March 9, 1954 RS. CHILDS 2,671,892 MEASURING DEVICE Filed Nov. 6, 1948 3 Sheets-Sheei 1

INVENTOR ROBERT s. GHILDSS' BY

ATTORNEYS March 9, 1954 ‘ R. s. CHILDS 2,671,892 ' MEASURING D-EVICE Filed Nov. 6; 1948 s Sheets-Sheet 2

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Ill" I ATTORNEYS March 9, 1954 . R. s. cHlLos 2,671,892 MEASURING DEVICE Filed Nov. 6, 1948 3 Sheets-Sheet 3 Fig. l0

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INVENTOR ROBERT S. CHILDS BY/ W ,4‘ ATTORNEYS Patented Mar. 9, 1954 2,671,892

UNITED STATES PATENT OFFICE 2,671,892 MEASURING DEVICE Robert S. Childs, South Sudbury, Mass., assignor, by mesne assignments, to Edward G. Martin, Cambridge, Mass. Application November 6, 1948, Serial No. 58,651 10 Claims. (Cl. 340-195) 1 2 The present invention relates to improvements switching may use the same apparatus as both in the measuring device described in my co-pend the coarse and ?ne measuring elements, both of ing application Serial No. ‘794,192, ?led Decem which are necessary in some servomechanism ap ber 27, 1947, now Patent No. 2,650,352, dated plications. The sectors of the stator according August 25, 1953. More particularly this inven to this invention can readily be connected either tion involves a means for reducing or eliminating to utilize the capacitive output voltage for coarse undesirable capacitive coupling between the in (sometimes called low-speed) measurement, or put and output of the measuring device. in a manner whereby the capacitive voltage is The aforementioned. device depends for its op substantially eliminated and the inductive volt eration upon inductive coupling between a rotor, age remains for the ?ne (sometimes called high to which in the preferred embodiment high fre speed) measurements. quency voltage is applied, and a stator. For rea A still further object is to provide simple means sons explained in said application a non-ferrous for leading in the input voltage to or leading out core is used for the stator and rotor and the in the error signal voltage from the rotor of the ductively induced voltage in the secondary mem 15 apparatus in such a way that no torque reaction ber is a small fraction of the voltage imposed on is exerted on the delicate prime mover to which the primary. At the same time the windings of the rotor may be attached for measurment pur the stator and rotor which oppose each other poses. The present invention contemplates across a narrow air gap constitute a substantial means for eliminating the usual brush and slip distributed capacitance, and at the high fre- ' rings with, their resultant inaccuracy due to quency (preferably of the order of 100 kilocycles counter-torque in the measuring device. Elimi per second) used in this device, the capacitive nation of this counter-torque makes possible a coupling between stator and rotor becomes corre servomechanism system that is able to take full spondingly large; that is, the capacitive voltage advantage of the inherent precision of the multi appearing in the secondary output becomes sub conductor, non-capacitive device about to be de stantial. As a result of the small inductive volt scribed. age and of the high stator-to-rotor capacitive In the accompanying drawing, with the aid of coupling, the capacitive component of the output which I will describe my invention, Fig. 1 is a voltage may be of such size as to mask the induc sectional elevation view of the device described tive voltage. Furthermore, the magnitude of the in my prior application; Fig. 2 is a detail view capacitive component varies with the relative po of the type of winding used on both rotor and sitions of the stator and rotor. The apparatus stator of that device; Fig. 3 is a view of the wind is primarily for the purpose of distinguishing the ing and connections according to this invention; angle of rotor rotation by the magnitude of in Fig. 4 is a view of the windings of the section-- ' ductive output voltage. However, the effect of ‘ alized member, preferably the stator, which is the large capacitive voltage is to render it dif here shown as a two-phase device; Fig. 5 is a ficult to determine the inductive voltage ac graph showing the variation in capacitive voltage curately. output with rotor angular displacement for the The principal object of this invention is to re device of my prior application; Fig. 6 is a graph dues or eliminate distributed capacitive effects of the capacitive output voltage vs. rotor dis without impairing the inductive coupling upon placement for the closed diameter-connected which the accurate measurement of small angu rotor of my present invention when employed‘ in lar displacement depends. conjunction with a stator having a winding of With this object in view, the present invention the type shown in Fig. 2; Fig. 7 is a graph of the comprises an improvement in the apparatus de capacitive output voltage vs. rotor displacement scribed in my prior application, said improve for the closed diameter-connected rotor when ment consisting in the use of a continuous rotor employed in conjunction with a multiple-section winding