- In a measurement system the transducer (or a combination of transducers) is the input element with the critical function of transforming some physical quantity to a proportional electrical signal. So, selection criteria of transducer play an important role for having accurate results.
- The first step in the selection procedure is to clearly define the nature of quantity under measurement (measurand) and know the range of magnitudes and frequencies that the measurand is expected to exhibit.
- Next step will be to examine the available transducer principles for measurement of desired quantity. The type of transducer selected must be compatible with the type and range of the quantity to be measured and the output device.
- In case one or more transducer principles are capable of generating a satisfactory signal, decision is to be taken whether to employ a commercially available transducer or build a suitable transducer.
- If the transducers are available in the market at a suitable price, the choice will probably be to purchase one of them, otherwise own transducer will have to be designed, built and calibrated.
Selection Criteria of a Transducer
The points to be considered in determining a transducer suitable for a specific measurement are as follows
The range of the transducer should be large enough to include all the expected magnitudes of the measurand.
The transducer should give a sufficient output signal per unit of measured input in order to yield meaningful data.
- Electrical Output Characteristics:
- The electrical characteristics-the output impedance, the frequency response, and the response time of the transducer output signal should be compatible with the recording device and the rest of the measuring system equipment.
- Physical Environment:
- The transducer selected should be able to withstand the environmental conditions to which it is likely to be subjected while carrying out measurements and tests.
- Such parameters are temperature, acceleration, shock and vibration, moisture, and corrosive chemicals might damage some transducers but not others.
- The errors inherent in the operation of the transducer itself, or those errors caused by environmental conditions of the measurement, should be small enough or controllable enough that they allow meaningful data to be taken.
- However, the total measurement error in a transducer-activated system may be reduced to fall within the required accuracy range by adopting the following techniques.
- Calibrating the transducer output against some known standards while in use under actual test conditions. This calibration should be performed regularly as the measurement proceeds.
- Continuous monitoring of variations in the environmental conditions of the transducer and correcting the data accordingly.
- Controlling the measurement environment artificially in order to reduce possible transducer errors. Artificial environmental control includes the enclosing of the transducer in a temperature-controlled housing and isolating the device from external shocks and vibrations.
Basic Requirements of a Transducer
The main function of a transducer is to respond only for the measurement under specified limits for which it is designed. It is, therefore, necessary to know the relationship between the input and output quantities and it should be fixed. Transducers should meet the following basic requirements.
- Ruggedness: A transducer should be capable of withstanding overload and some safety arrangement should be provided for overload protection.
- Linearity: Its input-output characteristics should be linear and it should produce these characteristics in symmetrical way.
- Repeatability: The ability to produce same output for same input at different times and environment is called repeatability. A transducer should reproduce same output signal when the same input signal is applied again and again under fixed environmental conditions e.g., temperature, pressure, humidity etc.
- High Output Signal Quality: The quality of output signal should be good i.e. the ratio of the signal to the noise should be high and the amplitude of the output signal should be enough.
- High Reliability and Stability: The output of a transducer should not be affected by temperature, vibration, other environmental conditions, with minimal error.
- Good Dynamic Response: It is the ability to change output quickly for dynamic variation in input with respect to time. Transducer output should be faithful to input when taken as a function of time. The effect is analyzed as the frequency response.
- No Hysteretic: It should not give any hysteretic during measurement while input signal is varied from its low value to high value and vice-versa.
- Convenient Instrumentation: It should have high analog output, high Signal to noise ratio (SNR), direct measurement or after amplification.
- Good Mechanical Characteristics: The ability to withstand deformation and deterioration in performance caused by various mechanical strains.