CN101530974A - Thermal displacement correcting method of a machine tool and a termal displace ment correcting device - Google Patents

Abstract

A thermal displacement correction method and thermal displacement correction device of a machine tool are used for correcting errors caused by the thermal displacement of a ball screw mechanism generated during the operation of the machine tool. The first calorific value computing unit computes a first calorific value generated by the nut on the screw shaft based on the rotational speed detected by the speed detector. The second calorific value calculating unit calculates a second calorific value generated on the screw shaft by the servo motor based on the temperature rise detected by the temperature detecting unit. The temperature distribution calculation unit calculates the temperature distribution in a plurality of sections formed by dividing the screw shaft in the longitudinal direction based on the first calorific value and the second calorific value. The thermal displacement calculation unit calculates the thermal displacement in each section based on the temperature distribution calculated by the temperature distribution calculation unit. The correction amount calculation unit calculates the correction amount of the machining data for calculating the feeding amount of the nut based on the thermal displacement amount calculated by the thermal displacement amount calculation unit.

Description

Thermal displacement correction method and thermal displacement correction device of machine tool

technical field

The invention relates to a thermal displacement correction method and a thermal displacement correction device of a machine tool. More specifically, the present invention relates to a method and an apparatus for correcting errors caused by thermal displacement of a ball screw mechanism during operation of a machine tool.

Background technique

Ball screw mechanisms are becoming popular as positioning mechanisms for machine tools. The temperature of the ball screw mechanism rises due to the frictional resistance between the screw shaft and the nut, the frictional resistance between the screw shaft and each part of the bearing, and the heat generated by the servo motor. The ball screw mechanism generates thermal displacement (elongation) due to the above-mentioned temperature rise. The current control mode of CNC machine tools is generally semi-closed loop. In a semi-closed-loop CNC machine tool, the thermal displacement of the screw shaft is directly expressed as a positioning error. As a countermeasure against the above situation, there is a pretension method. The pretension method absorbs thermal expansion by applying pretension to the screw shaft. Recently, CNC machine tools use thick screw shafts, and the feed speed has become very fast. Therefore, the calorific value increases, and when the pretension method is used, a very large tension has to be applied. As a result, there are problems that the structural body of the ball screw mechanism is deformed, and unreasonable force is applied to the thrust bearing to cause sintering.

In the thermal displacement correction method of the screw shaft proposed in Japanese Patent Laid-Open No. 256336 of 1988, unreasonable pretension is not applied to the screw shaft, and a special measuring device is not required. In this method, the thermal displacement is corrected in-process. Specifically, in the first step, the heat generation value of the screw shaft is obtained using the product of the armature current and the voltage of the servo motor. In the second step, the temperature distribution is obtained using the calorific value using a model in which the screw shaft is divided into sections. In the third step, the amount of thermal displacement of the screw shaft is estimated moment by moment based on the temperature distribution. In the fourth step, the amount of thermal displacement is given to the numerical control device as pitch error correction.

Japanese Patent Application Laid-Open No. 240045 in 1992 focuses on the problem that the heat generated by the method of the above-mentioned publication (Japanese Patent Laid-Open No. 256336 in 1988) includes acceleration and deceleration energy of the servo motor itself. In the thermal displacement amount correction method disclosed in this publication (Japanese Patent Laid-Open No. 240045, 1992), the heat generation amount in each section of the screw shaft is calculated using the rotational speed of the servo motor. With this method, the correction amount calculated from the rotational speed of the servo motor that does not affect the acceleration/deceleration energy can be approximated to the actual elongation of the screw shaft.

In the method of Japanese Patent Application Laid-Open No. 240045 in 1992, only the rotational speed is used to calculate the calorific value of the servo motor. The above method does not consider the fact that the amount of heat generated differs depending on the load on the servo motor. The above methods also do not disclose the conditions for calculating the correction amount at the initial stage of operation of the servo motor and after a certain period of time has elapsed. Therefore, the above method may not approximate the actual elongation (thermal displacement) of the screw shaft with the correction amount calculated in the initial stage of motor operation, that is, in the transient state.

