Patent GB-615535-A (GB)
Title: Improvements in or relating to apparatus for detecting and/or interpreting signals from a leader cable
IPC: G01S1/02
Filed: 1945-10-26
Abstract: 615,535. Apparatus for detecting signals from a leader cable. WILLIS, H. F., WRIGHT, P. M., BAKER, P. T. W., and WRIGHT, C. S. Oct. 26, 1945, No. 28320. [Class 40 (i)] [Also in Groups XXXVI and XL (c)] In apparatus for interpreting the electromagnetic signals from a leader cable carrying an alternating current at a medium frequency, the output signals from three mutually perpendicular coils, comprising a horizontal coil C arranged athwartship of a craft, a horizontal coil B arranged fore-and-aft the craft and a vertical coil A, each capable of developing an E.M.F. proportional to the component of magnetic field along the direction of its axis, are transmitted to the x and y plates of two cathode-ray oscillographs 28, 29. The tube 29 is connected through amplifiers B<SP>1</SP>, C<SP>1</SP> to the coils B, C, so that the spot produced on the screen of the tube 29 describes a linear trace, the inclination of which to the y axis of the screen is equal to the angle between the cable and the fore-and-aft line of the craft. The tube 28 is connected by the amplifiers A<SP>1</SP>, B<SP>1</SP>, to the coils B, A, the spot then describes an elliptical path in one sense of rotation if the leader cable is on the right of the craft and in the other sense if the leader cable is on the left of the craft. In order to discern the direction of rotation of the spot on the tube 28: (a) the intensity of the spot is pulse modulated to give additional intensity for very short periods of time and at a regular frequency which differs by a small constant amount from the cable frequency. The screen of the cathode-ray tube then exhibits a stationary elliptical track upon which is superposed a bright spot which slowly moves round the track. The direction of movement of this spot provides the required indication of direction. Provision is made by which the frequency of modulation automatically adjusts itself so as to differ by a small constant amount from the frequency of the leader cable as follows: (i) Some of the signal from one of the amplifiers A<SP>1</SP>, B<SP>1</SP> or C<SP>1</SP> is mixed with the signal from the local modulator oscillator and the difference frequency is separated out and applied to a discriminator network, the output from which is rectified and smoothed and the resulting direct current voltage is applied to the control grid of a local oscillator, so that the local oscillator is self-stabilized at a frequency differing by a few cycles per second from the cable frequency; or (ii) an amplified signal from any of the detection coils is split into suitable phases and applied to the polyphase windings of a dynamo electric motor. The motor is then driven independently at the desired difference frequency causing the rotor winding to develop a voltage whose frequency is the difference, or sum of, the cable frequency and the frequence of rotation of the rotor. The output is then used to control the frequency of a pulse circuit modulating the brightness of the cathode-ray tube 28; (b) in the second method of discerning the direction of rotation of the spot the ellipse is modulated by a periodic voltage wave or recurring voltage pulse of equal frequency to the cable. This modulating voltage is applied either to the brilliancy control of the tube 28 causing intensity modulation or to the deflection plate causing displacement modulation. In one form the recurring unsymmetrical voltage pulse shown in Fig. 4, derived by using some of the signal applied to the y-plates of tube 28 to trigger a discharge tube, whereby a condenser is discharged through a resistance, is used for modulating the brightness of the spot, causing the ellipse to appear as in Figs. 4a 4b. In a second form part of the voltage applied to the y-plates of the tube 28 is phaseshifted by differentiation, thus providing a suitable voltage for intensity modulation and causing the ellipse to have characteristic asymmetry. In a third form part of the voltage applied to the y-plates of the tube 28 is changed to a flat-topped wave-form, and then differentiated, thus providing a series of alternately positive and negative pulses. When used for intensity modulation this produces on the ellipse a spot of excessive brightness on one side and a blank spot on the other. In each of these cases the direction of rotation is determined by the symmetry of the ellipse. An indication of the horizontal distance of the cable from the craft in terms of the vertical depth of the cable from the craft is obtained by measuring the angle of the major axis of the ellipse with the vertical (Fig. 1, not shown).