Method for inspecting a metal concrete-prestressing element of a pipeline, and inspection device

Abstract

The invention relates to a method for inspecting a metal concrete-prestressing element of a pipeline, in particular a water-conducting pipeline, wherein: an alternating magnetic field is generated by means of at least one exciter unit, preferably comprising an exciter coil, of an inspection device; during a movement of the inspection device along the concrete-prestressing element, first measurement data are recorded on the basis of a resulting magnetic field at at least one first position by means of at least one first magnetic field sensor, preferably comprising a first sensor coil, of the inspection device; during the movement of the inspection device along the contrete-prestressing element, second measurement data are recorded on the basis of the resulting magnetic field at at least one second position by means of at least one second magnetic field sensor, preferably comprising a second sensor coil, of the inspection device; as viewed in a longitudinal direction of the pipeline, the magnetic alternating field is generated downstream of the first position and in particular upstream of the second position; and a signal arising from the first measurement data and the second measurement data is evaluated with regard to the magnitude and / or phase of the resulting magnetic field.

Description

[0001]Method for Inspecting a Metallic Concrete Tensioning Element of a Pipeline and Inspection Device. The present invention relates to a method for inspecting a metallic concrete tensioning element of a pipeline, particularly one carrying water, for example, a concrete tensioning wire that is spirally embedded in the pipeline, particularly in its concrete. The invention further relates to an inspection device with which such a method can be carried out. Concrete pipelines are often used for transporting water. These pipelines have a cylindrical metal layer or a metal cylinder that is coated with concrete on the inside and outside. On the outside, the concrete is reinforced by one or more concrete tensioning elements, which are designed as wires or strands of wire. Such concrete tensioning elements absorb the tensile stresses acting in the concrete and significantly increase the strength of the pipeline.Over time, the concrete prestressing elements can break, for example due to corrosion, leading to an increased risk of pipeline failure. Due to the complex and inhomogeneous structure of the pipeline wall, conventional non-destructive testing methods (NDT methods) based on ultrasound or magnetic flux leakage measurements are not accurate enough. This is especially true for inspection devices moving inside the pipeline. Inspecting the concrete prestressing elements from outside the pipeline is also often not feasible with reasonable effort, as the pipelines are buried underground. A method frequently used for pipeline inspection is inline inspection (ILI). In this method, the inspection tool, which is typically a pig, serves as a platform for measuring instruments.The pig is positioned inside the pipeline and is driven by the flow of the medium being transported. This allows inspection to be carried out without interrupting the flow of the medium. However, the ILI (Inspection and Testing Institute) places additional demands on the measuring instruments used, which further complicates their selection. Such measuring instruments must be compact enough to be embedded in the pig. Resistance to vibrations and dirt, for example, is also necessary. Furthermore, the pipeline construction can vary considerably. For instance, it may be designed with or without metal cylinders. The concrete tensioning elements can be cord-shaped, wire-like, or stranded, consisting of several wires. The concrete tensioning element can also be in the form of a spiral wire or individual, closed wire rings.Therefore, flexibly deployable inspection methods and devices for use in different pipeline constructions are desirable. US Patent 6,127,823 describes an attempt to locate defects, particularly fractures, in cord-like concrete prestressing elements, such as those designed as concrete prestressing wires, using so-called "remote field eddy current" methods. Due to the distance of two to three pipe diameters between the excitation unit and the sensor, the device's suitability for use in pipes is very limited. The accuracy of the measurement is also affected by the distance between the excitation unit and the sensor. Since the recorded signal corresponds to a combination of all anomalies from the area defined by the distance, it is difficult to interpret. EP 2458376 A2 further discloses the use of two wall-mounted sensors to measure a resulting magnetic field that varies due to a defect.This type of measurement is also very susceptible to interference. Furthermore, due to the sensors' proximity to the wall in commonly used pipes with so-called butterfly valves, a corresponding inspection device cannot be used. The object of the present invention is to increase the accuracy of investigations into the integrity of concrete prestressing elements and to provide a suitable device for this purpose. This object is achieved by a method according to claim 1, by an inspection device according to claim 14, and by an arrangement according to claim 25. Advantageous embodiments of the invention can be found in the dependent claims relating to these claims and in the following description.In the inventive method for inspecting a metallic concrete tensioning element of a pipeline, particularly one carrying water, an alternating magnetic field is generated by at least one excitation unit of an inspection device, preferably comprising an excitation coil. Furthermore, during movement of the inspection device along the concrete tensioning element, at least one first position, initial measurement data are recorded by means of at least one first magnetic field sensor of the inspection device, preferably comprising a first sensor coil, based on a resulting magnetic field. Additionally, during movement of the inspection device along the concrete tensioning element, at at least one second position, at least one second magnetic field sensor of the inspection device, preferably comprising a second sensor coil, records further measurement data based on the resulting magnetic field.In this process, an alternating magnetic field is generated in a longitudinal direction along the pipeline behind the first position and, in particular, in front of the second position. Furthermore, a signal derived from the first and second measurement data is evaluated with regard to the magnitude and / or phase of the resulting magnetic field. In the case of a defect in the concrete tensioning element, this signal, when used with the inspection device, provides a characteristic feature from which the presence of the defect can be inferred. The signal derived from the measurement data can comprise the measurement data itself or be an analog or digital signal generated from the measurement data, for example, through superposition and / or evaluation.The resulting magnetic field arises from the superposition of the direct alternating field acting directly from the excitation unit in the area of ​​the magnetic field sensors, and from the fields induced by metallic parts of the pipeline. The latter arise in particular from interactions within the metal cylinder, if present, and within the concrete prestressing element, or within any multiple concrete prestressing elements. The invention is based on the understanding that currents are generated