Nov. 19, 1946. G. E; KING 2,411,162 CONTROL SYSTEM Filed Deo. 5, 1944 je / la one/2%# ATTORNEY ~Patented Nov. 19, 1946 UNITED `STATES `PATENT OFFÍCE 2,411,162 CONTROL SYSTEM George E. King, Swissvale, Pa., assigner to West inghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania l Application December 5, 1944, Serial No. 566,723 5 Claims. (Cl. 172-439) 2 My invention relates Áto electric motor con trol systems for grinding machines, milling ma chines and other machine tools in which a “feed motor” is used to -move workpiece and tool rela tive to each other while the tool performs its cut ting or other deforming operation on the work provided with a feed motor and a cutter motor denoted as a whole by FM and CM, respectively. The feed motor is energized, under control by an electromagnetic contacter CR., from a power transformer TR through two rectifier tubes TI and T2 of the gaseous discharge type, for in piece. . stance, as known under the name Thyratron. For certain applications of such machine tools These rectifier tubes are controlled by three elec it is desirable to vary the speed of the feed motor tronic tubes preferably of the high Vacuum type. in such a manner as to maintain the load on the 10 These tubes are denoted by T3, T4 and T5, re tool or cutter below a safe limit in order to pre spectively. vent «damaging the tool. In practice, the feed More in detail, the milling machine MS car ries on its stationary supporting or bed struc ture I a horizontally displaceable bed 2 for ac motor is usually operated at a speed lower than the maximum permitted by the tool, because. it is necessary to avoid overloading the tool or cut The milling tool or cutter 4 of the machine is connected by a loads occur due to unevenness of surface or ir- suitable transmission with the cutter motor CM. regularities in hardness of the workpiece. Very This motor is energized through a switch 5 from often, the operator, in order to avoid ruining the an alternating current line 6. Motor CM may be tool, will use a slower speed than necessary for 20 operated at constant speed and a control appa a suflicient' margin of safety. The customary ratus (not shown) may be provided for adjust overload relays installed in the motor control ing this speed to a desired value. _ system ofthe machine tool for protecting the The displaceable bed 2 of the milling machine electric equipment are normally set for a fixed is connected by a Suitable transmission to 4the maximum load value and neither designed nor» 25 armature 'I of the feed motor FM. The apper accessible to the operator for permitting a change taining ñeld winding is denoted by 8.' ` in adjustment to suit particular working condi The power transformer TR has a primary wind tions. Consequently, such machine tools are apt ing 9 connected to the alternating current line ter at those places _along the cut where increased to be operated below optimum capacity. 15 commodating a workpiece 3. 6 and is equipped _with three secondary windings It is an object of my invention to provide an 30 IIJ, II and I2. Winding l0 is connected through electronic system for controlling the drive mo a rectiñer I3`with ñeld winding 8 of motor FM tors of machine tools in such a manner as to so that this field winding is excited by 4direct ~ vary the motor speed automatically in dependence upon the load imposed on the tool and with the effect of permitting' an operation at or near optimum capacity without the danger of over loading the -tool at places of increased cross sec tion or hardness of the material. to be worke current of constant voltage during the operation of the system. The armature circuit of motor FM extends through two contacts I4 and I5 of contacter CR whose .control coil i6 actuates also four additional relay contacts denoted by Il, I8, I9 and 20, respectively. Relay coil I6 is con nected to line 6 through a normally open control Another object of my invention is to .provide an 40 contact 2| and a normally closed control contact electronic control system for machine tools that 22. These contactsl are preferably designed as upon. - - . ' permits setting the motor speed at a desired nor pushbutton switches and serve, respectively, to mal value and acts automatically to reduce the start and stop the operation of motor FM. That motor speed below this normal value if and when is, when contact 2l is depressed by the operator, the load onA the tool exceeds a magnitude which. 