MACHINE TOOL AND METHOD FOR DETERMINING AN ACTUAL STATE OF A MACHINE TOOL

Abstract

A machine tool comprises a measuring device, which is arranged on the machine tool, a control device, and a tool unit. The measuring device comprises at least one structure-borne sound sensor. T control device is coupled to the measuring device and to the tool unit. The control device is configured to acquire, by means of the measuring device, structure-borne sound signals caused by the machine tool and to determine a state variable, which describes an actual state of the machine tool, by forming a differential spectrum from a broadband reference spectrum and a broadband actual spectrum.

Claims

1. A machine tool comprising: a tool unit, a measuring device arranged on the machine tool, and a control device that is coupled to the measuring device and to the tool unit, wherein the measuring device comprises at least one structure-borne sound sensor, wherein the control device is configured to acquire, by means of the measuring device, structure-borne sound signals caused by the machine tool, involving an acquisition of a broadband reference spectrum and a broadband actual spectrum, and determine a state variable by forming a differential spectrum from the broadband reference spectrum and the broadband actual spectrum, wherein the state variable describes an actual state of the machine tool on the basis of structure-borne sound signals.

2. The machine tool as claimed in claim 1, wherein the differential spectrum has a power, and wherein the control device is further configured to evaluate a time behavior of the power of the differential spectrum.

3. The machine tool as claimed in claim 1, wherein the control device is configured to determine the broadband reference spectrum and the broadband actual spectrum by a transformation of the structure-borne sound signals into the frequency domain.

4. The machine tool as claimed in claim 3, wherein the control device is configured to determine at least one of the broadband reference spectrum and the broadband actual spectrum by one of a Fourier transformation and a fast Fourier transformation of the structure-borne sound signals into a frequency domain.

5. The machine tool as claimed in claim 3, wherein the control device is configured to determine the broadband reference spectrum and the broadband actual spectrum by a transformation of the structure-borne sound signals into the frequency domain.

6. The machine tool as claimed in claim 1, wherein the control device is configured to record, as the reference spectrum, a transformation of the structure-borne sound signals in a state in which the machine tool is in operation, but a workpiece is not yet being machined.

7. The machine tool as claimed in claim 1, wherein the control device is configured to determine a new reference spectrum before each machining of a workpiece.

8. The machine tool as claimed in claim 2, further comprising an output unit that is supplied with the time behavior of the power of the differential spectrum from the control device, and configured to output the time behavior of the power of the differential spectrum.

9. The machine tool as claimed in claim 1, wherein the machine tool comprises a tool unit having a tool spindle that supports and drives a tool, wherein the control device is configured to control the tool unit on the basis of the structure-borne sound signals, wherein the machine tool is arranged as a grinding machine, and wherein the tool unit comprises at least one grinding wheel.

10. A machine tool comprising: a tool unit, a measuring device arranged on the machine tool, and a control device that is coupled to the measuring device and to the tool unit, wherein the measuring device comprises at least one structure-borne sound sensor, wherein the control device is configured to acquire, by means of the measuring device, structure-borne sound signals caused by the machine tool, and determine a state variable by forming a differential spectrum from a broadband reference spectrum and a broadband actual spectrum, wherein the state variable describes an actual state of the machine tool on the basis of structure-borne sound signals.

11. A method for determining an actual state of a machine tool, the method, comprising the following steps: providing a machine tool comprising a tool unit, a measuring device arranged on the machine tool, and a control device that is coupled to the measuring device and to the tool unit, wherein the measuring device comprises at least one structure-borne sound sensor, determining an actual state of the machine tool on the basis of structure-borne sound signals, comprising: acquiring structure-borne sound signals of the machine tool, and determining a state variable by forming a differential spectrum from a broadband reference spectrum and a broadband actual spectrum, wherein the state variable describes an actual state of the machine tool, and wherein the actual state of the machine tool is determined on the basis of the differential spectrum.

12. The method as claimed in claim 11, wherein the differential spectrum has a power, and wherein a time behavior of the power of the differential spectrum is evaluated in its.

13. The method as claimed in claim 11, wherein at least one of the broadband reference spectrum and the broadband actual spectrum is determined by a transformation of the structure-borne sound signals into a frequency domain.