in the circumferential direction within the concrete prestressing element, thereby generating an alternating magnetic field.While it is difficult to generate significant currents in a spirally wound concrete clamping element, which is not inherently a closed conductor, the invention has demonstrated that the conductivity of spirally wound concrete clamping elements, along with any axially laid concrete clamping elements or a potentially present metal cylinder, is high enough to electrically connect the sections of the spirally wound wire. This connection enables the induction of current even in spirally wound concrete clamping elements. In non-spirally wound, particularly round and closed, concrete clamping elements that span a plane perpendicular to the pipeline axis, the induced currents are already sufficiently large. A defect in the concrete clamping element will therefore cause a change in the measured signal.This is known in principle, whereby the alternating field signal generated by any existing metal cylinder and / or the directly measured signal is, in principle, significantly larger than the signal of interest based on the concrete prestressing element. Accordingly, with the setups known from the prior art, it has not been possible to measure the signal based on a fracture of the concrete prestressing element with sufficient accuracy compared to the much larger signal based on the alternating magnetic field acting directly from the excitation unit towards the magnetic field sensors, as well as a field induced in any existing metal cylinder. The provision of a second magnetic field sensor should at least substantially compensate for the signals recorded by the magnetic field sensors.This applies to the case of an intact concrete tension wire, which, like the metal cylinder or similarly acting metallic parts of a concrete pipeline, can be used to generate currents directed, for example, in the circumferential direction. Normally, the signals of the receiving coils align for any electrically conductive pipe structure that is symmetrical to the excitation coil of the inspection device. A defect destroys this symmetry, and the signals of the coils no longer match, resulting in a fault signal. In particular, alternating fields with a frequency in the range of 1 kHz to 25 kHz are generated, preferably for pipelines without a metal cylinder.In a frequency range below 200 Hz, and particularly below 100 Hz, and especially preferably between 10 Hz and 30 Hz, the excitation unit, according to a further development of the invention, can generate marker signals separately or superimposed on the signals provided for the examination of the concrete prestressing element. These marker signals are used to indicate the position of the inspection device and not for evaluation with respect to the concrete prestressing element, and can be received, for example, by a portable device outside the pipeline. Thus, the excitation unit simultaneously serves as a marker, and a transmitting coil of an otherwise separate marker can be omitted. If the pipeline under investigation, and in particular the concrete prestressing element, is intact, the signals recorded by the first and second magnetic field sensors are therefore at least largely identical. From these at least largely identical signals, or rather,The small differential signal in this case then indicates that no defect is present. However, if the structure is no longer intact due to a fracture of the concrete tensioning element, the measurement data recorded by the magnetic field sensors differ in their magnitude and / or phase, and a characteristic differential signal arises, particularly when the first and second measurement data are differentiated. This signal stands out clearly from the other measured signals and varies depending on the position relative to the fracture point. Since the significantly stronger signals are still largely compensated for, such a method exhibits high sensitivity for the desired defect-sensitive signal. According to the present analysis, each sensor signal contains a defect-sensitive (^. ^ ) Component and a defect-insensitive (^ ^ ) Component. The components of the first magnetic field sensor signal are represented by ^^ ^ and ^ ^ ^ The components of the second magnetic field sensor signal are accordingly denoted by ^ ^ ^^ and ^ ^ ^^ The defective component is described as follows: Both the defect-sensitive and the defect-insensitive components are caused by complex interactions of electromagnetic fields and currents in different parts of the pipeline. However, the defect-sensitive component depends essentially on the currents induced in the concrete prestressing element. The defect-insensitive component depends essentially on the alternating magnetic field generated directly by the excitation unit and the eddy currents induced in the cylindrical metal of the pipeline. The signal ^ ^ is significantly larger than ^ ^ , making it difficult to interpret the signal ^ ^ in the presence of ^ ^to reliably detect. By providing two magnetic field sensors, especially those switched differentially or evaluated with opposite signs, compensation of ^ can be achieved. ^ ′ and ^ ^ The alternating magnetic field can be represented at the first and second positions by a complex vector whose amplitude and its phase ^ ^ can be described as: Thus, the recorded measurement data can be displayed as a function of amplitude and phase over time or position along the pipeline. The phase and amplitude of ^ ^ ^ and ^ ^ ^The phase difference varies depending on the distance of the respective magnetic field sensor from the excitation unit and the frequency of the excitation signal. This effect is particularly noticeable in pipelines with metal cylinders. By selecting a specific frequency for the excitation signal and a specific distance between the magnetic field sensors and the excitation unit, a phase difference between these signals can be enforced. Depending on the phase difference between ^ ^ ^ and mainly affects the signal, the phase or amplitude of the signal ^ ^ ^ In practice, the signal is ^ ^ ^ − ^^ ^^ non-zero and usually larger than the variation of the signal ^ ^ ^ The smaller |^ ^ ^ − ^^ ^^ The more | it is, the easier it is to detect the change in the signal caused by the defect ^ ^ ^ to record. However, if the value of If the phase of the signal is too small, it is ^ ^ ^ difficult to determine. If the phase of the signal ^ ^ ^ To ensure accurate measurement, one of the magnetic field sensors can be adjusted to receive a slightly stronger signal, for example by changing the number of wire loops in the preferred coil, thereby enabling stable phase calculation of the signal. This is ensured. Compared to the prior art, the method according to the invention and the design of an inspection device described below allow, in some cases, a significant reduction in the measurement setup by at least a factor of two. At the same time, the method is considerably more sensitive with regard to detecting defects. Since comparatively small distances can be used between magnetic field sensors and the excitation unit, improved resolution and a particularly pipe-compatible device are achieved. Compared to inspection methods and inspection devices from the prior art in which several excitation units are used, the inspection device according to the invention also has a particularly low energy requirement. Coils are used in particular to generate the alternating electromagnetic fields; these are preferably arranged axially, i.e.,The magnetic field sensor consists of a magnetic field sensor with windings arranged