1

TRANSDUCERS AND SENSORS

• A transducer is a device that convert one form of energy to other form. It converts the measurand to a usable electrical signal.

• In other word it is a device that is capable of converting the physical quantity into a proportional electrical quantity such as voltage or current.

2

BLOCK DIAGRAM OF TRANSDUCERS

• Transducer contains two parts that are closely related to each other i.e. the sensing element and transduction element.

• The sensing element is called as the sensor. It is device producing measurable response to change in physical conditions.

• The transduction element convert the sensor output to suitable electrical form.

3

Functions of Transducer1. To sense the presence, magnitude, change in, and frequency of measurand.

2. To provide an electrical output that, when appropriately processed and applied to readout device, gives accurate quantitative data about the measurand

Transducer Electrical outputMeasurand

Excitation

Measurand – refers to the quantity, property or condition which the transducer translates to an electrical signal.

4

Ruggedness. It should be capable of withstanding overload and some safety arrangement

should be provided for overload protection.

Linearity. Its input-output characteristics should be linear and it should produce these

characteristics in symmetrical way.

Repeatability. It should reproduce same output signal when the same input signal is applied

again and again under fixed environmental conditions e.g. temperature, pressure, humidity

etc.

CHARACTERISTICS OF TRANSDUCERS

5

• High Output Signal Quality. The quality of output signal should be good i.e. the

ratio of the signal to the noise should be high and the amplitude of the output

signal should be enough.

• High Reliability and Stability. It should give minimum error in measurement for

temperature variations, vibrations and other various changes in surroundings.

• Good Dynamic Response. Its output should be faithful to input when taken as a

function of time. The effect is analyzed as the frequency response.

6

• No Hysteretic. It should not give any hysteretic during measurement while input

signal is varied from its low value to high value and vice-versa.

• Residual Deformation. There should be no deformation on removal of local after

long period of application.

7

TRANSDUCERS SELECTION FACTORS

1. Operating Principle: The transducer are many times selected on the basis of operating principle used by them. The operating principle used may be resistive, inductive, capacitive , optoelectronic, piezo electric etc.

2. Sensitivity: The transducer must be sensitive enough to produce detectable output.

3. Operating Range: The transducer should maintain the range requirement and have a good resolution over the entire range.

4. Accuracy: High accuracy is assured.5. Cross sensitivity: It has to be taken into account when measuring mechanical quantities.

There are situation where the actual quantity is being measured is in one plane and the transducer is subjected to variation in another plan.

6. Errors: The transducer should maintain the expected input-output relationship as described by the transfer function so as to avoid errors.

8

7. Transient and frequency response : The transducer should meet the desired time domain

specification like peak overshoot, rise time, setting time and small dynamic error.

8. Loading Effects: The transducer should have a high input impedance and low output

impedance to avoid loading effects.

9. Environmental Compatibility: It should be assured that the transducer selected to work

under specified environmental conditions maintains its input- output relationship and does

not break down.

10. Insensitivity to unwanted signals: The transducer should be minimally sensitive to

unwanted signals and highly sensitive to desired signals.

9

CLASSIFICATION OF TRANSDUCERS

The transducers can be classified as:

1. Active and passive transducers.

2. Analog and digital transducers.

3. On the basis of transduction principle used.

4. Primary and secondary transducer

5. Transducers and inverse transducers.

10

Active transducers

These transducers do not need any external source of power for their operation. Therefore they

are also called as self generating type transducers.

1. The active transducer are self generating devices which operate under the energy conversion

principle.

2. As the output of active transducers we get an equivalent electrical output signal e.g.

temperature or strain to electric potential, without any external source of energy being used.

ACTIVE AND PASSIVE TRANSDUCERS

11

Passive Transducers

1. These transducers need external source of power for their operation. So they are not self

generating type transducers.

2. A DC power supply or an audio frequency generator is used as an external power source.

3. These transducers produce the output signal in the form of variation in resistance, capacitance,

inductance or some other electrical parameter in response to the quantity to be measured.