14. The method as claimed in claim 13, wherein at least one of the broadband reference spectrum and the broadband actual spectrum is determined by one of a Fourier transformation and a fast Fourier transformation of the structure-borne sound signals into the frequency domain.

15. The method as claimed in claim 11, wherein the broadband reference spectrum and the broadband actual spectrum are determined by a transformation of the structure-borne sound signals into the frequency domain.

16. The method as claimed in claim 11, wherein the reference spectrum is determined in a state in which the machine tool is in operation, but a workpiece is not yet being machined.

17. The method as claimed in claim 11, wherein, upon a renewed recording of a reference spectrum, the new reference spectrum is compared with the stored reference spectrum, in order to detect changes in the state of the machine tool.

18. The method as claimed in claim 11, wherein the control device is configured to determine a new reference spectrum before each machining of a workpiece.

19. The method as claimed in claim 11, wherein the time behavior of the power of the differential spectrum is provided at an output unit of the control device.

20. The method as claimed in claim 11, wherein a new reference spectrum is recorded before each machining of a workpiece.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Further features and advantages of the disclosure are disclosed by the following description of a plurality of exemplary embodiments, with reference to the drawings, wherein:

[0079] FIG. 1 shows a perspective view of a machine tool that is arranged as a grinding machine and comprises an enclosure;

[0080] FIG. 2a shows a perspective top view of a machine tool;

[0081] FIG. 2b shows a schematic block diagram of components of the measuring device;

[0082] FIG. 3a shows an example of a broadband reference spectrum;

[0083] FIG. 3b shows an example of a broadband actual spectrum;

[0084] FIG. 3c shows an example of a broadband differential spectrum, formed from a difference of an actual spectrum and a reference spectrum;

[0085] FIG. 4 shows a schematic representation of a power peak, for example in a differential spectrum or a reference spectrum;

[0086] FIG. 5 shows, schematically and exemplarily, the time behavior of the power values in the differential spectrum; and

[0087] FIG. 6 shows a schematic, simplified flow diagram of an exemplary method for determining an actual state of a machine tool.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0088] In FIG. 1, a machine tool is represented in perspective view and denoted as a whole by 10. FIG. 2 shows a corresponding top view, for instance of the machine tool 10 according to FIG. 1, wherein various components are not represented for reasons of clarity.

[0089] The machine tool 10 in the present case is arranged as a grinding machine, for instance as a cylindrical grinding machine, in general also as a horizontal grinding machine. The machine tool 10 comprises an enclosure 12, which acts as a housing. The enclosure 12 may also be provided with a viewing window 14. The enclosure 12 in this case defines a process space, which is closed, or closable, to the outside, at least in certain embodiments. The enclosure 12 provides for a safe delimitation of the process space of the machine tool 10, for instance in the case of automated machining operations. In this way, in principle, the hazard presented by moving components can be minimized. Moreover, lubricant, coolant, chips or, for example, sparks can be prevented from unwantedly escaping into the surroundings. To render the process space of the machine tool 10 accessible, the enclosure 12 may be appropriately provided with doors or flaps.

[0090] In the case of particular operating modes, it may be necessary for the viewing window 14 to be arranged as a type of protective door, in order that the interior of the machine tool can be reached from the outside by an operator. For this purpose, the viewing window 14 may be moved, or swiveled, laterally, for example, in order to release a previously closed opening. An arrow denoted by 16 indicates a possible opening movement of the protective door.

[0091] Operating modes that necessitate access into the interior of the machine tool 10 may be, for example, tool-setting operations, setting-up operations, truing operations, or generally tool-change or workpiece-change operations. It is understood that, depending on the degree of automation of the machine tool 10, differing operating modes may necessitate manual access into the interior of the machine tool 10.

[0092] Also indicated in FIG. 1, in the interior of the machine tool 10, is a tool unit 22 that comprises a spindle head 18. There is a tool 20 mounted on the spindle head 18. This tool 20 may be, for instance, a grinding tool, for instance a grinding wheel.

[0093] The machine tool 10 further comprises a workpiece mount 26, which is configured to support a workpiece 24. For reasons of clarity, FIG. 1 does not show a workpiece 24. For the purpose of machining a workpiece, the spindle head 18 can be moved axially relative to the tool receiver 26.

