WORK SPINDLE WITH A MONITORING DEVICE AND METHOD FOR MONITORING A WORK SPINDLE

Description

[0001] The invention relates to a work spindle with a monitoring device and a method for monitoring a work spindle.

[0002] On machine tools, contamination on the tool contact surfaces can cause suboptimal tool clamping. Contamination can consist in particular of material chips that inevitably arise when machining a workpiece. As a result, misalignment or axial offset of the tool can lead to defective machining. This effect is further exacerbated by the use of long tools. In order to improve production quality and reduce the proportion of scrap, it is desirable to be able to detect the occurrence of faulty tool clamping before machining begins. It has therefore already been proposed to check the quality of the tool clamping on machine tools by monitoring the flat contact of the tool. EP 1 889 685 B1 discloses a monitoring device with a measuring device provided on the circumferential surface of a spindle head, which is connected to the jacket or face.Vibrations or deformations occurring on the circumferential surface due to imbalance or contamination are detected. This can be a mechanical dial gauge whose probe rests on the circumferential surface of the spindle head, or the sensor can be an inductive or capacitive sensor or in the form of a strain gauge arranged on the spindle head. Furthermore, a mechanized, electrical, electromechanical, acoustic, or optical measuring device is mentioned as a possibility. With the exception of the mechanical dial gauge, the specific implementation of the aforementioned types of measuring devices is not discussed. The sensor is arranged on a section of the rotor that protrudes from the front of the stator of the work spindle.

[0003] Mechanically scanning the circumferential surface of a work spindle's rotor head with a dial gauge requires the rotor to rotate slowly and then remove the probe from the rotor surface at normal operating speed. Active measuring systems with sensors on the rotor and with power and data transmission between the stator and rotor are complex to manufacture, thus expensive, and difficult to replace. Installing sensors in the rotor reduces its mechanical rigidity. Measuring systems that measure runout near the tool protrude into the machining area. A sensor is undesirable there because chips can collect there and the sensor is difficult to protect. An additional measuring ring on the rotor is difficult to mount in such a way that it experiences all deformations.When measuring a runout error further away from the tool, there is a correspondingly smaller deformation of the rotor, which impairs the measurement accuracy.

[0004] DE 10 2016 112924 A1 discloses a machine tool unit with a stator unit and a rotor unit, which comprises a tool clamping device for fixing and clamping a tool. At least two clamping force sensors are provided to detect the clamping force. The clamping force sensors are designed as proximity sensors for detecting a distance between the rotor unit and the stator unit in order to measure the clamping force via a deformation of a part of the rotor unit, wherein the clamping force sensors are arranged at mutually different angular positions around the axis of rotation in order to determine the position and / or orientation of the axis of rotation during a machining operation of the machine tool. At least two sensors are provided, offset by 90 degrees with respect to the axis of rotation of the spindle, each with radial or axial sensitivity, which detect the width of a radial or axial displacement.axial air gap, whereby the radial sensor can be a magnetic sensor. The installation of at least two sensors at two positions offset by 90 degrees in the circumferential direction and their cabling are complex.

[0005] Based on this, it is the object of the invention to provide a work spindle with a monitoring device and a method for monitoring a work spindle, which overcome the disadvantages of the prior art and make it possible to detect a faulty tool clamping with high accuracy and reliability by measuring a deformation of the rotor head of the work spindle caused by clamping a tool or tool holder.

[0006] This object is achieved according to the invention by a work spindle having the features of claim 1 and by a method having the features of claim 11. Advantageous embodiments of the invention are specified in the respective subclaims.

[0007] According to the invention, in a work spindle with a monitoring device which has at least one magnet arranged on the stator or on the rotor of the work spindle and at least one magnetic field sensor arranged on the stator radially opposite a measuring section of the rotor head, the measuring section is penetrated by the magnetic field of the magnet and, when a tool or tool carrier is clamped on the rotor head by means of a tool clamping device arranged in the rotor, is penetrated by an axial force flow.The rotor head has a magnetizability or magnetization that varies in the axial direction in its measuring section, and the magnetic field sensor is direction-sensitive and connected to an evaluation device that is configured to determine a measure of the axial deformation of the rotor head as a function of the angle of rotation from the strength of the magnetic field detected by the magnetic field sensor during one revolution of the rotor as a function of the angle of rotation.

[0008] In this way, both the average axial deformation of the rotor head in the circumferential direction can be determined, which depends on the clamping force of the tool clamping device, thus enabling its monitoring, and a variation in the deformation in the circumferential direction can be detected, which is a clear indication of the presence of contamination in the contact surface between a clamped tool or tool holder and the rotor head, particularly in the form of a chip on the frontal flat contact surface of the rotor head. A magnetic field sensor operates without contact and requires no signal transmission between the rotor and stator. The sensor and the entire signal processing electronics can be housed inside the stator in a location where they are protected from contamination and mechanical influences of any kind.

