JP2009214283A - Thermal displacement correction method of machine tool, thermal displacement correction device and program for thermal displacement correction of the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermal displacement correction method of a machine tool, a thermal displacement correction device and a program for thermal displacement correction of the same in which an amount of correction can be approximated to an actual amount of extension of a ball screw shaft in a transition state to stabilize temperature increase of the machine tool. <P>SOLUTION: Saturation temperature is calculated from current and rotation speed in a temperature distribution calculation circuit 19 to calculate temperature increase of an X-axis motor 71. A heat generation amount of a front bearing part is calculated from the temperature increase and temperature of a shaft end part in an X-axis motor side, and a heat generation amount of each section of the ball screw shaft 81 is calculated. Afterwards temperature distribution of each heat source is calculated from these heat generation amounts and various data stored in a parameter memory 20. In a period until the heat generation amount and a heat radiation amount of the motor are balanced, an accurate amount of correction following change of temperature of the motor can be obtained by using a second heat generation amount based on the temperature increase of the motor for calculation to derive a correction value. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a thermal displacement correction method for a machine tool, a thermal displacement correction apparatus, and a thermal displacement correction program, and more particularly to a configuration configured to correct an error due to thermal displacement of a ball screw mechanism that occurs during operation of a machine tool.

  The ball screw mechanism is widely used in machine tools as a positioning mechanism. The ball screw mechanism causes thermal expansion due to a rise in temperature due to frictional resistance between the ball screw shaft, the nut, and each part of the bearing and heat generation of the servo motor, thereby causing thermal displacement. In the current NC machine tool, a semi-closed loop type is generally used, but in this type of NC machine tool, the thermal displacement of the ball screw shaft appears as a positioning error as it is. For this reason, a method of applying pretension to the ball screw shaft and absorbing thermal expansion has been used as a countermeasure. However, recently, the use of a thick ball screw shaft and the feeding speed has become very fast, so the amount of heat generation has increased, and when trying to cope with the pre-tension method, a very large tensile force has to be applied. There have been problems such as deformation of the structure of the screw mechanism and seizure due to excessive force applied to the thrust bearing.

  Therefore, a method for correcting thermal displacement of a ball screw shaft that does not give excessive pretension to the ball screw shaft and does not require a special measuring device has been developed. This is because the temperature distribution is obtained in a model in which the ball screw shaft is divided into a plurality of sections from the heat generation amount of the ball screw shaft obtained from the product of the electric current and voltage of the servo motor, and further, the amount of thermal displacement of the ball screw shaft. This is a method of correcting in-process by predicting this moment by moment and giving this thermal displacement amount to the NC device as pitch error correction.

  Patent document 2 pays attention to the problem that the acceleration / deceleration energy of the servo motor itself is included in the calorific value in the method of patent document 1, and the calorific value of each section of the ball screw shaft is determined from the rotation speed of the servo motor. A method for correcting the amount of thermal displacement to be calculated is proposed. According to this method, the calculated correction amount can be approximated to the actual elongation of the ball screw shaft.

JP-A 63-256336 Japanese Patent Laid-Open No. 4-240045

  However, in the method proposed in Patent Document 2, the heat generation amount of the servo motor is calculated only by the rotation speed, and no consideration is given to the point that the heat generation amount varies depending on the load of the servo motor. Furthermore, since there is no disclosure about the initial condition of servo motor operation and the condition for calculating the correction amount after a certain period of time, the correction amount calculated in the transient state at the initial operation of the motor and the actual ball screw shaft There is a possibility that the elongation cannot be approximated.

Therefore, the servo motor is actually operated to measure the temperature of the end of the ball screw shaft, and the heat distribution model proposed in Patent Document 2 is created to determine the actual temperature and heat distribution of the end of the ball screw shaft. A comparative experiment with the temperature calculated by the model was performed.
As shown in FIG. 14, a mechanism for measuring an end temperature of an actual ball screw shaft includes a servo motor 201, a ball screw shaft 203 connected to the servo motor 201 via a coupling 202, and the ball screw. A nut 204 that is screwed to the shaft 203 and can be moved back and forth by the rotation of the ball screw shaft 203 and a table 205 that can move back and forth integrally with the nut 204, and the ball screw shaft 203 is a fixed bearing installed on a support base. 206 and a movable bearing 207 are rotatably supported. The temperature measurement portion 208 was set near the center portion of the fixed bearing 206.

The conditions for this heat input experiment were set as follows.
(Condition 1)
The average current flowing through the moving servo motor 201 and the average number of rotations are kept constant, and reciprocating at a constant speed is repeated until the temperature measurement value at the fixed position 209 is stabilized.
(Condition 2)
The table 205 is moved at a position sufficiently away from the fixed bearing 206 so that the heat generation of the nut 204 does not affect the temperature measurement portion 208, and the temperature of the temperature measurement portion 208 is limited to the influence of the servo motor 201 and the fixed bearing 206. .

  Next, a heat distribution model proposed in Patent Document 2 was created. Since the average current flowing through the servo motor 201 and the average rotation speed are constant, the heat input from the servo motor 201 to the end of the ball screw shaft is assumed to be constant, and the unsteady heat conduction equation in Patent Document 2 is solved, The temperature at the time point was calculated.