symmetrically around the longitudinal axis or longitudinal center axis of an inspection device. The coil axis is located, in particular, along the longitudinal center axis of the inspection device. Hall effect sensors or GMR sensors can be used as magnetic field sensors. If the magnetic field sensor has a coil, its coil axis is preferably arranged parallel to, and especially coincident with, the longitudinal center axis. It is understood that for such an inspection device, which is designed to fit in pipelines, appropriate electrical components or electronics are provided. The magnetic field sensor is connected to these electronics for data acquisition.Furthermore, the inspection device according to the invention is equipped with other conventional means for autonomous travel in a pipeline, which may include energy storage means, data storage means, and, in particular, means for determining the position of the device in a pipeline. The inspection device is preferably designed as an autonomous inspection device and, in particular, for use in a method described above or below. The device is specifically designed as an autonomous inspection device such that it can travel through a pipeline without a cable connection for energy and / or data communication and thus without being tethered. It is designed for use in a method according to the invention, as described above or below.In a preferred embodiment of the invention, the first magnetic field sensor measures in the longitudinal direction of the pipeline, particularly the inspection device, upstream of the excitation unit, and the second magnetic field sensor measures in the longitudinal direction of the pipeline, particularly the inspection device, downstream of the excitation unit. For this purpose, the first magnetic field sensor is arranged upstream of the excitation unit in one longitudinal direction of the inspection device, and the second magnetic field sensor is arranged downstream of the excitation unit in the longitudinal direction of the inspection device. This enables a symmetry between the first and second measurement data in a structurally simple manner, and thus a compensation of these measurement data.The compensation is further simplified by arranging the magnetic field sensors one after the other in a longitudinal direction of the inspection device, and in particular symmetrically around a longitudinal central axis of the inspection device, in a further preferred embodiment of the invention, and by recording the resulting magnetic field at several positions located one after the other in the longitudinal direction of the inspection device. The magnetic field sensors are preferably spaced apart from each other in the longitudinal direction of the inspection device or record their respective measurement data at positions that differ from each other in the longitudinal direction. Preferably, the magnetic field sensors and the excitation unit are arranged one after the other in the longitudinal direction of the inspection device, with the excitation unit being arranged longitudinally behind the first magnetic field sensor and longitudinally in front of the second magnetic field sensor, or vice versa.Thus, the alternating magnetic field is generated by the excitation unit longitudinally behind the first magnetic field sensor and longitudinally in front of the second magnetic field sensor, i.e., between the first and second positions. Preferably, the excitation unit is also arranged symmetrically around the longitudinal center axis of the inspection device and generates, in particular, a full-circumference alternating magnetic field. When the inspection device moves along the concrete clamping element, its longitudinal direction is preferably parallel to, or lies on, the longitudinal center axis of the pipeline. The inspection device is preferably designed for movement within a pipeline of diameter D, and the distance between the magnetic field sensors is less than 2*D and, in particular, less than 1.5*D. This results in a particularly high degree of pipe maneuverability for the inspection device.Furthermore, the number of potential anomalies that could influence the measurement signal is reduced compared to the state of the art. Additionally, the spacing of the magnetic field sensors is particularly greater than 0.25*D and especially preferably greater than 0.38*D, preferably with the diameter of the coil-shaped magnetic field sensors being between 0.3*D and 0.75*D and especially between 0.55*D and 0.66*D. Viewed in the longitudinal direction of the inspection device and especially also in the longitudinal direction of the pipeline, the area spanned by the first magnetic field sensor, and viewed from the opposite direction, especially also the area spanned by the second magnetic field sensor, overlaps the area spanned by the excitation unit at least partially, preferably at least predominantly, and particularly completely.This particularly improves the bending capability of the magnetic field sensors, which are preferably located directly adjacent to any shielding elements or their mounting. Preferably, the axial component of the alternating magnetic field, relative to the longitudinal center axis of the pipeline, is measured. However, the measurement can also be performed for other components of the alternating magnetic field, such as the radial component. A supplementary measurement of the radial component can provide additional information about the defect, thus increasing accuracy. In a further preferred embodiment of the invention, the alternating magnetic field is axially aligned, i.e., it is generated in the axial direction. Axial alignment here refers to an alignment that is at least substantially axial with respect to the longitudinal direction of the pipeline.The alternating magnetic field can be generated, for example, by windings of a coil around a longitudinal central axis of the inspection device. This coil may also have a magnetic core extending along or encompassing the longitudinal central axis. Such a setup is particularly advantageous for spirally arranged concrete tensioning elements, which can be considered axially oriented structures, as the coils can be built sufficiently large and, in particular, can have a diameter of up to 75% of the diameter of the pipeline under consideration. During an inspection or measurement run with an inspection device, the measurement data recorded by the magnetic field sensors are either stored directly in the inspection device, for example, in the form of a pig, and processed there. Alternatively or additionally, the data can also be stored after completion of a measurement run or measurement run.The measurement data from the inspection device can be read out during a measurement run and evaluated in a standard computer system. Such an electronic data processing system includes, among other things, the usual components such as a processor, transistors, permanent memory, input and output interfaces, and means for controlling program sequences and evaluating the data. The computer system is designed to evaluate the measurement data with regard to amplitude and phase. The computer system can also be part of the inspection device, so that the detected defects and, if applicable, the associated raw data can be read out. Furthermore, the inspection device can include means for determining the position and / or orientation of the inspection device. The orientation and position data, for example, from gyroscopes or an odometer, can be combined with the measurement data from the magnetic field sensors to identify the location of a defect.Advantageously, an