DE102016219953B4 - Method and device for measuring the pretensioning force of a mechanical connection, in particular a screw connection - Google Patents

Abstract

Method for measuring and / or monitoring the pretensioning force of a mechanical connection that is established between a component and an object or other component via at least one connecting element that is subjected to tensile stress, in which the pretensioning force is measured with a capacitive pressure sensor that is placed in a flat base element ( 1) is integrated with a through opening (2) for the connecting element, the flat underlay element (1) being inserted during the production of the mechanical connection between a head or counterpart of the connecting element and the component, characterized in that a capacitive pressure sensor is used which Has at least three pressure-sensitive capacitors (CS1, CS2, CS3) around the through opening (2), capacities of the capacitors (CS1, CS2, CS3) which are established after the mechanical connection has been established for measuring the pretensioning force, and also the Capacitance of a reference capacitor s (CR1) is detected, which is arranged in an area of the base element (1) that is not loaded by the mechanical connection.

Description

Technical application area

The present invention relates to a method for measuring and / or monitoring the pretensioning force of a mechanical connection, in particular a screw connection, according to the preamble of claim 1. The invention also relates to a device designed to carry out the method.

In many technical fields, individual components have to be mechanically connected to one another or to, for example, fixed objects. Screw connections are frequently used for this purpose, in which the screw is loaded as a connecting element in tension. Monitoring of the mechanical connection over time is especially necessary for safety-relevant connections. For example, when building structures, neighboring components are often joined together by screws. These have to be loaded with a pre-tensioning force, which can be achieved by using torque wrenches when making the screw connection. If the screw jams when tightening, the set torque does not correspond to the preload force in the screw. The screw can also come loose over time.

State of the art

Up to now, the preload force for screw connections has in most cases only been set using a torque wrench. However, techniques are also known for measuring the pretensioning force during or after the screw connection has been produced using suitable sensors.

So it shows DE 10 2012 020 932 A1 a force measuring plate which has a plate element and a sensor system for detecting forces acting perpendicular to the plate surfaces. The sensor system is formed by at least one strain sensor, which is arranged on a peripheral surface of the plate element. In one embodiment, the force measuring plate is designed as an annular washer, which is inserted between the screw head or the nut and the component during the production of the screw connection. Since the washer slightly deforms when the screw connection is made, the preload force of the screw connection can be measured with the strain sensor.

From the DE 10 2009 043 267 A1 a system for monitoring a screw connection is known, in which in one embodiment a capacitive pressure sensor is used which is integrated in a washer for the screw connection. The prestressing force of the screw connection can then be determined by detecting the capacitance of the capacitive pressure sensor.

The DE 198 54 062 C1 describes a device and a method for determining and logging the prestressing force in screw connections. This involves logging the tightening process and compliance with the pretensioning force (during the tightening process). A capacitive pressure sensor is used to measure the preload, which is integrated in a washer for the screw connection.

WO 2006/136 182 A1 describes a pressure sensor which does not work according to the capacitive but according to the piezoelectric principle and has a large number of piezoelectric transducer elements.

The DE 198 31 372 A1 shows a device for checking non-positive connections, in which a measuring system is at least partially integrated in a washer-shaped element. Attention is also drawn to the use of a capacitive pressure sensor with an elastic dielectric.

In the DE 101 38 261 A1 describes a sensor device for monitoring the force required for a non-positive connection between workpieces. Here, reference is only generally made to a pressure-sensitive capacitive sensor element which is designed as a washer for screw connections.

The DE 44 19 009 A1 shows a preload force transducer for screw connections, in which the washer is provided with a radial bore, the deformation of which is detected by capacitive sensors.

However, many of the previously available solutions for measuring the preload force of a screw connection do not enable sufficiently cost-effective production to be produced and used in large quantities. Checking the preload is also often unreliable, as incorrect measurements can occur due to the screws becoming jammed.

The object of the present invention is to provide a method and a device for measuring the pretensioning force of a mechanical connection via a connecting element which is subjected to tension, which can be implemented inexpensively and enable reliable detection of the pretensioning force even over a longer period of time.