The inventors of the present invention actually operated the servo motor and measured the temperature of the end of the screw shaft. The inventors of the present invention created the heat distribution model disclosed in Japanese Patent Application Laid-Open No. 240045 in 1992, and conducted a comparison experiment between the actual temperature at the end of the screw shaft and the temperature calculated using the heat distribution model. As shown in FIG. 12 , the mechanism for measuring the actual temperature at the end of the screw shaft includes: a servo motor 201 , a screw shaft 203 , a nut 204 , and a table 205 . The servo motor 201 is connected to the screw shaft 203 through a coupling 202 . The nut 204 is screwed together with the screw shaft 203 . The nut 204 is movable in the front-rear direction (left-right direction in FIG. 12 ) according to the rotation of the screw shaft 203 . The workbench 205 is fixed on the nut 204 . The table 205 is movable in the front-rear direction integrally with the nut 204 . The screw shaft 203 is rotatably supported by a fixed bearing 206 and a movable bearing 207 provided on the support base.

The conditions of the experiments performed are as follows.

(Condition 1)

The average current and the average rotational speed flowing through the servo motor 201 are constant during the movement of the table 205 . The temperature measurement point 208 (measurement position) is set as the fixed position 209 which is the end of the screw shaft. The table 205 repeatedly reciprocates at a constant speed until the measured temperature value at the fixed position 209 becomes stable.

(Condition 2)

The table 205 moves to a position sufficiently separated from the fixed bearing 206 so that the heat generated by the nut 204 does not affect the temperature measurement portion 208 . In other words, only the servo motor 201 and the fixed bearing 206 affect the temperature of the temperature measurement portion 208 .

Next, the inventors of the present invention produced a heat distribution model. Since the average current and average rotation speed flowing through the servo motor 201 are constant, the input heat transferred from the servo motor 201 to the end of the screw shaft is constant. The unsteady heat conduction equation calculates the temperature at each moment.

FIG. 13 shows experimental values of thermal displacement at the fixed position 209 (see FIG. 12 ) and calculated values calculated from the thermal distribution model. The vertical axis of the graph represents the temperature at the end of the screw shaft (fixed position 209 ), and the horizontal axis of the graph represents the elapsed time. The solid line is the experimental value, and the dashed line is the calculated value. From this result, the following conclusions can be drawn. After the temperature rise at the end of the screw shaft stabilizes, the experimental value is close to the calculated value. During the transitional state until the temperature rise stabilizes, the temperature rise of the calculated value is faster than the temperature rise of the experimental value. Therefore, the above method cannot make accurate predictions.

Contents of the invention

The object of the present invention is to provide a thermal displacement correction method and a thermal displacement correction device for a machine tool which can make the correction amount approximate to the actual elongation of the screw shaft in the transitional state after the machine tool is operated until the temperature rise is stabilized.

The method for correcting thermal displacement of a machine tool according to claim 1 includes a ball screw mechanism for driving a feed having a screw shaft and a nut, and calculates the feeding amount of the screw shaft to the nut based on machining data. The thermal displacement correction method of a machine tool of a feed amount control device, a servo motor that drives the rotation of the screw shaft, and a speed control device that controls the rotation speed of the servo motor based on the machining data includes: The first step of solving the first calorific value generated on the screw shaft due to the movement of the nut; detecting the temperature rise of the servo motor and solving the temperature rise from the The second step of the second calorific value transmitted by the servo motor to the screw shaft; calculating a plurality of calorific values formed by dividing the screw shaft along the lengthwise direction according to the first calorific value and the second calorific value A third step of temperature distribution of the intervals; a fourth step of calculating the thermal displacements of the plurality of intervals based on the temperature distribution; and a fifth step of calculating the correction amount of the processing data based on the thermal displacements step.

When adopting the thermal displacement correction method of the machine tool of claim 1, not only the first calorific value generated on the ball screw shaft due to the movement of the nut, but also the second calorific value corresponding to the temperature rise of the servo motor is used to calculate the ball The temperature distribution of multiple sections of the screw shaft, from which the correction amount is obtained. Therefore, even in the transient state until the temperature of the servo motor is stabilized, the correction amount can be approximated to the actual elongation amount of the ball screw shaft. The input heat transferred from the servo motor to the ball screw shaft is affected by the temperature rise of the servo motor, and the temperature rise of the servo motor itself is caused by the influence of operating factors such as load. In particular, the temperature of the servo motor changes moment by moment until the heat generation and heat dissipation of the servo motor are balanced. The thermal displacement correction method of the machine tool can obtain the correction amount following the temperature change of the motor by using the second calorific value corresponding to the temperature rise of the servo motor in the calculation for obtaining the correction value. In addition to the input heat transferred from the servo motor to the ball screw shaft, the heat displacement amount is calculated for each of the multiple sections using the first heat generated on the ball screw shaft due to the movement of the nut . Therefore, the thermal displacement correction method of the machine tool can obtain a high-precision correction amount without installing additional sensors.