  FIG. 15 shows experimental values of thermal displacement at a fixed position 209 (see FIG. 14) and calculated values based on a heat distribution model. The vertical axis represents the temperature at the end of the ball screw shaft, the horizontal axis represents the elapsed time, the solid line represents the experimental value, and the broken line represents the calculated value. From this result, after the temperature rise at the end of the ball screw shaft is stabilized, the experimental value and the calculated value are approximated, but the calculated value is experimental during the period of transient state until the temperature rise is stabilized. It was confirmed that the temperature rises faster than the value and accurate prediction cannot be made.

The object of the present invention is to provide a transient state until the temperature rise is stabilized after the machine tool is operated.
It is an object of the present invention to provide a thermal displacement correction method for a machine tool, a thermal displacement correction apparatus, and a program for correcting the thermal displacement that can approximate the correction amount to the actual elongation amount of the ball screw shaft.

The method for correcting a thermal displacement of a machine tool according to claim 1 includes a ball screw mechanism for feed drive, a feed amount control means for calculating a feed amount of a nut by a ball screw shaft of the ball screw mechanism based on machining data, and the ball screw. A method having a servo motor for rotating a shaft and a speed control means for controlling the rotation speed of the servo motor based on machining data, and is generated from a nut to a ball screw shaft based on the rotation speed of the servo motor. A first step for obtaining a first heating value; a second step for detecting a rising temperature of the servo motor; and a second heating value generated from the servo motor to the ball screw shaft based on the rising temperature; and a first heating value; A third step of calculating a temperature distribution of a plurality of sections obtained by dividing the ball screw shaft in the length direction from the second calorific value;
A fourth step of calculating a thermal displacement amount of each section of the ball screw shaft from a temperature distribution and a fifth step of calculating a correction amount of machining data based on the thermal displacement amount are provided.

  According to a second aspect of the present invention, there is provided a method for correcting a thermal displacement of a machine tool, wherein the rising temperature of the servo motor is detected based on a rotation speed of the servo motor and a drive current value.

  A thermal displacement correction device for a machine tool according to claim 3, comprising: a feed drive ball screw mechanism; a feed amount control means for calculating a feed amount of a nut by the ball screw shaft of the ball screw mechanism based on machining data; and a ball screw shaft And a speed control means for controlling the rotation speed of the servo motor based on the machining data, the speed detection means for detecting the rotation speed of the servo motor, and the servo motor From temperature detection means for detecting the rising temperature, a first heat generation amount generated from the nut to the ball screw shaft based on the rotational speed of the servo motor, and a second heat generation amount generated from the nut to the ball screw shaft based on the rising temperature of the servo motor. Temperature distribution calculating means for calculating a temperature distribution of a plurality of sections obtained by dividing the ball screw shaft in the length direction; Wherein while calculating the thermal distortion of each section of the ball screw shaft, characterized in that a correction quantity calculating means for calculating a correction amount of the processed data based on the thermal displacement amount based on.

  According to a fourth aspect of the present invention, there is provided the thermal displacement correcting device for a machine tool according to the third aspect, wherein the temperature detecting means detects the rising temperature based on the rotational speed of the servo motor and the drive current value.

  A program for correcting thermal displacement of a machine tool according to claim 5 comprises: a feed screw ball screw mechanism; a feed amount control means for calculating a feed amount of a nut by a ball screw shaft of the ball screw mechanism based on machining data; A computer having a servo motor for rotationally driving the shaft and a speed control means for controlling the rotational speed of the servo motor based on the machining data, a first calorific value obtained based on the rotational speed of the servo motor and the servo motor A temperature distribution calculating means for calculating a temperature distribution of a plurality of sections obtained by dividing the ball screw shaft in the length direction from a second calorific value obtained based on the rising temperature of the temperature, and a temperature distribution obtained by the temperature distribution calculating means To calculate the amount of thermal displacement for each section of the ball screw shaft, and to calculate the amount of machining data correction using this amount of thermal displacement. Wherein the function as the correction amount calculation means.

According to the first aspect of the present invention, not only the first heat generation amount generated from the nut to the ball screw shaft but also the second heat generation amount based on the rising temperature of the servo motor is used to obtain the temperature distribution of the plurality of sections of the ball screw shaft. Since the correction amount is obtained by calculation, the correction amount can be approximated to the actual elongation of the ball screw shaft even in a transient state until the temperature of the servo motor is stabilized. That is, heat input from the servo motor to the ball screw shaft is affected by the rising temperature of the servo motor, but the rising temperature of the servo motor itself is caused by the influence of operating elements such as a load. In particular, during the period until the amount of heat generated and the amount of heat released by the servo motor are balanced, the temperature of the servo motor changes every moment, so the amount of heat generated based on the temperature increase of the servo motor should be used for the calculation to obtain the correction value. As a result, a correction amount following the change in the motor temperature can be obtained.
In addition to heat input from the servo motor to the ball screw shaft, the amount of thermal displacement can be calculated for each of a plurality of sections using the first heat generation amount generated from the nut to the ball screw shaft, and a separate sensor is provided. Thus, a highly accurate correction amount can be obtained.

