US2909929A - Gyro control system - Google Patents

Description

1959 CARL-ERIK GRANQVIST GYRO CONTROL SYSTEM 7 Filed Dec. 7, 1953 s Sheets-Sheet 1 D-l I J5 I I IN VENTOR BY WRQW ATTORNEY 7, 1959 CARL-ERIK GRANQVIST 2,909,929

GYRO CONTROL SYSTEM Fi led Dec. 7, 1953 3 Sheets-Sheet 2 INVENTOR MEL-167?? @P/VNQVASZI ATTORNEY United States Patent" O GYRO CONTROL SYSTEM Carl-Erik Granqvist, Lidingo, Sweden,

Svenska Aktiebolaget Gasaccumulator, Lidingo, near Stockholm, Sweden, a corporation of Sweden Application December 7, 1953, Serial No. 396,692

Claims priority, application Sweden October 26,1953

12 Claims. (Cl. 745.2)

With the advance of aeronautical science increasing demands are being made on the precision and dependability of the instruments used in flight navigation. It was found already long ago that the ordinary devices that depended on gravity for their operation, such as levels, pendulums and the like had a precision and a dependability during aerobatic manoeuvres that were far from sufficient. This led to the introduction of gyroscopes in various combinations. However, with the high velocities and the consequently increased demands made in aerobatic manoeuvres that have been caused by the advent of jet propulsion and the break-through of the sound barrier, even normal gyroscopes have proved insufficient for many purposes. It is well known to be a common occurrence upon the completion of an aerobatic manoeuvre that a gyroscope that was in correct position before the manoeuvre has acquired appreciable errors and must herefore be controlled and possibly readjusted as soon as possible after the manoeuvre has been completed.

The present invention is based on an investigation of the possibilities of an error occurring in the gyroscopes during aerobatic flying conditions. The investigation has proved that a very large proportion of these errors is caused by the fact that a gyroscope, in order to be able to take part in all the performed manoeuvres, should have complete freedom of rotation in each of the gimbal mountings, so that rotation can take place by an arbitrary number of turns.

Normally there are two and in certain cases three gimbal mountings, since the gyroscope should be able to stay in its position with regard to the earth irrespective of the manoeuvres of the aircraft, which may take place in any one of the directions of the three-dimensional space coordinate system. Such a system may be referred to for instance by three mutually perpendicular planes of rotation. The rotor of the gyroscope rotates in one of said planes, and the gyroscope must then be free to move in each of these planes. It is not inherently necessary for the planes to be mutually perpendicular but this leads to the simplest arrangement and is therefore common practice. The consequence of this is that if a manoeuvre is undertaken, in which an angle of 90 is exceeded, it cannot be avoided that two of the shafts, which allow for the gimbal movement, or for the rotation of the gyro rotor, and which are. mutually perpendicular during straight horizontal flight, coincide for a short time interval in the 90-position. Experiments have shown that the danger zone in this respect is at an angular displacement somewhere between the angles of a little more than 80 and a little less than 100. The 90 manoeuvre may for instance comprise a vertical dive-or climb or it may occur in the course of a roll or a looping manoeuvre. In any event there will be a tendency in the vicinity of the 90- position for the above mentionedshafts to rotate simultaneously about both of the shafts that are instantly codirectional. However, only one of these shafts is supposed to take up the change in direction, or the rotation, re-

assignor to other shaft, this shaft will however takeup'onlya of the change in angle that it should have taken up,

whereas the other shaft, which should not take up any angle, will yet do'so and assume a'faulty position. It.

therefore cannot be avoided during a manoeuvre in which a momentary coincidence .of the shafts is possible that both shafts acquire errors so that the gyro gives a wrong.

indication after the completion of the manoeuvre.

The present invention has for an object to avoid this possibility of a wrong indication.

In order to illustrate the various situations that may arise during aerobatic flying and which may give rise to. a

wrong position of the gyroscope of the type referred to above, reference is made to Figs. 1-6 of the accompapy-. ing drawings, in each of which figures there is shown at the left an aircraft in a certain position. and at the right a vertical view in the longitudinal direction of the aircraft'of the corresponding position of the gyro of the Figs. 7 and 8 of the accompanying drawings The aircraft is provided with a gimbal-mounted gyro for ease of manoeuvre, the rotor rotating about one first shaft, and the mounting being such as to allow adjustment of the gyroabout two mutually perpendicular shafts which are also perpendicular to the rotor shaft as is apparent from the right hand part of Fig. 1. This figure illustrates schematically the gyro rotor, which is des'igQ' nated 10. It rotates rapidly about a shaft 11 and is journalled for this purpose in an inner gimbal frame 12, which is in its turn suspended in an outer gimbal frame 13. Both frames are rotatable relative to each other by means of a shaft 14, which is perpendicular to the plane of Fig. 1, and the outer gimbal frame is rotatably journailed by means of a shaft 15.

