Patent GB-868070-A (GB)
Title: Improvements relating to control devices for automatic machine tools
Inventor: SPENCER ROLF EDMUND
IPC: G05B19/4105
Filed: 1956-07-18
Abstract: 868,070. Electric copying machines. ELECTRIC & MUSICAL INDUSTRIES Ltd. July 16, 1957 [July 18, 1956], No. 22184/56. Class 38 (6). [Also in Group XXXVI] A control device for automatic machine-tools comprises means for deriving first signals variable to represent two co-ordinates of a series of points on a desired locus with respect to predetermined axes, means for resolving a signal representing the radius of a tool, or a departure of the radius of such tool from a nominal dimension, to produce signals representing the components of said radius in said two coordinate directions, means for adding said component signals to said first signals to derive control signals such as can be used to control the movement of said tool relative to a workpiece, means for discontinuing the derivation of said first signals and for substituting signals of fixed values and means for varying said radius signal during a period of discontinuation to cause the control signals produced by adding the component signals from said resolving means to the respective signals of fixed value to represent two co-ordinates of points on a required circular arc extending from said locus and having a radius determined by the change of said radius signal. In a control system for an automaticallycontrolled machine-tool of the kind described in Specification 830,462, responsive to interpolated X and Y signals representing Cartesian coordinates referred to a given origin, of points on the profile to be machined, with correction for cutting tool radius as described in Specification 830,461 (Fig. 1, not shown), utilizing a resolver for each co-ordinate direction, wherein the rotor is maintained at angle # between the cutting- tool movement and the particular co-ordinate direction, is fed with a signal representing cutting tool radius r; and the profile to be machined is a circular arc of radius R, the controlling X and Y signals are replaced by fixed signals representing Xc, Yc defining the centre C (Fig. 2) of the circular arc described by the cutter centre moving from C 1  to C 2 , which are stored to provide constant output during description of the arc while the cutting-tool radius correcting-means is fed with a signal representing R+r. The resolver rotor is rotated at a predetermined velocity, defined by suitable programme control signals in dependence on (R+r), or controlled automatically in response to resolved errors in the cutting tool position servomotors and the instantaneous rotor velocity, between the values # 1  and # 2  of # at the beginning and end of the circular arc; the length of which is determinable by storing the co-ordinates of such beginning and end and thus the starting-point of the next span, which are utilized in conjunction with the normal position feed-back signals representing cutter displacements X and Y to resolve the difference vector between cutter position and end point of the span into the common tangent to cutter and profile; normal working being restored when the vector is zeroized. Sine and cosine potentiometers resolve the difference vector. Alternatively, the interpolators, which need generate no co-ordinates for the profile during the cutting period of the circular arc, are employed to generate for such period successive values of angular displacement of the cutter radius corrector to describe a parabola blending with the arc at the ends thereof. The programme control information tape has an additional track containing no information while the remaining tracks contain X and Y information. At the beginning of a circular arcuate cut, the additional track contains information of angle #1 at a point defined in the corresponding positions of the X,Y tracks. In the next track position, the additional track contains the radius R of the required arc and the X, Y tracks contain co-ordinates Xc, Yc of the arc centre. In the succeeding position the additional track contains the angle #2 at the end of the arc at a point defined in the corresponding positions of the X, Y tracks. At the starting- point the interpolated X, Y signals are overridden by Xc, Yc signals switched in from the appropriate stores while signal R is added to signal r already fed to the tool radius corrector; the end point of the arc and the angle # 2  being also stored at this time as described in Specification 830,462. A linear or non-linear interpolator fed with signals representing # 1 , # 2  derives # for the drive of of a resolver employed in generation of the arc by rotating the cutting- tool about Xc, Yc as centre. In Fig. 3 a punched tape 1 carrying the recorded information tracks passes a reader 2 deriving X-co-ordinate signals of successive points along a required curve, which are cyclically transmitted to five storage units 4 to 8 controlled by programme unit 3, each comprising an auto-transformer (not shown) whose tap is selectively