Clamping device with a sensor arrangement

Abstract

Clamping device (1), in particular support element, with a) a movable element (3) for clamping or supporting a workpiece or a tool, in particular in a machine tool, b) a fluid drive with a cylinder (5) in which a piston is movable, c) an integrated sensor arrangement (6, 7, 8), c1) wherein the sensor arrangement (6, 7, 8) comprises several deformation sensors (6, 7) each attached to the cylinder (5) and measuring a strain (ε) on the cylinder (5), c2) wherein the deformation sensors (6, 7) are spatially distributed on the clamping device (1) in order to be able to measure the strain (ε) with a spatial resolution, c3) wherein the sensor arrangement (6, 7, 8) comprises an electronic evaluation unit (9) which evaluates the output signals of the deformation sensors (6, 7), characterized in that d) that the evaluation unit (9) determines a strain profile during normal operation with a spatial and temporal resolution of the strain, e) that the evaluation unit (9) contains a stored reference profile for the strain profile with a spatial and temporal resolution of the strain, and f) that the evaluation unit (9) compares the strain profile determined during normal operation with the stored reference profile, g) that the deformation sensors (6, 7) are arranged in pairs opposite each other in order to compensate for strains caused by external forces by averaging, and h) that the evaluation unit (9) evaluates the output signals of the deformation sensors (6, 7) by averaging them.

Description

[0001] The invention relates to a clamping device (e.g., swing clamp, support element) with a movable element (e.g., clamping arm) for clamping or supporting a workpiece or a tool. For example, the clamping device can be a support element, a lever clamp, a swing clamp, a bore clamp, a vice, or a clamping cylinder. Such clamping devices are generally known from the prior art and are described, for example, in DE 102 52 549 A1, DE 33 34 401 A1, DE 197 52 671 A1, and DE 102 05 601 A1.

[0002] However, various problems can occur during operation of these known clamping devices, which are briefly described below.

[0003] For example, the hydraulic drive usually provided for the mechanical drive of such clamping devices can suffer a pressure loss, which can be caused, for example, by chips in the oil circuit or other oil contamination.

[0004] In addition, the hydraulic drive can become leaky, which can be caused, for example, by chips penetrating from outside or by excessively high temperatures.

[0005] Furthermore, the problem can occur that the position of the workpiece to be clamped is incorrect.

[0006] In addition, a collision may occur between the moving element (e.g. clamping iron) of the clamping device and an obstacle.

[0007] Finally, vibrations that exceed the permissible level can occur during processing.

[0008] For the technical background of the invention, reference should also be made to US 4 513 979 A, ​​DE 195 30 131 C1, DE 10 2011 116 808 A1, DE 196 02 712 A1, DE 10 2010 062 695 A1, DE 10 2006 013 935 A1 and DE 103 51 347 A1.

[0009] The invention is therefore based on the object of creating a correspondingly improved clamping device which at least partially solves at least one of the problems mentioned above.

[0010] This object is achieved by a clamping device according to the invention according to the main claim.

[0011] The invention is based on an otherwise conventional clamping device, such as a support element, a lever clamp, a swing clamp, a bore clamp, a vise, or a clamping cylinder, whereby the invention is not limited to these types of clamping devices. It should be noted that such clamping devices are generally known and are described in the publications cited above, the content of which is incorporated in its entirety into the present description, so that a detailed description of the otherwise conventional design and operation of the clamping device can be omitted.

[0012] It should only be mentioned at this point that the clamping device according to the invention has a movable element (e.g. clamping iron) in order to clamp a workpiece or a tool.

[0013] It should be noted that the clamping device according to the invention can be used either for clamping a workpiece or for clamping a tool, which, however, is also known per se from the prior art and therefore does not need to be described in more detail.

[0014] In contrast, the clamping device according to the invention is characterized by a structurally integrated sensor arrangement with multiple sensors. The sensors enable the determination of an operating parameter (i.e., deformation (elongation)), which allows conclusions to be drawn about the operating behavior of the clamping device during operation. In this way, the problems described above can be identified or even avoided by modifying the clamping device's control depending on the measured operating parameter.

[0015] Within the scope of the invention, the sensor arrangement can also comprise a temperature sensor, a pressure sensor (e.g., strain gauge), a deformation sensor (e.g., strain gauge), and / or a timer that provides the current operating time. In a preferred embodiment of the invention, the sensor arrangement comprises a combination of several sensor types, such as at least one deformation sensor and one pressure sensor. However, various combinations of different sensor types are possible within the scope of the invention.

