Patent GB-825772-A (GB)
Title: Electrical system for generating signals for controlling motion along separate co-ordinate axes
IPC: G05B19/25
Filed: 1956-10-19
Abstract: 825,772. Programming for machine-tools; copy-milling. GENERAL DYNAMICS CORPORATION. Oct. 19, 1956 [Nov. 2, 1955], No. 31867/56. Class 83 (3). [Also in Group XXXVIII] A system for generating signals for controlling motion along separate co-ordinate axes to describe, as a resultant of all said co-ordinate motions, the desired path, said system comprising first means 320, 322 to generate a separate train of control pulses for each co-ordinate axis, each pulse in a train representing an increment of motion along the co-ordinate, second means 332, 334 for establishing the correct number of control pulses in the first means to provide a required motion along each co-ordinate axis for describing said path, and means 308, 310, 338, 304, 306, 336 and 300 for controlling each of the first means for establishing the number of control pulses for deriving separate trains of control pulses for each co-ordinate axis. The slope m, Fig. 1, of the line to be followed by a milling cutter is calculated from the expression Y2-Y1/X2-X1. The subtractions of the co-ordinates   of the points P 1 , P 2  are carried out in a modified adding-machine, wherein the highest-digit indicator 214, Fig. 2, always registers 0 for positive answers. To indicate a positive answer, therefore, a fixed brush 218 is provided which can only engage an enlarged contact at the zeroposition. Of two other fixed brushes 219, 220, the latter is connected to a re-set switch 221, which is depressed when it is desired to read an answer, and the former brush indicates a negative answer as it is connected to the brush 220 by a bus-bar 217 whenever the brush 218 does not contact the zero digit. Each other digit-indicator 210, 212 has a brush 224, 226 similar to the brush 220, no zero contact, and an endless bus-bar 217. The brushes 224, 226 are respectively connected to a units and a tens reversible decimal counter 228, 230. When enabled by an " enter-switch " 234, an oscillator 232 generates pulses which are applied to plus-, and minus-, gates 240, 242 one of which is opened by a flip-flop 244 triggered, either by an output from a stop-gate 236 (gate 242), or by an output from the negative lead (gate 240). Pulses passed through the minus gate 242 are subtracted from the count already present in the units counter. Similar gates 240<SP>1</SP>, 242<SP>1</SP> between the counters are controlled also by the flip-flop 244, and to these are applied "carryover " pulses from the units counter, which pulses are derived when the counter passes through a count of nine. Thus the pulses from the oscillator 232 are either added to or subtracted from the total count of all the stages. If 15 is subtracted from 32, the indicators are set at 017. The reset switch 221 is pressed, and the counters respectively store one and seven pulses. When the store-key 234 is pressed, the oscillator 232 delivers pulses to the counters until they are zeroed; at which time the stopgate 236 stops the oscillator 232 and the operation is completed with 17 pulses stored in a binary counter 238. (Other types of adding- machine may be adapted for use, in which the indicators rotate in different directions, and it is possible to eliminate the need for the flip-flop and gates.) In a straight-line generator shown in Fig. 3, a switch 302 enaoles the adding- machine 300 first to calculate #x and then #y. The numerical values of #x and #y are passed to separate relay registers 306, 310, and thence to dividers. Upon a signal from a start source 318, a clock-pulse generator 316 supplies pulses to the dividers at a frequency F. The number of pulses is divided by the quantities #x and #y, and sign-sensing circuits control gates to direct the output pulses of the dividers to the proper x and y co-ordinate channels. (In a modification, two adding-machines are used. Also the apparatus may be decimal-operating instead of binaryoperating and the registers may be of relay, magnetic or electronic type.) These pulses are used directly or recorded on paper or magnetic tape or on magnetic drums. Two sample recordings of one operation, and the corresponding tool-paths, are illustrated in Figs. 4A and 4B. Each set of pulses is counted by a summing- counter. In modified circuits, circles can be generated, using a binary or decimal system. In this case, the quantity by which each dividing counter divides pulses from the generator is varied in time. The slope m is dy/dx = - x-a/y-b, so that the dividing counters divide the clock-pulse rate by (x - a) and (y - b). A switch determines whether a circle is generated in a clockwise or anti-clockwise direction. In a decimal system circuit, the dividing and summing counters employ tubes with a common anode, ten main cathodes and ten pairs of guide-cathodes, there being a glow-discharge passing in succession along the main cathodes. A typical portion of tape 900, Fig. 9, shows the pulses necessary to generate the upper half of a circle centred at 0, the y-pulses changing from the positive to the negative track. Straight-line and circle generators can be combined in a circuit having switches to change from one to the other. Other curves can also be generated, e.g. a parabola in which dy/dx = p/y-k, or an ellipse in which     dy/dx = (x-h)b<SP>2</SP>/(y-k)a<SP>2</SP> , the circuit for the latter requiring three counters in series to divide by b, b and (x - h). Three-dimensional curves may also be generated. The control signals produced by the circuits described, or recorded on a tape &c. are used to operate servomotors, one for each co-ordinate movement. Thus for moving a work-table 1422, Fig. 14, along one axis, the pulses are passed through a dynamic error register 1412, being a reversible counter, a digital-to-analogue-converter 1414, generating a voltage of amplitude proportional to the count of the counter, and a power amplifier 1416 to the servomotor 1418, which rotates a screw 1420 or a gear train. A motion digitizer 1424 feeds back the motion of the table, and may be an anti-backlash step-up gear train and an instrument synchro transformer, or may employ linear photographic gratings and photo-cells, or a magnetic system whereby the pulses are prerecorded on a magnetic medium 1500, Fig. 15A, attached to the table, and flux-responsive transducer heads 1506, 1508 pick up pulses from the moving table and are made effective, according to the direction of movement thereof, by a pair of flip-flops. Thus one flip-flop 1532 has a stable condition in one direction of movement, and the other flip-flop 1548 in the other direction. The error register 1412 is a gas tube having a single anode and ten cathodes, similar to the counters described above, and requires two pulses to change its count. The count is positive if the pulses have a sequence (G 1 -G 2 ), and negative if the sequence is (G 2 -G 1 ), the sequence determining the direction of travel of the discharge. The circuit is arranged so that, if pulses are received at the register simultaneously, the control pulse receives priority, and the pulse from the motion-digitizer is temporarily stored. The digital-to-analogue conversion is performed by a series of resistors connected as a voltage-divider, between the cathodes of the units and the tens counter. The pulse rate for any movement is determined from the formula where v is the required speed and # is the distance the table must move for the transducer to generate a pulse. Instead of employing an adding-machine(s) as an input device, the calculated differences can be stored on punched cards or tape. If a shortage or surplus of pulses occurs due to tool wear &c. an alarm may be operated. The system may be used to control a pen in curve-drawing equipment. Speed control.-The speed of the table is slowed to <SP>1</SP>/10 the highest permissible cutting velocity when the end of a cut is near, or when the table is to stop. To do this, 0.100 is subtracted from x 2  when it is entered into the adding-machine, and x 1  is subtracted from this difference. The difference, x 2  - 0.100 - x 1 , is then stored in an extra register. A set of coincidence gates can recognize when the quantity in an x-true-value counter, and the value of the number in the extra register, coincide. The output of the gates is used to slow down the pulse generator by a factor of ten, or to activate a counter which divides the pulse output by ten. The control pulses are thus spaced further apart, and the table velocity slowed down. Similarly, the table speed can be accelerated when a new path is commenced.