Patent GB-652325-A (GB)
Abstract: 652,325. Automatic control systems for aircraft. SPERRY CORPORATION. July 9, 1948, No. 18502. Convention date, July 16, 1947. [Class 38 (iv)] [Also in Group XL (c)] The receiver, mounted on a craft for use in steering it to approach and follow a position line defined by a radio navigation system includes means for deriving a first control signal varying in magnitude and sense in accordance with the departure of the craft from the position line, means for obtaining a rate signal dependent on the rate and direction in which the craft departs from the position line, and means combining the two signals algebraically, with the departure signal limited so as not to exceed the rate signal, the resultant signal being used in steering the craft to approach and follow the .required position line. It is shown that if the distance of the craft from the desired position line is large, so that the departure signal is greater than the rate signal, the craft will be directed to fly in circles and that the limitation of the departure signals to not exceed the rate signal enables the craft to approach the position line from any distance, the final stage of the approach being asymptotic. The invention may be used in conjunction with hyperbolic path difference contour radio navigation systems, for example " Loran." In such a system using spaced transmitters 10 the present position of the craft A may be on lines 12345 and 03215 of the two intersecting contour systems, these two values are indicated on the respective indicators 13 and 12 of the craft receiver 11. Suppose it is desired to approach and follow line 27643 of the first system and also to know when position B (position 27643, 19237) is reached. Position line value 27643 is set in manually on indicator 15 and the difference between the value of indicators. 13 and 15 constantly determined by differential 16 and used to adjust a special form of variable potentiometer 20. This comprises a cylinder with a helical groove in which a wire is wound, the wire being of low resistance over the end portions and of high resistance at the centre 22. A slider 21 moves along the wire as the cylinder 20 is turned by the differential 16. Potentiometer 20 is associated with variable resistors 24, batteries 25 and motor 23 in a bridge circuit arranged so that motor 23 rotates at a speed and in a direction determined by the deviation of slider 21 from the centre of the potentiometer. This is dependent on the departure of the craft from the required position line but is limited in value by the extent of the resistive portion 22 ; the limitation is also dependent on the setting of resistors 24 which are preferably set by knob 26 in accordance with the ground speed of the craft. A feed-back circuit from generator 35 through amplifier 36 ensures that the speed of the motor is proportional to the input. A direct drive from meter 13 rotates one input of differential 34 at the rate of change of the indicator 13, that is the rate at which the lines of position are crossed, and the other input to the differential is from motor 23 and is dependent on the deviation. The output of the differential 34 may be combined with that from directional gyro 31 and is fed by Selsyn 27 as an electrical control signal to steering unit 28 operating rudder 30. The craft at A thus turns until the rate and deviation signals are equal and opposite, following the path shown in Fig. 2, the contact 21 moving from the full line position shown towards the centre as the craft progresses. When the craft moves to a point at which slider 21 reaches the resistance portion 21 of the potentiometer the distance signal decreases and the craft is steered to approach line 27643. asymptotically. The other coordinate of position B may be manually set on indicator 14 and an alarm given when this point is reached. The alarm circuit consists of two parts, a relay which does not release to complete the circuit until the output on lines .37 is almost zero, so that the craft is on the desired position line, and a set of contacts associated with indicators 12 and 14 and only made when these read the same (Fig. 4,' not shown). A modification (Fig. 5, not shown) is also described in which the position line is defined by a radio localizer beacon.