IL46339A - System for controlling heading indication of an aircraft - Google Patents

Description

SYSTEM FOR CONTROLLING HEADING INDICATION OF AN AIRCRAFT 52 -EE-0-139 This invention relates to a system, which is includible in apparatus aboard an aircraft for controlling and/or indicating the attitude and heading of the aircraft. Such control and/or indication is dependent on correct operation of so-called gyroscopic or inertial- ·' devices. Under normal conditions, these devices are slaved to still other devices, such as a magnetic compass, which may be considered primary or fixed reference devices; the slaved inertial devices are also commonly and legitimately considered "reference devices", but because of their slave -state, they are in a certain sense secondary references. During turns of the aircraft, and particularly at high rates of turn, continued slaving of some inertial devices tends to introduce dynamic errors in their operation. The present invention 5 is directed to a system for cutting off the slaving under such circumstances. More particularly, the system of this invention . . . is designed to cut-off slaving of the heading gyro (gyroscope) to the magnetic compass, and slaving of the attitude gyro to a gravity sensing d e i c e under conditions of -high rate of turn.

To facilitate the understanding of the purpose, and of the operation,of the devices mentioned in the following portions of the specification introduction, the reader may wish to refer and 3 preliminarily to Figs. l/ of the accompanying drawings (as regards both previously proposed approaches and the system of the present invention) with the following correspondence: :! 52-EE-O-l 39 Gerald L.- Sullivan |i It has been well-known in the past to provide some arrangero^i-fc^for I; interrupting the slaving^of the directional, gyro and the erection of the ji attitude sensing- ertical gyro during turns which exceed a certain pre- : determined. ate to prevent introduction of these unwanted errors. Basically, the arrangement for avoiding errors due to excessive turn rates, involves the :' · Gf i; interruption or cutoff of slaving and/roll erection by interrupting the 1; circuit' betwee the sensing and control signal' generating means and the force j: or torque applying apparatus associated with the gyroscope. At one time in .j: the past, this was achieved by manual means in that the slaving and/o roll ; ir erection channels are rendered ineffective simply by operating a turn control ■ i i ■ ■ ■ .. . . - ■ '■ ' ' ■,. ·' ■ - · ' !; knob or switch which disconnects the input to the torque motors associated i; with, the gyroscopes. The pilot upon initiating 'a turn simply actuated th jj turn control knob thereby, initiating cutoff of directional gyro slaving and ![ vertical gyro roll erection. Typical of such an approach is the arrangement j! shown and described in U.S. Patent No. 2,998,727 issued September 5, 1961 jj which shows such ah arrangement in Figure.1 thereof and describes this manual approach in Columns 5, lines 70-75 and Column.6, lines 1-5.

Obviously, such a manual cutoff arrangement is primitive in. the extreme and suffers various shortcomings Which make more sophisticated automatic. ji cutoff approaches imperative. One prior art approach for automatically '{·:■ cutting off slaving and roll erection employs a separate'Vate gyroscope" J: which is gimbaled for onfl¾fgree of rotational freedom at right angles to its i,; I' spin axis and is constrained by springs towards a normal position.' Turning !· · separate rate gyro's ji of the vehicle on which the /rate gyro is suspended causes tne/marn girnbal jj . to tilt against the force of the springs, and if the rate of turn exceeds I; a predetermined magnitude, electrical contacts associated with j; , , . .directional and vertical gyros' Γ are caused to open, deenergizing the/ torque motors a¾d halting i; of corrective forces, thereby automatically cutting off slaving or roll j; separate ' . erection as the turn rate exceeds a predetermined rate. Alternatively, /rate '!! gyros could be used to actuate relays or other switching devices to interrupt or cut off slaving or roll erection rather than utilizing directly actuated l: separate : mechanical electrical contacts. However, in either event, /rate gyros, even in their simplest form, add considerably to the cost and weight of a heading and attitude reference system since obviously, another inertia! device has been added to the overall system. Hence, a need was felt for an automatic separate cutoff system during turns which avoided the use of/ rate gyros because of • the weight, complexity, and cost that added to the. system. 5': Slavin -to -the -ma netic com 52 -EE-0-139 The reader may wish to make further preliminary reference to Figure 3, for' a appreciation of the following part of the specification introduction, with the following parts correspondence:.

Device .. In Figure 3 Directional Gyro 1 .