evaluation detects a defect in the concrete prestressing element based on two successive, and especially differing, extrema of the magnitude and / or phase of the resulting magnetic field. Such an oscillation-like signal waveform allows for simpler detection of the fracture in the presence of noise and is also easier to detect in the measurement data than the signals generated using prior art methods. For example, such oscillation-like signals can be detected using wavelet analysis based on the Haar wavelet. Wavelet analysis allows for consideration of potential signal elongation, for example, due to multiple successive disturbances or fractures of the concrete prestressing element.In particular, the received data are analyzed for wavelets that arise when the inspection device moves past the fracture point of the concrete prestressing element. A wavelet is typically defined as a wave-like oscillation of a signal, i.e., the attainment of a minimum with a direct transition to a maximum, or vice versa. When considering the real and imaginary parts of the measurement signal, regardless of any background noise, the movement is around the origin. Preferably, the alternating magnetic field is generated centrally between the first and second magnetic field sensors. For this purpose, the excitation unit is arranged centrally between the magnetic field sensors, which are spaced apart from each other in the longitudinal direction of the inspection device.A central arrangement refers to an arrangement in a central area between the magnetic field sensors, and in particular an arrangement that encompasses the exact center between the magnetic field sensors. Specifically, the first magnetic field sensor is at the same distance from the excitation unit as the second magnetic field sensor is from the excitation unit. The first measurement data is then acquired at the same distance from the excitation unit as the second measurement data. This promotes a symmetrical arrangement of the magnetic field sensors relative to the excitation unit and symmetrical acquisition of the measurement data, and simplifies the compensation of the respective defect-insensitive components of the first and second measurement data. Magnetic parts of the inspection device and / or manufacturing inaccuracies can lead to asymmetrical first and second measurement data despite a symmetrical arrangement of the magnetic field sensors.This imbalance can be at least partially corrected by shifting at least one of the magnetic field sensors longitudinally along the inspection device. During the acquisition of measurement data, the inspection device moves autonomously through the pipeline. Due to the resulting freedom from cables, large distances can be covered. Since, with a corresponding setup of the inspection device according to the invention, energy consumption can also be optimized due to the significantly better detectable signal compared to the prior art, even long distances of more than one kilometer of pipeline can be inspected without any cables. The movement of the inspection device is primarily passive, driven by the flow present in the pipeline, especially the water. Alternatively, it can also be an inspection device that moves actively through the pipeline using its own propulsion system.Combinations of actively and passively driven inspection devices are also conceivable. These devices initially perform only a preliminary, rough inspection and then, upon detecting relevant signals, switch from passive to active mode to return to the point against the flow for a more detailed inspection. In a preferred embodiment of the invention, the resulting magnetic field is measured in the center of the pipe by means of the first magnetic field sensor and / or the second magnetic field sensor. For this purpose, the first magnetic field sensor and / or the second magnetic field sensor is positioned relative to the center of the pipe during operation of the inspection device. The center of the pipe refers to its longitudinal midpoint. This arrangement allows the use of larger magnetic field sensors, particularly coils.Alternatively, the resulting magnetic field is measured using a first magnetic field sensor and / or a second magnetic field sensor located outside the center of the pipeline. This can be advantageous for detecting particularly small defects. In a further advantageous embodiment of the invention with multiple first magnetic field sensors and / or multiple second magnetic field sensors, the resulting magnetic field is measured using at least several first or second magnetic field sensors arranged radially offset from one another. The radial direction refers to a direction radial to the longitudinal center axis of the pipeline. For this purpose, the inspection device includes at least one further first and / or at least one further second magnetic field sensor, which is arranged radially offset from the at least one first or second magnetic field sensor.Preferably, several first magnetic field sensors and / or several second magnetic field sensors are arranged symmetrically around the longitudinal center axis of the inspection device. The sensors, distributed circumferentially around the longitudinal center axis, can partially intersect it or, at least in the longitudinal direction, can at least partially overlap each other. This allows the position of a defect to be determined, particularly additionally in the circumferential direction. This improves the accuracy of the method. Alternatively or additionally, several excitation units arranged radially offset from one another can be provided. The excitation units can generate alternating magnetic fields that differ in time or in their excitation frequency, and these fields can be detected separately by each magnetic field sensor.If the signal of one of the excitation units deviates relative to at least one other excitation unit, conclusions can also be drawn about the position of the defect, particularly in the circumferential direction. In a preferred embodiment of the invention, the inspection device has several first magnetic field sensors spaced longitudinally apart from each other upstream of the excitation unit and / or several second magnetic field sensors spaced longitudinally apart from each other downstream of the excitation unit. This allows different first measurement data to be recorded at several first positions and different second measurement data to be recorded at several second positions. This enables the measurement of not only the amplitude and phase of the magnetic field, but also the change in the magnetic field along the longitudinal center axis of the pipeline.Individual first and / or second magnetic field sensors can be arranged centrally or eccentrically with respect to the longitudinal center axis. This reduces misinterpretations and provides additional information about the defect structure. Preferably, the alternating field generated by the excitation unit is attenuated by means of at least one shielding element in the direction of the at least one first magnetic field sensor and / or in the direction of the at least one second magnetic field sensor. This allows, in particular, the amplitude of the defect-insensitive component to be reduced. ^ ^ The shielding element can be made of a conductive and / or magnetic material. The shielding element is specifically positioned between the first magnetic field sensor and the excitation unit and / or between the second magnetic field sensor and the excitation unit. Due to the lower amplitude of ^ ^ ^A more precise balance is achieved between the magnetic field