In the thermal displacement correction method of a machine tool according to claim 2, the temperature increase of the servo motor is detected based on at least one of the rotation speed of the servo motor and the drive current value. When the thermal displacement correction method of the machine tool according to the technical solution 2 is adopted, it is not necessary to use a sensor or the like separately, and an existing sensor can be used to obtain a highly accurate correction amount.

The thermal displacement correction device for a machine tool according to claim 3 is a device that includes a ball screw mechanism for feeding drive having a screw shaft and a nut, and calculates the feed amount of the screw shaft to the nut based on machining data. The feed amount control device, the servo motor for driving the screw shaft to rotate, and the speed control device for controlling the rotation speed of the servo motor according to the machining data are included in the thermal displacement correction device of the machine tool, including: a speed detecting device for detecting a speed; a temperature detecting part for detecting a temperature rise of the servo motor; a first calorific value calculation unit for the first calorific value generated on the screw shaft; and calculates a second calorific value transmitted from the servo motor to the screw shaft based on the temperature rise detected by the temperature detection unit the second calorific value calculation unit; calculate the a temperature distribution calculation unit for temperature distribution of a plurality of sections formed by dividing the screw shaft in the longitudinal direction; a thermal displacement calculation unit for calculating a thermal displacement amount of each of the plurality of sections based on the temperature distribution; and A correction amount calculation unit that calculates a correction amount of machining data from the thermal displacement calculated by the thermal displacement calculation unit.

When the thermal displacement correction device for a machine tool according to claim 3 is adopted, a temperature detection unit that detects a temperature rise of the servo motor is provided, and the temperature distribution in a plurality of sections of the ball screw shaft is calculated using the second calorific value corresponding to the temperature rise. , thus obtaining the correction amount. Therefore, even in the transient state until the temperature rise of the servo motor stabilizes, the correction amount can be approximated to the actual elongation amount of the ball screw shaft. In addition to the input heat transferred from the servo motor to the ball screw shaft, the amount of thermal displacement can be calculated for each of the multiple sections using the heat generated on the ball screw shaft due to the movement of the nut. Therefore, a highly accurate correction amount can be obtained without providing a separate sensor.

In the thermal displacement correction device for a machine tool according to claim 4 , the temperature detection unit detects the temperature increase based on at least one of a rotation speed of the servo motor and a drive current value. When the thermal displacement correction device of the machine tool according to claim 4 is adopted, it is not necessary to use additional sensors and the like, and a high-precision correction amount can be obtained using existing sensors.

Description of drawings

FIG. 1 is an overall perspective view of a machining center M according to an embodiment of the present invention.

FIG. 2 is a front view centering on the spindle head 5 and the tool changer 7 of the machining center M. As shown in FIG.

Fig. 3 is a structural diagram of a ball screw mechanism.

FIG. 4 is a block diagram of a control system of the machining center M. As shown in FIG.

Fig. 5 is an explanatory diagram for calculating the calorific value by dividing the screw shaft.

FIG. 6 is an explanatory diagram of the data area of the temperature distribution calculation circuit 19 .

7 is an explanatory diagram showing the relationship between the temperature of the motor body and the elapsed time when the rotation speed and current of the motor are kept constant.

8 is an explanatory diagram for explaining the calculation method of the temperature of the motor body. FIG. 8(A) is a diagram showing the relationship between the temperature of the motor body and the elapsed time from 0 to t1 after the start of driving. FIG. The relationship between the temperature of the motor body and the elapsed time from t1 to t2 after the start, Fig. 8 (C) is the relationship between the temperature of the motor body and the elapsed time from t2 to t3 after the start of driving, Fig. D) is a graph showing the relationship between the temperature of the motor body and the elapsed time from 0 to t3 after the start of driving.

FIG. 9 is a diagram for explaining the temperature of each part and the calorific value input into each section.

Fig. 10 is a graph showing the temperature rise rate at each position.

FIG. 11 is a flowchart of position correction control of the machining center M. As shown in FIG.

Fig. 12 is a schematic diagram of an experimental device for measuring the temperature of an end portion of a screw shaft.

FIG. 13 is a graph showing calculated values obtained by the method of the prior art and experimental values obtained by the experimental device of FIG. 12 .

