Avionics, Electronics, Instrumentation, Sensors

Avionic sensors and instruments (Part 1)

Avionics are electronic systems and devices used in aircratfs. This post is the first part about the main sensors and instruments used in aviation.

Measuring rotations per minute (rpm)

The instrument which measure rotations is called tachometer. All aircraft needs this instrument to measure the power turbine’s percentage, the helicopter rotor’s rpm and speed of any important machine. A three-phase alternator is coupled with gearbox, supply a AC synchronous motor, the later turns a permanent magnet inside a metal drag cup. Foucault currents are generated, in its turn produce a torque in the drag cup against the spiral string and turning the pointer.

In more modern tachometers, the electric signals from alternator go to a processer, which counts the frequency and convert in a digital signal to appear in a display.

Measuring temperature

It is necessary to measure the temperature from engines and turbine’s exaust. The aircraft’s thermometers use thermocouples, which are good to measure high temperatures. Two wires made of different metals linked in a point called sensor or hot junction. These two wires also are linked with other wires made of other metal in another extremity called reference or cold junction. The temperature difference between the junctions creates a difference of electric potential, this is the Seebeck or thermoelectrical effect.

These are examples of thermocouple sensors used in aircrafts.

Measuring the quantity of fuel

The main fuel meters used in aircraft use a variable resistor or capacitor. In the resistive indicators, a float object is linked to a potentiometer and the float’s motion varies the resistence in the potentiometer, by its turn changes the current in the indicator.

The signal can be converted in the digital form to show in a display.

Are implemented compensation potentiometers to make adjusts when the tank is empty or full, improving the precision. Another type of system uses an aluminium variable capacitor which stays in the tank, these are the capacitor’s plates.

The capacitance changes when the tank’s level change, because the dielectrical constant is higher than the air. This system is a AC bridge with a reference capacitor. To know how circuit bridges work, click in the button below.

Circuit bridgesClick here

Compensator potentiometers are put in parallel with transformer’s secondary winding. The output signal is amplified or converted in the digital form to be displayed in the indicator.

In the next part, I will show other avionic sensors and instruments, which measure other important variables.

 

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About Pedro Ney Stroski

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