Gyro Magnetic Compass

Wg Cdr Rajagopal

Gyro Magnetic Compass

Index

Introduction to Gyro Magnetic Compass

  • Remote indicating compass reduces limitations of direct reading compass
  • In accurate headings during acceleration and turns
  • High deviation due to magnetic sensing element situated in the cockpit
  • Magnetic headings cannot be provided to other instrument
  • Gyro magnetic compass has a directional gyro monitored by compass
  • Drift of gyroscope is corrected by magnetic detector
  • Gyro has a slow precession rate of 3 degrees per minute
  • Slow precession rate damps flux valve errors of magnetic detector

Construction of Gyro Magnetic Compass

  • Gyro Magnetic compass is also called Magnetic heading reference system
  • Gyro-Magnetic compass uses the advantages of compass and gyroscope
  • Gyro magnetic compass has a horizontal gyro and magnetic flux valve detector
  • Compass dial shows headings using a precession amplifier and motor
  • Manual as well as automatic rapid synchronisation system is provides
  • Rapid synchronisation button enables quick sync of gyro and flux valve

Magnetic Flux Valve

  • Magnetic flux valve detects the location of magnetic north
  • Three spoke flux valve is located on underside of wing tip or tail fin
  • This location is far away from electrical sources
  • Flux valve is mounted pendulously using a Hooke’s joint
  • Hooke’s joint enables flux valve to hang level during pitch and roll

Principle of Magnetic Flux Valve

  • Magnetic flux valve works on the principle of Faradays law
  • Faradays law states that an electro-motive force is induced with change in flux
  • Induced EMF is proportional to rate of change of magnetic lines of flux
  • Flux detector has three spokes around a centre spoke
  • Alternating current in centre spoke creates opposite fields in top and bottom
  • Resultant current is zero in the absence of any external magnetic field
  • Effect of earth magnetic field saturates magnetic flux and shifts resultant flux
  • Earth’s magnetic field produces EMF in the circuit as per Faraday law

Transmission of Magnetic Heading

  • Selsyn unit transmits information using a transmitting and receiving device
  • The rotor of the transmitter rotates with the heading drive shaft
  • The orientation of the rotor is the heading information
  • The rotor induces a equivalent magnetic field in the stators
  • Stators are connected to stator arms of the repeater
  • Repeater’s stators induce a current which turns its shaft
  • The repeater shaft will turn until no voltage is detected by its stators
  • The correct heading is indicated if no voltage is detected
  • Selsyn unit generates an error signal on all incorrect headings

Signal Processing to Compass Dial

  • Selsyn unit generates an error signal on all incorrect headings
  • Error signal is amplified, phase detected and converted to direct current
  • Direct current is provided to the precession motor which rotates the dial
  • Compass dial indicates direction of directional gyro corrected by flux valve
  • The dial has a compass card with lubber line showing present heading
  • A bug is provided to manually set the heading required to turn
  • The warning flag indicates failure of magnetic flux detector

Control Panel of Gyro Magnetic Compass

  • Free or Slave button selects gyro only or compass slaved modes of operation
  • Slave mode is the normal operation mode of gyro magnetic compass
  • The directional gyro heading is corrected by magnetic monitoring
  • Annunciator indicates detection of magnetic monitoring
  • Free mode indicates gyro heading without magnetic monitoring
  • Near the poles, free mode is used due to high magnetic dip angle
  • CW and CCW buttons are used to manually select initial heading

Gyro Wander

  • Gyro wander consists of gyro drift and gyro topple
  • Drift is reduced by slaving the gyro to the magnetic flux valve output
  • Topple is overcome using a mercury levelling switch and a torque motor
  • Both these systems precess the gyroscope to return to the horizontal
  • Slow precession rate avoids reaction to temporary fluctuations
  • Temporary fluctuations result from turning and acceleration errors
  • Selsyn unit provides transmits information to RMI, HSI and Auto-pilot

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