Presentation of the invention

The object is achieved with the method and the device according to patent claims 1 and 9. Advantageous embodiments of the method and the device are the subject of the dependent claims or can be found in the following description and the exemplary embodiment.

In the proposed method, the prestressing force is measured with a capacitive pressure sensor, which is integrated in a flat base element with a through opening for the connecting element, which is inserted between a head or counterpart of the connecting element and the component during the production of the mechanical connection, in particular a screw connection . When a screw connection is produced, the screw thus represents the connecting element, the underlay element being inserted between the screw head and the component or between a nut as a counterpart of the screw and the component. Of course, one or more washers can also be present as intermediate elements. In the method and the proposed device, the through opening can represent an opening (hole) which is completely enclosed by the underlay element, but can be open towards a narrow side of the through element, that is to say form a slot in the underlay element which is open on one side. In this embodiment, the washer element is thus designed as a fork washer, which has an outer shape similar to a fork cable lug. With this configuration, it is possible to insert the support element laterally over the connecting element only after the connecting element has been loosely connected to the counterpart (e.g. screw with nut). The proposed method is characterized in that a capacitive pressure sensor is used to measure the pretensioning force, which has at least three pressure-sensitive capacitors around the through-opening, capacities of the at least three pressure-sensitive capacitors that arise after the mechanical connection has been established, directly or indirectly, for example via the frequency of an oscillating circuit formed with these capacitors.

By using a capacitive pressure sensor with at least three pressure-sensitive capacitors around the through-opening, the proposed method enables a more reliable measurement of the pretensioning force, since a tilting of the connecting element can be recognized immediately. In the case of a uniform connection, for example uniform tightening of a screw connection, the at least three pressure-sensitive capacitors are loaded to the same extent, whereas tilting is present when the load is different. By integrating the capacitive pressure sensor into a flat base element that remains between the head or counterpart of the connecting element and the component, the pretensioning force of the mechanical connection can also be measured or monitored over a longer period of time. In this way it can be determined whether the pretensioning force has decreased immediately after the connection has been made or has decreased over time.

The device proposed for carrying out the method is correspondingly designed as a flat base element with a through opening for the connecting element and has the capacitive pressure sensor. The underlay element can also be composed of several parts. The capacitive pressure sensor has at least three pressure-sensitive capacitors around the through opening and electrical connections via which the respective capacitors of the capacitors can be detected.

In the proposed method and the associated device, the underlay element is designed in such a way that it also has an area that is not loaded by the mechanical connection and is not influenced by the pretensioning force of the mechanical connection. In this unloaded area, another capacitor is arranged as a reference capacitor, the capacitance of which is also detected. Capacitance changes can be compensated for by forming the ratio of the capacitances of the capacitors in the loaded area, hereinafter also referred to as measuring capacitors, or the quantities, in particular frequencies, measured to detect the capacitances and the capacitance of the reference capacitor in the unloaded area or the correspondingly measured size that are not caused by the mechanical connection. This applies in particular to the compensation of temperature effects. The unloaded area is preferably additionally mechanically decoupled from the loaded area by a suitable partially circumferential slot in the underlay element or at least one of the carrier plates of the underlay element (when using the above sandwich structure). The slot encloses the unloaded area with the reference capacitor by at least 180 °.

In a preferred embodiment, the capacitive pressure sensor is formed by a dielectric between two metallic layers, of which at least one first metallic layer is separated into a plurality of regions, so that the plurality of pressure-sensitive capacitors, hereinafter also referred to as, through the two metallic layers with the intermediate dielectric Measuring capacitors referred to be formed around the through hole. The dielectric applied to the capacitors is deformed by the force applied when the mechanical connection is established, and the capacitance of the capacitors is thus changed. The respective capacitance or a change in the capacitance can be recorded and evaluated via the electrical connections. For example, the recorded capacitance can be converted into the applied force, ie the pretensioning force, using a calibration curve. There is also the possibility of forming a stack of several of these capacitive structures made of first and second metallic layers with an interposed dielectric in order to achieve higher capacitances by connecting the measuring capacitors one above the other in parallel. The capacitive structures are separated from one another in the stack by an intermediate insulation layer.