Detailed ways

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

Example 1

The configuration of the machining center M (machine tool) will be described with reference to FIGS. 1 to 4 . The machining center M shown in FIG. 1 is a machine tool capable of performing desired machining (such as "milling", "drilling", and "cutting") on a workpiece by relatively moving a workpiece and a tool. The machining center M mainly includes: a base 1 made of cast iron, a machine tool body 2 arranged on the upper part of the base 1 , and a protective baffle (not shown) fixed on the upper part of the base 1 . The machine tool body 2 performs cutting processing of workpieces. The protective shutter is a box-shaped cover that covers the machine tool body 2 and the upper part of the base 1 .

The base 1 is a substantially cuboid cast product long in the Y-axis direction. Height-adjustable feet are respectively arranged in the four corners of the lower part of the base 1 .

Next, the machine tool body 2 will be described. As shown in FIG. 1 , the machine tool body 2 mainly includes: a column 4 , a spindle head 5 , a spindle (not shown), a tool changing device 7 , and a workbench 8 . The column 4 is fixed on the upper surface of the column seat part 3 and extends vertically upward. The column seat part 3 is provided at the rear part of the base 1 . The spindle head 5 can be raised and lowered along the front surface of the column 4 . The spindle head 5 rotatably supports the spindle therein. The tool changer 7 is arranged on the right side of the spindle head 5 . The tool changing device 7 replaces the tool holder mounted on the front end of the spindle with another tool holder. The tool holder is fitted with a tool 6 . The workbench 8 is arranged on the top of the base 1 . The table 8 fixes the workpiece in a detachable manner. A box-shaped control box 9 is provided on the rear side of the column 4 . The control box 9 has a numerical control device 50 for controlling the operation of the machining center M inside the control box 9 .

Next, the movement mechanism of the table 8 will be described. Servomotor is X-axis motor 71 (referring to Fig. 4) and Y-axis motor 72 (referring to Fig. 4) to make table 8 along X-axis direction (the left and right direction of machine tool body 2 of Fig. 1) and Y-axis direction (machine tool body 2 depth direction) to move. The moving mechanism of the table 8 has the following structure. A rectangular parallelepiped support table 10 is provided below the table 8 . The support table 10 has a pair of X-axis feed rails extending in the X-axis direction on its upper surface. A table 8 is movably supported on a pair of X-axis feeding guide rails. As shown in FIG. 3 , a nut portion 8 a is arranged on the lower surface of the table 8 . The nut portion 8 a is screwed to an X-axis screw shaft 81 extending from the X-axis motor 71 , thereby constituting a ball screw mechanism. The fixed bearing 91 a fixed to the support base 10 supports the end portion 81 a of the X-axis screw shaft 81 on the X-axis motor 71 side. The movable bearing 91b supports the end portion 81b of the X-axis screw shaft 81 on the opposite side to the X-axis motor 71 .

A support table 10 is movably supported on a pair of Y-axis feeding guide rails extending in the longitudinal direction on the upper portion of the base 1 . The X-axis motor 71 arranged on the support table 10 is sent along the X-axis to the guide rail to drive the table 8 to move along the X-axis direction. The Y-axis motor 72 arranged on the base 1 drives the table 8 to move along the Y-axis along the Y-axis feed guide rail. The moving mechanism of the Y-axis is also a ball screw mechanism (see FIG. 3 ), as in the moving mechanism of the X-axis.

The telescopic retractable telescopic cover 11,12 covers the X-axis feeding guide rail on the left and right sides of the workbench 8. The telescopic cover 13 and the Y-axis rear cover respectively cover the Y-axis feeding guide rails at the front and back of the support table 10 . Even when the workbench 8 moves in any one of the X-axis direction and the Y-axis direction, the telescopic covers 11, 12, 13 and the Y-axis rear cover can always cover the X-axis feeding guide rail and the Y-axis feeding guide rail. Therefore, the telescopic covers 11, 12, 13 and the Y-axis rear cover can prevent chips, coolant, etc. scattered from the machining area from falling onto the guide rails.

Next, the elevating mechanism of the spindle head 5 will be described. A guide rail (not shown) extending vertically on the front side of the column 4 guides the spindle head 5 through a linear guide (not shown) so that it can be freely raised and lowered. A nut (not shown) connects the spindle head 5 to a Z-axis screw shaft (not shown) extending in the vertical direction on the front side of the column 4 . The Z-axis motor 73 (refer to FIG. 4 ) drives the Z-axis screw shaft to rotate in forward and reverse directions, so that the spindle head 5 is driven up and down in the vertical direction. The Z-axis control unit 63 a drives the Z-axis motor 73 based on a control signal from the CPU 51 of the numerical control device 50 . Driven by the Z-axis motor 73, the spindle head 7 is driven up and down.

