WO2011024840A1

WO2011024840A1 - Method for thermal displacement correction in machine tool, and thermal displacement correction device

Info

Publication number: WO2011024840A1
Application number: PCT/JP2010/064346
Authority: WO
Inventors: 治夫小林; 初倉橋
Original assignee: ブラザー工業株式会社
Priority date: 2009-08-28
Filing date: 2010-08-25
Publication date: 2011-03-03
Also published as: CN102481674B; CN102481674A; JP5387838B2; JP2011045986A

Abstract

A numerical control device obtains the generated heat in a plurality of intervals into which a ball screw shaft is segmented, said obtainment being performed every 50 ms on the basis of control data and the rotation speed of an X-axis motor, and computes the temperature increase of a bearing holder every 50 ms. The temperature distribution of the plurality of intervals is computed every 6400 ms on the basis of an unsteady heat conduction equation that uses the temperature increase of the bearing holder and the total amount of generated heat that is 6400 ms of cumulative amounts of generated heat of the plurality of intervals. The magnitudes of thermal displacement of the plurality of intervals of the ball screw shaft are computed every 6400 ms on the basis of the temperature distribution. A compensation value is computed every 6400 ms for each compensation interval into which the net range of motion is segmented, said computation being performed on the basis of the magnitude of thermal displacement of the plurality of intervals.

Description

Thermal displacement correction method and thermal displacement correction apparatus for machine tool

The present invention relates to a thermal displacement correction method and a thermal displacement correction apparatus for machine tools.

Machine tools use a ball screw mechanism as a positioning mechanism. The ball screw mechanism causes a pitch error between the amount of rotation of the ball screw shaft and the amount of movement of the nut due to manufacturing tolerances and the like. Therefore, the machine tool corrects the pitch error based on a preset table of pitch error correction amounts.

¡The ball screw mechanism causes thermal expansion due to temperature rise due to the frictional resistance between the ball screw shaft, the nut, and each bearing, causing thermal displacement. The thermal displacement of the ball screw shaft results in a positioning error. In order to solve the above-described positioning error problem, the machine tool uses a system in which pre-tension is applied to the ball screw shaft to absorb thermal expansion.

¡The machine tool uses a thick ball screw shaft and the feed rate is very fast, so the heat generation increases. When using the pre-tension method, the machine tool must apply a very large tension to the ball screw shaft. When a very large tension is applied to the machine tool, there is a problem that the ball screw mechanism is deformed. Furthermore, there is a problem that an excessive force is applied to the thrust bearing and seizes.

The method for correcting the thermal displacement of the ball screw shaft disclosed in Patent Document 1 does not apply excessive tension to the ball screw shaft and does not require a special measuring device. In the method of Patent Document 1, the amount of heat generated by the ball screw shaft is calculated from the product of the armature current and voltage of the servo motor. Based on the heat generation amount of the ball screw shaft and the ratio (heat distribution coefficient) obtained in advance, the heat generation amount of nut movement due to the movement of the nut, the heat generation amount of the front bearing due to the rotation of the front bearing, and the rotation amount of the rear bearing Calculate the rear bearing heat value. The temperature distribution is obtained from the unsteady heat conduction equation based on the calculated calorific value.

The method of correcting the thermal displacement of the ball screw shaft while the machine tool is operating predicts the thermal displacement amount of the ball screw shaft from moment to moment, and gives the predicted thermal displacement amount to the numerical controller as a correction amount for correcting the pitch error correction amount. This method has a problem that the correction amount becomes larger than the actual elongation when the number of times of acceleration / deceleration of the servo motor is large.

Patent Document 2 focuses on the fact that the acceleration / deceleration energy of the servo motor itself is included in the calorific value as a factor of the problem in Patent Document 1. The thermal displacement correction method disclosed in Patent Document 2 calculates the heat generation amount of the ball screw shaft from the rotation speed of the servo motor, and based on the heat generation amount of the ball screw shaft, the nut movement heat generation amount, the front bearing heat generation amount, and the rear bearing Calculate calorific value.

JP 63-256336 A JP-A-4-240045

The method of Patent Document 2 does not take into account the amount of heat generated by the servo motor when a drive current flows through the servo motor. The amount of heat generated by the servo motor is applied to the ball screw shaft through the front bearing. Therefore, in order to perform highly accurate thermal displacement correction, not only the nut movement heat generation amount, the front bearing heat generation amount, and the rear bearing heat generation amount are calculated, but also the servo motor heat generation as in the method of Patent Document 1. It is necessary to consider the amount.

In the ball screw mechanism shown in FIG. 3, a fixed bearing 18 is fixed to a support base 10 via a bearing holder 20. The fixed bearing 18 supports an end portion 81 e (end portion on the X-axis motor 71 side) of the X-axis ball screw shaft 81. The movable bearing 19 is movable in the axial direction of the X-axis ball screw shaft 81. The movable bearing 19 supports an end portion 81 f of the X-axis ball screw shaft 81 on the side opposite to the fixed bearing 18.

A path for radiating a part of the heat generated by the X-axis motor 71 includes a path for radiating heat to the periphery of the X-axis motor 71, a path for radiating heat from the X-axis motor 71 to the support base 10 via the bearing holder 20, and an X-axis motor. 71 is a path for radiating heat from 71 to the surroundings via the bearing holder 20. The amount of heat radiation is determined by a temperature difference between two objects, such as a temperature difference between the motor temperature and the ambient temperature. The method of Patent Document 1 does not take into consideration that a part of the heat generated by the servo motor radiates heat.

When the X-axis motor 71 is driven, the temperature of the X-axis motor 71 rises. The increased temperature enters the bearing holder 20, and the temperature of the bearing holder 20 increases. The temperature of the bearing holder 20 enters the motor side end 81e of the ball screw shaft 81, and the temperature of the motor side end 81e rises. The temperature rise of the motor side end portion 81e is after a predetermined time has elapsed since the X-axis motor 71 was driven.