In a preferred embodiment, the metallic layers of the capacitive pressure sensor are formed in a ring around the through opening. The first metallic layer is preferably separated into at least three ring segments, through which the individual measuring capacitors are formed in connection with the second metallic layer and the dielectric. When using the stack described above, this separation into ring segments accordingly affects all the first metallic layers of the respective capacitive structures.

The design of the capacitive pressure sensor with the two metallic layers and the interposed dielectric enables the proposed device to be produced very inexpensively. In this way, the two metallic layers can each be produced on separate carrier plates or boards, between which the dielectric is then inserted during the final assembly of the sensor. The entire underlay element is preferably formed in the form of a sandwich structure from the two carrier plates or boards with the dielectric in between. The non-segmented metallic layer can also be realized by a metallic housing part into which the circuit board with the segmented metallic layer is inserted. There is also the possibility of realizing the non-segmented metallic layer with a spring washer or spring washer.

It is also possible to manufacture a single-layer to multilayer board in which the carrier material forms the dielectric and the copper surfaces and segmented copper surfaces form the capacitor plates. Furthermore, there is the possibility of already applying signal processing electronics to one or both carrier plates or circuit boards or to the single-layer or multi-layer circuit board, hereinafter also referred to as sensor board, by means of which the respective capacitors of the capacitors are detected and provided in a suitable form at an output connection. This can be done, for example, by embedding a chip with a microcontroller in the sensor board. The microcontroller can also contain a temperature sensor, which can be used to carry out an accurate temperature measurement for a fine calibration of the pressure sensor, if required. There is also the option of making the transmission of the corresponding measurement signals contactless, for example by using RFID technology. For this purpose, a corresponding RFID unit is then applied or integrated on one or both carrier plates or boards or on the single or multi-layer board. Alternatively, the above microcontroller can also have an RFID function. Automatic capacitor switching, phantom power or other techniques for operating the pressure sensor can also be implemented in order to minimize the number of connecting lines to the pressure sensor.

The signal processing electronics are preferably designed in such a way that they each convert the detected capacitances into a frequency. This advantageously eliminates capacitive influences for further signal processing.

The proposed method and the proposed device can be used particularly advantageously for checking and / or monitoring the pretensioning force in the production of screw connections. However, there is also the possibility of using the method and the device for other types of mechanical connections which are produced via a connecting element which is subjected to tension. An example of this is rivet or pin connections.

Figure list

• 1 eine schematische Darstellung einer als Unterlegscheibe ausgebildeten Vorrichtung gemäß der vorliegenden Erfindung;
• 2 ein Beispiel für die Ausgestaltung der beiden Metallisierungsschichten auf zwei Platinen, aus denen die vorgeschlagene Vorrichtung zusammengesetzt wird;
• 3 ein Beispiel für eine Seite einer der beiden Platinen der 2, auf der die elektronische Schaltung zur Signalverarbeitung sowie die elektrischen Anschlüsse integriert sind;
• 4 ein Beispiel für eine elektronische Schaltung zur Umsetzung der Kapazitäten in Frequenzen; und
• 5 ein Beispiel für eine Realisierung des vorgeschlagenen Unterlegscheibensensors mit Gehäuse.

The proposed method and the associated device are explained in more detail below using an exemplary embodiment in conjunction with the drawings. Here show:

• 1 a schematic representation of a device designed as a washer according to the present invention;
• 2nd an example of the design of the two metallization layers on two boards from which the proposed device is assembled;
• 3rd an example of one side of one of the two boards of the 2nd , on which the electronic circuit for signal processing and the electrical connections are integrated;
• 4th an example of an electronic circuit for converting the capacities into frequencies; and
• 5 an example of a realization of the proposed washer sensor with housing.

Ways of Carrying Out the Invention

The proposed device is explained in more detail below using an exemplary embodiment in conjunction with the drawings. 1 shows a schematic representation of an example of the configuration of the device in the form of a washer sensor, which is inserted in the proposed method, for example between the screw head and the component to be connected. In the upper part of the illustration there is a top view of this washer sensor 1 shown the corresponding through opening 2nd through which the screw is passed. Around this opening 2nd The pressure-sensitive measuring capacitors are formed around them, which cannot be seen in this illustration. The same applies to the signal processing electronics and the reference capacitor, which are in the lower area in the figure in the washer sensor 1 are integrated. In the present example, the washer sensor consists of two superposed boards, between which a film or layer of a dielectric is embedded. This sandwich construction is schematic in the cross-sectional representation of the lower part of the 1 to recognize the the first board 3rd , the dielectric 4th as well as the second board 5 shows.