Magnetic Compass 2 • Torque Motor for Directional Gyro 56, Vertical Gyro 5 Gravity Sensor 6 . Torque Motor for Vertical Gyro 73 . System heading output shaft .. ' \ . 34 Servomotor . which is drive source for 48 the System Heading Output Shaft ■ ' · '. ; ' .. Reduction gears from 49 Servomotor to the system heading output Shaft One improved automatic system, which avoided the use of the separate "rate gyroscope" is described in U. S. Patent No. 2, 866, 934, issued September 30, 1958 in the name of Harold S. Whitehead, entitled, "Directional System Sensitive to Rates of Turn. " In this Whitehead patent specification, 52 -EE -0-139 ' .- . ' . .'■ · ' ' ' . · ■' " ■' . . '' ■ "· " 'ί' ■ ■ a "rate" signal generator, suc as a tachometer generator, is utilized to generate a signal which is proportional to the rate of turn. The tachometer generator is driven, by a servomotor which also drives, indirectly and via reduction gearing, a shaft which drives the craft's heading indicator. The latter shaft is hereinafter also referred to as the "system heading output shaft" or more simply "heading shaft. " In the Whitehead system, the heading shaft rotates at a speed proportional to the instantaneous rate of turn of the craft upon . which the directional gyroscope is mounted. Hence the tachometer . generator produces an output signal proportional to the aircraft's rate of turn. This rate of turn signal is then utilized to actuate the cutoff device whenever the output from the "rate" generator exceeds a predetermined amplitude indicating that the rate of turn of the craft has exceeded a given rate. However, in many aircraft systems it is required that slaving and roll erection be cut off when the system headin output shaft rotates at a relatively low rate -value, for example a value which lies in the range of from three degrees o o ( 3 ) per minute to six degrees (6 ) per minute. For such a rate of o 3 per minute, the heading shaft turns through one complete revolution every 120 minutes or once every 2 hours. Consequently, in order -to produce a usuable output signal from the tachometer, it is necessary to drive the tachometer generator directly, by a high speed servo motor so as to obtain usuful signal from the tachometer generator, and to use . a gear reducer with a high gear reduction ratio to gear the output heading shaft down to the actual 52 -EE -0-139 ■■ ; ,' - of the type described in the above identified Whitehead patent are useful and effective in many situations where turn rate cutoff is . required, and. is able to perform this function effectively without the use of rate gyros and the like, the necessity for a high gear ratio between the motor driving the rate generator and the output shaft can in some circumstances, introduce difficulties^, ... With the advent of very high performance aircraft, high rotational speed turns are not uncommon and place stringent demands on the heading- shaft-driving- servo system, as it must typically follow turn rates of the aircraft as high as 300°/ second , or 18000 /minute, equivalent to 50 heading-shaft revolutions per minute. Thus, although slaving will have been cut off at the o . 3 /minute turn -rate, the heading shaft should, when required^ o" turn at the 300 / second rate, or 50 RPM rate, "unslaved".

The maximum speed of commercially available servo motors of the size and weight applicable to heading attitude and . .· . reference systems is roughly 7500 RPM. To reduce the. 7500 RPM rate of the servomotor to the 50 RPM rate of the heading -output - shaft, a gear reduction of 150:1 is necessary, and the 150:1 reduction ratio is just about the maximum ratio tolerable for the 7500 RPM servomotors in a system which must track turn . o rates as high as 300 / second. If a still higher gear ratio were to be used, the system would be unable to track at the maximum o ; 52-EE-0- 39 Gerald L. Sullivan The two differentiated signals are then vectorially summed to produce a single output rate signal' which' is proportional to the rate at which ¾%v¥n¾frt is rotating and hence, the rate of turn of the vehicle. This rate signal is ·: then compared to a reference signal to produce a control or error, signal . whenever the turn rate exceeds a predetermined level. The error signal actuates circuitry which cuts off or interrupts slaving of the directional gyro to the magnetic compass and interrupts, roll erection of the vertical gyro ·-- - The invention will be better, 'understood by" ·.,"· reference to the following description taken in connection with the ; accompanying drawings in which: Figure 1 is a block diagram representation of the instant invention . showing the relationship, between the magnetic compass,, the slaved directional gyro, the vertical gyro, and the cutoff signal generating channels; Figure 2 is a block diagram of the cutoff signal generating channel showing the .· sequence of signal processing to produce a rate signal proportional to the rate of turn of the aircraft from heading information in the form of a shaft angle Θ.

Figure 3 illustrates partially in schematic and partially in perspecti e form a preferred embodiment. of the invention utilizing a. turn rate cutoff system. ' '.··".1 Figure 4 is a schematic illustration of an alternative embodiment of the system for producing the signals which are representative of the sine and cosine of the shaft angle for processi g in the cutoff signal generator channel .

