Introduction to sensors and transducerss

Sensors

1. Sensors and Transducers

2. ACCURACY

3. Range and Span (Example) Example: a sensors is designed for:  C to +80  C to output 2.5V to 1.2V Range:  C and +80  C Span: 80  (  30)=110  C Input full scale = 110  C Output full scale = 2.5V-1.2V=1.3V Dynamic range=20log(140/30)=13.38db CBIT, Hyderabad

4. Range and Span Range: lowest and highest values of the stimulus Span: the arithmetic difference between the highest and lowest values of the input that being sensed. Input full scale (IFS) = span Output full scale (OFS): difference between the upper and lower ranges of the output of the sensor. Dynamic range: ratio between the upper and lower limits and is usually expressed in db CBIT, Hyderabad

5. Classification of Errors Gross Errors Systematic Errors A) Instrument Errors i) Inherent Shortcomings of Instruments ii) Misuse of Instruments iii) Loading Effects B) Environmental errors C) Observational errors

6. General Errors in Sensor Measurement Absolute Error Relative Error Limiting Error Propagation of Errors Gross Errors Systematic Errors Random Errors Loading Effect Statistical Analysis of Measurement and Probable Error

7. Absolute Error Absolute error is the magnitude of difference between a measured value and a true or exact value. Absolute Error = |True Value – Measured Value| Mean absolute error is average of absolute errors Relative Error shows the size of the error relative to the true value. Relative Error = Absolute Error/True Value Percent Error shows the what percent error is the true or accepted value. Percent Error = Relative Error x100%

8. For Example A sensor gives a measurement as 53 and the true or known value is 55. Find the Absolute Error, Relative error and Percent Error? A sensor gives the measurements of 58, 59 60 units at three different instances. The true measurement is 60 units. What is the mean absolute error of the measurements.

9. Classification of Errors Gross Errors Systematic Errors A) Instrument Errors i) Inherent Shortcomings of Instruments ii) Misuse of Instruments iii) Loading Effects B) Environmental errors C) Observational errors

10. Gross Errors The main source of Gross errors is human mistakes in reading or using instruments and in recording and calculating measured quantity. As long as human beings are involved and they may grossly misread the scale reading, then definitely some gross errors will be occurred in measured value.

11. Example, Due to an oversight, Experimenter may read the temperature as 22.7C while the actual reading may be 32.7 C 0. The complete elimination of gross errors is maybe impossible, one should try to predict and correct them. Some gross errors are easily identified while others may be very difficult to detect.

12. Gross errors can be avoided by using the following two ways Two, three or even more readings should be taken for the quantity being measured by using different experimenters and different reading point (different environment condition of instrument) to avoid re-reading with same error. So it is suitable to take a large number of readings as a close agreement between readings assures that no gross error has been occurred in measured values.

13. Systematic Errors Systematic errors are divided into following three categories. Instrumental Errors Environmental Errors Observational Errors

14. Instrumental Errors These errors are arises due to following three reasons Due to Inherent Shortcoming of instruments Due to misuse of the instruments Due to loading effects of instruments Elimination or reduction methods of these errors The instrument may be re-calibrated carefully The correction factors should be applied after determing the instrumental errors. The procedure of measurement must be carefully planned.

15. Environmental Errors Due to external environmental conditions of the instrument, such as effects of temperature, pressure, humidity, dust, vibration or external magnetic or electrostatic fields . Elimination or reduction methods of these undesirable errors are Arrangements should be made to keep the conditions as nearly as constant as possible. Example, Example, variations in resistance with temperature can be minimized by using very low resistance temperature co-efficient of resistive material . Employing techniques which eliminate the effects of these disturbances. For example, the effect of humidity dust etc., can be entirely eliminated by tightly sealing the equipment.

16. Environmental Errors The external or electrostatic effects can be eliminated by using magnetic or electrostatic shield on the instrument . Applying computed corrections: Efforts are normally made to avoid the use of application of computed corrections, but where these corrections are needed and are necessary , they are incorporated for the computations of the results.

17. Observational Errors There are many sources of observational errors. As an example, the pointer of a voltmeter rests slightly above the surface of the scale. Thus an error on account of PARALLAX will be acquired unless the line of vision of the observer is exactly above the pointer.

18. Limiting Error The limited deviation of the measured value from the true value is known as the limiting error or guarantee error. Limiting Errors can be minimised by Make sure to keep the ambient physical conditions constant. For example, placing the instrument in a temperature-controlled enclosure ensures the ambient temperature to be kept constant. Use instruments which have ample immunity to effects of environmental changes. Use different techniques, for example sealing the instrument, to eliminate the effects. Use computed corrections.

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