As shown in FIGS. 1 and 2 , the tool changing device 7 includes a tool magazine 14 and a tool changing arm 15 . The tool magazine 14 houses a plurality of tool holders supporting the tools 6 . The tool changing arm 15 holds the tool holder and other tool holders attached to the main shaft, and performs transport and replacement. The tool magazine 14 includes a plurality of tool seats (tool pots) (not shown) and a transfer mechanism (not shown) inside. The tool holder supports the tool holder. The transport mechanism transports the tool holder within the tool magazine 14 .

FIG. 4 shows the electrical structure of the machining center M. The control device 50 as a control unit includes a microcomputer. The control device 50 includes an input/output interface 54, CPU51, ROM52, RAM53, axis control units 61a-64a, 75a, servo amplifiers 61-64, differentiators 71b-74b, and the like. Servo amplifiers 61 to 64 are connected to X-axis motor 71 , Y-axis motor 72 , Z-axis motor 73 , and spindle motor 74 , respectively. The axis control unit 75 a is connected to a library 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 magazine motor 75 is a motor for rotationally moving the tool magazine 14 . The spindle motor 74 is a motor for rotating the above-mentioned spindle. Hereinafter, the X-axis motor 71 , the Y-axis motor 72 , the Z-axis motor 73 , and the spindle motor 74 are collectively referred to as motors 71 to 74 . The motors 71 to 74 include encoders 71a to 74a, respectively.

The axis control units 61a to 64a receive movement command amounts from the CPU 51, and output current commands (motor torque command values) to the servo amplifiers 61 to 64, respectively. Servo amplifiers 61 to 64 receive current commands and output drive currents to motors 71 to 74, respectively. The shaft control units 61a to 64a receive position feedback signals from the encoders 71a to 74a, respectively, and perform position feedback control. The differentiators 71b to 74b differentiate the position feedback signals output from the encoders 71a to 74a to convert them into speed feedback signals, and output them to the axis control units 61a to 64a as speed feedback signals.

The axis control units 61a to 64a perform speed feedback control based on the speed feedback signals output from the differentiators 71b to 74b, respectively. The current detectors 61b to 64b detect drive currents output from the servo amplifiers 61 to 64 to the motors 71 to 74, respectively. The current detectors 61b to 64b feed back drive currents to the axis control units 61a to 64a, respectively. The shaft control units 61a to 64a perform current (torque) control based on the fed-back drive current.

The axis control unit 75 a receives a movement command from the CPU 51 and drives the library motor 75 .

The RAM 53 stores parameters related to the mechanical structure, parameters related to physical properties, heat distribution coefficients (ratio) η _N , η _B , etc. to be described later. Parameters related to the mechanical structure include, for example, the length and diameter of the screw shaft 81 , a reference position to be described later, and the like. As parameters related to physical properties, there are, for example, density, specific heat, linear expansion coefficient, heat capacity, heat transfer coefficient, γ used in formula (3) and formula (4), and the like. As shown in FIG. 6 , the RAM 53 has a data area for storing heat generation and a data area corresponding to the total heat generation and the total rotation speed of the servo motor corresponding to sections 1 to n of the later-described nut portion movement sections.

Next, a calculation method of the thermal displacement amount used in the numerical control of the machining center M will be described. For convenience, the X-axis ball screw mechanism will be described as an example, but the Y-axis ball screw mechanism and the Z-axis ball screw mechanism are basically the same. In this calculation method, the amount of heat generated in the three areas of the front bearing part of the screw shaft, the movement area of the nut part, and the rear bearing part are calculated. The movement section of the nut portion is divided into a plurality of sections. For the above-mentioned multiple intervals, the calorific value of each interval is solved.

(calculation of total calorific value)

As shown in FIG. 5 , the nut portion moving section (the length is represented by L) is divided into n pieces. In this embodiment, it is judged in which section the nut portion exists every time a certain period of time (for example, 50 ms) elapses, and the calorific value (first calorific value) is obtained from the actual rotation speed (feed speed) of the servo motor. The calculated calorific value is stored in the data area of RAM53. The calorific value can be found using the following equation.