In the method of Patent Document 1, the front bearing heat generation amount is calculated based on the heat generation amount supplied from the X-axis motor 71 to the ball screw shaft 81 and the heat distribution coefficient, but the predetermined time is not taken into consideration. The heat distribution coefficient is a constant value. Therefore, the method of Patent Document 1 cannot accurately calculate the temperature change of the motor side end portion 81e and cannot accurately correct the thermal displacement.

An object of the present invention is to provide a thermal displacement correction method and a thermal displacement correction apparatus for a machine tool capable of performing highly accurate thermal displacement correction.

The machine tool thermal displacement correction method according to claim 1 is a ball screw mechanism for feed driving, a servo motor that rotationally drives a shaft into which a nut of the ball screw mechanism is screwed, and a control device that controls the servo motor based on control data And calculating a heat generation amount generated in a plurality of sections obtained by dividing the overall length of the shaft into a plurality of sections at predetermined intervals based on the rotation speed of the servo motor and control data. A first step of calculating the temperature rise of the bearing holder that rotatably supports the motor side of the shaft via the predetermined time, a total calorific value obtained by accumulating the calorific values of the plurality of sections for a predetermined period, Based on the unsteady heat conduction equation using the temperature rise of the bearing holder instead of the temperature rise of the motor side end, On the basis of the second step of calculating the thermal displacement amount of the plurality of sections of the shaft from the temperature distribution, the third step of calculating the thermal displacement amount of the plurality of sections of the shaft for each predetermined period from the temperature distribution, And a fourth step of calculating a correction amount for correcting the control data for each of the predetermined periods for each of a plurality of correction sections obtained by dividing the nut movement range of the shaft.

In the thermal displacement correction method for a machine tool according to claim 1, the unsteady heat conduction equation uses an increase in the temperature of the bearing holder that is increased by the amount of heat input from the servo motor. Therefore, the amount of heat displacement can be calculated using the amount of heat generated in consideration of the amount of heat released from the amount of heat generated by the servo motor. The temperature rise of the bearing holder is calculated every predetermined time. Therefore, it is possible to calculate the amount of thermal displacement taking into account the time delay from when the current flows to the servo motor until the end of the shaft on the motor side is thermally affected by the servo motor. Since the thermal displacement correction method of the machine tool can calculate the thermal displacement amount considering the heat radiation of the servo motor and the time delay, the thermal displacement correction can be performed with high accuracy.

In the thermal displacement correction method for a machine tool according to claim 2, the temperature rise of the bearing holder is calculated based on a rotation speed and a drive current value of the servo motor.

The thermal displacement correction method for a machine tool according to claim 2 calculates the temperature rise of the bearing holder based on the rotation speed of the servo motor and the drive current value. Therefore, highly accurate thermal displacement correction can be performed using an existing sensor without newly providing a sensor or the like.

A thermal displacement correction device for a machine tool according to claim 3 is a ball screw mechanism for driving driving, a servo motor that rotationally drives a shaft into which a nut of the ball screw mechanism is screwed, and a control device that controls the servo motor based on control data In a thermal displacement correction device for a machine tool having a speed detection device for detecting the rotational speed of the servo motor, and the amount of heat generated in a plurality of sections obtained by dividing the overall length of the shaft, the rotational speed of the servo motor A calorific value calculation unit to obtain every predetermined time based on the control data, a temperature calculation unit to detect a temperature rise of the bearing holder that rotatably supports the motor side of the shaft via a bearing, at each predetermined time; Instead of the total calorific value of the calorific value of the plurality of sections accumulated for a predetermined period and the temperature rise at the motor side end of the shaft, the bearing ho A temperature distribution calculation unit that calculates a temperature distribution of a plurality of sections for each of the predetermined periods based on an unsteady heat conduction equation using a temperature rise of the power source, and a thermal displacement amount of the plurality of sections of the shaft from the temperature distribution. A correction amount for correcting the control data for each of a plurality of correction sections obtained by dividing a plurality of nut movement ranges of the shaft based on the thermal displacement amount calculation section for each predetermined period and the plurality of sections. And a correction amount calculation unit that calculates the value for each predetermined period.

The thermal displacement correction device for a machine tool according to claim 3 includes a temperature calculation unit for calculating a temperature rise of the bearing holder every predetermined time, a total heat generation amount obtained by accumulating a heat generation amount of a plurality of sections for a predetermined period, and a motor side of the shaft A temperature distribution calculation unit that calculates the temperature distribution of a plurality of sections for each predetermined period based on the unsteady heat conduction equation using the temperature increase of the bearing holder instead of the temperature increase of the end portion. 1 has the same effect.

In the thermal displacement correction apparatus for a machine tool according to claim 4, the temperature calculation unit calculates a temperature rise of the bearing holder based on a rotation speed and a drive current value of the servo motor.

In the thermal displacement correction device for a machine tool according to claim 4, the temperature calculation unit calculates the temperature rise of the bearing holder based on the rotation speed of the servo motor and the drive current value. Therefore, the manufacturing cost can be reduced by using an existing sensor without newly providing a sensor or the like.

1 is an overall perspective view of a machine tool M. FIG. The side view of the machine tool M. The block diagram of an X-axis ball screw mechanism. The block diagram of the control system of the machine tool M. Explanatory drawing of the several calculation area which divided | segmented the ball screw shaft into multiple. Explanatory drawing of memory | storage data, such as the total calorific value of several calculation area. Explanatory drawing of the calorific value and temperature distributed to the bearing holder and the dividing position of a plurality of calculation sections. The graph which shows the relationship between time t and the temperature rise (theta) ₀ (t) of a bearing holder. Explanatory drawing of the calculation method of the inclination of temperature rise (theta) _i (t) of a bearing holder. Explanatory drawing of the correction area for pitch error correction. Explanatory drawing which shows the thermal displacement amount in each calculation area division | segmentation position from a fixed bearing. The flowchart of a thermal displacement correction control program. The flowchart of a correction amount calculation processing program.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, modes for carrying out the present invention will be described. The configuration of the machine tool M will be described with reference to FIGS. The lower right of FIG. 1 is the front of the machine tool M. In the machine tool M, the workpiece (not shown) and the tool 6 are independently moved relative to each other in the XYZ rectangular coordinate system to perform desired machining (for example, “milling”, “drilling”, “ Cutting "etc.). The X-axis direction, Y-axis direction, and Z-axis direction of the machine tool M (machine main body 2) are the left-right direction, front-rear direction, and vertical direction of the machine tool M (machine main body 2), respectively.

