• Transducer is a device that receives energy in one system and transmits to other in a different form.• Will generate signal related to the input received which energizes the transducer.• The output may be mechanical or electrical• The energy transmitted may be mechanical or electrical or acoustical.• There are two types- electrical and mechanical
Sen sorSensing element Transduction ele element Electrical signal me nt
Parameters of transducer• Linearity – relationship between input and output must be linear.• Dynamic range –should be wide.• Physical size –minimum weight and volume.• repeatability- i/p and o/p relationship must be predictable.• Accuracy – ideal value of the variable being measured.• Physical size – have minimal weight and
ELECTRICAL TRANSDUCER• It converts non electrical signal into electrical signal.• The output can be easily used for transmission and storage.• Effects of friction are minimized.• Electrical amplification and attenuation can be easily done.• Can be controlled with a very small power level.
• Electrical Transducer is a sensing device by which a physical , mechanical or optical quantity to be measured is transformed directly into electrical voltage or current proportional to the input.• Mechanical Transducers respond to change in physical conditions and give output in different form.
• The output can be modified.• The output can be recorded at a distance from the sensing medium.• It can be classified into two major categories• Active• Passive• Active transducer generates electrical signal in response to input• Does not require any power source for operation
• Example- piezoelectric cells• Photovoltaic cells• Passive transducers depend on the change in electrical parameters such as R,L,C.• Examples- strain gauge, thermistors• Electrical transducers contains sensing element and transduction element.
SELECTION OF TRANSDUCER• 1.loading effect- should have a minimum loading effect to keep the errors to minimum.• Nature of measurement• Environmental conditions pressure ,shock, size and mounting restrictions• Operating range• Electrical parameters
RESISTIVE TRANSDUCER• Material in which the resistance due to a change in some physical phenomenon.• Potentiometer-it consists of a resistance element provided with a sliding contact called wiper.• Movement can be in rotational and translational or combination of both.
• It is a method of converting mechanical displacement into electrical output.• The electrical output is proportional to movement.• Based on the position of the pot the resistor will be added.
ADVANTAGES• They are inexpensive• Simple to operate.• Disadvantages-large force is required to move sliding contact• Can generate noise and misaligned.
Resistance pressure transducer• It is based on the fact that a change in pressure results in a resistance change in sensing element.• The element made by the pressure change is made to cause a change in resistance.• The change of resistance can be made part of a bridge circuit
• Fluid or gas whose pressure is to be measured to pass through pressure sensitive element.• Causes mechanical displacement.• Output is proportional to the displacement.• Bellows is a sensitive element thin walled cylindrical shells.• Materials used are phosphor bronze.
Strain gauge• It is an example of passive transducer.• It uses the variation in electrical resistance in wires to sense the strain produced by the force on the wires.• If a metal conductor is stretched its resistance changes.• It is mounted on material which is subjected to force.
Resistance wire gauge• Unbounded resistance wire gauge and bonded strain gauge• In first one a wire is stretched between two points in an insulating medium• The diameter is 25 micro meter.• It is connected to bridge circuit.• When external load is applied resistance changes and causes unbalance in bridge circuit
• In bonded strain gauge its looped back and forth on a carrier• The wire is covered on top with a thin material• A stress tends to change the length of wire and length increases.• The strain gauge is normally used in bridge arrangement.• Only one of the gauge is active producing the output proportional to force.
Types of strain gauge• The rosette type is having three or four arms.• It is used when the direction of force is unknown• Gauge elements are arranged in either 45 degree or 60.• Torque and helical type are the other type of gauge transducer.• Advantages.• It should have a high value of gauge factor.• The ressitance should be as high as possible.
• It should not have hysteresis effect,.• The variation in resistance should be having linear characteristics.
Foil strain gauge• The strain is detected using am metal foil.• The metals and alloys used are nichrome ,constantan etc.• It is based on peizo resistive effect.• They can be fabricated on large scale and at any shape.• It can be mounted on remote and places and curved surfaces.• The resistance values is generally 50 and 1000.
Semiconductor strain guage• These are used when high value of gauge factor is necessary.• They have gauge factor 50 times as high as wire strain gauges.• Materials used are germanium and silicon.• It consists of strain material and leads that are placed in a box.• Semiconductor wafer which have thickness of 0.05 mm are used.
advantages• It can be used to measure very small strains.• Very small in size ranging from .7 to 7.0mm• But they are more expensive and linearity of these type is poor.
RESISTANCE thermometer• The resistance of a conductor changes when its temperature is changed.• It is used to measure the resistance in terms of temperature.• Its main part is the sensing element.• It exhibits a relatively large resistance change with change in temperature.• Platinum, nickel, copper are the metals most commonly used to measure temperature.
• The measurement is very accurate.• It has a lot of flexibility with regard to choice of measuring equipment.• Stability of performance over long period of time.• But they have higher cost need for bridge circuit and power source.
INDUCTIVE TRANSDUCER• IT IS A DEVICE WHICH converts physical motion into change in inductance• It works on one of the following methods• Variation of self inductance• And variation of mutual inductancce.• It is mainly used for the measuremnt of diaplacement.• It is arranged to cause the variations in any one of the variables• Number of turns• Geometric configuration.
Variable reluctance type transducer• It consists of a coil or winding wound on a a ferromagnetic core.• The displacement which is to be measured is applied to ferromagnetic target.• The target does not have any contact with he core on which it is mounted.• When the target is near the core length is small and therefore the reluctance is large.
• When the target is away from the core the reluctance is large resulting in smaller self inductance value.• Thus the inductance of the coil is a function of displacement.
• In variable reluctance bridge a saperate col is wound on each outside leg of E core and iron bar is pivoted on center leg.• The moving member is attached on one end of the iron bar.• It is balanced only when the inductance of the two transducer coils are equal
Thermistor• Thermistor is as a semiconductor component which made by pure oxides of iron, cobalt, nickel, magnesium, with other material.• It has negative temp coefficient.• It is mainly used for temperature measurement .• It is found in different shapes like disc type, rod type ,washer type.• Disc type – 10mm diameter• It is mainly used for temperature control.• Thermistors can be connected in series or parallel combination• It is also used in regulator circuits , memory units.
Differential output transducer• It consists of a coil which is divided into to parts.• In response to displacement the inductance of one part increases form L to delta and other part from L-deltaL
• The output represents a change of self inductance due to change of response.
LVDT• The transformer consists of single primary winding P1 and two secondary windings s1 and S2 wound on hollow cylindrical former.• The primary is connected to ac source.• The displacement to be measured is applied to arm attached to the soft iron core.• When the core is its normal position equal voltages are induced in the two secondary windings.
• If the core is moved to the left of the null position more flux links with windings s1 AND LESS WITH WINDINGS S2.• Hence output voltage es1 of the secondary winding s1 is greater than es2.• If the core is moved to right position the flux linking with winding s2 becomes greater than the linked winding S1.
• The Rotational Variable Differential Transformer (RVDT) is used to measure rotational angles and operates under the same principles as the LVDT sensor.• Whereas the LVDT uses a cylindrical iron core, the RVDT uses a rotary ferromagnetic core. A schematic is shown below.
• Relative low cost due to its popularity. - Solid and robust, capable of working in a wide variety of environments. - No friction resistance, since the iron core does not contact the transformer coils, resulting in an very long service life. - High signal to noise ratio and low output impedance. - Negligible hysteresis.• The core must be in contact (directly or indirectly) with the measured surface which is not always possible or desirable.•
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