  According to the invention of claim 2, since the rising temperature of the servo motor is detected based on the rotation speed of the servo motor and the drive current value, an existing sensor can be used without using a separate sensor or the like, and an accurate correction amount can be obtained. Obtainable.

According to the invention of claim 3, the temperature detecting means for detecting the rising temperature of the servo motor is provided, and the correction amount is calculated by calculating the temperature distribution in a plurality of sections of the ball screw shaft using the heat generation amount based on the rising temperature. Therefore, the correction amount can be approximated to the actual elongation of the ball screw shaft even in a transient state until the temperature rise is stabilized.
In addition to heat input from the servo motor to the ball screw shaft, the amount of heat displacement can be calculated for each section using the heat generated from the nut to the ball screw shaft, and there is no need to provide a separate sensor. A numerical control device for a machine tool capable of obtaining a highly accurate correction amount is obtained.

  According to the invention of claim 4, since the temperature detecting means detects the rising temperature based on the rotation speed of the servo motor and the drive current value, an existing sensor can be used without using a separate sensor, etc. The quantity can be obtained.

  According to the fifth aspect of the present invention, the temperature distribution calculation for calculating the temperature distribution from the first heat generation amount obtained based on the rotation speed of the servo motor and the second heat generation amount obtained based on the rising temperature of the servo motor. Correction amount calculation that calculates the amount of thermal displacement of a plurality of sections of the ball screw shaft using the temperature distribution obtained by the means and the temperature distribution calculation means, and calculates the correction amount of the machining data using the amount of thermal displacement Since it functions as a means, the same effects as in the first or third aspect are obtained.

  Hereinafter, the best mode for carrying out the present invention will be described based on examples.

The configuration of the machining center M (machine tool) will be described with reference to FIGS.
As shown in FIG. 1, the machining center M moves a workpiece and a tool relative to each other in an XYZ orthogonal coordinate system independently to perform desired machining (for example, “boring”, “milling”). ”,“ Drilling ”,“ cutting ”, etc.). The machining center M includes a base 1 that is a cast iron base, an upper part of the base 1, a machine body 2 that performs cutting of a workpiece, and is fixed to the upper part of the base 1. It is mainly composed of a box-shaped splash cover (not shown) that covers the upper part.

  The base 1 is a substantially rectangular parallelepiped casting that is long in the Y-axis direction. Legs whose heights can be adjusted are provided at the four corners of the lower part of the base 1, and the machining center M is installed by installing these legs on the floor of a factory or the like.

Next, the machine body 2 will be described.
The machine body 2 includes a column 4 fixed to the upper surface of the column seat 3 on the rear portion of the base 1 and extending vertically upward, a spindle head 5 that can be raised and lowered along the front surface of the column 4, and the spindle head 5. A main shaft that is rotatably supported inside, a tool changer (ATC) 7 that is provided on the right side of the main shaft head 5 and attaches a tool holder of the tool 6 to the tip of the main shaft, and is provided at the top of the base 1. The table 8 is mainly composed of a table 8 for detachably fixing the workpiece. A box-shaped control box 9 is provided on the back side of the column 4, and a numerical controller 50 that controls the operation of the machining center M is provided inside the control box 9.

Next, the moving mechanism of the table 8 will be described.
As shown in FIG. 3, the table 8 is driven in the X-axis direction (left-right direction of the machine body 2 in FIG. 1) by an X-axis motor 71 (see FIG. 4) and a Y-axis motor 72 (see FIG. 4), which are servo motors. The movement is controlled in the Y-axis direction (the depth direction of the machine body 2). This moving mechanism has the following configuration. First, a rectangular parallelepiped support 10 is provided below the table 8. A pair of X-axis feed guides extending along the X-axis direction are provided on the upper surface of the support base 10, and the table 8 is movably supported on the pair of X-axis feed guides.
As shown in the block diagram of the ball screw mechanism in FIG. 3, a nut portion 8 a is disposed on the lower surface of the table 8, and this nut portion 8 a is screwed with an X-axis ball screw shaft 81 extending from the X-axis motor 71. By doing so, a ball screw mechanism is configured. An end portion 81a on the X-axis motor 71 side of the X-axis ball screw shaft 81 is supported by a fixed bearing 91a fixed to the support base 10, and an opposite end portion 81b is supported by a movable bearing 91b.

The support base 10 is provided on an upper portion of the base 1 and is movably supported on a pair of Y-axis feed guides extending along the longitudinal direction of the base 1. The table 8 is driven to move in the X-axis direction along the X-axis feed guide by the X-axis motor 71 provided on the base 1, and along the Y-axis feed guide by the Y-axis motor 72 provided on the base 1. It is driven to move in the Y-axis direction.
The Y-axis moving mechanism is also a ball screw mechanism, similar to the X-axis.

