Patent GB-1062466-A (GB)
Title: Improvements in apparatus for measuring the time interval between the pulses of a recurring pair
IPC: E21B47/08
Filed: 1964-07-02
Abstract: 1,062,466. Time interval measurement. SOC. DE PROSPECTION ELECTRIQUE SCHLUMBERGER. July 2, 1964 [July 3, 1963], No. 27411/64. Heading H4D. The invention broadly relates to apparatus for measuring the time interval between the pulses of a recurrent pair when the second pulse of the recurrent pair may be randomly absent especially where the pulses are a transmitted radio or acoustic pulse and its associated echo pulse. The trailing edge of a measuring pulse commenced in synchronism with the transmitted pulse is caused to track the mid-point of the echo by sampling the echo pulse with early and late gates abutting either side of the trailing edge. A gating pulse wide enough to completely enclose the echo is also caused to centre itself around the trailing edge of the measuring pulse and acts to gate through to the early and late gate circuitry only the required echo pulse in the presence of unwanted signals. Loss of the echo pulse causes the capture width of the gating pulse to increase. In the described embodiment which is an ultrasonic oil well diameter measuring device an unwanted echo (S20b), Fig. 3 (not shown), is caused by reflection of the transmitted pulse (S20a) from a pressure shell surrounding the transducer 34, Fig. 2. The required echo (S20c) is produced by the wall of the oil well. The echo pulses are converted into pulses (S38) in shaper 38 and the required pulse (S38c) is gated through AND gate 56 by means of the gating pulse (S42). The measuring pulse (S44) is produced by a monostable multivibrator 44 triggered by a pulse (S30) which also triggers the transmitted pulse and has its duration controlled by a control voltage applied to the multivibrator at point 71. Pulse (S44) is fed from the 1 output of multivibrator 44 to one input of AND circuit 62, whilst the inverse of pulse (S44) is fed from output T of multivibrator 44 to one input of AND circuit 64. The other inputs of the AND circuits are fed with the pulse (S38c), such that the AND circuits produce outputs (S62) and (S64) representing the respective overlaps of the pulse (S38c) with the measuring pulse and its inverse. The pulses (S62) and (S64) then pass to a current generator 66 which delivers a positive current to a condenser 68 during pulse (S62) and a negative current during pulse (S64). The net charge on condenser 68 (E68), represents the difference in the widths of the pulses (S62) and (S64) and is used to control the width of the measuring pulse until the trailing edge of that pulse coincides with the pulse (S38c) as required. A pulse (S40) is produced by a monostable multivibrator 40 and like pulse (S44) is started by pulse (S30) and controlled in duration by a signal applied at point 55. The termination of pulse (S40) triggers a similar duration controlled multivibrator 42 to produce the gating pulse (S42). The gating pulse (S42) is fed to AND gates 46 and 48 together with the measuring pulse (S44) and its inverse respectively, to produce output pulses (S46) and (S48) respectively. These are fed to a current generator 50, similar to generator 66, such that condenser 52 and integrator 54 produces a control signal (E52) whereby the width of the pulse (S40) is adjusted until the gating pulse (S42) is aligned with the end of measuring pulse (S44). The gated echo pulses (S56) are integrated by means 58 to produce a signal proportional to the average repetition rate of the echo pulses which controls the width of the gating pulse. The arrangement is such that when the echo pulses are lost for a time and their average repetition rate thus falls, the control signal correspondingly falls causing the width of the gating pulse to increase such that there is greater chance of the echo being picked up again if it has moved when it reappears. The transducer may be formed with a rear wave damping system comprising granules of glass embedded in a block of epoxy resin.