45 coil I6 is energized and closes contacts I4 and I5; can be selected by the operator to suit the work at the same time contact Il is closed and com piece and tool used under particular workingv pletes _a self-holding circuit so that coil I6 re conditions. mains energized after contact 2l is released. The ' 'I‘hese and other'objects will be apparent from actuation of .contact 22 interrupts the self-hold the following description of the control system 50 ing circuit and hence returns relay CR into the for a milling machine shown diagrammatically illustrated inoperative position. in the drawing as an embodiment of my inven The secondary winding II of transformer TR tion. is center tapped. »The center point is connected The drawing illustrates schematically a mill with contact I4 of relay CR, while the two .ter ing machine denoted asa Whole MS which yis minals of winding II are attached to the anodes 2,411,162 . i 3 of rectifier tubes Tl and T2, respectively. The cathodes of tubes Tl and T2 are interconnected and attached to contact I5 of relay CR through a series resistor 23. Hence when contactor CR is energized and closes its contacts I4 and l5, the armature 'l of -feed motor FM is energized by rec `titled current derived from the secondary Il of transformer TR through the plate` circuits of tubes TI and T2. The energizing direct current traverses the series resistor 23 and develops across this resistor a voltage drop which is a, measure of the >current intensity. A calibrating rheostat 4 ing if the function of tubes T4 and T5 is _at iirst disregarded in the following discussion of the control operation performed by the above-men tioned grid circuit portion A--B-C-D-E-F-G of tube T3. . This circuit portion includes in series the three vresistors or rheostats 24, 2l and 4I. Each of these three resistors is impressed by a voltage that originates from outside voltage source and hence represent each a separate source of a com ponent voltage for the grid circuit. The rheo stat 4I develops along its effective portion a volt age drop (El) which originates from the con stant voltage source 40 and has a magnitude se to permit measuring an adjustable proportion of the total voltage drop of resistor 23. 15 lected by the operator in accordance with the ~ 24 is connected in parallel to resistor 23 in order The control grids of rectifier tubes Tl and T2 ^ are denoted `by 25 and 25, respectively. These grids are interconnected through two lresistors 2l and~28 and the center-tapped secondary 29 desired speed of feed motor FM. The resistor 4l,l as explained previously, ‘develops across its ter minals a Voltage drop (E2) which Varies in ac cordance with the rectifier termina1 voltage im pressed on motor FM. That portion of rheo stat 24 which is series connected in the grid of transformer 30 is connected to the secondaryv circuit/»imposes thereon a voltage component (E3) i 32 of a phase shift transformer 33 whose primary which represents an adjusted proportion ofthe 34 is supplied with constant excitation from the voltage drop occurring across resistor 23. Since secondary i2 of the power transformer TR. '_I‘he connection includes a resistor 35 and a capacitor 25 resistor 23' lies in series between the rectifier 36 and functions to impose a component alter- ` cathodes and the armature l, its voltage drop and hence the component gridvoltage across the nating-currentvoltage on the grid circuit of the effective portion of rheostat 24 is proportional to rectifier tubes Ti and T2 under maintenance of the IR drop -in the armature circuit of motor a given phase displacement between the cycle of ~ this voltage component and that of the alternat 30 FM. The resultant grid voltage imposed on the con ing-current voltage to be rectified by the tubes trol grid 45 of tube T3 is `thus dependent upon Tl and T2. of a transformer 30. The primary winding 3l _ the behavior of three components. One compo The grid circuit of the rectifier tubes are fur nent (El) remains constant as long as the speed ther impressed by a variable direct-current volt age component. To this end, the.y tap point of 35 setting of rheostat 4l remains undisturbed., The othertwo components (EZ'Yand E3) tend to change the transformer secondary 29 is connected to the during the operation of the system in accordance - slider of a rheostat 3l which extends across awith the varmature voltage and the IR drop, _re voltage source 38 in order to provide an adjust spectively. The polarities of Áthe three compo able constant grid bias. Rheostat 31 isÍ also con nected with the plate circuit of tube T3. This. 40 nent voltages are so chosen that the resultant circuit contains a resistor 39 and is energized from a source of constant voltage, denoted by 4D, which is connected across a series arrange ment of resistors'ßll, 42 and 43. Resistor 4I con sists of an adjustable rheostat. and serves to se lect a desired speed for motor FM as will be ex voltage isr the algebraic sum of the negative value of El, the positive value of E2 and the negative value of E3. As a result, the condition of con trol grid 45 is basically determined by the set ting of rheostat 4l and changes in proportion to the value of the motor voltage minus the IR drop occurring in the armature circuit during plained in a later place. A