Figure 5 is yet another alternative embodiment of a system fo producing a signal proportional to the sine and cosine of the shaft angle.

Figure 6 is yet another embodiment of a portion of a system for producing signals representative of the shaft angle.

The heading and attitude system as shown in block form diagram in Figure 1 includes a stable directional element 1 in the form of a directional gyroscope, a magnetic azimuth detector 2 such as a magnetic compass which is coupled to the directional gyroscope by a slaving channel 3 and a slaving channel interrupt circuit 4. As will be pointed out in detail later, slaving channel 3 consists of a servo system in which the output of the stable directional device is continuously compared to the output from the magnetic compass to drive the directional gyro into correspondence with the alignment of the compass. Also forming part of the system is a stable that two signals, which are proportional to the sine and cosine of the output :of a gyro shaft angle are produced in any suitable manner so that: E-] = Ej,, Sin Θ, and (1) · 52-EE-0-139 Gerald L. Sullivan · -■ input signal multiplied by the sine and cosine, respectively, of the angle between the stator and rotor at any instant. Alternatively, the converter may consist of a Scott-Tee transformer connection which receives a three-phase synchro input signal and converts, that signal to a two-phase signal having the desired sine and cosine relationship. Yet another embodiment of the converter may be a simple potentiometer in which the wiper is driven by the shaft to produce an output which is a sine and cosine function of the shaft angle. : . . " '"- .'.·;;·.

The two signals E-| and E2 from converter 20 are differentiated in the different ating networks 21 and 22 to produce the rate signals and R2> The output of the differentiating networks are respectively. ·.·„■'"-"' dO + E M, Cos Θ ^ and It is to be noted that if the input signals to the differentiating networks are: from a resolver, polyphase synchro or a Scott-Tee network, the input signals are amplitude modulated signals which have to be demodulated to remove the carrier before differentiation so that the input to each different ating network is a varying unidirectional voltage which varies as a function of the sine and cosine of the shaft angle. If, on the other hand, the converter is simply a sine-cosine potentiometer, which is exten%ecf¾y DC/ driven directly by the shaft, no demodulation is necessary before application of the signal to the differentiating network.

Rate signals R.j and R2 are applied to a vector summer 23 to produce an output signal which is proportional to the total rate signal R. Vector summer 23 produces a signal which is equal to the square root of the sum of the square of two input signals so that the output from vector summer 23 as shown by Equation (8) is proportional to the rate of shaft rotation d0 R = E.

Devices which perform vector summation are well-known and there ' ■. are a variety available commercially. Some are electromechanical systems using resolver chains and others are logic circuits which compute the square root of the sum of two or three squares:^ An example of the electromechanical resolver chain type is shown and described in Figures 172 and 176 (pp 61-63) of the Publication Technical Information for the Engineer, No. 1 , - Motors, Motor Generators, Synchros, Resolvers, Electronics, Servos, 10th Edition, published by Kearfott Products Division-General Precision S stems Inc. Little Falls, New Jerse • ■■ · 52-EE-0-139 Gerald L. Sullivan . · proportional to the difference in angular orientation between the rotor 33 axi magnetic compass. This signal from winding 33 is an amplitude modulated, single side band, carrier suppressed signal whic is demodulated in .J demodulator 53. Demodulator 53, which may, for .example, be a synchronous, demodulator, has a carrier reinsertion reference signal applied thereto to produce a demodulated output which is a DC signal that varies with the sine o the angular position difference. The demodulated signal is amplified in an amplifier 31 and is applied through slaving interrupt relay network 4 and leads 55 to a torque motor 56 positioned on the inner gimbal of the. directional gyro.. Torque motor 56 applies a torque to the inner gimbal which causes precessional movement of the gyro in the proper direction to return gyroscope to its predetermined orientation so that the output of. roto winding 39 of differential 37 coincides with the orientation of the magnetic compass. This, in turn, drives shaft 34 through the servoloop to : rotor windings to null positions. ' - r V -.

