Patent GB-1144880-A (GB)
Title: Inertial instruments
Inventor: ERDLEY HAROLD FREDERICK
IPC: G01C19/22
Filed: 1968-01-29
Abstract: 1,144,880. Gyroscopic apparatus; accelerometers. LITTON INDUSTRIES Inc. Jan.29, 1968 [Jan.30, 1967], No.4573/68. Headings G1C, G1K and G1N. An inertial instrument for detecting angular displacement and linear acceleration comprises at least one and preferably four intertial elements 10, 12, 14, 16, Fig.3, each mounted on a torsion bar 22, 24, 26, 28 for rotation about a spin axis 59 of a shaft 18 and for vibrating motion about the axis of the respective torsion bar, the torsion bar axis being disposed at right angles to the axis 59 and the centre of mass 60, 66, 64, 62 of each element being radially displaced from the axis 59, and means for sensing vibrating motion of the element or elements. Each element 10, 12, 14 and 16 is in the form of a segment of a circular member and is provided with a radial slot through which the respective supporting torsion bar projects. Pairs of capacitive pick-off and torque plates (not shown) are provided for each inertial element, the pick-off's sensing the vibrating motion of the elements and the torques being used to damp this motion. Alternatively electomagnetic pick-offs and torques may be used. The instrument is made sensitive to linear accelerations normal to the axis 59 by displacing the centres of mass of the inertial elements from the axis of their respective torsion bar in a direction parallel with the spin axis 59. By displacing the centres of mass of opposed pairs of inertial elements 10, 14 and 12, 16 in opposite directions parallel to the axis 59 and in alignment with the parts of inflexion of the respective torsion bar, linear accelerations will cause opposite effects on the elements of each pair and thus the pick-offs will detect the difference between signals caused by linear accelerations and those caused by angular displacement about axes normal to the axis 59, which will be the same for the elements of each opposed pair. In operation the shaft 18 is rotated with an angular frequency N which is the fundamental frequency of oscillation of the torsion bars 22, 24, 26, 28 and this gives the greatest response. By use of the four inertial elements shown it is not only possible to separate signals caused by angular displacements and linear accelerations but also to eliminate the effect of disturbing 2N frequency displacements of the shaft 18, e.g. shaft wobble. The mathematical treatment is given in the Specification. Constructional features Fig .11 In Fig.11 the four inertial elements and two annular plates which carry the pick-off and torque electrodes are shown as an assembly 208 coupled to the shaft 18 and mounted inside an evacuated housing 260 which also rotates with the shaft 18. The shaft 18 is rotated by a hysteresis motor 30 and the shaft is supported along its length by a gas or air journal bearing 222 and axial thrust bearing 220. The bearing 222 is formed between a stack of ceramic wafers forming a rotor 292 on the shaft 18 and a stack of ceramic wafers 290 forming a stator in the instrument housing. Some of the ceramic wafers are spacers and others support capacitive electrodes serving to supply power to the pick-offs and torques for the inertial elements in the assembly 208. A mercury cup connector 70 is used to maintain the desired potential on the inertial assembly 208 and a similar connector is used to ground the shaft 18. The hub 208 of the motor 30 carries a pair of magnetic tracks 211, 213 which interact with stationary heads 207, 209 to generate reference signals indicating the angular position of the shaft 18. If desired rotary transformers may be used in place of the capacitive arrangement described to supply power to the assembly 208. Capacitive pick-off arrangement Fig. 9 In Fig.9 there is diagrammatic representation of a pick-off having a pair of plate electrodes 48, 49 co-operating with one of the inertial elements 10 carried by the torsion bar 22. Torquer plates 50, 52 are also shown. 32 and 34 represent two of the electrodes formed in the ceramic wafer stacks 290, 292 surrounding the shaft 18, and power is supplied by a square wave oscillator 106. During the positive half cycle of voltage of the oscillator 106, capacitor 102, the capacitance between the elements, 32, 34 and the plate 48 are charged through diode 108. When the oscillator goes negative capacitor 110 becomes charged while the pick-off capacitor at the plate 48 discharges through coupling 32, 34 capacitor 102 and resistors 116, 118. The pick-off capacitor is again charged on the next positive cycle whilst capacitor 110 discharges. Under normal conditions positive and negative half cycles across resistor 118 are of equal amplitude but when element 10 is displaced one half cycle increases in amplitude whilst the other decreases. The torsion bars are arranged to have long time constants whereby angular rate information is really angular rate times time, i.e. angular displacement, and acceleration information is acceleration times time i. e. velocity. To obtain true acceleration and angular rate information, the pick-off output signals are fed back to the torques in a closed loop damping arrangement. In this way an angular excursion of each element is continually torqued back to null, and the torquing voltages contain true angular rate and acceleration information.