As shown in FIG. 1, the machine tool M includes a base 1, a machine body 2, and a cover (not shown) as main constituents. The base 1 is a substantially rectangular parallelepiped casting that is long in the Y-axis direction. The machine body 2 is provided on the upper part of the base 1. The machine body 2 performs a cutting process on the workpiece. The cover is fixed to the upper part of the base 1. The cover has a box shape covering the machine body 2 and the upper part of the base 1.

The machine body 2 will be described. The machine body 2 includes a column 4, a spindle head 5, a spindle 5A, a tool changer 7, and a table 8 as main constituents. The column 4 has a substantially prismatic shape and is fixed to a column seat portion 3 provided at the rear portion of the base 1. The spindle head 5 can be moved up and down along the column 4. The spindle head 5 rotatably supports the spindle 5A. The tool changer 7 is provided on the right side of the spindle head 5. The tool changer 7 exchanges the tool 6 at the tip of the spindle 5A and the tool stored in the tool magazine 14. The table 8 is provided on the upper part of the base 1. The table 8 fixes the work so as to be detachable. The control box 9 is box-shaped. The control box 9 is provided on the back side of the column 4. The numerical control device 50 (see FIG. 4) is provided inside the control box 9. The numerical controller 50 controls the operation of the machine tool M. The numerical control device 50 has a function as a thermal displacement correction device.

The moving mechanism of the table 8 will be described with reference to FIGS. An X-axis motor 71 that is a servomotor drives the table 8 to move in the X-axis direction. The X-axis motor 71 is provided on the rectangular parallelepiped support base 10. A Y-axis motor 72, which is a servo motor, drives the table 8 to move in the Y-axis direction. The Y-axis motor 72 is provided on the base 1. The support base 10 is provided below the table 8. The support base 10 includes a pair of X-axis feed guides (not shown) extending along the X-axis direction on the upper surface thereof. The pair of X-axis feed guides support the table 8 movably on the upper part thereof.

As shown in FIG. 3, the nut portion 8 a is arranged on the lower surface of the table 8. The nut portion 8a is screwed with the X-axis ball screw shaft 81 to constitute an X-axis ball screw mechanism. The X-axis ball screw shaft 81 is connected to the X-axis motor 71 via the coupling 17. The fixed bearing 18 is fixed to the support base 10. The fixed bearing 18 supports a fixed-side end portion 81e of the X-axis ball screw shaft 81 on the X-axis motor 71 side (fixed side). The movable bearing 19 supports the movable side end 81f. The movable end 81f is on the opposite side (movable side) of the fixed end 81e. The movable bearing 19 is movable along the axial direction of the X-axis ball screw shaft 81.

A pair of Y-axis feed guides (not shown) are provided on the upper side of the base 1 and extend along the Y-axis direction. The pair of Y-axis feed guides support the support 10 so as to be movable. The table 8 moves in the X-axis direction along the pair of X-axis feed guides when the X-axis motor 71 is driven. The support base 10 is moved in the Y-axis direction along the pair of Y-axis feed guides when the Y-axis motor 72 is driven. The Y-axis moving mechanism is a ball screw mechanism, similar to the X-axis moving mechanism.

Covers

11 and 12 cover the X-axis feed guide on the left and right sides of the table 8. The

covers

11 and 12 can be expanded and contracted. The cover 13 and the Y-axis rear cover (not shown) cover the Y-axis feed guide on both the front and rear sides of the support base 10, respectively. Even when the table 8 moves in either the X-axis direction or the Y-axis direction, the

covers

11, 12, 13 and the Y-axis rear cover always cover the X-axis feed guide and the Y-axis feed guide. The

covers

11, 12, 13 and the Y-axis rear cover prevent chips and coolant liquid scattered from the processing area from falling on the rails of the respective feed guides.

The lifting mechanism of the spindle head 5 will be described with reference to FIGS. The column 4 supports a Z-axis ball screw shaft (not shown) extending in the vertical direction. The nut portion (not shown) is screwed with the Z-axis ball screw shaft. The nut portion supports the spindle head 5. A Z-axis motor 73 (see FIG. 4) rotates the Z-axis ball screw shaft in forward and reverse directions. Based on the forward / reverse rotation drive of the Z-axis ball screw shaft, the spindle head 5 is driven up and down in the Z-axis direction. The axis controller 63a (see FIG. 4) drives the Z-axis motor 73 based on a control signal from the CPU 51 (see FIG. 4) of the numerical controller 50. Therefore, the spindle head 5 is driven up and down by driving the Z-axis motor 73.

1 and 2, the tool changer 7 includes a tool magazine 14 and a tool change arm 15. The tool magazine 14 stores a plurality of tool holders (not shown) that support the tool 6. The tool exchange arm 15 grasps a tool holder attached to the main shaft 5A and another tool holder, and conveys and exchanges them. The tool magazine 14 includes a plurality of tool pots (not shown) and a transport mechanism (not shown) inside. The tool pot supports the tool holder. The transport mechanism transports the tool pot in the tool magazine 14.

The electrical configuration of the numerical control device 50 will be described with reference to FIG. The numerical controller 50 includes a microcomputer. The numerical controller 50 includes an input / output interface 54, a CPU 51, a ROM 52, a flash memory 53, axis control units 61a to 64a and 75a, servo amplifiers 61 to 64, differentiators 71b to 74b, and the like. The axis controllers 61a to 64a are connected to the servo amplifiers 61 to 64, respectively. The servo amplifiers 61 to 64 are connected to an X-axis motor 71, a Y-axis motor 72, a Z-axis motor 73, and a main shaft motor 74, respectively. The shaft control unit 75 a is connected to the magazine motor 75.