It is apparent that if the aircraft is made to dive or to climb so that it does no longer follow a horizontal path, the gyroscope will in the conventional way turn, about the shaft 14, and if the aircraft is made to bank, it will turn about the shaft 15. Normally these two degrees of freedom are suflicient to make it possible to obtain the indications from the gyro required for an artificial horizon, but for certain purposes a gyro has been constructed to give at the same time an indication in a third plane of reference, for instance by being combined with an induction compass. In this case it may be necessary to suspend the structure 16, in which the shaft 15 is journalled; by

- means of a shaft 17, which is journalled in the frame 18 spectively. Owing to the simultaneous rotation of the of the aircraft. The shaft 17, usually, is not freely rotatable but controlled by'a servomotor. I

In such a case the shafts 11 and 17 will coincide and it cannot be avoided then'that the shaft 17,'which' should in level flight properly be at a standstill, acquires now, if made free to rotate a certain amount of rotation by taking up some of the rotation of the shaft 11, unless the shaft 17 is locked against rotation in the vicinity of the position where it coincides with the shaft 11.

If now the aircraft should momentarily come into a Patented Oct. 27, 1959.

but the shaft 17 is now driven by the coupling in the opposite direction to that corresponding to Fig. 1.

It is also possible that the loop is not completed but the pilot tries instead to return to level flight by means of a half-roll.- He will then momentarily pass the. position indicated in Fig. 4, in which, although the coupling. between the shafts 11 and 17 has again been released, there. is instead a coupling established between the shafts 14 and 17.

When upon completion of the manoeuvre the aircraft is back in level flight according to Fig. 5, the direction of the aircraft being, opposite to that shown in Fig. 1, the shafts will be essentially in the same positions as in Fig. 1, with the. difference that the. shafts 14 and 15. are now in the opposite direction to that of Fig. 1.

It might also be desirable for the aircraft when having assumed. the vertical climbing position illustrated in Fig. 2 to. return to a course deviating by 90 from the original one instead of to a course opposite to the original one, as illustrated in Fig. 3. The aircraft may then be brought through a vertical quarter roll into the position shown in Fig. 6, whereupon the initated looping manoeuvre is either retracted or completed. In the latter case it should be followed by a horizontal half-roll/ In the position shown in Fig. 6 the state of affairs is insofar complicated as not only the shafts 14 and 17 are co-directional and therefore coupled but also a corresponding co-direction and coupling occurs between the shafts 11 and 15.

In any case in which two shafts have acquired a mutual coupling owing to co-direction of the shafts it is possible that the shaft that should take up all of the rotation actu-. ally acquires only a part thereof whereas the other shaft coupled with it takes up the remaining part of the rotation. Upon completion of the manoeuvre and with. the aircraft returned to horizontal flight in accordance with Fig. 1 there may then be an error with regard to any. one of the three fundamental shafts 14, and 17, which leads to an error in the gyro indication. Due to the socalled autocoupling between the two gimbal shafts, this indication error immediately divides upon the two gimbal shafts. If thus, according to Fig. 2, a coupling effect has occurred between the shaft 11 and the shaft 15, then after the aircraft has been brought into the position shown in Fig. 5, the shaft 14 will also have an error of indication, which may cause the pilot (believing he is in level flight) with a continuously increased speed to fly right down on the ground. Within a moment he will black out due to the acceleration force, and both crew and aircraft are lost without any possibility of saving. It is true that it would be possible to eliminate the emanating error by climbing or diving into a position in which the horizon is well observable and caging the gyro during level flight with reference to the horizon, whereupon the gyro is restarted. However, even if the accident mentioned above should not occur such manoeuvre requires some time and, above all, the aircraft and its crew will be in evident jeopardy during it if the manoeuvre is performed under combat or in non-military traffic when there is appreciable risk of collision with surrounding terrain formations, for instance in alpine flying. Furthermore the pilot has as a rule lost some orientation during the immediately preceding aerobatic manoeuvre and even if the plane is provided with an induction compass under gyro control, even thi s compass will show an error owing to the vertical component of the earths magnetism becoming active when the gyro for controlling the compass has acquired anerror. V

It is therefore of the greatest importance that rotation be prevented in those shafts-which at; the instant of a coupling should notrotate, sothat all of the rotation-is taken up by. the shaft intended for that purpose.