Q＝K ₁ ×F ^T …(1)

Here, Q is the calorific value, F is the feeding speed, and K ₁ and T are coefficients.

In this embodiment, the amount of heat generated by the movement of the nut portion in each section is calculated every time a certain time elapses within a certain period of time. In this embodiment, the calorific value is calculated 128 times at intervals of 50 ms within 6400 ms, and the calculated calorific value is added for each section, and stored in a file corresponding to each section 1 to n. within the data area of RAM53. The data area of the RAM 53 stores the total calorific value Q _TTL and the total rotational speed _NTTL of the calorific values 1 to n of the respective sections 1 to n generated within 6400 ms.

(distribution of total calorific value 1)

The allocation method of the total calorific value Q _TTL shown below is based on the same method as in Japanese Patent Application Laid-Open No. 240045 in 1992. That is, the movement section of the nut portion, the front bearing portion, and the rear bearing portion do not conduct heat with each other, and are regarded as approximately independent from a thermodynamic point of view. The ratio of the calorific value of each heat source part to the total calorific value is substantially constant regardless of the change in the feeding speed.

According to the method described above, the present embodiment uses the following equations to calculate the calorific value Q _N of the movement section of the nut portion and the calorific value Q _B of the rear bearing portion.

Q _N =η _N ×Q _TTL

Q _B ＝η _B × Q _TTL

The ratio η _N is the ratio of the calorific value in the movement section of the nut portion to the total calorific value. The ratio η _B is the ratio of the calorific value of the rear bearing portion to the total calorific value. As shown in the above method, since the ratios η _N and η _B are constant, the ratios η _N and η _B are calculated in advance by measuring Q _N and Q _B on site.

(Distribution of heat generation in each section of the movement section of the nut part)

Next, in this embodiment, the calorific value of each section in the movement section of the nut portion is calculated. The calorific value stored in the RAM 53 is the total value of the calorific value calculated at intervals of 50 ms for 6400 ms. Therefore, after obtaining the average calorific value at intervals of 50 ms for each section, the existence probability X ₁ ...X _i of the nut portion in each section is obtained using the following formula from the average calorific value and the total calorific value Q _TTL ... X _n .

X ₁ = average calorific value of section 1/Q _TTL

          :

X _i = average calorific value of interval i/Q _TTL

          :

X _N = average calorific value of interval N/Q _TTL

In this embodiment, after the existence probabilities X ₁ ... _Xi ... X _n of the nut parts in each interval are obtained, the distribution is obtained by using the following formula according to the existence probability and the calorific value Q _N of the movement interval of the nut part Distributed calorific value Q _N1 ... Q _Ni ... Q _Nn to each section 1 to n.

Q _N1 =X ₁ ×Q _N

    :

_QNi ＝ _Xi × _QN

    :

Q _Nn = X _n × Q _N

(distribution of total calorific value 2)

Next, in this embodiment, the amount of heat generated by the front bearing portion Q _F is calculated. The heat generated by the front bearing part Q _F is generated by the input heat generated by the temperature rise of the servo motor. Therefore, this embodiment calculates the temperature of the servo motor body. In this embodiment, the heat input to the end of the screw shaft, that is, the heat generated by the front bearing portion Q _F (second heat generated) is obtained from the difference between the calculated temperature and the temperature of the end of the screw shaft.

The following describes how to calculate the temperature of the servo motor body. Referring to FIG. 7 , the temperature change of the servo motor when the rotational speed and the drive current of the servo motor are constant will be described. When the machining center M starts to be driven, the temperature _ΘM of the motor body rises along the curve 150 and saturates at a certain temperature. The temperature at this saturation is referred to as saturation temperature L _1a . The saturation temperature L _1a can be represented by the following formula.

L _1a =K ₂ ·ω+K ₃ ·i ² ...(2)

K ₂ and K ₃ are constants inherent to the servo motor, ω is the rotational speed of the motor, and i is the driving current of the servo motor.

The curve 150 representing the increase in the temperature _ΘM of the motor body can be represented by the following equation.

Θ _M ＝ L _1a ·{1—exp(—γ·t)} …(3)

γ is a constant inherent to the servo motor, and t is an elapsed time from the start of driving. When the machining center M is stopped after the motor body temperature _ΘM reaches the saturation temperature _L1a (time t=8 hours in FIG. 7), the motor body temperature _ΘM falls along the curve 151. The curve 151 can be represented by the following formula.

