JP6719906B2 - Eddy current sensor and tool holder mounting state detection device including the same - Google Patents

Description

The present invention relates to a tool holder attachment state detection equipment equipped with the eddy current sensor and the eddy current sensor, in particular relates to a tool holder mounting state detection equipment equipped with the eddy current sensor and the eddy current sensor has linear approximation output ..

In machine tools, when holding a tool on a spindle via a tool holder, an eddy current sensor is widely used to detect whether or not the tool is correctly held with respect to the rotation axis, based on the distance between the tool and the tool holder. Has been. The eddy current sensor has the advantages of simple structure and high detection sensitivity. An example of such an eddy current sensor is described in Patent Document 1.

In order to provide an eddy current sensor which can be measured without increasing the size of the sensor head and without lowering the sensor sensitivity even if the cable is lengthened, the sensor head having an inductance element is disclosed in Patent Document 1. The eddy current sensor includes a cable connected to both sides of the inductance element of the sensor head and an amplifier circuit connected between two terminals of the cable. Further, the amplifier circuit has an oscillation circuit and an amplifier capacitance element, and the sensor head has a head capacitance element, and the inductance element, the head capacitance element, the cable, and the amplifier form a resonance circuit.

By the way, the output of the eddy current sensor changes depending on the distance to the axis or the like that is the measurement target, but this output is a function of the distance, and if the measurement target is a plane, it is considered to be inversely proportional to the distance. ing. However, in reality, it is not necessarily inversely proportional to the distance depending on the shape of the measurement object, and much less linearity is obtained, and calibration curves are individually created. In order to make the calibration curve easy to use, a method is used in which the distance from the measurement target is divided into a plurality of sections, and the calibration curve is linearly approximated for each section to improve the measurement accuracy. An example of linearly approximating the calibration curve of such a sensor output is described in Patent Document 2.

The sensor output linearization circuit described in Patent Document 2 is provided with a division circuit for calculating the reciprocal of the detection signal inversely proportional to the measured value, for the purpose of expanding the linear change range by simply adding a simple circuit. , The detection signal is converted into a signal proportional to the measured value. When the detection signal is greater than or equal to the predetermined value, the first correction signal is generated according to the detection signal, and when the detection signal is less than or equal to the predetermined value, the second correction signal is generated according to the detection signal. The detection signal is changed according to the first and second correction signals to expand the range in which the detection signal is proportional to the measured value.

It has been conventionally performed to mount an eddy current sensor in the vicinity of a tool mounting portion (tool holder device) of an automatic processing machine such as a machining center or an NC machine tool, and monitor a defect in tool mounting such as runout. 3 discloses an example of a tool holder mounting state detection device. In the device described in this publication, in order to accurately detect a mounting error of the tool holder on the spindle of the machine tool, the position of the outer peripheral surface of the cylindrical tool holder having a notch is detected by an eddy current sensor and detected. Data on the cutout portion is removed from the data, corrected detection data is created by interpolating the removed portion, and the corrected detection data is Fourier-transformed to determine the mounting state of the tool.

International publication number WO2012/133569 Japanese Patent Laid-Open No. 10-267687 JP 2008-93750 A

In the eddy current sensor, an AC magnetic flux due to a high-frequency current is applied to a measurement object, which is a conductor, and a magnetic flux change is caused inside the measurement object by Faraday's law of electromagnetic induction to generate an electromotive force. The electromotive force causes an eddy current to flow on the surface of the object to be measured to create a pseudo secondary coil, and the inductance and resistance of the sensor coil of the eddy current sensor change according to the distance from the object to be measured. The distance is calculated by detecting the change in the impedance of the sensor coil.

As described above, since the eddy current sensor can detect the distance at the contact position or in the non-contact state, when the eddy current sensor is used in the tool holder portion of the machine tool, the mounting state of the tool can be easily detected in the non-contact state. However, the sensor output is smaller as the distance to the object to be measured is smaller and increases as the distance from the object is increased. Since the relationship is not linear, the sensor output value cannot be used as it is as the distance.