[0009] Furthermore, it is advantageous if the evaluation device is configured to determine a measure of the axial position of the rotor head relative to the stator from the strength of the magnetic field detected by the magnetic field sensor during one rotation of the rotor as a function of the angle of rotation. In this way, the same sensor can also be used to detect the systematic axial displacement of the rotor relative to the stator during operation of a work spindle, known as spindle growth. The extent of this displacement must be known for its compensation during axial tool positioning.

[0010] Preferably, the at least one magnet is arranged on the stator radially opposite the measuring section of the rotor head, with its poles radially aligned. The measuring section of the rotor head has at least two grooves on its surface extending in the circumferential direction and spaced apart from one another in the axial direction. This design makes it easy to cause a strong distortion of a magnetic field at the location of the sensor through axial deformation and / or displacement of the rotor head.

[0011] Such grooves preferably have, at least at one location in the circumferential direction of the measuring section of the rotor head, a marking of a predetermined length in the circumferential direction in the form of an interruption or an axial offset of a predetermined magnitude. This creates a zero-point reference for the angle of rotation of the rotor head, which can be detected by the magnetic field sensor itself, thus eliminating the need for additional sensors. If an axial offset of a predetermined magnitude is used as the marking, this also provides the possibility of self-calibration of the magnetic field sensor by assigning an axial displacement of a known magnitude to the signal change occurring when passing through the offset groove section.

[0012] As an alternative to a magnet on the stator and slots in the rotor head, the at least one magnet can consist of at least three magnets arranged axially consecutively in the measuring section of the rotor head or of at least three axially consecutive magnetized regions of the rotor head, with the magnetic poles each aligned radially and two axially consecutive magnets or magnetized regions having an inverted radial arrangement of the magnetic poles. This also results in an axial distortion of a magnetic field penetrating the magnetic field sensor in the event of an axial deformation or displacement of the rotor head.

[0013] By analogy with an interruption or an axial offset of a predetermined extent of grooves, magnets or magnetized regions arranged in the measuring section of the rotor head can have, at least at one location in the circumferential direction of the measuring section of the rotor head, a marking of a predetermined length in the circumferential direction in the form of an interruption of the magnetization or an axial offset of a predetermined extent of the magnetization in order to create a zero point reference for the angle of rotation of the rotor or to enable self-calibration of the magnetic field sensor.

[0014] Preferably, the magnetic field sensor is a magnetoresistive sensor having at least one bridge circuit of resistors whose value depends on the strength and direction of a magnetic field passing through them, the axial variation of the magnetizability or magnetization of the measuring section of the rotor head is periodic, and the geometric arrangement of the resistors is adapted in the axial direction to this axial variation such that an axial displacement of the rotor head relative to the stator by a period length of the axial variation results in a periodic course of the output signal of the bridge circuit as a function of the axial displacement of the rotor head with the same period length.

[0015] Magnetoresistive sensors have high sensitivity and a pronounced directional dependence, as required for the application according to the invention. This allows position and deformation measurements with very high resolution in the nanometer range. The coordination between the sensor structure and the axial variation of the magnetizability or magnetization greatly simplifies the conversion of a change in the sensor signal into a change in position or deformation.

[0016] The evaluation device preferably contains a memory in which a reference signal is stored, which was derived from at least one sensor signal recorded during one rotation of the rotor with a correctly clamped tool or tool holder. It is configured to determine the axial deformation of the rotor head based on the deviation between a signal derived from at least one sensor signal recorded during one rotation of the rotor with a clamped tool or tool holder and the stored reference signal. This allows interference such as noise in a recorded measurement signal (which here refers to spatial noise rather than temporal noise) or a systematic offset in a recorded measurement signal to be largely eliminated.

[0017] Preferably, the evaluation device is configured to derive a signal from the axial deformation of the rotor head, indicating a faulty clamping state of a tool or tool holder. Such a faulty clamping state can result either from contamination of a tool contact surface of the rotor head or from insufficient clamping force due to a defect or excessive wear of the tool clamping device.

[0018] A plurality of direction-sensitive magnetic field sensors can also be arranged on the stator radially opposite the measuring section of the rotor head, spaced apart from one another in the circumferential direction of the stator, and each connected to the evaluation device. This can accelerate the measurement because a full rotation of the rotor is not required to detect the axial position of the measuring section of the rotor head over the entire circumference of the rotor head, or the measurement can be performed simultaneously at a plurality of points along the circumference of the rotor head while the rotor is stationary.

[0019] A method according to the invention for monitoring a work spindle according to the invention comprises the following steps: a) Recording at least one signal which is derived from at least one signal emitted by a magnetic field sensor within one revolution of the rotor using a tool or tool holder clamped to the rotor of the work spindle; b) Calculating a position signal from the recorded signal which indicates the axial position of a measuring section of the rotor head radially opposite the magnetic field sensor as a function of the angle of rotation of the rotor within one revolution of the rotor; c) Calculating a difference signal as a function of the angle of rotation of the rotor within one revolution of the rotor from the calculated position signal and a reference signal stored in a memory;d) Determining at least one measure of the axial deformation of the rotor head from the calculated difference signal, e) Comparing the determined measure of the axial deformation of the rotor head with a limit value and issuing a status signal which indicates a faulty clamping state of a clamped tool or tool holder, depending on the comparison result.

