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Learn about transducers and their classification based on principles. Understand the characteristics and choice of transducers, including input and transfer characteristics, as well as output characteristics. Explore factors that influence the choice of transducers.
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UNIT 5 TRANSDUCERS AND DATA ACQUISITION SYSTEMS
TRANSDUCERS • It’s a device which convert one form of energy to another form • Non electrical quantity is converted into an electrical form by a transducer.
Classification of Transducers • Classification based upon principle • As primary and secondary transducers • As passive and active transducer. • As analog and digital transducer. • As transducers and inverse transducers.
Classification based upon principle of transduction • Resistive • Inductive • Capacitive etc Eg piezoelectric, thermoelectric, magneto restrictive, electro kinetic and optical
Active Transducer • Thermoelectric • Piezoelectric • Electro magnetic
Passive and Active Transducer • Active Transducer: Also known as self generating type, develop their own voltage or current from the physical phenomenon being measured. Velocity , temperature , light intensity and force can be transduced with the help of active transducer.
Conti.. • Passive Transducer: Also known as externally powered transducers, i.e., derive the power required for energy conversion from an external power source. e.g. POT (Potentiometer)-used for the measurement of displacement .
Analog and Digital Transducer. • Analog Transducers : It converts the input quantity into an analog output which is a continuous function of time. • E.g. LVDT, Thermocouple or a thermistor (gives output which is continuous function of time)
Conti.. • Digital Transducer: Converts input quantity into an electrical output which is in the form of pulse.
Transducers and Inverse Transducers • Transducer: Non electrical to electrical quantity • Inverse transducer: Electrical quantity into non electrical quantity.
Characteristics and Choice of Transducer • Input Characteristics • Transfer Characteristics • Output Characteristics.
Input Characteristics • Type of Input and Operating Range • Loading effect. • Type of Input :The type of input, which can be any physical quantity, is generally determined in advance . • Operating Range : Choice of transducer depends upon the useful range of input quantity.
Conti.. • Loading Effect : The transducer, that is selected for a particular application should ideally exact NO force, power or energy from the quantity under measurement in order that is measured accurately.
Transfer Characteristics • Transfer function. • Error. • Response of transducer to environmental influences.
Transfer function. • The transfer function of a transducer defines a relationship between the input quantity and the output. The transfer function is Where are respectively output and input of the transducer.
Conti.. • Sensitivity, • Scale Factor, Inverse of sensitivity.
Error • The error in transducer occur because they do not follow, the input output relationship. • Example.. Instead of qo, we might get a output as qo’, then the error of the instrument is
Three components of error • Scale error. • Dynamic error • Error on account of noise and drift.
Scale error. • Zero error. • Sensitivity error • Non conformity. • Hysteresis.
Practical Curve. Output Theorectical Curve. Input Zero error • Output deviates from the correct value by a constant factor over the entire range of the transducer.
Practical Curve. Output Theorectical Curve. Input Sensitivity Error • Observed output deviates from the correct value by a constant value.
Practical Curve. Output Theorectical Curve. Input Non conformity • Transfer function deviates from the theoretical transfer function for almost every input.
Decreasing input Output Increasing input Input Hysteresis
Response of transducer to environmental influences. • It should not be subjected to any disturbances like stray electromagnetic and electrostatic fields, mechanical shocks and vibrations temperature changes, pressure and humidity changes, changes in supply voltage and improper mechanical mountings.
Output Characteristics • Type of Electrical Output. • Output Impedance • Useful Range.
Type of Electrical Output. • The type of output which may be available from the transducers may be available from the transducers may be a voltage, current , impedance or a time function of these amplitudes.
Output Impedance • Ideally the value of output impedance should be zero if no loading effects are there on the subsequent stage. • Since zero output impedance is not possible , it should be kept as low as possible, since it determines the amount of power that can be transferred to the succeeding stages of the instrumentation system.
Useful Output Range • The output range of a transducer is limited at the lower end by noise signal. • The upper limit is set by the maximum useful input level.
Factors Influencing the choice of Transducer. • Operating Principle • Sensitivity • Operating Range • Accuracy • Cross sensitivity • Errors • Transient and frequency response
Conti.. • Loading effects. • Environmental compatibility • Insensitivity to unwanted signals • Usage and Ruggedness • Electrical aspects • Stability and Reliability • Static characteristics.
Operating Principle: The transducer are many times selected on the basis of operating principle used by them. The operating principle used may be resistive, inductive, capacitive , optoelectronic, piezo electric etc. • Sensitivity: The transducer must be sensitive enough to produce detectable output. • Operating Range: The transducer should maintain the range requirement and have a good resolution over the entire range.
Accuracy: High accuracy is assured. • Cross sensitivity: It has to be taken into account when measuring mechanical quantities. There are situation where the actual quantity is being measured is in one plane and the transducer is subjected to variation in another plan. • Errors: The transducer should maintain the expected input-output relationship as described by the transfer function so as to avoid errors.
Transient and frequency response : The transducer should meet the desired time domain specification like peak overshoot, rise time, setting time and small dynamic error. • Loading Effects: The transducer should have a high input impedance and low output impedance to avoid loading effects.
Environmental Compatibility: It should be assured that the transducer selected to work under specified environmental conditions maintains its input- output relationship and does not break down. • Insensitivity to unwanted signals: The transducer should be minimally sensitive to unwanted signals and highly sensitive to desired signals.
Usage and Ruggedness: The ruggedness both of mechanical and electrical intensities of transducer versus its size and weight must be considered while selecting a suitable transducer. • Electrical aspects: The electrical aspects that need consideration while selecting a transducer include the length and type of cable required. • Stability and Reliability : The transducer should exhibit a high degree of stability to be operative during its operation and storage life.
Static Characteristics :Apart from low static error, the transducer should have a low non- linearity, low hysteresis, high resolution and a high degree of repeatability.
Any method of varying one of the quantities involved in the above relationship can be the design basis of an electrical resistive transducer. • The translational and rotational potentiometers which work on the basis of change in the value of resistance with change in length of the conductor can be used for measurement of translational or rotary displacement.
Strain gauge work on the principle that the resistance of the conductor or a semiconductor changes when strained. This property can be used for measurement of displacement, force and pressure. • The resistivity of the material changes with change of temperature thus causing a change of resistance. This property may be used for measurement of temperature.
Potentiometers • POT • Resistive potentiometer used for the purposes of voltage division is called POT. • Resistive potentiometer consist of a resistive element provided with a sliding contact. • Sliding Contact-Wiper
POT • It’s a Passive Transducer. • Linear Pot –Translational Motion • Rotary Pot-Rotational Motion • Helipots- Combination of the two motions (translational as well as rotational). • In Electrical Measurement , Standard potentiometer are used to measure the unknown voltage by comparing it with a standard known voltage.
