Complex control systems all make use of signals that can be
easily measured and altered remotely. Automatically operated
machines or actuators need to be powered either by electrical
motors, pneumatic or hydraulic cylinders.
The most common signals sent out from a transducer are
electrical current or voltage and pneumatic pressure. These
signals are easily translated by controllers. Visual measuring
devices like liquid bulb thermometers are not able to be used
in automatic control systems because there are no electrical
or pneumatic signals.
However, when some other component is added to it to enable it
to give a corresponding signal, then the measuring unit becomes
a transducer.
A transducer is a device to convert a signal (representing a
physical quantity) of one form into a corresponding signal of
another form, retaining the amplitude variations of energy
being converted.
Just as an example, a microphone is a sound transducer (acoustic
to electrical) and a loudspeaker is an electrical transducer
(electrical to acoustic). A transducer may be an integral part
of the measuring unit, for example pressure to displacement in
a Bourdon pressure gauge. It may also be a separate unit
converter especially suitable to change the signal to a better
form for remote transmission, e.g. displacement to electrical
in a differential transformer.
How do these transducers work?
To convert physical movement to electrical signals, some devices
make use of variable resistors. Any movement will cause a slider
contact to change position against a resistance wire causing a
change in electrical current or voltage. Other devices may make
use of the movement to alter the position of an induction coil
relative to the magnetic core, causing a change in the induced
current in the circuit. Others may make use of the movement to
change the air gap between two capacitance plates, causing a
change in the current in the circuit.
In the case of the loudspeaker, the opposite effect takes place.
Electrical signals are converted to movement.
Some pneumatic transducers make use of changing air signals to
cause a corresponding mechanical movement in the flapper and
linkages. This movement is then converted to electrical signals
by the methods above. So it takes 3 steps of conversion before
we can get the electrical signal.
Once the electrical signal is obtained, it can be used to compare
to the set value in the controller, and a whole control system can
be designed from it.
Well folks, get your signals and be in control!
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