Slaving interrupt network 4 includes a pai of normally "closed contacts 57, a pair of normally open contacts 58 connected to torque motor leads 55, and a pair of moveable relay armatures 59 and 60 connected to the output of amplifier 54.. The- relay inding 61 is usually energizedjits magnetic effect on .... -· ..eonnedtion -amplifier armatures 59 and 60 is .indicated by a syrr olfc caTWli:irr62 , The wmding 61 ads t> usually/ 54 to torque motor 56 during usual; slaving operation and to disconnect the amplifier from the torque motor whenever the turn rate (as determined by comparator 10 and the turn rate cutoff signal channel 9)exceeds a predetermined value. When relay 61 is energized, armatures 59 and 60 are positioned against normally open contacts 58 thereby connecting the output of amplifier 54 to leads 55 and to torque motor 56 associated with the inner gimbal of the 'Viusually ) di ectional ,gyro. When relay winding 61 is./deenergized as is the case durin turn rate cutoff, armatures 59 and 60 are positioned against normally closed lat contacts 57 and slaving is interrupted. Deenergization of winding 61 is descri Relay winding 61 is selectively controlled by comparator 10 through par of relay switching element 8 so that relay winding 61 is energized during normal operation. Whenever the turn rate exceeds a predetermined value, the . output from comparator 10, actuates relay switching circuit 8 to deenergize relay winding 61. When relay winding 61 is deenergized, armatures 59 and 60 against the normally closed contacts 57 thereby disconnect ng leads 55 from the output of amplifier 54 thus interrupting slaving of the directional gyro to the magnetic compass. system heading The servo dri en/output shaft 34, in addition to positioning rotors 33 and 45 provides the system heading output and may be used directly to actuate a pointer of a direction indicating mechanism. In the usual case, however, 52-EE-0-139 Gerald L. Sullivan In summary, under usual- slaving conditions, if the position of the . directional gyro las represented by shaft 34 coincides with the magnetic compass orientation, no signal is induced in winding 33. Any movement of the directional gyro away from alignment with the compass drives shaft 34· through the servoloop so that rotor winding 33 is no longer al gned with the electrical angle of the signal from the compass and a signal is nduced in rotor winding 33. The signal from rotor winding 33 is demodulated, amplified and applied via leads 55 to the torque motor 56 mounted on the. inner gimbal of the directional gyro to apply a torque to the inner gimbal which causes the directional gyro rotor to prece'ss until it aligns itself in azimuth with the magnetic compass. This slaving of the directional gyrolto the magnetic compass continues as long as the slaving interrupt network is not actuated. : When the. craft goes into a turn and the turn rate exceeds' the preset .· value, a control signal from turn rate cutoff signal generating channel 9 actuates the slaving interrupt circuit 4 to terminate slaving of the ■''"' directional gyro by interrupting the output control signals from the magnetic compass to torque motor 56 so that the directional gyro operates .··''. in the free gyro mode and is no longer slaved to the compass .

VERTICAL GYRO ERECTION '. ; i Vertical gyro 5 comprises a spinning rotor element 70*mounted in an 171 ■ inner gimbal 71· which has a single degree of freedom about the pitch axis/and an outer gimbal 72 which has a single degree of freedom about the roll axisl72 A torque motor 73 is mounted on the inner gimbal to apply precessing torques which cause the gyro rotor to precess and maintain it aligned properly.

Mounted on the outer gimbal is a gravity sensing element shown generally at 6 which is aligned with the Earth's vertical and is utilized to correct any roll axis misalignment between the spin axis of the gyro and the Earth's. vertical . Misal gnment between the gyro spin axis and the Earth's vertical may be due . to drift of the gyro, random torques that may be exerted about the gimbals, and the Earth's rotation. The pendulous device 6 which lines up with the Earth's vertical will thus produce an electrical signal whenever the spin axis of the gyro departs from the Earth's vertical. Pendulous device 6 is mounted on the roll axis and therefore, senses movement about the roll axis to produce an electrical output signal responsive to any departure of the spin axis from the vertical. This signal is utilized to control torque motor 73 which applies torque in the proper direction to cause the gyro to precess and. eturn the spin axis to the proper vertical alignment.

Pendulous device 6 may be any one of a numbe of available gravit sensing devices. It is, however, preferably of the type which is extremely, the gyro case rotates with the turn of the craft upon which the gyroscope is mounted. The changing angular position of the case with respect to the directi ; gyro shaft,' may be utilized to derive a rate signal proportional to 52-EE-0-139 Gerald L. Sullivan I differential transformer 37 consists of three phase displ aced signals I proportional to. the directional gyro shaft angle Θ plus a fixed difference I angle entered, by rotor shaft 40 and is in effect, proportional to shS^i. angle : Θ. This angular signal is' converted in a Sin/Cos converter 90 to two. signals ! proportional respectively to the sine and cosine of the heading angle. j Converter 90 may be a Scott-Tee transformer connected to output winding 39 of j control differential transformer 37. The Scott-Tee is a well-known device' I · ·' ■' · " . ■·. ■' . ■ i for transforming from two-phase input, to three-phase output, or conversely, i as shown, from a three-phase input to a two phase output. ' When the conversio j is from three-phase to two phase, the two phase output are respectively signa i which are proportional to the sine and cosine of the input signal. Scott-Tee j transformers are well-known devices for achieving such transformations and : j reference is hereby made to the textbook "Alternating Current Machinery" LV Bewley, MacMillan Co. , N.Y. 1949, and particularly pages 89, through 91 thereof which describe the basic characteristics of the so-rcalled Scott-Tee ·: connection. The output from Scott-Tee 90 is a pai of signals proportional' t the sine and cosine of the shaft angle Θ, i.e., E-j = Ej(| Sin wt Cos Θ y ■.