Θ _M ＝L _1a ·exp(—γ·t) …(4)

γ is a constant inherent to the servo motor, and t is the elapsed time from the drive stop.

From the above formula (3), the motor body temperature _ΘM1a after a minute from the start of driving the machining center M can be expressed by the following formula.

Θ _M1a ＝ L _1a ·{1—exp(—γ·a/60)}

From the above formula (4), the motor body temperature ΘM _-1a after a minute from the stop of the driving of the machining center M can be expressed by the following formula.

Θ _M-1a = L _1a ·exp(—γ·a/60)

The temperature change of the servo motor when the rotation speed and drive current of the servo motor are constant has been described above, but when the machining center M is actually driven, the rotation speed and drive current of the servo motor are not always stable. Especially in the transient state at the beginning of operation, the rotation speed and drive current are not stable. Therefore, in this embodiment, every time a predetermined elapsed time (6400 ms specifically), the actual rotation speed and drive current (specifically, the rotation speed and drive current measured at intervals of 50 ms) average value), use equation (2) to solve the saturation temperature of the servo motor. In this embodiment, the temperature change of the servo motor body is obtained using the above-mentioned equations (3) and (4) from the saturation temperature and the elapsed time. In this embodiment, the actual temperature of the motor main body is obtained by adding the obtained temperature changes.

Next, a method of calculating the temperature of the actual servo motor body will be described with reference to FIG. 8 . In the following description, it is assumed that time t1, t2, . . . (minutes) elapses after the drive of the machining center M is started. That is, each interval between times 0, t1, t2, . . . is the elapsed time of each process.

In this embodiment, the motor body temperature _ΘM first increases according to the above-mentioned equation (3) within the above-mentioned elapsed time, and then decreases according to the above-mentioned equation (4). As shown in FIG. 8(A), the motor body temperature θ _Mt1 based on the elapsed time from time 0 to time t1 forms a curve 301 that rises from time 0 to time t1 and falls after time t1. The value Θ _Mt1-1 of the motor body temperature Θ _Mt1 at the time t1 can be calculated as follows according to the formula (3).

Θ _Mt1-1 ＝L _t1 ·{1—exp(—γ·t1/60)}

L _t1 is the saturation temperature calculated from the actual rotation speed and drive current of the servo motor from time 0 to time t1. Since the motor body temperature Θ _Mt1 decreases according to equation (4) after time t1, the value Θ _Mt1-2 of the motor body temperature Θ _Mt1 at time t2 can be calculated as follows.

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 can be calculated as follows, respectively, according to Equation (4).

Θ _Mt1-3 ＝Θ _Mt1-1 ·exp{—γ·(t3—t1)/60}

Θ _Mt1-4 ＝Θ _Mt1-1 ·exp{——γ·(t4—t1)/60}

As shown in FIG. 8(B), the motor body temperature θ _Mt2 based on the elapsed time from time t1 to time t2 forms a curve 302 that rises from time t1 to time t2 and falls after time t2. Since the saturation temperature L _t2 can be calculated from the actual rotation speed and drive current of the servo motor from time t1 to time t2, the motor body temperatures at time t2, t3, and t4 are Θ _Mt2-1 , Θ _Mt2-2 , Θ _{Mt2- 3} can be calculated as follows using Equation (3) and Equation (4), respectively.

Θ _Mt2-1 ＝L _t2 ·[1—exp{—γ·(t2—t1)/60}]

Θ _Mt2-2 ＝Θ _Mt2-1 ·exp{—γ·(t3—t2)/60}

Θ _Mt23 ＝Θ _Mt2-1 ·exp{—γ·(t4—t2)/60}

As shown in FIG. 8(C), the motor body temperature θ _Mt3 based on the elapsed time from time t2 to time t3 forms a curve 303 that rises from time t2 to time t3 and falls after time t3. The motor body temperatures Θ _Mt3-1 , Θ _Mt3-2 , and Θ _Mt3-3 at times t3, t4 _, and _t5 can be obtained in the same manner as Θ Mt1 and Θ Mt2 described above.