with external connections made across parallel-connected stator winding; Figs. 8 and 9 a diameter and, in addition, the splitting of the are circuit diagrams showing how a two-phase stator winding into a number of equal sections device according to the present invention would which may be parallel-connected. This arrange be used in a servomechanisms application, Figs. ment results in eliminating or at least substan 8 and 9 showing the coarse and ?ne connections, tially minimizing capacitive voltages. respectively; Fig. 10 is a view, partly in section, Another object is the development of a self of apparatus incorporating the present inven synchronous device which by means of simple tion; Fig. 11 is a plan view of a part of the device 2,671,892 4 shown in Fig. 10; and 12 is the circuit dia each formed of zig-z-ag conductor bars having gram equivalent of the same device. the same angular bar spacing as the rotor. The The illustrated embodiment of the invention purpose of the two windings is to provide a two (Fig. 1) comprises a primary member 5 and a phase system, and to that end the bars of wind secondary member 8, of which one, preferably the ing P2 are angularly displaced from those of P1 primary, is a rotor, while the other is a stator. by one-half the angular bar-spacing. Both As shOWIl in Fig. 1, the stator and rotor may com windings are in inductive relationship with the prise disks, preferably of glass, arranged face to rotor, and accordingly the bars of each stator face with as small an air gap between them as winding are only about half the radial length possible. The rotor is suitably mounted on a hub 10 of each rotor bar. or spider I0 which in turn is mounted on a shaft It will be understood that the bars are pref l2. The stator carries a deposited metal con ferably great in number, say of the order of 1000 ductor indicated by heavy lines 64 in Fig. 1 and for , each winding, but only a relatively small the rotor carries a deposit It opposed to deposit number are shown in the drawing. M. 15 One of the stator windings, say P1, is formed The windings described in my prior applica' in three sections 28, 30 and 32, each spanning tion are shown in Fig. 2 for comparison with the 120°. One end of each section is connected to a windings in accordance with my improvement common terminal 9, while the other ends of the shown in Figs. 3 and It. ,Both the stator and rotor several sections are brought out to terminals a, deposits comprise grid~like structures ar 20 b and 0, respectively. ranged around the peripheries of the disks. The The second-phase winding P2 is likewise formed conductor l8 comprises a single conductor ar in three sectionsv 3.4-, 36 and 38. These three sec ranged in zig~zag or back-and-forth fashion tions are all connected in parallel by connections whereby there is formed a succession of juxta leading to terminals 40. posed series—connected “bars.” In this form of 25 Considering now the winding P2, in which, all the invention the bars are radially disposed. The of the sections are connected in parallel, it can be individual bars are connected at their ends by shown that this winding, when used with a rotor short connectors 25. The deposition of the con winding like that of Fig. 3, is insensitive to ca duotor may be effected in any desired way, as by pacitance-coupling and is responsive only to evaporation of metal, such as aluminum, in the inductive coupling between the rotor and stator. desired pattern determined by a mechanical or To show this theoretically, consider ?rst that photographic process, as will be understood by the rotor and stator are capacitively equivalent to those skilled in the art. two conducting bands opposing each other across As shown in Fig. 2 the conductors on both rotor a narrow gap. Starting. with the case. of my prior and stator of the device of my prior application 35 application in which is used a single-path rotor were inter-connected to form a loop broken at like that shown in Fig. 2 hereof, it can be‘shown one point to lead out two terminal connections mathematically that there is induced a capaci 2|. The changes made by the present invention tively-coupled voltage, between stator terminals will be apparent from a comparison of Fig. 2 which is represented by the curve E0 in Fig. 5. In with Figs. 3 and 4 showing the conductor con that ?gure, a is the angular displacement of the nections according to the present invention. The rotor with respect to the stator. The inductive “primary” member (Fig. 3) which in servo voltage E1. is also show-n on this diagram and mechanisms applications will preferably be the varies with 0 in a manner depending on the bar rotor, has the important differences that the spacing. The inductive voltage En is usually winding is closed to form an unbroken 100p and v1 Cl smaller than, and may be masked by,‘ the capaci the terminal connections 22 and 23 are made at tance voltage EC. ‘ diametrically opposed points on the rotor. The By substituting a rotor having a winding of the result of this change is to cause the capacitive type shown in Fig. 3, in which the terminals are