The pronounced inertia effects that are present in rotating shafts make theresulting coupling very small-sothat the shafts are not exactly co-di-rectional or are eodirectional to a very small degree. In the experiments forming the basis of the present invention it was found that no or practically no coupling is present between co-directional shafts when the angular difference between the shafts is larger than 10. On the contrary, all of the coupling occurs at difference angles that are between 0 and an angle smaller than 10, and in most cases even smaller than 5; The amount of the latter angle cannot be generally stated, as it is dependent upon the mass. of the gyro rotor as well as on its velocity and on the friction of the various shafts.

The present invention has for its object to prevent coupling between co-directional shafts by providing for the one shaft, namely that shaft which should not rotate, to be locked against rotation during a predetermined angle on each side of the position corresponding to exact co-direction, this angle being small relative to the quarter turn that-separates the position of maximum coupling from the position of zero coupling (no account is taken here of the special so called gyro coupling mentioned above in other connection, and which makes a deviation of a shaft in one plane lead to an automatic displacement of another shaft in a perpendicular plane). In accordance with a preferred embodiment, a shaft is placed in the same position as the shaft 17, mentioned above, but this shaft now is not used for an induction compass but for registering the movement, which should have been afforded by the locked shaft, although it was not due to the locking of the shaft. According to another preferred embodiment, the angle during which the shaft is locked against rotation is less than 10 on each side of the maximum coupling position. However, in many cases it is enough if the angle during which locking occurs isonly 5 or less on each side of the maximum coupling position.

In the flight position shown in Fig. 1, which corresponds to normal flight conditions, the above-mentioned additional shaft, which is hereinafter referred to as the shaft 17 is locked against rotation. The gyro then has only the degrees of freedom present in normal gyros corresponding to a rotation of the gyro rotor about its own shaft 11, the rotation of the inner gimbal frame 12 about} the shaft 14 in its journalling in the outer gimbal frame 13,- and the rotation of the outer gimbal frame about the shaft 15 relative to the part 16, which is fixed relative to the frame of the aircraft since the shaft 17 is, locked against rotation. The freedom of rotation about the various shafts is however unlimited as contrasted to what has hitherto been usual, each of the said' freely rotatable shafts being able to perform unlimited rotations.

If the gyro is used as a central gyroscope, in which electrical or magnetical generators or transmitters controlby a. remote control action instruments disposed at a distance from the. gyro, it is obvious that the electric conductors must be brought out by means of slip rings and. brushes. or by some equivalent device past each point of rotation. corresponding to a shaft and its attached parts on the one hand and the journalling and its attached parts on the other hand. A suitable arrangement is to mount slip ringsv on the shaft and brushes or contacts on the journallings so. as to make contact with the slip rings.

Fig. 7 in the'drawings illustrates the arrangement for avoiding mutual coupling between co-directional shafts. The gyro rotor is designated 10 as before and its shaft 11. For the rest, the various shafts 14, 15 and 17 have beenshown in the same mutual position asin the schematic Fig. 1 the designations of Fig. 1 being employed in Fig. 7 also.

0n the shaft 14 there is arranged a knob 19 in a position so as to control a resilient contact means 20 to close its'c'onta'ctwhen the aircraft is climbing at an angle of approximately for instance within the aforesaid interval of 10 on each side of the said vertical position. A'sirnilag kuob 2 1 is disposed on the, shaft opposite to the knob 19 to actuate the contact means within degrees on each side of the position corersponding to a vertical dive. Within 10 on each side of the vertical position the contact means 20 therefore close a circuit comprising the positive pole of a battery 22, the contact means 20, a relay winding 23, a second relay winding 24 and the negative pole of the battery 22. The relay winding 23 actuates its armature in such a way that a brake shoe 25 is made to abut the knurled or otherwise uneven rim of a disc 26 which is keyed on to the shaft 15, whereby this shaft is braked against further rotation. This prevents the shaft from rotating when it is in a position in which a coupling might cause a rotation not corresponding to the actually prevailing conditions of the gyro. It is true that this also prevents a possibly desirable rotation of the shaft 15 in accordance with the gyro conditions but the'probability of such rotation occurring is extremely small. An examination of Fig. 2 actually shows that the shaft 15 is vertical in this particular position, which means that if a rotation were to be initiated by movement of the gimbal frames the aircraft would have to be in a vertical roll movement. Such movement during a climb is very unusual, and a spin during a vertical dive is one of the manoeuvres that the pilot must at all cost avoid, as it is very difficult and in some cases impossible to get out of it and it generally results in an uncontrollable dive and crash.