If the measurement object is a flat plate microscopically and macroscopically, and the sensor end face is a flat surface, the sensor output can be theoretically obtained using Faraday's law, but general measurement Since the object is a round bar or a cylindrical surface with a notch, the flow of magnetic flux is not axisymmetric, or so-called magnetic flux escape occurs, so it is possible to theoretically determine the output characteristics. Have difficulty. Therefore, the sensor is usually calibrated in advance for the object to be measured. The calibration curve of the sensor output is a monotonically increasing function whose output increases as the distance between the sensor and the measurement object increases. In the actual operation, this monotonically increasing function depends on the shape or the like of the measuring object, and cannot be easily used as it is. Therefore, the distance between the sensor and the measuring object is divided into a plurality of sections, and the calibration curve of the sensor is linearly approximated for each section. In that case, it is also adopted to convert the numerical data into a look-up table and interpolate the intermediate points. Alternatively, a linearly approximated circuit is provided as a linearizer, and the sensor output itself is linearized for each section and corrected and output.

In the measurement using such a conventional eddy current sensor, since the calibration curve is divided into a plurality of sections, it is necessary to store data for the plurality of sections in a circuit device or a sensor head to which the sensor is connected. However, an analog circuit requires a large number of circuits, which becomes an obstacle when miniaturization is desired. In particular, when the number of sections is increased in order to improve the accuracy of the calibration curve that approximates the output of the eddy current sensor, this inconvenience becomes remarkable.

Although Patent Document 1 is a prior application of the inventor of the present invention, it has an advantage that the eddy current sensor can be downsized without deteriorating the detection sensitivity. However, how to convert the detection data into the distance data easily and with high accuracy is not sufficiently considered.

Further, in Patent Document 2 described above, in order to convert the output of the capacitance sensor or the eddy current sensor into a straight line, a division circuit that is inversely proportional to the output is provided. However, in the linearization circuit described in this publication, it is premised that "the detection signal is inversely proportional to the distance from the tip surface of the sensor to the surface position of the object", and a general measurement object causes a considerable error. There is a fear.

Further, in the above-mentioned Patent Document 3, it is disclosed that data of a tool holder having a notch is interpolated with high accuracy by using an eddy current sensor. However, even in Patent Document 3, how to easily and highly accurately use the obtained calibration curve of the sensor is not sufficiently taken into consideration.

The present invention has been made in view of the above-mentioned problems of the conventional technology, and an object thereof is to reduce the time required for calibration of an eddy current sensor in measurement using an eddy current sensor and to linearize the output of the eddy current sensor. It is to make the conversion efficient and simple. Another object of the present invention is, in addition to the above object, to facilitate and speed up the output conversion of the eddy current sensor and prevent the eddy current sensor from becoming large.

A feature of the present invention for achieving the above object is to have a sensor main body having a sensor coil and having a flat measurement end, and a circuit device (control device) electrically connected to the sensor main body. In the eddy current sensor, the circuit device uses a calibration curve obtained by approximating an output of the sensor coil with respect to the measurement object by a single exponential function in the use of the sensor body with respect to one or a plurality of measurement objects. To convert it.

Further, in this feature, the circuit device may include a linearization circuit corresponding to the single exponential function instead of converting using the calibration curve approximated by the single exponential function. However, it may have a logarithmic amplifier. The single exponential function is represented by βe ^αx +γ, where α, β, and γ are constants, and x is the distance between the sensor body end face (measurement end face) and the object (described later). (corresponding to g)). Further, the sensor body may have a memory means, and the memory means may store the constants α, β, γ of the single exponential function in advance.

Another feature of the present invention that achieves the above object is to provide a tool holder for attaching a tool to a spindle of a machining center or an NC machine tool, and a tool holder that is detachably arranged at the tip of the spindle, and the vicinity of the tool holder. In the tool holder mounting state detecting device including the tool holder and an eddy current sensor arranged apart from each other, the tool holder has a cylindrical surface whose outer periphery is substantially cylindrical, and the eddy current sensor has the cylindrical shape. The eddy current sensor is arranged so as to face the surface, and the eddy current sensor includes a sensor body having a sensor coil and a flat measurement end portion, and a circuit device electrically connected to the sensor body. The device consists in transforming the output of the sensor coil with respect to the tool holder in the use of the sensor body for a plurality of tool holders by means of a calibration curve approximating with a single exponential function.

And in this feature, the distance between the tool holder and the end surface of the sensor body is preferably shorter than the outer diameter of the measurement end surface of the sensor body, and is converted using the calibration curve approximated by the single exponential function. Alternatively, the circuit device may include a linearization circuit having a logarithmic amplifier corresponding to the single exponential function. The single exponential function is represented by βe ^αx +γ, where α, β and γ are constants, and x is the distance between the sensor body end face (measurement end face) and the object. Is desirable.