  In the X-axis feed guide, telescopic covers 11 and 12 that contract in a telescopic manner are provided on both the left and right sides of the table 8. The Y-axis feed guide is provided with a telescopic cover 13 and a Y-axis rear cover before and after the support base 10, respectively. Even if the table 8 is moved in either the X-axis direction or the Y-axis direction by these plural covers, the X-axis feed guide and the Y-axis feed guide are always the telescopic covers 11, 12, 13 and the Y-axis rear cover. Covered by. That is, it is possible to prevent the chips scattered from the processing area, the splash of the coolant, and the like from falling on each rail.

Next, the elevating mechanism of the spindle head 5 will be described.
As shown in FIGS. 1 and 2, the spindle head 5 is supported by a guide rail extending in the vertical direction on the front side of the column 4 so as to be movable up and down via a linear guide. The spindle head 5 is connected to a Z-axis ball screw shaft extending in the vertical direction on the front side of the column 4 by a nut. By rotating the Z-axis ball screw shaft in the forward and reverse directions by a Z-axis motor 73 (see FIG. 4), the spindle head 5 is driven up and down in the vertical direction. Accordingly, the spindle head 7 is driven up and down by driving the Z-axis motor 73 by the Z-axis drive circuit 63 based on a control signal from the CPU 51 of the numerical controller 50.

  As shown in FIGS. 1 and 2, the tool changer 7 grips a tool magazine 14 that stores a plurality of tool holders that support the tool 6, a tool holder attached to the spindle, and another tool holder. It is composed of a tool change arm 15 for conveying. Inside the tool magazine 14 shown in FIGS. 1 and 2, a plurality of tool pots for supporting the tool holder and a transport mechanism for transporting the tool pots in the tool magazine 14 are provided.

FIG. 4 shows an electrical configuration of the machine tool.
The control device 50 as a control means includes a microcomputer, and includes an input / output interface 54, a CPU 51, a ROM 52, a RAM 53, axis control circuits 61a to 64a and 75a, and servo amplifiers 61 to 64. Differentiators 71b to 74b are provided. 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 axis control circuit 75 a is connected to the magazine motor 75.

  The X-axis motor 71 and the Y-axis motor 72 are for moving the table 8 in the X-axis direction and the Y-axis direction. The magazine motor 75 is for rotating the tool magazine 14. The main shaft motor 74 is for rotating the main shaft. 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 control circuits 61a to 64a receive a movement command amount from the CPU 51 and output a current command amount (motor torque command value) to the servo amplifiers 61 to 64. The servo amplifiers 61 to 64 receive this command and output a drive current to the motors 71 to 74. The axis control circuits 61a to 64a receive position feedback signals from the encoders 71a to 74a and perform position feedback control. The differentiators 71b to 74b differentiate the position feedback signals input from the encoders 71a to 74a to convert them into speed feedback signals, and output the speed feedback signals to the axis control circuits 61a to 64a.

  The axis control circuits 61a to 64a receive speed feedback signals from the differentiators 71b to 74b and control the speed feedback. The drive current output from the servo amplifiers 61 to 64 to the motors 71 to 74 is detected by the current detectors 61b to 64b. The drive current detected by the current detectors 61b to 64b is fed back to the axis control circuits 61a to 64a. The axis control circuits 61a to 64a perform current (torque) control using the fed back drive current.

  In general, since the drive current flowing through the motors 71 to 74 and the load torque applied to the motors 71 to 74 are approximately the same, the current detectors 61b to 64b that detect the drive current flowing to the motors 71 to 74 are used. Can be detected. The axis control circuits 61 a to 64 a receive the movement command amount from the CPU 51 and drive the magazine motor 75.

Next, a method for calculating the amount of thermal displacement used in numerical control of the machine tool will be described.
In this calculation method, the calorific value of the three regions of the front bearing portion and nut portion of the servo motor side ball screw shaft and the rear bearing portion of the anti-servo motor side ball screw shaft is obtained, and further, the nut portion movement interval Is divided into a plurality of sections, and the calorific value for each section is obtained.

(Calculation of total heat)
As shown in FIG. 5, the section from the end 81a to the end 81b (the length is indicated by L) of the ball screw shaft is divided into n. It is determined in which section the nut portion is present at regular time intervals (for example, 50 ms), the amount of generated heat is obtained from the actual rotation speed of the servo motor, and stored in a data area of a temperature distribution calculation circuit described later. The amount of generated heat is determined by the following equation.

[Equation 1]
Q = K ₁ × F ^T (1)
Here, Q: generated heat amount, F: feed rate, K ₁ , T: coefficient.

As shown in FIG. 6, the amount of heat generated by the movement of the nut portion in each section is calculated every 50 ms for a certain time (for example, 6400 ms), that is, 128 times. Are stored in the data area corresponding to .about.n. The total heat quantity Q _TTL of the heat quantities 1 to n in each section 1 to n generated during 6400 ms and the total rotation speed N _TTL of the rotation speeds in each section 1 to n are stored in the data area.

(Total heat distribution 1)
In the distribution method of the total heat quantity Q _TTL shown below, heat transfer to the other parts does not occur in the nut part moving section and the front and rear bearing parts, as in Patent Document 2, and is thermally and approximately independent. It is based on the knowledge that the ratio of each heat source part to the total calorific value is almost constant regardless of the feed rate.