It was mentioned above that the shaft 17 is normally locked against rotation. This is as it should be, since in horizontal flight according to Fig. 1 the shaft 17 represents a degree of freedom which is not required in view of the normal cardanic movements of the gyro rotor. When the shaft 15 is now locked against rotation, the number of degrees of freedom is diminished by one relative to what is required for the gyro rotor and since the shaft 17 will assume in the vertical position of the aircraft the same orientation as that of the shaft 15 in level flight, the shaft 17 should be released for rotation at the same time as the shaft 15 is locked in order for the gyro to have the required minimum number of degrees of freedom. This is done by means of the relay 24 which upon energization disengages the braking member 27 from the disc 28 mounted on the shaft 17.

If now a rotation should occur during the interval in which the shaft 17 is free to rotate, an error may be caused in the instruments indicating the position of the gyro. This is particularly the case if the gyro is arranged in the manner of a so called central gyroscope and provided with synchro transmitters at the various gimbal turning points that are coupled to corresponding synchro receivers in the indicating instruments. To avoid an erroneous indication on these instruments a cam disc 29 is mounted on the shaft 17 and provided with a recess, which corresponds in its position with a roller 30 when the shaft 17 is in its normal position. The roller 30 actuates a contact 31 which is inserted in the circuits transmitting the synchro information to the indicating instruments and cause momentary breaking of these circuits. It is true, that in this case, the instruments will not move at all, but this is preferable to permitting the instruments to move in an incorrect Way, because the pilot will observe that the instruments are standing still when they should be in movement. However, the pilot will usually not understand, that there is something wrong if the instruments move according to a false indication.

A second cam disc 32 is mounted on the shaft 17 and co-operates with a roller 33, which controls a changeover contact 34. In dependence upon the direction in which the shaft 17 has rotated, one or the other of the contacts of the changeover contacts means 34 is closed, whereby current is applied from the positive pole of the current source through contacts 39 and 34 to either the ductors are connected to a reversible motor 37. Al-

though this motor may be of any known type it has been illustrated for simplicity on thedrawing as a motor having an armature winding and two oppositely wound field windings, each of which is connected to one of the conductors 35 and 36. It is apparent from this that the motor 37 will start running as soon as the contact device 34 closes the circuit provided that the shaft 17 has been subjected to a coupling movement and also that the direction in which the motor turns is dependent upon which half of its complete revolution the cam disc 32 is inrelative to the roller 33. The arrangement is so chosen that the motor is coupled to the shaft'17 by means of the driving means 27, which has the form of a friction roller adapted to cooperate with the friction disc 28 and that the motor drives the shaft 17 the shortest way back to its inoperative position by means of the friction roller 27 and the friction disc 28.

The friction roller 27, however, only contacts the friction disc 28 upon release of the relay 24, i.e. after the I shaft 15 has been released for free rotation, and thereafter the shaft 17 is locked against any rotation other than that transmitted by the motor 37. Since the shafts 17 and 15 are mutually perpendicular, any rotation of the shaft 17 will cause through the so called gyro coupling a corresponding rotation of the shaft 15. When the shaft 17 has been brought back into its normal position, the shaft 15 will therefore also be in a position which it should have assumed if it had not been locked but the shaft 17 had instead been locked against rotation all the time. At this instant the current is broken at the contact 34 through the motor 37 and the shaft 17 remains locked against rotation until the next time the aircraft is Within the danger angle of about 10 or less on each side of the vertical direction.

If however the aircraft should temporarily be in the position indicated in Fig. 6, in which not only two shafts coincide but also two additional shafts coincide, it might occur that the shaft 15 will rotate completely erratically without any dependable control from the shaft 17. In order to prevent the shaft 17 from acquiring a rotation under such circumstances there is arranged on the shaft 15 an additional cam disc 38 cooperating with a contact means 39, which is inserted in the circuit of the motor 37 and is adapted to break this circuit when the shaft 15 is in the position corresponding to Fig. 6.

