Experiment 9: Measurement of water level and angular displacement using capacitive transducers.

This experiment has two setups, one for angular displacement and other for water level measurement. So it has two sub experiments. The objective of this experiment is to teach about the capacitive transducers used in real field.

Experiment 1: Measurement of angular displacement usingcapacitive 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.
  

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.

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

 Experiment 2: Measurement of water level usingcapacitive transducer.

Setup:

• The capacitive transducer used here is cylindrical in shape. It has an inner metallic rod insulated with Teflon covering and an outer stainless steel pipe is acting as an outer electrode.
• There are some small holes at the bottom of the outer pipe through which the water enters in to the cylindrical capacitor. 
• Capacitor is formed with inner rod acting as one electrode, water acting as the dielectric and outer stainless steel pipe acts as other electrode.
  
• The water acts as the dielectric, which gets changed when water level is changed.  
• The cylindrical capacitor is placed inside a plastic container, where water is poured for level measurement. There is an mm marking scale pasted over the container and a pipe is fitted at the bottom of the container with a flow control switch attached with it, to drain the water. 
• Two knobs are there in the signal conditioning board for calibration purpose (zero and span adjustment).
  

Working:

• When the water is poured in the plastic container it goes inside of the capacitive transducer through the hole at the bottom of the transducer. 
• As per the level of the water it occupies the space as dielectric in the transducer. 
• As the level increases or decreases the dielectric area occupied by water changes. Hence the capacitance of the transducer gets changed with the level of the water. 
• The signal conditioning and processing circuit used to detect the change in capacitance is same as in case of angular displacement measurement. The DC output voltage after the filter circuit is directly calibrated in terms of level of the water inside the plastic container.
  

Procedure:

• Connect the terminal of the capacitive transducer to the signal conditioning board at the place named “Transducer”. Connect a patch cord between T4 and T5. Switch ON the board.
• Drain all the water from the container by the water flow control switch connected to the outlet pipe.Tune the zero adjust pot to show zero reading on the display.
• Pour the water inside the container up to 200mm level and adjust the adder pot to show 200 on the display.
• Connect a dual trace oscilloscope at T1 and T2 and a multimeter at T3 in DC range.
• Now change the water level 10/15mm in step and note down the data. 
• Trace or save three to five wave forms. Follow the link how to save wave form in pen drive.

Observation:

Sl no	Level over the container	Level on Display	Pulse width/ duty cycle at T2		DC voltage at T3