MEASURING METHOD FOR DETERMINING THE STRUCTURAL AND WEAR CONDITION OF A COMPONENT

Description

[0001] The invention relates to a measuring method for determining a structural and wear condition of components, as specified in claim 1.

[0002] Growing cost pressure in global competition is forcing companies to vigorously implement efficiency-enhancing measures. Trouble-free operation must be ensured. Unplanned machine malfunctions must be avoided. The service life of systems, components, and structures must be optimally utilized. To achieve these goals, continuous and reliable monitoring of components and / or parts is of particular importance. Faults and signs of wear must be diagnosed in a timely manner, maintenance measures must be scheduled in a timely manner, and unexpected downtimes and / or equipment damage must be avoided.

[0003] Depending on the material used, the intended use, and the resulting load conditions, various wear phenomena and failure scenarios occur in components or building elements (e.g., steel beams, reinforcement in concrete). There are many different approaches to material monitoring in this regard.

[0004] WO 2018 / 143616 A1 describes a monitoring method in which an industrial furnace is provided with a refractory material mounted inside the furnace and a cable module integrated therein, the properties of which are monitored.

[0005] US 2016 / 0146697 A1 also describes a monitoring method in which a structure is monitored using integrated electrical conductors.

[0006] DE 10 2006 056 235 A1 describes a method for monitoring a cable protection hose of a robot.

[0007] JP 2000-002503 A describes another method for monitoring buildings with an integrated structural element with electrical cables.

[0008] JP H07 286982 A discloses a method for monitoring a ceramic component in which a conductor is placed on the component. The monitoring takes into account the skin effect, which is supposed to change when material changes such as cracks develop in the component.

[0009] US 3,272,003 describes a material fatigue indicator comprising a resistive element manufactured to exhibit properties specifically correlated with the fatigue life of a base structure whose fatigue life is to be indicated. This involves a grid of conductive material in the form of a foil, film, or wire applied to a base structure to be tested, and the change in resistance or temperature coefficient of resistance of the grid material is used to determine the fatigue life of the structure.

[0010] Wear phenomena that are easily measurable and follow predictable processes, such as the increase in mechanical play or the wear of a tool, can be monitored relatively reliably. An example of this is DE 10 2008 045 470 A1, which discloses a method for determining the wear condition of toolholders and cutters on milling drums.

[0011] Hidden structural damage and signs of wear in the form of material fatigue, for example, under dynamic loads, are more difficult to detect. DE 10 2004 023 824 B4 discloses a non-destructive acoustic testing method for examining largely homogeneous series components, such as railway wheels, for wear and structural damage.

[0012] Another common approach for monitoring dynamically mechanically loaded components is the use of force-torque sensors (strain gauges). Force-torque sensors are electronic components that can measure strain and compression in specific locations. Monitoring larger components or structures can be achieved by coupling several force-torque sensors into a network. DE 10 2017 213 667 A1 discloses such a network with a monitoring system for detecting the failure or malfunction of individual sensors in the network.

[0013] Certain components and structures require particularly high levels of effort, especially when it comes to detecting hidden structural damage and material fatigue. These include very large components and structures, such as pipeline installations, which require a large number of measurement points.

[0014] Components such as various flexible load-bearing spring and damping elements can currently only be examined to a limited extent for material fatigue and structural damage using non-destructive testing methods.

[0015] Components such as hydraulic or pneumatic media hoses cannot be monitored with force-torque sensors in terms of their dynamic load due to their inherent flexibility.

[0016] For all of the components mentioned, it has so far been the case that in-situ component monitoring during ongoing operation is more complex to carry out than regular inspections in designated workshops or laboratories.

[0017] What is desirable is a system with which large structures and components with a high degree of inherent flexibility can be monitored in situ during ongoing operation in an economically viable, safe and non-destructive manner.

[0018] Based on the testing and monitoring methods described, the present invention aims to propose a measuring method that enables an economically viable, non-destructive and reliable determination of the structural or material condition of dynamically mechanically loaded components with high inherent flexibility.

[0019] This object is achieved according to the invention by a method having the characterizing features of claim 1, which uses insulated electrical cables as indicators to detect structural damage and wear phenomena of components due to dynamic mechanical stress.

[0020] Advantageous embodiments are the subject of respective subclaims.

[0021] The method according to the invention uses indicators whose material properties in the surface region or on the surface, in particular their surface roughness, increase under dynamic mechanical stress. Tests have shown that electrically conductive components of electrical cables, including many data transmission cables, are suitable for evaluating this effect for the desired measurement. Electrical cables, connected to a component in a manner according to the invention as a type of indicator, move at least largely synchronously with the component. The greater the movement or the longer the movement lasts, the greater the surface roughness of the electrical conductor of the electrical cable becomes in correlation with the component to be monitored. The increasing surface roughness of the electrical conductor reflects the progressive wear of the component.