Still another feature of the present invention that achieves the above object is, in a tool holder mounting state detection method using an eddy current sensor, the tool holder is made of steel having a substantially cylindrical outer peripheral surface, and at least 3 From the calibration data of the eddy current sensor detected at the point distance, a single logarithmic curve (βe ^αx +γ: where α, β, γ are constants, x is the sensor body end face (measurement end face) and the object The distance between the eddy current sensor and the tool holder when the tool holder is actually attached is obtained using the obtained calibration curve, and the calculated distance is within a predetermined range. If it exceeds, the circuit device will judge that it is not suitable.

According to the present invention, in the measurement using the eddy current sensor, the distance from the end surface of the eddy current sensor to the surface of the measurement target is represented by a monotonically increasing exponential function, so that only the exponential function parameter is stored in the circuit device. By doing so, the time required for the calibration of the eddy current sensor can be reduced, and the linear conversion of the output of the eddy current sensor can be efficiently and simplified. Further, the output conversion of the eddy current sensor can be facilitated and speeded up, and the size increase of the eddy current sensor can be prevented.

It is a schematic diagram showing a tool holder wearing state detecting device of one example of a machining center concerning the present invention. 3 is a flowchart of shake detection in the tool holder mounting state detection device shown in FIG. 1. FIG. 2 is a schematic diagram of an embodiment of an eddy current sensor included in the machining center shown in FIG. 1. FIG. 4 is an equivalent circuit diagram of the eddy current sensor shown in FIG. 3. It is a flow chart of displacement or distance measurement using an eddy current sensor. It is a figure which shows typically the behavior of the eddy current sensor in cylindrical surface measurement and plane measurement. It is a figure which shows an example of the calibration curve and approximate curve of an eddy current sensor. It is a circuit diagram showing an example of a drive control circuit of an eddy current sensor. It is a circuit diagram which shows an example of a logarithmic circuit.

The present inventor has examined calibration curves for various measurement objects accumulated so far, and as a result, whether the measurement object has a flat surface with a small curvature or a cylindrical surface with a large curvature, it is practical. It was found that the calibration curve increases exponentially in a wide range of use. Then, the parameter of the exponential function that approximates the calibration curve corresponding to the measurement object is obtained, and the parameter is stored in the circuit device (also referred to as a control device in the present invention). The distance to the surface can be easily and quickly obtained. An example of the present invention based on this finding will be described below.

In the following description, a machining center MC will be taken up and described as an example of a device including the tool holder mounting state detection device 10. However, the present invention is not limited to the machining center MC, and is applicable to general machines such as NC machine tools that automatically or manually exchange processing tools.

An embodiment of a machining center MC according to the present invention will be described below with reference to the drawings. FIG. 1A is a schematic view partially showing the tool holder mounting state detection device 10 of the machining center MC, and is a front view. FIG. 1B is a view of the tool holder mounting state detection device 10 as viewed from below.

The machining center MC automatically selects various tools according to the machining process and automatically mounts them on the spindle to perform various kinds of machining. In the machining center MC, the tool 1 is automatically replaced by using an automatic tool change (ATC: automatic tool change) device (not shown). The ATC device automatically takes out the tool holder 2 to which the tool 1 is attached from a tool magazine (not shown) and automatically attaches it to the spindle 3 held by a vertically formed head 5.

The tool holder 2 to which the tool 1 is attached has a conical fitting portion 2A, and the fitting portion 2A is fitted into a conical fitting portion 3A formed on the spindle 3 to be mounted. To be done. At that time, if chips or the like adhere between the fitting portion 2A and the fitted portion 3A of the tool holder 2 and the main shaft 3, respectively, the shaft of the tool 1 is bent and installed unless the correction means is provided. .. When the machining is started in this state, the tool 1 is shaken and the machining accuracy of the work is significantly reduced.

Such a mounting error of the tool holder 2 can be determined by, for example, irradiating the tip of the tool 1 of the tool holder 2 mounted on the spindle 3 with laser light. However, since a large amount of cooling liquid (coolant) is supplied to the tip of the tool 1, the laser light is blocked by the coolant and the detection accuracy decreases.

Therefore, in the present invention, the outer peripheral surface of the flange 2B of the tool holder 2 is formed in a cylindrical shape, and the eddy current sensor body 12 for detecting the displacement of the outer peripheral surface is provided via the bracket 6. There is. Then, the detected displacement data of the outer peripheral surface of the flange 2B is analyzed to detect the mounting abnormality (chuck error) of the tool holder 2.