12

CLASSIFICATION OF ACTIVE TRANSDUCERS

13

CLASSIFICATION OF PASSIVE TRANSDUCERS

14

PRIMARY AND SECONDARY TRANSDUCERS

• Some transducers contain the mechanical as well as electrical device. The mechanical device converts the physical quantity to be measured into a mechanical signal. Such mechanical device are called as the primary transducers, because they deal with the physical quantity to be measured.

•The electrical device then convert this mechanical signal into a corresponding electrical signal. Such electrical device are known as secondary transducers.

15

CLASSIFICATION OF TRANSDUCERSAccording to Transduction Principle

16

CLASSIFICATION OF TRANSDUCERSTransducer and Inverse Transducer

TRANSDUCER:

•Transducers convert non electrical quantity to electrical quantity.

INVERSE TRANSDUCER:

• Inverse transducers convert electrical quantity to a non electrical quantity

17

Liquid Crystal Display• A Liquid Crystal Display (LCD) is a thin , flat panel used for electronically

displaying information such as text ,images and moving picture.

• Its uses include monitor for Computers, Televisions , Instrument panels Gaming devices etc.

• Using polarization of lights to display objects.

18

Different types of LCDs

• Passive Matrix LCDs (AMLCD) and Active Matrix LCDs (AMLCD)

• Passive Twisted Nematic Displays (TNLCD)

• Super Twisted nematic LCD (STNLCD)

• Thin Film Transistor LCD (TFT LCD)

• Reflective LCD

• Rear Projection LCD

19

Liquid Crystal Display

Solid

Liquid crystal fourth state of matter

Liquid

Gas

20

Liquid Crystal – a stable phase of matter characterized by anisotropic properties without the existence of a 3-dimensional crystal lattice – generally lying between the solid and isotropic (“liquid”) phase.• light passes through liquid crystal changes when

it is stimulated by an electrical charge.

What is a Liquid Crystal?

21

Isotropic

vs.

Anisotropic

Liquids and gases(uniform properties in all

directions).

Liquid Crystals have orientational order

22

Nematic, Smectic & Cholesteric

Anisotrpic substances may go through one or several Liquid Crystal Phases

Liquid Crystal Phases

23

Liquid Crystal Displays• Consists of an array of tiny segments (called pixels) that can be manipulated to present

information.

• Using polarization of lights to display objects.

• Use only ambient light to illuminate the display.

• Common wrist watch and pocket calculator to an advanced VGA computer screen

• A typical nematic liquid crystal produces a 90- degree shift in the polarization of the light passing through when there is no electric field present.

• When a voltage is applied, an electric field is produced in the liquid, affecting the orientation of the molecules. This causes the polarization shift to be reduced.

24

Polarization

25

Polarization of light

• When unpolarized light passes through a polarizing filter, only one plane of polarization is transmitted.

Two polarizing filters used together transmit light differently depending on their relative orientation.

Online Offline

26

LCD WORKING PRINCIPAL

27

LCD WORKING

28

LCD WORKING

• TFT Glass has as many TFTs as the number of pixels displayed.

• A Color Filter Glass has color filter which generates color.

• Diffuser distributing the light evenly across the viewing area

• Liquid crystals move according to the difference in voltage between the Color Filter Glass and the TFT Glass.

• The amount of light supplied by Back Light is determined by the amount of movement of the liquid crystals in such a way as to generate color.

29

LCD WORKING

30

LCD WORKING

• With no voltage applied across the pixel, the LC molecules twist to align to the rubbing of the glass plates. Light entering the first polarizer is twisted and can exit the second polarizer --> pixel is ON

• With a voltage applied across the pixel, the LC molecules untwist to align with the electric field. Light entering the first polarizer cannot exit the second polarizer --> pixel is OFF.

31

LCD WORKING

32

BACKLIGHT REFLECTIVE

33

Reflective And Backlight Display• Liquid Crystal material emit no light of their own. For illumination of light -

backlight and reflective method used.• Reflective- Use external light reflected by a reflector behind the display.

Example: watch, calculatorThis is achieved by combining a reflector with rear polarizer

• Backlight- : light source is from a backlight, and viewed from the front. Example: Computer display

built in fluorescent tubes above ,besides and sometimes behind the LCD.