The X-axis motor 71 and the Y-axis motor 72 are motors for moving the table 8 in the X-axis direction and the Y-axis direction, respectively. The Z-axis motor 73 is a motor for driving the spindle head 5 up and down in the Z-axis direction. The magazine motor 75 is a motor for rotating the tool magazine 14. The main shaft motor 74 is a motor for rotating the main shaft 5A. The X-axis motor 71, Y-axis motor 72, Z-axis motor 73, and main shaft motor 74 are provided with encoders 71a to 74a, respectively.

The axis controllers 61a to 64a receive the movement command amount from the CPU 51 and output a current command (motor torque command value) to the servo amplifiers 61 to 64. The servo amplifiers 61 to 64 receive a current command and output a drive current to the motors 71 to 74. The axis controllers 61a to 64a receive position feedback signals from the encoders 71a to 74a and perform position feedback control. Differentiators 71b to 74b differentiate the position feedback signals output from the encoders 71a to 74a and convert them into speed feedback signals. Differentiators 71b to 74b output speed feedback signals to the axis controllers 61a to 64a.

The axis controllers 61a to 64a receive the speed feedback signal from the differentiators 71b to 74b and control the speed feedback. The current detectors 61b to 64b detect drive currents output from the servo amplifiers 61 to 64 to the motors 71 to 74. The current detectors 61b to 64b feed back the detected drive current to the axis controllers 61a to 64a. The shaft controllers 61a to 64a perform current (torque) control based on the drive current fed back by the current detectors 61b to 64b. The shaft control unit 75 a receives the movement command amount from the CPU 51 and drives the magazine motor 75.

The ROM 52 is a main control program for executing a machining program for the machine tool M, a control program for thermal displacement correction control (see FIG. 12), and a control program for correction amount calculation processing for calculating the correction amount of the pitch error correction amount (FIG. 13). Etc.) are stored. The flash memory 53 stores parameters relating to the mechanical structure, parameters relating to physical properties, heat distribution coefficients (ratio) η _N , η _F and η _B , a table of pitch error correction amounts, and the like. The parameter relating to the mechanical structure is, for example, the length and diameter of the ball screw shaft 81. Parameters relating to physical properties are, for example, density and specific heat. The flash memory 53 includes a data area for updating and storing the heat generation amount, the total heat generation amount, the rotation speed and driving current of the motor 71, and the distributed heat generation amount shown in FIG. The flash memory 53 also appropriately stores a plurality of machining programs for machining various workpieces.

The pitch error correction amount table is a table for correcting pitch errors of the ball screw mechanisms of the X axis, the Y axis, and the Z axis. The pitch error of the ball screw mechanism is caused by manufacturing tolerances. In this embodiment, the pitch error between the rotation amount of the ball screw shaft 81 and the movement amount of the nut portion 8a is corrected based on a preset pitch error correction amount table.
The thermal displacement correction method of this embodiment is performed by correcting the pitch error correction amount using the calculated thermal displacement when correcting the thermal displacement.

This embodiment is an example in which the thermal displacement of the X-axis ball screw shaft 81 is corrected, but basically the same applies to the Y-axis ball screw mechanism and the Z-axis ball screw mechanism.

As shown in FIG. 10, the nut portion 8 a is movable in the nut portion moving range 81 b of the X-axis ball screw shaft 81. Pitch error correction is performed for each correction section. The correction sections are 15 sections obtained by dividing the nut portion movement range 81b by a set length of 20 mm, for example. The pitch error correction amount for correcting the pitch error is a value obtained by the following procedure in the adjustment stage before shipment. The nut portion 8a moves from the position X0 to the position X300 for each correction section at intervals of 20 mm in the X-axis direction according to the movement command value. In this embodiment, an error (target value−actual movement amount) with respect to the movement command value is accurately measured. In this embodiment, a table of pitch error correction amounts is created based on the measurement results. In this embodiment, the created table is stored in advance in the flash memory 53 and shipped. In the present embodiment, a table of pitch error correction amounts is similarly created for the Y-axis and Z-axis directions.

熱 Explain how to calculate thermal displacement. The amount of thermal displacement is generated along with the numerical control during the operation of the machine tool M. As shown in FIG. 5, in this embodiment, the amount of heat generated in three regions of the front shaft portion 81 a of the ball screw shaft 81, the nut portion moving range 81 b, and the rear shaft portion 81 c of the ball screw shaft 81 is obtained.

As shown in FIG. 5, in this embodiment, the section from the end 81e to the end 81f of the ball screw shaft 81 is divided into four. In this embodiment, for example, the calculation sections 1 to 3 are divided into 120 mm (six times the set length of the correction section), and the calculation section 4 is divided into 140 mm (seven times the set length of the correction section). To do. However, the four divisions are merely an example, and the present embodiment is not limited to the four divisions. In the present embodiment, the calorific value for each calculation section is obtained for each of the plurality of calculation sections at a predetermined time (for example, 50 ms).

As shown in FIGS. 5 and 6, the flash memory 53 is a predetermined time period (e.g., 6400ms) calorific value of the total generated in the arithmetic section 1-4 Q ₁ ~ Q _4, the total heat generation of the calorific value Q ₁ ~ Q ₄ A data area for storing the quantity Q _T , the rotational speed ω of the X-axis motor 71 and the drive current i is provided.

[Calculation of total calorific value]
In the present embodiment, it is determined in which calculation section the nut portion 8a exists based on the X-axis feed data (control data) of the machining program every predetermined time (for example, 50 ms). In this embodiment, the calorific value is calculated according to the following equation based on the feed speed of the table 8. The feed speed of the table 8 is determined based on the actual rotational speed of the X-axis motor 71. The actual rotational speed of the X-axis motor 71 is determined based on the detection signal of the encoder 71a. The data area of the flash memory 53 stores the calculated heat generation amount. The calorific value is calculated according to the following equation.

