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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