Irrespective of the presence of the latter arrangement, the position shown in Fig. 6 is a hazard to the dependability of the gyro, for it must also be taken into account that in this particular position the gyro has not two diverse degrees of freedom, each being at right angles to the direction of rotation of the gyro rotor, but there is only one such degree of freedom, since the shaft 15 is parallel with the shaft 11 of the gyro rotor at the same time as the shafts 14 and 17 coincide. This may bring about various undesirable errors in the gyro position, which may remain for a long time after the plane has left the unstable position shown in Fig. 6, so that the pilot not only loses the guidance of the gyro he would normally have but may even be deceived by the error in the gyro position.

For this reason, the motor 37 is made to operate under predetermined conditions to cause an intentional angular error in the shaft 17 instead of restoring the correct angle of this shaft as would be in accordance with the normal function of the motor. The corresponding arrangements comprise two cam discs, each mounted on one of the shafts 14 and 15. The arrangements being otherwise similar, only one of them is illustrated in Fig. 8. The two cam discs are designated in this figure as 40 and 41. Each is provided with a cam 42 or 43, respectively, which cooperates with a contact spring assembly 44 or 45, re-

spectively. It is immediately apparent that this arrangement operates within a restricted angular interval, which may for instance be Within 8-l2 on each side of the danger point forjthe shafts, to close. a contact, at the contact device shownin Fig. Scan therefore be said to fonna shaft-controlled change-over contact and the con tact means controlled by the shafts 14 and 15 are accordingly illustrated in Fig. 7- in the form of changeover contacts, the contact corresponding to the shaft 14 being designated 46 and that corresponding to the shaft 15' being designated 47. p

A current source48 indicated schematically in the form of a battery is provided with a mid point tapping, which is connected through a conductor 49 with the mid point of the winding 50 of a polarized relay. Theterminals of the source are connected with the opposed contacts of the change-over device 47 and the corresponding terminals of the relay winding are connected with the opposed contacts of the change-over device 46. The members illustrated schematically in Fig. 7 as contact springs of the change-over device are coupled together. The polarized relay 50 is in turn provided with a change-over contact adapted to cause the relay spring 51 to make contact with one of the contacts whenthe relay is magnetized in one direction and with the other contact when the relay is magnetized in the opposite direction. The two opposed contacts of the relay spring 51 are connected each to a field winding of the motor 37, the relay spring being connected to the positive pole of the voltage source 22.

The last-described arrangement operates as follows. If the aircraft should approach. the position shown in Fig. 6, in; which for the aforesaid reasons there is an instability of the gyro, a tendency will be formed for the shafts 14 and 17 to be rotationally coupled, the shaft 14 striving to take up a part of the rotation that should normally be taken up by the shaft 17 and the shaft 15 being made to take up some rotation from the shaft 11 of the gyro rotor. Depending upon the direction of the rotation of each of the separate shafts, either of the contacts 46 and either of the contacts 47 will then be closed. If for instance the contact 47 is displaced to the left and the contact 46 upwards as shown in the figure, current will flow from the positive half of the battery through the upper half of the relay winding 50 and the relay will be energized to close in one direction. The relay 50 will close in the same direction also if the contact 47 moves to the right and the contact 46 downwards. If however either of the contacts should move in the direction opposed to that cited in the above example and the other contact moves in the same direction as in the example, current will flow from the negative half of the battery through the upper half of the winding of the relay 50 or, conversely, from the positive half of the battery through the lower half of the winding of the relay and the relay will be energized in either case in the opposite direction to that of the first example.

Depending upon the direction in which the relay 50 is energized, the motor 37 will be made to rotate momentarily to turn the shaft 17 until the gyro is brought back into a safe position. Clearly, this position will be erroneous and for this reason the contact 31 is now also closed so, as to switch off the indicating instruments. The time during which the plane may stay in the position shown in Fig. 6 must be very short owing to the nature of this position. When the plane has left the dangerous position after a; few seconds at the utmost, the relay 50 gets no current and the motor 37 operates under the exclusive influence of the circuit already described, under the control of the cam disc 32'and the roller 33. When the cam disc has returned to its normal position, in which the contact device 34 is not closed, the rotation of the shaft 17 stops. This rotation however, owing to the gyro coupling, has simultaneously effected the return of the shafts 14 and 15 to their normal positions. At the same time the cam disc 29 in cooperation with the roller 30 has again switched on the indicating instruments and theseare adjusted rapidly under the influence of; the synchro coupling to their correct positions.

It is. apparent from the above that it is possible to. achieve in this manner a gyro system which is operative in practically all attitudes of the aircraft and in which the sources of disturbance that have occurred in earlier gyro scopes on account of temporary coincidence of two. shafts are effectively removed.