That is, assuming that the total heat generation amount Q _TTL , the nut portion moving section heat generation amount Q _N , and the rear bearing portion heat generation amount Q _B , the heat generation amount of each heat source unit is expressed by the following equation:

[Equation 2]
Q _N = η _N × Q _TTL
Q _B = η _B × Q _TTL
Is calculated from Here, the ratios η _N and η _B are constant according to the above knowledge, and the ratios η _N and η _B are obtained in advance by measuring Q _N and Q _B with an actual machine.

(Distribution of heat quantity to each section of nut moving section)
Next, the amount of heat in each section of the nut section movement section is obtained. Since the heat quantity stored in the data area is a total value calculated every 50 ms, after obtaining the average heat quantity every 50 ms for each section, for each section from the average heat quantity and the total heat quantity, Existence probabilities X ₁ ... X _i ... X _n are obtained.

[Equation 3]
X ₁ = Average heat value for section 1 / Q _TTL
:
X _i = average heat for interval i / Q _TTL
:
X _N = Average heat value for section N / Q _TTL
In this way, when the existence probabilities X ₁ ... X _i ... Xn for each section are obtained, the distribution heat quantity Q _N1 for each section is calculated from the existence probability and the nut movement movement calorific value Q _N by the following equation. ... Q _Ni ... Q _Nn is obtained.

[Equation 4]
Q _N1 = X ₁ × Q _N
:
Q _Ni = X _i × Q _N
:
Q _Nn = X _n × Q _N

(Total heat distribution 2)
Then, to calculate the front bearing unit calorific value Q _F. Since the amount of heat generated at the front bearing is due to heat input due to the rising temperature of the servo motor, the temperature of the servo motor body is calculated, and the ball screw shaft end is calculated from the difference between this value and the ball screw shaft end temperature. heat input section, it is possible to obtain a so-called front bearing unit calorific value Q _F.

As illustrated in FIG. 7, when the maximum saturation temperature is L _1a , the motor body temperature Θ _M during driving of the machine tool draws an asymptotic line 150 with respect to the straight line l = L _1a . When the machine tool is stopped after the motor main body temperature Θ _M reaches the maximum saturation temperature L _1a (at time t = 8 hour in FIG. 7), the motor main body temperature Θ _M draws an asymptotic line 151 with respect to the straight line l = 0. . Asymptote 150 is
[Equation 5]
L _1a = K ₂ · ω + K ₃ · i ² (2)
Θ _M = L _1a · (1−exp (−γ · t)) (3)
The asymptotic line 151 can be expressed as

[Equation 6]
Θ _M = L _1a · exp (−γ · t) (4)
It is represented by Here, i is the current flowing through the servo motor, ω is the motor speed, L _1a is the saturation temperature, and γ, K ₂ , and K ₃ are constants specific to the servo motor. The motor body temperature Θ _M1a after a minute from the start of driving of the machine tool is
Θ _M1a = L _1a · {1-exp (−γ · a / 60)}
It becomes. In addition, the motor body temperature Θ _M-1a a minutes after the machine tool stops is
Θ _M−1a = L _1a · exp (−γ · a / 60)
It becomes. The motor body temperature Θ _M during the elapsed time is calculated mainly using the equation (3). In the following description, it is assumed that time t1, t2,... (Minutes) have elapsed since the machine tool was driven. That is, the interval between times t1, t2,... Is the elapsed time in each process.

In this embodiment, when calculating the motor body temperature theta _M based on the elapsed time, the motor body temperature theta _M is considered to decrease as subsequent formula (4). That is, as illustrated by a curve 301 in FIG. _8A , the value Θ _{Mt1-1 at} the time t1 of the motor body temperature Θ _Mt1 calculated based on the elapsed time from the time 0 to the time t1 is as described above. like,
Θ _Mt1-1 = L _t1 · {1-exp (−γ · t1 / 60)}
It becomes. However, L _t1 is the maximum saturation temperature calculated based on the elapsed time from time 0 to time t1. Then, the value Θ _Mt1-2 of the motor body temperature Θ _Mt2 at time t2 is _obtained from the equation (4):
Θ _Mt1-2 = Θ _Mt1-1 · exp {−γ · (t2−t1) / 60}
Similarly, the values Θ _Mt1-3 and Θ _Mt1-4 of the motor body temperature Θ _Mt1 at times t3 and t4 are
Θ _Mt1-3 = Θ _Mt1-1 · exp {−γ · (t3−t1) / 60}
Θ _Mt1-4 = Θ _Mt1-1 · exp {−γ · (t4−t1) / 60}
It becomes. Similarly, if the maximum saturation temperature L _t2 is calculated based on the elapsed time from time t1 to time t2, the motor body temperature Θ _Mt2 corresponding to the maximum saturation temperature L _t2 is illustrated by a curve 302 in FIG. Θ _Mt2-1 , Θ _Mt2-2 , Θ _{Mt2-3 at} the times t2, t3, t4 are _respectively
Θ _Mt2-1 = L _t2 · [1-exp {−γ · (t2−t1) / 60}]
Θ _Mt2-2 = Θ _Mt2-1 · exp {−γ · (t3−t2) / 60}
Θ _Mt2-3 = Θ _Mt2-1 · exp {−γ · (t4−t2) / 60}
It becomes. FIG. _8C shows the temperature change of the motor main body temperature _ΘMt3 , and _ΘMt3-1 , _ΘMt3-2 , and _ΘMt3-3 can be obtained in the same manner as described above.

