are used for spacing. As long as the monitor is functional, an electric current flow occurs across the screen’s two layers. When a touch is made on the screen surface, the flexible layer push down and touches the glass (bottom) layer. Due to this, a variation in the electric field is detected. The coordinates of touching point is evaluated by controller section. When the coordinates are analyzed, the corresponding driver interprets the touch in to readable indications for the operating system to understand and react accordingly.                         

The Resistive Touchscreen transmits only 75% of light from the display monitor. The resistive touch screen further more split in to 4 wired, 5 wired, 6 wired, 7 wired and 8 wired resistive touch screen. The design and construction of all the types of resistive touch screen technologies are similar, but the method of determining the co-ordinates of touch is different from each other.

1. Four-wire Resistive Touch screen:

The four wire resistive touch screen technology is the simplest one regarding the others. In this four wire resistive touch screen, both the upper and lower layers are used to determine the axes locations (X and Y coordinate) of the touch. By developing a voltage gradient on the flexible layer, we can easily determine the X coordinate point of the touch. Similarly the Y coordinate point can be evaluated by developing a voltage gradient on the second layer.

Five-wire Resistive Touch screen:

For calculating the touch coordinates in the five wire resistive touch screen, it uses only the stable glass layer instead of the flexible upper layer. The lower glass layer is equipped with all the touch position sensing capabilities. In this five-wire resistive touch screen design system, one wire proceeded to the flexible layer and the other four wires are expanded to the four of the bottom or the second layer. The flexible layer operates as a voltage calibrating probe.

3. The Six and Seven wire resistive touch screen:

To improve the overall performance of the six-wire resistive touch screen technology, an additional ground layer is included behind the glass layer. But in the seven-wire resistive touch screen technology, there are two sense lines provided on the bottom layer.

4. Eight-wire Resistive Touch screen:

The Eight-wire Resistive Touchscreen is an alternative for four-wire Resistive Touch screen with an addition of four sensing points. These four sensing points are used to decrease the drift created by the changes in the environment and also to provide stabilization to the system. These Eight-wire Resistive Touchscreen systems employed in 10.4” sizes or larger sizes according to the significance of the drift.

Fig: Layers of Resistive Touch Screen  and Resistive touch screen panel

Most devices with resistive screens are used in manufacturing, ATMs and kiosks, and medical devices. This is because in most industries the users need to wear gloves when using the touchscreens.                                                       

Advantages of resistive touch screen technology:

• Resistive touch screen provides typically high resolution around 4096 x 4096 DPI.
• Resistive touch screen provides accurate touch controls.
• In Resistive touch screen, contact is made with fingers, any other pointed devices such as stylus or gloved fingers.
• The Resistive touch screen technology can be made to support multi touch inputs.
• Resistive touch screen technology offers relatively low cost.
• Resistive touch screen provides high resistance to water and dust.
• For handwriting recognition, Resistive touch screen is the best suited technology.

Disadvantages of Resistive Touchscreen technology:

• In Resistive Touchscreen, the finger touching required little pressure since the Resistive Touchscreen is not too sensitive.
• Relatively the Resistive Touchscreen may be more prone to damage.
• The Resistive Touchscreen transmits only 75% of light from the display monitor
• Resistive touch screen can be easily damaged by poking, scratching and by any impact.
• A periodic recalibration is required for the resistive touch screens.

REVIEW QUESTIONS

1. What is meant by a transducer? How they are classified?

2. What is meant by an active transducer? Give example.

3. What is meant by a passive transducer? Give example.

4. What are the basic requirements of a transducer?

5. What is meant by forced running device? Explain with neat sketch.

6. What is a displacement transducer? Explain with example.

7. Explain the working principle of Resistive transducer.

8. Explain the working principle of Capacitive transducer.

9. Explain the working principle of LVDT.

10. Explain the working principle of Piezo-electric transducer.

11. Explain the working principle of Capacitive touch screen Technology.

12. Explain the working principle of Resistive touch screen Technology.

TWO MARKS QUESTIONS AND ANSWERS

1. What is a transducer ?

The broader definition of transducer is that, it is an element which converts one form of energy into another.

2. What are two basic types of a transducer ?

1. Active transducer

2. Passive transducer

3. What is an active transducer?

The active or self generating transducer converts energy directly from one state to another without the need for an external power source or excitation. For example, thermocouple.

4. What is a passive Transducer?

A passive transducer does not convert energy directly but derives the power re- quired for energy conversion from an external source. For example, potentiometer.

5. What is an LVDT?

Linear variable differential transformer is a magnetic displacement transducer. The magnitude of voltage induced in secondary would be the indicative of the amount of displacement that had taken place.

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6. Give four examples for active transducers.

1. Thermocouple

2. Moving coil generator

3. Photovoltaic cells

4. Piezo-electric pick up

7. Give four examples for passive transducers.

1. Potentiometer

2. Strain gauge

3. Photoconductive cell

4. Thermistor

8. List four basic requirements of transducers.

1. Range: Must be operable over minimum to maximum values of the parameter to be measured.

2. Accuracy: Confirmity of indicated value to an accepted value.

3. Repeatability: The closeness of the output value among a number of consecutive measurements.

4. Sensitivity: Should be high enough

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