Patent GB-863456-A (GB)
Title: Vertical reference defining apparatus
Inventor: WING WILLIS G
IPC: G01C19/44
Filed: 1957-03-22
Abstract: 863,456. Navigation systems. SPERRY RAND CORPORATION. March 22, 1957, No. 9580/57. Class 40 (1). Apparatus for defining in a moving craft a vertical reference and from which the velocity of the craft may be computed, includes a platform which is stabilized about three mutually perpendicular axes by gyroscopic means which are controlled by two linear accelerometers, mounted horizontally at right angles on the platform, from which angular velocity signals are derived by a computer which takes into consideration the mean radius of the earth and the latitude of the craft. In Fig. 1, a platform ring 12, mounted with three degrees of freedom in gimbal rings 14, 15, 16 on a moving craft, carries three gyros A, B, C which tend to maintain the platform automatically in a desired position in space by applying electric signals from E-type pick-offs 24, 31, 164 to torque motors 34, 35, 165 rotating appropriate rings 14, 15, 12. A double resolver 36 divides the signal from each of the pick-offs 24, 31 between motors 34, 35 in the manner required to ensure proper control. Integrating accelerometers 47, 48 mounted on platform 12 produce electric signals corresponding to components of velocity of the craft in two perpendicular directions, and these signals, transformed into angular velocities about the earth's centre, control torque motors 23, 30 associated with gyros A, B to ensure that the axis 13 of platform 12 is maintained aligned with an apparent vertical corresponding to the vector resultant of centrifugal force due to the earth's rotation and the force due to gravity, despite rotation of the earth and movements of the craft over the earth's surface. Craft velocity may then be computed from the accelerometer signals. As shown in Fig. 5a, a circuit for converting the linear velocity signal from integrating accelerometer 47 into an angular velocity signal includes: (a) a mixer 69 which introduces a fixed factor dependent on the mean radius of the earth; (b) a potentiometer 63 which is adjusted to correct for the variation of earth's radius with latitude by a servo-system controlled by a compensated latitude computer 53, 54; (c) a potentiometer 62 which is adjusted to correct for the effect of altitude on the effective radius; (d) a potentiometer 65 which is adjusted to correct for the orientation of platform 12 about its axis 13 by an initial setting member 59 and a servo-system 79 controlled by an earth's rate computer 80 and a rate of change of latitude computer 86 both modified in dependence on latitude by resolvers 82, 87. A similar circuit including appropriately adjusted potentiometers 73, 74, 76 converts signals from accelerometer 48. The converted signals from the accelerometers besides controlling torque motors 23, 30 are also applied to craft velocity computers, that for accelerometer 47 being mixer 181. To provide longterm accuracy, craft velocity is also measured by Doppler radar 95, Fig. 5b, signals from which are converted into angular velocity signals by computer 131 which introduces factors dependent on earth's radius, latitude and altitude. A term dependent on the earth's rate is added from a computer 93 and the combined signal corrected at 128 for orientation of platform 12 about its axis 13. The resultant signal is compared in a mixer 101 with the shortterm velocity signal derived from the output of mixer 69 through lead 121. Any error is effective through a servomotor 104 to position a follow-up potentiometer 106 correcting the craft velocity signal from mixer 181. Damping is provided by a rate of change of velocity error signal applied to mixer 69 from a tachogenerator 105. This damping may be critical (heavy), normal, or disconnected by operation of a switch 112 controlling the circuit constants and connections between generator 105 and mixer 69. If radar cannot be used, signals from an airspeed meter 96 with or without wind correction may be substituted, but in view of the decreased accuracy normal damping is used and a follow-up potentiometer 168 is substituted for potentiometer 106 so that further correction is not applied to the craft velocity computer 181. The damping period of the system is normally that of Schuler tuning but it may be reduced by operation of a switch 137 to apply the error signals from mixer 101 to mixer 69 through line 146. These signals are substituted for the rate of change of error signal if the airspeed meter is being used but additional if radar is in use. Under these conditions a potentiometer 139 is substituted as the follow-up potentiometer for servomotor 104. The last-mentioned arrangement may be used for rapid erection of the system during which correction signals to velocity computer 181 are disconnected. Specifications 863,457 and 863,458 are referred to.