As shown in FIG. 1A, the tool holder mounting state detection device 10 automatically detects a chuck error of the tool holder 2 mounted on the spindle 3 in the ATC device. The tool holder mounting state detection device 10 mainly includes an eddy current sensor body 12 and a control device (circuit device) 14. The tool holder 2 to which the tool 1 is attached has a conical fitting portion 2A, and the fitting portion 2A is fitted to a conical fitting portion 3A formed on the main shaft 3, The tool 1 is attached to the tool holder 2.

The eddy current sensor 100 is a sensor that is configured to include the eddy current sensor main body 12 and the control device 14, and is capable of highly accurately measuring a minute variation in the distance (gap) in the adjacent section, and is attached to the spindle 3. The distance g to the outer peripheral surface of the flange 2B of the tool holder 2 is detected as an electric signal. The controller 14 detects a chuck error of the tool holder 2 based on the detection data detected by the eddy current sensor body 12. The control device 14 includes a signal detection circuit 15, an A/D converter 16, a CPU 18, a memory 20, an input/output circuit 22, and the like.

As will be described later in detail, the A/D converter 16 converts the electric signal indicating the distance g output from the eddy current sensor body 12 into a digital signal and outputs the digital signal to the CPU 18. The CPU 18 calculates the amount of eccentricity of the tool holder 2 based on the detection data of the eddy current sensor body 12 converted into this digital signal. The calculated eccentricity amount is compared with the allowable value stored in advance in the memory 20, and if the eccentricity amount exceeds the allowable value, it is determined that the chuck is missed. Then, the determination result is output to the machining center control device 24 which controls the machining center MC via the input/output circuit 22. Here, the eddy current sensor 100 is configured to transmit only the data of the distance g to the MC control device, calculates the eccentricity amount of the tool holder 2 described above and also later, and determines whether or not there is a chuck error. The processing may be performed by the MC controller 24 instead of the controller 14 of the eddy current sensor 100.

The CPU 18 stores the detection data of the distance g output from the eddy current sensor 100 in the memory 20 for one rotation corresponding to the rotation angle θ of the tool holder 2, and frequency-analyzes the detection data by Fourier analysis or the like. The intensity of each frequency component is calculated. Here, since the amplitude value of the fundamental wave frequency component (single peak component) of each frequency component can be regarded as twice the eccentric amount of the tool holder 2, it is compared with an allowable value to determine whether or not there is a chuck error. judge.

FIG. 1B is a bottom view of the tool holder 2. The eddy current sensor body 12 is arranged near the outer peripheral surface of the flange 2B of the tool holder 2. Since the eddy current sensor 100 uses a magnetic field for detection, it has an advantage that it is not affected by non-conductive contaminants such as dust, water, and oil, and the position of the tool holder 2 that is contaminated by cutting fluid or the like. It is suitable for measurement. Further, the eddy current sensor body 12 tends to increase in output as the distance from the outer peripheral surface of the tool holder 2 increases, and has a characteristic that the output becomes substantially constant when the distance exceeds a predetermined distance. .. The eddy current sensor body 12 is in the shape of a round bar having a diameter of φd on the surface facing the flange 2B of the tool holder 2 at the axial end, and the measurement range of the mating material is 3 mm of this diameter φd. The range is twice (more precisely, three times the inner coil diameter).

In addition, two or more notches may be provided on the outer peripheral surface of the flange 2B of the tool holder 2 for chucking. In that case, it is necessary to interpolate the detection data of the distance g output from the eddy current sensor body 12 at the notch. For this interpolation, for example, the method described in Patent Document 3 can be used.

A procedure for determining the state of the tool holder 2 after mounting the tool holder 2 on the spindle 3 using the tool holder mounting state detection device 10 will be described with reference to FIG. FIG. 2 is a flowchart illustrating the processing contents of the tool holder mounting state detection device 10 having the eddy current sensor 100.

In the machining center MC, many tools 1 are used according to the processing content. Therefore, the number of tool holders 2 is increased according to the number of tools 1, and the eddy current sensor 100 is required to be able to handle the large number of tool holders 2. The eddy current sensor 100 depends on the material of the measurement target, and the performance thereof changes greatly depending on the magnetic/nonmagnetic classification of the measurement target. That is, the magnetic field of the eddy current sensor body 12 invades the object to be measured and attracts the eddy current, and generates a magnetic field opposite to the magnetic field of the eddy current sensor body 12. The strength of the eddy current generated and the strength of the resulting magnetic field depend on the magnetic permeability and the conductivity of the measurement target.

