Patent GB-962669-A (GB)
Title: Improvements in inertial navigation
IPC: G01C21/16
Filed: 1960-12-30
Abstract: 962,669. Inertial navigation. SVENSKA A.B. GASACCUMULATOR. Dec. 30, 1960 [Jan. 21, 1960], No. 44748/60. Heading G1C. In an inertial navigation system for a vehicle moving over the curved surface of the earth the position of a pendulum is compared with a direction which is constant with reference to inertial space, obtained for instance by means of a gyroscopically stablized platform, any difference between the two being used to change the frequency of an oscillator, the change of frequency being a measure of the speed of the vehicle. As shown in one embodiment, Fig. 3, a gyroscope 15 has a rotor axis 16 and inner and outer gymbal axes 18, 17, so that outer gimbal ring 19<11> forms a stable platform as regards its freedom of rotation about shaft 17. A disc 20 rotatably mounted on shaft 17 is rotated by a gear wheel 34 driven by the output shaft 33 of a differential 31 having inputs from two synchronous motors 29, 32, whose speeds are determined by the frequencies of oscillators 36, 39, respectively. Whilst the vehicle is at rest, oscillators 36, 39, deliver the same frequency to motors 29, 32 which thus rotate at the same speed and shaft 33 is not driven, so that disc 20 is rigid with the stable platform, represented by gimbal 19<11>, and a pendulum 21 is in a fixed position relative to disc 20. When the vehicle moves over the curved surface of the earth, stable platform 19<11>, and thus disc 20, remain fixed with respect to inertial space whereas pendulum 21 moves so that it still points to the centre of the earth, and, through a sensing device 22, such as a photo-cell or electro-magnetic detector, a signal is applied to amplifier 43, servomotor 45 and gearing 46 to change the frequency of oscillator 39, such as by adjusting a capacitor 41 or alternatively subjecting the crystal 40 to mechanical forces. The consequent change of speed of motor 32 causes rotation of shaft 33 and disc 20 so that the latter is realigned relative to pendulum 21. The oscillators 36, 39, are also connected to a differentially wound motor 53 which revolves at a speed controlled by the difference frequency of the two oscillators and thus at a speed proportional to the speed of the vehicle. The total number of revolutions as measured by counter 54 represents the distance travelled in the direction in which gimbal ring 19<11> forms a stable platform. Two such arrangements are placed at right angles to form a complete navigational system, a third gyroscope ensuring that the two systems are kept aligned in the correct directions e.g. N-S and E-W, respectively, and corrections are supplied to allow for rotation of the earth. A portion of the output from amplifier 43 is fed via line 47 to a braking device comprising a disc 48 and coils 49, 50 acting in the manner of an induction motor, to damp out movements of the pendulum due to temporary accelerations of the vehicle. In a modification, Fig. 4, the stator of a synchronous motor 55 is mounted on the shaft of gimbal ring 19<11> whilst its rotor is connected with the shaft 56 carrying disc 20. A beam of light (not shown) passing through the gaps between teeth 57 on disc 20 impinges on a photo-cell 58 carried by pendulum 21, the resultant pulses being amplified at 60 and compared at 61 with the frequency from crystal controlled oscillator 62 after it has been divided in frequency divider 65. The frequencies are arranged to be the same when the vehicle is stationary. When it moves, however, since the stator of motor 55 is mounted on the shaft of gimbal ring 19<11>, the disc 20 will rotate faster or slower relative to pendulum 21, the frequencies as detected at 61 will differ and the output is applied through amplifier 66 and transformer 67 to amplifier 70 feeding a servomotor 71 which controls the frequency of oscillator 75. The motor 55, which is controlled by oscillator 75 through frequency divider 76, thus has its speed correspondingly adjusted. A tachometer generator 69 coupled to servomotor 71 delivers a frequency in phase opposition to the voltage induced in the secondary of transformer 67 so that when the tachometer generator output equals the voltage induced in the transformer secondary, servomotor 71 will have performed a number of revolutions which is proportional to the current appearing in the output of detector 61. The damping device 68 is similar to that 48-50 of Fig. 3. The frequencies of oscillators 62, 75 are compared at 79 and used to operate a distance counter in the same way as 53, 54, Fig. 3. The motor 55 and gyroscope 15 may be replaced by a speed-controlled gyroscope since the moment of precession is directly proportional to its speed and if the rotor motor is directly controlled by the oscillator the moment of precession can be made directly dependent on the frequency of oscillator 75. Such an arrangement is shown in Fig. 5 wherein the spring 82 causes a constant torque to be applied so that when the rotor is at constant speed the gimbal 19<11> and its shaft 56 carrying disc 20 precess at a constant speed. The pulses from photo-cell 58 when compared at 61 with the frequency from oscillator 62 cause no output when the vehicle is stationary. When however the vehicle moves, the detector 61 provides an output to amplifier 66 and in a similar way to the arrangement of Fig. 4, the frequency of oscillator 75 is varied. This controls, through frequency divider 76 and amplifier 81, the rotor speed and consequently the rate of precession of gimbal 19<11>, shaft 56 and disc 20 is altered until balance is achieved. The differential phase motor 79 compares the frequencies of oscillators 62, 75, as before, to operate a distance counter., The gyroscope provides a reference by virtue of the fact that its speed of precession is constant only when related to an imaginary stable inertial co-ordinate system. To compensate the effects of such features as internal friction in the bearings and eccentricities in the distribution of mass in the rotor a stabilizing feedback system is provided. This comprises a pick-off 83 detecting relative movement of the inner and outer gimbals 19<1>, 19<11>, supplying signals to an amplifier 84 controlling a torque motor 85. Since, at take-off, high acceleration may be encountered by the pendulum, a Doppler system may be provided to check the speed as given by the inertial navigator when the vehicle has assumed a steady speed.