Oct. 1, 1946. J. J. MÓYNIHAN 2,408,472 ELECTRICAL SYSTEM FOR THE CONTROL OF MECHANICAL DEVICES Filed Aug. 28, 1941 Ú@ 2 Sheets-Sheet l Oct. 1, 1946.r J. J. MoYNlHAN 2,408,472 ELECTRICAL SYSTEM FOR THE CONTROL OF MECHANICAL DEVICES Filed Aug. 28, 1941 2 Sheets-Sheet 2 mdr. anon ’ll YHIXOHddY Uli/0.17.? JNO R w Jo «n n. M. wnm m m/.aM ¿i w, 2,408,472 Patented Oct. 1, 1946 `UNITED STATES PATENT. OFFICE 2,408,472 ELECTRICAL SYSTEM FOR THE CONTROL OF DIECHANICAL DEVICES John J. Moynihan, Boston, Mass., assigner of one-half to Joseph B. Brennan, Cleveland, Ohio Application August 28, 1941, Serial No. 408,658 19 Claims. (Cl. Z50-_2) 1 2 This invention relates as indicated to control devices, and more especially to a novel method and apparatus for the comparison of com mensurable properties of a plurality of entities, reached, until the correction has been achieved within the desired limits. Other objects of my invention will appear as such comparison being accomplished by cyclically varying a parameter of one such, and determin ing whether the parameter of the other lies with in the range of such variation. Not only does this invention relate to such method and appa ratus for making the comparison, but it also pro vides an improved method and apparatus for so changing one of the variables, when upon com the description proceeds. To the accomplishment of the foregoing and related ends, said invention then comprises the features hereinafter fully described and partic ularly pointed out in the claims, the following description and the annexed drawings setting -iorth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principle oi the invention may be employed. parison in the mode stated it is found to lie out In said annexed drawings: side of said range of variation, as to bring it Fig. 1 is a schematic diagram of a remote con within such range. trol system for airplanes, such system compris This invention has a wide ñeld of application ing one embodiment of my invention which will may be observed from the foregoing statement be illustrative of its wider field of application as oi its general principles. The invention can best will be presently described. be illustrated by having reference to one specific embodiment thereof. This specific embodiment, 20 Fig. 2 is a schematic diagram similar to Fig, 1 showing the incorporation in the system of Fig. l which is only one of its many possible adaptations of a protective expedient preventing accidental will be hereinafter more particularly pointed or intentional outside interference; out, pertains to the remote control of devices, Fig. 3 is a representation of the type of emission such as aircraft and the like, from a control sta tion, such a problem giving rise, not only to the 25 from the local airplane oscillator. Referring now more specifically to the draw need for a comparison of a commensurable prop ings, and more especially to Fig. 1, the system erty or condition in both the control station and illustrated in this figure may be divided into the airplane, but also giving rise to the need for two parts, A and B. maintaining within limits at all times such prop That part designated by the ordinal A is a erties or conditions even though one such property 30 schematic representation of the equipment at the or condition, as for example at the control sta ground or control station from which the air tion, may be varied with the intent that a corre plane, carrying system B, is to be controlled. I sponding variation will occur at the controlled shall first indicate the nature of the apparatus station. which is thus shown assembled in Fig. 1, and It is a principal object of my invention to pro then follow such description with a detailed ex vide a method and apparatus of the character planation of its mode of operation by having above described which is characterized by a high reference to several selected sets of conditions. sensitivity without, however, requiring the em The control station A includes a control stick ployment of extremely sensitive instruments and 40 I which, for convenience, may correspond in its the like. By having reference to the specific embodi ment of the invention, which is to be presently described, it is a further object of this invention to provide method and apparatus of the char construction and general mode of manipulation to the control stick l' in the airplane. It will be kept in mind that it is one of the objects of this invention to provide control mechanism actor described which will make possible not only 45 whereby the operator on the ground may manipu late the stick I at his station, and have the stick the desired sensitive control, but also the accom I’ in the airplane follow precisely, and within plishment of such control by apparatus which is deiinite and known limits, the same manipula not only readily available, but which, not being of tions. extreme sensitivity, may be used practically under Mechanically connected with the stick I is a conditions where rough usage is encountered. 50 conventional audio frequency oscillator 2. It is a îurther object of this invention to pro Throughout the following description, reference vide a method and apparatus of the character will Vbe had to certain standard radio equipment. described in which the corrective force, applied I have not attempted to illustrate the precise when the system determines the necessity there of, is such that it does not decrease in intensity 55 construction of such pieces of equipment. At a later point in this specification, and after all of as the neutral point ofthe control'system is 2,408,472 4 such conventional pieces of radio equipment have been identified by reference characters, I shall set forth a table in which such pieces of equipment are cross-referenced to a standard treatise on The effect of the periodic connection of con denser Il into the resonant circuit of the fixed frequency section of the beat-frequency oscil lator is to modify the audio frequency output of radio engineering in which the precise construc tion and particular mode of operation of such individual pieces of equipment is fully explained. the oscillator which now emits a signal for any The oscillator 2 is mechanically connected to on the size of the condenser I‘I. The output of the fixed frequency section I6 is fed through a the stick I by means of a link 3. When the stick I, or, as is now common practice, the control wheel of the airplane, is moved in one direction or other, the link 3 will cause an adjustment of the oscillator 2 so that the latter is caused to send out audio waves of different frequencies. Generally, and in particular where apparatus such as an airplane is to be controlled, it will be necessary that the output of the oscillator be a continuous frequency spectrum. from one end of its range to the other. The audio frequency out put of the oscillator 2 is applied to a modulator stage 4 which modulates the output of a conven tional radio frequency transmitter 5. The out put of the transmitter is fed to a suitable an tenna 6. Referring to the apparatus, B, the modulated