When using this washer sensor, when the screw is tightened, a compressive force corresponding to the prestressing force of the screw is applied to the dielectric 4th exercised. This force changes the geometric strength of the dielectric 4th and thus the distance between the respective metallizations or metallization layers, between which the dielectric is located 4th located. The metallizations are on the two boards 3rd , 5 and each form with the intermediate dielectric 4th the measuring capacitors. The dielectric becomes due to the applied force 4th deformed and thus the capacitance of the respective measuring capacitor changed. This capacity or change in capacity is measured and can be converted into the applied force, for example, using a calibration curve. The washer sensor also enables permanent monitoring of the respective preload.

To determine a tilting of the screw, at least one of the metallizations is divided into several areas or segments, as a result of which several measuring capacitors are formed. If tightened evenly, the respective segments or capacitors are equally loaded, whereas if they are loaded differently, they tilt.

2nd shows an example of the design of the metallizations of the first board 3rd and the second board 5 . The figure shows the dielectric 4th facing sides of these boards. The first metallization 6 the first board 3rd is divided into three segments. The second metallization 7 the second board 5 is, however, as a closed ring around the through opening 2nd educated. When joining the two boards 3rd , 5 with an intermediate dielectric 4th are three measuring capacitors around the through opening 2nd formed around, whose capacities are measured via a signal processing electronics, not shown in this figure. In the lower part of the two boards 3rd , 5 is another metallization 8th to recognize by which a reference capacitor is formed for the measurement. This reference capacitor is located in an area that is not stressed by the screw connection to be measured. This is in addition in the second board 5 a partially circumferential slot 9 introduced the dielectric 4th additionally decoupled from the mechanical load by the screw connection in this area.

1. 1. Kupferfläche (Schirm),
2. 2. Platinenmaterial,
3. 3. segmentierte Kupferflächen (erste metallische Schicht bzw. Platte der Messkondensatoren),
4. 4. Dielektrikum verformbar,
5. 5. Kupferfläche (zweite gemeinsame metallische Schicht bzw. Platte der Messkondensatoren),
6. 6. Platinenmaterial,
7. 7. Kupferfläche (Schirm, kann entfallen).

The capacitive pressure sensor can also have further layers, for example for shielding against interference. For example, the capacitive pressure sensor could have the following stratification:

1. 1. copper surface (screen),
2. 2. circuit board material,
3. 3. segmented copper surfaces (first metallic layer or plate of the measuring capacitors),
4. 4. dielectric deformable,
5. 5. copper surface (second common metallic layer or plate of the measuring capacitors),
6. 6. circuit board material,
7. 7. Copper surface (screen, can be omitted).

The copper surfaces are all connected to the GND (ground), the segmented surfaces carry the signal. In this example, the two outer copper surfaces only serve as shielding against interference.

1. 1. Kupferfläche (Schirm),
2. 2. Platinenmaterial,
3. 3. segmentierte Kupferflächen (erste metallische Schicht bzw. Platte der Messkondensatoren, 1. Schicht),
4. 4. Dielektrikum verformbar,
5. 5. Kupferfläche (zweite gemeinsame metallische Schicht bzw. Platte der Messkondensatoren, 1. Schicht),
6. 6. Platinenmaterial,
7. 7. segmentierte Kupferfläche, (erste metallische Schicht bzw. Platte der Messkondensatoren, 2. Schicht),
8. 8. Dielektrikum verformbar,
9. 9. Kupferfläche (zweite gemeinsame metallische Schicht bzw. Platte der Messkondensatoren, 2. Schicht),
10. 10. Platinenmaterial,
11. 11. Kupferfläche (Schirm, kann entfallen).

