A device which converts a physical quantity into the proportional electrical signal is called a transducer. The electrical signal produced may be a voltage, current or frequency. A transducer uses many effects to produce such conversion. The process of transforming signal from one form to other is called transduction. A transducer is also called pick up. The transduction element transforms the output of the sensor to an electrical output, as shown in the Fig
The input in any instrumentation system makes its first contact with a primary detection element this includes process variables like temperature, pressure and the flow rate which are widely employed in the process and production plants. The measurand also includes electrical quantity like current, voltage, resistance, inductance, capacitance,frequency, phase angle, power and magnetic quantity like flux, flux density, reluctance all this quantity required a primary detection element or a transducer to be converted into analogous format which is the acceptable by the later stages of the measurement system. The measurand or input signal is called as information of the measurement system and it is in the form of physical phenomena or it could be the electrical signal this energy may be extracted from the measurand but it also loads error the efforts should be made two supplies energy required for conversion from outside sources so that the measure and is not distorted during the process of conversion in order that ate be faithfully reproduced in its analogous form .
Mechanical devices as primary detectors: In order to get the information from the mechanical systems only mechanical displacement or velocity can be used some of the commonly used mechanical sensing elements are Springs which converts the force or torque into a displacement a diaphragm, capsule ,or Bourdon tube which converts pressure into displacement a bimetallic strip converts temperature into displacement mass damper system is used for measurement of acceleration velocity and displacement some input devices may involve more than one mechanical conversion for example fluid flow measurements may involve conversion of fluid rate into pressure differential using an orifice Venturi tube or pitot tube and then in turn this pressure is converted into displacement for the purpose of measurement.
Mechanical spring devices:
Torsion bars and shafts
spring fixture pivotes
Pressure Sensitive Primary Devices:
An input device
A signal conditioning or processing device
Advantages of electrical transducers:
Electrical amplification and detonation can be done easily and that too with the static devices.
The effects of friction are minimized.
The electrical or electronic system can be controlled with a very small power level
Battery is used in almost all sophisticated measurement systems
There has been an explosive development in the field of Electronics components and devices. This development is on account of the fact that electronic devices are very amenable to miniaturization .
Classifications of Transducer:
On the basis of transduction form used
Primary and secondary transducers
As passive and active transducers
As analogue and digital transducers
Transducers and Inverse transducers
Classification based on the principle of the transducer:
Resistance Strain Gauge: The change in value of resistance of metal semi-conductor due to elongation or compression is known by the measurement of torque, displacement or force.
Resistance Thermometer: The change in resistance of metal wire due to the change in temperature known by the measurement of temperature.
Resistance Hygrometer: The change in the resistance of conductive strip due to the change of moisture content is known by the value of its corresponding humidity.
Hot Wire Meter: The change in resistance of a heating element due to convection cooling of a flow of gas is known by its corresponding gas flow or pressure.
Photoconductive Cell: The change in resistance of a cell due to a corresponding change in light flux is known by its corresponding light intensity.
Thermistor: The change in resistance of a semiconductor that has a negative coefficient of resistance is known by its corresponding measure of temperature.
Potentiometer Type: The change in resistance of a potentiometer reading due to the movement of the slider as a part of an external force applied is known by its corresponding pressure or displacement.
Variable capacitance pressure gauge- Principle of operation: Distance between two parallel plates is varied by an externally applied force Applications: Measurement of Displacement, pressure.
Capacitor microphone Principle of operation: Sound pressure varies the capacitance between a fixed plate and a movable diaphragm. Applications: Speech, music, noise.
Dielectric gauge Principle of operation: Variation in capacitance by changes in the dielectric. Applications: Liquid level, thickness
Applications: Force, pressure, sound
Voltage and current Transducers:
Self-Generating Transducers (No External Power)
Active Transducers They do not require an external power, and produce an analog voltage or current when stimulated by some physical form of energy.
Primary Transducers and Secondary Transducers
These transducers convert the input quantity into an analog output which is a continuous function of time. Strain Gauge , LVDT, Thermocouple . Thermistor
These transducers convert the input quantity into an electrical output which is in the form of pulses. Glass Scale can be read optically by means of a light source,an optical system and photocells .
Transducers and Inverse Transducers
A Transducer can be broadly defined as a device which converts a non-electrical quantity into an electrical quantity. Ex:-Resistive,inductive and capacitive transducers -An inverse transducer is defined as a device which converts an electrical quantity into a non-electrical quantity. Ex:-Piezoelectric crystals.
Advantages of Electrical transducers:
Mostly quantities to be measured are non-electrical such as temperature, pressure, displacement, humidity, fluid flow, speed etc., but these quantities cannot be measured directly. Hence such quantities are required to be sensed and changed into some other form for easy measurement. Electrical quantities such as current, voltage, resistance, inductance and capacitance etc. can be conveniently measured, transferred and stored, and, therefore, for measurement of the non-electrical quantities these are to be converted into electrical quantities first and ten measured. The function of converting non-electrical quantity into electrical one is accomplished by a device called the electrical transducer. Basically an electrical transducer is a sensing device by which a physical, mechanical or optical quantity to be measured is transformed directly, with a suitable mechanism, into an electrical signal (current, voltage and frequency). The production of these signals is based upon electrical effects which may be resistive, inductive, capacitive etc. in nature. The input versus output energy relationship takes a definite reproducible function. The output to input and the output to time behavior is predictable to a known degree of accuracy, sensitivity and response, within the specified environmental conditions. Electrical transducers have numerous advantages. Modern digital computers have made use of electrical transducers absolutely essential.
