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 5Q ' Fig. 8 52 /——M~\ o 9 r-_*——\ l8, . ls‘ g) SIGNALERROR 5| I v I l8 l8‘ Q) SIGNAL,{:RROR 5| \ INVENTOR Ill" I ATTORNEYS March 9, 1954 . R. s. cHlLos 2,671,892 MEASURING DEVICE Filed Nov. 6, 1948 3 Sheets-Sheet 3 Fig. l0 Fig. ll A IL ll ' ‘ \72 Fig. l2v [74 A IL ll 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.