Calibrating resistor the operation of the motor. This resultant value 44 is connected in the circuit of the control grid is proportional to the motor speed. ' 45 of tube T3. Connected across the plate cir By virtue of the above-described connection of cuits of the rectifier tubes TI and T2 are two 50 resistors 46 and 4l, arranged in series relation tube T3 with the grid circuit of the rectifier tubes to ,each other. When the motor is in operation, TI and T2, the direct-current component of rec the voltage drop across resistors 46 and 41 is pro tifier grid voltage varies also in .accordance with portional to the rectified or terminal voltage ap « the speed of motor FM. These variations have plied to the feed motor. Consequently, the Dro 55 the effect of displacing the firing point of _each portion of voltage drop appearing across resistor rectifier with respect to the cycle of the alter 4l presents a measure of the motor Voltage. nating current to be rectified thus causing a cor -The just-mentioned circuit connections of tube responding change in the rectified output voltage T3 include a composite grid circuit. A complete and hence a change in the speed of motor FM. loop portion of this circuit extends from control 60 Inthis manner, a decrease in motor speed below the value ,set by rheostat 4I has> the effect of vthrough resistor 44 to the point marked B, thence producing a còntro1 action in the rectifier tubes through a portion of rheostat 24.to points C and Tl and T2 which is in proportion to the de D, thence through resistor 4l to points E and F»A parture of the speed from the desired value and and through the effective portion of the speed 65 causes lan increase in the rectified output volta-ge, adjusting rheostat 4i to point G thus ending at thereby restoring the motor speed to thedesired the cathode of tube T3. It will be recognized that value. An increase inmotor speed above the the plate circuits of tubes yT4 and T5 are tied -desired value has a similar effect except that it up with the just-mentioned portion of the-grid causes a decrease in rectified Áoutput voltage in circuit of tube T3. As will be explained later, 70 order to reduce the motor speed to the intended however, the tubes T3 and T4 have no «effect on value. the operation of the above-mentioned loop por Referring now to the „tubes T4 and T5, it will tion of the grid circuit as long as certain operat be seen that-the plate circuits' of these tubes ing conditions are prevailing. `Hence it appears contain a resistor 50 or 5i which is connected to to be logical and conducive to easier understand 75 the positive pole of they above-mentioned current l grid 45, through the point marked A, thence 2,411,162 source 40. Both tubes are pentodes having a sharp cut-off characterstic. The cathodes 52 with the current source 56 so that the displace ment cui-_rent iiowing into the capacitor upon the and 53., respectively, of tubes T4 and T5 are in closure of contacter CR delays the building up of terconnected and are also in connection to a the blocking potential across the rheostat 55. point, marked H, of the armature circuit of mo 5 The capacitor 58 is discharged through contact tor FM. Consequently, the source 40 which pro I9 when the feed motor is stopped by depressing vides the plate voltage for tube T3 serves also as the pushbutton 22. a plate voltage source for tubes T4 and T5. The The control grid 52 of tube T4 is connected plate circuit of tube T4, for instance, extends through a resistor 60 to the slider of a rheostat fromy the appertaining anode through resistor l0 6| which extends across a constant voltage source 50, voltage source 40, points F and E, resistor 41, v|52. The positive pole of source 62 is connected points D and C, resistor 23, and point H to the cathode of the tube. The plate circuits of tubes T4 and T5 are thus interlinked withLthe above described grid circuit of tube T3. Consequently, 15 the appearance of a voltage drop across either resistor 5D or 5| has the effect of imposing a bias on the grid voltage of .tube T3 in super~ to a resistor 63 which, at point K, is in connec tion with the cathode of tube T4. Resistor 63 is connected across a rectifier 64. This rectiñer is energized by a current transformer 65 which measures the load current of the cutter motor CM. Hence, the voltage drop developed across resistor 63 is a measure of the load imposed on position upon the above-mentioned resultant of the cutter 4 of the milling machine. The circuit the voltage components El, E2 and E3. The 20 of grid 52 extends through resistor 60 and the occurrence and magnitude of such voltage drop selected portion of rheostat 6|, thence -through across resistors 50 and 5| is controlled by the control grids 52 and 53, respectively, of tubes T4 and T5. - Grid 53 of tube T5 is connected through a re~ sistor 54 to the slider of a rheostat 55 