[0020] By forming the difference between the position signal calculated from recorded measured values ​​and a stored reference signal, interference effects can be largely eliminated and a reliable measure of the deformation of the rotor head can be determined, based on which it can be easily assessed by comparing it with a limit value whether the clamping state of the tool or tool holder is correct or faulty.

[0021] Preferably, the reference signal is determined by applying steps a) and b) with a faultless clamping of a similar tool or tool holder and stored in the memory. This easily defines a criterion for evaluating a current clamping state as correct or faulty.

[0022] When performing step c), the respective mean value of the position signal and the reference signal is preferably calculated, and the mean value is first subtracted from each of the two signals. Subsequently, to calculate the difference signal, the zero-mean reference signal is subtracted from the zero-mean position signal, and the status signal indicates the presence of contamination between a clamped tool or tool carrier and a surface of the rotor head. By subtracting the respective mean values, a systematic offset in the sensor signal plays no role in the accuracy of calculating the deviation between the current position signal and the reference signal.

[0023] If, when determining the reference signal, steps a) and b) are carried out a first time without axial force being applied to the rotor head and a second time with a correctly clamped, similar tool or tool holder, the reference signal is calculated by subtracting the position signal obtained in the first implementation from the position signal obtained in the second implementation, when carrying out step c) the respective mean value of the position signal and the reference signal is calculated and, to calculate the difference signal, the mean value of the reference signal is subtracted from the mean value of the position signal, a status signal can be generated which indicates the application of insufficient clamping force by the tool clamping device.Using the same measuring device, another type of tool clamping malfunction can be detected simply by performing an additional measurement and expanding the signal processing.

[0024] By performing steps a) and b) without applying axial force to the rotor head in a defined reference state of the work spindle, calculating the mean value of the obtained position signal and storing it as a reference mean value, performing steps a) and b) without applying axial force to the rotor head in a current operating state of the work spindle and calculating the current mean value of the obtained position signal, an axial displacement of the rotor head relative to the stator can be calculated by subtracting the reference mean value from the current mean value and transmitted to a higher-level control unit.Using the same measuring device, simply by extending the signal processing, further status information that is important for the operation of a work spindle in the form of an axial displacement of the rotor head can be recorded and a deterioration in the quality of the workpiece machining can be avoided.

[0025] An embodiment of the invention is described below with reference to the drawings, in which Fig. 1the front part of a work spindle with a clamped tool carrier, Fig. 2an enlarged view of section A of Fig. 1 , Fig. 3 an electrical block diagram of a device according to the invention, Fig. 4 an enlarged view of section B of Fig. 2 , Fig. 5 the two measuring signals of the sensor of a device according to the invention, Fig. 6 the measuring signals of Fig. 5 for three different cases after low-pass filtering, Fig. 7, which is obtained from the filtered measurement signals of Fig. 6 calculated axial position of the rotor head, Fig. 8, which is derived from the position of Fig. 7 and a stored reference signal calculated axial deformation of the rotor head and Fig. 9 a representation of the inventive processing of sensor signals for detecting contamination in the form of a program flow chart.

[0026] Fig. 1 shows the front end section of a work spindle 1, the main components of which are a stator 2 and a rotor 3 mounted therein. Located in the rotor 3 is a tool clamping device 4, by means of which a tool carrier 6 is clamped to a front end section 5 of the rotor 3, which is referred to below as the rotor head 5. This structure of a work spindle 1 is known in specialist circles and therefore requires no further explanation here. The tool clamping device 4 has a plurality of collets 7 along its circumference, which exert a clamping force on the tool carrier 6 that is evenly distributed in the circumferential direction and acts on the rotor head 5 in the axial direction via the flat contact surface 8.

[0027] The presence of a contamination, particularly in the form of a chip on the flat contact surface 8, causes an irregularity in the clamping force distribution in the circumferential direction and thus also a corresponding irregularity in the elastic deformation of the rotor head 5. This irregularity in the deformation of the rotor head 5 has both a radial and an axial component, whereby the terms axial and radial here always refer to the rotational axis R of the work spindle. According to the invention, the axial component of such an irregular deformation of the rotor head 5 is measured. The structure of the measuring device is described below with reference to Fig. 2 described, which enlarges the Fig. 1 shows the section marked A of the front end section of the work spindle 1.

[0028] On the side of the rotor head 5, the measuring device consists of Fig. 2 shows, from a series of axially spaced grooves 9 in the circumferential direction on the outer surface of the rotor head 5. Each two adjacent grooves 9 are separated from each other by a web 10 that also runs in the circumferential direction. All grooves 9 have the same width. This also applies to the webs 10, so that overall a periodic sequence of grooves 9 and webs 10 results in the axial direction. In the Fig. 2 In the example shown, four grooves 9 and three webs 10 are provided. The grooves 9 and webs 10 together define a measuring section 11 of the rotor head. It is essential that the measuring section 11 is located in an area of ​​the rotor head 5 through which a force flow in the axial direction occurs when a tool or tool holder 6 is clamped, so that said clamping causes an elastic deformation of the measuring section 11 in the axial direction. With error-free tool clamping, this deformation is approximately uniform along the circumference of the rotor head 5, whereas in the case of contamination it varies irregularly along the circumference of the rotor head 5.

