GB1163209A - Accelerometer - Google Patents

Abstract

1,163,209. Measuring acceleration electrically. LITTON INDUSTRIES Inc. Sept. 20, 1967, No.42892/67. Heading G1N. An accelerometer comprises an annular body 84, Fig. 8, attached to a fixed annular member 42 by diametrically arranged flexible hinges 92, 94, 98, 101, Fig.10, so that the body is free to pivot about diametrical axis 170 through its centre of mass. An additional mass, e.g. 150, is mounted on the body so that the centre of mass of the composite body is not coincident with the axis 170, an applied acceleration thus producing a turning force on the body. The position of the mass on the body depends on the plane of the applied acceleration, e.g. for accelerations transverse to the plane of the body, the mass is arranged round the circumference or equally at the ends of the diameter perpendicular to the axis 170; for accelerations in the plane of the body, the mass is arranged at one point on the circumference. As shown, ceramic body 84 and member 42 are hinged together by pieces of metal foil 92, 94, 98, 101, e.g. electrolessly deposited nickel, which are also used as electrical connections to and from the pivoting body 84. Movement of the body is sensed by changes in capacitance between two pieces of metal foils 91, 93, Fig. 10, on the lower surface of the body and a magnet body 50 forming a third common capacitor plate. The two capacitors (156, 158, Fig. 14, not shown), are connected in adjacent arms of an A. C. bridge circuit whose output after demodulation at (168) is fed to a force balance coil 82 attached to the circumference of the lower surface of the body 84. The coil is suspended in the flux gap between the annular magnet 50 of aluminium nickel alloy magnetized so that its poles lie at the ends of a diameter transverse to axis 170, and a ring 78 of magnetic material forming part of the return path of the magnetic flux. Thus application of current to the coil 82 produces a resolving torque in the coil which returns the body 84 to its zero position. This current is also indicative of the acceleration. Temperature compensation for magnet 50. The field strength of the magnet 50 tends to decrease with increasing temperature. Thus, to maintain the flux in the gap between magnet and ring 78 constant at the poles, the gap must be varied inversely with temperature. To this end an annular disc 52 of e. g. aluminium provided with an insulating layer 70 on its upper surface, is provided, having projections (79, 80, Fig. 13, not shown), at opposite ends of a diameter parallel to axis 170, which bear against the ring 78. As the temperature increases the disc 52 expands producing an elongation of the diameter of the ring 78 parallel to axis 170 and corresponding contraction of its diameter perpendicular to axis 170 (see dot-dash line (182)) producing the required reduction in the flux gap at the poles of the magnet. The above described temperature compensated magnetic system is also described and claimed in Specification 1,163, 210.