output voltage to. be substantially a sinusoidal diametrically connected, so that there are, two function of the rotor angular displacement. As paths from one terminal to the other, the stator will be explained, this fact can be put to use to voltage E0 across any portion of the‘stator» be provide a coarse system which uses the capacitive comes a function of 0, as shown in Fig. 6. In voltage for measurement. . Fig. 6, E0 is essentially a sinusoidal function of It is, to be noted that the rotor connections are the angle 0. ‘ . made in such a way that the current directions With the rotor winding of Fig. 3 and a multi in successive conductor bars are the same as in ple-section stator winding, such as P2 each sec the winding of Fig. 2,. Instantaneous current tion. of the stator winding acts the same as, a directions are shown by arrows in Figs. 2 and 3. whole stator, so far as the capacitance-coupled In Fig. 3 the current comes from terminal 22 on voltage is concerned.‘ The capacitive voltage in a. conductor bar 24 and then divides to follow two any stator'section will be zero when the diameter parallel paths toward the other terminal 23. A through the, rotor terminals bisectsthe stator single conductor 26 leads to the terminal 23. section. Rotation, of the rotor causes the: ca Thus the conductors 24 and 25 serve as joint pacitive voltage to change to a maximum, back conductors for the two paths. By this arrange to zero, to a maximum in the reverse phase, vand ment, currents in adjacent conductors are in ~" back to zero again, as shown in Fig. 6. Each opposite radial directions; more generally, any of the sections goes through the same variation two conductors spaced by an even number have but since successive sections are displaced in po currents in the same radial direction, and con sition, their voltages are likewise displaced. In ductors spaced by an odd number have currents Fig. 7 the capacitively-induced voltages, in the in opposite radial directions. The result is to " three sections are shown at Ec34,.Ec36 and EcIiB give the same general inductive relationship be as functions of. tween rotor and stator bars as exists in the wind It will be understood, that the diagrams of Figs. ing. of Fig. 2. 6 and 7 represent the relative magnitudes of the The secondary» member, preferably the, stator, several alternating- voltages as functions, of angu has two. windings, designated generally P1 and P2, ” lar: position. and‘ that; these. voltages‘ change in 2,671,892

"time at the frequency of the input voltage; in may be switched in for measurement within the other words, the three voltages shown in Fig. 7 particular quarter-cycle of the inductive output are in the same time phase but differ in space voltage selected by the coarse system. phase by 120°. The circuit diagrams of Figs. 8 and 9 represent Since the sections 134, 36 and 38 are uniform a servomechanism utilizing two of the units here and symmetrical, it can be shown that the pres tofore described, and capable by simple switch ence of three equal voltages displaced in space ing of being converted from a “coarse” to a “fine” ‘phase by 120° will result in zero capacitance volt control system. One unit 5|] (which may be age at the output terminals 40, and that all cur termed the transmitter) has a rotor I8 fed from rents due to the capacitance voltages are local 10 any suitable alternating current supply 5|, and ized within the stator sections. Thus the effects a stator having the windings P1 and P2. The ' of capacitive coupling are substantially elimi other unit 52 (which may be termed the receiver nated, and only the inductive voltage E1. appears or control transformer) is identical, but the cor at the terminals 40. responding rotor winding and stator sections are Some residual capacitance e?ects may appear primed in Fig. 18. The voltage across the rotor of ‘because of unavoidable asymmetry of the wind unit 52 is utilized as an "error signal” which is 'ings or departure of E0 from a true sinusoid, but used'for positional control in a manner familiar they are small. Furthermore, they are still fur to those skilled in the servomechanism art. ther reduced because the diameter-connected For a coarse control system, the capacitive rotor results in doubling of the current in the variation of voltage in the rotor winding P1 is windings by paralleling the two halves of the utilized. The terminals a, b, and 0, respectively, rotor. The current increase more than doubles of the two units are connected together. The [the ratio of inductively-coupled voltage to the windings P2 are not connected into the system. capacitively-coupled voltages. With this connection of the windings P1, the sec The foregoing explanation applies also to the tions 28, 28', the sections 30, 30’ and the sections winding P1 if .the terminals a, b and c are all con 32, 32' are all independently connected. The nected together. Elimination of capacitance winding sections of each stator constitute three coupling effects occurs because the three sections separate “phases” in the sense of space-phase thereof act identically with those of