[0022] The surface roughness of an electrical conductor is measured using the so-called skin effect. An electrical signal in the form of a high-frequency alternating current is introduced into the electrical conductor of an electrical cable. The current flow is concentrated on the surface of the electrical conductor. The respective surface roughness of the conductor through which it flows changes the resistance value of the electrical conductor, particularly in the peripheral area close to the surface. The output signal is used to determine the current surface roughness of the electrical conductor.

[0023] In a particularly preferred embodiment of the method according to the invention, an alternating current with a frequency of 800–1000 MHz is used, which flows along the circumference of an electrical conductor in a region of approximately 2 µm thickness. Using such an alternating current, surface roughnesses of an electrical conductor down to approximately 2 µm were reproducibly measured. A surface roughness of approximately 2 µm on an electrical conductor means that the respective electrical conductor has completed approximately 80% of its service life and is therefore ready for replacement. The typical increase in surface roughness occurs at every point of dynamic mechanical loading of an electrical conductor.

[0024] This applies, for example, to copper. With other conceivable sensor materials following the inventive concept (e.g., to adjust a different sensor sensitivity), the same effect (roughness) will be stronger or lesser and—going beyond the inventive concept—is detected with an individual indicator frequency according to particularly preferred embodiments.

[0025] In another preferred embodiment, an electrical cable acts as a sensor along its entire length. This results in a particular advantage of a method according to the invention: it can detect wear or structural damage to a component not just at specific points, but over an entire distance. The course of the distance is determined almost arbitrarily by appropriately laying the electrical conductor. The length of the electrical conductor(s) acting as a sensor cable can vary according to the invention from a few millimeters to several hundred meters in order to provide a suitable monitoring response for various monitoring tasks. For example, the positions of local damage to the sensor cable(s) can be determined using time-distance evaluations of the measured values ​​used in the invention.

[0026] Complex interconnections of sensors for measuring a distance or area are not necessary. The method according to the invention is therefore suitable for a wide variety of components and structures. For example, it is possible to monitor hose lines for hydraulic or pneumatic systems on highly mobile industrial robots. At the same time, the methods according to the invention are also suitable for monitoring, for example, masts or antennas exposed to dynamic loads with not precisely quantifiable load peaks caused by wind pressure, or for monitoring load-bearing components such as steel girders.

[0027] The small installation space requirement and the comparatively low costs for the sensor cable(s) allow the construction of redundant systems by laying two or more electrical conductors in parallel.

[0028] The electrical cables acting as wear sensors can be attached both to the surface of the components to be monitored and inside the component. With the method according to the invention, sensor cables can be integrated into the component during production, thus enabling non-destructive monitoring of the component's internal structure throughout its entire service life.

[0029] The connection of a sensor cable according to the invention to a component must be designed in such a way that component movements are transmitted to the sensor cable. For example, the connection can be made by gluing, clamping, or other known fastening solutions.

[0030] The method according to the invention can be applied in all areas where electrical cables can be used. This particularly includes working environments that only allow the use of sensitive sensors to a limited extent, such as in areas exposed to aggressive substances, high or low temperatures, increased dust development, or strong vibrations.

[0031] Due to the relatively small installation space requirement for the sensor cable(s), the method according to the invention is suitable for components with limited installation space; in many cases, subsequent installation into existing installations is also possible.

[0032] Prefabricated individual components or spare parts can be provided with pre-assembled sensor cables. Electrical cable types suitable for conducting an electrical wave, such as coaxial cables or twisted-pair cables, are used in particular according to the invention. The type of electrical cable used as an indicator has a significant influence on the significance of the monitoring measurement performed. The use of electrical cable types suitable for conducting at least one electrical wave leads to measurement results, i.e., output signals from the sensor cable(s), that can be directly compared with reference signals. This enables a component monitoring mode that allows for meaningful wear diagnosis even with regular measurements lasting only a few seconds.The use of electrical cable types that are not suitable for carrying at least one electrical wave produces measurement results, i.e., output signals whose signal characteristics must be evaluated. Continuous monitoring is required for condition diagnosis.

[0033] The method according to the invention thus allows both continuous monitoring of components and structures and, when using suitable sensor cables, regular querying of the component's condition. Continuous monitoring with evaluation devices constantly connected to the sensor cable(s) provides direct information about the component's condition and also about changes in the wear pattern. For example, an accelerated increase in surface roughness and thus in the measured values ​​indicates increased component movement. This can indicate incipient structural failure, for example, due to delamination or incipient crack formation.

