Method for determining clamping force

Abstract

The present invention relates to a method for determining the clamping force of a powered, rotatable clamping device (2) of a machine tool (1), the method comprising the steps of: clamping an object (5) with an operating force F; and determining, in a control unit (6) of the machine tool (1), a clamping force F that can be estimated based on the operating force F and on operating parameters of the machine tool (1). calc and a step of calculating the clamping force F acting on the object (5) in the clamping device (2) on a model basis. real at a time interval I, and measuring the measured value F real is wirelessly transmitted to the control unit (6) of the machine tool (1), and the measured value F real and the calculated clamping force F calc and adapting the measurement interval if the comparison indicates a deviation that reaches a threshold.

Description

[Technical field]

[0001] The invention is formed by a method for determining the clamping force of a powered, rotary-drivable clamping device of a machine tool, the method comprising the following steps: - tightening the object with an operating force F; The clamping force F can be estimated in the control unit of the machine tool based on the operating force F and the operating parameters of the machine tool. calc and calculating, on a model basis, - Clamping force F acting on the object in the clamping device real at time intervals of a measurement interval I; Measurement value F real The measured value F is wirelessly transmitted to the control unit of the machine tool. real and the calculated clamping force F calc and comparing Adapting the measurement interval if the comparison indicates a deviation that reaches a threshold.

[0002] For the safe operation of a machine tool with its assigned clamping device, the clamping force with which the workpiece or tool is clamped in the clamping device must be known. This realization led to the Directive 2006 / 42 / EC of the European Parliament and of the Council on Machinery, which requires a certain level of functional safety in operation. In this case, some safety-relevant signals must be provided redundantly.

[0003] In this case, the clamping force of the clamping device is preferably measured directly, and the use of a clamping device on a machine tool means that this is usually formed by a clamping chuck with clamping jaws, in which the clamping force measured at the clamping jaws must be transmitted from the rotating system to the machine tool. For this purpose, wireless data transmission is provided, but this means that energy must be provided in the rotating system for the detection of the measured values ​​and for the wireless transmission of the measured values.

[0004] DE 102011081122 describes a clamping device with a chuck body having at least one clamping element and a measuring device for detecting the clamping force applied by the clamping element to a workpiece clamped in the workpiece clamping device, where the measuring device, which also has an evaluation unit, is provided in the upper jaw. There is also an operating device for applying the desired clamping force to the workpiece clamped in the workpiece clamping device. In the evaluation unit, a comparison is made between the determined required clamping force and the clamping force detected by the measuring device.

[0005] The object on which the invention is based is to reduce the energy requirements required in a rotary drivable clamping device whilst ensuring the required level of safety.

[0006] This problem is solved by the method mentioned at the beginning. In this method, a given clamping force is determined in two ways, on the one hand by a model-based calculation of the clamping force, which can be estimated on the basis of the operating forces and the operating parameters of the machine tool, and on the other hand by a direct measurement of the clamping force. Here, the direct measurement of the clamping force is not performed continuously, but rather clocked at the time interval of the measurement interval. In this case, however, the duration of the measurement interval is not set constant, but rather can be extended to save energy in relation to the detection of the measured value in order to obtain a good agreement between the calculated, estimable clamping force and the actually measured clamping force. On the other hand, if the model-based calculation of the clamping force leads to excessively large values ​​and the associated determination of the individual operating parameters is insufficient and requires correction, a shortening of the measurement interval can also be performed to ensure the required safety level. Operating parameters of this kind result from a stress transmission chain with the generation of the operating forces, which are converted into clamping forces via a mechanical transmission ratio. The mechanical transmission ratio depends mainly on the angle ratio or the leverage ratio of the clamping mechanism. Further operating parameters are the rotational speed of the clamping device with the resulting centrifugal forces, as well as the coefficient of friction at the functional surface of the transmission system, which is influenced by frictional effects such as wear or the lubrication state.

[0007] As provided by this method, wireless data transmission, which alone cannot meet the functional safety requirements of the Machinery Directive, is ensured by a model-based calculation of the estimable clamping force and makes it possible to adapt the measurement intervals to optimize energy consumption based on the reliability of the estimable clamping force.

[0008] It is envisaged here for the method that, in order to increase safety, the measurement interval is extended below a threshold value and / or shortened above a threshold value. Thus, by using an optimization with respect to energy consumption, it is ensured that the model-based calculation of the estimable clamping force reproduces the actually applied clamping force with sufficient accuracy in order to guarantee the required safety level.

[0009] With regard to ensuring a sufficient level of safety, the magnitude of the change in the measurement interval is also important. real It is assumed that the measurement interval is proportional to the interval between the threshold and the F threshold, i.e., if the interval between the threshold and the F threshold is large, the measurement interval is changed accordingly. real F calc The threshold for deviations smaller than F real F calc It is also possible that the actual clamping force is below a threshold of deviation greater than 0.05, i.e., a different threshold is used with respect to the required measure of safety, and a more accurate monitoring is performed when the actual clamping force is less than the calculated clamping force, since this indicates a greater potential risk.

[0010] However, F real F calc It can also be taken into account that the deformability of the object to be clamped is taken into account when determining the threshold deviation value greater than , so that deformable workpieces, such as thin-walled sleeves, are protected from being destroyed by excessively high clamping forces in practice.

[0011] It has proven expedient if the threshold value is a deviation of less than 2%, in particular a deviation of 1.5%, preferably a deviation of 0.5%.