[0016] In addition, the integrated sensor arrangement also includes an electronic evaluation unit that evaluates the output signal of the sensor arrangement. According to the invention, this evaluation unit is also structurally integrated into the clamping device.

[0017] In a preferred embodiment of the invention, the evaluation unit derives at least one of the following variables from the output signal of the sensor arrangement: - Force (e.g. clamping force) with which the clamping device clamps a workpiece or a tool, - pressure, in particular hydraulic pressure of a hydraulic drive of the clamping device, - Position of the clamping device, in particular the angular position of a movable element (e.g. clamping iron), - Speed ​​of adjustment of the clamping device, in particular clamping speed.

[0018] However, the invention is not limited to the variables derived from the output signal of the sensor arrangement. Rather, the evaluation unit can also derive other operating variables from the measured sensor variables.

[0019] According to the invention, the clamping device has a fluid drive (e.g. pneumatic drive, hydraulic drive) which is driven by a pressurized drive fluid (e.g. compressed air, hydraulic oil).

[0020] In such a fluid drive, a pressure sensor is preferably provided which measures the pressure of the drive fluid (e.g. hydraulic oil, compressed air).

[0021] In addition, a temperature sensor can also be provided which measures the temperature of the drive fluid (e.g. hydraulic oil).

[0022] Furthermore, it must be taken into account that the fluid drive experiences a strain depending on the applied force, which can be measured within the scope of the invention by a strain gauge (DMS) or another strain sensor.

[0023] In a fluid drive, the drive fluid (e.g. hydraulic oil) is usually supplied with a specific flow rate, whereby the following fault-free or faulty operating states can occur: - The pressure build-up of the hydraulic pressure in the fluid drive is OK, ie no error occurs, - the pressure of the drive fluid is too low, ie the operating condition is faulty, - the pressure of the drive fluid is too high, ie the operating condition is faulty, - the drive fluid is supplied with too large a flow rate, which can lead to a pressure peak, ie the operating condition is faulty, - the pressure build-up of the fluid pressure of the drive fluid is too slow, ie the operating condition is faulty, - When the clamping device moves, inadmissible pressure peaks occur, which can be caused, for example, by a collision with an obstacle or by internal stiffness of the clamping device or the fluid drive.

[0024] In the preferred embodiment of the invention, the evaluation unit evaluates the output signal of a pressure sensor or a strain sensor (e.g. strain gauge) and distinguishes or recognizes at least some of the above-mentioned fault-free or faulty operating states.

[0025] Furthermore, the invention provides that the evaluation unit determines a profile of the measured sensor variable (strain) during operation (e.g., during a clamping process) with a spatial and temporal resolution. The spatial resolution is achieved by measuring the strain using several deformation sensors that are spatially distributed on the clamping device and therefore measure the strain at different positions on the clamping device. The temporal resolution of the profile of the measured sensor variable can be achieved by measuring the sensor variable at different times during a clamping process, preferably continuously or intermittently.

[0026] This allows for a reference profile to be measured during a test run, which is then saved. The reference profile is thus determined during a fault-free clamping process. During normal operation of the clamping device, the saved reference profile is then compared with the actual measured profile of the sensor size. By comparing the measured profile with the saved profile, a variety of conclusions can be drawn regarding possible error states and causes.

[0027] It was already mentioned above that the clamping device has a fluid drive (e.g., hydraulic drive). In this case, it is possible for a temperature sensor to compare the temperature of the drive fluid (e.g., hydraulic oil) with a specified maximum value and / or a specified minimum value in order to detect an error, which could, for example, be manifested by overheating of the hydraulic oil. The evaluation unit can then generate an error message if the measured temperature of the drive fluid lies outside the permissible temperature range. The evaluation unit can also contain a timer that specifies the current time, so that the error message then contains not only a corresponding error code for the respective error (oil temperature too high / too low), but also the current time at which the error occurred.

[0028] As briefly mentioned above, the sensor arrangement comprises multiple deformation sensors to enable strain measurement with local resolution. The deformation sensors are arranged in pairs opposite each other to compensate for strains caused by external forces by averaging. External forces typically result in strain on one side and a corresponding compression on the opposite side, so averaging the measured strains makes it possible to calculate external forces. The deformation sensors are therefore connected to the evaluation unit, which evaluates the output signals of the deformation sensors, namely by the averaging process mentioned above.