It is obvious that the invention is not restricted to the. embodiment described and illustrated in the drawing but that various modifications are possible within the spirit and scope of the invention.

What is claimed is:

1. A gyro system, particularly for flight navigation, including a rotor rotatable about a first shaft journallcd in an inner gimbal ring, the inner gimbal ring being rotatable about a second shaft journalled in an outer gimbal ring and. the outer gimbal ring having a third shaft which is rotatable with respect to a structure normally having a fixed position relative to the aircraft, and means.- for locking said third shaft against rotation when itcomes within a predetermined angular range of coincidence with said first shaft.

2. Gyro system as claimed in claim 1, including a fourth shaft journalled in the frame of the aircraft on which the system is mounted, said fourth shaft rotatably mounting said relatively fixed structure and means normally locking said fourth shaft against rotation, means for releasing said locking when the third gimbal shaft;- comes within a predetermined angular range of coincidence with said third shaft, said third shaft simultaneously being locked against rotation.

3. Gyro system as claimed in claim 1, said means for locking a shaft against rotation being operative when the corresponding angular range of coincidence is smaller than 10.

4. Gyro system as claimed in claim 1, including a servomotor operatively connected to said fourth shaft and means for actuating said servomotor to return said fourth shaft to its normal position after rotation of said shaft occurring in a release interval.

5. Gyro system as claimed in claim 4, said last-named means including a cam disc means mounted on said fourth shaft and indicating the normal position of said fourth shaft.

6. Gyro system as claimed in claim 5, said cam disc means, being responsive to the angular position of the fourth shaft, switch means operatively connected to said cam disc means to actuate the servomotor to rotate to return the said fourth shaft the shortest way to its normal position.

7. Gyro system as claimed in claim 4 in which the servomotor has a friction roller operatively connected therewith, said friction roller in engagement with a friction disc mounted on the said fourth shaft, the said friction roller being adapted to drive the said disc and thereby return said fourth shaft to its normal position.

8. A gyro system particularly for flight navigation including a rotor rotatable about a first shaft, said first shaft being journalled in an inner gimbal ring, said inner gimbal ring being rotatable about a second shaft, said second shaft being journalled in an outer gimbal ring, said outer gimbal ring having a third shaft, said third shaft being rotatable with respect to normally fixed structure relative to the aircraft, means for locking said third shaft against rotation when it comes Within a predetermined angular range of coincidence with said first shaft, a fourth shaft journalled in the frame of the aircraft, said normally fixed structure being rotatably mounted on said fourth shaft, means normally locking said fourth shaft against rotation, means for releasing said locking means when said third shaft comes within a predetermined an? gular-range of coincidence with said first shaft, said third shaft simultaneously being locked against rotation, a servomotor, a friction roller operatively connected with said servornotor, a friction disc mounted on said fourth shaft, said friction roller adapted to drive said disc, means for actuating said servomotor to return said fourth shaft to its normal position after rotation of said shaft, and electromagnetic means operatively connected to said fric tion roller to disengage said roller from said disc when said third shaft is locked against rotation.

9. Gyro system as claimed in claim 8 including electromagnets for controlling the release of said fourth shaft and the braking of said third shaft and a contact responsive to the rotation of the said second shaft controlling said electromagnets.

10. Gyro system as claimed in claim 8 including an additional cam disk-controlled contact operatively controlled by said fourth shaft, the said contact being included in an electric circuit adapted to effect, during the intervals when the said fourth shaft is not in its normal position, switching off of the indicating instruments controlled by the gyro.

11. Gyro system as claimed in claim 8 including a pair of contacts operatively associated with said second and third shafts, said contacts included in a circuit to operate said servornotor when the said fourth shaft is approximately in coaxial position with said second shaft and said third shaft is approximately in coaxial position with said first shaft.

12. Gyro system as claimed in claim 11, in which a polarized relay is connected with the said contacts responsive to the gimbal shafts as to cause attraction of the relay in one direction upon a simultaneous movement of the said contacts in the same direction and to cause the relay to be attracted in the opposite direction upon a simultaneous movement of the said contacts in opposite directions, the servomotor being reversible so as to rotate in direction dependent upon the direction of attraction or" the polarized relay.

References Cited in the file of this patent UNITED STATES PATENTS 2,450,875 Braddon et al. Oct. 12, 1948 2,469,782 Phair May 10, 1949 2,524,553 Wendt Oct. 3, 1950 2,561,367 Haskins July 24, 1951

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