[0012] Furthermore, preferably, a sensor is present for detecting the operating force, the measurement data of which can be transmitted by wire to the control unit. In this method, two separate sensors are used which detect the forces acting at different points of the force transmission chain and which represent a significant increase in safety through the fusion of the two sensor data for the redundant transmission path.

[0013] If the clamping device is formed by a clamping chuck, the clamping force F real A sensor for measuring is preferably assigned to one of the clamping jaws.

[0014] The invention will now be explained in more detail with reference to exemplary embodiments shown in the drawings. [Brief description of the drawings]

[0015] [Figure 1] Schematic diagram of a process for determining a clamping force having a required safety level.

[0016] FIG. 1 shows a machine tool 1 to which a clamping device is assigned, specifically a three-jaw clamping chuck with three clamping jaws 3, which in the illustrated embodiment are evenly distributed over the circumference and radially adjustable relative to the chuck axis. The adjustment of these clamping jaws 3 is carried out via a drive 4, which in the illustrated embodiment is electric, by means of which an operating force F is provided. This operating force is converted via a mechanical transformation in a stress transmission chain into a clamping force, which is applied from the clamping jaws 3 to the workpiece 5, in the illustrated embodiment to a sleeve held under external tension. The clamping force resulting from the operating force F is further influenced by the centrifugal forces during the rotational operation of the clamping device, the design and quality of the functional surfaces of the clamping jaws 3, as well as the wear and lubrication state of the functional surfaces. From the operating force F, in turn, a clamping force F can be determined, which can be estimated on the basis of the operating force F and the operating parameters of the machine tool 1. calc A model-based calculation of the clamping force F acting on the object in the clamping device is performed in the control unit 6 of the machine tool 1. real at the time interval of the measurement interval and transmitting the measured value F real The control unit 6 wirelessly transmits the measured value F real and the calculated clamping force F calc A comparison is made with.

[0017] If the comparison indicates a deviation that reaches a threshold value, an adaptation of the measurement interval is performed, whereby the measurement interval is extended if the threshold value is below and shortened if the threshold value is above. As a result, the measurement value F realIn the case of a good match, the radio signal for the wireless transmission of the measurement F can be transmitted much less frequently. This significantly extends the life of the accumulator battery installed in the clamping jaw 3 for providing the required electrical energy. real During the transmission pauses between transmissions, the clamping force is calculated according to a model 7 on the basis of the operating force F and additional measured variables, such as the number of revolutions of the machine spindle 8, the mass of the jaws 10, the number of accumulated cycles 11, and further important parameters 9 such as wear and lubrication state.

[0018] F real F calc The smaller deviation threshold is F real F calc It is noted that it does not have to be the same as the threshold value of the larger deviation, and in particular can be selected to be smaller. When determining the threshold value, the deformability of the workpiece 5 to be clamped or to be clamped can also be taken into account. Finally, it is noted that the actuation force F can not only be derived from the actuation force generated, but can rather also be controlled by means of a sensor, the measurement data of which are transmitted by wire to the control unit 6. For redundant control of the clamping state, two different sensors can thus be used, which transmit their measurement data to the control unit 6 by different paths. [Explanation of symbols]

[0019] 1 Machine tools 2 Clamp chuck 3 Clamp jaw 4 Drive unit 5 Work 6. Control Unit 7 Models 8 Rotational Speed 9 Further parameters 10 Joe's mass 11 Number of cycles F Operating force F calc Estimated tightening force F real Measured clamping force

Claims

1. A method for determining the clamping force of a powered, rotationally drivable clamping device (2) of a machine tool (1), comprising the steps of: A step of clamping an object (5) with a clamping force obtained by mechanically converting an operating force F provided by a drive unit (4) of the clamp device (2); A clamping force F that can be estimated in a control unit (6) of the machine tool (1) based on the operating force F and the operating parameters of the machine tool (1). ｃａｌｃ and calculating, on a model basis, The measured value F of the clamping force acting on the object (5) in the clamping device (2) ｒｅａｌ at time intervals of a measurement interval I; The measured value F ｒｅａｌ to the control unit (6) of the machine tool (1), and the measured value F ｒｅａｌ and the calculated tightening force F ｃａｌｃ and comparing adapting the measurement interval I if the comparison indicates a deviation that reaches a threshold value, The operating parameter is one resulting from a stress transmission chain or a friction coefficient in the functioning of the transmission system, method.

2. 2. The method of claim 1, wherein the measurement interval I is extended if the threshold is not satisfied and is shortened if the threshold is exceeded.

3. The magnitude of the change in the measurement interval I is the measurement value F ｒｅａｌ 3. The method of claim 1, wherein the step of determining whether the threshold is exceeded is proportional to the interval between the threshold and the step of determining whether the threshold is exceeded.

4. The measured value F ｒｅａｌ is the calculated clamping force F ｃａｌｃ The smaller deviation threshold is the measurement F ｒｅａｌ is the calculated clamping force F ｃａｌｃ The method of claim 1 , wherein the deviation is less than or equal to a threshold value of greater deviation.

5. The measured value F ｒｅａｌ is the calculated clamping force F ｃａｌｃ 5. The method according to claim 1, further comprising taking into account the deformability of the object to be clamped when determining the threshold deviation value greater than .

6. The method of claim 1 , wherein the threshold is a deviation of less than or equal to 2%.

7. 7. The method according to claim 1, further comprising the step of transmitting measurement data of a sensor for detecting the actuating force F by wire to the control unit (6).

8. The clamping device (2) is formed by a clamping chuck, and the measurement value F ｒｅａｌ 7. The method according to claim 1, further comprising the step of: providing a sensor for measuring the position of the clamping jaws (3) at one of the clamping jaws (3).

Images