[0029] In the preferred embodiment of the invention, the clamping device has a replaceable attachment, such as a clamping iron, a clamping lever, a pressure piece, or vice jaws. The attachment is preferably mounted to the clamping device by a screw connection, wherein the screw connection is tightened to a specific tightening torque when the attachment is installed. However, the tightening torque not only leads to a desired fastening of the attachment to the clamping device, but also to an undesirable mechanical load on the attachment or the clamping device. It is therefore preferably provided that the evaluation unit checks, by querying the deformation sensors, whether the tightening torque leads to a mechanical overload of the clamping device. If this is the case, the evaluation unit can generate an error message so that the tightening torque is not increased any further.In this way, damage can be prevented when attaching the attachment (e.g. clamping iron) to the clamping device (e.g. swing clamp).

[0030] Furthermore, within the scope of the invention, it is possible for the evaluation unit to detect various operational malfunctions by querying the deformation sensors, such as tool breakage, vibration, a measurement deviation, a workpiece inclusion, or a leak in the clamping device. This can be done, for example, by comparing the actually measured current profile with a stored reference profile, as already described above.

[0031] As briefly mentioned above, the clamping device has a fluid drive. Such a fluid drive has a cylinder in which a piston is displaceable, which is known per se from the prior art. During operation, forces therefore arise on the cylinder of the fluid drive, and these forces can be measured by the deformation sensors attached to the cylinder. For example, the deformation sensors can be attached to the outer surface of the cylinder. It is also possible to arrange several deformation sensors distributed around the circumference of the cylinder in order to measure the strain on the cylinder with local resolution.

[0032] Furthermore, within the scope of the invention, it is also possible for the evaluation unit to determine the piston position of the fluid drive from the sensor output signal. Thus, within the piston's end positions, variable forces act on the cylinder, which can be measured by the deformation sensor, which in turn allows the piston position to be determined.

[0033] Furthermore, within the scope of the invention, it is possible for the evaluation unit to derive the stiffness of the tool or workpiece from the measured strains. In this case, a strain profile can be measured with spatial and temporal resolution and compared with a stored reference profile.

[0034] Furthermore, it should be mentioned that the invention also claims protection for a corresponding operating method for such a clamping device, the details of this operating method being already apparent from the above description, so that a separate description of the operating method can be dispensed with.

[0035] Other advantageous developments of the invention are characterized in the subclaims or are explained in more detail below together with the description of the preferred embodiments of the invention with reference to the figures. They show: Fig. 1 a simplified schematic representation of a swing clamp according to the invention with a sensor arrangement, Fig. 2 a flow chart explaining the temperature measurement on the swing clamp, Fig. 3 a flow chart explaining the monitoring of the assembly of the clamping lever on the swing clamp to avoid mechanical overload, Fig. 4A is a flow chart explaining the determination of a reference profile of the measured strains with spatial and temporal resolution, Fig. 4B is a flow chart explaining the comparison of an actually measured profile with the stored reference profile during a clamping cycle, Fig. 5A a flow chart to illustrate the differentiation of different operating states depending on measured sensor variables, as well as Fig. 5B is a pressure diagram illustrating the various faulty operating conditions.

[0036] Fig. 1 shows a simplified, schematic representation of a swing clamp 1 according to the invention, as is known per se from the prior art (e.g. DE 102 52 549 A1), so that with regard to the details of the structural design and the functioning of the swing clamp 1, reference can be made to the prior art.

[0037] It should only be mentioned at this point that the swing clamp 1 has a base plate 2 which can be attached to a surface.

[0038] On its upper side, the swing clamp 1 has a conventional clamping iron 3 which can be pivoted about a pivot axis 4, as is known from the prior art.

[0039] The clamping arm 3 is moved in a conventional manner by a hydraulic drive housed in a housing 5 of the swing clamp 1. However, the forces generated by the hydraulic drive act not only on the clamping arm 3, but also on the housing 5 of the swing clamp 1 and the other components of the swing clamp 1.