E2 = EM Sin wt Sin Θ where EM = the peak voltage . Θ = shaft angle E Sin wt = the excitation voltage for the gyro position transmitter 35 j The and cosinusoidal signals must be demodulated to remove the I carrier and produce DC signals which vary with the sine and cosine of the j shaft angle. The signals from Scott-Tee 90 are therefore applied to a pair j of demodulators .93 and 94 which are respectively coupled to a common referenc j signal source 95. The demodulators are preferably synchronous demodulators j since the signals from the gyro pickoff and control differential transformers I are typically single side band, suppressed carrier signals so that the carrier must be reinserted for demodulation purposes.

The demodulated signals, which are proportional to the sine and cosine of a shaft angle, are converted to angular rate s gnals R| and Rg by differentiating the signals in the differentiating netv/orks 96 and 97 which are coupled respectively to the outputs of demodulators 93 and 94.

Differentiati g netv/orks are well-known circuits for producing an output whic is the first derivative of an input signal and networks 96 and 97 may take various forms including that of a simple R-C circuit. The differentiated si nals are a lied to low ass filters 98 nd 9 jj . 52-EE-0-139 Gerald L. Sullivan ·. • il , - ' ' . .·. "' ; /·;.■·.¾ ;i with the magnetic compass. Similarly, any deviation of the vertical- gyro/spin \> axis from the Earth's, gravity vector results in an output signal from he ■! gravity sensing, pendulous device 6. This signal is amplified and applied to ' * 5 ;'; torque motor: 73 which precesses the gyro /to align the spin axis vertically. j! Simultaneously, the output from the directional gyro/ is fed through synchro 37 j.to ^ the.turn-rate-cutoff-signal-generatmg-signal-chamei 9. Although channel 9 ·;. . . - ■ , _ , .- ■ . it produces an output simal ; receives no input Signal traceable to the rate of rotation of the heading shaft 34 j; proportional to the rate of shaft rotation, if any, and. hence, to the rate of ii turn of the craft..., ' In the absence of the output from the turn rate cutoff j! output . . j! signal channel 9 or i f sictyis be! ow a predetermined amplitude;, slaving relay : |S winding 61 associated with slaving channel interrupt network 4 is energized and the armatures 59 and 60 associated therewith are positioned against . the normally open contacts 58 thereby applying the output from amplifier 54 to torque motor.56 and slaving the directional gyrol to the magnetic compass.2. !j Similarly, relay winding 80 associated with roll erection interrupt channel 8 ( · ■■ . .. .. . . ' is in the deenergized state so that armature 79 is positioned against the ■.""// • normally closed contact 77 so that lead 13 from the roll erection amplifier 7.4 i connected to the ίΓό ε motor. Consequently, the vertical gyro 5 is precessed j response to the output from the gravity sensing pendulous device 6 mounted on i| the outer gimbal. Whenever the angular rate of rotation of directional gyro [i ' H transmitter output exceeds a predetermined rate such as 3° per minute" : -/ jj indicating that the rtate of turn also exceeds 3° min. a control signal, from j; comparator 10 is generated which energizes relay winding 80. When relay Ii winding 80 is energized relay armature 79 is moved from the normally closed j; contact 77, interrupting the connection between amplifier 74 to torque moto Ii 73 thereby terminating roll erection of the vertical gyro 5in response to outpulj signal from the vertical gravity sensing element 6.

With the energization of relay winding 84, its armature 8 is also^ j; actuated and moves away from the normally closed contact 82 against the jj normally open contact 83. The positive supply voltage from mode control switclj j! 86 is disconnected from slaving relay winding 61, deenergizing the winding 61. j; When relay winding 61 is deenergized armatures 59 and 60 in · slaving- .intemipt-:: network 4" move from the normally open contacts 58 to normally closed contacts ! 57 thereby disconnecting the output of amplifier 54 from torque motor 56 so \ that directional gyro 1 operates in the free gyro mode and is no longer slaved to the output of magnetic compass 2.

