GB1293751A - Method and apparatus for locating targets - Google Patents

Abstract

1293751 Optical position finding ELTRO GmbH & CO 19 Dec 1960 [18 Dec 1959] 43674/60 Heading H4D A method of locating a target by forming-an image of the target and determining the polar co-ordinates of the image with respect to an axis, which method comprises deriving a signal in accordance with the presence of the image, generating first and second reference signals each of predetermined frequency and phase, periodically interrupting the image so that the derived signal is a modulated signal having two components whose respective frequencies are the same as those of the reference signals and whose phases relative thereto are functions of the respective polar co-ordinates of the image' with respect to said axis, and comparing said components respectively with said reference signals. The apparatus for carrying out the method includes an optical system for examining the target area and forming an image of the target in the plane of a rotating disc 30, the polar co-ordinates P, # of the target image being measured with reference to the disc centre and the O-co-ordinat Fig. 4a. One half of the disc 30 is formed with curved apertures 38 whereby the target image may be received by a detector placed behind the disc and connected to a phase comparator where the signal is compared with a reference signal. The reference signal I, Fig. 6a, is generated by a generator running in synchronism with the rotating disc. A target displaced at an angle #begins to produce a signal when the disc 30 has rotated through the angle # in the direction of the arrow so that the first aperture is aligned with the target image, the target signal being denoted by II and shown in Fig. 6b. Thus by measuring the displacement #, Fig. 6b, the polar co-ordinate # is directly determined. The target signal II extends through a rotation # and is divided into 2<SP>n</SP> subpulses by the apertures 38 and webs 36. As shown, n=3 and the curvature of the webs is such that their peripheral extremities are displaced by an amount equal to their maximum width so that through the extent of the disc radius the phase of modulation changes by it s half-period. In order to determine the polar co-ordinate p a sine wave III, Fig. 6c, is generated from the recurring pulse II and from signal III a signal IV, Fig. 6d, is derived by frequency multiplication, the multiplication factor being equal to 2<SP>n</SP>. From signal II a sine wave signal V, Fig. 6e, is derived. The signals IV and V are of the same amplitude and frequency but are phase shifted relatively to one another in dependence upon the co-ordinate p. To measure co-ordinate p signals IV and V are added to produce signal VI, Fig. 6f, the addition being such that for p = 0 the signals are in anti-phase and cancel one another out. Thus the amplitude of signal VI determines the co-ordinate p.