Patent GB-589562-A (GB)
Title: Improvements in or relating to frequency determining arrangements
IPC: G01P3/48
Filed: 1944-12-28
Abstract: 589,562. Frequency measurements. P. R. T. LABORATORIES. Ltd., ROWE, J. C. L., LEONARD, G. H., and SOLLEY, B. J. Dec. 28, 1944, Nos. 26034/44 and 31790/45. [Class 37] The speed of a recurrent moving part or the frequency of a pulsating current is measured by applying pulses of one polarity, derived from a pulsed current generated in synchronism with a moving part, as a triggering input to a relaxation circuit of the " flip flop " type adapted to generate output pulses, of predetermined amplitude and time duration, which are applied to a frequency calibrated meter. A current of waveform, Fig. 2 (a), derived from the low-tension side of an engine ignition system, is applied to terminals 10, 11, Fig. 1. Resistance R1 and condenser C1 form an integrating circuit which filters-out the high frequencies and produces a waveform (b) across C1. The input pulses are put in the same sense by the rectifier W1, amplified by a transformer T1, and then applied to a second integrating R2, C2 which filters out the remaining undesired frequencies and produces a waveform (c). The pulses are then applied to the grid of valve V 1 Which is arranged with resistors R3, R4, R5 and condensers C3, C4 to form a high gain resistance-capacity coupled amplifier, the resultant waveform at the anode being as shown at (d). This waveform is applied to condenser C5 and resistance R6, which form a differentiating circuit and a waveform (e) is produced at their junction. The positive pulses are removed from this waveform by the diode V3 and its load resistance R7, the waveform (f) being available at the anode of V3. Pulses of this waveform are applied as a triggering input to the suppressor grid of a transitron " flip flop " circuit comprising a pentode valve V2 with its feed-back condenser C6, and resistors R8, R9, R10, the diversion of space current due to the screen causes immediate cut-off. After a time interval determined by the circuit constants the suppressor potential rises sufficiently to allow anode current to flow, whereupon there is an immediate reversal to the original pre-pulse condition. The resultant waveform (g) is a positive square pulse whose amplitude and duration are dependent only upon the circuit constants and are independent of the duration and shape of the input triggering pulse. The output from the anode V2 is fed by way of condenser C7 to a rectifier W3 the D.C. output of which is applied to a moving coil galvanometer. Due to the mechanical inertia of the meter movement, the deflection is proportional to the number of pulses supplied in a given time. The meter scale may be calibrated in R.P.M. In order to check the calibration of the meter the switch S1 is reversed and the arrangement is supplied by the A.C. mains, if necessary the meter indication is corrected by adjustment of the shunt R11. The initial current pulses may be obtained by other means such as : (1) A disc of insulating material coupled to the moving part carrying a contact segment which when in contact with a stationary brush completes the circuit of a battery across the terminals 10, 11 ; (2) A fly-wheel provided with a light reflecting zone which when in one particular position during each cycle reflects light rays from a suitable source onto a photo-electric cell. (3) A piezo-crystal subject to pressure pulses in a fuel feed line of a diesel engine ; (4) A microphone which is responsive to mechanical noise, the resultant current being applied after rectification to the terminals 10, 11. In a modification for measuring the frequency of electrical oscillations having a sinusoidal waveform the pulses are amplified, their waveform changed to a squared form and a narrow steep-fronted pulse is derived by means of a differentiating circuit. The pulses are then applied as a triggering input to a relaxation circuit of the " flip flop " type which is adapted to generate an output current pulse of predetermined amplitude and time duration for each triggering pulse. These output pulses are applied to a frequency calibrated meter. In this embodiment the frequency checking is obtained from a stable frequency oscillator comprising a stabilised Hartley circuit.