Introduction
Capacitive transducers are passive transducers that determine the quantities like displacement, pressure, temperature etc. by measuring the variation in the capacitance of a capacitor.
For circular plate capacitor
Therefore we can write
From above equation it is found that, capacitance can be varied with _{ε_0,\ A,\ d,\ θ}
So, capacitive transducers ca be used as displacement transducer in two ways,
i) Transnational and ii) Rotational
Capacitive Transnational Transducers
Variation in capacitance for this type of transducers are due to
i) Change in the area of overlapping between the two plates i.e., change in A
ii) Change in the distance, d between the two plates and
iii) Change in relative permittivity ε of dielectric material present in between the two plates.
Change in Area
Change in Distance
In this type of displacement sensor, the sensitivity is non-linear, which makes the calibration task complex. To overcome this non-linear response, a push-pull arrangement provides a linear response.
Push Pull arrangement
Here a third plate is inserted in between the two plates, and this central plate can move in either direction vertically.
If the central plate is equidistant from the outer plates, for which x is zero and output is also zero. Otherwise It is seen that the output varies linearly.
Change in Relative Permittivity/Dielectric Medium
In this configuration, the dielectric slab can move horizontally in either direction and thus produce a change in capacitance.
Therefore, capacitance now varies linearly with the length of the dielectric slab that enters between the capacitor plates.
Capacitive Rotational Transducers
In reference to FIG. 2, it can be written for circular plate capacitors that,
Signal Conditioning Units
Primary sensing element for the capacitive transducer is a capacitor which undergoes changes in capacitance under the influence of the physical parameters like displacement, pressure, moisture, proximity etc.. To enable the instrument to perform the desired function, this variable may need to be converted to another more proper variable while keeping the information content of the original signal. The unit/element performs such function is called signal conditioning unit or conversion element.
Signal conditioning unit (SCU) performs the tasks of converting change in capacitance into voltage, frequency as per the desire and some additional jobs like attenuating, filtering, isolating etc. The common SCUs are:
- AC Bridge circuits
- Charge amplifiers
- Oscillator Circuits
AC Bridge Circuits
AC bridge circuit is used to sense change in capacitance and provides equivalent voltage change. Consider the AC bridge circuit as shown below:
Let the unknown capacitor C3 is placed at Z3 and a known capacitor C1 is placed at Z1. Also assume Z2 = Z4= R. Then
Therefore, from eq. (1)
Now, unknown capacitor C3 is
Therefore, by eq. (2), we get
Eq. (3) shows non-linear response.
If we use capacitors in push-pull configuration as shown in Fig. 8, then
Therefore, by eq. (2), we get
Eq. (4) shows linear response.
Charge amplifiers
A typical charge amplifier circuit is as shown below:
A charge amplifier uses a capacitor, C f in the feedback loop.
A parallel resistor, R f is used to minimize the gain at very low frequencies and reduce the DC offset voltage at the output. We will assume this resistor has sufficient resistance to not affect the passband gain.
Sensor is connected at the input as shown in the above circuit. Output of the amplifier 1 is given by
Output of the amplifier 2 is thus
Therefore, as the capacitance of the transducer changes accordingly the equivalent charge q will change and hence the output voltage will also change.
It is clearly shows that the output voltage of a charge amplifier depends only on the charge input and the feedback capacitance. Input and cable capacitances have no influence on the output signal. This is a significant fact when measuring with different cable lengths and types.
Oscillator Circuits
•In astable mode, the 555 timer IC generates very stable and highly accurate free-running waveforms, the output frequency of which can be adjusted using an externally connected RC tank circuit made up of only two resistors and a capacitor.
•The capacitor then charges to 2/3Vcc (the upper comparator limit) as determined by the 0.693(R1+R2)C1 combination and discharges to 1/3Vcc (the lower comparator limit) as determined by the 0.693(R2*C1) combination. This produces an output waveform with a voltage level close to Vcc – 1.5V and output “ON” and “OFF” time periods determined by the capacitor and resistor combinations. As a result, the individual times required to complete one charge and discharge cycle of the output are as follows:
•The duration of one full timing cycle is thus equal to the sum of the two individual times that the capacitor charges and discharges, which is given as:
The output frequency of an Astable 555 Oscillator as:
Applications of Capacitive Transducers
- Pressure measurement
- Liquid level Measurement
- Moisture Measurement
- Proximity Measurement
- Capacitive touch Screen
Pressure Measurement
Gauge pressure measurement:
The pressure with respect to atmospheric pressure is known as gauge pressure. The capacitive pressure transducer consists of a pair of conductive plates, one of which is a diaphragm from a closed chamber, as shown in the figure below. As the pressure inside the chamber rises, the diaphragm moves closer to the fixed plate, causing a change in capacitance. An appropriate SCU converts the change in capacitance into an equivalent output voltage that can be calibrated in terms of pressure.
Liquid level Measurement
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Moisture Measurement
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Proximity Measurement
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Capacitive Touch Screen
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