winding P2. displacement. By reasoning similar to that ap On the other hand, if capacitance coupling effects ' plicable to synchro theory, it can be shown that are desired, they may be obtained either by con the voltage across the terminals of rotor I8’ is an necting the three sections of P1 in series or by “error signal,” which is a function of the angu ‘utilizing the eifects of the windings independ lar displacement between the two rotors I8 and ently. i8’. This error signal may be utilized for con The principles outlined above apply to a di trol purposes, as will be understood by those vision of the stator windings into any integral familiar with the servomechanism art. number of equal sections. Thus, for 11. sections, To convert to a ?ne control system, the sec where n is any integer equal to or greater than 2, tions of each winding P1 are all connected in the several stator voltages are displaced in space parallel by jumpers 54 and 56 tying together the phase by 360/n degrees. The elimination of ca 40 terminals a, b and c of ‘the windings P1 of the pacitance coupling effects is accomplished for all respective units. This results in the elimination such values of n. For n=2, the explanation of of capacitance e?‘ects heretofore described. The operation is simple, since the two voltages in each windings P1 of the two units are connected to stator winding are merely displaced 180° in space gether by a wire 58, and the windings P2 are con phase. Where only the elimination of capaci nected by a wire 53. By reason of the half-bar tance coupling effects is desired, the use of a displacement of the two windings P1 and P2 of two-section stator is preferred for simplicity. each stator, this results in a two-phase system, However, another feature of the invention resides whereby an error signal appears across I8’ as a v‘in the use of the same units for both “coarse” function of the relative angular position of the and "?ne” control, and for the coarse control, 11. rotors. must be greater than 2 in order to obtain an out It will be observed that the coarse-control con put signal as a function of angular position. nections of Fig. 8 provide a three-phase system The feature of utilizing the units heretofore in which each phase covers 120° of the stator, described for either coarse or ?ne control will while the ?ne-control connections provide a two now be described. It is well-known in the art 55 phase system wherein the phases are separated of servo-mechanisms that best control is ob by 90 electrical degrees (the spacing between tained from the combination of two systems of two adjacent bars representing 180 electrical de diiferent “speeds,” i. e. different rates of voltage grees). Thus the coarse system executes a com change with the same rate of rotor angular dis plete cycle upon 360" relative movement between placement, more properly referred to as coarse rotors, While the ?ne system executes a complete and ?ne controls. The inductively-induced volt cycle upon a relative movement of only twice the age E1. passes through a complete cycle with a angular bar spacing. very small rotation of the rotor; this voltage is As is apparent to those skilled in the art, the well adapted for the ?ne-control system. On the rotors of devices 50 and 52 are usually made in other hand, the capacitive output voltage of a de tegral with the respective input and output de vice using the diameter-connected rotor (Fig. 3) vices, between which the circuit measures the and a stator winding of the type shown in Fig. 2 error angle, or angular difference. Electrical con passes through only a single cycle with a 366° nections must be made to the movable rotor. rotation of the rotor; this voltage is adapted for The usual brush and slip»rings impose too much the coarse-control system. Operation may thus vtorque reaction for high-precision systems. To be on the coarse control (that is, on the system take full advantage of the accuracy ‘of angular using the capacitive-coupled signal) to the limit measurement that is possible with the measuring 1of its accuracy, which will necessarily be within apparatus herein described, substantially a the nearest quarter of a cycle of the fine informa torqueless take-off from the rotor is required. tion; then the ?ne or inductive-signal system Figs. 10 and 11 show such a takeuoff embodied in acvraca 7 a measuring device. The. error voltage-produc loop in which adjacent conductors carry currents ing members of the apparatus are the stator 60 opposite in direction and terminal connections and the rotor 62. Windings of the type de on the same member at diametrically opposite scribed earlier are placed on the opposing sur points on said closed loop, and interconnections faces 64. Another stator plate 66 is positioned on the other member series-connecting adjacent on the opposite side of rotor 62 from‘ the ?rst conductors to form n separate groups positioned stator, the two stator plates being clamped to a 360/n degrees from one another, in which ad supporting and spacing ?ange 10. On the op jacent conductors carry currents opposite in di posing surfaces 