one position of the control stick I', periodically varying in frequency by an amount dependent radio frequency filter 2U, in the conventional manner, to a mixer stage 2|, and the low fre quency heterodyne output of the mixer stage is fed to an audio frequency amplifier 22 and a band pass filter 23, all in a manner well-known in the art. The output of the airplane beat-frequency os cillator I3 is now fed by suitable connections C to the heterodyne mixer stage I2 simultaneously with the incoming signal as heretofore described. The heterodyne signal from mixer stage I2 is fed through a low pass filter 24 and a high pass filter 25 to an audio frequency amplifier 25, The numerical value of the cut-off point of the low pass filter 24 will be approximately equal to the difference between the lowest frequency emitted radio wave emitted by the ground station an~ by the ground station oscillator 2, and the highest tenna 6 is received at the airplane antenna 'I, frequency emitted by the airplane oscillator I3. and fed, if necessary, through a current limiting The numerical value of the cut-off point of the network 8, the purpose of which is to prevent a high pass filter 25 will be approximately equal to destructive amount of energy being applied to 30 the difference between the signals emitted by the the radio receiver 9 when the aircraft is in the ground station oscillator 2, and the airplane os immediate vicinity of the ground station. cillator I3, for corresponding positions of the After passing through the current limiting net control stick. The audio frequency amplifier 26 work 8, a signal is impressed on a standard radio must be capable of amplifying linearly any sig receiver 9 and, after detection, is amplified by a nals which pass through both the low pass filter standard audio frequency amplifier I3. The 24 and the high pass filter 25. After the hetero characteristics of the current limiting network dyne signal has been ampliñed, it is applied to a 8, radio receiver 9, and the audio frequency am rectiñer stage 21, the output of which is fed to plifier IIJ, should be such that audio signals over the coil 28 of a single-pole, dcuble-throw relay the entire spectrum used by the ground control 40 29. stations should be amplified substantially linear Referring again to the motor driven switch I9, ly; that is, the system should comprise a high f1 there is provided an auxiliary switch 3D connect~ delity receiver system over the range of audio fre ed in synchronism with switch I9, and so ar quencies used. ranged that when switch I9 is closed, switch 30 After amplification, the signal is passed to the » is closed onto contact 3|, and when switch I9 is band pass filter II, the cut-off points of which open, switch 39 is closed on contact 32. Relay correspond to the extreme frequencies emitted 29 is arranged so that when the coil 23 is ener by the ground station oscillator with the control gized, the switch element is closed on contacts stick in either extreme position. The incoming 33, and when the coil 28 is not energized, the audio frequency signal is now passed to the het- 50 switch element is closed on contacts 34. erodyne mixer stage I2 where a locally generated A local source of current such as a storage bat signal is also applied. tery 35 is provided. The terminal I 90 of the stor The locally generated signal is produced as age battery 35 is connected by means of the lead follows: Connected to the control stick I’ of the airplane is a beat-frequency oscillator I3 by means of a link I4. One section of the beat-fre quency oscillator I3 consists of an oscillator I5, the output frequency of which is capable of being varied. The other section of the beat frequency IIII to the arm of switch 39. The other terminal |02 of the battery 35 is connected by means of a lead IIJ3 to one pair of contacts 33 and 34 of the relay 29. A lead IM connects the other contact 34 to the contact 32 of the switch 30. A lead IU5 connects the other Contact 33 to the contact 3I of the switch 39. It will be observed that put frequency of which is substantially constant the coil of relay 3l is connected in series in line except when periodically varied as described |04 and the coil of relay 36 is connected in series later. in the line Illl‘». It will be observed that when The resonant circuit of oscillator I6 has con contacts 33 of relay 29 are closed, and contact nected therein a small condenser I'I in series with 3! of switch 30 is closed, solenoid valve coil 35 is a single pole switch I9, and so arranged that it energized; and when contacts 34 of relay 29 are may be connected in shunt with the main tun closed, and contact 32 of switch 33 is closed, ing condenser IB for one position of the switch solenoid valve coil 3l is energized; and under I9. all other combinations of switch and relay posin The switch I9 is actuated by a motor driven 70 tions, solenoids 33 and 3l are deenergized. mechanism. A motor I9a is coupled thereto by Solenoid coils 36 and 3l operate to open sole means of an eccentric arm or pitman C whereby noid valves 38 and 39 respectively when ener rotation of the motor continuously in one direc gized, and a double acting compressed air cylin tion will cause the switch to be alternately der 40 is provided, so arranged that when valve opened and closed. 75 38 is opened by the energizing of coil 35, the oscillator I3 consists of an oscillator I6 the out 2,408,472 5 oiîset from the ground station oscillator as shown in the table. It will also be apparent that it is undesirable for the highest frequency emitted by the ground station oscillator to exceed twice the lowest frequency, if the control frequencies airplane stick I’ is pushed forward, and when valve 39 is operated by the energizing of coil 3l, the airplane stick l’ is pushed backward by means of the cable assembly generally indicated at 4I. A line for the admission of compressed air to are selected according to the above considera the cylinder 40 is shown at 42, and will be con tions. The exact values of the frequency allo nected to some suitable source of compressed air cations are not critical and do not constitute a supply, not shown. part of the invention. The system shown in Fig. 1 is complete as to The band pass filter II will, in all cases` have the control of any one function of the airplane. 