[0094] Machine tools 10, for instance grinding machines, usually have a worker interface or operator interface 28, arranged outside of the interior of the machine tool 10. Consequently, an operator can control, program or adjust the machine tool 10 or, for example, perform diagnostics without coming into contact with the interior of the machine tool 10. The operator interface 28 is, in certain embodiments, an operating unit, which comprises at least one input device 30 for inputting control commands. The operator interface 28 may further comprise an output unit 32, for example a monitor screen. Moreover, it is conceivable to use a so-called touchscreen, i.e. a combined input and output unit.

[0095] In addition, a status indicator 34 may be provided, which for example comprises a red lamp 34a, an orange or yellow lamp 34b and a green lamp 34c, in order to represent the actual state of the machine. In other words, the status indicator 34 may be of a design somewhat similar to that of a set of traffic lights. Other designs of the status indicator 34 are readily conceivable.

[0096] Also schematically represented in FIG. 1 is a sensor 36, for instance a piezoelectric acceleration or structure-borne sound sensor. This sensor 36 is, in certain embodiments, arranged close to the tool 20 and connected, wirelessly or by cable, to a measuring device 38 that is not represented. The measuring device 38 may be integrated in a control device 40, in certain embodiments, and the control device 40 may be integrated into the operator interface 28, in certain embodiments.

[0097] Alternatively or additionally, sensors 36 may be provided, which are configured as microphones or acoustic transducers and which cover a broadband frequency spectrum, for instance in the audible sound range (20 Hz to 20 kHz) or even above, also in the infrasonic and/or ultrasonic range. Sensors 36 configured in such a manner may also be arranged at a distance from the tool 20 or other moving components of the machine tool 10.

[0098] Clearly, it is also conceivable to arrange a plurality of these sensors 36 on the machine tool 10, for instance close to the workpiece 24 to be machined.

[0099] FIG. 2a shows a simplified, perspective top view of a machine tool 10, which in principle may correspond to, or at least be similar to, the machine tool 10 according to FIG. 1. For reasons of clarity, the design represented in FIG. 2a does not have an enclosure 12 or an operator interface 28 or set of traffic-signal type lights 34.

[0100] FIG. 2a shows the workpiece mount 26 in simplified form. It is arranged on a workpiece carrier 42, which can be moved axially along a guide 44. It is further conceivable to provide a further workpiece carrier, or tailstock 42, having a further workpiece holder 26, at an axial end of the guide 44 that is opposite the workpiece carrier 42, in order thus to fix the workpiece 24 in position between the workpiece holders 26 and 26, for the purpose of machining the tool 20.

[0101] In the present case the tool 20 comprises a tool casing 46, this tool casing 46 being arranged on the spindle head 18 and at least partly surrounding the tool 20. An acceleration or structure-borne sound sensor 36 is represented schematically on the spindle head 18. A corresponding structure-borne sound sensor 36 may also be arranged, for instance additionally, on the workpiece carrier 42 or on the workpiece carrier 42.

[0102] Represented schematically in FIG. 2b are electrical connections and/or wireless connections of the measuring device 38 to one or more structure-borne sound sensors 36. Also shown are connections of the control device 40 to the operator interface 28, which is not represented in FIG. 2a, and a connection, indicated exemplarily by a broken line, to the status indicator in the form of a set of traffic lights 34.

[0103] Further shown in FIG. 2b is a connection 48 to the (higher-order) control system of the machine tool 10.

[0104] For the purpose of machining a workpiece 24, the workpiece is first inserted in the workpiece holder 26 and fixed in position, for instance clamped, such that the workpiece 24 is held by the workpiece holder 26. The tool 20 and the spindle head 18 are configured to be movable, such that the tool 20 can be moved to the workpiece 24 in order to machine it. It is conceivable in this case that the tool 20, for instance the entire spindle head 18, is configured to be movable by more than one spatial direction, in order to ensure comprehensive machining of the workpiece 24, at least in some embodiments.

[0105] In a state in which the tool 20 is already rotating, but the workpiece 24 is not yet being machined, the control device 40 can initiate recording of a reference spectrum 50 (or background spectrum). In this case, the control device 40 reads-out directly, or indirectly, i.e. via the measuring device 38, the signals of the at least one structure-borne sound sensor 36, and transforms the signals, recorded in time series, into a frequency representation. There are a multiplicity of algorithms available for this purpose, the fast Fourier transform algorithm (FFT) being used in the present case, at least in certain embodiments, and not to be understood in a limiting sense. Such a reference spectrum 50 is represented schematically in FIG. 3a.