sensors, leading to higher sensitivity and allowing for a smaller distance between them, which in turn improves the measurement resolution. Preferably, alternating magnetic fields of different frequencies are generated sequentially and / or simultaneously, and the resulting measurement data are recorded by at least one first magnetic field sensor and / or at least one second magnetic field sensor. This approach is based on the understanding that the symmetry of the magnetic field recorded by the first and second magnetic field sensors can be disturbed, for example, by movement of the inspection device or by changes in the pipe wall. Such disturbances result in changes in the amplitude and phase of the signal in the measurement data, which can then be misinterpreted as a break signal of the concrete prestressing element.Such misinterpretations can be detected and internally eliminated by measuring at different excitation frequencies. Changes due to fractures in the concrete prestressing element differ significantly in their signal shape at different frequencies compared to changes due to, for example, tool movement or the wall structure. Measurements at different frequencies result in different fault signals at the same fault location. Thus, a defect acquires a specific pattern composed of signals at different frequencies, distinguishing it from noise. Within the scope of the invention, it was found that combined measurements at different frequencies not only enable better fault detection but also a better assessment of the number of fractures.When using different frequencies, a primary frequency can be selected as the one that, based on calibration data, produces the strongest interference signal in the event of a concrete prestressing element failure—for example, the one that exhibits the strongest phase shift. A second frequency can then be selected lower than the primary frequency, for example, a frequency that primarily results in an amplitude difference in the measurement signal. To identify other structural changes beyond a concrete prestressing element failure, at least one second frequency, also higher than the primary frequency, can be selected, as described above, to amplify skin effects of any existing metal cylinder and thus exclude any interaction with the concrete prestressing element.To generate different frequencies, the same excitation unit or multiple excitation units operating at different frequencies can be used on the same inspection device. It is also possible to combine inspection runs performed sequentially at different frequencies. Advantageously, in data analysis, an interference signal is reduced by subtracting the scaled amplitude values ​​from the phase values. For this purpose, corresponding optimal scaling factors for the amplitude signal, as well as any parameters of the measurement system, such as the excitation frequency, can be determined during a calibration process. This is based on the understanding that the interference signal resulting from a fracture of the concrete prestressing element manifests itself differently in amplitude and phase.For this purpose, the complex signal representation must be considered in the evaluation, since amplitude and phase are components of a complex-valued signal. To subtract any disturbance from the measured amplitude values, i.e., to be able to use the pure phase information, the signal is first normalized. The normalized or corrected complex signal is then used for further analysis. In an advantageous embodiment of the invention, the first and second measurement data are acquired using differentially connected first and second magnetic field sensors, or a difference between the first and second measurement data is calculated to evaluate the resulting magnetic field. Crucially, this involves compensation of the identical parts of the measurement data, particularly the defect-insensitive components. Thus, higher sensitivity with respect to the defect-sensitive component can be achieved in a simple manner.The use of an inspection device in which the distance between the magnetic field sensors is variably adjustable is particularly advantageous. This allows the inspection device to be adapted to different pipelines. Adaptability is achieved, for example, through an interchangeable central body, interchangeable modules of the central body on which the sensors are or can be mounted, or through a suitable mounting system, such as one based on a thread or a plug-in system, which allows the longitudinal distance between the magnetic field sensors to be adjusted. Finally, the problem formulated at the outset is solved by an arrangement according to claim 25.Such a device comprises an inspection apparatus according to the invention as described above or below, as well as an electronic data processing device, and is configured to carry out the method according to the invention as described above or below. This allows for a better identification of fractures in concrete prestressing elements in preferably water-carrying pipelines than previously possible in the prior art, thus enabling the operator of a pipeline to initiate targeted repair measures and carry out repairs to the pipeline. Further advantages and details of the invention can be found in the following description of the figures. Fig. 1 shows an object according to the invention in a perspective view, Fig. 2 shows the object according to Fig. 1 in a side view, Fig. 3 shows the object according to Fig. 1 in an operating position in a water-carrying pipeline, Fig. 4 shows an illustration of signal generation, Fig.Figure 5 illustrates a sensor setup with circumferential position resolution, Figure 6 shows a further illustration of the sensor setup for circumferential resolution, and Figure 7 shows another embodiment according to the invention. Individual technical features of the embodiments described below can also be combined with previously described embodiments and the features of one of the independent claims and any further claims to form objects according to the invention. Where appropriate, functionally identical elements are provided with identical reference numerals. An inspection device 2 according to the invention has an excitation unit 4 arranged concentrically to the longitudinal central axis 24, with an excitation coil 6 extending symmetrically around a longitudinal central axis of the inspection device 2 (Figures 1 and 2).The excitation unit 4 is controlled and operated by electronics located in the central body 22 of the inspection device 2, indicated by a dashed box 25. The excitation coil 6 generates an alternating magnetic field in the axial direction, i.e., in the direction of the longitudinal center axis 24. A first magnetic field sensor 12 is located in front of the excitation unit 4. This sensor has a coil 14 that also extends symmetrically around the longitudinal center axis 24. The diameter of the coil 14, viewed in the direction of the longitudinal center axis 24, can be, in particular, 75% to 100% of the diameter of the excitation coil 6. The coils 14 and 15 can have a greater length in the longitudinal direction of the inspection device (contrary to the schematic representation) than the excitation coil 6.A second magnetic field sensor 13 with a second coil 15, also arranged symmetrically around the longitudinal center axis 24 of the device 2, is positioned longitudinally offset from and behind the excitation unit 4. Shielding elements 20 are arranged between the excitation unit 4 and the magnetic field sensors 12, 13. As in other embodiments, these can generally be placed directly on the excitation unit 