The servo network which is controlled by control differential transformer 37 and repeating control transformer 44 continues to operate to align I '· the system heading output shaft 34 so that it follows the directional gyro 1 j -EE-0-139 turn of the aircraft without introducing acceleration induced dynamic .' ■ errors, in magnetic heading and roll. . .

In the system illustrated in Figure 3, the angular position of the shaft of directional gyro 1 is converted into sine and cosine functions for further processing in the turn-rate-cutoff-signal-generating channel 9 by means of the Scott-Tee transformer 90, which converts a three-phase signal from the differential transformer 37 into the desired sine and cosine functions. In Figure 4, a different arrangement, which includes a so -called transolver., is utilized to provide the sine and cosine functions for processing in the turn-rate -cutoff -signal- generating-channel 9. In Figure 4, and also in Figures 5, 6 and 7: (1) parts which are alike to corresponding parts of Figure 3, are given the same reference numeral as in Figure 3, and (2) parts which are not so alike, but which are analogous to parts of Figure 3, are given the same reference numeral as in Figure 3 followed by a suffix, e. g. , "transolver" 37a of Figure 4 is analogous to synchro 37 of Figure 3. Thus, in Figure.4, the signals 36 representative of the directional gyro's shaft angle from the directional gyro' s pickoff 35 of Fig. 3, are impressed on the input terminals of transolver 37a. Transolver 37a includes -EE -0-139 a polyphase stator winding 38a and a pair of quadrature rotor windings 39a 1 and 39a 2. . The two pairs 109, 110 of. output signals from rotor winding pairs 39a 1, 39a 2, are respectively proportional to the sine and cosine of the same sum of two angles as in Fig. 3, namely the sum of the spin-axis angle of gyro 1, and of the angle of shaft 40, on which three rotor windings are mounted. Thus, as in Fig. 3, the angle-sum represents the desired system heading shaft angle. The signal pairs 109, U0 are applied to output terminals 108 which are coupled to the demodulators 93 and 94 of Figure 3.

The signal pairs 109 and 110 are also applied to a resolver 44a which controls the servoloop for positioning the system heading shaft 34. Resolver 44a has the signals 109, 110 (proportional to the sine and cosine) applied respectively to the quadrature stator windings 43a 1 and 43a 2 to produce a voltage in rotor winding 45a which represents the vector sum of the two input signals 109, 110. ' A voltage is induced in rotor winding 45a, only if there is a difference between the angular output of transolver 44a and the system heading shaft 34. This signal is applied through the summing node 46 to the input of servo amplifier 47. Onward of amplifier 47, the system of Figure 4 is the same as the system of Figure 3. : 52 -EE-0-139 " ;'/ '.· It will be apparent from the description of Figure 4 that mechanization illustrated in Figure 4, i. e. , the use of a transolver, performs the same function as the Scott -Tee arrangement utilized in Figure 3. Depending upon the application and environment involved, one or the other may be preferred. It is to be understood, however, that these are merely alternative ways of providing the sine and cosine functions which are then processed electrically to produce a signal proportional to the rate of shaft rotation and hence, to the rate of turn of the aircraft.

In the heading, attitude, reference systems illustrated in Figure 3 or 4, it is the resolver 37 or transolver 37a, which provides the signals proportional to the sine and cosine of the shaft angle Θ, and these sin9 and cos9 signals are used for two purposes, namely firstly for driving the servomotor 48 and hence the system heading output shaft 34, and secondly for providing input signals to the turn-rate-cut-off-signal-generating channel 9. In order that the differentiators 96 and 97 perform their functions in a better way, the channel 9, under certain circumstances may require input signals which vary more rapidly with time, than the signal available from the device 37 or 37a. For example, when the system heading output shaft 34 is turning at approximately the cut-out-turn-rate of 3°/ second, the signal 36 from the position transmitter 35 to the resolver 37 or transolver 37a, will vary relatively slowly, and since the devices 37 and 37a are essentially 52 -EE -0-139 system of Fgiure 5 provides input signals for channel 9, which vary more rapidly wi th time than the signals available from resolver 37.