which is energized from a constant voltage source 56 in series connection with a current limiting resistor the load measuring resistor 63 to point K and the cathode of tube T4. This grid circuit in cludes two sources of grid voltage. One source is represented by the adjusted portion of rheo stat 6I and provides a grid bias which~ is highly negative so that tube T4 is normally prevented from conducting plate current. The second 51 and under control by the above-mentioned source of component grid voltage for tube T4 is contacts I8, I9 and 20 of contactor CR. The formed by the load measuring resistor 63. The positive pole of source 56 is connected to point corresponding voltage component acts in op D of the rectifier output circuit so that the grid position to the negative grid- bias provided by circuit of tube T5 includes the load responsive rheostat 6|. Consequently, when the load on measuring resistor 23 in the following manner. the cutter 4 increases, thereby increasing the Starting at grid 53. the appertaining grid circuit 35 voltage drop across resistor 63, this will tend to extends through resistor 54, a portion of rheo make the control grid 52 of tube T4 less nega stat 55, the point marked I, points D and C, re tive and in this way will cause the plate current sistor 2'3, point H, and back to the cathode of through resistor 50 to increase. This in turn tube T5. Consequently, the potential of grid 53 results in an increase of `voltage drop across re depends on two voltage components. One com 40 sistor 50, and the control grid 45 of tube T3- will ponent is represented by the voltage drop be become less negative. The plate current of tube tween the slider of rheostat 55 and point I. The T3 increases accordingly. Consequently, the other voltage component is the voltage drop firing angle of rectifier tubes TI and T2 is de developed across resistor 23 in accordance with layed so that the rectified output voltage de the rectified load current supplied to the arma creases and causes a slow down of the feed motor. ture 1 of motor FM. The component deter Due to the high negative voltage bias imposed on mined by the adjustment of the rheostat 55 is so grid 52 by rheostat 6I, tube T4 does not conduct chosen that the potential of grid 53_is normally until the voltage drop across resistor 63 reaches highly negative and thus maintains the tube T5 a definite value set by the potentiometer 6|. normally in Iblocked condition. The voltage 50 The operating point at which the control effect of drop across resistor 23 acts in opposition to the tube T4 becomes operative under increasing cut adjusted bias. Hence when the load current of ter loads is, therefore, determined by the selected armature 1 increases beyond a value determined adjustment of potentiometer 6|. Hence, -by a by the setting or rheostat 55, the grid 53 be proper selection of this adjustment, the operator comes suiliciently less negative to permit the 55 can limit the cutter load in order to suit the cut passage of current through the plate circuit. ter and workpiece for any'particular working op Then the resistor 5| will develop a voltage drop eration of the machine tool. ‘ which, as explained above, superimposes a con The potentiometer rheostat 6| can be calibrated trol bias on the grid circuit of tube T3 with the in terms of cutter load current and a chart can -be . result` of causing the rectifier tubes TI and T2 60 prepared to show the operating current for each to reduce the voltage of their output current. In ' cutter. 'The operator can then' set the potenti this manner, the speed of motor FM is reduced ometer 4|rfor the speed best 'suited for the cutter and 'the load correspondingly diminished. In to enter the workpiece and then readjust it to summary, tube T5, in conjunction with the as~ -the maximum permissible speed and let the cutter sociated tube circuits, serves to prevent the feed 65 -load regulate the speed as set on the potentiom motor from exceeding a maximum load of ad- . justed value by reducing under overload condi tions vthe motor speed below the normal value , eter 6 l . This permits obtaining optimum produc tion from the machine without fear of ruining the cutters. selected at rheostat 4|. When the overload As explained in the foregoing, the tubes T4 and conditions disappear, the correct feed motor, 70 T5 operate independently of each other. The speed as determined by rheostat 4| is automati speed regulation in dependence upon the cutter cally established. In order to prevent tube T5 load is effected merely by tube T4 and the asso from causing undesired transient phenomena ciated circuits so that tube T5 may be omitted during the starting and stopping periods of the if a speed control in dependence upon the feed machine too1 drive, a capacitor 58 is connected 75 motor load is unnecessary. The fact that the two _ 2,411,162 7 8 tubes T4 and T5 operate in parallel and inde pendently of each other shows also that instead. rent exceeds a value determined by the selected adjustment of said potentiometer means. 