Q = K ₁ × F ^T (1)
Q is a calorific value. F is the feed speed of the table 8. K ₁ and T are predetermined constants.

In the present embodiment, using the above calorific value calculation formula, the calorific value due to the movement of the nut portion 8a in each calculation section is calculated 128 times every 50 ms for a predetermined period (for example, 6400 ms). In the present embodiment, the calorific values calculated during a predetermined period are totaled for each computation interval, and calorific values Q ₁ to Q ₄ for each computation interval are calculated. As shown in FIG. 6, in this embodiment, the heat generation amounts Q ₁ to Q ₄ are stored in the flash memory 53 in association with the calculation sections 1 to 4. In this embodiment, the total calorific value Q _T is calculated and stored in the flash memory 53. The total calorific value Q _T is a calorific value obtained by summing the calorific values Q ₁ to Q ₄ . In this embodiment, the data of the rotational speed ω (that is, ω ₀ , ω ₁ ,..., Ω ₁₂₇ ) of the X-axis motor 71 every 6400 ms during 6400 ms and the drive current of the X-axis motor 71 every 50 ms during 6400 ms. The data of i (that is, i ₀ , i ₁ ,... i ₁₂₇ ) is stored in the flash memory 53 respectively.

[Distribution of total calorific value]
The distribution method of the total calorific value Q _T shown below is based on the same method as that of Japanese Patent Publication No. 1992-240045. That is, the nut portion moving range 81b, the front shaft portion 81a, and the rear shaft portion 81c of the ball screw shaft 81 are regarded as being thermally independent from each other without causing heat conduction to other portions. The ratio of each heat generating portion to the total heat generation amount Q _T is substantially constant regardless of the change in the feed rate.

The CPU 51 calculates the distributed heat generation amount of each heat generating part according to the following equation.
Q _F = η _F × Q _T
Q _N = η _N × Q _T
Q _B = η _B × Q _T
The calorific value Q _F is the calorific value of the front shaft portion 81 a due to the rotation of the fixed bearing 18. The heat generation amount Q _N is the heat generation amount of the nut portion moving range 81b. The heat generation amount Q _B is the heat generation amount of the rear shaft portion 81 c due to the rotation of the movable bearing 19. Ratio eta _F is the ratio of the calorific value Q _F with respect to the total heat generation amount Q _T. Ratio eta _N is the ratio of the calorific value Q _N to the total heat generation amount Q _T. Ratio eta _B is the ratio of the calorific value Q _B to the total heat generation amount Q _T. The ratios η _F , η _N and η _B are constant as shown in the method. Therefore, the ratios η _F , η _N , and η _B are values obtained in advance by measuring Q _F , Q _N , and Q _B using an actual machine.

[Distribution of calorific value to each calculation section in nut movement range]
In the present embodiment, the heat generation amount Q _N of the nut portion movement range 81b is distributed to the bearing holder 20 and four calculation section separation positions (positions corresponding to θ ₂ to θ ₅ in FIG. 5). In the present embodiment, distribution ratios X ₁ to X ₄ are obtained according to the following equations based on the calorific values Q ₁ to Q ₄ and the total calorific values Q _T of the four calculation sections stored in the data area. The distribution ratios X ₁ to X ₄ are ratios for distributing the calorific value Q _N to the calorific values of the four calculation sections. The heat generation amounts Q ₁ to Q ₄ and the total heat generation amount Q _T are stored in the data areas, respectively.

X ₁ = calorific value Q ₁ / Q _{T of} calculation section 1
:
X ₄ = calorific value of calculation section 4 Q ₄ / Q _T
In the present embodiment, the distribution heat generation amounts Q _N1 to Q _N4 for the four calculation sections 1 to 4 are calculated from the distribution ratios X ₁ to X ₄ of the four calculation sections and the distribution heat generation amount Q _N of the nut portion movement range 81b by the following equation. Is calculated.

Q _N1 = X ₁ × Q _N
:
Q _N4 = X ₄ × Q _N
In the present embodiment, using the above result, the distribution heat generation amount at the bearing holder 20 and the four calculation section break positions can be expressed as shown in FIG.

[Calculation of temperature distribution]
In this embodiment, after calculating the distribution heat generation amount at the bearing holder 20 and the four calculation section break positions, the temperature distribution is calculated from each distribution heat generation amount. The temperature distribution can be obtained by solving the following unsteady heat conduction equation. However, the initial condition is {θ} _{t = 0} = {θ _a }, and θ _a is the initial temperature.

[C] d {θ} / dt + [H] {θ} + {Q} = 0 (2)
[C] is a heat capacity matrix, [H] is a heat conduction matrix, {θ} is a temperature distribution, {Q} is a calorific value, and t is time. In this embodiment, the temperature distribution multiplied by the heat conduction matrix is called a temperature rise matrix. The temperature distribution {θ} and the calorific value {Q} can be expressed by the following equations, respectively.

Q _F is the amount of heat generated by the front shaft portion 81a. Substituting the above equation into equation (2) yields the unsteady heat conduction equation as follows:

The conventional method using the unsteady heat conduction equation of Equation (3) uses a calorific value Q _F calculated based on a calorific value supplied from the X-axis motor 71 to the ball screw shaft 81 and a heat distribution coefficient. . The heat distribution coefficient is a constant value. Therefore, the conventional method cannot accurately estimate the temperature change of the ball screw shaft end portion 81e. The conventional method does not consider the time delay from when the current flows to the X-axis motor 71 until the temperature of the ball screw shaft end portion 81e rises. The conventional method does not take into consideration that part of the heat generated by the X-axis motor 71 radiates heat.

In the present embodiment, a point that a part of the heat generation amount of the X-axis motor 71 dissipates heat and the time delay are considered. In this embodiment, the temperature rise θ ₀ of the bearing holder 20 is used instead of the temperature rise θ ₁ of the ball screw shaft end portion 81e in the temperature rise matrix of the expression (3). For the temperature rises θ ₀ and θ ₂ to θ ₅ , a time variable is added as will be described later. The unsteady heat conduction equation is as follows.

