What is Capacitive Transducer : Working and Its Applications

Energy, we can’t destroy it but we can change the energy from one form to another form. In some situations, we need to take help from one form of energy to change it into another form. So the energy conversion process can be done by using “Transducers”. There are various types of transducers like pressure transducers, piezoelectric transducers, ultrasonic transducers, temperature transducers, capacitive transducers, etc. In this article, we are going to know about what is a capacitive transducer, its working principle, circuit diagram, types, and its applications, advantages, and disadvantages.

What is a Capacitive Transducer?

Transducers are categorized into two types such as active transducers and passive transducers. Active transducers are one kind of transducers which do not require any kind of power for their operation. Whereas passive transducer requires external power for their operation in the energy conversion process. These transducers have come under passive transducers.

The definition of the capacitive transducer is to measure the displacement (how much distance it covers), pressure and other several physical quantities, these transducers are preferred. In these transducers, the capacitance between the plates is varied because of the distance between the plates, overlapping of plates, due to dielectric medium change, etc.

Capacitive Transducer Working Principle

The above diagram indicates the capacitive transducer. The working principle of a capacitive transducer is variable capacitance. As per its structure, these are having two parallel metal pates which are maintaining the distance between them. In between them, dielectric medium (such as air) can be filled. So, the distance between these two metal plates and positions of the plates can change the capacitance. So, variable capacitance is the principle of these transducers. The basic difference between the normal capacitors and capacitive transducers is, the capacitor plates are constant in normal capacitors wherein these transducers, capacitor plates are the movable condition.

The capacitance of the variable capacitor can be measured by this formula.

In this formula:

C indicates the capacitance of the variable capacitance
εo indicates the permittivity of free space
εr indicates the relative permittivity
A indicates the area of the plates
D indicates the distance between the plates

So according to the formula, the variable capacitance value is dependent on four important parameters. They are the distance between the plates of the variable capacitor, occupying area of the plates, permittivity of the free space, relative permittivity and dielectric material. These parameters can be varying the capacitance value of the variable capacitor.

• Change in dielectric constant can vary the capacitance of this transducer.
• The area of the plates of these transducers can vary its capacitance value.
• Distance between the plates can vary the transducers’ capacitance value. This method is mostly used. In this method, the dielectric medium and area of the plates are kept constant. When the plates are moving then the distance is varied and this results in the changing of the capacitance of the capacitive transducer.

These above three methods are used to change the capacitance value of this transducer.

Capacitive Circuit Diagram

The above circuit diagram indicates the equivalent circuit diagram of a capacitive transducer. The difference between the variable capacitor to the normal capacitor is, the capacitance of the variable capacitor is varied whereas in a normal capacitor, the capacitance value is fixed & it cannot be changed.

Types of Capacitive Transducer

According to the structure of the capacitive transducer, they are four types which are discussed below. They are

• Parallel plate capacitance with rectangular plates.
• Cylindrical capacitor transducer.
• Semi circular parallel plates.
• Change in dielectric between parallel plates.

Parallel Plate Capacitance with Rectangular Plates

This is also called a flat type of capacitive transducer. In this type of transducer, one plate is fixed and the other plate can be moved. By this variation, the distance d or area A can be varied. This results in the capacitance value of this transducer.

If area A varies and the capacitance value C would be when the plates are having the distance of x, then

C= ε(A-wx)/d

Cylindrical Capacitive Transducer

Considering the length of the cylinder is to be L, then the capacitance

Semi Circular Capacitive Transducer

This type will provide the highest capacitance value when the two capacitive plates are overlapped to each other. These are preferable when the circuit requires the maximum capacitance.

In this type capacitive transducer, area A = πr^2/2  and the capacitance C=ε πr^2/2d

Change in Dielectric Medium between Parallel Plates

When the dielectric medium varies between the two parallel plates of this transducer, that also varies the capacitance of the transducer.

Therefore capacitance C= εo (ε1*L1*w + ε2*L2*w)/d

Here – L1 and L2 are indicating the length of the 1st and 2nd plates length.

W indicates the width of the plate

D indicates the distance between the plates

Advantages

The capacitive transducer advantages are discussed below. They are

• These transducers offer high input impedance. So the loading effects value will be too low.
• The frequency response of these transducers is extremely high.
• These transducers are highly sensitive.
• These are consuming low power to operate. So, these transducers are called low power consuming devices.
• High resolution can be possible by using these transducers.

Disadvantages

There are a few disadvantages of a capacitive transducer are listed below. They are

• It has a high output impedance. Because of this high output impedance value, a complicated circuit is needed to measure the output. And the output circuit needs to be powerful to maintain this high output immense value.
• These transducers exhibit non-linear behaviors due to edge effects.
• These are temperature-dependent. The external temperature value can affect this transducer capacitance value.

 Applications

The Applications of the capacitive transducer are

• This transducer has a wide range of applications in determining the quantities like temperature, displacement, and pressure, etc. Capacitive transducer applications are listed below.
• These transducers have applications in the field of linear and angular displacement with the sensitivity factor.
• One of the best applications of this transducer is to find the humidity level. As the humidity value changes the capacitance value of this transducer also changes. By this value, we can measure the change in humidity.
• The variable capacitance pressure transducer is applicable to find the pressure variations by using the variable capacitance.

Thus, capacitive transducers are useful for converting one form of energy into another form of energy by taking the change in capacitance value. These are the passive transducers because this requires external power to operate them. And with the help of these transducers, we can measure the pressure, temperature, and displacement, etc.