The Capacitance Transducers

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The Capacitance Transducers Powered By Docstoc
					                Kuwait University Electrical Engineering Department
                   Measurement & Instrumentation Laboratory
                    The Capacitance Transducers
                                                                   By Eng. Ahmad Haitham
Objectives:
                •    Variation of capacitance value by varying dimensions.
                •    Get frequency variation or voltage variation due to varying
                     capacitance value (transducers).

Knowledge Required:

                •    Understand the resonance circuit.
Theory:

       Methods of measuring small changes in capacitance will be investigate,




It is very difficult to measure small values of capacitance using normal Wheatstone
bridge techniques; therefore some other method is needed when transducers whose
operation depends on detecting small capacitances are used.

If capacitance position transducer is used, i.e. varying the area or distance, knowing the
actual capacitance of the transducer is less important than knowing the relationship the
thing is being measured and the output. So we are looking to transducers that depend on
variation of capacitance to vary parameters of an electronics circuit.

Perhaps the most common of such circuits is the inductance-capacitance resonant circuit
shown in figure 1.




                                        Figure 1.
The resonant circuit can be used, together with an amplifying circuit to form another type
of circuit which is known as an oscillator. The oscillator produces an AC sinusoidal
output at a frequency governed by the resonant frequency of the tuned circuit of the
oscillator which is also changed by varying the value of the capacitance.

The TK2941 contains such an oscillator circuit. The external component to the left can be
added as variable capacitance of variable inductance.

Also in the oscillator circuit there is inside a variable capacitance which is controllable by
the “frequency knob”. The oscillator also contains an amplifier can be controlled by
“output voltage pk-pk knob” to give a variable output ac voltage.

Equipment Required:
         TK2941A module, function generator Agilent 3320A, DC voltmeter, Tk294H
variable capacitor (Area) sub unit, TK294J variable capacitor (Distance) sub unit, TK294
linear transducer test rig, and digital frequency meter.

Frequency variation due to variable area capacitance:
Setup the circuit in figure2.




                                          Figure 2.
The lower “external tuning comp” socket should be connected to TK294H and set SW9
to C position.

Set the slider to 45mm, so that the steel rod is positioned half way through the capacitive
transducer.

Set the oscillator frequency to give 465k Hz at the output. Fill table 1.
                              Slider position    Frequency
                                   (mm)            (k Hz)
                                     20
                                     25
                                     30
                                     35
                                     40
                                     45
                                     50
                                     55
                                     60
                                     65

                                           table 1
Frequency variation due to variable distance capacitance:
Setup the circuit in figure 3.




                                          Figure 3.
The lower “external tuning comp” socket should be connected to TK294J and set SW9 to
C position.

Set the slider to 12.5mm, so that the steel rod is positioned half way through the
capacitive transducer.

Set the oscillator frequency to give 465k Hz at the output.
Fill table 2.



                             Slider position     Frequency
                                 (mm)              (k Hz)
                                   23
                                   24
                                   25
                                   26
                                   27
                                   28

                                          table 2

				
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Description: Capacitive sensors are often known as the capacitance level meter, capacitance level meter capacitive sensing element is a cylindrical capacitor according to the principle of work, by the two capacitor plates insulated coaxial cylindrical inner and outer electrodes electrodes, between the two cylinders filled with a dielectric constant e of the electrolyte, the capacitance between two cylinders for the C = 2ΠeL/lnD/d, where L is the overlapping part of two tube length; D electrode diameter for the outer tube; d is the diameter of the inner cylinder electrode; e is the dielectric constant of the intermediate medium. In the actual measurement D, d, e is basically unchanged, so you can know the level measured at the level of C, which is easy to use with capacitive sensors, a simple structure and high sensitivity, low price and so one of the reasons .