Different Types of Transducers and Displays

1.INTRODUCTION

A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. A thermocouple converts temperature to an output voltage which can be read by a voltmeter. For accuracy, all sensors need to be calibrated against known standards. Sensors are used everywhere such as touch sensitive mobile phones, laptop's touch pad, touch controller light, etc. Examples of daily applications are in the field of medicine, machines, cars, aerospace, robotics and manufacturing plants. Sensors are selected based on the requirement and the sensitivity. The sensitivity of the sensors is the change of sensor's output when the measured quantity changes. To measure very small changes, the sensors should have very high sensitivity.

A transducer is a device, usually electrical, electronic, electro-mechanical, electromagnetic, photonic, or photovoltaic that converts one type of energy or physical attribute to another (generally electrical or mechanical) for various measurement purposes including measurement or information transfer (for example, pressure sensors). The term transducer is commonly used in two senses; the sensor, used to detect a parameter in one form and report it in another (usually an electrical or digital signal), and the audio loudspeaker, which converts electrical voltage variations representing music or speech to mechanical cone vibration and hence vibrates air molecules creating sound.

2.CHARACTERISTICS OF A TRANSDUCER

The main objective of a transducer is to react only for the measurement under specified limits for which it is designed. It is, therefore, necessary to know the relationship between the input and output quantities and it should be fixed. A transducer should have the following basic requirements:

2.1. Linearity : Its input vs output characteristics should be linear and it should produce these characteristics in balanced way.

2.2. Ruggedness: A transducer should be capable of withstanding overload and some safety arrangements must be provided with it for overload protection.

2.3. Repeatability :The device should reproduce the same output signal when the same input signal is applied again and again under unchanged environmental conditions, e.g., temperature, pressure, humidity, etc.

2.4. High Reliability and Stability: The transducer should give minimum error in measurement for temperature variations, vibrations and other various changes in surroundings.

2.5. 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.

2.6. No Hysteresis :It should not give any hysteresis during measurement while input signal is varied from its low value to high value and vice versa.

2.7. Residual Reformation: There should not be any deformation on removal of input signal after long period of use.

3.TYPICAL SENSOR APPLICATIONS:

4.SELECTION OF TRANSDUCER

Following are the factors which need to be considered while selecting a transducer

High input impedance and low output impedance, to avoid loading effect.
Good resolution over is entire selected range.
Highly sensitive to desired signal and insensitive to unwanted signal.
Preferably small in size.
High degree of accuracy and repeatability.
Selected transducer must be free from errors.

5.DIFFERENT TYPES OF TRANSDUCERS

5.1.Strain Gauges

The strain gauge is an electrical transducer; it is used to measure mechanical surface tension. Strain gauge can detect and convert force or small mechanical displacement into electrical signals. On the application of force a metal conductor is stretched or compressed, its resistance changes owing to the fact both length and diameter of conductor change. Also, there is a change on the value of resistivity of the conductor when it is strained and this property of the metal is called piezoresistive effect. Therefore, resistance strain gauges are also known as piezoresistive gauges. The strain gauges are used for measurement of strain and associated stress in experimental stress analysis. Secondly, many other detectors and transducers, for example the load cell, torque meter, flow meter, accelerometer employ strain gauge as a secondary transducer.

Strain is expressed as the ratio of total deformation to the initial dimension of the material body in which the forces are being applied. Mathematically, strain,

where L = the original length of the material body

l= Final length and

ΔL = Change in length.

A strain gauge is a device used to measure the strain of an object. It consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.

The most popular electrical elements used in force measurements include the resistance strain gage, the semiconductor strain gage, and piezoelectric transducers. The strain gage measures force indirectly by measuring the deflection it produces in a calibrated carrier. Pressure can be converted into a force using an appropriate transducer, and strain gage techniques can then be used to measure pressure. Flow rates can be measured using differential pressure measurements which also make use of strain gage technology.

The resistance of the conductor is proportional to its length and inversely proportional to its area of cross-section. The resistance of the gauge increase with positive strain.

Let us consider a strain gauge made of circular wire. The wire has a resistivity p, the resistance of untrained gauge,

where L = length of conductor

A = area of conductor

Lets the tensile stress is applied to the wire. The length is increase and area is decrease as shown in figure1

Lets differentiate the R with respect to stress S, we get,

From the above equation we see that the per unit change in resistance is due to per unit change in length, per unit change in area and per unit change in resistivity.

The possion’s ratio is given by

Substituting the values of in equation (ii) we get,

= G_f × ε

Where ϵ = Strain =

The poisson’s ratio of all metals is between 0 and 0.5.

5.2. Electrical Resistance Strain Gauge:

In an electrical resistance strain gauge the device consists of a thin wire placed on a flexible paper tissue and is attached to a variety of materials to measure the strain of the material. The gauge position will be in such a manner that the gauge wires are aligned across the direction of the strain to be measured. When a force is applied on the wire, there occurs a strain that increases the length and decreases its area. Thus, the resistance of the wire changes. This change in resistance is proportional to the strain and is measured using a Wheatstone bridge. There are three different ways of connecting strain gauge in bridge circuit they are given below:

5.2.1. Full Bridge :

Figure 2 shows a full bridge circuit. It has all four of its gauges active. A full bridge circuit is used in applications where complimentary pair of strain gauges is to be bounded to the test specimen. A full bridge circuit is said to be more linear than other circuits.

