Patent GB-861341-A (GB)
Title: Accelerometers
IPC: G01P15/10
Filed: 1959-07-16
Abstract: 861,341. Accelerometers. SPERRY RAND CORPORATION. July 16, 1959 [July 16, 1958], No. 24542/59. Class 40 (1). [Also in Group XXXV] In an accelerometer of the type described in Specification 789,611 in which acceleration displaces a mass connected between two wires so altering their tensions and hence their natural vibration frequencies these frequencies being detected and compared to determine the acceleration, the mass includes a compound mass comprising two masses connected resiliently together so as to have a relative oscillation frequency substantially equal to the mean vibration frequencies of the wires so as to reduce vibratory intercoupling between the wires. A preferred embodiment comprises, Fig. 1, masses 16 and 17 suspended by ligaments 18 from rings 12, 13, 14 and 15 which form part of the supporting frame. From the outer ends of the masses wires 25 and 26 run to clamps on the end plates 10 and 10<SP>1</SP>, the inner ends of the masses being coupled by circular spring 40 which serves to tension wires 25 and 26. Permanent magnets 51, 52 and 53 provide magnetic fields in which the wires vibrate at frequencies dependent on their natural fundamental vibration frequencies due to electric currents in the wires derived as described below under " Detection circuit." One or both of masses 16 and 17 is constructed as shown in Fig. 2, i.e. it comprises a main body 37 having a second mass 65 suspended in a recess by a circular spring 66. The mass and spring being so arranged that the fundamental vibration frequency of mass 65 relative to body 37 is the same as the mean natural fundamental frequency of vibration of the wires 25 and 26. Detection circuit.-The wires 25 and 26 are maintained in oscillation by arranging each wire in a bridge circuit, the input voltage to the bridge being supplied from an amplifier whose input voltage is the unbalance signal of the bridge. This arrangement maintaining wire 26, provided with permanent magnet 51 midway along its length, vibrating at its natural fundamental frequency and wire 25, provided with magnets 52 and 53 a quarter and threequarters along its length, vibrating at twice its natural fundamental frequency. The wire oscillator circuits are connected as shown in the block diagram in Fig. 5 to produce a voltage proportional to the difference frequency of the natural fundamental frequencies of the wires, which voltage is proportional to the acceleration applied to the accelerometer. Alternative forms of the invention comprise a rotary accelerometer, Fig. 7, in which masses 116 and 117 are constrained against rotation by wires 125 and 126 which are tensioned by spring 122, masses 116 and 117 including secondary masses 131 on flexible spokes 127, the combination of 131 and 127 having a vibration frequency equal to the wire fundamental frequency. An alternative device for tensioning the wires in the arrangement of Fig. 1 replaces circular spring 40 with a solenoid energized from a D.C. source producing a constant force irrespective of the displacement of the core over the limited travel required of the core which system also tends to isolate the vibrations of the two wires.