Before solving the detection method of metal pipeline under complex conditions, it is necessary to understand the basic working principle and basic structure of metal pipeline detector.
A primary alternating field source is applied to the metal pipeline through the emitting device, and an induced current is generated by excitation on it, and a secondary magnetic field is generated around it. See Figure 1.
The magnetic field closest to the pipeline is the strongest, and there is only a horizontal componentThe direction of the magnetic field away from the pipeline is tangential and can be decomposed into vertical and horizontal directions. The secondary magnetic field and its spatial distribution are measured on the ground by the receiving device, and then the location (planar and vertical) of the underground pipeline is judged according to the distribution characteristics of this magnetic field. See Figure 2
Figure 1: Schematic diagram of the working principle of the electromagnetic method.
Figure 2 Characteristics of the magnetic field distribution.
The transmitter consists of a transmitter coil and a set of electronic circuits, which are used to apply a signal current or magnetic field of a certain frequency to the pipeline. According to the principle of electromagnetic induction, there is an alternating magnetic field in the space around an alternating electromagnetic field, and if there is a conductor passing through the alternating magnetic field, an induced electromotive force will be generated inside the conductor, and if the conductor forms a loop, an electric current will be generated in the conductor. As shown in Figure 3.
Figure 3: Schematic diagram of the operating principle of the transmitter.
The signal application can be inductive, direct, clamping, etc., as shown in Figure 4 and Figure 5.
Figure 4 Direct Method.
Figure 5 Clamp method.
The induction method is the most widely used, and can be divided into three types: horizontal launch, vertical launch, and oblique emission according to the state between the coil and the ground.
1. Horizontal launch
The central axis of the emitting coil is parallel to the ground and perpendicular to the axis of the pipeline, as shown in Figure 6. Coupling to the pipeline is strongest when it is located directly above the pipeline.
Figure 6: Horizontal launch
Figure 7: Vertical launch.
2. Vertical launch
The central axis of the emitting coil is perpendicular to the ground, as shown in Figure 7When located directly above the pipeline, it is not coupled to the pipeline.
3. Tilt launch
Between horizontal and vertical launches, the axis of the emission coil is at a certain angle to the ground.
The receiver is composed of a receiving coil and a set of corresponding electronic circuits and signal indicators, which are used to detect the current signal of a specific frequency applied by the transmitter to the pipeline above the pipeline, as shown in Figure 8.
Figure 8: Schematic diagram of receiver measurement.
Fig.9. Single-horizontal coil
1. Single-horizontal coil receiver
The horizontal component generated by the main receiving line is shown in Figure 9. The instrument response is greatest when the coil shaft centerline is parallel to the ground perpendicular to the line and directly above the pipeline, and when the coils are on both sides of the pipeline, the instrument's response becomes smaller as the coils move away from the pipeline. This is because the farther away from the pipeline, the weaker the magnetic field at the coil is located, and the direction of the magnetic field is not perpendicular to the coil, and the magnetic flux through the coil decreases.
2. Single vertical coil receiver
The receiver coil mainly receives the vertical component of the magnetic field generated by the line, as shown in Figure 10. When the coil axis is perpendicular to the ground and directly above the pipeline, the instrument response is minimal, when the coils are located on both sides of the pipeline, the instrument response gradually increases as the coil moves away from the pipeline, and the response signal gradually decreases when the extreme value is reached.
Fig.10. Single vertical coil.
Figure 11: Double-horizontal coils.
3. Double-horizontal coil structure receiver
There are two horizontal coils parallel to each other in the receiver, and the position of the pipeline is determined by measuring the induced electromotive force difference of the upper and lower coils, and the buried depth of the pipeline is calculated, see Figure 11.
4. Three-coil structure receiver
The receiver has two horizontal coils parallel to each other, one vertical coil. The measurement of the above three types of coil structure receivers can be realized, as shown in Figure 12.
Figure 12: Common pipeline detector structures.
At present, the mainstream receiver has developed a multi-coil structure receiver on the basis of the three-coil structure receiver, which adopts the secondary difference and has stronger anti-interference ability, but its basic measurement principle is the same as that of the three-coil structure receiver, such as the VLOCPRO2 receiver has 5 signal receiving coils
Lower horizontal coil, lower vertical coil, upper horizontal coil, upper vertical coil, compass coil. Different combinations of 5 coils are used for signal reception in different positioning modes.
The double horizontal coil combination (lower horizontal coil + upper horizontal coil) has two functions:
1. Measure the peak positioning signal.
A double horizontal coil is used to measure the difference signal of the horizontal magnetic field of the pipeline perpendicular to the direction of the receiver meter, and the peak positioning signal is displayed in the form of numbers, bar graphs, audio, etc
2. Measure the direct reading depth.
The function of the double vertical coil combination (lower vertical coil + upper vertical coil) is to measure the positioning direction arrow signal of the pipeline, use the double vertical coil to measure the difference signal of the vertical magnetic field of the pipeline, indicate the direction of the pipeline relative to the receiver in the form of left and right positioning arrows, and display the distance of the pipeline relative to the receiver in the form of the number of positioning arrows.
The function of the compass coil is to measure the horizontal magnetic field signal of the pipeline parallel to the direction of the receiver meter, and to determine the deviation angle between the direction of the receiver and the direction of the pipeline by comparing it with the horizontal magnetic field signal of the pipeline perpendicular to the direction of the receiver meter, so as to indicate the direction of the pipeline with the guide pointer, and to indicate whether the direction of the receiver deviates from the direction of the pipeline with the color compass.
The function of the lower horizontal coil is to measure the broad peak positioning signal, and the lower horizontal coil is used to measure the horizontal magnetic field signal of the pipeline perpendicular to the direction of the receiver meter, and the wide peak positioning signal is displayed in the form of numbers, bar charts, audio, etc.
The function of the lower vertical coil is to measure the valley positioning signal, and the lower vertical coil is used to measure the vertical magnetic field signal of the pipeline perpendicular to the direction of the receiver meter, and the valley positioning signal is displayed in the form of numbers, bar charts, audio, etc.