Patent GB-791778-A (GB)
Title: Improvements in and relating to function generators
IPC: G06K11/02
Filed: 1954-10-27
Abstract: 791,778. Analogue electric calculating. CONTRAVES AKT.-GES. Oct. 27, 1954 [Oct. 28, 1953; Oct. 7, 1954], No. 31009/54. Class 37. [Also in Group XL (b)] In apparatus for producing an output -yrelated to an input x by the equation y = f(x) the equation is plotted in " polydromic " representation on a moving strip the position of which represents x, a sensing device producing an error voltage in dependence on the y coordinate of the curve to reduce the error and thus indicate the " y " co-ordinate. General.-Fig. 1 shows a curve K (dotted) and its corresponding polydromic representation is shown below it on a strip F which may be advanced by toothed wheels co-operating with perforations in the edge of the strip. The continuous curve K is represented as a series of discontinuous parts K0, K<SP>1</SP>, K2, no one of which exceeds the width in the y direction. Thus the ordinate of a particular point, say P, is determined by adding the product of the width y and the number of " jumps " to the distance of the point from the centre MM of the track. Thus the ordinate of the part y is 3y - y<SP>1</SP>, while that of P2 is 2y + y¢.  Optical sensing.-Figs. 2 and 3 show an arrangement in which the function appears in polydromic representation as a series of trans-. parent segments on an opaque film 21. The film is advanced in accordance with rotational movement of a shaft 23 which movement is converted by toothed wheels 22 into abscissa movement of strip 21. The film is illuminated by a lamp 24 below and its light transmitted by the section of film at the examination position is transmitted by a polygonal prism, received by an objective lens 28, and directed to a pair of differentially connected photo-cells 28, 29. The outputs of these cells drive a motor M through an amplifier V and reposition the polygon until the light falls equally on both. The position of the polygon as then indicated against an appropriate scale represents the -yco-ordinate of the curve in so far as the angular rotation of the prism is linearly related to the ordinate of the part of the curve under examination. To take account of this the " y " values are pre-distorted by being recorded by an apparatus which is the optical reverse of the present one. Reading of fast-moving tape. The Specification explains that the necessary speed of rotation of prism 40 increases with the slope of the curve at the point of examination and the velocity of the film. This is because the servo system has an inherent time lag. Since motor speed is proportional to the magnitude of the output signal, if the speed of operation of the system is to be increased it is necessary that an error Sy must exist between the position of the image curve and the optical centre of the image screen diaphragm. Fig. 5 shows an image plane diaphragm for use with a quick-acting system. The diaphragm comprises two triangular apertures 37, 38 meeting at the centre of the diaphragm and bounded by straight lines of slope determined by the maximum speed of the strip, the maximum slope of curve and the response time of the servo system. This is because all curved segments K reproduced in the plane of the image screen diaphragm originates in a point P which lies in the X axis and is displaced by a constant value regardless of the slope at P. The other boundary lines of the opening are curves such that the amount of light received by the cell is proportional to the displacement of the curve from the optical centre. Instead of shaped diaphragms shaped photo-cells may be used. Other constructions of diaphragms are described (Figs. 7 and 8, not shown) in which the optical centre appears at one edge. This procedure will lead to an error since the prism 40 will not have assumed the position representing the ordinate of the curve. Accordingly, provided the speed of the film strip is known in advance, the displacement error can be taken into account by correcting the individual y-values of the curve y = f(x). Recording function in polydromic representation (Fig. 26).-Light from as ource 250 passes through a pair of apertures in a mask 291<SP>1</SP> these apertures being spaced by 2y, that is the width of the track in which the function is to be depicted. The input representing the abscissa is made by advancing the film to an extent indicated by a scale 233 and the polygonal prism 400 is set so that its angular position represents f(x). A shutter 212 is then opened and the film exposed. Modification.-In the arrangement described. with reference to Figs. 17 to 22, only Fig. 21 of which is shown, for " reading " the curve the differential photo-cell arrangement consists of semi-circular selenium coatings 303a, 303b on a metal disc 301, these coatings being separated by a strip 302. A resilient contact plate engages the back of strip 301 and a metal contact coating 304a, 304b is deposited on the edges by vaporization and makes contacts with semicircular ring segments (not shown). An octagonal prism is provided for deflecting the light beam. Circuit.-In the circuit described as suitable for the arrangement of Fig. 2 the photo-cells PH1, PH2 are energized in anti-phase through phase-correcting circuits. The unbalance voltage has a polarity the phase of which depends on which cell receives the greater amount of light. This voltage is amplified and drives a servomotor. A generator coupled to the motor supplies a feedback voltage to the amplifier. In the arrangement described in the modification referred to above the photo-cells are alternately connected by an A.C. vibrated contact to the grid of an amplifying valve which drives the servomotor.