[0029] On the side of the stator 2, the measuring device contains a magnetic field sensor 12, which is constructed in the form of a chip on a circuit board 13, which is installed in a cavity in the stator 2 such that the magnetic field sensor 12 is radially opposite the measuring section 11 of the rotor head 5 and is separated from it only by a narrow radial gap. On the radially outer side of the circuit board 13, a permanent magnet 14 is arranged symmetrically to the magnetic field sensor 12 such that its magnetic field runs in the radial direction within it. Fig. 2 In the example shown, the south pole of the permanent magnet 14 is located at its radially inner end and the north pole at its radially outer end. Accordingly, without the special shape of the measuring section 11 of the rotor head 5, the magnetic field lines through the circuit board 13 and the sensor chip 12 would also run approximately radially and at least symmetrically to a common radial center axis M of the magnetic field sensor 12 and the permanent magnet 14.

[0030] Due to the grooves 9 and webs 10 of the measuring section 11, the magnetic field of the permanent magnet 14 is distorted in the axial direction in the area of ​​the magnetic field sensor 12. However, without any deformation of the rotor head 5 or with a uniform deformation of the rotor head 5 in the axial direction along the circumference of the rotor head 5, the distortion is uniform along the entire circumference of the rotor head 5. If the axial deformation of the rotor head 5 in the case of contamination, such as the presence of a chip on the flat contact surface 8 ( Fig. 1 ) when clamping a tool or tool holder 6, is uneven along the circumference of the rotor head 5, then so is the distortion of the magnetic field by the measuring section 11.

[0031] The distortion of the magnetic field depends on the position of the grooves 9 if the material of the rotor head 5 has a high magnetic permeability, in particular if it is ferromagnetic, which can generally be assumed for a rotor head 5 of a work spindle 1 of a machine tool. To avoid contamination, the grooves 9 can be filled with a non-magnetic material, in particular a plastic.

[0032] The axial distortion of the magnetic field through the measuring section 11 is measured according to the invention using the magnetic field sensor 12, which has a strongly direction-dependent sensitivity. The circuit board 13 with the magnetic field sensor 12 and the permanent magnet 14 is installed in a housing 15, which in turn is installed in a cavity 16 in the stator 2, and on the rear of which a signal processing electronics 17 is mounted. A cable 18 ( Fig. 1 ) from the stator 2 of the work spindle 1 to the machine control of the processing machine, of which the work spindle 1 is a component. Signal transmission from the rotor 3 to the stator 2 is therefore not required.

[0033] A known type of direction-sensitive magnetic field sensor 12 is a magnetoresistive sensor, in which the value of an electrical resistance strongly depends on the direction of a magnetic field passing through it. Such magnetoresistive sensors are known as such and are commercially available, preferably in the form of four resistors connected to form a bridge circuit such that their output signal is proportional to the strength of a magnetic field in the sensitivity direction of the magnetic field sensor 12. Typically, two such bridge circuits of magnetic-field-sensitive resistors are provided together on a chip in a specific geometric arrangement, which is explained below.

[0034] Fig. 3 shows a section of the measuring section 11 of the rotor head 5 with a period of length L of the sequence of grooves 9 and webs 10. The magnetic field sensor 12 contains two magnetoresistive bridge circuits, the respective resistances S1 to S4 and C1 to C4, as in Fig. 3 shown schematically, are arranged in such a way that they are nested in the direction of the position of the measuring section 11 to be measured, ie here in the axial direction of the rotor head 5, that in this direction a resistance Sn of one bridge circuit is always followed by the corresponding resistance Cn of the other bridge circuit, two resistances of each bridge circuit being arranged one above the other.

[0035] Here, the length of the entire resistor arrangement in the direction of the position of the measuring section 11 to be measured and the period length L of the sequence of grooves 9 and webs 10 are coordinated such that when the measuring section 11 is shifted by a period length L, one bridge circuit comprising resistors S1 to S4 outputs one period of a sinusoidal output signal and the other bridge circuit comprising resistors C1 to C4 outputs one period of a 90° phase-shifted, i.e., cosinusoidal output signal. In this case, using the well-known arctan2 function, an output signal that is linearly dependent on the axial position of the measuring section 11 within a period length L can be generated from the two bridge output signals.

[0036] This concept of position measurement is known per se and therefore requires no further explanation here. It is typically used to measure large displacements, which are a multiple of the period length L, or to measure the rotational speed of gears. According to the invention, however, the concept is used to measure the smallest deformations in the nanometer range along the circumference of a rotor head 5 of a work spindle 1, which represents a massive misuse compared to previously known applications. This requires a different type of signal processing than the one required to measure large displacements or rotational speeds. This special type of signal processing is described below.