10 cut-ods at the extreme limits of the frequency In this case, I have chosen to illustrate the con values of the oscillator 2. The purpose of the oí trol of the stick fore-and-aft. For complete con setting of the local oscillator by a constant num trol of the airplane, provision must be made for ber of cycles from the corresponding positions control of the motion of the stick right-and-left, of the ground station oscillators is to provide and control of the rudder bar position, in ac a directive characteristic to the control system; cordance with the established principles of con that is, the function of the onset is to cause a trolling the motion of> an airplane in pitch, roll discrimination between the signal produced (be and yaw. Auxiliary controls are also necessary. yond the heterodyne mixer stage I2) by the mo For example, the engine speed should preferably be controlled. Similarly, such auxiliaries as 20 tion of the ground control stick to a position, say, flaps, brakes, landing gear manipulating devices, and >armament may require to be controlled. In such cases, additional control channels will be re quired, said channels being tied in after the audio frequency amplifier I0. Two such additional control channels |06 and 200 cycles higher than neutral, land a similar motion to a position, say, 20D cycles lower than neutral. The control frequency channels should prefer ably be separated from one another by dead spots 25 in the audio spectrum so that the band pass filters II do not have to discriminate sharply |01 are shown in Fig. l in skeleton form for the between received signals. control of the motion of the stick left-and-right For the study of the operation of the device, and for the rudder bar control. For purposes certain instantaneous positions of some of the of establishing an illustrative design, the follow 30 elements thereof will be assumed. First, we will ing numerical values have been assigned to the assume that the ground control stick is neutral, frequency of the oscillators and filters in the sys the airplane control stick is neutral, switch I9 tem shown in Fig. 1. is open, and switch 30 is on contact 32. This 35 will be, for purposes of convenience, referred to Stick Stick Stick Local osc. forward neutral back offset by Ground BFO ________ __ 1, 250 1,600 1, 950 } Airplane BF O _______ _ _ 1, 750 2, l0() 2, 450 hereinafter as “Condition A.” Condition A 500~ Stick Stick Stick Local csc. left neutral right offset by-- Ground BFO ________ __ 2, 500 3, 200 3, 900 } Airplane BF O ____ _ _'. _ _ 3, 500 4, 200 4, 900 Rudder Rudder Rudder left neutral right 1 OOON L ’ Local osc. offset by The first thing we will show is that under these circumstances, the mechanism in the air plane does not tend to throw the airplane con trol off the neutral position. The same thing ap plies Veven if the two sticks are not in neutral, provided their positions relative to their respec tive neutral positions are the same. That is, if the ground control stick is in a dive position, and the airplane control stick is in the corre» sponding dive position, the mechanism of the airplane will not tend to change the position of the airplane stick. Ground BFO ________ __ 5, 000 6, 500 8, 00() } Airplane BFO _______ __ 6, 000 7, 500 9.000 1 OOON ’ For purposes of description, we will assume that the ground control oscillator delivers a note of 1600 cycles when the ground stick is in neutral, and that the output of the airplane beat fre ous table headed “Local osc. Oiïset by” are in n quency oscillator is 2100 cycles for the neutral each instance the difference between the fre position of the airplane control stick. This 2‘100 quency of the signal emitted by the ground beat cycles is the basic note delivered by the airplane frequency oscillator and the airplane beat fre beat frequency oscillator. The mechanism in the airplane is arranged to change this basic note quency oscillator. Certain practical considerations are responsi 60 periodically by any of several means, the most convenient of which is probably to shunt a small ble for the choice of the frequencies. Since it condenser Il, similar to an ordinary radio trim is desired that the corrective control frequency mer condenser, across the tuning condenser I8 and any possible positions of ground stick and of either section of the beat frequency oscillator, airplane stick be unique, it is necessary that the but preferably across the fixed frequency tuning range of difference frequency produced by the condenser. The reason for this preference is that heterodyne mixer stage I2 should not include under these circumstances, the addition of a fixed either the radio control signal frequency or the condenser to the tuning section will detune the locally generated signal frequency. Therefore, fixed frequency section a constant amount, and the low pass filter, after the mixer stage I2, will have a cut-off for each channel equal to the .dii 70 will change the note delivered by the entire beat frequency oscillator by a constant amount, say ference between the lowest frequency generated 100 cycles, for all positions of the beat frequency by the oscillator 2, and the highest frequency oscillator. generated by the oscillator I3. , This is not necessarily true if thevariaible fre The high pass ñlter 25 will have a cut-off ,equal to the frequency by which the ‘local oscillator is 75 quency section of the beat frequency oscillator The figures given in the last column of the previ 2,408,472 7 is shunted by this condenser, that is, under these latter circumstances, the note might vary, say 100 cycles, at the low frequency end of the set 8 a value not exceeding about 21/2% of the value of signals which are not attenuated. A high-pass ñlter is, of course, one which ting, and 150 cycles at the high frequency end. Driven in synchronism with the switch I9 which passes frequencies above its cut-off point, and which attenuates frequencies below the cut-off cuts the modifying condenser in and out, is an other single-pole, double-throw switch 30 whose function will appear later. In operation, the 1600 cycle note emitted by the ground station oscillator is used to modulate a standard radio transmitter. The standard radio transmitter car rier is received at the aircraft antenna, and if the airplane and the transmitter are to be oper ated close to one another, a current-limiting net work is inserted after the antenna, as the high power field in the vicinity of the transmitter might cause damage in the first stages of the radio receiver. point. The design of the filter must be such that it will attenuate the frequencies below the cut off point so that they will be incapable of oper The signal having passed through the current ating any device beyond the filter, and I have assigned an arbitrary figure of 21?