Capacitive type transducers

basic working,construction,types,advantages,disadvantages and alternatives are discussed in this ppt

1. CAPACITIVE TYPE TRANSDUCERS

2. Introduction Work Principle Parameters of capacitive transducer Application of capacitive transducer Advantages and Disadvantages

3. Introduction “Capacitive type means using capacitance and transducer means a device that converts variations in a physical quantity, into an electrical signal” Moreover, the capacitive transducer or sensor is nothing but the capacitor with variable capacitance

4. The capacitive transducer comprises of two parallel metal plates that are separated by the material such as air, which is called as the dielectric material. In the typical capacitor the distance between the two plates is fixed, but in variable capacitance transducers the distance between the two plates is variable. Working Principle

5. The capacitance C between the two plates of capacitive transducers is given by: C= ( ε x ε r x A) / D Where, C - The variable capacitance ε - absolute permittivity ε r - relative permittivity The product of ε & ε r is also called as the dielectric constant of the capacitive transducer. A - area of the plates D - distance between the plates

6. Parameters of Capacitive transducers Changing Dielectric Constant type of Capacitive Transducers Changing Area of the Plates of Capacitive Transducers Changing Distance between the Plates of Capacitive Transducers

7. Changing Dielectric Constant type of Capacitive Transducers In these capacitive transducer the dielectric material between the two plates changes, due to which the capacitance of the transducer also changes. When the input quantity to be measured changes the value of the dielectric constant also changes so the capacitance of the instrument changes. This capacitance, calibrated against the input quantity, directly gives the value of the quantity to be measured. This principle is used for measurement of level in the hydrogen container, where the change in level of hydrogen between the two plates results in change of the dielectric constant of the capacitance transducer. Apart from level, this principle can also be used for measurement of humidity and moisture content of the air.

8. Changing Area of the Plates of Capacitive Transducers The capacitance of the variable capacitance transducer also changes with the area of the two plates. This principle is used in the torquemeter , used for measurement of the torque on the shaft. This comprises of the sleeve that has teeth cut axially and the matching shaft that has similar teeth at its periphery.

9. Changing Distance between the Plates of Capacitive Transducers In these capacitive transducers the distance between the plates is variable, while the area of the plates and the dielectric constant remain constant. This is the most commonly used type of variable capacitance transducer. For measurement of the displacement of the object, one plate of the capacitance transducer is kept fixed, while the other is connected to the object. When the object moves, the plate of the capacitance transducer also moves, this results in change in distance between the two plates and the change in the capacitance. The changed capacitance is measured easily and it calibrated against the input quantity, which is displacement. This principle can also be used to measure pressure, velocity, acceleration etc.

10. Capacitive type transducers has wide range of applications in which s o me of them are: In Microphones In Semiconductor Industries In Detecting Dynamic Motion Application of Capacitive transducer

11. In Microphones An increase of distance between the parallel plates will decrease the capacitance value correspondingly. This transducer is used to convert the value of displacement or change in pressure in terms of frequency.

12. As shown in the figure , A capacitive transducer has a static plate and a deflected flexible diaphragm with a dielectric in between. When a force is exerted to the outer side of the diaphragm the distance between the diaphragm and the static plate changes. This produces a capacitance which is measured using an alternating current bridge or a tank circuit. A tank circuit is more preferred because it produces a change in frequency according to the change in capacitance. This value of frequency will be corresponding to the displacement or force given to the input.

13. Capacitive sensors are used for the precise positioning of semiconductor down to nanometer level for exposure in different electrical and electronic circuits. Semiconductor industry

14. Capacitive transducers are ideal for the measurement of dynamics of a continuously moving target such as rotating spindles or vibrating element like in disc drive spindles. Detecting Dynamic Motion

15. Liquid level measurement by changing the capacitance between two conductive plates. Capacitors are common force sensors. Commonly used for Thickness measurement in a noncontact fashion.

16. Advantages High sensitivity Can detect upto 10 -14 m displacements Low power requirement High resolution Low cost Easy to integrate into ICs

17. Disadvantages Performance is severely effected by dirt and other contaminants. The metallic part must be insulated from each other. Affected by temperature and humidity Sensitivity to noise Difficulties in designing Non-linearity

18. References www.electronicdesign.com www.instrumentationtoday.com http ://www.brighthubengineering.com/hvac /53147-how-capacitive-transducers-work/ http://kalpanatamvada.tripod.com / https://circuitglobe.com/capacitive-transducer.html