In FIG. _8D , the motor body temperature is calculated by adding the values of the motor body temperatures Θ _Mt1 , Θ _Mt2 . For example, assuming that the motor body temperature Θ _M illustrated by curves 301, 302, 303... In FIG. 8D is calculated based on the elapsed time between times t1, t2, t3,. The temperature Θ changes as illustrated by the curve 304 in FIG.
In order to calculate the front bearing heating value Q _F using the motor body temperature Θ, the following equation:

[Equation 7]
Q _F = K ₄ (Θ−Θ _S ) (5)
It becomes. Here, K ₄ : coefficient, Θ _S : ball screw shaft end temperature.
(Calculation of temperature distribution)
When the heat generation amount of each heat source unit is obtained as described above, the temperature distribution is calculated from this heat amount. The temperature distribution is the following unsteady heat conduction equation:

[Equation 8]
[C] d {θ} / dt + [H] {θ} + {Q} = 0 (6)
Is solved under the initial conditions {θ} _{t = 0} , d {θ} / dt _{t = 0} . Here, [C]: heat capacity matrix, [H]: heat conduction matrix, {θ}: temperature distribution, {Q}: calorific value, t: time.

(Calculation of thermal displacement)
When the temperature distribution of each heat source part of the ball screw shaft is obtained, the amount of thermal displacement is calculated therefrom. The amount of thermal displacement is

[Equation 9]
_{^{ΔL = ∫ L 0 β × θ}} (L) dL ... (7)
It is requested from. Here, ΔL: thermal displacement amount, β: linear expansion coefficient of the ball screw shaft material.

  Next, the configuration of numerical control of the machine tool will be described with reference to the functional block diagram shown in FIG. For convenience, the description will be given by taking an X-axis ball screw mechanism as an example, but basically the same processing is performed for the Y-axis and the Z-axis.

  The interpolation control circuit 16 is a circuit for calculating the feed amount of the ball screw mechanism based on the machining data stored in the ROM 52. The signal distribution means 17 distributes the feed amount signal corresponding to the feed amount of the ball screw mechanism to each axis and gives the signal to the shaft speed circuit 61a. This feed amount signal is also given to the position register 18 stored in the RAM 53, and the position data of the nut portion 8a is stored. The rotational speed of the X-axis motor 71 is always detected by the encoder 71a, and the detection signal is input to the axis control circuit 61a and the temperature distribution calculation circuit 19. The broken line indicates the control device 50.

The parameter memory 20 stored in the RAM 53 includes parameters relating to the mechanical structure such as the length and diameter of the ball screw shaft 81, parameters relating to physical properties such as density, specific heat, γ used in the equations (3) and (4), and The heat distribution coefficient (ratio) η _N , η _{B and the} like are input. The temperature distribution calculation circuit 19 calculates the heat generation amount of the nut portion moving section of the ball screw shaft 81 every 50 ms from the rotation speed detection signal of the X-axis motor 71 based on the equation (1), and after 6400 ms, each heat source unit is calculated from the total heat generation amount. Calculate the calorific value distribution.

  Further, with respect to the heat generation amount of the front bearing portion, the temperature distribution calculation circuit 19 calculates the saturation temperature of the X-axis motor 71 from the current from the current detector 61b and the rotation speed of the X-axis motor 71 based on the equation (2). The temperature of the X-axis motor 71 is calculated based on (3) and equation (4). Based on this temperature and the ball screw shaft end temperature Θ obtained from the equation (6), the front bearing portion calorific value is calculated based on the equation (5).

The temperature distribution calculation circuit 19 calculates the existence probability of the nut, calculates the heat generation amount of each section of the ball screw shaft 81, and then calculates Equation (6) from these heat generation amounts and various data stored in the parameter memory 20. Solve and calculate the temperature distribution of each heat source. Specifically, after the machine tool is driven (t = 0), the temperature distribution when the time elapses as t1, t2,... (Minutes) is calculated as follows.
From FIG. 5, the temperature of each part and the amount of heat input to each section can be expressed as shown in FIG. Using this FIG. 10, the equation (6) can be expressed by the following equation:

It can be expressed as

Since the temperature {θ} of each part of the ball screw at the time t = 0 and the motor body temperature Θ are known, Q _F can be obtained from Equation (5). Further, Q _{N1 to} Q _Nn and Q _B are also known from the equation (1). By substituting these values into the right side of the equation (8), the speed at which the temperature rises at each position (d {θ} _{t = 0} / dt), that is, the slope can be obtained as shown in FIG. From this inclination, the temperature {θ} of each part at t = 1 can be obtained by the following equation.
{Θ} _{t = t1} = {θ} _{t = t0} + (d {θ} _{t = 0} / dt) × t1

{Θ} Q _{F at} t = 1 is obtained from Equation (5) from the ball screw shaft end temperature Θ _S at _{t = t1 and} the motor body temperature Θ obtained from Equations (3) and (4). Substituting these values into equation (8) to obtain d {θ} _{t = 1} / dt, the temperature of each part at t = 2 is {θ} _{t = t2} = {θ} _{t = t1} + (d {θ} _{t = 1} / dt) × (t2−t1)
It is obtained by In this way, the temperature at t = t3,... Can be obtained in the same manner.