¿% because it is easy to make the relays to be operated compara tively insensitive. The 500 cycle signal having been passed by the high-pass filter 25 is amplified by a conventional audio frequency amplifier 26 and rectified or de tected by a power rectifier 21. The function of the power rectifier is to enable the use of the energy contained in the control signal for oper ating a direct current relay 29 by energizing its limiting network 8 is impressed on a standard radio receiver, probably a good high-grade ama coil 28. If the energy in the 500 cycle signal were not rectified, the signal being alternating teur communication receiver 9, and, having been detected in the radio receiver, is amplified, pref erably with automatic volume control operated from the audio signal. This arrangement is dif lay 29. An arbitrary figure of 10 milliamperes ferent from the automatic volume control ordi narily employed in radio receivers in that the conventional control operates on the carrier in order to maintain a constant radio signal in the radio receiver prior to detection. After amplification, the received signal is split up by means of band pass filters into the control signals required for controlling the flight path of the airplane along its three axes, and for such other controls as may be desired. We are con current would not be able to operate the D. C. re of D. C. current has been assigned as the value of the current output of the rectifier. The only reason for assigning this value is that it is with in the range of ordinary operating currents, and it allows us to discriminate in this discussion between conditions where no current is flowing, 30 and where current sufiicient to operate the re lay is flowing. Since under the assumed conditions current is flowing in the relay coil 2‘8, the contacts 33 of relay 29 are closed. Now referring back to the 35 synchronously driven switch 3D, associated with the switch I 9, we find that during the interval sidering only the control of the stick fore-and aft. The band pass ñlter II for this control under consideration, the modifying condenser I1 channel will correspond to the oscillator control is not cut into the circuit, switch I9 being on an limits of the ground stick oscillator. That is, only open contact, and switch 30 being on contact 32. those frequencies will be effective in this channel if) Since contacts 33 of relay 29 are in series with beyond the band pass filter II which lie within contact 3I of switch 30, there is no current flow the limits of the oscillator at the ground station ing at the instant under consideration in any corresponding to the stick fore-and-aft control, part of the network comprising contacts 33 and in this case assumed to be 1250 to 1950 cycles. 34 of relay 29, contacts 3I and 32 of switch 30, Since the control frequency' 1600 cycles lies and the two solenoid operating valve coils 35 and within the pass iband of the iilter II, it is passed 31 which comprise the network. There is thus unattenuated to the heterodyne mixed stage I2. at the instant under consideration no tendency The output of the airplane beat-frequency oscil for the compressed air cylinder to be energized lator, in this case 2100 cycles, is also passed to to change the position of the control stick which the heterodyne mixer stage. When the two sig is assumed to be in neutral position. nals are mixed there will appear in the output The foregoing description has been given for four signals, viz.1 the two signals which are ap the instantaneous condition which maintains plied to the input, a signal frequency equal to when the ground control stick and the airplane their sum, and another signal frequency equal control stick are both in neutral position, and to their difference. This is the case in all heter- ; the switch I 9 is open. With both ground and odyne systems. The applied signals and the sig airplane control sticks still in the neutral posi nal frequency equal to the sum of the applied tion and with motor I9a operating continuously, signals are of no value in the control of the air during the next instant (approximately one-half plane. They are, therefore, rejected by means second later for example) the motor I9a will of a low-pass filter 24 which rejects all frequen 60 have closed the switch I9, and simultaneously cies above 1200 cycles for this particular channel. the switch 30 will be on contact 3|. This will The reason for the choice of 1200 cycles is that be referred to for convenience hereinafter as it represents the difference-frequency corre “Condition B.” sponding to a heterodyne between a 1250 cycle Condition B note and a 2450 cycle note, the former represent The ground signal is the same as that out ing the low limit of the ground oscillator, and the lined in Condition A, that is, the stick is in latter the high limit of the airplane oscillator. neutral and the ground oscillator is emitting a Since the difference-frequency under the as 1600 cycle note. The synchronously driven sumed conditions of both sticks being in the same switches I9 and 30 in the airplane, however, are relative position is 500 cycles, for this channel, 70 in the position shown in Fig. 1, that is, the modi this is passed through the low-pass filter unat fying condenser I1 has been cut into the beat tenuated. This 500 cycle signal is applied to a frequency oscillator tuning circuit. The beat fre high-pass filter, which passes all frequencies quency oscillator, therefore, is delivering a signal greater than 500 cycles unattenuated, and which equal to 2000 cycles instead of the 2100 cycles as attenuates all frequencies less than 500 cycles to 75 sumed previously. 9 2,408,472v The exact value of the difference between that delivered by the airplane frequency oscillator the ground stick has been moved forward sulfi cient to Vcause a change of A200 cycles in the fre quency emitted by the ground station oscillator under Condition A and under Condition B is not important, the only function of this difference in frequency being to provide a tolerance for the operation of the device. 2, and with the airplane stick in the neutral posi tion, and at the instant in the cycle when switch I9 is open and switch 30 is on contact 32 (the I have assumed a 100 cycle difference in fre quency when the switch I9 is cut in. This fre quency may be different for the three control same as in Condition A above). Condition C In the case assumed, the signal emitted by the ground station is 1400 cycles, and we are assum ing that the stick in the- airplane is in neutral. We will consider the situation under the same channels without affecting the operation of the device. The condenser Il may change in value, and the frequency emitted by the control oscil lator in the airplane may change in value, with out affecting the operation of the device. The only important thing is that the note delivered by the airplane oscillator should change, when the switch I9 is closed, sufficiently to enable ordinary cheap, conventional, rugged filters to time circumstances as the first discussion, that is, cycles instead of 500 cycles as before. are the same as outlined previously. with switch I9 open and switch 30 on contact 32. Under these circumstances, a signal of 2100 cycles is fed into the mixer together with a 1400 cycle signal from the receiver. The output of the mixer discriminate with respect to the two notes includes the difference-frequency, 700 cycles, emitted. 