5.2.2. Half Bridge:

Figure 3 shows a half bridge. It has two of its gauges active and thus uses two precise value resistors. A half-bridge configuration is one where two strain gauges are mounted on opposite surface of the test specimen with one in stretched strain and the other as compressed strain. The other two are dummy resistance that experience no strain and are temperature insensitive Therefore, the effects of temperature change will be cancelled, and the circuit will suffer minimal temperature-induced error.

5.2.3.

Quarter Bridge:

Figure 4 shows a quarter bridge. It has only one gauge and the rest of the resistors will be precise in value. The bonding is difficult in a quarter bridge circuits. It is mostly used for strain gauge measurements.

5.3. Wire Strain Gauge :There are two type of wire strain gauges:

(i)unbonded metal strain gauge and

(ii) bonded strain gauge.

5.3.1. Unbonded Metal Strain Gauge:

The unbonded metallic strain gauge is based on the principle that the electrical resistance of a metallic wire changes due to the change in the tension of the wire. This type of strain gauge consists of a stationary frame and a movable platform. Relative motion between the platform and the frame increases the tension in two loops, while decreasing tension in the other two loops. These four elements are connected approximately to a four arm Wheatstone bridge. These type of strain gauges are used for measurement of acceleration, pressure, force etc. Figure 5 shows the unbonded metal strain gauge. It consists of stretch wire between two points in an insulating medium such as air, and a stationary frame with an armature supported at centre of the frame. The armature can move only in one direction. It travels in that direction and is limited by four filaments of strain sensitive wires. The wire made up of copper, nickel, chrome nickel or nickel iron alloys.

5.3.2. Bonded Strain Gauge :Bonded strain gauge is connected to a paper or a thick plastic film support. The measuring leads are soldered or welded to the gauge wire. The bonded strain gauge with the paper backing is connected to the elastic member whose strain is to be measured. Bonded strain gauges are of two type wire type or a foil type they are explained below:

Bonded Metal Wire Strain Gauge :The bonded metallic type of strain gauge consists of a strain sensitive conductor (wire) mounted on a small piece of paper or plastic backing. In us this gauge is cemented to the surface of the structural member to be tested. The wire grid may be flat type or wrap-around. In the flat type after attaching the lead wires to the ends of the grids, a second piece of paper is cemented over the wire as cover. In the wrap-around type, the wire is wound around a cylindrical core in the form of a close wound helix. This core is then flattened & cemented between layers of paper for the purpose of protection and insulation. Formerly only wrap-around gauges were available, but generally flat grid gauges are preferred as they are superior to wrap-around gauge in terms of hysterisis, creep, elevated temperature, performance, stability & current carrying capacity.

Figure 6 shows the bonded metal wire strain gauge. It consists of grid of a resistive element which is cemented to the base which may be thin sheet of paper. A thin sheet made up of Bakelite or Teflon. It used for both stress analysis and for construction of transducers. The spreading of wire permits a uniform distribution of stress over the grid.

b) Bonded Metal Foil Strain Gauge :This is the extension of the bonded metal wire strain gauges. In this the strain is detected using metal foil. The metal and alloys used for the foil and wire are nichrome, constantan, isoelastic, nickel and platinum. Fig. 7 show the construction of strain gages. The fine-wire strain gages are used for special purposes, such as at high temperatures. Foil strain gages are usually made by a printed-circuit process. Since the foil used in a strain gage must be very fine or thin to have a sufficiently high electrical resistance. Some foil use has been made of wire filaments in strain gages, but this type of gage is seldom used except in special or high temperature applications. In order to handle this foil, it must be provided with a carrier medium or backing material, usually a piece of paper, plastic, or epoxy. The backing material performs another very important function in addition to providing ease of handling and simplicity of application. The cement provides so much lateral resistance to the foil that it can be shortened significantly without buckling; then compressive as well as tensile strains can be measured. Lead wires or connection terminals are often provided on foil gages A protective coating, recommended or supplied by the manufacturer, is usually applied over the strain gage, especially where the lead wires are attached. The foil strain gauge, it made up of thinner than comparable wire units. Also they are more flexible. The foil can be mounted in remote and restricted places and especially on curved surface.

5.4. Resistive Transducers:

In such material the resistances of the transducer get varied according to the measured. The resistance of any metal conductor is expressed by a simple equation,

where, L = length of the conductor

A = Cross Section area of the conductor

ρ = resistivity of conductor

From the above equation we see that the resistance of the material depends on L, A and ρ. Any method of varying one of the quantities involved in the above relationship can be the design basic of electrical resistive transducer

6.APPLICATIONS OF TRANSDUCER

The following are the application of the transducers.

It is used for detecting the movement of muscles which is called acceleromyograph.
The transducer measures the load on the engines.
It is used as a sensor for knowing the engine knock.
The transducers measure the pressure of the gas and liquid by converting it into an electrical signal.
It converts the temperature of the devices into an electrical signal or mechanical work.
The transducer is used in the ultrasound machine. It receives the sound waves of the patient by emitting their sound waves and pass the signal to the CPU.
The transducer is used in the speaker for converting the electrical signal into acoustic sound.
It is used in the antenna for converting the electromagnetic waves into an electrical signal.

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