Equation (4) can be transformed and expressed as the following equation (5).

Equation (5) is obtained by using a time variable with respect to the temperature of the bearing holder 20, the ball screw shaft end portion 81e, and four calculation section separation positions (positions corresponding to θ ₂ to θ ₅ in FIG. 5). It can be expressed as (6).

In this embodiment, a known value is substituted for the right side of the equation (6). In the present embodiment, it is possible to calculate the gradient of the temperature rise at the ball screw shaft end portion 81e and the four calculation section break positions at the time nΔt on the left side.

In this embodiment, the temperature rise θ ₀ (t) of the bearing holder 20 is calculated. In this embodiment, the temperature rise θ ₀ (t) of the bearing holder 20 is calculated based on the drive current of the X-axis motor 71 and the rotational speed of the X-axis motor 71. The current detector 61b detects the drive current of the X-axis motor 71. The rotational speed of the X-axis motor 71 is based on the detection signal of the encoder 71a. In this embodiment, as shown in FIG. 8, the calculation cycle is t = Δt (for example, Δt = 50 ms), and the temperature rise θ ₀ (t) of the bearing holder 20 is calculated by the following equation.
θ ₀ (Δt) = θ ₀ (0) + k1 × ω (Δt) + k2 × (i (Δt)) ² −k3 ×
θ ₀ (0)
θ ₀ (2Δt) = θ ₀ (Δt) + k1 × ω (2Δt) + k2 × (i (2Δt)) ²
−k3 × θ ₀ (Δt)
θ ₀ (3Δt) = θ ₀ (2Δt) + k1 × ω (3Δt) + k2 × (i (3Δt)) ² −k3 × θ ₀ (2Δt)
θ ₀ (4Δt) = θ ₀ (3Δt) + k1 × ω (4Δt) + k2 × (i (4Δt)) ² −k3 × θ ₀ (3Δt)
:
θ ₀ (nΔt) = θ ₀ ((n−1) Δt) + k1 × ω (nΔt) + k2 × (i (nΔt)) ² −k3 × θ ₀ ((n−1) Δt)
k1, k2, and k3 are values obtained by experiments (specific constants based on the X-axis servomotor 71 and the bearing holder 20). ω (nΔt) is the average speed of the X-axis motor 71 at t = 0 to nΔt. i (nΔt) is an average value of the drive current of the X-axis motor 71 at t = 0 to nΔt. θ ₀ (0) is the initial temperature rise of the bearing holder 20 at t = 0.

In the present embodiment, the initial temperature rise and the distribution heat generation amount at the bearing holder 20 and the four calculation section break positions are substituted into the right side of Expression (6). As shown in FIG. 9, in this embodiment, it is possible to calculate the gradient of the temperature rise between the ball screw shaft end portion 81 e and the four calculation section break positions at t = 0. The following equation (7) can be obtained from the calculated inclination. Equation (7) can calculate the temperature rise at the ball screw shaft end portion 81e and the four calculation section break positions at t = Δt.
θ _i (Δt) = θ _i (0) + dθ _i (0) / dt × Δt (7)
i = 1 to 5, θ _i (0) is the initial temperature rise at t = 0.

In the present embodiment, by repeating the same calculation, it is possible to calculate the temperature rise of the ball screw shaft end portion 81e and the four calculation section break positions at t = 2Δt, t = 3Δt,. In this embodiment, the temperature distribution {θ} with the passage of time can be calculated at the ball screw shaft end portion 81e and the four calculation section break positions.

[Calculation of thermal displacement]
In this embodiment, the temperatures θ ₁ to θ ₅ of the ball screw shaft end portion 81e and the four calculation section break positions are calculated. In this embodiment, based on the calculated temperatures θ ₁ to θ ₅ , the thermal displacement amounts of the ball screw shaft end portion 81e and the four calculation section break positions are calculated. The thermal displacement amount between the ball screw shaft end portion 81e and the four calculation section break positions is calculated from the following equation (8).

^{_{ΔL = ∫ L 0 β × θ}} (L) dL ··· (8)
ΔL is the amount of thermal displacement. β is the coefficient of linear expansion of the ball screw shaft material. The integration symbol indicates integration over a range of 0 to L. L indicates the length up to the four calculation section break positions. Specifically, the integration over a range of 0 to 120, 0 to 240, 0 to 360,.

[Calculation of correction amount]
In this embodiment, after calculating the thermal displacement amounts at the four calculation section break positions of the ball screw shaft 81, the correction amounts for correcting the pitch error correction amounts of the 15 correction sections are calculated. Since the nut moving range is X0 to X300 (300 mm range) and the length of each correction section is 20 mm, there are 15 correction sections. The correction amounts of the 15 correction sections are calculated using FIG. 11 and a correction amount calculation formula described later.

FIG. 11 is an explanatory diagram for calculating a correction amount for correcting the pitch error correction amount. As shown in FIG. 11, the vertical axis represents the amount of thermal displacement based on the position of the fixed bearing 18. The upper horizontal axis is the position of each part of the ball screw shaft 81 with respect to the fixed bearing 18. The horizontal axis on the lower side is a delimiter position (X0, X20..., X300) of 15 correction sections.
D _F1 is a thermal displacement amount in the calculation section 1.
D _F2 is the total amount of thermal displacement in the calculation section 1 and the calculation section 2.
:
D _F4 is the total amount of thermal displacement in the calculation section 1 to the calculation section 4.

