PLT 202– Instrumentation and Measurement By: Chanuri a/p Charin

1. PLT 202– Instrumentation and Measurement By: Chanuri a/p Charin Chapter 3: Concept and Principles of Transducers and Sensors

2. Course Outcome (C03) At the end of the lesson, students should be able to: • define basic concept of transducers and sensors • Ability to DESIGN and apply the working principles of various sensors

3. Objectives: 1. To get familiarize with several types of transducers and selection criteria. 2. Able to apply basic principles of operation and application of common transducer.

4. Definition of Transducers Transducer - device that converts one form of energy into another form of energy • Sound > Electric • Electrical > Sound • Mechanical motion > Electrical Signal

5. Type of Transducers Electrical Transducers • Converts the input measurand into an electrical voltage/current Mechanical Transducers • Converts the input measurand into a mechanical energy

6. Application of Transducers Electrical Sensor • Device that capable to detect electrical signal and sent it to another measurand electrical output TRANSDUCER excitation

7. Advantages of Electrical Transducers • Electrical amplification and attenuation can be easily done • Mass-inertia effects are minimized • Effect of friction are minimized

8. Advantages cont… • The output can be indicated and recorded remotely at a distance from the sensing medium • The output can be modified to meet the requirements of indicating or controlling units • The signal can be conditioned or mixed to obtain any combination with outputs of similar transducers or control signal

9. Classification of Transducers • Active Transducers • Passive Transducers

10. Active Transducer • Generates electrical signal directly in response to physical parameter • Do not requires external power for its operation. Can generate electric current • Self generating device, operate under energy conversion principle • Example piezo electric sensor, photo voltaic cell, thermocouple measurand Active Transducer electrical output

11. Passive Transducer • Operates under energy controlling principle • Require external power source to operate • Can’t generate electric current • Depend upon change in electrical parameters ( R,L and C) • Example : strain gauge, thermistor measurand Passive Transducer electrical output external power

12. Selecting a Transducers

13. Parameter for Transducers

14. PASSIVE TRANSDUCERS Resistive transducer PASSIVE TRANSDUCERS • Potentiometer • Resistive Position transducer Strain Gauges THERMISTOR Capacitive transducers LVDT RTD

15. Potentiometer • Electromechanical device containing a resistance element that is contacted by a movable slider • The motion of the movable slider may be translatory or rotational.

18. The output voltage of the position of the movable slider and is

19. Example: A displacement transducer with a shaft stroke of 30 cm is applied to the circuit. The total resistance of the potentiometer is 5k Ω . The applied voltage VS is 5V. Calculate the output voltage when the wiper is 9cm from B.

22. Resistive Position Transducer • Example: A potentiometer transducer with a shaft stroke of 8cm is used in circuit below. The applied voltage is 10V. The total resistance of potentiometer R1 and R2 is 6kΩ. The total resistance of the potentiometer R3 and R4 is 4kΩ. The initial position to be used as a reference point is set such that R1 is 4.5 cm and R3 is 3.5cm of the shaft stroke length (from point A). • i. Calculate the values of R1, R3 and VE at initial position • ii. Calculate the displacements of potentiometer R3 and R4 in the case that VE =0. Then identify the direction of the displacement

24. Exercise: A displacement transducer with a shaft of 2.0mm is used in the circuit as shown in figure below. The total resistance of the potentiometer R1 and R2 is 5000Ω and the applied voltage is 5.0V. The total resistance of the potentiometer R3 and R4 is also 5000Ω.The initial position to be used as reference point is set such that R1 = R2 (i.e. when the shaft is at mid-stroke). Initially, potentiometer R3 and R4 is adjusted so that the bridge is balanced (i.e. VE = 0). Assuming the shaft of the potentiometer R3 and R4 will be moved 0.5mm towards A, what is the value of VE?

26. Potentiometer senses displacement by means of sensing shaft, which is mechanically connected to the point or objects whose displacement, is to be measured. Example: Petrol-tank level indicator. In this case, potentiometer is used to indicate/sense the petrol level in a tank as shown in Figure below. The output signal (voltage) is proportional to the petrol level.

28. Advantages & Disadvantages of Potentiometer

29. Strain Gauge • Strain gauge - passive transducer that uses the variation in electrical resistance in wires to sense the strain produced by force on wires. • It is used for measuring weight, pressure, mechanical force and displacement. • A bonded strain gauge consists of a fine wire looped back and forth on a mounting plate which is usually cemented to the member undergoing stress as shown below:

30. Bonded strain gauge

31. Strain gauge is generally uses as one arm of a bridge is shown Figure below. • This method is capable to measure the change in resistance when the wire is under strain.

32. In some cases, strain gauges are used in pairs (active gauge and dummy gauge) to provide temperature compensation as in Figure below. However, only the active gauge will respond to stress. • The dummy gauge is mounted in an insensitive orientation to provide some compensation for temperature effects.

33. Strain gauge 1 is stretch Strain gauge 2 is compressed

35. Example of Strain Gauge

37. The strain will cause: • i) The change in length ΔL • ii) The change in gauge resistance ΔR

42. Capacitive Transducer • A capacitor consists of two parallel plates separated by an air space or by a dieletric (insulating material) as shown in figure below • The capacitance of the pair of plates is measure of the amount of charge that can be transferred before a certain voltage is reached. • If the capacitance is large, more charge is needed to establish a given voltage difference

43. a capacitor

44. Advantages : 1. Required extremely small forces to operate them and hence are very useful for use in small systems. 2. Extremely sensitive. 3. A good frequency response as high as 50kHz and useful for dynamic studies. 4. High input impedance therefore the loading effects are minimum. 5. The force requirements is very small and therefore require small power to operates them • Disadvantages: 1. The metallic parts of the transducer must be insulated from each other in order to reduce the effects of stray capacitance, the frames must be earthen. 2. The output impedance of the capacitive transducers tends to be high on account of their small capacitance value this leads to loading effects.

45. Uses of Capacitive Transducer 1. It can be used for measurement of both linear and angular displacements. 2. It can be used for measurement of force and pressure. The force and pressure to be measured are first converted to displacement which caused a change in capacitance. 3. It can be used for measurement of humidity in gases since the dielectric constant of gases changes with change in humidity thereby producing a change in capacitance. 4. It is commonly used in conjunction with mechanical modifiers for measurement of volume, density, liquid level, weight and etc.

46. Applications

47. Example: A capacitive transducer is used for the measurement of linear displacement, X, as shown in below. The parallel plate has a dimension of 5.0cm X 5.0cm and is separated by a distance of 1.0cm. The space between the plates is filled with a dielectric material of 1.0cm thick, which has a dielectric constant of 4.0. If the dielectric constant for air is 1.0cm, determine the value of the capacitance when x is equal to: (i) 0.0cm (ii) 2.0cm

49. Exercise:Figure below shows a capacitive transducer used for measurement of linear displacement, x. the parallel plates have a dimension of (4.0 cm x 4.0cm) and separated by a distance of 10 mm. the space between plates is filled with a dielectric material with constant of 3.0.If the dielectric constant for air is 1.0, determine the value of the capacitance when x is equal to: • i) 0.0 cm • ii) 2.0 cm • iii) 4.0 cm

50. iv) What is the effect of capacitance if high dielectric constant is used? Given εo = 8.854 x 10-12 F/m.