Transducers & Pressure Sensors

Transducers

• Electrical or electronic components sense physical variables
• Sensors provide the detected physical variable as a displayed output
• Physical variables, like temperature or pressure, becomes an electrical signal by conversion
• Transducers are sensors which convert physical variables into electrical signals.
• Transducers send an output voltage to instruments and converted back into a readable display

Transducer Applications

• Pressure sensors are used for hydraulic system pressure indications.
• Level sensors are used as fuel quantity indicators
• Position sensors are used as flap position indicators
• Speed sensors are used as propeller/turbine speed indicators

Pressure Sensors

• Pressure sensors help monitor hydraulic or air pressure systems by use of a potentiometer device
• The sensor has a "sensing" component and a section to convert physical variables into electrical output.
• A mechanical connection links the diaphragm to a potentiometer's wiper.
• Voltage applied to the potentiometer determines the output voltage tapped off

Level sensors: Resistive Method

• Used for fuel quantity indication (FQI) in systems, particularly on small aircraft

• Direct current is supplied through two inductors (A and B) that behave as electromagnets

• Magnetic flux impacts a permanent magnet attached to a gauge

• Inductor A's magnetic flux remains constant, while inductor B's is variable

• A float connects mechanically to a variable resistor's wiper

• When the tank is full, variable resistance is low

• The result is more current to inductor B, increasing magnetic flux.

• This pulls the indicator towards "full"

• When tank level is low, the flow increases resistance

• Reducing current through inductor B, with magnetic flux of inductor A overpowering that of B

• The indicator is pulled toward empty.

Level sensors: Capacitive Type Probe

• A more reliable level sensor for aircraft, mainly in FQI systems on large aircraft

• Exploits the difference in dielectric constant fuel relative to air

• The tank probe is a capacitor in the tank

• Air's dielectric constant approximates 1, jet fuel at 1.7 and gasoline registering around 2.0

• Jet fuel has a stronger dielectric, allowing more charge storage

• Higher fuel level means more fuel between capacitor plates and a higher voltage

• Dropping fuel levels lead to more air, lowering dielectric and voltage

• The system compares tank probe readings to a reference capacitor with only air

• The difference is output as an indication

Linear Variable Differential Transformer (LVDT)

• A sensing device to detect linear motion

• LVDTs use induction to produce a voltage output

• An AC voltage applied to a primary coil which creates a magnetic field

• Magnetic field is transferred via an iron core to two secondary coils

• When the iron core is centred (rest position), flux transmits equally to each secondary coil

• When secondary coils are wound in opposition the voltage output is zero, induced voltages cancel

• When the iron core is displaced flux distributes unequally

• Resulting unequal voltages lead to an output voltage for reading

• An LVDT gives two data quantities simultaneously

• Signal strength correlates to amount of deflection/movement

• Signal phase (positive or negative) correlates to direction of deflection/movement

Rotary Variable Differential Transformer (RVDT)

• A variation on the LVDT
• Senses rotary motion rather than linear, while the principle of operation is the same
• Electromagnetic field transmits through a rotating mechanism.
• Rotating mechanism directs magnetic flux toward one secondary coil

Torque Synchros

• Transmit position information from one point to another

• Used to sense a rotary position and relay it to an instrument display

• They can be used by an indicator in the cockpit for flap position deployment angle

• The transmitter are the sensing components, while a schematic's right half indicates components

• AC voltage is applied to both the sensing transmitter's rotor and the indicator pointer

• AC voltage causes current to flow through the rotors, setting up a magnetic field

• The transmitter stator windings induce a corresponding magnetic field

• Stator windings transmit an equivalent magnetic field to the indicator

• The indicator rotor matches the magnetic field of the indicator stator windings

• Rotation in the transmitter changes the magnetic field

• This changes the magnetic field in the indicator, and the position indicator turns to match

Torque Synchro Units

• Synchros are generally labelled by operation in the system
• The synchro unit sensing and transmitting the signal is the Torque Transmitter (TX)
• The receiver unit is called the Torque Receiver (TR)

Differential Synchros

• A synchro system can be modified by adding a differential unit

• A torque differential transmitter (TDX) adds or subtracts a signal prior to its output

• A TDX allows magnetic field transmitted by stator windings to be modified between transmitter and receiver

• Using TX and TR units allows mechanical inputs from two separate sources

• Providing a single differential output for comparison, like an asymmetrical flap deployment warning

Control Synchros

• Synchros are intended for indication systems, not requiring heavy loads.

• The resulting currents from induced voltages are too low for significant outputs.

• Control synchros are used when a heavy load must be driven

• These units are known as the control transmitter (CX) and control transformer (CT)

• Mechanical input is sensed by the CX and the CT

• System outputs an error signal to an amplifier that drives a motor to move a load

• Rotor windings in the CX are 90 degrees out of phase with the windings in the CT

Wheatstone Bridge

• A Wheatstone bridge is a common sensing circuit for many applications

• It provides very accurate readings and allows relatively easy calibration

• This circuit configuration is bridge circuit

• Parallel pathways (A-B-C and A-D-C) are bridged with a sensing device (VG from B-D)

• Parallel pathways form voltage dividers through resistors (R3/RX and R1/R2 )

• When the ratio of R3/RX equals R1/R2, voltages at B and D are equal

• Then there is no current flow between B and D, and VG reads zero

• RX is the sensing resistor, it changes resistance by physical condition such as temperature

• As RX changes, above ratios are unequal, causing voltage difference between B and D

• VG will give a reading proportional to differences

• R2 functions as a calibration resistor, that can be set to ensure R3/RX equals R1/R2

• Allows a known set of conditions (ambient temperature) to be set to zero reading

• Any resistor in the network can be a sensing resistor, so long as the calibration resistor is chosen accordingly