[0037] A block diagram of a magnetic field sensor 12 with two bridge circuits on a chip is provided with a signal processing electronics 17 according to the invention in Fig. 4 Both bridge output signals are first amplified by an amplifier 19, then converted by an analog / digital converter 20 into digital signals 21 and 22, respectively, which are fed to a digital signal processing unit 23. This unit is connected to a memory 24, in which a reference signal to be defined later is stored, and to a control unit 25 of the processing machine, of which the work spindle 1 is a component.

[0038] An example of measured curves of these raw signals 21 and 22 as a function of the angle of rotation of the rotor 3 of the work spindle 1 over a little more than one complete revolution is shown in Fig. 5 It can be seen that both signals 21 and 22 are highly noisy, which results from the irregularities in the surface of the measuring section 11 of the rotor head 5. These irregularities affect both signals 21 and 22 equally, as the clearly visible similarity of the two signals 21 and 22 shows. These raw signals 21 and 22 are not yet proportional to an axial deformation of the measuring section 11 of the rotor head 5. The first operation performed in the digital signal processing unit 23 is low-pass filtering of the sensor signals 21 and 22, whereby the variable here is not time, but the angle of rotation of the rotor 3, i.e., this is a spatial filtering to suppress spatial noise.

[0039] An example of the curves of signals 21 and 22 as a function of the angle of rotation of the rotor 3 of the work spindle 1 over a little more than one full revolution after the low-pass filtering is shown in Fig. 6 , where three different signal curves are shown, namely as a continuous line the curve with trouble-free tool clamping, ie without contamination of the flat contact surface 8, as a dashed line the curve with contamination of the flat contact surface 8 by a small chip and as a dash-dotted line the curve with contamination of the flat contact surface 8 by a medium-sized chip. As can be seen from Fig. 6 As can be seen, the strong noise is largely eliminated by the low-pass filtering and there are significant deviations between the curves of the three different cases, particularly in the rotation angle range between approximately 220° and 320° for both signals.

[0040] From the two low-pass filtered signals of Fig. 6 a position signal is then calculated in the digital signal processing unit 23 using the arctan2 function, the course of which as a function of the angle of rotation of the rotor 3 of the work spindle 1 is again shown for the three previously mentioned cases in Fig. 7 The results obtained from the two individual signals in Fig. 6 noticeable deviations between the three different cases occur in the position signal of Fig. 6 becomes even clearer, revealing that the deviations are not limited to the range between approximately 220° and 320°, although they are greatest there. While the scales of the ordinates in the Figuren 5 und 6 nor digitized voltage values, the unit of the ordinate of Fig. 7 Nanometers, where it is less the absolute value that is important, but rather its change over one revolution of the rotor 3, i.e. over a rotation angle of 360°.

[0041] As from Fig. 7 As can be seen, the position signal is periodic and has a similar curve in all three cases, whereby the deviation caused by a chip clamped on the flat contact surface 8 compared to the curve with contamination-free tool clamping depends on the size of the chip. In order to extract this deviation solely as a signal, the mean value of the position signal is first Fig. 7 over a complete revolution of the rotor 3 and from the position signal of Fig. 7 subtracted to obtain a position change signal with a mean value of zero. From this mean-free position change signal, a value is then stored in a memory 24 ( Fig. 3 ) stored reference signal is subtracted. This is the signal resulting from the Fig. 7 as a continuous line shown position signal by subtracting its mean value, ie the mean-free position change signal in the case of a contamination-free tool clamping.

[0042] The result of this operation is in Fig. 8 shown. In the case of correct tool clamping, the signal curve logically results in the zero line, since in this case the reference signal is subtracted from itself. In the case of contamination, significant deviations of the signal curve from the zero line occur over an entire revolution of the rotor 3, even with contamination from a small chip. The unit of the ordinate is also in Fig. 8 Nanometers, which means that even the smallest axial deformations can be reliably and accurately detected.

[0043] For the accuracy of the Fig. 8 shown curve of the difference between the position change signal and the stored reference signal, it is of essential importance that the signal to be analyzed is correctly synchronized with the stored reference signal, i.e. that pairs of values ​​of both signals are subtracted from one another which are assigned to exactly the same angular position of the rotor 3. This can be ensured by calculating the cross-correlation function for each of the two signals for different displacements of the two signals relative to one another, based on a known approximation of the correct point-by-point assignment of the two signals, and selecting the assignment of pairs of values ​​for which the cross-correlation function has a maximum for the subtraction.

[0044] An approximation of the correct point-by-point assignment of the two signals can be obtained, for example, using a marking at a location on the circumference of the measuring section 11 of the rotor head 5, which generates a characteristic pulse in the output signal of the magnetic field sensor 12. A simple type of such a marking is a short interruption of the grooves 9 or a short axial offset of the grooves 9 and webs 10. Both structures result in a short section of characteristic shape in both a signal to be analyzed and a stored reference signal during one rotation of the rotor 5, which provides approximate information about the correct point-by-point assignment of the two signals.