2. A machine tool control system, comprising of, or in addition to tube T5, one or several other tube arrangements may be used which are re a. cutter motor and a direct-current feed motor, alternating current supplymeans for said feed sponsive to cutter loads in accordance with the motor, electronic rectifier meansof the gaseous operating principle of tube T4. Such use Of 2» discharge type connected to said supply means plurality of tubes arranged similar to that of tube for energizing said feed motor by rectified cur T4 is of advantage for machines in which differ rent derived from said supply means, said recti-ent types or sizes of cutters are to be used at the same time. The different tubesyall similar to 10 fier means having a. control circuit for varying the voltage of said rectified current and thereby tube T4, are allotted- to the diiferentrespective the speed of said feed motor, an electronic vacu cutters and their grid circuits are controlled in um tube having a plate circuit connected with dependence upon the load current of the respec said control circuit and a grid circuit for con tive cutter motors. This permits the operator to trolling said plate circuit. an adjustable potenti ometer disposed in said »grid circuit for imposing' adjust each potentiometer, according to potenti ometer 6| of tube T4, to the proper load setting for its associated cutter motor or cutter. 'I'hen thereon a component voltage in accordance with a speed value for said feed motor determined l the feed motor will operate automatically at that `speed which is proper for the cutter motor carry by the selected adjustment of said potentiometer, When` a 20 circuit means also disposed in said grid circuit fragile cutter is cutting, this cutter will determine the maximum rate of feed, but if this cutter does not happenl to be cutting, Isome more rugged cut cordance with the speed of said feed motor so as to maintain said speed normally at said se ing the greatest proportional load. for imposing thereon a voltage variation in ac lected value, and'load-responsive circuit means ter or one of the cutter motors carrying thelarg _est percentage of its rated load will determine 25 connected with said grid circuit for superimpos 'ing thereon -another voltage variation in depend the rate of feed. Thus, the machine will always ence upon the l'oad current of the cutter motor be caused to operate at the highest rate of feed so as to reduce the speed of the feed motor below -suitable for the conditions under which the ma said selected value when said load current ex chine is operating at any moment. In this man ner the feed can be maintained at that rate which 30 ceeds a ~given magnitude. » 3. A machine >tool control system comprising will not overload any of the motors and will not a tool operating motor and a direct-current feed put too heavy a duty upon any of the cutters. In motor, alternating current supply means for said case the rate of feed is lower than that proper feed motor, electronic rectifier means of the for the work being done by the cutter or the motor least able to withstand the load,l the feed 35 gaseous discharge type connected to said supply means for energizing said feed motor by rectified rate will be increased automatically until the current derived from said supply means, said rec optimum load for that cutter o_r motor is reached. tifier means having a control circuit for varying It will -be understood by those skilled in the the voltage of said rectified current and thereby art that other'modiñcations or alterations are possible in systems according to the invention as 40 the speed of »said feed motor, an electronic vacu um tube having a plate circuit connected with ` described in the foregoing without departing from said control circuit and a grid circuit for con the gist of the invention or from the essential trolling said. plate circuit, an adjustable potenti features of the invention as set forth in the claim ometer disposed in said grid circuit for impos attached hereto. ing` thereon a component-Voltage in accordance I claim as my invention: -with a speed value, for said feed motor determined 1. A machine tool control system, comprising a tool operating motor, a direct-current feed by the selected adjustment of said potentiometer, circuit means also disposed in said grid circuit for imposing thereon a voltage variation in ac cordance with the speed of said feed motor so as motor, alternating current supply means for' said ` feed motor, electronic rectifier means of the gas eous discharge type connected to’ said supply to maintain‘said speed normally at said selected value, and load-responsive circuit means connect ed with said grid