  The correction data calculation circuit 21 calculates a correction amount from the temperature distribution calculated by the temperature distribution calculation circuit 19 based on the equation (7). The correction signal generation means 22 gives a correction signal corresponding to the correction amount calculated by the correction data calculation circuit 21 to the axis control circuit 61a. The calculation by each circuit is processed by the CPU 51.

Next, a specific procedure of numerical control of the machine tool will be described with reference to a flowchart shown in FIG.
A matrix necessary for calculation by the finite element method is set from setting data such as parameters, and the target model is divided into a finite number of sections as shown in FIG. The area of the heat distribution model is formed by dividing the section. (S1)
Corresponding to each section i, the RAM 53 is provided with a memory area in which the current outside air temperature θ _air , initial position, current position, displacement, linear expansion coefficient, heat capacity, heat transfer coefficient, and the like are stored.

Next, an initial temperature {θ} _{t = 0} is set in each section i of the heat distribution region model set in the step S1. This initial temperature {θ} _{t = 0} can be set individually for each section. However, when the machining center M can be treated as being coincident with the outside air temperature θ _air , the initial temperature for all sections is set. {Θ} _{t = 0} can be set to the outside air temperature θ _air . In addition, when a temperature difference is generated between the sections due to driving of the machining center M or the like, an initial temperature can be set for each section. This initial temperature {θ} _{t = 0} is stored in the memory. (S2) Also, a reference value such as an initial position other than the initial temperature {θ} _{t = 0} is measured and stored.

Every 50 ms, the nut position current position and feed rate data are input to the temperature distribution calculation circuit 19, and the amount of heat generated for each section of the nut portion 8a is obtained based on the equation (1). (S3)
When a certain time (6400 ms) has elapsed, the temperature distribution calculation circuit 19 calculates the existence probability of the nut portion from the total heat quantity Q _TTL and distributes it to the divided sections using the calorific value obtained in S3. (S4)

The saturation temperature is obtained based on the equation (2) using the current flowing through the servo motor and the motor speed, and the temperature rise of the servo motor body is obtained from the saturation temperature and the equations (3) and (4). (S5)
Based on the temperature rise of the servo motor body and the temperature at the end of the ball screw shaft, the heat input to the divided section adjacent to the servo motor, that is, the so-called front bearing heat generation amount is calculated based on the equation (5). (S6)

The temperature distribution of each section is obtained using the calorific value of each section obtained in S4 and S6 and the unsteady equation (6). (S7)
The amount of thermal displacement in each section is calculated using the equation (7) based on the temperature distribution obtained in S7 (S8), and the amount of thermal displacement from the reference position stored in S2, that is, the correction amount used for machining control is calculated. . (S9)
If the correction mode is ON, a feed amount signal corresponding to the correction amount obtained in S9 is sent from the correction signal generating means 22 to the axis control circuit 61a. (S10)
When the process up to S10 is completed, the process returns to S1 and continues the calculation periodically.

Next, another embodiment will be described with reference to FIG.
The difference from the first embodiment described above is that the temperature rise of the servo motor is not obtained from the current and the rotational speed but the temperature rise of the servo motor is obtained by the temperature sensor attached to the servo motor and the room temperature sensor for measuring the room temperature. It is a point.

A temperature distribution calculation circuit 19 is composed of the temperature Θ _M0 of the servo motor main body detected by the temperature sensor 101 separately installed in the X-axis motor 71 and the Θ atm detected by the room temperature sensor 24 for detecting the outside air temperature installed in the machining center M. Sent to. In the temperature distribution calculation circuit 19, the temperature rise Θ of the servo motor body is expressed by the following equation:

[Equation 11]
Θ = Θ _M0 −Θatm
Ask for. Since the heat generation amount of the front bearing part is due to heat input due to the temperature rise of the servo motor,

[Equation 12]
Q _F = K ₅ (Θ−Θ _S )
It is also possible to ask for. By using this calculation, the accuracy of detecting the amount of heat input to the end of the ball screw shaft determined by the thermal displacement correction algorithm is improved, and the accuracy of thermal displacement correction can be further improved. K ₅ : coefficient, Θ _S : ball screw shaft end temperature rise from time t = 0.