[0106] Understood herein as a time series is the vibration amplitude of the machine tool 10, i.e. the structure-borne sound.

[0107] A representation in which the amplitudes of the vibrations of the machine tool 10, i.e. the structure-borne sound, are sensed/calculated/represented in respect of their frequency components is understood as a frequency domain. The frequency domain provides information on the amplitude and frequency at which the machine tool 10 is vibrating.

[0108] When the workpiece 24 is being machined, for instance ground or polished, by the tool 20, the signals that can be sensed by the structure-borne sound sensors 36, i.e. the structure-borne sound of the machine tool 10, change. The control device 40 in this case can record a so-called actual spectrum 54, i.e. can read-out the signals of the structure-borne sound sensors 36 and transform them into the frequency domain. The reference spectrum 50 can then be subtracted from the thus obtained actual spectrum 54, in order to obtain a differential spectrum 56. A corresponding actual spectrum 54 is shown exemplarily in FIG. 3b. A corresponding differential spectrum 56 is shown exemplarily in FIG. 3c.

[0109] Usually, such spectra have differing so-called peaks 52. These peaks 52 show how much power of the structure-borne sound is present in a certain frequency (band). The peaks 52 are produced primarily as the result of the occurrence of a periodic motion such as, for example, the rotation of the tool 20. The peaks 52 show dominant or characteristic, structure-borne sound frequencies that can occur during operation of the machine tool 10. Usually, these peaks 52 are not sharp, but have a certain lack of definition, i.e. width, in the frequency domain. This is associated, for instance, with the fact that the structure-borne sound signals are partially damped, and for instance a certain dispersion of the structure-borne sound signals in the machine tool 10 occurs as the structure-borne sound propagates from the source of the structure-borne sound to a structure-borne sound sensor 36.

[0110] It is understood that the reference spectrum 50 may be stored in a storage unit of the control device 40 in order to enable a rapid calculation of the differential spectrum 56.

[0111] In certain embodiments, during operation of the machine tool 10, actual spectra 54 are determined continuously and subtracted from the reference spectrum 50, in order to obtain corresponding differential spectra 56.

[0112] The power contained in the differential spectra 56, i.e. the area under the curve of a differential spectrum 56, is added up.

[0113] In this case, the power contained in a peak 52, as represented schematically in FIG. 4 by an ideal-characteristic peak 52, can be determined as follows: The area content of an area that is defined by the lines 58 and 60 and the peak 52 can be calculated in a manner known per se. In this case the lines 58 and 60 are arranged symmetrically around a peak maximum 62. All areas obtained in such a manner are then added up.

[0114] It is understood that this method is cited only by way of example. It is also conceivable to add up each individual discrete value of the differential spectrum 56, in the manner of a numerical integration. A value of the differential spectrum is multiplied by the corresponding interval width, also in this case referred to as the resolution of the spectrum, in order to determine a sub-area below the spectrum. The thus obtained subareas are then added in order to determine the area contained in the spectrum, and thus the power.

[0115] Then, as represented schematically in FIG. 5, the power contained in the differential spectra 56 can be plotted over time. In FIG. 5, power contained in the differential spectrum 56 is plotted along the ordinate, with time being plotted along the abscissa. In this way, a type of pulsation 64 can be determined, this pulsation 64 providing information on the magnitude of the power of the broadband structure-borne sound signal in relation to the background noise, i.e. the reference spectrum 50.

[0116] This means, in other words, the lesser the amplitude of the pulsation 64, the less additional structure-borne sound of the machine tool 10 has been acquired. In a state in which a workpiece 24 is not yet being machined by the machine tool 10, there is no additional structure-borne sound present, at least in certain embodiments. This means that the pulsation 64 has a relatively low amplitude, for instance close to 0. Such as state is shown, for example, by the references 66 and 68 in the case of the pulsation 64 in FIG. 5.

[0117] As the intensity of machining of a workpiece 24 increases, the additional structure-borne sound also increases. As a result, the power contained in the differential spectra 56 increases, and ultimately the amplitude of the pulsation 64. Such a state is shown, for instance, by the reference 70 in the case of the pulsation 64 in FIG. 5.