circuit for superimposing there-. means for energizing said feed motor by rectiñed current derived from said Supply means, said rectifier means having a control circuit for vary on another voltage variation in dependence upon A ing the voltage of said rectified current and there- . by the speed of said feed motor, an electronic vac uum tube having a plate circuit connected with the load current of said tool operating motor, said grid circuit for Isuperimposing another voltage is- reduced below said' selected value when said load current exceeds a magnitude determined byv 65 _load-responsive circuit means comprising an elec tronic tube havingA a plate circuit connected with said control circuit and a grid circuit for con said control circuit and having a grid circuit trolling said plate circuit, adjustable circuit equipped with adjustable potentiometer means means connected with said grid circuit for impos ing thereon a voltage adjusted in accordance with 60 for providing a cut-'off bias of selective magnitude and with a circuit member for providing opposite a selected value of motor speed, and electric cir ly acting bias variable in accordance with said cuit means disposed for measuring the load of load current, whereby the` speed of the feed motor said tool operating motor and connected to said thereon so as to vary said rectifiedvoltage in de the selected adjustment of said potentiometer pendence upon said load current, said circuit means comprising an electronic tube having a plate circuit connected with said control circuit ' means. . 4. A machine control system, comprising a di rect current drive motor, circuit means for sup » and having a grid circuit equipped with adjust able potentiometer means for providing a cut-olf 70 plying alternating current, electronic rectifier means of the gaseous discharge type disposed be bias of `selective magnitude and with a circuit member for` providing oppositely acting bias vari able inìaccordance with the load current of the cutter motor in order to cause a reduction in volt age of said rectified current when said load cur tween said circuit means and said motor for en ergizing said motor by rectified current, said rec tiñer means having a contra; circuit for. varying 75 the voltage of said rectified current and thereby 2,411,162 10 the speed of said motor, circuit means connected ' - - said plate> circuit, an adjustable potentiometer _ to said control circuit for causing it to vary said disposed in said grid circuit for imposing thereon rectiñed voltage in dependence upon load magni a component voltage inpaccordance with a speed value for said feed motor determined by the se tudes, said circuit'means comprising a plurality of electronic tubes all having their respective .plate circuits connected to said control _circuit and each having a grid circuit equipped With adjustable potentiometer means for providing a Íblocking bias of selective magnitude and with a load-responsive circuit member for providing a variable bias in_opposition tosaid blocking bias, whereby the speed of said motor is reduced when lected adjustment of said potentiometer, circuit means also disposed in said grid circuit for im-u posing thereon a voltage variation in accordance with the speed of said feed motor so as to main tain said speed normally at said selected value, and load-responsive circuit means connected with said grid circuit for superimposing thereon an other voltage variation in dependence upon load the load acting upon any one of said circuit mem- ' conditions of said operating motor, said load-re- . bers exceeds a magnitude `determined by the se sponsive circuit means comprising a plurality of lected adjustment of said respective potenti ometer means. 5. A machine control system, comprising an operating motor and a direct-current feed motor, alternating current supply means for said feed 15 electronic tubes all having their respective plate , circuits connected to said control circuit and each having a grid circuit equipped with ad justable potentiometer means for providing a Y blocking bias of selective magnitude and with a motor, electronic rectifier means of the gaseous 20 load-responsive circuit member for providing a discharge type connected to said supply means for energizing said feed motor. by rectified current derived from said supply means, said rectiñer _variable bias in opposition to said blocking bias, whereby the speed of s_aid drive motor is reduced means having a control circuit for varying the when the load acting upon any one of said circuit members exceeds a magnitude determined by the voltage of said rectified current andv thereby the selected adjustment of said respective potenti speed of said feed motor, an electronic vacuum ometer means. tube havingA a plate circuit connected with said control circuit and a grid circuit for controlling GEORGE E. KING.