[0040] The swing clamp 1 therefore has a structurally integrated sensor arrangement comprising several deformation sensors 6, 7, e.g., strain gauges. Only the two deformation sensors 6, 7 are shown in the drawing. In practice, however, numerous deformation sensors 6, 7 can be arranged distributed around the circumference of the housing 5, which makes it possible to measure a mechanical strain ε on the housing 5 with local resolution.

[0041] In addition, the sensor arrangement of the swing clamp 1 also includes a pressure sensor 8, which measures a pressure p of the hydraulic oil of the hydraulic drive.

[0042] Furthermore, the structurally integrated sensor arrangement can comprise further sensor types, such as a temperature sensor that measures the operating temperature of the hydraulic oil of the hydraulic drive.

[0043] The measured sensor variables are fed to an electronic evaluation unit 9, which is also structurally integrated into the swing clamp 1. The evaluation unit 9 can then draw conclusions about various error causes or detect specific error cases from the current measured values, as described in detail below.

[0044] In the following, the flow chart according to Fig. 2, which provides for temperature monitoring.

[0045] In a first step S1, a current temperature T IST of the hydraulic oil in the hydraulic drive.

[0046] In a step S2 it is then continuously checked whether the current temperature T IST a given maximum value T MAX exceeds.

[0047] If no temperature exceedance is detected, a check is carried out in step S3 to determine whether the current temperature T IST a given minimum value T MIN falls below.

[0048] If this is not the case, the current temperature T IST in a permissible temperature range between the minimum value T MIN and the maximum value T MAX .

[0049] Otherwise, however, there is a temperature overshoot or undershoot and an error message is generated in step S4, which indicates the actually measured value of the temperature TIST as well as the date and time of the limit violation.

[0050] This error message is then visually displayed on a display device in step S5 and saved in step S6.

[0051] In addition, the evaluation unit can make a suggestion for error correction in a step S7, whereby this suggestion can be read from a database depending on the error message.

[0052] The following is the flow chart according to Fig. 3, which provides for monitoring the assembly of the clamping iron 3 on the swing clamp 1.

[0053] In a first step S1, the clamping iron 3 is first provided so that the clamping iron 3 can be mounted on the swing clamp 1 by means of a screw connection.

[0054] The screw connection between the clamping arm 3 and the swing clamp 1 is then tightened in step S2. This generates considerable forces that not only tighten the screw connection but also act on the clamping arm 3 and the swing clamp 1.

[0055] In order to avoid mechanical overload, the strain ε is then continuously measured by the deformation sensors 6, 7 when tightening the screw connection in a step S3.

[0056] In a step S4, it is then continuously checked whether the measured strain ε exceeds a predetermined maximum value ε MAX exceeds.

[0057] If this is the case, an error message is generated in step S5 and the tightening of the screw connection between the clamping arm 3 and the swing clamp 1 is stopped in order to avoid mechanical overload.

[0058] The following flow charts are shown according to the Fig. 4A and Fig. 4B. The flow chart according to Fig. Figure 4A first describes a so-called teach-in operation, which is used to determine a target profile (reference profile) of the strains that occur during a correct clamping cycle. The flow chart according to Fig. 4B then explains a normal clamping operation in which the currently occurring actual profile of the strains is compared with the stored reference profile (target profile).

[0059] In the following, the teach-in operation is described first according to Fig. 4A explained.

[0060] In a step S1, a clamping cycle with several phases (adjusting, clamping / tightening, unclamping, resetting) is initiated.

[0061] During this clamping cycle, in a step S2, the strains ε i(t) is measured by n deformation sensors. The measurement by several spatially distributed deformation sensors enables temporal and spatial resolution during the measurement.

[0062] The strains ε measured during the clamping cycle i (t) are then combined in a step S3 to form a target profile (reference profile), which is then stored in a step S4 to serve as a comparison during later normal operation.

[0063] In normal operation according to Fig. 4B, a clamping cycle is then initiated again in a step S1.

[0064] In a step S2 the strains ε i (t) of the n deformation sensors.

[0065] The strains ε measured in this way i (t) are then combined in a step S3 to form an actual profile that contains a spatial and temporal resolution of the strains.

[0066] In step S4, the currently measured actual profile is compared with the stored target profile (reference profile). From this comparison, the speed v of the clamping process can then be derived.

[0067] In a step S5 it is then checked whether the current speed v of the clamping process is within a permitted value range between a minimum value v MIN and a maximum value v max lies.

[0068] If this is not the case, an error message is generated in step S6.

