Patent GB-1469764-A (GB)
Title: Accelerometers
IPC: G01P15/13
Filed: 1973-06-30
Abstract: 1469764 Accelerometer FERRANTI Ltd 28 June 1974 [30 June 1973] 31259/73 Heading G1N A force balance accelerometer is as described in 1362121, Division G1, having an inertial mass displaceable by acceleration forces, an electromagnetic coil 11 responsive to current flow therein to exert a compensating force on the mass and a switch 12  which makes contacts 12a, 12c when the mass is displaced by the acceleration force and makes contacts 126, 12c when the force due to the coil overcomes the acceleration force. The coil 11 connects to a current source 13 including an oscillator 14 counter 15 and binary scaled resistor network 16 having resistors R, 2 R, ... nR and supplied with a reference voltage so for each stage of the count in 15 of oscillator pulses the current from the network to coil increases by a step unit of Y/nR, see Fig. 2. With no acceleration applied contacts 12b, c are made, synchronizing latch 24 is reset so no enable signal is applied to counter 15. Acceleration of the mass changes-over contact 12c to make with 12a, latch 21 is set to produce a signal on line 22 when acceleration counter 20 counts pulses from the oscillator 14 and synchronizing latch 24 is set enabling counter 15 to count oscillator pulses the while the current in coil increases in steps 44. When coil current is such, as I' for example, that force balance obtains when the current I' is proportional to the acceleration of the mass, switch 12 changesover again. Latch 21 is triggered to stop the acceleration counter 20. The counter 15 continues counting until it reaches a reference level, corresponding to current I", at which gate 17 opens and counter 15 is reset. The synchronizing latch is also reset from gate 17 so the counter 15 is disabled and stays at zero. Resetting of the synchronizing latch 24 provides a trigger signal to control 29 which causes the contents of the register 37, intially zero, to be added in adder 36 with the count in counter 20, the total returning to the register 37. The time for the current to rise from zero to the value I" is a fixed reference time period. When counter 15 is reset, the coil current falls to zero and switch 12 is re-made to begin the cycle again. The current again increases in steps and at force-balance the switch is again broken. Once again the acceleration counter 20 holds the count until the first counter 15 is reset. The new count in the acceleration counter is summed with the previous total stored in shift register 37 the output of which connects to a "velocity" output terminal 42. Similarly a further adding element 38 and shift register 39 connects with a "distance" output terminal 43. In an alternative embodiment Fig. 3 (not shown) the acceleration counter is substituted by a serial input/ output shift register 46 so that a reset signal from bi-stable latch 21 on line 23, caused by breaking of the switch 12, transfers the count from counter 15 into register 46. Gate 47 prevents shift-out of register contents while they are changing. Analogue operation may be provided by a signal on line 22 switching an output voltage of given magnitude which reduces to zero when the switch is broken. In any one reference time period the fraction of the period that an output voltage is produced is proportional to the acceleration.