It is also possible to stack a plurality of such capacitive structures of first and second metallic layers with an interposed dielectric in order to achieve higher capacities. With this stacking, however, the pressure sensor should no longer be composed of individual parts but are formed in one piece, whereby electrical connections between the layers in the unloaded part of the sensor are generated via vias. An example of a double layering according to this system:

1. 1. copper surface (screen),
2. 2. circuit board material,
3. 3. segmented copper surfaces (first metallic layer or plate of the measuring capacitors, first layer),
4. 4. dielectric deformable,
5. 5. copper surface (second common metallic layer or plate of the measuring capacitors, first layer),
6. 6. circuit board material,
7. 7. segmented copper surface, (first metallic layer or plate of the measuring capacitors, second layer),
8. 8. dielectric deformable,
9. 9. copper surface (second common metallic layer or plate of the measuring capacitors, second layer),
10. 10. circuit board material,
11. 11. Copper surface (screen, can be omitted).

The copper surfaces are again connected to GND (ground). The copper segments lying one above the other are connected to each other (parallel connection of the capacitors). This results in double capacitance for the four capacitors.

In the example of the 1 and 2nd is the capacitance of the respective capacitors via signal processing electronics 10th converted to a frequency to exclude capacitive influences for further signal processing. This signal processing electronics 10th is in the first board in this example 3rd integrated like this into 3rd is indicated schematically. 3rd shows a top view of the dielectric 4th opposite side of the first board 3rd . The area of integrated signal processing electronics 10th is only indicated schematically here. Electrical connection surfaces are also shown in this illustration 11 for the power supply, the output signal of the signal processing electronics 10th as well as the activation of the sensor.

In this example, the capacitance of the individual capacitors is measured or signal processed with the aid of a multiplexer, with which the capacitance of the capacitors is recorded in chronological order. 4th shows an example of such signal processing electronics 10th . In the present example, this signal processing electronics has a voltage regulator 13 for the voltage supply of the individual components via a power supply input 12th is fed. The capacitances of the measuring capacitors CS1, CS2 and CS3 as well as the reference capacitor CR1 are via the analog multiplexer 14 measured sequentially in time. The analog multiplexer 14 is in turn controlled via external binary switches, via the corresponding control inputs 15 be connected to the signal processing electronics. The capacities measured in each case are via a Schmitt trigger 16 converted to frequencies at the frequency output 17th the signal processing electronics of the washer sensor are provided. In the circuit diagram of the 4th C1 to C6 capacitances, R1 to R5 resistors and L represent an inductive filter element. The respective output signal corresponds to a frequency which decreases with increasing pressure on the dielectric. The corresponding preload can be determined from the frequency using a calibration curve.

The washer sensor described in this example can be realized, for example, with a thickness of only about 1 mm and also significantly less. Depending on the dielectric used and the area of the metallizations, the capacitance for the reference capacitor and the measuring capacitors in the unloaded state can be, for example, in the range between 10 and 100 pF.

5 shows in plan view and in cross section an example of a realization of the proposed washer sensor, in which the sensor board 18th with the reference and the measuring capacitors is surrounded by a protective housing. This housing is through an upper housing part 19th and a lower housing part 20 formed between which the sensor board 18th with the integrated, deformable dielectric. The housing parts are designed as specially shaped washers and with an anti-rotation device 21 secured against twisting when tightening. In the present example, the upper part of the housing 19th as an anti-twist device 21 the two lugs recognizable in the figure, which in corresponding receptacles on the lower housing part 20 intervention.

Reference list

1

Washer sensor

2nd

Through opening

3rd

first board

4th

dielectric

5

second board

6

first metallization

7

second metallization

8th

Metallization for reference capacitor

9

slot

10th

Signal processing electronics

11

electrical connection pads

12th

Power input

13

Voltage regulator

14

Analog multiplexer

15

Control inputs for external binary switches

16

Schmitt trigger

17th

Frequency output

18th

Sensor board

19th

Upper part of the housing

20

Lower part of the housing

21

Anti-twist device

Claims (16)