As shown in FIG. 11, the following equation can calculate the correction amount at the delimiter positions (X20,..., X300) of the 15 correction sections.
[Correction amount calculation formula]
Correction amount of X0 = thermal displacement amount of calculation section 1 × {(length between left delimiter position of calculation section 1 and X0) / length of calculation section 1}
Correction amount of X20 = thermal displacement amount of calculation section 1 × {(length between the left delimiter position of calculation section 1 and X20) / length of calculation section 1} −correction amount of X0 correction amount of X40 = calculation section 1 Heat displacement amount + heat displacement amount in computation interval 2 × {(length between left separation position of computation interval 2 and X40) / length of computation interval 2} −correction amount of X20 X60 correction amount = calculation interval 1 Thermal displacement amount + thermal displacement amount in computation interval 2 × {(length between left delimiter position of computation interval 2 and X60) / length of computation interval 2} −correction amount of X40 correction amount of X80 = calculation interval 1 Thermal displacement amount + thermal displacement amount in computation interval 2 × {(length between left delimiter position of computation interval 2 and X80) / length of computation interval 2} −correction amount of X60:
Correction amount of X300 = thermal displacement amount of computation section 1 + thermal displacement amount of computation section 2 + thermal displacement amount of computation section 3 + thermal displacement amount of computation section 4 × {(between the left separation position of computation section 4 and X300 Length) / length of calculation section 4} −X280 correction amount

With reference to FIG. 12, an embodiment of the thermal displacement correction control executed by the numerical controller 50 will be described. The numerical control device 50 functions as a thermal displacement correction device of the present invention. In the figure, Si (i = 1, 2,...) Indicates each step. The thermal displacement correction control executes the above-described thermal displacement correction method. Therefore, since this control has many parts which overlap with the above-mentioned content, it demonstrates easily. Numerical control of machining on the workpiece is executed in parallel with thermal displacement correction control.

The CPU 51 of the numerical controller 50 first performs initial setting in S1 (S1). In the initial setting, a matrix and an initial temperature necessary for calculation by the finite element method are set from setting data such as parameters. In the initial setting, the CPU 51 executes processing such as clearing the related area of the flash memory 53. As shown in FIG. 5, the CPU 51 divides the ball screw shaft 81 into four calculation sections 1 to 4 (S2).

The CPU 51 sets the counter I to 0 (S3). The CPU 51 reads the X-axis feed data, the detection signal of the encoder 71a, and the drive current value of the current detector 61b (S4). The CPU 51 calculates the amount of heat generated every 50 ms in the calculation sections 1 to 4 and the temperature θ ₀ of the bearing holder 20 and stores them in the flash memory 53 (S5). The CPU 51 adds “1” to the counter I (S6). The CPU 51 determines whether or not the counter value of the counter I is “127” (S7). While determining that the counter value of the counter I is not “127” (S7: NO), the CPU 51 returns to S4 and repeats the processes of S4 to S6. When the CPU 51 determines that the counter value of the counter I is “127” (S7: YES), the CPU 51 calculates the heat generation amounts Q ₁ to Q ₄ and the total heat generation amount Q _T for 6400 ms in the calculation sections 1 to 4. The data is stored in the flash memory 53 (S8).

The CPU 51 calculates the calorific values Q _N and Q _B of each part described above, stores them in the flash memory 53, and computes the calorific values Q _N1 to Q _N4 distributed to the calculation sections 1 to 4 distributed to the calorific values Q _N. And stored in the flash memory 53. The CPU 51 also calculates the distribution heat generation amount shown in FIG. 7 for the bearing holder 20 and the four calculation section break positions and stores them in the flash memory 53 (S9). The CPU 51 calculates the temperatures θ ₁ to θ ₅ of the ball screw shaft end portion 81e and the four calculation section break positions based on the distributed heat generation amount shown in FIG. 7, and stores them in the flash memory 53 (S10).

The CPU 51 calculates the thermal displacement amount at the calculation section break position for the four calculation sections based on the above formula (8) and stores it in the flash memory 53 (S11). The CPU 51 calculates the correction amounts at the 16 correction section break positions as described above based on the correction amount calculation formula described above (S12). The CPU 51 uses the correction amount calculated in S12 to execute correction processing for the preset pitch error correction amount for the 16 correction section break positions, and performs feed amount correction processing using the corrected pitch error correction amount. (S13). The CPU 51 determines whether or not to end the thermal displacement correction process (S14). When it is determined that the thermal displacement correction process is not to be ended (S14: NO), the CPU 51 returns to S3 and repeatedly executes the processes after S3. When it is determined that the thermal displacement correction process is finished (S14: YES), the thermal displacement correction control is finished.

With reference to the flowchart of FIG. 13, the correction amount calculation processing for calculating the correction amount for correcting the pitch error correction amount in S12 will be described. In the figure, Si (i = 20, 21...) Indicates each step. The CPU 51 resets the counter n to 0 (S20). A correction amount ΔM _n for the position Xn is calculated by the following equation (S21).

ΔM _n = D _F + ΔD _n × {(Xn−X _F ) / L _n } −ΔM _n−20 , n = 0, and the correction amount ΔM ₀ of the position X0 is obtained. The following formula simply represents the correction amount calculation formula described above.
D _F : The total amount of thermal displacement generated in the calculation section on the fixed side with respect to the position Xn.
ΔD _n : thermal displacement amount generated in the calculation section including the position Xn.
X _F : Left separation position of the calculation section including the position Xn.
L _n : the length of the calculation section including the position Xn.
ΔM ₋₂₀ used for obtaining ΔM ₀ is 0.

The CPU 51 adds 20 to n (S22). The CPU 51 determines whether n is 320 (S23). If n is not a 320 (S23: NO), it is determined that not finished operation on the correction amount to the position X300, it calculates a correction amount .DELTA.M _n positions Xn returns to S21. Until calculates a correction amount .DELTA.M ₃₀₀ position X300 is repeatedly executes S21 ~ S23. After calculating the correction amount ΔM ₃₀₀ (S21), if n = 320 in S22 (S23: YES), the CPU 51 ends this process and proceeds to S14 in FIG.