[0118] By evaluation of this pulsation 64, the actual state of the machine tool 10 can be determined in a simple manner. In this case, for example, threshold values may be defined for the obtained pulsation 64 and, if a corresponding threshold value is exceeded, for example an alarm signal may be output to an operator.

[0119] It is further possible to switch off the machine tool 10 in an automated manner upon exceeding of a threshold value, in order thus to prevent damage to the workpiece 24 or to the machine tool 10, or even to prevent any hazard to an operator.

[0120] Moreover, it is possible to regulate the infeed or machining speed of the machine tool 10, such that the machining of a workpiece 24 is controlled according to the contained power in the differential spectrum 56, i.e. according to the additional structure-borne sound, and ultimately in dependence on the pulsation 64.

[0121] Illustrated in a highly simplified manner in FIG. 6, on the basis of a schematic flow diagram, is an exemplary method for determining an actual state of a machine tool 10. In this case, in a first step 72, a measuring device 38 is provided, which comprises at least one structure-borne sound sensor 36, for instance a piezoelectric sound sensor 36. In a following step 74, the structure-borne sound signals of the machine tool 10 are acquired, the actual state of the machine tool 10 being determined, in a following step 76, by means of the acquired structure-borne sound signals. The differential spectrum 56 is formed from a broadband reference spectrum 50 and a broadband actual spectrum 54, wherein the actual state of the machine tool 10 is determined on the basis of the differential spectrum 56. The actual state of the machine tool 10 can then be output, in a following step 78.

[0122] In certain embodiments, the power of a differential spectrum 56 is evaluated between the acquisition of the structure-borne sound signals of the machine tool 10 in step 74 and the determining of the actual state of the machine tool 10 in step 76. This is to be explained in greater detail in the following.

[0123] In a step 80, a reference spectrum 50 is compiled on the basis of the acquired structure-borne sound signals, wherein in this case the structure-borne sound signals are sensed while the machine tool 10 is in operation, but the workpiece 24 is not yet being machined, at least in certain embodiments. In a further step 82, an actual spectrum 54 is determined on the basis of acquired structure-borne sound signals, the structure-borne sound signals being recorded while the machine tool 10 machines the workpiece 24. In the determination of the spectra, i.e. the actual spectrum 54 and the reference spectrum 50, the structure-borne sound signals are transformed into the frequency domain, for instance by means of FFT.

[0124] In a subsequent step 84, the reference spectrum 50 is subtracted from the actual spectrum 54, and as a result a differential spectrum 56 is determined, and the power contained in the differential spectrum 56 is added up. In a following step 86, the power contained in the differential spectrum 56 is represented/evaluated as amplitude over time. Then, in step 76, the actual state of the machine tool 10 can be determined on the basis of this amplitude.

[0125] The structure-borne sound/vibration of the machine tool 10 is acquired continuously or quasi-continuously, at least in certain embodiments, wherein the reference spectrum 50 is subtracted from the thereby obtained actual spectra 54, in order to determine, at each time-point, a differential spectrum 56, for instance the power in the differential spectrum 56, and thus the additional structure-borne sound. The thus obtained characteristic of the power in the differential spectra 56 over time is equal to a pulsation 64.

[0126] The actual state of the machine tool 10 may be determined at each time-point. In general, it is thus possible to determine the actual state of the machine tool 10 directly, or with only a slight delay, such that malfunctions can be identified at an early stage and damage to the machine tool 10 can reliably be prevented.

[0127] Moreover, in this way, predictions become possible. For example, as a result of determination of the instantaneous rise of the pulsation 64, the further characteristic can be estimated. It is thus possible to react accordingly, even before the machine tool 10 is operated outside of the admissible range.

[0128] It is further conceivable to create a self-regulating machine tool 10, since controlled variables, such as the rotational speed of the tool 20 or the infeed speed of the workpiece 24, can be regulated on the basis of the actual state such that the structure-borne sound signals obtained, for instance the amplitude of the obtained pulsation 64 remains, as far as possible, within the admissible range. The self-regulating machine tool 10 can thereby be controlled more rapidly and with greater precision than a machine tool 10 that is set-up by an operator.