[0034] The periodic inspections of components that are possible when using electrical cables suitable for guiding at least one electrical wave also reveal the wear progress of a component or structure, since the dynamic mechanical stress accumulated in the surface roughness of the electrical conductor of the sensor cable can be measured and evaluated using the method according to the invention.

[0035] A device (not the subject of the present invention) for carrying out the measuring method according to the invention consists in a preferred embodiment of a power supply, a signal generator, a signal evaluation unit, a data logger for recording relevant data, in particular the RF resistance, and one or more interfaces for contacting the sensor cable(s).

[0036] Device components of preferred embodiments are provided in a portable form with their own power supply, for example, via batteries, for the flexible implementation of in-situ measurements. For parallel implementation of the measurement method according to the invention on multiple electrical conductors, the measuring device must be equipped with a corresponding number of input and output interfaces for the measurement signals.

[0037] Preferably, coaxial cables are used in the device (not subject of the present invention) and the method according to the invention.

[0038] When using an electrical cable suitable for conducting an electrical wave as a sensor cable, a regular resistance measurement can be performed. The dynamic mechanical stress accumulates in the described structural changes of the electrical conductor, which in turn changes the resistance of the conductor. Based on the measured resistance value, the depth of the structural changes and thus the degree of wear of the conductor can be calculated.

[0039] When using electrical cables that are not suitable for conducting electrical signals, the resistance value is also measured for diagnosis. However, in this case, it is not the measurement result itself that is meaningful, but rather the progression of the signal changes. This means that continuous monitoring is required to observe the progression of the signal changes.

[0040] The device (not the subject of the present invention) comprises, in addition to the sensor cable(s), at least one measuring device, preferably including a power supply, a signal generator / signal source, a signal evaluation device, a data logger and, if necessary, physical and electronic interfaces for separating the sensor cables from the rest of the device and / or for connecting the device to a central computer or a machine control system.

[0041] The sensor cables can run on the surface of components or inside the component.

[0042] Sensor cable lengths can range from a few millimeters to several hundred meters (final limits not yet determined).

[0043] The sensor cables can be laid either in a loop or, if a terminal resistor is used, simply.

[0044] The resistance value of a terminal resistor attached to the end of the sensor cable corresponds to the internal resistance of the monitoring cable in question.

[0045] A monitoring device can be designed in a stationary or portable version.

[0046] Another exemplary monitoring device (not subject of the present invention) monitors several sensor cables in parallel, assuming a corresponding number of measuring inputs and outputs.

[0047] A particularly advantageous use generally provides that an electrically measured value on the component serves to indirectly describe the component wear.

[0048] The invention is explained in detail below with reference to the following drawings. Figure 1 an electrical conductor with a schematic representation of the surface damage, Figure 2a detailed view of surface damage, Figure 3 a conductor cross-section with schematically drawn penetration depth δ (skin depth), Figure 4 a schematic representation of an example component, with load vectors entered, Figure 5 a schematic representation of an example component with a sensor cable and terminal resistor laid on the component surface, Figure 6 a schematic representation of an example component with a sensor cable laid as a loop on the component surface and Figure 7 a schematic representation of an example component with integrated sensor cables.

[0049] Figure 1shows a perspective view of a section of an electrical cable 1, in an embodiment particularly preferred as a sensor cable for the method according to the invention as a coaxial cable, suitable for guiding at least one electrical wave. Shown are an electrical conductor 2 inside the cable structure and insulation 3 surrounding the cable structure. The electrical conductor 2 consists of a metallic material commonly used for this purpose, preferably copper or aluminum or suitable alloys based on these metals. Schematically shown are surface damage 4 of the electrical conductor 2, which occurs when the electrical cable 1 is subjected to dynamic mechanical stress and increases with continuous dynamic mechanical stress on the electrical cable. Possible directions of movement 5 of the electrical cable 1 as a result of dynamic mechanical stress are indicated by arrows.