[0045] If this marking has the form of an axial offset of precisely defined extent of the grooves 9 and webs 10, then it can be used not only for approximate synchronization between a currently detected position signal and a stored reference signal but also for automatic calibration of the position determination, since in this case it is precisely known which axial position change of the rotor head 5 the change in the sensor signal occurring at the offset flank of the grooves 9 and webs 10 would correspond to.

[0046] Finally, the Fig. 8 The differential signal shown must be evaluated to determine whether its deviation from the zero line is so significant that it indicates the presence of contamination on the flat contact surface. A simple evaluation criterion is the magnitude of the differential signal exceeding a threshold value at any value of the angle of rotation. Another possible criterion is the mean value of the magnitude of the differential signal over an entire revolution exceeding a threshold value. These criteria can also be combined in the form of a logical operation (e.g., AND, OR) to increase the reliability of error detection.

[0047] In summary, the functionality of the inventive processing of the signals detected by the magnetic field sensor 12 in the digital signal processing unit 23 for detecting contamination is shown in Fig. 10 in the form of a program flow chart. When a measuring cycle has been started, which can be triggered in particular by a corresponding control signal from the control unit 25, in step 26 the two signals 21 and 22 of the magnetic field sensor 12 are recorded over one complete revolution of the rotor 3 of the work spindle 1. In step 27 the signals are subjected to spatial low-pass filtering and in step 28 a single signal is calculated from the two phase-shifted signals, which indicates the axial position of the measuring section 11 of the rotor head 5. In step 29 the mean value of this position signal over one complete revolution of the rotor 3 is subtracted from this position signal.

[0048] The resulting zero-mean position change signal is synchronized in step 30 using cross-correlation with a zero-mean reference signal stored in memory 24. In step 31, the difference between the synchronized signals is calculated by subtracting the reference signal. In step 32, the difference signal is evaluated for the presence of contamination, and in step 33, the evaluation result is reported to the control unit 25 of the processing machine.

[0049] If contamination is reported in step 33, the control unit 25 initiates the release of the tool or tool holder 6 by the clamping device 4, its removal from the rotor head 5 by the tool changer, and automatic cleaning of the flat contact surface 8. If this is unsuccessful, and contamination is detected again after the tool or tool holder 6 is re-clamped, a corresponding error message is displayed, prompting intervention by the machine operator. If no contamination is detected during an inspection by the operator, the detected uneven deformation of the rotor head 5 indicates a defect in the tool clamping device 4 in the form of wear or breakage of an individual collet.

[0050] The determination of the reference signal stored in the memory 24 is also carried out with the aid of the device according to the invention, wherein a check before clamping a tool or tool carrier 6 ensures that no contamination is present, and the process according to Fig. 9 only steps 26 to 28 are executed. After step 28, the signal present at this point is stored as a reference signal in memory 24.

[0051] It is advantageous because the simplest method is always to measure at a constant speed of rotor 3. Such a measurement can be performed, for example, while the work spindle 1 is approaching the machining position after a tool change. The speed can be significantly below the normal operating speed. In principle, a measurement while the rotor 3 is accelerating to the operating speed is also possible, but this requires greater signal processing complexity due to the variation in the relationship between time and angle of rotation.

[0052] If, as a result of carrying out the complete process according to, no significant non-uniformity of the deformation of the rotor head 5 along its circumference can be detected, it can be assumed that there is no contamination between the rotor head 5 and the tool or tool carrier 6.

[0053] By means of the device according to the invention, it can also be determined whether the clamping force of the tool clamping device 4 lies within the permissible range. For this purpose, during a tool change, either in the released state of the tool clamping device 4 or in the clamped state thereof without the presence of a tool or tool carrier 6, i.e. when the rotor head 5 is not axially deformed due to the lack of force flow, a position measurement of the measuring section 11 of the rotor head 5 is carried out by means of the magnetic field sensor 12 during one revolution of the rotor 3. The output signals of the magnetic field sensor 12 are processed according to steps 26 to 28 of the sequence shown in Fig. 10. Subsequently, as in step 29, the mean value of the position signal is calculated, but is not subtracted from the position signal, but instead stored.After clamping the tool or tool holder 6, the normal position measurement of the measuring section 11 of the rotor head 5 is carried out by means of the magnetic field sensor 12 during one revolution of the rotor 3 and the steps 26 to 28 of the signal processing according to Fig. 10 are also carried out and then the mean value of the position signal is also calculated.

[0054] The difference between the mean position values ​​is then calculated, which indicates the mean axial deformation that the measuring section 11 of the rotor head 5 has experienced due to the clamping of the tool or tool holder 6. This entire process is run for the first time when a new tool clamping device 4 is commissioned, and the difference between the mean position values ​​obtained before and after the clamping of the tool or tool holder 6 is stored as the reference value for the axial deformation. When a tool is changed during operation of the tool clamping device 4, the mean position values ​​before and after the clamping of the tool or tool holder 6 are determined in the same way and then subtracted from the stored reference value.

