Patent GB-1134650-A (GB)
Title: Acceleration measuring system
IPC: G01P15/08
Filed: 1967-12-14
Abstract: 1,134,650. Digital accelerometers. GENERAL MOTORS CORP. 14 Dec., 1967 [13 Jan., 1967], No. 56836/67. Heading G4H. [Also in Division G1] In a linear accelerometer a mass 14 (Fig. 1) is oscillated at a constant frequency (615 cps), with a displacement period of fixed length controlled by synch pulses G (615 cps) from a computer 28, about a central reference position toward and away from a pick-off 26, the oscillation of the mass being caused by reversal, by means of a change over switch 24, of the direction of constant magnitude D.C. pulses applied to a forcer coil 20 carried by the mass. The amplitude modulated pick-off output A (Figs. 2 and 3) is amplified and fed to a demodulator 46 which produces an alternating polarity envelope signal B the positive portion of which indicates mass displacement to the left of the reference position and the negative portion displacement to the right. The signal B is filtered at 48 to produce a direct voltage signal C the magnitude of which is proportional to the displacement of the average position of the mass from the reference position and the polarity of which indicates the direction of acceleration. Measuring the D.C. signal.-The time between two consecutive synch pulses G also establishes a measurement period corresponding in duration to the displacement period and the D.C. voltage signal is measured digitally by counting the number of clock pulses E (78À5 kc.s), from the computer, between the instant a ramp voltage D (from a wave generator 54 and initiated by a first synch pulse) reaches the D.C. signal level, as signified by a gate pulse F from a comparator 50, and the end of the measurement period which occurs at the next synch pulse. The gate pulse also terminates the ramp voltage. At zero acceleration (Fig. 2) the gate signal will occur mid-way between two synch pulses i.e. 64 clock pulses from the end of the period. The difference between the actual count and 64 indicates the magnitude and direction of acceleration. For instance when the mass is acelerated to the left the average position of the mass is shifted to the left of the reference line and as shown in Fig. 3 the clock pulse J count is less than 64. Forcer unit.-The mass 14 carries a forcer coil 20 which is energized from a constant magnitude current source 22 in a selected direction determined by the position of a change over switch 24 and which coil in co-operation with permanent magnets in a unit 18 operates to produce a force in a selected direction along axis 16 acting to return the mass to the reference position. The displacement period is initiated by the 1st synch pulse fed to reset input R of flip flop 36. Coincidence of a clock pulse and a voltage at the " 0" output of 36 produces an output from AND gate 38 and a voltage from the " 0 " output of a flip-flop 40 to operate switch 24 and send current in a first direction through coil 20. The current in the coil is reversed when gate pulse and clock pulse are received simultaneously at AND gate 56 whereupon the "1" output of flip-flop 36 stops the ramp voltage via line 52 and supplies a first signal to AND gate 58 which also receives clock signals via line 22 resulting in an increased voltage at the "1" output of flip-flop 40 to reverse the current in the coil. Counting.-The "1" output of 40 is also fed to an AND gate 60 together with clock signals from the computer 28. The clock signals are passed on to a buffer register 62 which accumulates the signals and the computer subsequently counts them to provide an indication of the acceleration forces on the mass.