Slaved Gyro Compass Principle

Principle of the Slaved Gyro Compass

The slaved gyro compass principle combines the short-term stability of a directional gyro with the long-term accuracy of a magnetic flux detector. The gyro unit, with two degrees of freedom and a horizontal spin axis, resists changes in orientation due to its gyroscopic properties. However, gyros alone drift over time due to factors like Earth rotation and aircraft movement.

A flux detection unit (often a flux valve or flux gate) senses the Earth's magnetic field and provides a reference for magnetic north. This signal is compared to the current heading indicated by the gyro. Any difference (error) between the two is detected by an error detector (selsyn system), which generates an error current.

Automatic Correction and Components

The error current is amplified by a precession amplifier and sent to a precession coil or torque motor. This motor applies a force to the gyro, causing it to precess (rotate) about its vertical axis. The system automatically realigns the gyro and heading display to match magnetic north, correcting at a typical rate of about 2° per minute. This continuous correction keeps the displayed heading accurate over long flights.

Key components include:

• Flux detection unit: Senses magnetic north.
• Gyro unit: Provides stable short-term heading reference.
• Error detector: Identifies misalignment between gyro and magnetic north.
• Precession amplifier and coil: Drives the correction process.
• Display unit: Shows heading, often with a compass card and controls for manual alignment or mode selection.
• Annunciator: Indicates slaving loop status.

Modes and Manual Alignment

Pilots can select between slaved (COMP) and free (DG) modes. In slaved mode, the system automatically corrects heading errors. In DG mode, the gyro operates independently, requiring manual realignment. Manual synchronization is also possible if the system becomes significantly misaligned, using a knob to quickly set the correct heading.

Advantages and Disadvantages

Compared to direct-reading magnetic compasses, slaved gyro compasses offer:

• Greater accuracy and stability
• Reduced susceptibility to aircraft magnetism and attitude errors
• Easier integration with other avionics (e.g., RMI, autopilot) However, they require electrical power and are more complex and heavier than simple compasses.