POTENTIOMETER
&
LINEAR VARIABLE
DIFFERENTIAL
TRANSFORMER
Engr. Raymond J. Pidor, PECE
TRANSDUCER
a device that converts energy from one form to
another
common examples include microphones,
loudspeakers, position, light intensity, speed,
acceleration, temperature and pressure sensor
INPUT TRANSDUCERS
All industrial control depends on the ability to
accurately and swiftly measure the value of controlled
variable.
It has been found that the best way to measure the
value of a controlled variable is to convert it into an
electrical signal of some sort and to detect it with the
electrical measuring device.
This approach is superior to converting the value of
the controlled variable into a mechanical signal
because electrical signals have certain advantages
over mechanical signals:
electrical signals can be transmitted from place to place
much more easily than mechanical signals
electrical signals are easier to amplify and filter than the
mechanical signals
electrical signals are easy to manipulate to find out such
things as the rate of change of variable.
POTENTIOMETER
A potentiometer (also known as a 'pot') is a modified
resistor.
It can be used to allow a change in the resistance in a
circuit or as a variable voltage divider (in the case of a
volume control).
It is the most common electrical transducer where it
can be used alone or attached to a mechanical sensor
to convert a mechanical motion into an electrical
variation.
A potentiometer generally has 3 terminals. 2 of the
terminals are connected to the opposite ends of a
resistive element. The 3rd terminal (usually, is
physically in-between the other 2 terminals) is called
the wiper. The wiper is a contact (actually, generally
many very small contacts) that slides along the
resistive element.
SCHEMATIC SYMBOL
POTENTIOMETER LINEARITY
The term linear means that a given mechanical
movement of the wiper produces a given change
in resistance no matter where the wiper happens
to be on the element. It also means that the
resistance is evenly distributed along the length
of the element.
POTENTIOMETER RESOLUTION
Resolution in a pot means it is the smallest
possible change is resistance as the wiper moves
along the element.
A POTENTIOMETER AS A VARIABLE
VOLTAGE DIVIDER:
A pot connected as it is in
the following diagram will
act to divide the voltage
like the 2 individual
resistors.
In the middle of travel,
the resistance from the
sliding terminal to either
of the other terminals is
half of the total
resistance.
The output is simply the
voltage at the point where
the wiper contacts the
resistive element.
RESISTANCE TAPER
The potentiometer had a 'linear' taper. These are
general-purpose potentiometers and may be used
for controlling DC voltage or to control the levels
of the individual bands on an equalizer.
For volume controls you need to use a
potentiometer with a 'log' (short for logarithmic)
taper. This is because the human auditory
system works logarithmically. If we'd use a linear
taper pot for a volume control, the low end of the
volume control's range would be 'touchy'.
ROTARY POTENTIOMETERS:
A single turn pot will only
travel through about 270º of a
complete revolution.
There are other potentiometers
that have some sort of
mechanism to increase the
number of times the shaft must
be turned to travel through its
entire range of motion. This is
generally done with gears or
bearings. The multi-turn
potentiometers are used to
make it easier to precisely set
the output level.
WHEATSTONE BRIDGE CIRCUIT
LINEAR VARIABLE DIFFERENTIAL
TRANSFORMER (LVDT)
A common type of electromechanical transducer
that can convert the rectilinear motion of an
object to which it is coupled mechanically into a
corresponding electrical signal.
A passive inductive transducer that require an
external source of power.
It consists basically of a primary winding and two
secondly windings, wound over a hollow tube and
positioned so that the primary is between two of
its secondary windings.
LVDT
An iron core slides within the tube and therefore
affects the magnetic coupling between the
primary and two secondary windings.
When the core is in the centre , the voltage
induced in the two secondary windings is equal.
When the core is moved in one direction of centre,
the voltage induced in one winding is increased
and that in the other is decreased. Movement in
the opposite direction reverse this effects.
the winding
is connected ‘series opposing’
-that is the polarities of V1
and V2 oppose each other
as we trace through the circuit
from terminal A to B.
Consequently, when the core
is in the center so that V1=V2,
there is no voltage output,
Vo = 0V.
ASSIGNMENT
In a one whole sheet of newsprint:
What types of industries and applications use
LVDTs?