Flux Valve and Gyro-Magnetic Compass: Slaved and Free Modes
The Flux Valve (or Flux Detector) is a sensor used in remote-reading compass systems to detect the horizontal component of the Earth's magnetic field without the moving parts and errors of a direct reading compass.
Construction and Operation
It consists of three spokes (legs) arranged at 120°, made of soft iron, with excitation and pick-off coils.
- Sensing: It detects the horizontal magnetic field electronically.
- Location: Installed in a location with minimal magnetic interference from the aircraft (e.g., Wing tip or Vertical Stabiliser).
- Advantage: It "senses rather than seeks". It does not rotate to align with North physically; instead, it outputs an electrical signal representing the magnetic heading.
Gyro-Magnetic Compass
The Flux Valve is the sensing element of a Gyro-Magnetic Compass (or Remote Indicating Compass).
- Principle: It combines the long-term accuracy of the magnetic north reference (Flux Valve) with the short-term stability of a gyroscope.
- Flux Valve: Senses magnetic north (Long-term reference).
- Gyro: Provides stability during turns and acceleration (Short-term reference).
- Error Detector & Amplifier: Compares the gyro heading with the flux valve heading and uses a torque motor to "slave" or correct the gyro drift.
Modes of Operation
- Slaved Mode: Normal operation. The gyro is automatically corrected to Magnetic North by the Flux Valve signals.
- Free Mode (DG Mode): Used if the flux valve fails or in regions of extreme magnetic variation/dip (near poles). The gyro works as a standard Directional Gyro (DG) and must be checked/reset manually against a standby compass periodically.