Even if the same material is used, these physical property values microscopically change due to compositional variations, cracks, voids, and the like. By the way, there are many factors that hinder the uniformity of the physical property values as described above in the whole material, but if only in the vicinity of the surface of the material, the uniformity of the material is considerably increased by the mechanical processing such as heat treatment or surface finishing. To be retained.

Therefore, the penetration depth of the magnetic field is made shallow so that the material can be regarded as a material having substantially uniform physical properties in the measurement range, and the output of the eddy current sensor 100 is made to be the same in many tool holders 2. Generally, the tool holder 2 is made of steel from the requirements of fitting with the spindle 3, fitting with the tool 1 by an impact force, strength for stably holding the tool 1, and the like. If the material to be measured is a ferromagnetic metal such as steel or iron-based material, the penetration depth of the magnetic field is the very surface of the object to be measured, and for example, a high frequency current of about 400 kHz was applied to the coil of the eddy current sensor body 12. In this case, the magnetic field penetrates only up to about 3 to 4 μm from the surface of the measuring object.

Taking advantage of this feature, as a preliminary step of measurement by the eddy current sensor 100, it is commonly used for all tool holders 2 made of ferromagnetic metal (the material of the tool holder may be steel or iron-based material). The approximate calibration curve to be obtained is obtained in advance for the machining center MC with the cylindrical surface as the calibration target (step S10). That is, a calibration curve for each tool holder 2 is unnecessary. This calibration can be performed collectively by the manufacturer of the machining center MC, can be performed once by the purchaser of the machining center MC, or can be already performed at the time of shipment. Since the calibration curve changes if the eddy current sensor body 12 changes, the first calibration shown in step S10 is necessary when the eddy current sensor body 12 is replaced. In the calibration of step S10, three coefficients of an exponential function, which will be described in detail later, are obtained, and these values are stored in the memory 20 included in the control device 14, or a logarithmic amplifier is provided for the processing in the A/D converter 16. Then, the gain of the logarithmic amplifier is adjusted to a value corresponding to the obtained exponent.

Although not shown, in FIG. 1A, the eddy current sensor body 12 is provided with a memory means of a small capacity different from the memory 20 of the control device 14, and the memory means stores three coefficients of an exponential function. You may let me. In that case, the memory 20 of the controller 14 reads the exponential constants α, β, γ stored in the memory means of the eddy current sensor body 12. As a result, the three constants α, β, γ of the exponential function can be set in the eddy current sensor body 12 before the eddy current sensor 100 is shipped, so that it is not necessary to calibrate the eddy current sensor after shipment from the user. The sensor 100 is easier to use. Further, since the eddy current sensor main body 12 has a memory means and exponential constants α, β, γ are stored in this memory means, the eddy current sensor main body 12 can be easily replaced and the eddy current sensor After replacement of the body 12, calibration, which is typically performed at the source, is no longer required.

Next, the machining center MC is actually operated. When the machining center MC is activated, the tool holder mounting state detection device 10 is activated (step S12). When the tool is automatically changed in the automatic tool changer (step S14), the machining center controller 24 rotates the spindle 3 at a preset rotation speed (step S16).

The eddy current sensor body 12 detects a signal corresponding to the distance g to the outer peripheral surface of the flange 2B of the rotating tool holder 2 (step S18), and sends the detection signal to the control device 14 to which the eddy current sensor body 12 is connected. Send. If the logarithmic amplifier 41 is present (step S20), the control device 14 processes the signal detected by the eddy current sensor main body 12 by the logarithmic amplifier 41 (step S22), and then sends it to the CPU 18. Alternatively, after digitizing the signal detected by the eddy current sensor body 12, the CPU 18 converts the signal into a distance using the coefficient of the calibration curve stored in the memory 20 (step S24).

The distance detection by the eddy current sensor 100 is executed for one round over the entire circumference of the tool holder 2 (step S26), and the detection data for the entire circumference of the tool holder 2 is obtained. If a notch or the like for the attachment of the tool holder 2 is formed on the flange 2B of the tool holder 2 (step S28) and there is a sudden change in the data, that part is interpolated/corrected (step S30), The correction data is generated and stored in the memory 20.