R eferring to Figure. 5, it is noted that the resolver 37 is still utilized, but only for the purpose of providing drive signals for the servomotor 48 (via repeater 44, summing node 46 and; amplifier 47). For the purpose of providing the sin-θ and cosO input signals to the channel 9, the ultimate signal source is not the azimuth axis of directional gyro 2 (Figure 3) and its position transmitter synchro 35, but rather the system output heading shaft 34 which produces these sin9 and cos0 signals via the reduction gear train 49 ,and a resolver 146. Noteworthy is the fact that the rotor 149 of the resolver 146 is driven at a speed which is intermediate the high speed of the shaft 141 of the servomotor 48 itself, and of the relatively low speed of the system output heading shaft 34. For this purpose, the reduction gear train 49 is arranged in several stages, the initial stage 142 being driven by the shaft 141, and the final stage 144 driving the shaft 34. An intermediate stage 139 drives the intermediate -speed -shaft 143, which drives the rotor 149 ' of resolver 146. By the utilization of an intermediate -speed-shaft 143, it is assured that the sine9 and cose input signals to the channel 9: (1) vary more rapidly in time (2) have enhanced signal level, and yet ( 3) accurately reflect the rate of turn of the craft, and (4) maintain the desired relationship arid proportionality with the 52 -EE -0-139 ·.

To complete the descr iption of Figure 5, the rotor winding 149 of the resolver 146 is mounted on intermediate speed shaft 143 and is excited from a suitable source of alternating voltage ' applied to the rotor input terminals 145. A pair of quadrature stator windings . 147 and 148 have voltages induced therein which are respectively proportional to the product of the rotor voltage and the cosine and ■ sine of the rotor shaft angle.

Shown in Figure 5 are further units, which bear the same . reference numerals as corresponding units of Figure 3, and which are structurally and functionally identical to such respectively . corresponding units; these are not discussed. Furthermore, any · · . units not illustrated in Figure 5, may be assumed to be the same as in Figure 3, functionally and structurally.

In the system illustrated in Figure 3 or 4, the ultimate source of the sin0 and cos9 signals was the position transmitter 35 . associated with the azimuth axis of the directional gyro. This azimuth axis signal was processed in the resolver 37 or transolver with or without use of 37a, then further processed within the channel 9, / the Scott-Tee transformer 90, then in the demodulators 93 and 94, and then i the differentiators 96 and 97. In the system of Figure 5, the source of the sine O and cos Θ signals was the system heading output shaft 34; its shaft-position was processed in another resolver, namely the resolver 146, and then further processed within channel 9 in the ' manner stated in the preceding sentence. Furthermore the azimuth axis signal from the position transmitter 35 was utilized 52 -EE -0 -139 and cosG signals, for purposes of application to channel 9, directly the from the position transmitter 35, or from / simple repeater 37 thereof, without the use of intervening resblvers, transolvers,.

Scott-Tee transformers, provided the position transmitter 35 (or its repeater 37' ) is arranged as a three-phase device, known · in the art as a "three-wire synchro", and that the vector summing device 100 sum vectorially three input signals, rather than merely two (as in Figure 3). Figure 6 utilizes such an arrangement. * In Figure 6, although vector summation is performed on three signals, only two differentiators 96 and 97 are provided the same as in Figure 3, and not three differentiators. In arriving at the present invention, it was found that where a three-wire synchro signal is processed directly (rather than via resolver plus Scott-Tee), only two of the three, and not all three, signals need to be differentiated inasmuch as the 120° phase relationship among the three signals causes any one of the three to be the negative of the sum of the other two. Furthermore, polarity correction is not required because polarity becomes arbitrary in the vector summing squaring process. . Thus, if two of the three signals derived from synchro 35 (or its repeater 37 : ) are differentiated, the two differentiated signals are summed algebraically to produce a third signal, and the three signals, namely the two differentiated signals, and their algebraic summation signals are vector summed, the output signal is proportional to the rate of shaft rotation. This - · ■ 52 -EE-0-139 In Figure 6, the heading signals from the gyro position transmitter- 35 are reproduced in the repeater 37, (which also feeds re solver Tv/o of the repeater-output signals are applied to synchronous demodulators 93 and 94 along with a signal from reference source 95 to demodulate the single side band, carrier suppressed signals to produce varying DC voltages which vary with the sine of the o shaft angle Θ but are displaced in phase by 120 0 The demodulated signals are differentiated in differentiating circuits 98 and 99 which produce output rate signals, which : as in Figure 3 are filtered in a pair of low pass filters 98 and 99 to remove any high frequency components due to yaw oscillations. The filtered signals are coupled over leads 158 and 159 to the vector summing network 100 and also to the inputs to an algebraic summing network 161 to produce a third output signal, which, as pointed out above, is equal to the negative of the sum of the two differentiated signals.

The third signal (from summing network 161) is also applied to vector summing network 100 to produce an output total rate signal which is representative of the rate of change of the shaft angle and hence, of the turn rate of the aircraft carrying the headirg attitude and reference system. The rate signal from vector summing network 100 is applied to the comparator 10 as in Figure 3.

