Patent GB-1093559-A (GB)
Title: Apparatus for generating a curve from digital data
IPC: G06G7/30
Filed: 1964-11-25
Abstract: 1,093,559. Curve generating apparatus. INDUCTOSYN Ltd. Nov. 25, 1964 [Dec. 6, 1963], No. 47948/64. Heading G4G. [Also in Division G3] General Theory. In a modified curvilinear interpolation apparatus for controlling machine tools of the kind described in Specification 863,125, it is shown that the instantaneous co-ordinates X<SP>1</SP>, Y<SP>1</SP> of the centre of a rotary cutter are given by (Fig. 9): where X 1 , Y 1  are the coordinates of a data point; D is the distance of a data point X 2 , Y 2  along a chord joining X 1 , Y 1 ; X 2 , Y 2 ; S is the distance along the chord of the foot of a perpendicular to the cutting point P on the curve; H is the departure of the chord from the curve at the cutting point; #X, #Y are the abscissal and ordinal differences of adjacent data points; X R , Y R  are the X, Y components of the cutter radius. Positional electrical signals are developed to represent (1) the position of the start of the curve segment relative to a reference point X 1 Y 1 ; (2) the coordinate components of D; (3) the coordinate components of H; and (4) the coordinate components of tool radius R, and to derive an analogue control providing continuous analogue signals representing successive curve segments, derived from the successive digital bits representing data points of the required tool position. The coordinated binary gear mechanisms receiving X 1 , Y 1  signals from corresponding servomotors, and a cyclic shaft input, with electrical input voltages representing #X, #Y, to conduct mechanical multiplication and interpolation of #X, #Y over successive cycles of discrete input data points whereby shaft outputs S S represent X 1  + -#X and Y 1  + -#Y, shown D D in Specification 863,125, are each replaced by a multiplying rotary potentiometer having shaft and electrical inputs, operative to linearly interpolate the voltages representing #X, #Y over an operating cycle, to produce electrical signals S S representing -#X and -#Y; the potentio- D D meters comprising circular impedances with diametrical terminals and sliders, of which half a revolution corresponds to one cycle of the system and chordal length D; and an input reversing switch operating once per half revolution for continuous cycling without periodically returning the slider to its start point. At the end of a cycle when # 1 X = H#Y/D =   0, # 1 Y = H#X/D = 0 and S/D = 1 then and X 1  + #X, Yl + #Y for the end of one cycle is equivalent to X 1 , Y 1  for the commence- S#X ment of the next cycle, at which -= S#Y/D = 0À5.   A resolver 20 (Fig. 3) responsive to angle # from curve computer 406 develops analogue voltages representing tool radius components X R1 Y R  which are added to other analogue voltages representing the second and third terms in the above equations; the components # 1 X and # 1 Y being equivalent to the third terms in the equations (Fig. 9). Each coordinate axis is provided with a machine servomotor controlled from two alternately operative control servomotors, the active motor of which is energized by input representing the last three terms of the corresponding equation 1 or 2; the inactive motors being energized with preparatory input representing the appropriate tool radius component X R , Y R  so that this information is present in the servo on activation, for superimposition on the second and third terms of the equations. Signals representing the first terms of the respective equations are added alternately to the outputs of the pairs of control servomotors corresponding thereto; derived from a pair of digital/analogue converters alternately receiving successive bits of X 1  and Y 1  information and converting to electric analogue signals for the addition. Successive positional digital signals are converted by storage digital/analogue converters utilizing successively the bits previously stored, into a continuous analogue signal, and a new positional input is provided for the start of each segment without successive accumulations of #X, #Y and consequent errors. As in the prior Specification the input values of α, #, #X, #Y are computed digitally, and the H values of H, -, tan #, and # are continuously D derived therefrom, where α, # are chord to tangent angles at intersections thereof, # is chord to tangent angle at cutting point, # is slope of tangent at cutting point. The terms of the above equations are similarly continuously computed and combined, and the resultant values X<SP>1</SP>Y<SP>1</SP> are utilized to control a machine tool drive along the X and Y coordinate axes. Structure. Figs. 2 to 7 arranged in sequence correspond to Figs. 11 to 16 of the prior Specification, excited from a tape