Transducers

http://ei-notes.blogspot.in/2012/05/transducers.html

Transducers:

•Transform values of physical variables into equivalent electrical signals
or

•Converts a signal from one form to another form.

A measuring unit or device which measures and converts nonelectrical quantity into electrical quantity is known as transducer.

Transducers are classified into several types. However, these can be categorized into

five types as,

Types of Transducers:-

•Variable-Resistance Transducer

•Differential Transformer (LVDT)

•Capacitive Transducer

•Piezoelectric Transducer

•Photoelectric Transducer

•Photoconductive Transducer

•Photovoltaic Transducer

•Ionization Transducer

•Magnetometer Search Coil

•Hall-Effect Transducer

•Digital Displacement Transducer

Variable-Resistance Transducer:-

•Resistance Potentiometer, rheostat

•Converts linear or angular displacement into an electric signal

•Can be used to convert force and pressure to a displacement through mechanical methods

•The Moving contact of Potentiometer, rheostat on a slide-wire that moves through an angular displacement on a solid conductor (e.g. graphite)

•Fairly cheap –some can be expensive, though

Differential Transformer (LVDT):-

•Converts displacement into electric signal

•Three coils placed in a linear relationship with a magnetic core free to move inside the coils

•Alternating input voltage impressed in center coil, and output voltage from two end coils depends on magnetic coupling between the core and coils

•The coupling is dependent on the position of the core

•If the core of LVDT remains near the center of the coils ,then the output is very nearly linear.

•mainly the differential transformers are only operated in this linear range, and so these are called linear variable differential transducers (LVDT)

Capacitive Transducer:-

•Measures a change in capacitance through a change in distance
between 2 plates, d

•Measures capacitance change through a change in overlapping area of them, A, resulting from its plate movement or a change in dielectric constant, ε

• use0.225 , 0.0885 for ε = 1 for airε = 1 for plasticsdAC⋅⋅=ε225.02in2cm

Capacitive Transducer cont.

•For liquid level measurements two electrodes are placed in the liquid, and the dielectric constant varies between them according to the liquid level

•Thus, capacitance between electrodes is a direct indication of liquid level

Piezo-electric Transducer:-

•A force applied to a piezoelectric crystal between two electrodes produces
a stress and corresponding deformation in the crystal•Deformation produces potential difference at crystal surface.
•The stress on the crystal produces an output voltage:E = gtpwhere t is crystal thickness in m p is impressed force ing is voltage sensitivity•
Piezoelectric crystals used as pressure transducers for dynamic measurements.

Photo-electric transducer:-

•Converts a light beam into a usable electric signal•Light strikes cathode and releases electrons•Electrons are attracted toward anode, thereby producing an electric current in external circuit•Cathode and anode enclosed in glass or quartz, which is evacuated or filled with inert gas•Photoelectric tubes useful for measuring light intensity

Photo-conductive Transducers:-

•Voltage impressed on semiconductor material

•Light strikes semiconductor material and a resistance of it decrease in occurs, therefore producing an increase in current

•Useful for measurement of radiation at all wavelengths

Photovoltaic Transducers:-

•Sandwich design of a metal base plate and a thin transparent metallic layer with a semiconductor material in between

•Light strikes barrier between transparent metal layer and semiconductor material, and a voltage is generated

• photovoltaictransducer,that is most widely used application of photovoltaic cell, is the light exposure meter in photographic work

Ionization Transducer:-

•Converts displacement to a voltage through a capacitance change•As tube moves horizontally between the two plates, a dc potential difference is created between electrodes 1 and 2

Magnetometer Search Coil

•Transforms a magnetic field signal into a voltage

•A flat coil with a specified number of turns is placed in a magnetic field, and voltage is generated in it.