34

PASSIVE MATRIX DISPLAY ACTIVE MATRIX DISPLAY

35

• Uses a grid of vertical and horizontal conductors comprised of Indium Tin Oxide (ITO) to create an image

• There is no switching device.• Pixels are addressed one at a time by row and column matrix• Only used in low-resolution displays (such as watch, calculator)• Slow response time, poor contrast

PASSIVE MATRIX DISPLAY

36

Passive Matrix Display Problems

• Crosstalk occurs when neighboring pixel voltages affect each other, reducing the gray scale, contrast, and viewing angle.

• Submarining occurs when slow-to-respond LC materials cannot respond quickly enough and the picture can disappear temporarily.

• One Solution: placing a switch at each pixel, such as a transistor or diode --> pixel matrix becomes “active.”

37

Active Matrix Liquid Crystal Display

• It is based on Thin Film Transistor (TFT) Technology• Switching element at each pixel. Individual pixels isolated

from each other. Thin Film Transistors most commonly used. • Each row line is activated sequentially• Used in computer displays• A switching device and a storage capacitor are integrated at

the each cross point of the electrodes

38

Many Passive Display Problems Eliminated

• Pixel isolation eliminates crosstalk • Isolation from the column line permits the pixel

capacitor to remain charged, so that faster responding liquid crystals can be used.

• Due to switching action of transistors,only the desired pixel recieve a charge ,improving image quality over a passive matrix.

39

Advantage of Active Matrix Display

• Higher sizes• Higher contrast• Higher gray scale• Higher resolution• Higher viewing angle• Faster response. Eliminates “ghosting”• Better control of the color

40

Color Display

Each pixel has three sub-pixels: Red, Green and Blue.

Different intensities of the sub-pixels give perception of whole spectrum of colors.

If each red ,green and blue sub-pixel can display 256 different intensity of their respective color , then each pixel can produce a possible palette of 16.8 million (256*256*256) colors.

41

Advantage Of LCD Over CRT

• Smaller size —LCDs occupy approximately 60 percent less space than CRT displays an important feature when office space is limited.

• Lower power consumption—LCDs typically consume about half the power and emit much less heat than CRT displays.

• Lighter weight —LCDs weigh approximately 70 percent less than CRT displays of comparable size.

• No electromagnetic fields —LCDs do not emit electromagnetic fields and are not susceptible to them. Thus, they are suitable for use in areas where CRTs cannot be used.

• Longer life —LCDs have a longer useful life than CRTs.

42

ApplicationsA) Thin Film Transistor (TFT) • Constructed on a glass surface using a photolithographic process.

• The source and gate are the control electrodes. The drain electrode connects to the liquid crystal pixel. The thin layer of amorphous silicon is the semiconducting material that allows the TFT to function. The capacitor is attached to the pixel electrode, but is not an integral part of the TFT.

43

B) Alpha-numeric display

• Digital letters can be displayed by blocking the lights in different plates we place.

• For applications such as digital watches and calculators, a mirror is used under the bottom polarizer. With no voltage applied, ambient light passes through the cell, reflects off the mirror, reverses its path, and re-emerges from the top of the cell, giving it a silvery appearance.

• When the electric field is on, the aligned LC molecules do not affect the polarization of the light. The analyzer prevents the incident light from reaching the mirror and no light is reflected, causing the cell to be dark. When the electrodes are shaped in the form of segments of numbers and letters they can be turned on and off to form an alpha-numeric display.

44

C) Back lighting systems

• Alpha-numeric displays are not very bright because the light must pass through multiple

polarizers which severely cut down on the intensity of the light, in addition to the various

layers of the display which are only semi-transparent.  Therefore a more intense source is

employed in the form of a back lighting system.

For brighter displays

• Light bulbs mounted behind

• At the edges of the display replace the reflected ambient light.

• Disadvantage : very power intensive.  Back lighting systems are used in more complex displays

such as laptop computer screens, monitors, LCD projectors, pda, digital devices such as

digital camera and DV.