### Rotary Variable differential transformer (RVDT)

Rotary Variable differential transformer (RVDT)

RVDT transducer is similar to the LVDT in their work but just used to measure angular displacement. inside of this, we use a rotary shaft instead of the movable iron rod. RVDT is an electromechanical transducer whose output is the ac Voltage and this output voltage is directly proportional to the angular displacement. In RVDT there are no electronic components inside it, it is an AC-controlled device. RVDT has a very high accuracy level.

Rotary Variable differential transformer

RVDT is designed to measure the angular displacement so we can also be called it an angular position sensor. it is a passive transducer.

• Compact and strong construction.
• Long life span.
• Very high resolution.
• High durability.
• Easy to handle.
• The performance is repeatable.

• It has moveable parts so the need for periodical maintenance
• It has bulky in size
• It can affect  by surrounding stretch magnetic or electric field

Capacitive transducers:-  A capacitive transducer is also used to measure the displacement of any physical body we can measure the displacement up to serval meter using the capacitive transducer with the good stability and high speed under various environmental conditions and variations
Generally, we use the parallel-plate capacitor as a transducer. And as we know the capacitance formula of a capacitor is

So here, there is three type of variation that happens in the capacitive transducer so we can measure the displacement by varying these 3 variations let's see one by one to each variation.

$C=\frac{\in A}D$
Here
ϵ is the permittivity of the medium,
A is the area of the plates
d is the distance between two plates.

Changing the gap between the plate:- As we know that each capacitor is consist of a movable parallel plate, we can change the distance between them using some external configuration, and this variation in distance between the two plates is shown in the displacement and due to the change in the distance of parallel plate there is a change in the capacitance because capacitance is directly propositional to the change in distance, and we can measure the capacitance using some circuit arrangement.

Changing the gap between two parallel plate

• Changing the area between the plate:- here we know that the area between the two parallel plates of capacitors is directly proportional to the Capacitance. So we can measure the

Changing the area between the two plates of capacitor

displacement by changing the area between two movable plates.
here we can establish some mechanism to measure the linear displacement, so simply we can connect any moving element that causes the change in the area of these two capacitive plates.
In this capacitive transducer, one plate is fixed in its position and the other is variable concerning the other plate, and this process affects the overlapping area of capacitive transducers.

Changing the permittivity of capacitor:- Here we can change the capacitance of the transducer by varying the dielectric material is called the permittivity of the capacitive transducer (ϵ). AS we know that the permittivity of the capacitive transducer is directly proportional to the capacitance of the transducer. here dielectric material is filled between the two parallel plates of capacitors so we can change the capacitance of the transducer by moving the arms, which are directly connected to the dielectric medium.

Changing the dielectric medium of capacitor

• The capacitive transducer has very high sensitivity.
• It requires very less power for operation.
• It has a good frequency response.

It can be effect by a stray magnetic field.
It requires some extra electrical circuitry to measure the change in capacitance.
Capacitance can be affected by changes in temperature variation

Measurement of Strain & Force
Now a day to measurement of strain and force is very essential to determine and examine the various parameters of any physical body. Measuring the strain and force in underground cavities, tunnels, buildings, concrete, bridges, and embedment in soil/concrete, etc is very important. The main purpose of measuring the strain of any physical body is to determine the variation with time, and quantitatively. So for the measurement of strain and force of any physical body use a strain gauge, here we will discuss every point of the strain gauge.

Strain gauge:- Strain gauge is a resistor-based electronics transducer that is used to measure the strain and force of any physical object. it's a passive electrical Transducer. The strain gauge was invented in 1938 by Edward E. Simmons & Arthur C. Ruge. The strain gauge is a very important and significant sensor that is used to measure the amount of strain of any physical object. When we applied any external force and strain on any physical object. So there is a change in the shape of an object. It's called the deformation of an object.  and this deformation of any object happens in both compression and tensile of the object's body. This tensile and compression are measured by the strain gauge sensors.
The deformation of any physical object happens in its limited elasticity. Either object size or shape is increased, narrower and longer and becomes shorter and broadens As a result of it, there is a change in resistance of the object.

Working and Construction of strain Gauge
The strain gauge sensor is made of a thin metallic strip, which arranges in a zigzag pattern on a non-conducting material called the carrier, it is capable to increase the small amount of stress applied to it due to the group of parallel lines, and it can measure stress with very high accuracy, mainly the strain gauge resistive transducer id consists of the four-arm bridge circuit and in this four-arm, we connect a resistor and one is strain gauge thin metallic strip which is the main part of the sensor. Now here we can understand the strain gauge sensor with an appropriate circuit diagram

Strain gauge circuit diagram
strain gauge transducer works on the principle of piezo-resistance effect. In this we can see that, there are the 3 resistors with equal values and one is the strain gauge strip is connected, Resistor R1 & R3 are the ratio arm and it has equal to each other in their resistance value, similar to resistor R2  is the rheostat arm which has equal value to the strain gauge resistance. Initially, the bridge circuit is balanced and there is no change in their resistance and the output voltage is remaining 0 concerning the bridge circuit but when applied any force any strain so there is a change in the shape of the thin metallic strip and causes use the change in the resistance of the circuit and our bridge circuit become unbalanced, Now here we can measure the variation in the voltage due to change in the resistance of the bridge circuit. This output voltage can be further amplified to send to a remote area or control room.

Deformation in a thin metallic strip of strain guage

Temperature effect on the strain gauge or metallic plate:- As we know that the temperature is a major problem in any circuital device because when the temperature value goes up there is a little bit change in electrical parameter or factor so same problem occurs in the strain gauge transducer, in single plate strain gauge temperature is affect it and change in temperature cause the change in resistance as well, so this problem here we connect one extra thin metallic plate which overcomes this temperature problem in the strain gauge and keep balance the whole bride circuit,

Classification of strain gauge:-
Wire wound strain gauge:- It is composed of fine wire. The resistance range of this transducer is 100 to 500 ohm. This is called a resistance wire strain gauge.

Foil type strain gauge:-  This transducer has made up of metal foil, it has a great surface area in size, and high dissipation capacity with a high-temperature range it can measure the resistance between 50 to 1 k ohm, the current range of this transducer is 30A.

Semiconductor strain gauge:- This semiconductor-type strain gauge works on the piezoresistance-based principle. It means a change in resistance value due to a change in resistivity of the element. It consists of silicon and germanium material, it has a high

gauge factor and a small envelope is needed, it provides the great Teflon insulation.

Capacitive Strain gauge:-  Capacitive strain gauge transducer work on the variable capacitance principle, here we applied strain on the top of the plate of the capacitive transducer. here the charge is the distance between the curved electrodes, and their result is a change in then capacitance. This can use to measure a wide range of measurements. It has a temperature range is 300 degrees celsius and the capacitance is 0.5 picofarad

• It does not have any moving parts.
• It is small and inexpensive.
• It can produce accurate measurements of the strain.
• Different sizes and materials of gauge allow the correct one for the application to be chosen.