20 which is passed through the low-pass filter un Switch 30 synchronously driven with switch attenuated, and through the high-pass filter un I9 is now in the 31 position. The incoming sig attenuated since the signal lies vbetween the cut nal as before is received at the aircraft antenna, off points of each filter. The signal is amplified passed through 9 and I0 as a 1600 cycle signal, and rectified as before. A D. C. current of l0 and following the same reasoning as before, is 25 milliamperes assumed is applied to the relay 29, passed unattenuated through the band pass and the switch bar of relay 29 is on contacts 33. filter II. However, when the 1600 cycle signal However, no current will flow in the circuit, and from the receiver and the 2000 cycle signal from accordingly relay 36 will not be energized be the airplane beat frequency oscillator are ap cause switch 30 is on contact 32. As before, no plied to the mixer stage, the difference-frequency 30 operation of the solenoids 36 and 3l occurs, since in the output of the mixer is observed to be 400 the switch conditions of relay 29 and switch 30 The 400 cycle note is passed through the low-pass filter unattenuated. Condition D is assumed to be approximately The high-pass filter, however, one-half second after Condition C just described, attenuates the 400 cycle signal so that although 35 with the switch I9 in the closed position, switch it has some value, the audio frequency amplifier 30 on contact 3! and the ground stick in the posi is unable to build up the energy in the signal to tion described in Condition C and the airplane a point where, after rectification, sufficient D. C. stick still in the neutral position. can be derived to operate the relay 29. We will assume that a half milliampere of D. C. 40 is applied to the closing coil of relay 29, and that the sensitivity of the relay has been adjusted by Condition D In the succeeding half second, the switches I9 conventional methods so that a half milliarnpere and 3i) being in the same respective positions as assumed in our second discussion, the oscillator will not lift the armature, and, therefore, the output of the airplane oscillator being 2000 cycles switch bar of relay 29 will not be lifted from the 45 and the incoming signal 1400 cycles, a difference contacts 3d. For any current to flow in the cir frequency of 600 cycles is produced in the mixer cuit of the battery, it would be necessary, for this stage. The 600 cycle signal is' passed unattenu instant of operation, for the switch bar 30 to be ated through the lowépass and high-pass filter, closed on contact 32. However, since the switch amplified and rectified as before, and the current bar 30 is closed on contact 3 I, no current will flow 50 of 10V milliamperes D. C. derived from the rectifier in the relay coil 31. used to operate the relay 29.- Since the relay 29 The above description has shown, therefore, is energized, contacts 33 are closed, and since that under the conditions outlined with both contact 3| of switch 30 is also closed, the circuit sticks neutral, no tendency exists for a change is closed, energizing te solenoid operating valve in the position of the stick in the airplane. By a similar process of reasoning in which only the values of the frequencies emitted by the ground and airplane oscillators are changed, it can be shown that at any time when the two are in coin cident positions no tendency exists to change the position of the stick in the airplane. At this- point we might summarize the reasons for the difference by 500 cycles of the signals re spectively emitted by the ground station oscillator and the airplane station oscillator. The differ ence 015500 cycles is to provide a discriminating indication between a change of, for example, 100 cycles due to moving the ground stick forward, and the change of 100 cycles due to moving the ground stick backward; this will become evident in the description of what happens when the ground stick is moved forward by an amount suf ficient to cause a change’ of 200 cycles in the fre quency of the ground station oscillator. For Condition C, we willl assume the case where coil 30, allowing compressed air to be applied to one end of the air cylinder which, through me chanical linkages, moves the stick forward. AsY soon as the position of the stick in the air plane has reached a point where, when the switch 60 30 is on contact 3I a signal of less than 500 cycles is delivered by the mixer stage, the relay 29 will no longer lift the armature in closed circuit 33, and under these circumstances, the energizing circuit will no longer be closed, andthe stick will 65 tend to remain in the position it has adopted, which is the same relative toits neutral position as the position of the ground stick is to its neu tral position, until either the ground stick is moved or the airplane stick moves or drifts out 70 of such relative position. Condition E For Condition E, the ground stick is assumed to be moved backward to a pointv where a signal 75 of 1800 cycles is emitted from the ground sta 2,408,472 1l 12 tion, the airplane stick is assumed to be in neu the previous portion of this description relating' td tral, switch I9 is open, and switch 30 is on con tact 32. Under these circumstances, the output of the mixer stage is a 200 cycle note, which is at tenuated on passage through the high-pass filter, such apparatus. A single beat frequency oscillator is provided aboard the airplane, the output of which is ap plied to the heterodyne mixer stages I2 and I2a as described in connection with Figure l, and the operation of the entire duplicate set of con trol channels up to and includingr relays 29 and 29a through their connections c’ and c" is as previously described. In the wiring of the relays, and which on rectification delivers a D. C. cur rent insufficient to operate the relay 29. Con tacts 34 of relay 29 are, therefore, closed, and a circuit comprising contacts 34, contact 32 of switch solenoids, and switch 3B, however, the contacts SB, and solenoid 31, is closed. In each of the foregoing conditions, it has been assumed that the airplane control stick is in the neutral position and the ground control stick of the two relays 29 and 29a are connected in se ries so that simultaneous operation of the two either in a coincident or different position. It will . be obvious that the conditions of operation which have been described will be the same for all rela tive positions of the plane and ground stick, whether one or both are in neutral or off-neutral positions. relays is required to permit the energizing of the solenoids 36 and 3l. Thus, it will be seen that the legitimate control signal, being transmitted by both the control transmitters 5I and 5Ia, will operate the relays 29 and 29a simultaneously, and thus effect the control of solenoids 3B and 31. 