4, so that any inductive field resulting from an influence on the shielding elements 20 is attenuated as much as possible due to the resulting distance to the magnetic field sensors 12, 13. The inspection device 2 is equipped with a series of centering elements 18 in the form of spring-loaded arms with rollers at their ends, which center the device 2 as closely as possible in the middle of the pipeline as it travels through it.The inspection device 2 can be coupled to other functional components of an inspection pig via connection areas 16 and, for example, pulled through a pipeline by the pig. Alternatively, the inspection device 2 itself can also be designed as a pig with corresponding cups or discs for passive propulsion in a pipeline. The water-carrying pipeline 26 (Fig. 3) to be inspected has an interior space that is almost completely filled by the inspection device 2 in the longitudinal direction, i.e., in the direction of the longitudinal center axis 24. The pipeline 26 comprises a metallic, cylindrical layer in the form of a metal cylinder 28, which is covered on the inside by an inner concrete layer 30 and on the outside by an outer concrete layer 32.In the outer concrete layer 32, a concrete prestressing element 34, in the form of a concrete prestressing wire, is spirally wound around the longitudinal axis of the pipe, which ideally corresponds to the longitudinal center axis of the inspection device 2. This concrete prestressing wire is positioned close to the cylindrical metal layer 28 but does not touch it. Nevertheless, due to the small distance between the metal cylinder 28 and the concrete prestressing element 34, a sufficient current flow for measurement is possible. Additionally, connection points for the concrete prestressing wire may be present at the ends of the respective pipe segment of the pipeline 26. Depending on the position of the inspection device 2 in the pipeline 26, the presence of a defect 36 during passage through the pipeline results in a characteristic signal, shown in Fig. 4, with two successive extremes.The following section considers four different positions A, B, C, and D of the defect signal, each corresponding to a position of the inspection device 2, which is shown only schematically with the excitation unit 4 and the magnetic field sensors 12 and 13. At point A, an equilibrium exists in the pipeline 26 between the magnetic field sensors 12 and 13, and no relevant (differential) signal is measured. The induced current flows along the concrete prestressing element and partially passes into layer 28. The current through the metal layer 28 formed by the metal cylinder completes the circuit through the concrete prestressing element and that layer 28. Arrows 38 and 40, located outside layer 28, indicate the direction of current propagation through the concrete prestressing element. The spiral shape of the concrete prestressing element ensures that the current flow propagates along the pipe from one turn of the spiral to the other.The current flow induces a magnetic field, which is detected by the magnetic field sensors 12 and 13. Since the first and second measurement data recorded by these sensors are equal in magnitude, the effects cancel each other out, and initially no relevant (differential) signal is measured that could indicate a defect. Due to a defect 36, no current flow is possible in the subsequent section, and a difference exists between the first and second measurement data. This difference, or imbalance, in the recorded magnetic fields, primarily indicated by a current 40, manifests itself as a positive extremum marked by point B. This signal weakens again as the defect propagates through the pipeline. When the defect is located longitudinally at the same level as the excitation unit 4, symmetry is restored, and a zero crossing occurs at point C.Further movement of the inspection device in the pipeline, in this case towards the right edge of the figure, leads to an opposite signal, since the defect 36 now disrupts the propagation of the current in the concrete tensioning element in the area of ​​the magnetic field sensor 12. Due to the imbalance also indicated by arrow 38, a local minimum occurs in the evaluated measurement data and the resulting signal at point D. Further movement of the inspection device 2 towards the right edge of the figure then restores equilibrium, which is represented by a curve starting from point D that now again approaches the zero point. A possible sensor setup of the magnetic field sensors for determining the circumferential position of a defect is shown schematically in Fig. 5.Here, a plurality of five magnetic field sensors 12 are located longitudinally in front of, and a plurality of magnetic field sensors 13 are located behind, the excitation unit 4. The outer circumference of the respective coils of the magnetic field sensors 13 and 12 is shown schematically in Fig. 6, viewed longitudinally, i.e., in the direction of the longitudinal center axis 24. The middle coil is used to determine the defect position in the axial direction, i.e., in the direction of the longitudinal center axis. Analysis of the relative signal changes in the four other coils or magnetic field sensors 12 allows the determination of the position in the circumferential direction. For example, a comparison of the signals between the upper and lower magnetic field sensors 12 can be used to restrict the position of the defect to the upper or lower half of the pipeline.Another embodiment of an inspection device according to the invention is particularly suitable for pipelines carrying water or other media and equipped with butterfly valves. The inspection device 2 has a central body 22 which has a plurality of support elements 42 arranged articulated and / or fixed to the central body 22 (Fig. 7). If the support elements 42 are not articulated or do not have any bendable sections, the support elements 42 are at least sufficiently flexible so that, upon contact with a butterfly valve, the effective cross-section of the inspection device 2 is reduced to less than half the pipeline diameter. A propulsion element 44 is arranged at the forward end of the inspection device 2 in the direction of travel. This element is suspended in the medium and, due to the pressure exerted by the medium, causes propulsion and propagation of the device.This thrust element 44 also has a plurality of segments 48 spanned by tension rods 46, which, due to the articulated arrangement of the tension rods 46 on the central body 22, can also be bent, thereby reducing the effective cross-section of the inspection device 2 (viewed in its longitudinal direction). Curved sections 50 can be arranged at the ends of the support elements, which contact the inside of the pipeline to be inspected as gently as possible. Furthermore, odometers 52 can be arranged at the ends of the individual support elements 42 to better determine the position of the inspection device 2 in the pipeline. Within the central body 22, which can be modularly constructed with several sections, an excitation coil 6 (indicated by the dashed line 6) of an excitation unit 4 is arranged centrally around the longitudinal axis 24.A sensor coil 14 or 15 (indicated by the dashed lines 14, 15) of a magnetic field sensor 12 or 13 is arranged symmetrically around the longitudinal axis 24, respectively, before and after the excitation coil 6. These coils record first and second measurement data based on the resulting magnetic field, which are evaluated directly or indirectly in the form of a resulting signal regarding the magnitude and / or phase of the resulting magnetic field.