[0055] Assuming that the clamping force of a new tool clamping device 4 that has generated the reference value of the axial deformation is correct, then it can be determined whether the clamping force is still sufficient by comparing the deviation of the current difference of the position mean values ​​from the reference value with a suitable threshold. If the comparison proves that the mean axial deformation of the rotor head 5 during operation is significantly lower than the mean axial deformation with the correct clamping force, with the deviation between the two exceeding a threshold, then the clamping force must have decreased beyond the permissible level, indicating a defect or excessive wear of the tool clamping device 4.

[0056] An effect that overlays the position change of the measuring section 11 of the rotor head 5 due to the deformation caused by the clamping force of the tool clamping device 4 is the axial forward displacement of the entire rotor 3 of a work spindle 1, known as spindle growth, due to the heating of its bearings during operation. The resulting position change is not a deformation of the rotor head 5, but rather an axial displacement of the entire rotor 3. It plays no role in the detection of contamination, since in the latter case, the mean values ​​of both the current position signal and the reference signal are eliminated, and only the mean-free axial position change of the measuring section 11 of the rotor head 5 during one revolution is evaluated.

[0057] Spindle growth is potentially important for monitoring the clamping force of the tool clamping device 4, since the resulting change in position of the measuring section 11 of the rotor head 5 has the same direction as its position change due to a smaller axial deformation caused by a decreasing clamping force. However, spindle growth is a comparatively slow process, so it can be assumed that the resulting displacement of the rotor 3 remains almost constant during a tool change, which takes very little time on modern processing machines. Therefore, the measurement of the deformation of the measuring section 11 of the rotor head 5 due to the clamping force in the form of the difference between the positions with and without clamping force applied to the rotor head 5 is not distorted by spindle growth.

[0058] On the other hand, the spindle growth can be detected solely, for example, based on the previously mentioned position measurement of the measuring section 11 of the rotor head 5 by means of the magnetic field sensor 12 during one revolution of the rotor 3 during a tool change, either in the released state of the tool clamping device 4 or in the clamped state thereof without the presence of a tool or tool holder 6. For this purpose, a first measurement must be carried out in the cold state, i.e. at the start of operation of the processing machine, and the measured value obtained must be saved as a reference value. During operation, a position measurement can then be carried out with each tool change in the released state of the tool clamping device 4 or in the clamped state thereof without the presence of a tool or tool holder 6, and the deviation of the position value from the reference value can be calculated.Since there is no deformation of the rotor head 5 in these measurements, a detected change in position can only be due to spindle growth.

[0059] Alternatively, the mean position value can also be recorded with the tool or tool holder 6 clamped in the cold state, i.e., at the start of operation of the processing machine and during operation, if it can be assumed that any change in the position value during operation is caused almost exclusively by spindle growth. This is the case in practice, since the extent of spindle growth is significantly greater than the extent of the axial deformation of the rotor head 5 due to the clamping force, and the latter, moreover, generally changes only very slowly and slightly during operation.

[0060] By means of the work spindle according to the invention, depending on the type of signal processing, both contamination of a contact surface between the rotor head 5 and the tool or tool holder 6, as well as excessive wear or failure of the tool clamping device 4 can be detected, and the spindle growth can be detected without the use of an additional sensor.

Claims

1. Work spindle with a monitoring device, which has at least one magnet (14) arranged on the stator (2) or on the rotor (3) of the work spindle (1) and at least one magnetic field sensor (12) arranged on the stator (2) in a manner radially opposite a measurement portion (11) of the rotor head (5), wherein the magnetic field of the magnet (14) passes through the measurement portion (11) and, when a tool or tool holder (6) is clamped on the rotor head (5) by means of a tool clamping device (4) arranged in the rotor (3), an axial force flow passes through said measurement portion, wherein the rotor head (5) has, in its measurement portion (11), a magnetizability or magnetization that varies in the axial direction, wherein the magnetic field sensor (12) is direction-sensitive, and wherein it is connected to an evaluation apparatus (17) which is designed to determine a measure of the axial deformation of the rotor head (5) as a function of the angle of rotation from the strength of the magnetic field, as a function of the angle of rotation, detected by the magnetic field sensor (12) during one revolution of the rotor.

2. Work spindle according to claim 1, wherein the evaluation apparatus (17) is designed to determine a measure of the axial position of the rotor head (5) relative to the stator (2) as a function of the angle of rotation from the strength of the magnetic field detected by the magnetic field sensor (12) during one revolution of the rotor.

3. Work spindle according to claim 1 or 2, wherein the at least one magnet (14) is arranged on the stator (2), in a manner radially opposite the measurement portion (11) of the rotor head (5), with radial alignment of its poles, and wherein the measurement portion (11) of the rotor head (5) has, on its surface, at least two grooves (9) extending in the circumferential direction and spaced apart from one another in the axial direction.

4. Work spindle according to claim 3, wherein the grooves (9) have a marking of a predetermined length in the circumferential direction, in the form of an interruption or an axial offset of a predetermined extent, at least at one point in the circumferential direction of the measurement portion (11) of the rotor head (5).