20 A single interfering signal, however, even if on the frequency of one of the control transmitter carriers, will operate only one of the relays 29 or 29a, and unless the operation happens to co incide with the desired control, no effect will the plane stick into correspondence with the posi be had on solencids 3S and 31. It will, there tion of the ground stick. fore, lbe impossible for accidental or willful inter In the description of conditions B and D, it ference to produce miscontrol of the airplane has been stated that the airplane stick is in a unless such interference comprises two transmit neutral position, when in the case of condition ters operating on the same frequency as those of D, particularly, a corrective force may have been applied. The description of the conditions has 30 the control station, and modulated by identical signals different from those required for control. not taken into account the possible movement of It is obvious too, as an evidence of this inven the airplane stick in response tc- the corrective tion, that a plurality of transmitters and control force applied during the previous instant. It is channels greater than two may be employed, the obvious that the device will operate properly re When the two sticks are not in corresponding position, there is a corrective tendency on the part of the mechanism in the airplane to bring relay contacts being connected in series to pre vent operation unless three or more carriers are during the period when switch I9 is open, or dur received simultaneously. ing the period when switch I9 is closed, and it As previously indicated, the equipment illus will also be obvious, from a study of the descrip trated diagrammatically and referred to in gen tion, that corrective changes of the position of the airplane stick occur during opposite relative 4.0 eral terms in the foregoing description is all more or less standard radio equipment, and for a full halves of the cycle of switch I9, depending on understanding of the precise construction and whether the airplane stick is forward or aft, rela mode of operation of these various items refer tive to the position occupied by the ground stick. gardless of whether the ground signal changes Figure 2 is a disclosure of a modification of this system, the purpose of which is the prevention of accidental or willful interference with the op eration of the device when, as may happen, a modulated carrier is received on the same fre quency as the ground control transmitter, but emanating from some other transmitter in the vicinity. In particular, when the control is used for military purposes, the enemy may, by listening in on the lband in which the control transmitter is operating, determine the carrier frequency, and adjust his own transmitter to emit a carrier of the same frequency on which he may modulate a random frequency, different from that em ence may be had to the work on “Radio Engineer' ing” by Frederick Emmons Terman, second edi tion, McGraw-Hill Book Company, Inc., New York (1937). In the following table are given, first, a list of the reference characters employed in the foregoing description and in the drawings in the identification of particular equipment units; second, the figure number, if any, of the Terman book illustrating such unit in greater de tail; and third, the page of such book where such figure number occurs, and in certain instances, a description of the apparatus. Reference character Termen Fig. No. Tcrmau page No. 2 200 371 ployed by the original station, thus eliminating the original station as the controller. The meth od by which this interference may be circum vented comprises simultaneously modulating two radio transmitters having different carrier fre quencies with identical signals from oscillator 2. In Figure 2, the modulators and transmitters are shown as 50, 50a, 5I, and 5Ia. The control 65 equipment aboard the airplane has the same nu merals assigned to it as in Figure l for one chan nel, and the same numerals with the switch, a, for the other channel. For convenience in de scription, items 8, 9, I0 and II have been grouped into a block, and similarly, items 24, 25, 25 and 21 have been grouped into a block. For an illustration and for identification of the apparatus used between the antenna and relays (Various other types of audio-frequency oscilator are also suitable) 4 5 l 227 l 411 t (see rig. 5) 9 (Conventional amateur communications receiver) ~ n 101A 42 189 88 t0 92 25o, 251, 252 45o to 452 15 (Similar to Ref. 2) 16 (Similar to Ref. l5, with modifications shown on 2 drawinff) 2l 22 23 ~ a 24 114 250, 251, 252 101A 450 to 452 189 42 es to 92 24 (Similar to Fig. 268i) 25 (No illustration iu this text) 483 26 27 18€) 433 101A` 239 The design of the various components adapted from Terman’s text is intended to be merely illus 29 and 29a, reference may be had to Figure 1 and 75 trative as an aid in tracing the operation of the 2,408,472' ' 13 system, and it is not intended that the citation of these ñgures from this text are restrictive of the methods which may be employed to obtain my results; nor is it intended as a statement that the methods shown by Terman may be employed exactly as shown. Rather, it may be necessary to modify them to adapt them to use in this inven tion, but such modifications are apparent to those skilled in the art. In Figure 3, I have illustrated the character istics of the signal emitted by the airplane oscil lator. From the following description of Figure 3, the wider application of my invention than its restricted application on remote control airplanes, will become apparent to those familiar with the art. In Figure 3, the horizontal axisv of the graph is a time axis, and the graduations along the 14 tion of the stick in the airplane. For example, a wheel may be used on the ground to affect motion of a stick in the airplane. It is obvious that the cyclically varied signal may originate at either the control station or the station to be controlled provided there are means for synchronizing the signal varying mechanism with the relay 29. As readily seen, analogous conditions may be controlled similarly where there is in general a variable factor' at one point which is to be corre lated to and controlled by suitably varied con trolling means at another point, with transmission of signals between. Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the fea tures stated in any of the following claims or the equivalent of such be employed. horizontal axis are marked according as the switch I, therefore, particularly point out and dis I9 is opened or closed at the time under con 20 sideration. When the switch I9 is open, the fre quency emitted by oscillator I3 which is dotted along the vertical ordinate of the graph will be higher than the frequency emitted when switch tinctly claim as my invention: ~ l. In apparatus for the comparison. of two com mensurable properties, within a predetermined range of variation of a parameter thereof, the Thus, for any one setting of the 25 combination of means for selecting from one a stand-ard of value and means for simultaneously control stick I', a periodically varying frequency comparing together values indicated by the two will be emitted by oscillator I3. The variation in respectively while cyclically varying the value of the emitted frequency will be of the order of the parameter of one of said properties and means 100 cycles when the switch is closed. This 100 cycle Variation in frequency is constant approxi 30 for determining whether the parameter of the other property lies within such range. mately at all positions of the control stick I". The 2. In apparatus for the comparison of two’ com basic frequency, that is, the frequency of the mensurable properties, within a predetermined oscillator I3 when condenser Iï'I is out of the range of variation of a parameter thereof, they circuit, will Vary according to the position of the ~ combination of means for selecting from one a control stick. The modified frequency, that is, standard of value and means for simultaneously the frequency emitted by the oscillator I3 when comparing together values indicated by the two condenser I1 is connected into the circuit, will respectively while cyclically varying the valueI of likewise vary as the position of the control stick the parameter of one of said properties over said I is varied; and the modified frequency will, for predetermined range and means for determining all positions of the control stick, be approxi whether the parameter of the other property lies mately 100 cycles lower than the basic fre within such range. quency. 