GB964581A - Improvements in or relating to the measurement of time intervals - Google Patents

Abstract

964,581. Electric voltage measurement; electric time measurement BRITISH IRON & STEEL RESEARCH ASSOCIATION. Aug. 9,1960 [Aug. 9, 1959], No. 27240/59. Heading G1U. [Also in Divisions G4 and H3] The speed of a moving strip 1, Fig. 1, e.g. in a rolling mill, is measured by generating similar signals f1(t) and f2(t), representing common characteristics of the strip material, in response to its passage past two spaced points along its path, the signal f2(t) being delayed by a time Ts (inversely proportional to the speed) relative to f1(t). From f1(t) is derived at least one further signal F1(t) delayed relative to f1(t) by a time T, and either T is adjusted or the spacing of the points is varied to adjust Ts to satisfy a condition of the cross-correlation function C(T) of f2(t) and F1(t). This function is shown to be the mean of f2(t) f2(t-T+Ts) which is equal to the auto-correlation function R(T1-T) of f2(t) and has a sharp maximum when T = Ts. Light signals reflected or emitted from the strip are picked up by photo-electric cells 4, 5 to form the signals f1(t), f2(t). F1(t) is generated by recording f1(t) on tape 8 by a recording head 9 and picking it up by a head 10 spatially adjustable with respect to 9 by a servo-motor 11, controlled by a signal corresponding to C(T). This signal is generated by a multiplier 14 and time averager 15 and compared with the output of an oscillator 18 in a phase-sensitive rectifier 16. The oscillator signal is also applied to the servo-motor 11 through an adding device 17, the motor 11 oscillating head 10 about an average position corresponding to T = Ts since the output of rectifier 16 is arranged to be zero when C(T) is a maximum. In the apparatus shown in Fig. 8, a pair of adjustable pick-up heads 30, 31 generating signals F1<SP>1>(t), F1<SP>11>(T) respectively are used and a zero seeking servo system 32, 33 is controlled by a comparison signal C<11>(T)-C<1>(T) where C<11>(T), C<1>(T) are the correlation functions of f2(t), F1<SP>11>(t) and f2(t), F1<SP>1>(t) respectively, the signal C<11>(T)-C<1>(T) being maintained at zero. Control circuits including relays V, E, F, G, S, T are provided to disconnect the inputs from servo-amplifier 33 to allow motor 32 to drive the heads 30, 31 in a direction to make T correspond more nearly to Ts, i.e. to the speed of the strip, should there be a very large, or very small difference between T, Ts. The operation of these is fully described in the Specification, also with reference to Figs. 9(A-E), Figs. 10(A, B), not shown. In order to avoid differences in the form of the signal f2(t) from that of f1(t), F1<SP>1>(t) or F1<SP>11>(t) owing to the recording of the latter, f2(t) can also be recorded on the magnetic tape (or drum) (Figs. 7A, 7B, not shown), and a signal F2(t) similar to, but delayed by a fixed time interval from, f2(t), be used instead of f2(t). In the embodiment of Fig. 17 an output which is a measure of the fractional reduction in thickness of the strip 1 in passing between rolls 252, 253 is obtained. Apparatus as already described is used to measure the intervals Tsi, Tso for the strip to pass between photo-electric cells 4, 5 and 4a, 5a at the entrance and exit, respectively of the rolling mill, the outputs being recorded on a drum 261 rotated at constant speed. Two pairs of pickoff heads 330, 331, 330a, 331a are driven by servomotors 32, 32a respectively forming parts of two zero-seeking servo systems so that the rotary position of gear wheels 263, 265 carrying these pickoff pairs, are measures of Ts1, Tso. A rotary pickoff 271 is connected to the output shaft of a differential 269 having an angular position which at any time corresponds to Ts1-Ts0 = (B); a rotary pickoff 272 gives a measure of Tsi = (C); B/C corresponds to the fractional reduction. A signal D=B+E1+ E2 is used instead of B to compensate for errors in pick-off head positions, E1 E2 being proportional to the servo motor speeds. The output D/C of a divider 274 can be compared with a command signal F if a certain fractional reduction is required any difference G changing the roller separation. Instead of photo-electric cells, optical or other transducers responsive to patterns, radiation, variation of a magnetic field, markings, variations in electromagnetic radiation such as X-rays or radioactivity may be used. It is stated that fluid speeds may be measured by networks according to the invention. Summer and trigger circuit, fig. 15. Two inputs at 206, 207 are summed'in an amplifier 209 the output of which is fed through a cathode follower detector 210 and a time averager 92 to a Schmitt trigger circuit 224, 225 so arranged that if the sum signal is less than a fixed value the circuit will trigger and operate a relay V. A.C. trigger circuit Fig. 16. Pairs of valves 237-238, 239-240 have their anodes supplied in antiphase from a transformer 243. Valves 238, 240 are thus cut off during negative half cycles and valves 237, 239 during positive half cycles. The grid of 238 is, however, connected through a potentiometer 246 to a positive biasing voltage, so that the grid potential of 240 will be kept too low for valve 240 to conduct during positive half cycles of the anode supply. Relay F therefore remains inoperated provided the input at 230 is less than the biasing voltage from the potentiometer. A similar arrangement prevents valve 239 from conducting and relay S remains inoperated. If, however, the input at 230 exceeds the biasing voltages in magnitude, one or other of valves 237, 238 dependent on the phase of the A.C. signal at the input 230 relative to the phase of the transformer supply will conduct and cause the appropriate one of valves 238, 240 to conduct and operate its associated relay.