The CPU 18 Fourier-transforms the correction data stored in the memory 20, extracts the basic frequency (the cycle is the time for one round of the tool holder 2), and calculates the amplitude value thereof. Since the amplitude value of the fundamental frequency is equal to twice the eccentricity amount of the tool holder 2, the eccentricity amount of the tool holder 2 is obtained (step S32). The CPU 18 compares the obtained amount of eccentricity with the allowable value and determines whether or not there is a mounting error (step S34).

The Fourier transform may be executed simultaneously with the measurement. The allowable value is input by the operator from the input device before the operation of the machining center is started. The input allowable value is written in the memory 20. In addition, the allowable value is set based on the machining accuracy required by the user, and the optimum value is appropriately selected and set within the allowable range of the shake based on the eccentricity of the tool holder 2.

The determination result is output to the machining center control device 24, and when the machining center control device 24 determines that the eccentricity amount is smaller than the allowable value and is normally mounted (step S40), the machining is started in that state (step S42). On the other hand, if the amount of eccentricity is larger than the allowable value and it is determined that there is a mounting error (step S36), a command to redo the automatic tool holder replacement is issued (step S38), and the process returns to step S14. After that, the above procedure is repeated. In the case of a mounting error, foreign matter may have been caught in the fitting portion between the tool holder 2 and the spindle 3, so the foreign matter is removed by air blowing or the like at this fitting portion.

Next, the eddy current sensor 100 included in the machining center MC will be described with reference to FIGS. 3 to 9. FIG. 3 is a schematic diagram of an embodiment of the eddy current sensor 100, and FIG. 4 is an equivalent circuit diagram of the eddy current sensor 100. FIG. 5 is a flow chart for explaining a distance measurement method using the eddy current sensor 100, and FIG. 6 is a diagram for explaining output changes of the eddy current sensor 100 depending on the shape of the measurement target. Further, FIG. 7 is a graph showing an example of the relationship between the output characteristic (calibration curve) of the eddy current sensor body 12 according to the present invention and its approximate curve, and FIG. 8 shows an example of the circuit configuration for realizing the present invention. It is a figure. FIG. 9 is a circuit diagram showing an example of a logarithmic amplifier in the circuit configuration shown in FIG.

The eddy current sensor body 12 shown in FIG. 3 has a disk-shaped coil 31 formed in a spiral shape on the side of a surface 32 (also referred to as a measurement end surface in the present invention) facing the object to be measured (tool holder 2). have. The coil 31 is connected to the coaxial cable 33. The coil 31 is flattened up to almost the outer diameter of the container to be held, and is made thin. The coil 31 is held in the shape of a plastic cap such as epoxy, and then held in a resin-made cap-shaped cylindrical container. In the eddy current sensor main body 12 used in this embodiment, typically, the outer diameter of the surface 32 facing the object to be measured is φ5 and the axial length is about 16 mm. The coil 31 has a thickness (axial length) of 1.2 mm.

An equivalent circuit of the eddy current sensor 100 is shown in FIG. 4, and a flowchart of its operation is shown in FIG. In the eddy current sensor 100, the high frequency oscillator of the control device 14 supplies the high frequency signal of the excitation voltage v _i at the excitation angular frequency ω to the coil 31 via the coaxial cable 33 (step S60). In this embodiment, the excitation frequency is about 400 kHz.

Upon receiving the high frequency signal from the high frequency oscillator, the coil 31 generates a high frequency magnetic flux (step S62). A magnetic field is formed by the high frequency magnetic flux, and the magnetic field penetrates into the surface of the measuring object. At that time, if the measurement target is an iron-based material, the penetration depth is about 3 to 4 μm, and if it is an aluminum-based material, the penetration depth is 100 μm or more. Eddy current is generated on the surface of the measurement object into which the magnetic field has entered (step S64). The magnitude of the generated eddy current changes depending on the distance and shape between the coil 31 and the measurement object (step S66).

FIG. 6 schematically shows the difference in the generation of the magnetic force lines 54 due to the difference in the shapes of the measuring objects 50 and 51 and the difference in the arrival of the magnetic force lines 54 at the measuring object 50 due to the change in the distance. In the case of the diagram (d) in which the measurement object 51 can be simulated as a flat surface, when the eddy current sensor body 12 approaches the measurement object 51, an eddy current is generated near the surface of the measurement object 51 (not shown). The interlinkage 56 in which the magnetic force line 54 penetrates into the measurement object 51 occurs. If the measurement object 51 is an iron-based material, since the penetration depth of the magnetic field is shallow, this interlinkage 56 is also negligible.