68 of stator 66 and rotor 62 are rection. placed identical pairs of concentric rings of con 10 3. Apparatus for electrical measurement com ducting material 12 and 14. These rings oppose prising two relatively movable non-ferrous mem each other across the air gap and can be made bers, a large number of conductors extending of suf?cient area to provide series capacitors back and forth across each of said members and through which to make connection to the rotor. arranged in accurately spaced array, the con Fig. 12 shows the equivalent circuit diagram ductors of the two members being in inductive of measuring apparatus with capacitive coupling relationship and having the same angular spac rings. The series capacitors which provide the ing, interconnections on one member to’ series coupling to the rotor winding are indicated at connect adj acent'conductors to form a closed» loop 12 and 14. The rotor can be made with very in which adjacent conductors carry currents op small inertia, and the capacitive lead-in coupling posite in direction and terminal connections on permits connection to the rotor without involving the same member at diametrically opposite points counter-torque. With the high frequencies ap on said closed loop, interconnections on the other plied to this apparatus, preferably of the order of member to series connect adjacent conductors to 100 kilocycles per second, this coupling is su?i form n separate groups positioned 360/11 degrees cient for practical use. from one another, in which adjacent conductors A further advantage accrues from eliminating carry currents opposite in direction, and con slip-rings by the orientation of parts shown in ducting members to connect the said n groups in Fig. 9. Although angular accuracy of the device parallel. described in my prior application is independ 4. In apparatus for electrical measurement, ent of small axial displacements between the ro 30 the combination comprising two relatively-mov tor and stator windings, some change in voltage able windings each made up of a large number of gradient or sensitivity will occur. However, with series-connected conductors extending'back and the rotor coupling capacities on the side of the forth in a plane, said conductors of the two wind rotor opposite from the measuring windings, an ings being in inductive relationship and at the axial shift of the rotor relative to the stator that 35 same angular spacing, each conductor of one increases. the gradient of the stator output volt winding being series interconnected with its next age tends to reduce the voltage’ applied through adjacent conductor to form one closed loop in the capacitive lead-in rings and vice versa; thus, which adjacent conductors‘ carry currents op a compensating action is obtained. posite in direction with terminal connections at It is to be understood that the form of my diametrically opposite points’ on said loop; each invention, herewith shown and described, is to be conductor of the other winding being series inter- ' taken as a preferred embodiment, and that vari connected with its next adjacent conductor to ous changes in the shape, size and arrangement form three separate groups in which adjacent of parts may be resorted to, without departing conductors carry currents opposite in direction from the spirit of my invention, or the scope of spaced at 120 degrees from one another. the claims below; For example, the elements of 5. In apparatus for electrical measurement, my invention might be used in a device in which the combination comprising two relatively-mov the, relatively rotatable members have concentric able windings each made up of a large number cylindrical opposing surfaces. of series-connected conductors extending back Having thus described my invention,I claim: and forth in a plane, said conductors of the two 1. Apparatus for electrical measurement com windings being in inductive relationship and at prising two relatively-movable non~ferrous mem the same. angular spacing, each conductor of one bers, a large number of conductors extending winding being series interconnected with its next back and forth across each of said members and adjacent conductor to form one closed. loop in arranged in accurately spaced array, the con- . which adjacent conductors carry currents op ‘ductors of the two members being in inductive posite in direction with terminal connections relationship and having the same angular spacing, at diametrically opposite points on said. loop; each conductor on one of said members being each conductor of the other winding being series series-connected with the next adjacent con interconnected with its next adjacent conductor ductor to form a closed loop in which adjacent 60 to form n ‘separate groups ‘spaced at 360/11 de conductors carry currents opposite in direction, grees from one another in which adjacent con said loop having terminal connections at di ductors carry currents opposite in direction and ametrically opposite points on the loop, and the the groups being. connected in parallel with one conductors on the other member being similarly