Claims

Claims 1. Method for inspecting a metallic concrete prestressing element (34) of a particularly water-carrying pipeline (26), wherein an alternating magnetic field is generated by at least one excitation unit (4) of an inspection device (2) comprising preferably an excitation coil (6), wherein, during a movement of the inspection device (2) along the concrete prestressing element (34), first measurement data are recorded at at least a first position by means of at least one first magnetic field sensor (12) of the inspection device (2) having preferably a first sensor coil (14), based on a resulting magnetic field.wherein, during the movement of the inspection device (2) along the concrete clamping element (34), second measurement data are recorded at at least a second position by means of at least one second magnetic field sensor (13) of the inspection device (2), preferably having a second sensor coil (15), based on the resulting magnetic field, wherein the alternating magnetic field is generated in a longitudinal direction of the pipeline (26) behind the first position and in particular in front of the second position, and wherein a signal resulting from the first measurement data and the second measurement data is evaluated with regard to the magnitude and / or phase of the resulting magnetic field.

2. Method according to claim 1, characterized in that the first magnetic field sensor (12) measures in the longitudinal direction of the pipeline (26) in front of the excitation unit (4) and the second magnetic field sensor (13) measures in the longitudinal direction of the pipeline (26) behind the excitation unit (4).

3. A method according to claim 1 or 2, characterized in that the magnetic field sensors (12, 13) are arranged one after the other in a longitudinal direction of the inspection device (2) and, in particular, symmetrically around a longitudinal central axis (24) of the inspection device (2), and the resulting magnetic field is recorded at several positions located one after the other in the longitudinal direction of the inspection device (2).

