Experiment 9(a): Measurement of Angular displacement using capacitive Transducer

Setup:

• In this setup a variable overlapping area capacitor has been used.
• A knob is connected to one of the plate which is free to move and another plate is fixed. The knob is placed over a protractor and there is a pointer which is also connected with the knob and moves over the protractor by tuning the knob.
• There are two other knobs also for calibration purpose, one for zero reading adjustment and other is for span adjustment.
• Four 7 segment LED display also there to display the angle directly in degree after proper calibration.

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Working:

• This capacitive transducer works on the principle of change of capacitance caused by change in overlapping area (A) of plates.
•  When the knob over the protractor is turned the plate attached to it moves. So the area between parallel plates changes which in turn changes the capacitance. In this capacitor semi-circular plates are used.
• There is an astable multivibrator used in the signal conditioning circuit, that generates square wave of frequency 4KHz. These square wave pulses are used as triggering pulses for a Monostable Multivibrator. 
• The ON time of the pulse available at the output of the monostable multivibrator depends on a time constant formed by a capacitor and a resistor (RC time constant). The capacitive transducer used here changes that time constant while the resistor has been kept at fix value. 
• Hence when the capacitance changes by turning the knob over the protractor the width of thepulse at the output of the monostable mutivibrator changes. 
• The output pulse of the monostable multivibrator is passed through a low pass filter, which gives a DC output voltage proportional to the Duty Cycle of the pulse. And this DC voltage is further processed to display the angle, indicated by the pointer over the protractor, directly on the display unit.

 Apparatus required:

• Dual trace oscilloscope.
• Multimeter
• Trace sheet or pen drive of 4 GB or less.

Procedure:

• Connect a patch cord between T4 and T5. Switch ON the board. 
• Turn the knob over the protractor so that the pointer will place over zero degree, then adjust zero adjust pot so that the display will show zero. 
• Now turn the knob to indicate 90 degree on protractor and adjust the adder pot to show 90 on the display. 
• Connect a dual trace oscilloscope at T1 and T2 and a multimeter at T3 in DC range. 
• Now change the angle over protractor from 0 to 90 degree, 10 degree in step, and record the pulse width/ duty cycle  at T2 and corresponding DC voltage at T3.
• Trace or save three to five wave forms. Follow the link how to save wave form in pen drive.

Observation:

Sl no	Angle over Protractor	Angle on Display	Pulse width/ duty cycle at T2		DC voltage at T3