On the other hand, in the cases (a) to (c) in which the measurement object 50 can be approximated by a cylindrical surface, as the eddy current sensor main body 12 moves away from the measurement object 50, the so-called escape of the magnetic force lines 54 increases and the eddy current is increased. The output of the sensor body 12 decreases. The rate of decrease in the output of the eddy current sensor body 12 due to the increase in the distance between the eddy current sensor body 12 and the measurement objects 50 and 51 is almost inversely proportional to the distance in the case of the plane shown in FIG. , (A) to (c), the magnetic field lines 54 escape to give a complicated function of distance. In addition, 55 in the (a)-(c) figure shows typically the eddy current generated in the surface of the measuring object 50.

Returning to FIG. 5, when the magnitude of the eddy current generated on the surface of the measurement target changes, the impedance Z(Lc, Rc) of the coil 31 including the measurement target changes (step S68). The change in the impedance Z (Lc, Rc) is detected as the voltage Vc in the equivalent circuit shown in FIG. 4 (step S70). Finally, the detected voltage Vc is adjusted by the detection circuit, and then a voltage proportional to the distance is output by the linearization circuit (step S72).

As described above, in the case of the present embodiment in which the measurement object is not a plane, it is difficult to use an analytical method for linearizing the output. Therefore, in the present invention, when the measurement object is a cylindrical surface, the distance to the measurement object with high accuracy is obtained by using a single approximate calibration curve without using a complicated configuration in which the configuration curves are individually approximated. Are seeking. The details will be described with reference to FIGS. 7 to 9.

FIG. 7 is a diagram showing the calibration curve (solid line) of the eddy current sensor body 12 and its approximate curve (broken line) when the measurement object 50 is a cylindrical surface. In FIG. 7, the output of the eddy current sensor body 12 is normalized to ±10V. That is, it is −10V at the closest point and +10V at the point at infinity. The calibration curve is obtained by using the least square method based on the output result of the eddy current sensor body 12 at a plurality of distances x. The calibration curve is a curve that monotonically increases as the distance x between the eddy current sensor body 12 and the measurement object 50 increases, and an approximate calibration curve is an approximation of this calibration curve with an exponential function. In FIG. 7, since the calibration curve and the approximate calibration curve are present at most positions, the two curves overlap and only the solid line appears.

The approximate exponential function is
Vout=βe ^αx +γ Here, α, β, and γ are represented by constants. In the example of FIG. 7, α=−0.0013, β=−44.08, and γ=7.596. The calibration curve and the approximation curve deviate slightly in the vicinity of the contact point where the distance x is x≦0.1, but almost match in the range of 0.1 to 3 mm. Therefore, the linearization process can be executed by configuring this approximate curve with a circuit or storing it in the memory as an arithmetic expression.

FIG. 8 and FIG. 9 are examples of linearization on a circuit. As shown in FIG. 8, a logarithmic amplifier 41 for linearization processing is added to the conventional control circuit of the eddy current sensor 100. That is, when the input signal V _in (corresponding to Vc _in FIG. 4) is input to the circuit including the plurality of stages of amplifiers A _{1 to} A ₃ , the FET, the transistor Tr, and the D/A converter 42, it is linearized. The output signal V _out is output. This corresponds to FIG. 7. As an example of the logarithmic amplifier 41, a circuit as shown in FIG. 9 may be used, or a commercially available logarithmic amplifier IC may be used.

The gain of the logarithmic amplifier 41 is adjusted using a value obtained by measurement on a cylindrical surface simulating an actual measurement object. As described above, if the measurement target is an iron-based material, the penetration of the magnetic field is about 3 to 4 μm. Therefore, as long as the measurement target is an iron-based material, individual differences in the surface of the measurement target occur. It is difficult to assume that the generated eddy currents exhibit almost the same behavior.

That is, the object to be measured is not microscopically uniform, and there may be compositional variations, cracks, voids, etc. inside, but these may occur within the range of 3 to 4 μm from the surface. Is almost none. Therefore, even if a large number of tools are provided for one machine, as in the case of a tool holder of a machining center, as long as the eddy current sensor is the same, the same approximate calibration curve can be obtained even if the tool holder is replaced. Can be used.