5. Work spindle according to claim 1 or 2, wherein the at least one magnet consists of at least three magnets arranged axially successively in the measurement portion (11) of the rotor head (5) or of at least three axially successive magnetized regions of the rotor head (5) wherein the magnetic poles are each aligned in the radial direction and every two axially successive magnets or magnetized regions have an inverted radial arrangement of the magnetic poles.

6. Work spindle according to claim 5, wherein the magnets or magnetized regions arranged in the measurement portion (11) of the rotor head (5) have a marking of a predetermined length in the circumferential direction, in the form of an interruption of the magnetization or an axial offset of a predetermined extent of the magnetization, at least at one point in the circumferential direction of the measurement portion (11) of the rotor head.

7. Work spindle according to any of claims 1 to 6, wherein the magnetic field sensor (12) is a magnetoresistive sensor which has at least one bridge circuit of resistors, the value of which depends on the strength and direction of a magnetic field passing through them, and wherein the axial variation of the magnetizability or magnetization of the measurement portion (11) of the rotor head (5) is periodic and the geometric arrangement of the resistors in the axial direction is adapted to this axial variation such that an axial displacement of the rotor head (5) relative to the stator (2) by a period length (L) of the axial variation results in a periodic waveform of the output signal of the bridge circuit as a function of the axial displacement of the rotor head (5) with the same period length (L).

8. Work spindle according to any of claims 1 to 7, wherein the evaluation apparatus (17) contains a memory (24) in which a reference signal is stored which was derived from at least one sensor signal recorded during one revolution of the rotor (3) with a tool or tool carrier (6) clamped fault-free, and wherein the evaluation apparatus (17) is designed to determine the axial deformation of the rotor head (5) on the basis of the deviation between a signal derived from at least one sensor signal (21, 22) recorded during one revolution of the rotor (3) with a clamped tool or tool holder (6), and the stored reference signal.

9. Work spindle according to any of claims 1 to 8, wherein the evaluation apparatus (17) is designed to derive a signal which indicates a faulty clamping state of a tool or tool carrier (6) from the axial deformation of the rotor head (5).

10. Work spindle according to any of claims 1 to 9, wherein a plurality of direction-sensitive magnetic field sensors (12) are arranged on the stator (2) in a manner radially opposite the measurement portion (11) of the rotor head (5) and at a distance from one another in the circumferential direction of the stator (2), and are each connected to the evaluation apparatus (17).

11. Method for monitoring a work spindle (1) according to any of claims 1 to 10, comprising the following steps: a) recording at least one signal (21, 22) which, with a tool or tool carrier (6) clamped on the rotor (3) of the work spindle (1), is derived from at least one signal emitted by a magnetic field sensor (12) within one revolution of the rotor (3); b) calculating, from the recorded signal (21, 22), a position signal which indicates the axial position of a measurement portion (11) of the rotor head (5) radially opposite the magnetic field sensor (12) as a function of the angle of rotation of the rotor (3) within one revolution thereof; c) calculating a difference signal as a function of the angle of rotation of the rotor (3) within one revolution thereof from the calculated position signal and a reference signal stored in a memory (24); d) determining at least one measure of the axial deformation of the rotor head (5) from the calculated difference signal, e) comparing the determined measure of the axial deformation of the rotor head (5) with a limit value and emitting a state signal which indicates a faulty clamping state of a clamped tool or tool carrier (6), depending on the comparison result.

12. Method according to claim 11, wherein the reference signal is determined by applying steps a) and b) in the case of fault-free clamping of a similar tool or tool carrier (6), and is stored in the memory (24).

13. Method according to claim 11 or 12, wherein when carrying out step c), the respective mean value of the position signal and the reference signal is calculated and the mean value is first subtracted from each of the two signals and, to calculate the difference signal, the mean-value-free reference signal is subtracted from the mean-value-free position signal, and wherein the state signal indicates the presence of contamination between a clamped tool or tool carrier (6) and a surface (8) of the rotor head (5).

14. Method according to any of claims 11 to 13, wherein when determining the reference signal, steps a) and b) are carried out a first time without axial force being applied to the rotor head (5) and a second time with a similar tool or tool carrier (6) clamped fault-free, wherein the reference signal is calculated by subtracting the position signal obtained in the first implementation from the position signal obtained in the second implementation, wherein, when step c) is carried out, the respective mean value of the position signal and the reference signal is calculated, wherein, to calculate the difference signal, the mean value of the reference signal is subtracted from the mean value of the position signal, and wherein the state signal indicates the application of insufficient clamping force by the tool clamping device (4).

15. Method according to any of claims 11 to 14, wherein steps a) and b) are carried out without axial force being applied to the rotor head (5) in a defined reference state of the work spindle (1) and the mean value of the obtained position signal is calculated and stored as a reference mean value, and wherein steps a) and b) are carried out without axial force being applied to the rotor head (5) in a current operating state of the work spindle (1), and the current mean value of the obtained position signal is calculated, and an axial displacement of the rotor head (5) relative to the stator (2) is calculated by subtracting the reference mean value from the current mean value, and is transmitted to a higher-level control unit (24).