set to derive a voltage analogous to the corresponding coded value, and the stored signals are alternately applied in successive groups of 3, wherein the third signal of one group is the first signal of the next group, over a selector switch 9 to a pair of quadratic interpolators of the kind described in Specification 796,994 comprising autotransformers 10, 13 each having ten linearly-spaced taps connected to respective sets of ten contact studs 11, 11<SP>1</SP> selectively through a series of windings 12, 14 inductively coupled to each other but not to the autotransformers, and having respective turns ratios quadratically related to the control winding. When input signals representing X co-ordinates of three reference points are applied to transformer 10 (or 13), the output signals at studs 11 (or 11<SP>1</SP>) represent the X-co-ordinates of closely-spaced points of a parabolic arc through the reference points. Alternate studs of the ranges form two concentric circles each engaged by two make-before-break contact brushes 15, 18; 16, 17; which overlaps each other radially and are individually spaced apart by the angular spacing of adjacent studs of the two circles regarded as one, so that contacts 15, 16 and 17, 18 alternately derive voltages representing successive intervals of the parabolic arc, to respectively energize two autotransformers 20, 21 having regularly-spaced tap contacts disposed in respective halves of a contact circle 22, having half contacts at the end of each semi-circle, and which is engaged by a single rotary brush 23 on shaft 24 synchronized with shaft 19 driving contacts 15 to 18 so that it rotates for movement of brushes 15 to 18 through the angular spacing of contacts 11 (or 11<SP>1</SP>) and moves from one semi-circle to the other as brushes 15 to 18 leapfrog. Brush 23 thus derives a voltage for machine tool control in the X axis which is linearly subinterpolated over successive intervals of the parabolic arcs defined by the interpolating transformers. Series windings 26, 27 of autotransformers 20, 21, respectively derive voltages representing the time differentials of successive X values which energize stator 28 of a resolver whose other stator 29 is similarly energized by a voltage representing the time derivative of successive values of Y obtained from a similar interpolating arrangement (not shown), giving a Y signal for machine-tool control along the Y axis in response to the tape track along which Y information is recorded. The output of rotor 30 energizes servo-system 32, 33 through normally-closed switch 60 to rotate shaft 31 and rotor 30 through angle   # = tan<SP>-1</SP> #Y/#X. Rotor 36 of a resolver compris-   ing stators 34, 35 is rotated by shaft 31 and is normally energized by a voltage r representing the tool radius, so that stator voltages E x , E y  are provided for addition to the X, Y control signals for cutter radius compensation, as described in Specification 830,461. An additional track of tape 1 passes reader 2<SP>1</SP> to cyclically feed information to stores 37 to 41 under control of programme unit 3 similarly to the operation of stores 4 to 8. As the left-hand interpolation transformers 10, 12 derive signals representing X co-ordinates of a parabolic arc passing through points given by reference signals X 1 , X 2 , X 3  from stores 4, 5, 6, signals representing co-ordinate Xc of the succeeding circular arc centre and co-ordinate Xs of the first reference point following the arc are supplied respectively to stores 7, 8. Simultaneously with storage of X 3 , Xc, X 5 , the signals representing # 1 , R and # 2  are derived from the additional track and stored in 39, 40, 41, respectively. The stored signals are alternately applied in successive groups of 3 (wherein the third signal of one group is the first of the next group) over selector switch 42 similar to 9 above to two groups of three output leads; the outside leads of which are respectively connected to the opposed ends of auto-transformers 43, 44 which have regularly-spaced taps connected to respective contacts of a circle 45 similar to 22 but having contact brush 46 rotated on shaft 47 in synchronism with contact 16 to derive signals representing the interpolation of # between # 1  and # 2 . As brush 16 leaves the bottom contact of semi-circle 11, brush 23 completes its corresponding sub-interpolation rotation over #/2 with a signal representing X 3 , and brush 46 commences interpolation at angle # between # 1  and # 2  on the right-hand side of contact circle 45; the interpolated value of # at brush 46 being compared in difference circuit 48 with a voltage analogue of the instantaneous position of shaft 31; the output energizing a servomotor 49 (over switch 62) to drive shaft 31 during the generation of the circular arc. The stored signal representing arc radius R is derived from the appropriate group centre lead from switch 42, selected by the switch 61 and earthed over windings 50, 52 coupled to winding 51 of the r signal input to add in the R signal.