Remote Indicating Compass

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Remote Indicating Compass

Post  Admin on Tue Dec 01, 2009 6:58 am

To understand how a remote indicating compass functions in comparison to the direct indicating compass, it is vital to observe and understand their differences. In doing so the function of the remote indicating compass becomes clear. The remote indicating compass is a wonder of technology; although complex, it plays a massive role in connecting to various other instruments on an aircraft. This table below outlines the major components of its operation in comparison to the direct heading indicator. The remote indicating compass is electromagnetically intricate, vastly more complex than the basic direct indicating compass.
The remote indicating compass utilizes a detector unit (usually located in the wing tips) known as the flux valve, which senses the Earth’s magnetic field and reduces it to a complex phase signal. This is achieved electromagnetically by means of a very sensitive transformer, which is excited by AC. The amount of induction which occurs in the three spokes of the transformer (flux valve) is relative to the Earth’s magnetic field. The secondary ‘pick-off coils’ which surround each spoke resolve the signals electromagnetically. Each of the valves pick-off coils out puts to its own transmission line which are received by a ‘coils receiver stator unit’, this creates a magnetic field in the receiver stator, representative of the Earth’s magnetic field. The receiver rotor, which is on the output shaft of the gyro, senses the resultant magnetic field and if not in line with the ‘stator field’, an error signal is produced. This is called the ‘self synchronous transmission system’ or ‘selsyn’; together with toque motors this model makes up the aircraft ‘Slaving’ and ‘transmission’ system. The HI can be slaved to the flux valve in order to retain a constant reliable heading indication, thus the HI is now a remote indicating compass.
The error signal is sent to the ‘slaving amplifier’ which activates a toque motor; the motor mechanically applies force to the appropriate gimbals, precessing the gyros heading until the error signal is completely reduced. The selsyn rotor is aligned with the stator field, called ‘null position’, which is why the rotor is sometimes referred to as a ‘null seeking rotor’, a mechanical representation of the Earth’s field. Perfect alignment only lasts for a few milliseconds; the entire process occurs 3 – 4 times a second, working to maintain the constant heading. An electronically attached unit called an ‘anunciator’ indicates to the pilot wether or wether not the SLAVE system is properly functioning. The anunciator uses a ‘SLAVE/FREE switch by which a pilot can select to slave the HI to the flux valve or simply use it as a normal HI; resetting it every 15 minutes. Note again that is it gyroscopic rigidity and not precession which enables a constant heading.

The normal slaving rate is approximately 2° per minute, ample for normal operation and reduces the risk of introducing turning error when the aircraft banks. Some systems use a ‘turn cut-out’ which temporarily disables the SLAVE system when the angle of the gyro gimbals exceeds 10°, preventing false input from the detector from creating turning error. The gyro is susceptible to topple during initial spin up, turbulence and violent manoeuvres, for this reason a ‘selsyn push button’ (fast erection/slaving) is provided. This button provides 60° of precession per minute as opposed to the normal 2°, allowing for faster erection of the gyros on start up to ensure the gyros are properly aligned and during turbulence and so on. Do not operate the fast slaving system for more than 15 seconds at a time, as this could melt or explode electrical circuitry.

The selsyn rotor provides a mechanical representation of the Earths field; this output is referred to as analogue information. This information is sent to other instruments on an aircraft either in analogue or digital form. By adding the use of radio navigation information the remote indicating compass is presented as an RMI (radio magnetic indicator) or HSI (horizontal situation indicator). A ‘head setting knob’ provides signals between the aircraft heading and the desired heading direct to the autopilot. The remote indicating compass is not just an RMI, but has been developed as an AHRS (attitude and heading reference system) and is the back bone of many navigation systems. One last advantage of the remote indicating compass is its ability to display as EFIS (electrical flight information software).

Author: David G. P. Martin
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