Method for measuring and / or monitoring the pretensioning force of a mechanical connection that is established between a component and an object or other component via at least one connecting element that is subjected to tension, in which the pretensioning force is measured with a capacitive pressure sensor that is inserted into a flat base element 1) is integrated with a through opening (2) for the connecting element, the flat underlay element (1) being inserted during the production of the mechanical connection between a head or counterpart of the connecting element and the component, characterized in that a capacitive pressure sensor is used which Has at least three pressure-sensitive capacitors (CS1, CS2, CS3) around the through opening (2), capacities of the capacitors (CS1, CS2, CS3) which are established after the mechanical connection has been established for measuring the pretensioning force, and also the Capacity of a reference capacitor rs (CR1) is detected, which is arranged in a region of the base element (1) that is not loaded by the mechanical connection. Procedure according to Claim 1 , characterized in that the capacitive pressure sensor has a capacitive structure made of a dielectric (4) between two metallic layers (6, 7), which deforms due to the production of the mechanical connection, wherein at least a first (6) of the metallic layers into several areas is separated in order to form the plurality of pressure-sensitive capacitors (CS1, CS2, CS3) around the through-opening (2) through the two metallic layers (6, 7) with the interposed dielectric (4). Procedure according to Claim 2 , characterized in that the capacitive pressure sensor has a stack of several of the capacitive structures, in which pressure-sensitive capacitors lying one above the other are connected in parallel. Procedure according to one of the Claims 1 to 3rd , characterized in that the capacitances of the capacitors (CS1, CS2, CS3, CR1) are converted into frequencies via an electrical circuit (10). Procedure according to one of the Claims 1 to 4th , characterized in that the pretensioning force is determined via a calibration curve from the capacities detected or from the quantities derived therefrom. Procedure according to one of the Claims 1 to 5 , characterized in that the prestressing force is measured continuously or repeatedly after the mechanical connection has been established. Procedure according to one of the Claims 1 to 6 , characterized in that by comparing the capacitances of the several capacitors (CS1, CS2, CS3) or variables derived therefrom it is checked whether the connecting element is tilted. Procedure according to one of the Claims 1 to 7 for measuring and / or monitoring the preload force of a screw connection. Device for measuring and / or monitoring the pretensioning force of a mechanical connection which is produced between a component and an object or further component via at least one connecting element which is subjected to tension, the device being a flat underlay element (1) with a through opening (2) for the Connection element is formed and has a capacitive pressure sensor, characterized in that the capacitive pressure sensor has at least three pressure-sensitive capacitors (CS1, CS2, CS3) around the passage opening (2) and electrical connections, via the capacitors (CS1, CS2, CS3) ) can be detected, and the base element (1) has a load-free area that is not loaded by the mechanical connection between the head or counterpart of the connecting element and the component when the base element (1) is inserted, a reference capacitor () in the unloaded area CR1) for the measurement is arranged. Device after Claim 9 , characterized in that the capacitive pressure sensor has a capacitive structure made of a dielectric (4) between two metallic layers (6, 7), at least a first one (6) of which is divided into a plurality of regions in order to pass through the two metallic layers (6, 7) with the intermediate dielectric (4 ) to form the plurality of pressure-sensitive capacitors (CS1, CS2, CS3) around the passage opening (2). Device after Claim 10 , characterized in that the capacitive pressure sensor has a stack of several of the capacitive structures, in which pressure-sensitive capacitors lying one above the other are connected in parallel. Device after Claim 10 or 11 , characterized in that the metallic layers (6, 7) are ring-shaped around the through opening (2). Device after Claim 12 , characterized in that the first metallic layer (6) is separated into at least three ring segments. Device according to one of the Claims 10 to 13 , characterized in that the underlay element (1) is formed in a sandwich construction from two carrier plates (3, 5) with the intermediate dielectric (4), the two metallic layers (6, 7) being formed on the two carrier plates (3, 5) are. Device according to one of the Claims 10 to 13 , characterized in that the base element (1) is made of a single or multilayer board which has a carrier material and copper surfaces, the carrier material the dielectric (4) and the copper surfaces the metallic layers (6, 7) for the capacitors (CS1 , CS2, CS3) form. Device according to one of the Claims 9 to 15 , characterized in that an electrical circuit (10) for detecting the capacitances of the capacitors (CS1, CS2, CS3, CR1) is arranged on the base element (1) or is integrated in the base element (1).

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