Following are the mathematical considerations which gover the performance of a three -wire synchro ( 35c in Figure 6). The outputs of a three wire synchro are three phase displaced sinusoidal voltages which ma be defined as follows: 52 -EE -0 -139 .

. .. ' · ,, ·; - In the various arrangements shown and described previously, the signals proportional to the sine and cosine of shaft angle have been ·... signals from synchros and the like and have to be demodulated to generate the DC signals representing the sine and cosine of the shaft angle. It will be appreciated by those skilled in the art, that it is also possible to obtain the full benefits of the instant invention by providing the DC signals representing the sine and cosine of the shaft angle directl by utilizing potentiometers driven from the gyro heading shaft or system heading output shaft thereby eliminating the need for demodulation of the signals. It will be apparent from the description of the various arrangements shown and described that a heading and attitude reference system has been described which includes a turn rate cutoff channel for interrupting slaving of the directional gyro to the magnetic compass and roll erection of the vertical gyro whenever the turn rate of the aircraft on which the system is mounted exceeds a predetermined rate. In this fashion, dynamic errors introduced by turns are eliminated, while at the same time, provided is a system which has extremely rapid response and is capable of following extremely rapid rates of turn. The servo system for the output heading shaft is therefore highly sensitive and capable of slewing (tracking or following-up)high angular-rates, as high as 300° per second. Furthermore, for purposes of generating the turn rate cutoff signal, avoided is the useage of electromechanical differentiating devices such as tachometer rate generators, which lack sensitivity at very low angular turn rates, and of rate gyros which are large, 52 -EE-0-139 . · :■ " ■■ ·■ ■ '. , ; ■■ rate of heading shaft rotation and hence, of the rate of turn of the aircraft. Furthermore, the components of the turn rate cutoff signal generating circuit lend themselves well . to useage of solid state, integrated circuit components, thereby offering further advantages in terms of reduced size and compactness as compared to electro mechanical devices such as tachometers, generators and such. Hence, a turn rate cutoff system for heading attitude and reference system has been provided which is small in size, is highly accurate and has good response time to permit cutoff at low rates of turn, while at the same time, permitting rapid and accurate response of the heading output shaft for very, rapid rates of turn.

Claims (8)

52 -EE -0 -139 . . ; ; : ■ What is claimed is:

1. A heading reference system for a movable craft comprising: (a) an inertial directional reference means which has a stable . orientation in space, and is provided with a heading shaft whose angular position is indicative of the azimuth orientation of the craft. . (b) a rotatable output heading shaft, (c) a servoloop responsive to the relative orientation in azimuth between the craft and said directional reference means for driving said outpu heading shaft in response to the turn of the craft. (d) slaving control means, for correcting the orientation of said directional reference means upon departure of said directional reference means from a predetermined orientation in azimuth, .(e) signal processing means for receiving position-type signals from one of said heading shafts, the received signals being proportional to angular heading position information, said processing means including me providing signals proportional to the sine and cosine of the heading angle, differentiating means for differentiating the signals proportional to the sine and cosine of the shaft angle, means for squaring the latter differentiated signals, and vector summing means for extracting the square root of the sum of the squares of the latter squared . differentiated signals, and (f) means responsive to said angular rate of change signal for interrupting the operation of said slaving control means when the craft rate of turn exceeds a predetermined level. 52 -EE-0-139

2. A system as claimed in Claim 1 or 2, wherein the differentiating , means is electronic.

3. A system as claimed in Claim 1 or 2 , including a sine/cosine converter which converts the position type-signal to signals representing the sine and cosine of the heading angle.

4. A system as claimed in Claim 3 wherein the converter, for producing signals proportional to the sine and cosine of the angle is a Scott-T ee transformer,, or a transolver.

5. A system as claimed in Claim 3 or 4 wherein the input position-type signal to the converter is proportional to the azimuth ; angular position of the inertial, directional reference means.

6. A system as claimed in Claim 3 or 4 wherein the sine/ cosine converter includes angular resolving means having a rotor coupled to the heading output shaft, and stator windings producing signals proportional to the sine and cosine of the heading angle.

7. A system as claimed in Claim 6 wherein the rotor of the angular resolving means is driven at a higher speed than the heading shaft but at a speed proportional to the heading shaft speed.

8. A system as claimed in Claim 1 or 2, wherein the position-type signal is supplied as a set of three-phase signals from the inertial directional reference means, and wherein the differentiating means differentiates two signals derived from the set of three-phase signals, and comprising algebraic summing means for summing algebraically the two differentiated signals to produce an algebraic sum signal, the vector summing means vectorially summing, in