•To measure a steady magnetic field, the search coil must be moving., in this circumstance.typically a rotating coil is used

Hall-Effect Transducers:-

•In the Hall-Effect Transducers , in the semiconductor plate an external current passes through it. .•When a magnetic field is impressed on a plate in a direction perpendicular to the plate surface, a voltage is generated•This voltage is the Hall voltage•where is the Hall coefficienttIBKEHH⋅=HK

Digital Displacement Transducers:-

•Used to measure angular and linear displacements

•For an angular measurement, a wheel rotates and light is alternately transmitted and stopped through small cutouts

•The signal is sent to the photodetector, amplified, then sent to a counter.

•The number of counts is proportional to angular displacement.

•Sensitivity can be improved by increasing the number of cutouts

Digital Displacement Transducers:-

•For a linear measurement, small reflecting strips are installed on a motion device.
•Light is then alternately reflected and absorbed with linear motion presenting a signal to
the photodetector
•Calibration with a known displacement standard must be performed

Transducers can classify on following basis :

1. on the basis of transduction principle used.

2. Active and passive transducers.

3. Analog and digital transducers.

4. Primary and secondary transducers.

5. Transducers and inverse transducers.

1. on the Basis of Transduction Principle .

This classification is done depending on the transduction principle i.e., how the inputvariable is being converted into capacitance, resistance and inductance values. (These are namedas capacitive transducer, resistive transducer and inductive transducer respectively).

As we can say following examples of Capacitive Transducer applications:-

1. Dielectric gauge.

2. Capacitor Microphone.

1.It is used to measure,

(i) Thickness and

(ii) Liquid level.

2.It is used to measure,

(i) Noise

(ii) Speech and Music

As we can say following examples of Resistive Transducer applications:-

1. Resistance thermometer.

2. Potentiometer device.

1. Used in the measurement or,

(i) Temperature and

(ii) Radiant heat.

2. (i) Used in displacement measurement and

(ii) Used in pressure measurement

As we can say following examples of Inductive Transducer applications:-

1. Reluctance pick up.

2. Magnetostriction gauge.

1. It is used to measure,

(i) Pressure

(ii) Vibrations

(iii) Position and

( iv) Displacement.

2. It is used to measure,

(i) Sound

(ii) Force

(iii) Pressure.

2. Active and Passive Transducers:-

Active Transducer

The transducer which does not requires any external excitation to provide their

outputs are referred as active transducer.

As we can say following examples of Active Transducer applications:-

1. Photo voltaic cell.:its used in light meters and Used in solar cells.

2. Thermocouple.:its used to measure, Temperature Radiation and Heat flow.

Passive Transducer:

The transducer which requires an external excitation to provide their output is referred as passive transducer.

Examples of Passive Transducer Applications

1. Capacitive transducers:its used to measure liquid level, noise,thickness etc.

2. Resistive transducers.:its used to measure temperature, pressure,displacement etc.

3. Inductive transducers:.its used to measure pressure, vibration,position, displacement etc.

3. Analog and Digital Transducers:-

Analog Transducer

The transducer which produces their outputs in analog form or a form which is a

continuous function of time is referred as analog transducer.

Examples of Analog Transducer Applications

1.Strain gauge:its used to measure, Displacement ,Force and Torque measurement etc.

2. Thermistor:its used to measure,Temperature and Flow measurement etc..

Digital Transducer:-

The transducer which produces their outputs in digital form or a form of pulses is referred

as digital transducers.

Examples of Digital Transducer Applications as Turbine meter used in for flow measurement

4. Primary and Secondary Transducers:-

Primary Transducer:

The transducer which sends the measurement and converts them into another

variables (like displacement, strain etc.) and whose output forms the input of another

transducer is called as primary transducer.

As we can say following examples of Primary Transducer applications,

1. Bourdon tube:its used in pressure measurment etc.

2. Strain gauge:its used in force and strain measurements

Secondary Transducer

The transducer which converts the output of first transducer into an electrical output

called secondary transducer.

As we can say following examples of Secondary Transducer Applications:

LVDT Used to measure,Displacement , Force, Pressure measurement and Position etc.

5. Transducers and Inverse Transducers:

Transducers:

A measuring device which measures and converts nonelectrical variable into electrical

variable is known as transducer.