3. In apparatus for the comparison of two I have indicated on the drawings, and elsewhere I9 is closed. in the specification, that the approximate period commensurable properties, the combination of of the motor driven switch is one second. It will means for selecting from one a standard> of value be appreciated that the exact value of this period is a matter for the judgment; of the designer. 'I‘he period should not be so short that switching dini culties will be encountered due to the mechan ical inertia or difliculty inl keeping switch 30 in . synchronism with switch I9; and ' the period should not be so long that the aircraft flies for an appreciable length of time in an undesired direc tion without a corrective tendency on the-part of the mechanism. In an analysis of the control system, it will be seen that we have here two variables, the position and means for simultaneously comparing together values indicated by a common characteristic of the two respectively while cyclically varying a characteristic of one of said' properties over a predetermined range, and means for determining whether a characteristic of the other property bears a particular relationship to such range. 4. In apparatus for the correlation of two com mensurable properties, within a predetermined range of variation of a parameter thereof, the combination of means for selecting from one a standard of value and means for simultaneously comparing together values indicated by the two of the ground stick, and the position of the air respectively while cyclically varying, from a nor plane stick; that each of these variables has a common parameter, for example the frequency 60 mal, the value of the parameter of one of said properties and means for adjusting the normal of the audio frequency signals emitted by the value of the parameter of one of said properties ground and aircraft oscillators respectively; that so that the said range of cyclic variation of the this parameter is a function of the respective magnitude of the parameter of one of said proper positions of the sticks; that these two functions are being constantly compared as to magnitude 65 ties includes the magnitude of the parameter of the other of said properties. by the cyclic variation of the parameter of one 5. In apparatus forthe correlation of two com of them, that is, the airplane control stick oscil mensurable properties, within a predetermined lator; and that the comparison of the two func range of variation of a parameter thereof, the tions of the parameter is utilized to obtain a corrective force for changing the position of the 70 combination of means for selecting from one a standard of value and means for simultaneously airplane control stick. It is obvious that the comparing together values indicated by the two principle here enunciated is of wide application. respectively while cyclically varying, from anor Retaining the airplane control system as a basis, mal, thevalue of the parameter of one of said it is obvious that the method of varying the ground stationV oscillator need not be similar to the Ino 75 properties, and means for adjusting the' said nor 15 2,408,472 mal and accordingly the said range of cyclic variation so that'l such range includes the param eter of the other of said properties. 6. In apparatus for the correlation of two com mensurable properties, within a predetermined range of variation of a parameter thereof, the combination of means for selecting from one a standard of value and means for simultaneously 16 signal bears’ a predetermined relationship to the signal of the other device. 12. In apparatus for the correlation of two remote mechanical devices actuating electrical devices respectively adapted to emit electrical signals having variable frequencies indicative of different states of physical adjustment of each said mechanical device, the combination of means comparing together values indicated by the two for selecting frequencies from one as control respectively while cyclically varying the value 10 and means for mixing together frequencies simul of the parameter of one of said properties and taneously from the two systems while cyclically means for adjusting the value of the parameter varying the frequency of the signal of one of said of the other of said properties so as to bring the devices over a predetermined range, and means same within the said range of cyclic variation. for adjusting the physical condition of said last '7. In apparatus for the correlation of two com 15 named device so that the range of cyclic varia mensurable properties, within a predetermined tion of the frequency of its signal bears a prede range of variation of a parameter thereof, the termined relationship to the signal of the other combination of means for selecting from one a device. standard of value and means for simultaneously 13. In apparatus for the correlation of two comparing together values indicated by the two 20 remote mechanical devices actuating electrical respectively while cyclically varying the value devices respectively adapted to emit electrical of the parameter of one of said properties and signals having variable frequencies indicative of means for adjusting the value of the parameter different states of physical adjustment of each thus cyclically varied so as to bring the range said mechanical device, the combination of means of such variation into predetermined relationship 25 for selecting frequencies from one as control and to the parameter of other of said properties. means for mixing together frequencies simultane 8. In apparatus for the comparison of two ously from the two systems while cyclically vary systems respectively adapted to emit electrical sig ing the frequency of the signal of one of said nals having commensurable characteristics, the devices over a predetermined range and means combination of means for selecting from one a 30 for adjusting the physical condition of said last standard of value and means for simultaneously named device so that the range of cyclic variation comparing together signals of the two systems of the frequency of its signal includes the signal While cyclically varying the said characteristic of the other device. of one of said signals and means for determining 14. In apparatus for correlating the physical the relationship of the said characteristic of the 35 condition .of an apparatus at a remote station other signal with respect to such range of cyclic with the physical condition of apparatus at a