Next, a modified example in which the above embodiment is partially modified will be described.
1] In the first embodiment, the example in which the temperature rise Θ of the servo motor body is obtained using the equations (3) and (4) has been described, but from the first-order lag system in which the temperature rise of the motor body is discretized, If K ₆ , K ₇ and K ₈ are coefficients,

[Equation 13]
L _Tn = K ₆ · ω + K ₇ · i ²
Θ _n = (1−K ₈ ) Θ _n−1 + L _Tn
It is also possible to ask for.

1 is an overall perspective view of a machine tool according to an embodiment of the present invention. It is a side view of this machine tool. It is a block diagram of an X-axis ball screw mechanism. It is a block diagram of the control system of a machine tool. It is explanatory drawing in the case of dividing | segmenting a ball screw shaft and calculating | requiring a calorie | heat amount. It is memory explanatory drawing in the case of calculating | requiring the amount of heat distribution of the area i. It is explanatory drawing which illustrates the time-dependent change of the thermal displacement amount corresponding to the maximum displacement. The relationship between the motor body temperature and the elapsed time is shown, (A) is a relationship diagram between the motor body temperature and the elapsed time from 0 to t1 after the start of driving, and (B) is the motor body temperature from t1 to t2 after the start of driving. (C) is a relationship diagram between the motor body temperature and the elapsed time from t2 to t3 after the start of driving, and (D) is a motor body temperature and elapsed time from 0 to t3 after the start of driving. FIG. 3 is a functional block diagram according to Embodiment 1. FIG. It is a figure explaining the temperature of each part, and the calorie | heat amount input into each area. It is a figure which shows the temperature rise rate in each position. It is a flowchart of position correction control of this machine tool. 10 is a functional block diagram according to Embodiment 2. FIG. It is the schematic of the experimental apparatus for comparing the calculation result of a prior art, and actual elongation amount. It is a graph which shows the calculation result and experiment result of a prior art.

Explanation of symbols

M Machining center 8a Nut 16 Interpolation control circuit 19 Temperature distribution calculation circuit 20 Parameter memory
21 correction data calculation circuit 22 correction signal generating means 50 control device 51 CPU
61a Axis control circuit 61b Current detector 71 X-axis motor 71a Encoder 81 Ball screw shaft 101 X-axis temperature sensor

Claims (5)

A ball screw mechanism for feed driving, a feed amount control means for calculating a nut feed amount by the ball screw shaft of the ball screw mechanism based on machining data, a servo motor for rotationally driving the ball screw shaft, and a servo motor of the servo motor A thermal displacement correction method for a machine tool having speed control means for controlling the rotational speed based on machining data,
A first step of obtaining a first heat generation amount generated from the nut to the ball screw shaft based on the rotation speed of the servo motor;
A second step of detecting a rising temperature of the servo motor and determining a second heating value generated from the servo motor to the ball screw shaft based on the rising temperature;
A third step of calculating a temperature distribution of a plurality of sections obtained by dividing the ball screw shaft in the length direction from the first calorific value and the second calorific value;
A fourth step of calculating a thermal displacement amount of each section of the ball screw shaft from the temperature distribution;
A fifth step of calculating a correction amount of the machining data based on the thermal displacement amount;
A thermal displacement correction method for machine tools, comprising:   The method according to claim 1, wherein the temperature rise of the servo motor is detected based on a rotation speed of the servo motor and a drive current value. A ball screw mechanism for feed driving, a feed amount control means for calculating a nut feed amount by the ball screw shaft of this ball screw mechanism based on machining data, a servo motor for rotationally driving the ball screw shaft, and rotation of this servo motor A thermal displacement correction device for a machine tool having speed control means for controlling the speed based on machining data,
Speed detecting means for detecting the rotational speed of the servo motor;
Temperature detecting means for detecting the rising temperature of the servo motor;
The ball screw shaft is moved in the length direction from a first heat value generated from the nut to the ball screw shaft based on the rotation speed of the servo motor and a second heat value generated from the nut to the ball screw shaft based on the rising temperature of the servo motor. Temperature distribution calculating means for calculating the temperature distribution of the plurality of divided sections;
And a correction amount calculation means for calculating a thermal displacement amount of each section of the ball screw shaft based on the temperature distribution and calculating a correction amount of the machining data based on the thermal displacement amount. Machine thermal displacement compensation device.   4. The thermal displacement correction device for a machine tool according to claim 3, wherein the temperature detecting means detects the rising temperature based on a rotation speed of a servo motor and a drive current value. A ball screw mechanism for feed driving, a feed amount control means for calculating a nut feed amount by the ball screw shaft of this ball screw mechanism based on machining data, a servo motor for rotationally driving the ball screw shaft, and rotation of this servo motor A machine tool computer having speed control means for controlling the speed based on the machining data;
A temperature distribution of a plurality of sections obtained by dividing the ball screw shaft in the length direction from a first calorific value obtained based on the rotation speed of the servo motor and a second calorific value obtained based on the rising temperature of the servo motor. Temperature distribution calculating means for calculating;
Correction amount calculation means for calculating a thermal displacement amount of each section of the ball screw shaft using the temperature distribution obtained by the temperature distribution calculation means, and calculating a correction amount of the machining data using the thermal displacement amount. A program for correcting a thermal displacement of a machine tool, characterized by functioning as