GB1432502A - Devices for measuring shaft rpm - Google Patents

Abstract

1432502 Measuring RPM LUMENITION Ltd 26 April 1974 18469/74 Heading G1A In an apparatus for measuring shaft RPM, e.g. in a car, the number of voltage pulses produced by a shaft transducer in a given time is cyclically determined by a counter, and the counter output is stored; the store is cyclically read out to illuminate some of a line of light emitting diodes so that the length of the line which is lit represents the RPM. In Fig. 1 the transducer has a blade 72 coupled to the shaft, and an infrared source 71 (e.g. an LED) and detector 73; these, with an amplifier 74 and pulse shaper 75, produce a squarewave of constant mark/space ratio at a frequency proportional to shaft RPM. At the start of each measurement cycle, pulses from shaper 75 pass to shift register 76; when the third pulse arrives, the register gives an output which causes the output of a monostable 77 to switch to a high value. The monostable output remains high for a known time, e.g. 30 ms, depending on its characteristics, and during this time a gate 78 is opened by the monostable output to pass pulses to a counter 80. At the end of the known time the monostable output drops, thus closing the gate and also resetting the register 76 so that a new cycle can start. When the count is terminated by the drop in the output of monostable 77, a store 81 receives the count and lights appropriate ones of a line of LED's 82; when, in the next cycle, the output of the monostable 77 rises, this not only clears counter 80 via monostable 79 but also causes store 81 to latch and hold the count of the previous cycle. As a result the LED's remain lit, without flicker, a change in the number lit occuring only if the RPM count changes. The shift register 76 may be replaced by a gated monostable or flip-flop; its function is to cause two pulses to be missed so as to allow the monostable 77 time to reset in each cycle. For the line of LEDs, gallium phosphide devices are preferred.