GB720399A - Improvements in or relating to speed measuring devices - Google Patents

Abstract

720,399. Frequency measurements. SUBMERGED LOG CO., Ltd., and SWANNELL, A. C. Dec. 16, 1952 [Dec. 20, 1951], No. 29882/51. Class 37. [Also in Groups XIX and XL (b)] In a stroboscopic device for indicating the frequency of a single-phase voltage induced in a pick-up, or for indicating the speed of a rotating device producing the voltage, the singlephase voltage is converted to polyphase and drives a polyphase synchronous motor which rotates an indicator member past a fixed scale, the indicator member being illuminated periodically by a flash tube a predetermined delay time atter a putse derived trom one of the motor phases has been passed thereto. As shown, for measuring the speed of a ship's log, a bar magnet a rotating with the impeller induces an alternating E.M.F. in a stationary coil C of magnitude and frequency proportional to the ship's speed. This voltage is amplified at 1 and squared at 2. The output from 2 is differentiated, the resultant negative pulses, only, triggering a bi-stable multivibrator 3 connected as a " scale of two " circuit. Two square wave outputs at half the input frequency and in phase opposition are taken from the anodes of the multivibrator 3, and applied, respectively, to further similar "scale of two " frequency dividers 4, 5, from each of which one output is taken, these outputs being in phase quadrature. These quadrature outputs are passed through respective cathode follower buffer stages 6, and respective power amplifiers 7, 8, which convert the rectangular waves into flat-topped triangular form, to the two phases e-f, g-h of a synchronous motor t having a permanent magnet rotor d which runs synchronously with the impeller magnet a and may operate a revolution counter q, Fig. 2. The rotor d, Fig. 2, carries a disc i slotted radially at j, and rotatable within a fixed concentric scale k. The output from one of the quadrature phases is taken from the buffer valve and fed to a timing circuit 9, 10, 11 which produces an output pulse delayed a predetermined time relative to the zero point of the respective phase. The delayed output pulse operates a flash tube 12 which illuminates the radial slot pointer and the scale k to indicate the speed stroboscopically. The pick-up, a-c, may use a four-pole magnet or a slotted disc may be used in lieu in conjunction with a photo-cell.