| 摘要 |
A method and apparatus are provided for fixing a geographical reference (a line marker) of a vehicle, such as a pipeline pig, travelling through a pipeline. …<??>In the apparatus, (Figure 1) a magnetically activated detector 1 detects the presence of an approaching pig 6 and sends a signal via line 3 to switch on a transmitter unit 2. The unit 2 supplies an AC current via lead 5 to an AC coil 4 which generates a magnetic field for a preselected period (say 17 seconds). The flux signal is then sensed by a pair of crossed coil sensors 8a, 8b on-board the pig 6. The sensed signal is then processed in a unit on-board the pig 6 and finally a reference pulse is output for recording on a tape-recorder 11 also on-board the pig 6. …<??>The transmitter (Figure 2) includes a monostable 22 which controls the period for which the unit generates the alternating current. …<??>Footage pulses from a transducer (not shown) (Figure 4) are counted in counters 58 and 59 as soon as the sensors 8 sense the magnetic field as the pig nears the transmitter coil 4. The pulses are divided by 4 in a divider 51. The counted pulses are further divided and stored in a word comparator 60. When the magnetic field can no longer be sensed by the sensors 8, the divider 51 is now programmed to divide by 2 and these pulses are further divided and stored in the comparator 60. When the stored pulse count A coincides with a preset count B input to the word comparator 60 from an input 61, a monostable 61 outputs a line marker pulse 62 which is recorded on a tape recorder. The line marker pulse 62 alwasy therefore occurs at a fixed distance from the transmitter coil 4. |