In the eddy current sensor 100 of this embodiment, which is linearized by using the logarithmic amplifier 41, the coil 31 is thinned as shown in FIG. 3, so that it is possible to measure to a position farther from the object to be measured than before. Become. However, when the eddy current sensor main body 12 is separated from the object to be measured, the sensitivity is also lowered, and when the distance is increased by 0.5 mm, the sensitivity is halved. In order to solve this problem, the number of measurements is increased and the gain is increased by AC. These are based on the advantage that the approximate calibration curve is represented by a single function, and the measurement accuracy can be improved by multiple measurements. As a result, although the distance between the tool holder and the eddy current sensor body 12 had to be set within the range of 0.9 to 1.1 mm in the related art, the sensitivity decreases from the shortest distance of 0.1 mm where the approximate calibration curve is effective. The distance measuring range can be expanded up to the limit of 2 mm, which can be compensated by the measurement frequency. Therefore, the installation margin of the eddy current sensor body 12 is improved.

1... Tool, 2... Tool holder, 2A... Fitting part, 2B... Flange, 3... Spindle, 3A... Fitted part, 5... Head, 6... Bracket, 10... Tool holder mounting state detection device, 12... Vortex Current sensor body, 14... Control device, 15... Signal detection circuit, 16... A/D converter, 18... CPU, 20... Memory, 22... Input/output circuit, 24... Machining center control device, 31... Coil, 32... Opposing surface , 33... Coaxial cable, 41... Logarithmic amplifier, 42... D/A converter, 50, 51... Measurement object, 54... Magnetic field line, 55... Eddy current, 56... Interlinking, 100... Eddy current sensor, MC... Machining center, g... Distance between sensor and tool holder, d... Sensor outer diameter of sensor surface

Claims (9)

What is claimed is: 1. An eddy current sensor, comprising: a sensor main body having a sensor coil, a measurement end surface of which is substantially flat; and a circuit device electrically connected to the sensor main body, the eddy current sensor being mounted with a tool for replacement. A plurality of objects to be measured of a ferromagnetic material used as a measurement are measured, and a state at a distance of 0.1 mm to 2 mm where chain linkage can be ignored with respect to the objects to be measured is measured. , in use of the sensor body relative to the plurality of measurement object, the output of the sensor coil with respect to the plurality of measurement object, and converting using a calibration curve approximated by a single exponential function Eddy current sensor. 2. The eddy current according to claim 1, wherein the circuit device includes a linearization circuit corresponding to the single exponential function, instead of converting using the calibration curve approximated by the single exponential function. Sensor. The eddy current sensor according to claim 2, wherein the linearization circuit has a logarithmic amplifier. The single exponential function is represented by βe ^αx +γ, where α, β, γ are constants, and x is a distance between the measurement end surface and an object. The eddy current sensor according to any one of items 1 to 3. The sensor body has a memory means, the single exponential function is βe ^αx +γ (where α, β, γ are constants, x is the distance between the measurement end face and the object), The eddy current sensor according to claim 1, wherein the single exponential constants α, β, γ are stored in advance in the memory means. A ferromagnetic tool holder which is provided to attach a tool to a spindle of a machining center or an NC machine tool and which is detachably arranged at the tip of the spindle, and is arranged near the tool holder and apart from the tool holder. A mounting state detection device for a tool holder including the eddy current sensor described above, wherein the eddy current sensor is applicable to a plurality of the tool holders that are used by being exchanged, and the eddy current sensor is linked to a measurement object. A state in which the distance is 0.1 mm to 2 mm which can be ignored is measured, and the outer periphery of the tool holder is a substantially cylindrical cylindrical surface, and the eddy current sensor is arranged so as to face the cylindrical surface. The eddy current sensor has a sensor body having a sensor coil and a measurement end surface being a substantially flat surface, and a circuit device electrically connected to the sensor body. In the use of the sensor body for a plurality of tool holders to be used interchangeably, the output of the sensor coil for the tool holder is converted using a calibration curve approximated by a single exponential function. Wearing state detection device. The tool holder mounting state detection device according to claim 6, wherein a distance between the tool holder and an end surface of the sensor body is shorter than an outer diameter of a measurement end surface of the sensor body. 7. The circuit device comprises a linearizing circuit having a logarithmic amplifier corresponding to the single exponential function, instead of converting using the calibration curve approximated by the single exponential function. 7. The tool holder mounting state detection device according to item 7. The single exponential function is represented by β e ^αx +γ, where α, β, γ are constants, and x is a distance between the measurement end surface and the object. The tool holder mounting state detection device according to claim 6.