[0050] Figure 2shows, in a schematic detailed view as a greatly enlarged excavation, the changes in a metal structure due to dynamic mechanical stress. The metal structure, consisting of grains 8, forms, under dynamic mechanical stress, starting at a surface 7 of the electrical cable 1, partial structural changes in the form of sliding zones 9. The material of the sliding zones 9 shifts relative to one another due to the dynamic mechanical stress and in doing so forms depressions referred to as intrusions 6 and elevations referred to as extrusions 10 relative to the surrounding material of the electrical conductor 2. Relevant for a wear diagnosis according to the method according to the invention is the average distance 11 between the highest points of the extrusions 10 and the lowest points of the intrusions 6 at various locations over a certain length of an electrical conductor to be assessed.If this average distance is 2 µm for a metallic electrical conductor 2, the respective electrical conductor 2 has completed approximately 80% of its service life and is therefore ready for replacement. Therefore, significant wear has already been diagnosed, and a conclusion can be derived from the method according to the invention.

[0051] Figure 3 shows a schematic cross-sectional view of an electrical cable 1. An electrical conductor and surrounding insulation 3 are clearly visible. A dashed line encircling the electrical conductor indicates the skin depth 15. Alternating currents concentrate, depending on their frequency, in a certain area along the surface of an electrical conductor 2. The value for the thickness or depth of the area flowing through, measured from the surface of the electrical conductor radially towards the center, is referred to as the skin depth 2.

[0052] Figure 4shows a schematic view of an exemplary component 20. Dynamic mechanical forces acting on the component 20 are represented as vectors 21. Areas of the component 20 that may be subject to elastic and / or plastic deformation and thus wear due to the acting mechanical dynamic forces are represented as deformation zones 22.

[0053] Figure 5shows schematically, using an exemplary component 20, an advantageous embodiment of a measuring device according to the invention for component monitoring, with the arrangement of an electrical cable 1 as a sensor cable on the outside of the component 20. The electrical cable 1 is laid along the component to be monitored, starting from a measuring device 35 arranged on the left in the view, and is connected to the component 20 in a suitable manner not shown in detail. The electrical cable ends in a resistor 31, which can prevent the electrical cable 1 from being fed back to the measuring device 35 in a loop. In the advantageous embodiment shown, the electrical resistance value of the resistor 31 corresponds to the internal resistance of the upstream electrical cable 1.

[0054] Figure 6shows schematically, in a further advantageous embodiment of the exemplary measuring device (not the subject of the present invention), an exemplary component 20 with an electrical cable 1 laid as a loop.

[0055] Figure 7 shows schematically, using an exemplary component, a further advantageous measuring device (not the subject of the present invention).

[0056] Schematically illustrated are particularly preferred components contained in a measuring device 35, a power supply 36, a signal generator 37, a signal evaluation 38 and a data logger 39. The components of the measuring device 35 are designed in the embodiment shown so that several electrical cables 1 can be connected. Four electrical cables 1 are clearly shown, which run from the measuring device 35 through an interface 40 into the component 20. The electrical cables 1 are laid simply and each end in a resistor 31. The interface 40 is designed so that a physical and electronic separation of the electrical cables 1 can take place there, for example in order to be able to use the measuring device 35 for the regular component monitoring of several components 20. An interface 40 could, for example, be implemented by a plug or a connection terminal.In the advantageous embodiment of the measuring device shown, the electrical cables 1 are laid inside the component 20, preferably in areas that are particularly relevant for component monitoring. List of reference symbols

[0057] 1Electrical cable 2Electrical conductor 3Insulation 4Surface damage 5Bending direction 6Intrusion 7Surface 8Grain 9Slip zone 10Extrusion 11Mean distance 15Skin depth 20Component 21Vector 22Deformation zone 31Resistance 35Measuring device 36Power supply 37Signal generator 38Signal evaluation 39Data logger 40Interface

Claims

1. Monitoring method with respect to wear or structural damage of components due to dynamic mechanical loading for the prediction of a residual service life of the component, in which method an electrical cable having at least one metallic electrical conductor is used as a sensor or indicator, wherein the electrical cable is attached to or arranged in a component to be monitored, with the result that dynamic mechanical loads and thus deformations of the component are transferred to the electrical cable, comprising the following steps: - introducing an electrical signal in the form of a relatively high frequency alternating current into the electrical conductor of the electrical cable in such a way that the current flow is concentrated on the surfaces of the electrical conductor through the skin effect; - measuring the resistance value of the conductor; - determining the surface roughness of the electrical conductor from the resistance value; - predicting the remaining service life of the component based on the value of the surface roughness of the electrical conductor.

2. Monitoring method according to Claim 1, characterized in that changes in a material surface in the form of elevations and depressions are monitored.

3. Monitoring method according to either of the preceding claims, characterized in that, in the case of copper as sensor cable material, alternating currents of a frequency between 800 and 1000 MHz are used for measuring.

4. Monitoring method according to one of the preceding claims, characterized in that a signal evaluation by means of an impedance measurement is carried out.