Encoder 71a corresponds to “speed detection device”. The CPU 51 that executes S3 to S7 corresponds to a “heat generation amount calculation unit” and a “temperature calculation unit”. The CPU 51 that executes S8 to S10 corresponds to a “temperature distribution calculation unit”. The CPU 51 that executes S11 corresponds to a “thermal displacement calculation unit”. The CPU 51 that executes S12 corresponds to a “correction amount calculation unit”.

The operation and effect of the numerical control device 50, which is a thermal displacement correction method and a thermal displacement correction device for the machine tool M, will be described. In this embodiment, the amount of heat generated in a plurality of sections obtained by dividing the entire length of the ball screw shaft 81 is calculated every 50 ms based on the rotational speed of the X-axis motor 71 and control data. The temperature rise of the bearing holder 20 is calculated every 50 ms. The present embodiment is based on the unsteady heat conduction equation using the total heat generation amount accumulated for 6400 ms and the temperature rise of the ball screw shaft end portion 81e instead of the temperature rise of the bearing holder 20 based on the unsteady heat conduction equation. The temperature distribution is calculated every 6400 ms.

In the present embodiment, the thermal displacement amount of a plurality of sections of the ball screw shaft 81 is calculated every 6400 ms based on the temperature distribution. Finally, a correction amount for correcting the control data is calculated every 6400 ms for each of a plurality of correction sections obtained by dividing the nut movement range 81b of the ball screw shaft 81 based on the thermal displacement amounts of the plurality of sections. Therefore, the present embodiment can obtain the following effects.

The unsteady heat conduction equation uses the temperature rise of the bearing holder 20 that has risen due to the amount of heat input from the X-axis motor 71. Therefore, in this embodiment, the heat displacement amount can be calculated using the heat generation amount considering the heat radiation amount in the heat generation amount of the X-axis motor 71. The temperature rise of the bearing holder 20 is calculated every predetermined time. Therefore, this embodiment can calculate the amount of thermal displacement taking into account the time delay from when the current flows to the X-axis motor 71 until the ball screw shaft end portion 81 e is thermally affected by the X-axis motor 71. In the present embodiment, the thermal displacement amount is calculated in consideration of the fact that a part of the heat generation amount of the X-axis motor 71 dissipates heat and the occurrence of the time delay. Therefore, highly accurate thermal displacement correction can be performed.

The temperature rise of the bearing holder 20 is calculated based on the rotational speed of the X-axis motor 71 and the drive current value. Therefore, in this embodiment, it is possible to perform highly accurate thermal displacement correction using an existing sensor without newly providing a sensor or the like, and to reduce the manufacturing cost.

A modification example in which the above embodiment is partially modified will be described. The above embodiment uses the drive current of the X-axis motor 71 and the rotational speed of the X-axis motor 71 when calculating the temperature rise of the bearing holder 20. The modification example may be calculated using only the drive current of the X-axis motor 71 or only the rotation speed of the X-axis motor 71. In the modified example, a temperature sensor may be attached to the bearing holder 20, and the temperature rise of the bearing holder 20 may be calculated using the detection result of the temperature sensor.

In the above embodiment, the calculation cycle for calculating the calorific value is described as an example, but the calculation cycle is not limited to 50 ms. The predetermined period of 6400 ms is only an example. In the modified example, for example, the predetermined period may be set in units of seconds longer than that of the present embodiment.

Claims

In a thermal displacement correction method for a machine tool having a feed screw ball screw mechanism, a servo motor that rotationally drives a shaft into which a nut of the ball screw mechanism is screwed, and a control device that controls the servo motor based on control data.
The amount of heat generated in a plurality of sections obtained by dividing the entire length of the shaft is obtained every predetermined time based on the rotation speed of the servo motor and control data, and the motor side of the shaft is rotatably supported via a bearing. A first step of calculating the temperature rise of the bearing holder every predetermined time;
Based on the total calorific value obtained by accumulating the calorific value of the plurality of sections for a predetermined period, and an unsteady heat conduction equation using the temperature rise of the bearing holder instead of the temperature rise of the motor side end of the shaft, A second step of calculating the temperature distribution of the section for each predetermined period;
A third step of calculating a thermal displacement amount of the plurality of sections of the shaft from the temperature distribution for each predetermined period;
A fourth step of calculating, for each predetermined period, a correction amount for correcting the control data for each of a plurality of correction sections obtained by dividing the nut movement range of the shaft based on the amount of thermal displacement of the plurality of sections;
A thermal displacement correction method for machine tools provided with
The method for correcting a thermal displacement of a machine tool according to claim 1, wherein the temperature rise of the bearing holder is calculated based on a rotation speed and a drive current value of the servo motor.
In a thermal displacement correction device for a machine tool having a feed screw ball screw mechanism, a servo motor that rotationally drives a shaft into which a nut of the ball screw mechanism is screwed, and a control device that controls the servo motor based on control data.
A speed detection device for detecting the rotational speed of the servo motor;
A calorific value calculation unit for obtaining a calorific value generated in a plurality of sections obtained by dividing the entire length of the shaft at a predetermined time based on a rotation speed of the servo motor and control data; and
A temperature calculation unit that calculates a temperature rise of a bearing holder that rotatably supports the motor side of the shaft via a bearing, for each predetermined time;
Based on the total calorific value obtained by accumulating the calorific value of the plurality of sections for a predetermined period, and an unsteady heat conduction equation using the temperature rise of the bearing holder instead of the temperature rise of the motor side end of the shaft, A temperature distribution calculator that calculates the temperature distribution of the section for each predetermined period;
A thermal displacement calculation unit that calculates the thermal displacement of the plurality of sections of the shaft from the temperature distribution for each predetermined period;
A correction amount calculation unit that calculates a correction amount for correcting each of the control data for each of a plurality of correction sections obtained by dividing a plurality of the nut movement ranges of the shaft based on the thermal displacement amount of the plurality of sections;
Thermal displacement correction device for machine tools equipped with
The thermal displacement correction device for a machine tool according to claim 3, wherein the temperature calculation unit calculates a temperature rise of the bearing holder based on a rotation speed and a drive current value of the servo motor.