Patent GB-1121899-A (GB)
Title: Floated inertial platform
IPC: G01C19/20
Filed: 1966-10-24
Abstract: 1,121,899. Inertial navigation systems. NORTHROP CORP. Oct.24, 1966 [Oct. 24, 1965], No. 47622/66. Headings G1C and G1N. An inertial platform comprises a ball-shaped platform member 11 with a spherical casing 12 surrounding the member 11, the casing 12 being in the form of two hemispheres joined together so as to be electrically insulated from each other, and the platform member 11 being floated in a liquid within the casing and centred by a plurality of roller wheels 20 mounted in supports on the member 11 for rotation about axes passing through the centre of the ball member, resilient electrical brushes 30 connecting with the casing hemispheres to provide an electrical supply to equipment in the member 11, which equipment comprises stabilizing gyroscopes and accelerometers for sensing vehicle travel, and a capacitive pick-off arrangement generating output signals indicative of the orientation of the platform member 12 relative to the casing 12. The two casing hemispheres are connected to the opposite poles of a D.C. supply. The electrical brushes 30 are so connected through a diode matrix that the positive and negative terminals for connection to the platform equipment remain of the same polarity whatever the relative orientation between the platform member 11 and casing 12. The D.C. supply also has A.C. signals modulated thereon. Torquer units 45 are provided on the surface of the ball member 11 to maintain the platform member 11 inertially stabilized within the casing 12. These units comprise two phase induction motor field windings and receive signals from the pick-offs on the stabilizing gyroscopes to induce eddy currents in the surfaces of the casing 12. The units 45 are arranged to provide torques about three orthogonal axes to completely stabilize the platform member 11. The capacitive pick-off arrangement comprises three excitation strips 41, 42, 43 which are all situated on a great circle of the platform member 11 and are orthogonally disposed and a pick-off ring 50 arranged at the join between the two hemispheres of the casing. The orientation of the platform member 11 with respect to the casing 12 is determined by measuring three angles α, #, and #. The angle α is measured between a reference point X o  on the strip 42 to the point where this strip intersects the ring 50. The angle # is likewise measured between a reference point Y<SP>o</SP> o <SP> </SP>on the strip 41 and the point where the strip 41 intersects the ring 50. The angle 6 is measured between a reference point Z o  on the ring 50 and the point where the strip 42 intersects the ring 50. The strip 43 is provided for use when one of the strips 42, 41 becomes co-planar with the ring 50, suitable switching, controlled by the polarities of each of the brushes 30 as fed to the diode matrix, being provided. The ring 50 comprises a plurality of individual capacitive plates 51, Fig. 11, insulated from each other and each of the excitation strips comprises two conductive lines from which interleaved conductive fingers extend, the fingers connected to one line being insulated from those connected to the other line. The fingers connected to one line increase from a minimum at one end of the strip to a maximum at the other and vice versa for the other line. The net result is that the area of the fingers presented to the plates 51 of the ring at any one point is a junction of the sine of the angle to be measured for one line and the cosine of this angle for the other line. This is schematically shown in Fig. 11 where the area of one set of fingers 68 decreases towards one end of the strip and vice versa for the other set of fingers 69. Each set of firgers is connected to be supplied with signals in phase quadrature so that the output signals on the plates 51 for each set of fingers are in phase quadrature and are of a magnitude dependent on the area of the fingers presented to the plates 51 and therefore representative of the angle to be measured. The signals from the plates 51 are fed to a summing circuit the phase of the output of which is dependent on the angle to be measured. This is compared with a reference signal from the source supplying the excitation signals to the fingers 68, 69 in a detector whose output represents the desired angle. The angles α and # are measured using different excitations frequencies for the strips 41, 42. A third frequency source is used for the measurement of the angle #, and this is connected to both sets of fingers of the strip 42. Each plate 51 is connected to a separate phase shifter unit to generate successively phase shifted signals from the reference point Z o . The outputs of the phase shifter units are fed to a summing unit, the phase output of which will be indicative of the angle # after comparison with the reference signal fed to the strip 42. By suitable conversion of the signals representing the angles α, # and #, signals indicative of angular displacements about three orthogonal axes are derived to provide platform attitude information.