100668788 Transducers Ppt

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  • Chapter-8 TRANSDUCERSTopics to be discussed: Mechanical, Electrical, Electronic Transducers its advantages & Disadvantages.Introduction: In a measurement system, the measurand (the quantity under measurement) makes its first contact with system through a detector. The measurand is converted into an analogous form by the detector. The measurand or input signal is called information for the measurement system. The information may be in the form of a physical phenomenon or it may be an electrical signal.

  • The function of the detector is to sense the information and convert it into a convenient form for acceptance by the later stages of the system. The process of detection & conversion of input signal from one form to another requires energy. This energy may be extracted from the signal, thereby, causing loading effects. If energy is extracted from the signal, it will not be reproduced after conversion leading to errors in measurement. Energy is supplied to the detector from external sources so that the input signal is converted into a usable output without drawing an undue amount of energy from the signal. The ideal conversion will be when absolutely no energy is extracted from the signal so that it is not distorted and the analogous output of the detector is a faithful representation of the input signal.

  • TRANSDUCER A transducer is a device used to covert motional, thermal and optical signals into electrical quantities, that may be amplified, recorded and otherwise processed in the instrumentation system. Transducers are also known as prime sensors, gauges, pickups and signal generators.TYPES OF TRANSDUCERSWith the fast developing technology various types of transducers have appeared for measurement of one particular quantity. The following are the types of transducers and the quantities to be measured.

  • 1.Capacitive Transducers for the measurement of Displacement, Thickness, Velocity, Acceleration, Pressure, Flow, Humidity, Viscosity, Level, etc. 2.Electron tube Transducers for the measurement of Displacement, Velocity, Acceleration & Pressure. 3.Inductive type of Transducers for the measurement of Mass, Displacement, Thickness, Velocity, Acceleration, Force, Pressure, and Flow. 4.Magneto-Electric Transducers for the measurement of Mass, Displacement, Velocity, Acceleration, Pressure and Flow.

  • 5.Photo Electric Transducers for the measurement of Displacement, Thickness, Velocity, Temperature, level and height. 6.Piezo Electric Transducers for the measurement of mass, Displacement, Thickness, Velocity, Acceleration, Force, Pressure, Flow, Viscosity, Level, etc. 7.Radio active Transducers - for the measurement of mass, Displacement, Thickness, Velocity, Force, Pressure, Flow, Level, etc. 8.Resistance Transducers - for the measurement of mass, Displacement, Thickness, Velocity, Acceleration, Force, Pressure, Flow, Viscosity, Level, etc.

  • The basic function of first stage, or sensor-transducer stage, is to detect or sense the input signal and transfer it to the second stage. Thus, in measurements, the information required is picked up from the source by the first stage detectors.

    Usually, the sensed information, i.e., the input is transduced into a convenient analogous form, like mechanical displacement into electrical voltage form. These devices which sense the input & convert them into an analogous form are called detector-transducer

  • Transfer Efficiency: The sensed input Iin and the transduced analogous form, Iout can be related as Iout = f (Iin) and transfer efficiency = Iout / Iin < 1Transfer efficiency will always be less than unity, because of the losses in sensing and processing the input. In measurements, a transfer efficiency as high as possible is desired for accurate results.

  • PRIMARY & SECONDARY TRANSDUCERS: The devices which sense the input, transform them to a convenient analogous form, and send the signal to the next stage are known as transducers. These are the basic elements of any measurement systems, which can be distinguished as primary and secondary transducers, based on the functions they perform. They act in combination one after the other.The primary transducer basically senses and sends the signal without any modification, like a displacement as a result of measurement of displacement, force, pressure, temperature, etc. This displacement is amplified in the second stage and used to indicate the reading in the third stage.A secondary transducer, works subsequent to a primary transducer receiving the input from it. For example, the displacement of the primary transducer used in a Linear Variable Differential Transformer (LVDT) to produce an electrical output which can be amplified and used at the recorder.