reader computer and rerecorder producing a tape bearing information signals representing α, #, X, Y, Z, #X, #Y (Fig. 10 of the prior Specification) which is passed through a tape reader (Fig. 2). Fig. 8 shows the curve computer 406 (Fig. 3) including α,# storage relays 11, α,# multipliers 12, gain control system 193, slope angle servomotor 21, together with associated elements of Fig. 3 and of Figs. 11, 12 of the prior Specification. The X 1 , X 2  and Y 1 , Y 2  digital to analogue converters (Fig. 2) are of the kind set forth in Specification 809,219 (Figs. 10, 11, 13, not shown). Corresponding references denote components common to the prior Specification, while in Figs. 2 to 8 new or modified components are denoted by references commencing with 401, and the signal lines from the stop and corner control (Fig. 2) are denoted by references 451 to 453. The X and Y servomotors of the prior Specification are replaced by pairs of alternate X 1 , X 2  servomotors 403, 404 and corresponding Y 1 , Y 2  servomotors (Fig. 6) while the servoamplifier is replaced by amplifier 405 and correspondingly for the Y servomotors. The X output of translator 422 excited from the distributer (Fig. 2) is alternately connectible by switch 423 operated by coil 200a to the active one of a pair of digital analogue converters 413, 414 for X 1  and X 2 , while the Y output is similarly alternately connectible to converters for Y 1  and Y 2  and the reference source 428 is switchably connected in alteration to the inactive X, and Y 1  converters. The input signals from translator 422 are stored in stepping switches of the digital analogue converters, which maintain the positions to which they have been set after alternative disconnection of the inputs, and reference signals are applied to activate the circuit for generation of the corresponding analogue outputs. A group of three pairs of lines 433, 434; 435; 436 from converter X 1  and similar groups of lines 437, 438, 439 from converters X 2 , Y 1  and Y 2  develop coarse, medium, and fine analogue signals representing the sin and cos values of X 1 , X 2 ; Y 1  Y 2  which are fed to the differential resolvers 67, 68, 69 for X 1 ; 416, 417, 418 for X 2 ; and corresponding resolvers For Y 1  and Y 2  (Fig. 6). The analogue value of #X from curve computer 406 (Figs. 3, 8) is fed through reversing switch 407 to annular potentiometer 401 mechanically driven by shaft 196 through ¢ revolution per system cycle. Switch 407 is operated by sequence control 215 and relay coil 200a (Fig. 4) together with switches 408, 409 to take up opposite positions in alternate cycles, and the slider linearly interpolates #X S#X over 1 cycle to a resulting signal - which is D applied over summation resistor 410 to the input of amplifier 26 (Fig. 2) where it is combined with signals representing # 1 X and X R . The amplifier drives servomotors 403, 404 alternately as selected by switch 409 operated by relay 200a in response to sequence control 215; and potentiometers 411, 412 driven thereby supply follow-up feedback signals to the amplifiers, alternately as selected by switches 408. The servomotor output thus satisfies equation 1. A similar arrangemeht of summation amplifiers, servomotors, feedback potentiometers and selection switches operating alternately, satisfies equation 2. The shaft 63 of X 1  servomotor 403 is rotated S to positionally represent -X + A 1 X + X R  D and operates differential resolvers 67, 68, 69 which add in the value X from the X 1  digital analogue converter 413 and at the same time the X 2  servomotor 404 is driven by amplifier 405 to develop value X R  in shaft 415 and its differential resolvers 416, 417, 418, which are thus rotatably set to position X R  in preparation for the next operative cycle, although their output is zero when X 1  servomotor 403 is active, since reference source 428 is disconnected from the X 2  converter 414 for this cycle. Converters 413, 414 are alternately activated by switch 423, so that at the end of the operative cycle of X 1 , the X 2  servomotor 404 is correctly positioned to start a new cycle X 2 , since #X and S -#X are then zero. At the commencement of D cycle X 2 , switches 407, 408, 409 change over, and the X 2  system effects the summation of S -#X + # 1 X + X R  while the X 1  system is D resetting to the value of X R . A corresponding sequence of operations occurs on switching from an X 2  cycle back to an X 1  cycle, and no interpolation errors accrue over successive cycles. The outputs of differentials 416, 417, 418 are combined in parallel with those of corresponding differentials 67, 68, 69 to operate the X feed rate drive of the machine element (Fig. 3). The operation of the Y 1 , Y 2  system for computing the second equation is generally similar.