Aviation Electronics Technician 1 - Historic Naval Ships Association

compensation is needed for the longitudinal axis andis provided for by the development of outriggercompensators of Permalloy near the detectingelement.Permanent field compensation must be done inthree dimensions rather than in two, and it isaccomplished by three compensating coils mountedmutually perpendicular to each other (fig. 4-25, viewA). The aircraft is rotated in 5-degree and 10-degreesteps around its three axes. Adjustment of the fieldstrength is accomplished by controlling the amount ofdirect current that flows through a particular coil.Figure 4-25, view B, shows a circuit for a singlecompensating coil.Compensation for the dc magnetic field isaccomplished by using electromagnetic compensatingloops. The loops are arranged to provide horizontal,vertical, and longitudinal fields, and they are adjustedto be equal and opposite to the dc magnetic fieldcaused by the load current. The compensating loopsare connected across a variable resistor for aparticular distribution center, and they are adjusted toallow current flow proportional to the load current forcorrect compensation. Different types of aircrafthave several sets of compensation loops, dependingupon the number of distribution centers. In neweraircraft, production changes have been made to useground return wires to minimize loop size.The procedure for adjustment of the dc compensationsystem makes use of straight and levelflight on the four cardinal headings. For example,actuation of a cowl flap motor will cause dc fieldchanges representative of those caused by any nacelleload. The load is energized, the size and polarity ofthe signal are noted, and the compensation control isadjusted. The load is reenergized, and the compensationcontrol is adjusted again. Adjustments arecontinued until the resulting signals from the dc fieldare minimized.Under ideal conditions, all magnetic fields thattend to act on the magnetometer head would becompletely counterbalanced. In this state the effecton the magnetometer is the same as if there were nomagnetic fields at all. This state exists only when thefollowing ideal conditions exist:1. The aircraft is flying a steady course through amagnetically quiet geographical area.2. Electric or electronic circuits are not turned onor off during compensation.3. Direct current of the proper intensity anddirection has been set to flow through the compensationcoils, so that all stray fields are balanced.To approximate these conditions, the compensationof MAD equipment is usually performed inflight, well at sea. In this way, the equipment iscompensated under operation conditions, whichclosely resemble those of actual ASW search flights.From the foregoing, it should be clear that theobjective of compensation is to gain a state of totalbalance of magnetic forces around the magnetometer.Thereafter, any sudden shift in one of the balancedforces (such as an anomaly in the earth’s field force)upsets the total balance. This imbalance is indicatedon the recorder. Unfortunately, a shift in ANY of thebalanced forces will be indicated. Shift in any of theforces other than the earth’s natural field are regardedas noise.MAJOR COMPONENTSFigure 4-25.-A. Arrangement of compensating colts.B. Compensating coil circuit.The MAD system consists of the AN/ASQ-81MAD set, AN/ASA-64 submarine anomaly detecting(SAD) group, AN/ASA-65 magnetic compensator4-20