TRANSDUCER

A transducer is a device that converts one form of energy into another. Usually, a transducer
converts a signal in one form to another.

Transducers are often employed at the boundaries of automation, measurement, and control
systems, where entire electrical systems (signals) are converted into some other physical
quantities. The process of converting one form of energy to another is known as transduction.

Simplified Classification of Transducers

1.​ Based on Energy Source:​

○​ Active Transducer – Generates output signal without external power.​

○​ Passive Transducer – Requires external power for operation.​

2.​ Based on Output Signal:​

○​ Analog Transducer – Gives continuous output (e.g., thermocouple).​

○​ Digital Transducer – Gives discrete output (e.g., encoder).​

3.​ Based on Function:​

○​ Primary Transducer – Directly senses the physical quantity.​

○​ Secondary Transducer – Converts output of primary into an electrical signal.​

4.​ Based on Transduction Principle:​

○​ Resistive​

○​ Capacitive​

○​ Inductive​

5.​ Based on Direction of Operation:​

○​ Transducer – Converts physical quantity into electrical signal.​

○​ Inverse Transducer – Converts electrical signal into physical quantity.

Passive Transducer:

A passive transducer is a device where the output signal is supplied entirely by the input
signals.

Examples:

●​ Spring-loaded pressure gauge​

●​ Differential transformer​

●​ etc.

Active Transducer

Definition:​
An active transducer is a device that requires an auxiliary power source to supply the majority
of the output power. The input signal contributes only a negligible portion of the output power.

Examples:

●​ Photovoltaic (PV) cell – Converts light into electrical energy.​

●​ Thermocouple – Converts temperature difference directly into voltage.​

Primary and Secondary Transducers

Primary Transducer

Definition:​
The sensing or detecting element that directly responds to changes in a physical phenomenon
such as pressure, temperature, etc.

Example:

●​ Bourdon Tube – The tube deforms in response to pressure and produces mechanical
displacement at its free end.​

Secondary Transducer
Definition:​
A device that converts the output of the primary transducer (such as displacement) into an
electrical signal.

Example:

●​ LVDT (Linear Variable Differential Transformer) – In a Bourdon tube system, the

mechanical displacement of the tube's free end is converted into a corresponding
electrical signal (voltage) by the LVDT.

Transducer vs Inverse Transducer

Type Function Examples

Transducer Converts a non-electrical quantity into an Strain gauge, LVDT,

electrical signal Thermocouple

Inverse Converts an electrical quantity into a Piezoelectric actuator,

Transducer physical quantity Solenoid valve

Analog vs Digital Transducers

Type Function Examples

Analog Converts input into a continuous - Strain gauge (voltage proportional

Transducer (analog) output to deformation)
- LVDT (linear displacement)
- Thermocouple (voltage vs.
temperature)

Digital Converts input into a discrete or - Optical encoder (digital pulses for
Transducer digital output position)
- Digital temperature sensor (outputs
via I2C/SPI)

Digital transducer:​
Converts the input quantity into the digital signal.​
The digital signal works on high to low power.

Piezoelectric transducer:​
When certain solid materials are deformed, they may generate within an electric charge.​
This effect is reversible in that way; if a charge is applied, the material will mechanically deform
in response. This action is called piezoelectric effect.​
The piezoelectric effect can be made to respond to mechanical deformations of the materials in
many modes such as thickness expansion, transverse expansion etc.

Metal electrodes are placed onto selected faces of the material, so that lead wires can be
attached. Since the piezoelectric materials are insulated, the electrodes also placed like the
plates of the capacitor.​
Mechanical deformation give rise to voltage change!

Translational velocity transducer:

The moving coil pickup is based on the law of induced voltage.

E0=(BLvr)⋅10−8

Where:

●​ E0: terminal voltage (V)​

●​ B: field density (T)​

●​ L: length of coil (cm)​

●​ vr​: relative velocity of coil & magnet (cm/sec)​

Since Band Lare constants, the output voltage shows the input velocity linearly and reversed
polarity when the velocity changes sign.

Some pickups are widely used in the measurement of vibration. Since flux density from
permanent magnet is of ~ 10^3, an increase in sensitivity is achieved mainly by increase in
length of coil (wire).

Relative Acceleration Measurement:

Transducers directly sensitive to relative acceleration are there, where shaft rotates at constant
speed eddy current generated in the conducting unit — are constant and rotating conductive
units. These currents produce a field proportional to speed.​
This current links to the two pickup magnetic fields which link to the coil, voltage is produced
because the field is steady, but no coil.

When the shaft accelerates, a changing magnetic field causes coil output voltage which is
proportional to acceleration.​
Speed of response is limited to the electric resonance in the coil, and operational mechanical
response.Digital differentiation of velocity signals may be provided in some areas.

Seismic (Absolute) Displacement Pickups:

Here, we employ an array of uniaxial transducers oriented along the axes (selected) to measure
orthogonal components of the vectors which are then combined by calculations, to define the
total vector magnitude and direction.

The general configuration of a seismic displacement pickup is for uniaxial translatory or rotatory
motion.

These devices are used almost extensively for measurement of displacement in most cases
where a fixed reference for relative-displacement measurement is not available.

The basic principle is to simply measure the relative displacement of a mass connected by a
soft spring to the vibrating body.

For frequencies above the natural frequency, this relative displacement is also very nearly the
absolute displacement since the mass tends to stand still.

LVDT (Linearly Variable Differential Transformer):

LVDT stands for linearly variable differential transformer. It is the most widely used industrial
transformer that converts the linear motion into electrical signals.

(Diagram: showing a primary coil, secondary coils, moving core, and input/output voltage
connections.)

As the primary coil is connected to an AC source, the AC voltage is produced in the

secondary of LVDT.​
The output in secondary S₁ is e1​, and in secondary S₂ is e2​.

The differential output: e out​=e1​−e2​