another. , series-connected in a loop opened at at least one 65 6. In apparatus for electrical measurement, a point to permit terminal connections. winding comprising a large number of series 2. Apparatus for electrical measurement com connected conductors extending back and-.iorth prising two relatively movable non-ferrous mem and arranged to form a closed conducting loop, bers, a large number of conductors extending in which adjacent conductors carry currents op back and forth across each of said members and posite in direction and terminal connections at arranged in accurately spaced array, the con diametrically opposite points on said closed loop ductors of the two members being in inductive to form two parallel conducting paths, the con relationship and having the same angular spac ductors connected to the terminals. forming joint ing, interconnections on one member series-con conductors. for both parts of the winding..- : :necting adjacent conductors vto form a closed 7. In apparatus for electrical measurement, a 2,671,892 9 10 winding comprising a large number of conductors spacing, the windings being inductively coupled disposed radially at regular angular spacing in a whereby an induced voltage appears at the termi single plane, conducting interconnections be nals of one winding when the other is excited with tween conductors to form with said conductors alternating current, and shunt means for elimi a continuous and closed loop, and terminal con nating voltages due to capacitive coupling be nections at diametrically opposite points on said tween the windings by connections within one closed loop, to form two parallel conducting winding. paths, the conductors connected to the terminals 10. Apparatus for electrical measurement com forming joint conductors for both parts of the prising two relatively movable members, each winding, a second winding, in inductive relation 10 having a winding consisting of a large number of ship with, and movable relative to, the ?rst, com conductors extending back and forth in accu prising an equal number of conductors disposed rately spaced array, the conductors being series radially at the same angular spacing in a single connected so that adjacent conductors carry cur parallel plane, and conducting interconnections rents of opposite direction, the conductors of the between conductors of the second winding to 15 two windings having the same spacing, the wind form with the conductors n separate groups ings being inductively coupled whereby an in spaced 360/11, degrees apart, each group compris duced voltage appears at the terminals of one ing conductors series-connected so that adja winding when the other is excited with alter cent conductors carry currents opposite in ra nating current, one of said windings being di dial direction. 20 vided into a number of equal sections connected 8. In apparatus for electrical measurement, a in parallel, said sections having induced therein winding comprising a large number of conduc capacitively coupled voltages in space phase, the tors disposed radially at regular angular spac currents due to said voltages being localized with ing in a single plane, conducting interconnec ing the sections. tions between conductors to form with said con 25 ROBERT S. CHILDS. ductors a continuous and closed loop, and termi nal connections at diametrically opposite points References Cited in the ?le of this patent on said closed loop, any two conductors spaced by an even number of spaces, carrying currents UNITED STATES PATENTS in the same radial direction, and any two con 30 Number Name Date ductors spaced by an odd number of spaces carry 264,643 Edison ______Sept. 19, 1882 ing currents in opposite radial directions, a sec 679,008 Steinmetz ______July 23, 1901 ond winding, in inductive relationship with, and 714,786 Day ______Dec. 2, 1902 movable relative to, the ?rst, comprising an equal 787,302 Latour ______Apr. 11, 1905 number of conductors disposed radially at the 35 1,268,712 Harle ______June 4, 1918 same angular spacing in a single parallel plane, 1,630,757 Perkins ______May 31, 1927 and conducting interconnections between con 1,763,966 Tanner ______July 1, 1930 ductors of the second winding to form with the 1,937,375 Woodward ______Nov. 28, 1933 conductors 11. separate groups spaced 360/n'de 2,038,059 Reichel ______Apr. 21, 1936 grees apart, each group comprising conductors 40 2,400,619 Woodward ______May 21, 1946 series-connected so that adjacent conductors 2,401,344 Espely ______June 4, 1946 carry currents opposite in radial direction. 2,402,603 Clark ______June 25, 1946 9. Apparatus for electrical measurement com 2,409,876 Martin ______Oct. 22, 1946 prising two relatively movable members, each 2,426,226 Labin ______Aug. 26, 1947 having a winding consisting of a large number 45 of conductors extending back and forth in ac FOREIGN PATENTS curately spaced array, the conductors being Number Country Date series-connected so that adjacent conductors 5,562 Great Britain ______Feb. 11, 1891 carry currents of opposite direction, the con of 1890 ductors of the two windings having the same 50