Electrical transducers suffer due to some draw-backs too, such as low reliability in comparison to that of mechanical transducers due to the ageing and drift of the active components and comparative high cost of electrical transducers and associated signal conditioners. In some cases the accuracy and resolution attainable are not as high as in mechanical transducers.
Some of the advantages are:
Factor to be considered while selecting transducer:
It should have high input impedance and low output impedance, to avoid loading effect.
It should have good resolution over is entire selected range.
It must be highly sensitive to desired signal and insensitive to unwanted signal.
Preferably small in size.
It should be able to work in corrosive environment.
It should be able to withstand pressure, shocks, vibrations etc..
It must have high degree of accuracy and repeatability.
Selected transducer must be free from errors.
The transducer circuit should have overload protection so that it will withstand overloads.
Requirements of a good transducers
Smaller in size and weight.
Ability to withstand environmental conditions.
Resistance of an electrical conductor is given by, R=ρl/A Where ,
R = Resistance in „Ω‟
Ρ = Resistivity of the conductor (Ω - cm)
l = Length of the conductor in cm.
A = Cross-sectional area of the metal conductor in cm2
It is clear from the equation that, the electrical resistance can be varied by varying,
Cross-sectional area and
Resistivity or combination of these.
Principle: A change in resistance of a circuit due to the displacement of an object is the measure of displacement of that object ,method of changing the resistance and the resulting devices are summarized in the following
Method of changing resistance
Length: Resistance can be changed varying the length of the conductor,(linear and rotary). Dimensions - When a metal conductor is subjected to mechanical strain, change in dimensions of the conductor occurs, that changes the resistance of the conductor.
Resistivity: When a metal conductor is subjected to a change in temperature and change in resistivity occurs which changes resistance of the conductor.
Resulting Device: Resistance potentiometers or sliding contact devices displacements ,Electrical resistance strain gauges.Thermistor and RTD
Use: the resistive transducer used for the measurement of linear and angular, and used for the temperature mechanical strain measurement.
Strain gauge is one of the most popular types of transducer. It has got a wide range of applications. It can be used for measurement of force, torque, pressure, acceleration and many other parameters. The basic principle of operation of a strain gage is simple: when strain is applied to a thin metallic wire, its dimension changes, thus changing the resistance of the wire. Let us first investigate what are the factors, responsible for the change in resistance.
Metallic Strain Gage Most of the strain gages are metallic type. They can be of two types: unbonded and bonded. The unbonded strain gauge is normally used for measuring strain (or displacement) between a fixed and a moving structure by fixing four metallic wires in such a way, so that two are in compression and two are in tension, as shown in fig. (a). On the other hand, in the bonded strain gauge, the element is fixed on a backing material, which is permanently fixed over a structure, whose strain has to be measured, with adhesive. Most commonly used bonded strain gauges are metal foil type. The construction of such a strain gauge is shown in fig. (b). The metal foil type strain gauge is manufactured by photo-etching technique. Here the thin strips of the foil are the active elements of the strain gauge, while the thick ones are for providing electrical connections. Because of the large area of the thick portion, their resistance is small and they do not contribute to any change in resistance due to strain, but increase the heat dissipation area. Also it is easier to connect the lead wires with the strain gauge. The strain gauge in fig.(b) can measure strain in one direction only. But if we want to measure the strain in two or more directions at the same point, strain gage rosette, which is manufactured by stacking multiple strain gauges in different directions, is used. Fig. shows a three element strain gage rosette stacked at 45 Degree .
Three Element Strain Gauge Rosette-45 Degree Stacked:
The backing material, over which the strain gage is fabricated and which is fixed with the strain measuring structure has to satisfy several important properties. Firstly, it should have high mechanical strength; it should also have high dielectric strength. But the most important it should have is that it should be non-hygroscopic, otherwise, absorption of moisture will cause bulging and generate local strain. The backing materials normally used are impregnated paper, fiberglass, etc. The bonding material used for fixing the strain gauge permanently to the structure should also be non hygroscopic. Epoxy and Cellulose are the bonding materials normally used.
Semiconductor type Strain Gage :Semiconductor type strain gauge is made of a thin wire of silicon, typically 0.005 inch to 0.0005 inch, and length 0.05 inch to 0.5 inch. They can be of two types: p-type and n-type. In the former the resistance increases with positive strain, while, in the later the resistance decreases with temperature. The construction and the typical characteristics of a semiconductor strain gauge are shown in fig. MEMS pressure sensors are nowadays becoming increasingly popular for measurement of pressure. It is made of a small silicon diagram with four piezo-resistive strain gages mounted on it. It has an inbuilt signal conditioning circuits and delivers measurable output voltage corresponding to the pressure applied. Low weight and small size of the sensor make it suitable for measurement of pressure in specific applications.