[0069] In the following, the embodiment according to the Fig. 5A and Fig. 5B explains. Fig. 5B is a pressure diagram showing the temporal progression of a hydraulic pressure p during a clamping cycle as a function of time.

[0070] A curve A shows the desired curve (target curve) of the pressure curve over time. Thus, the pressure p should initially increase relatively evenly and linearly during clamping, until the pressure p then reaches a permissible operating range between a minimum value p MIN and a maximum value p max reached (t1). Curve A should then flatten out so that the pressure p remains within the permissible range.

[0071] Curve B, on the other hand, shows a faulty control of the hydraulic drive, since the pressure p is above the permissible maximum pressure p even in the stationary state. MAX lies.

[0072] Curve C, on the other hand, shows an erroneous pressure curve, where pressure p is below the specified minimum value p even in the steady state. MIN lies.

[0073] Curve D, on the other hand, leads to a permissible pressure value after the transient processes have subsided. However, the pressure p initially rises above the permissible maximum value p during the clamping process. MAX and then subsides again. This error typically occurs when the hydraulic oil flow is too high.

[0074] Finally, curve E shows a pressure build-up that is too slow, since the pressure p is only within the permissible pressure range after the end of the clamping process (t2) and thus during the mechanical processing.

[0075] The invention enables a distinction between the above-mentioned operating states according to the curves AE.

[0076] In a step S1 according to Fig. 5A first initiates a clamping process.

[0077] During the clamping process, the hydraulic pressure p of the hydraulic oil in the hydraulic drive is then measured in a step S2.

[0078] In step S3 the following parameters are then determined: - Stationary pressure value p END after the decay of transient processes, - Time T until the minimum pressure p is reached MIN , - Maximum pressure p PEAK when clamping.

[0079] In a subsequent step S4, the various operating states already described above are then differentiated according to the curves AE.

[0080] In a step S5, the pressure control of the hydraulic drive is then adjusted according to the determined operating state if an error has been detected. List of reference symbols: 1 swing clamp 2 base plate 3 clamps 4 swivel axis 5 Housing of the swing clamp 6 strain gauges 7 strain gauges 8 Pressure sensor for measuring hydraulic pressure 9 Evaluation unit ε strain p Hydraulic oil pressure