  • CLASSIFICATION OF FIRST STAGE DEVICES:First Stage Devices can be broadly classified as follows 1. First stage elements used as detectors only. 2. First stage elements used as detector and single transducer 3. First stage elements used as detector with two transducersThe elements used as detectors may be very simple, like a mechanical spindle or a contact member that picks up and transmits the input to a secondary transducer. Generally first stage devices are a combination of detectors and transducers, since transducing is essential for further stages (e.g. Bourdon pressure gage-Bourdon tube with this linkage). Based on its two functions,

  • first stage can be represented schematically as shown in figure 1.

  • IAS Intermediate analogous signalADS - Analogous driving signalACTIVE AND PASSIVE TRANSDUCERS:Based on the input circuitry adopted and method of operation, the Electrical detector transducers can also be classified as active type and passive type.

    ACTIVE TRANSDUCERS: Active transducers are those, which are self-powered, hence require no external source of energy. For example, a piezoelectric accelerometer does not require any external source of energy, because it works on its own energy. Main advantage of this type is that its circuit design is simpler.

  • PASSIVE TRANSDUCERS: Passive transducers are those which require an external source of energy for their operation. For example, a simple bonded wire strain gage using resistance bridge requires an auxiliary source of energy. This type of transducer requires a complicated input design since it requires special arrangement to introduce the auxiliary energy.TYPES OF TRANSDUCERS WITH THEIR FUNCTIONS:The following table gives a list of some primary detector transducer elements generally used in measurements, along with their functions.

  • It can be seen from the list that, most mechanical sensors transduce the inputs to displacement, while electrical sensors transduce displacement to voltage/current/resistance change. In practice, the mechanical elements are used as primary transducers, while electrical elements are used as secondary transducers.

  • Q] LIST THE ADVANTAGES OF ELECTRICAL TRANSDUCERS OVER MECHANICAL TRANSDUCERSThe electrical transducer elements have many advantages over mechanical elements, which are: 1.Amplification or attenuation is easier 2.Mass-inertia effects are minimum 3.Friction problems are negligible 4.Output power of desired magnitude is possible 5. Remote indication/recording is feasible 6.These are lighter and smaller compared to mechanical elements.

  • ELECTRICAL TRANSDUCER ELEMENTS:These transducers converts mechanical displacement to voltage. The quantity of interest is first detected and transduced to displacement by some form of mechanical element; then the electric element serves as the secondary transducer, transforming the analogous displacement into an analog voltage or current. The basic electrical change may be resistive, inductive, Capacitive, etc., from which the voltage or current change results. The most commonly used principles of operation employed are 1. Variable resistance transducer elements. 2. Variable inductance transducer elements. 3. Variable capacitance transducer elements. 4. Piezoelectric transducers. 5. Photoelectric transducers.

  • ADVANTAGES OF ELECTRICAL TRANSDUCER ELEMENTS:The following are the advantages of Electrical Transducer Elements 1.Amplification or attenuation may be easily obtained. 2.Mass-inertia effects are minimized. 3. The effects of friction are minimized. 4. An output with sufficient power for control may be provided. 5. Remote indication or recording is feasible. 6. The transducers are usually susceptible to miniaturization.

  • 1. VARIABLE RESISTANCE TRANSDUCER: The resistance of an electrical conductor varies according to the following relation.

    R = L / AWhere, R = Resistance in ohms,

    L = length of conductor in cm A = cross-sectional area of conductor, cm2 = resistivity of material, ohms-cm.

    In variable resistance transducers, mechanical displacement is converted into electrical output, such as voltage or current. This is achieved by changing the value of L in the above equation.

  • A simple form of variable resistance transducer consists of a resistance element and sliding brush with guide [sliding contact resistive transducer].

    SLIDING CONTACT RESISTIVE TRANSDUCER: In a sliding contact resistive transducer as shown in figure [1], mechanical displacement is converted into electrical output, such as voltage or current. This is achieved by changing the value of L in the equation R = L / AWhere, R = Resistance in ohms, L = length of conductor in cm A = cross-sectional area of conductor, cm2

  • Fig-1 Variable resistance consisting of a wire and movable contractor or brush. This is often referred to as a slide wire.