variation. control station, with each such station adapted 9. In apparatus for the correlation of two to emit a variable frequency electrical signal systems respectively adapted to emit electrical indicative of the physical condition sought to be signals having commensurable characteristics, 40 correlated, the combination of means for select the combination of means for selecting from ing frequencies from one as control and means for one a standard of value and means for simul mixing together frequencies simultaneously from taneously comparing together signals of the two systems while cyclically varying the said char the two systems while cyclically varying the fre acteristic of one of said signals and means for stations, and means for adjusting the physical adjusting the said characteristic of the said cycllcally varied signal so as to bear a prede termined relationship to the said characteristic quency of the signal emitted from one of said condition of the apparatus at the last named sta tion so that the range of cyclic variation of the frequency of its emitted signal bears a prede of the other system. termined relationship to the frequency of the 10. In apparatus for the correlation of two 50 signal emitted from the other of said stations. systems respectively adapted to emit commensur l5. The process of correlating the physical con able electrical signals indicative of a state of dition of apparatus at a remote station with appa physical adjustment of the system, the combina ratus at a control station which comprises the tion of means for selecting from one a standard steps of simultaneously emitting from the con of value and means for simultaneously comparing 55 trol station a plurality of significantly different together signals of the two systems While cyclically electrical signals each indicative of the physical varying the signal of one of such systems over a condition of the apparatus at the control station, predetermined range, and means for adjusting receiving such signals at the remote station and the physical condition of said system so that the generating at least one signal at the remote sta range of cyclic variation of its signal bears a pre 60 tion variable in accordance with variations in the determined relationship tothe signal of the other physical condition of the apparatus at the remote system. station, mixing the respective signals together, 11. In apparatus for the correlation of two and adjusting the physical condition of the appa remote mechanical devices actuating electrical ratus at the remote station so that its signal bears devices respectively adapted to emit alternating 65 a respective predetermined relation to each of current signals having commensurable character said control station signals. istics indicative of a state of physical adjustment 1G. In a remote control device, the combina of each said mechanical device, the combination tion with a controller at a control station and a of means for selecting from one a standard of controlled element at a remote station, of means value and means for simultaneously comparing 70 functionally responsive to different positions of together signals of the two systems while cyclically varying the commensurable characteristic of the signal of one of said devices and means for adjust ing the physical condition of said last named the controller for ent signals, means tionally responsive controlled element, emitting significantly differ at the remote station, func to different positions of the for emitting cyclically varied device so that the range of cyclic variation of its 75 signiñcantly different signals, and mixing them 2,408,472 17 together with the signals received from the con trol station and means at said remote station functionally responsive to move said controlled element when a parameter of the signal emitted at the control station lies outside of the range of cyclic variation of the parameter of the signal emitted at the remote station. 17. In apparatus for correlating the position 18 signals and effective to adjustably maintain the said element at said remote station at such a posi tion that a parameter of the signal from the con trol station lies within the cyclic variation of the parameter of said signal generated at said remote station. 19. In a device for correlating the position of a movable arm at a remote station with the posi tion of a movable arm at a control station, the manually controlled lever at a ground station, 10 combination of a radio transmitter including a the combination of means at the ground station beat frequency oscillator at the control station and movements o-f a lever in an airplane and a for emitting radio signals signiñcantly different for each position of the lever at said ground sta tion, means actuated by the lever in the air plane for emitting cyclically varying signals sig nificantly different for each setting of such lever, means in the airplane for receiving the radio signals emitted by the ground station and mix ing them together with those generated on the airplane, and means functionally responsive to both sets of signals for moving the lever on the airplane so that a parameter of the ground sta tion signal will lie within the range of cyclic variation of the parameter of the signal emitted on the airplane. 18. In a remote control device for correlating movable elements respectively located at a control station and a remote station, the combination of means at the control station functionally re sponsive to different positions of the said element at the control station for emitting radio signals adapted, for various adjustments of said beat frequency oscillator, to send out radio signals in cluding significantly different frequencies, means connecting said arm at said control station with the said beat frequency oscillator so that move ment of said arm to different positions results in significantly different settings respectively of said beat frequency oscillator, means at said re mote station including an adjustable beat fre quency oscillator adapted upon various adjust ments of the latter to generate cyclically varying radio signals of significantly different reference frequencies respectively, means connecting said arm at said remote station with said last named beat frequency oscillator whereby movement of said last named arm to different positions will adjust to different settings said last named beat frequency oscillator, means at the remote station frequencies, for combining the said signal from said control station with the said signal generated at said remote station, and for setting up a signal which means at the remote station functionally respon sive to different positions of said element at said is a function of said combined signals, a relay energized by said latter signal, and power means including signiñcantly different remote station for emitting significantly different 35 controlled by said relay for moving the arm at cyclically varied radiosignals, and means at said remote station combining the aforesaid sets of said remote station. JOHN J. MOYNIHAN.