4. A method according to any one of the preceding claims, characterized in that the alternating magnetic field is axially aligned.

5. A method according to any one of the preceding claims, characterized in that a defect in the concrete tensioning element (34) is detected on the basis of two successive, and in particular different, extrema of the magnitude and / or phase of the resulting magnetic field. 6.A method according to any one of the preceding claims, characterized in that the alternating magnetic field is generated centrally between the first magnetic field sensor (12) and the second magnetic field sensor (13). A method according to any one of the preceding claims, characterized in that the resulting magnetic field is measured in the center of the pipeline by means of the first magnetic field sensor (12) and / or by means of the second magnetic field sensor (13).

8. A method according to any one of claims 1 to 6, characterized in that the resulting magnetic field is measured with a first magnetic field sensor (12) arranged outside the center of the pipe and / or a second magnetic field sensor (13) arranged outside the center of the pipe.

9. A method according to any one of the preceding claims with several first magnetic field sensors (12) and / or several second magnetic field sensors (13), characterized in that the resulting magnetic field is measured by means of at least several first or second magnetic field sensors (12, 13) arranged radially offset from one another.

10. A method according to any one of the preceding claims, characterized in that the alternating field generated by the excitation unit (4) is attenuated by means of at least one shielding element (20) in the direction of the at least one first magnetic field sensor (12) and / or in the direction of the at least one second magnetic field sensor (13). 11.Method according to one of the preceding claims, characterized in that alternating magnetic fields of different frequencies are generated successively and / or simultaneously and the resulting measurement data are recorded by the at least one first magnetic field sensor (12) and / or the at least one second magnetic field sensor (13).

12. Method according to claim 11, characterized in that, during data evaluation, an interference signal is reduced by subtracting the scaled amplitude values ​​from the phase values.

13. Method according to any of the preceding claims, characterized in that the first and second measurement data are acquired using differentially connected first and second magnetic field sensors (12, 13) or that a difference between the first and second measurement data is calculated to evaluate the resulting magnetic field. 14.Inspection device (2) for inspecting a metallic concrete tension element (34) of a pipeline (26), in particular a water-carrying pipeline, comprising at least one excitation unit (4) preferably comprising an excitation coil (6) for generating an alternating magnetic field, comprising at least one first magnetic field sensor (12) preferably having a first sensor coil (14) for recording first measurement data of a resulting magnetic field, and comprising at least one second magnetic field sensor (13) preferably having a second sensor coil (15) for recording second measurement data of the resulting magnetic field, wherein the at least one excitation unit (4) is arranged in a longitudinal direction of the pipeline (26) downstream of the first magnetic field sensor (12) and in particular upstream of the second magnetic field sensor (13).

15. Inspection device (2) according to claim 14, characterized in that the first magnetic field sensor (12) is arranged in a longitudinal direction of the inspection device. (2) is arranged in front of the excitation unit (4) and the second magnetic field sensor (13) is arranged in the longitudinal direction of the inspection device (2) behind the excitation unit (4).

16. Inspection device (2) according to claim 14 or 15, characterized in that it is designed as a self-contained inspection device (2) and in particular for use in a method according to one of the preceding claims 1 to 13.

17. Inspection device (2) according to one of claims 14 to 16, characterized in that the excitation unit (4) is arranged, in particular centrally, between the magnetic field sensors (12, 13) spaced apart from each other in the longitudinal direction of the inspection device (2). 18.Inspection device (2) according to any one of claims 14 to 17, characterized in that the inspection device (2) is designed for movement in a pipe (26) of a diameter D and the distance between the magnetic field sensors (12, 13) is less than 2*D and in particular less than 1.5*D.

19. Inspection device (2) according to any one of claims 14 to 18, characterized in that at least one of the magnetic field sensors (12, 13) is arranged centrally with respect to the longitudinal center axis (24) when viewed in the longitudinal direction.

20. Inspection device (2) according to claim 19 with multiple magnetic field sensors (12, 13), characterized by at least one further first and / or at least one further second magnetic field sensor (12, 13) which is arranged longitudinally and / or radially offset from the at least one magnetic field sensor (12, 13).

21. Inspection device (2) according to any one of the preceding claims 14 to 20, characterized in that a plurality of first and / or second magnetic field sensors (12, 13) are arranged around a longitudinal central axis (24) of the inspection device (2).

22. Inspection device (2) according to any one of the preceding claims 14 to 21, characterized in that the distance between the magnetic field sensors (12, 13) is variably adjustable.

23. Inspection device (2) according to any one of the preceding claims 14 to 22, characterized in that the first magnetic field sensor (12) and the second magnetic field sensor (13) are differentially connected. 24.Inspection device (2) according to one of the preceding claims 14 to 23, characterized in that a shielding element for attenuating the alternating magnetic field is arranged between the first magnetic field sensor (12) and the excitation unit (4) and / or between the second magnetic field sensor (13) and the excitation unit (4).

25. Arrangement comprising an inspection device according to one of the preceding claims 14 to 24 and an electronic data processing device, characterized by a configuration for carrying out a method according to one of claims 1 to 13.

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