Example of Transducer Applications as,

Thermocouple : its used to measure, Temperature, Radiation and
Heat flow measurement

Inverse Transducer:-A measuring device which measures and converts an electrical variable intononelectrical variable is known as inverse transducer.
Example of Inverse Transducers Applications as Piezo-electric crystal:as

used to measure, Pressure measurement, Vibration and acceleration measurement etc.

Parameters to be considered in the selection of a transducer for a particular application are.:-

1. Operating Principle:

Basically the transducers are selected based on their operating principle. Examples of

operating principles used by the transducers are resistive, capacitive, piezoelectric,

inductive, up to electronic principle etc.

2. Operating Range:

This factor is considered so that the transducer should be able to function within the

specified range with good resolution. Every transducer should be provided with some

rating within which there will be breakdown in its function.

3. Accuracy:

It is one of the most desired characteristic of any transducer. If the transducer doesn't

needs frequent calibration, it must have high degree of accuracy and repeatability.

Because errors may occur due to the sensitivity of the transducer to other stimulations.

4. Sensitivity

It is also a desired characteristic of a transducer. Every transducer should be sufficiently

sensitive to provide some output that can be sufficient and detectable.

5. Stability and Reliability

The transducer should have high degree of stability during its function and also storage

life. It should also have a high degree of reliability.

6. Usage and Ruggedness

The ruggedness, size and weight of a transducer should be chosen depending on the

application in which it is used.

7. Transient Response and Frequency Response

The transducer should have required time domain specifications such as, settling time,

rise time, peak over shoot and small dynamic error etc.

8. Loading Effects

The transducers should undergo minimum loading effect so that if can provide accurate

measurement. The parameters of a transducer are that, which characterize the loading

effect is its input and output impedances.lt is considered in order to get minimum loading

effects (Which can be neglected). For minimum loading effect the transducer should have

low output impedance and high input impedance.

9. Electrical Parameters

The type and length of cable required, signal to noise ratio in case the transducer is used

with amplifiers and frequency response limitations should also be considered.

10. Ability to be insensitive to unwanted signals (or the ability to be sensitive to desired

signals).

11. Environmental compatibility.

12. Static Characteristics

The selected transducer should have low hysteresis, high linearity and high resolution.

Difference between photoemmissive, photoconductive and photovoltaic transducers:-

Photo emissive Transducer	Photoconductive transducer	Photovoltaic transducer
1. When the cathode of photo emissive cell is subjected to radiations, it emits electrons. These electrons increase the current of the cell. Hence, this cell provides the measure of light intensity in terms of current. 2. It is a, passive transducer. 3. They have a moderate luminous sensitivity of 10- 100 uA/lm and 40-150 uA/lm. A very high sensitivity of 20 A/lm can also be achieved by photomultiplier tube. 4. The response time of these cells ' is less than that of photoconductive cells. 5. The spectral response of these cells range from visible to infrared region up to 220° nm. 6. Photo emissive cells possess good amount of stability.	-When the photoconductive semiconductor element is subjected to radiations, its resistance changes (i.e., if decreases). Due to this, the flow of current through the cell increases. -This cell is a passive transducer. -They are highly sensitive with small change in light intensity they exhibit high change in their resistance. These cells possess a short response time of 10-100 ps. These cells do not respond to radiations having wavelength less than 300 nm. Their spectral response extend from thermal radiation through visible, IR,UV up to X-rays and y-rays. The characteristics of photoconductive cells are affected by temperature.	-When the radiations fall on a photovoltaic cell, it generates a voltage whose value is proportional to the intensity of radiation incident on the cell. Thus, it convertselectromagnetic energy in to electrical energy. -It is an active transducer. They possess sensitivity of 1 mA/lm. -These cells have very short response time of 1-50 ps. The spectral response of these cells range from 200 nm - 2000 nm These cells are sensitive to, a-rays, (3-rays, y-rays and Xrays. Photovoltaic cells are also stable but, they are seriously affected by temperature. An increase in temperature leads to a rapid decrease in output voltage of these cells, usually few mV/°C.