Claims

[1] Clamping device (1), in particular support element, with a) a movable element (3) for clamping or supporting a workpiece or a tool, in particular in a machine tool, b) a fluid drive with a cylinder (5) in which a piston is displaceable, c) an integrated sensor arrangement (6, 7, 8), c1) wherein the sensor arrangement (6, 7, 8) comprises a plurality of deformation sensors (6, 7), each of which is attached to the cylinder (5) and measures a strain (ε) on the cylinder (5), c2) wherein the deformation sensors (6, 7) are mounted locally distributed on the clamping device (1) in order to be able to measure the strain (ε) with a spatial resolution, c3) wherein the sensor arrangement (6, 7, 8) comprises an electronic evaluation unit (9) which evaluates the output signals of the deformation sensors (6, 7), characterized by , d) that the evaluation unit (9) determines a strain profile during normal operation with a spatial and temporal resolution of the strain, e) that the evaluation unit (9) contains a stored reference profile for the strain profile with a spatial and temporal resolution of the strain, and f) that the evaluation unit (9) compares the strain profile determined during normal operation with the stored reference profile, g) that the deformation sensors (6, 7) are arranged in pairs opposite one another in order to compensate for strains caused by external forces by averaging, and h) that the evaluation unit (9) evaluates the output signals of the deformation sensors (6, 7) by averaging. [2] Clamping device (1) according to claim 1, characterized by that the sensor arrangement (6, 7, 8) comprises at least one of the following sensors: a) temperature sensor, b) pressure sensor (8), in particular with at least one strain gauge, c) strain gauges, d) Time measuring device. [3] Clamping device (1) according to one of the preceding claims, characterized by that the evaluation unit (9) derives at least one of the following variables from the output signal of the sensor arrangement (6, 7, 8): a) a force, in particular the clamping force, with which the clamping device (1) clamps a workpiece, b) a print, c) a position of the clamping device (1), in particular an angular position, d) a speed, in particular a clamping speed. [4] Clamping device (1) according to one of the preceding claims, characterized by that the fluid drive is fed with a pressurized drive fluid, in particular a) as a pneumatic actuator supplied with compressed air, or b) as a hydraulic drive powered by hydraulic oil. [5] Clamping device (1) according to claim 4, characterized by , a) that the pressure sensor (8) measures the pressure (p) of the drive fluid, and / or b) that the temperature sensor measures the temperature of the drive fluid, [6] Clamping device (1) according to claim 5, characterized by , a) that the drive fluid is supplied to the fluid drive with a specific flow rate, and b) that the evaluation unit (9) detects at least one of the following operating states (AE) depending on the output signal of the pressure sensor or depending on the strain: b1) Pressure build-up OK b2) too low pressure of the drive fluid, b3) excessive pressure of the drive fluid, b4) excessive flow of the drive fluid and b5) Pressure build-up too slow, b6) unacceptable pressure peaks during movement, for example due to a collision with an obstacle or internal stiffness, and / or c) that the evaluation unit (9) determines a profile of the tensioning process with a spatial and temporal resolution of the strain, and / or d) that the evaluation unit (9) determines the profile during a test run and then stores the determined profile as a reference profile, and / or e) that the evaluation unit (9) determines the profile during normal operation and compares it with the stored reference profile. [7] Clamping device (1) according to one of the preceding claims, characterized by , a) that the evaluation unit (9) compares the measured temperature of the drive fluid with a predetermined maximum value and / or with a predetermined minimum value, and / or b) that the evaluation unit (9) generates an error message if the measured temperature of the drive fluid exceeds the specified maximum value, and / or c) that the evaluation unit (9) generates an error message if the measured temperature of the drive fluid falls below the specified minimum value, and / or d) that the evaluation unit (9) contains a timer which specifies the time, and / or e) that the evaluation unit (9) generates and / or stores the error message together with the time. [8] Clamping device (1) according to one of the preceding claims, characterized by , a) that the clamping device (1) has a replaceable attachment (3), in particular a clamping iron (3), a clamping lever, a pressure piece or vice jaws, b) that the attachment part can be mounted on the clamping device (1) by means of a screw connection, wherein the screw connection is tightened with a specific tightening torque when the attachment part is mounted, c) that the tightening torque also leads to a mechanical load on the clamping device (1) which acts on the deformation sensors, d) that the evaluation unit (9) checks by querying the deformation sensors (6, 7) whether the tightening torque leads to a mechanical overload of the clamping device (1), e) that the evaluation unit (9) generates an error message if the evaluation unit (9) detects a mechanical overload of the clamping device (1). [9] Clamping device (1) according to one of the preceding claims, characterized by , a) that the evaluation unit (9) queries the deformation sensors (6, 7) in order to determine the strain (ε), and b) that the evaluation unit (9) determines, on the basis of the determined strain (ε), whether an operational fault has occurred, in particular a tool breakage, vibration, a measurement variation, a workpiece inclusion or a leak in the clamping device (1), and / or c) that the evaluation unit (9) determines the stored reference profile during a test run. [10] Clamping device (1) according to one of the preceding claims, characterized by that the deformation sensors (6, 7) are attached to the outer surface of the cylinder. [11] Clamping device (1) according to one of the preceding claims, characterized by , a) that the clamping device (1) has a piston drive with a movable piston, b) that the sensor is connected to an evaluation unit (9) which evaluates the output signal of the sensor, and c) that the evaluation unit (9) determines the piston position of the piston from the output signal of the sensor. [12] Clamping device (1) according to one of the preceding claims, characterized by that the evaluation unit (9) derives the stiffness of the tool and / or the workpiece from the measured strains (ε). [13] Clamping device (1) according to one of the preceding claims, characterized by that the clamping device (1) is one of the following clamping devices (1): a) support element, b) lever clamp, c) Swing clamp (1), d) bore clamp, e) vice, f) clamping cylinder. [14] Operating method for a clamping device (1) according to one of the preceding claims, comprising the following steps: a) Saving a strain profile with a spatial and temporal resolution of the strain as a reference profile, b) Normal operation of the clamping device (1), wherein during normal operation a strain profile of the clamping device (1) is determined with a spatial and temporal resolution of the strain by means of the sensor arrangement (6, 7, 8) integrated into the clamping device (1) with the strain sensors arranged locally distributed on the clamping device (1), and c) Comparing the stored reference profile with the strain profile determined during normal operation.

Images