The actual motor body temperature Θ is calculated by adding the motor body temperatures Θ _Mt1 , Θ _Mt2 , Θ _Mt3 . . . For example, it is assumed that the motor body temperatures Θ _Mt1 , Θ _Mt2 , Θ _Mt3 . In this case, the value _X1 of the motor body temperature Θ at time t1 is ΘMt1-1. The value X ₂ of the motor body temperature Θ at time t2 is Θ _Mt1-2 + Θ _Mt2-1 . The value X3 of the motor body temperature Θ at time t3 is Θ _Mt1-3 + Θ _Mt2-2 + Θ _Mt3-1 . When the value of the motor body temperature Θ is obtained at each time point in the same manner, the motor body temperature Θ changes as illustrated by the curve 304 (see FIG. 8(D)).

In the present embodiment, the heat generation amount Q _F of the front bearing portion is calculated using the motor body temperature Θ obtained by the above-mentioned method according to the following equation (5).

Q _F ＝K ₄ (Θ—Θ _S ) …(5)

_K4 is a coefficient, and _ΘS is the temperature of the end portion of the screw shaft on the servo motor side (the end portion 81a in the example of FIG. 3 ).

(calculation of temperature distribution)

After calculating the calorific value of each heat source portion as described above, the CPU 51 calculates the temperature distribution based on the calorific value. The temperature distribution is solved by solving the following unsteady heat conduction equation under the initial conditions of {θ} _t=0 , d{θ}/dtt ₌₀ .

[C]d{θ}/dt+[H]{θ}+{Q}＝0 …(6)

[C] is the heat capacity matrix, [H] is the heat conduction matrix, {θ} is the temperature distribution, {Q} is the calorific value, and t is the time.

Specifically, the temperature distribution at times t1, t2, ... (minutes) after the machining center M is driven (t=0) is calculated as follows.

When the movement section of the nut portion is divided as shown in FIG. 5 , the temperature of each section and the calorific value of each section can be input as shown in FIG. 9 . Using FIG. 9, equation (6) can be represented by the following equation.

The temperature distribution {θ} of each part of the screw shaft and the temperature Θ of the motor body at time t=0 are known. Therefore, in this embodiment, equation (5) is used to obtain the heat generation Q _F of the front bearing portion. The distributed heat generation values Q _N1 to Q _Nn and the heat generation value Q _B of the rear bearing portion in each of the nut portion movement intervals are also known from Equation (1). Substitute the obtained value into the right side of formula (7). As shown in FIG. 10 , in this embodiment, the rate of temperature rise (d{θ} _t=0 /dt), that is, the slope, of each part of the screw shaft can be obtained. According to the following formula, this embodiment calculates the temperature {θ} of each part at t=t1 according to the calculated slope.

{θ} _{t = t1} = {θ} _{t = 0} + (d{θ} _{t = 0} /dt) × t1

In this embodiment, the temperature Θ _S at the end of the screw shaft at {θ} _t=t1 and the temperature Θ of the motor body obtained according to equations (3) and (4) are used to solve Q at t=t1 according to equation (5). _F. Substitute the calculated value into formula (7), and solve d{θ} _t=t1 /dt. As a result, the temperature of each part at t=t2 can be obtained by the following equation.

{θ} _t＝t2 ＝{θ} _t＝t1 +(d{θ} _t＝t1 /dt)×(t2—t1)

The temperature at t=t3, ... can be solved in the same way.

(calculation of thermal displacement)

After solving the temperature distribution of the screw shaft, this embodiment calculates the thermal displacement according to the temperature distribution. The thermal displacement can be solved by the following formula.

ΔL＝ ^∫L ₀ β×θ(L)dL …(8)

ΔL is the thermal displacement, and β is the linear expansion coefficient of the screw shaft material.

Next, specific steps of the numerical control of the machining center M will be described with reference to the flow chart shown in FIG. 11 . The CPU 51 uses setting data such as parameters to set a matrix required for calculation by the finite element method. As shown in FIG. 5 , the CPU 51 divides the movement section of the nut portion of the screw shaft into a limited number of sections. By dividing the section, the CPU 51 forms the area of the heat distribution model ( S1 ).

Next, the CPU 51 sets an initial temperature {θ} _t=0 for each section of the heat distribution model set in the process of step S1. The initial temperature {θ} _t=0 is set individually for each section. However, when the temperature of the machining center M can be regarded as the same as the external air temperature θ _air , the initial temperature {θ} _t=0 is set to the external air temperature θ _air for all sections. Conversely, when a temperature difference occurs between the sections due to the driving of the machining center M or the like, the CPU 51 sets the initial temperature for each section. The CPU 51 stores the set initial temperature {θ} _t=0 and the reference position in the RAM 53 (S2).

The CPU 51 finds out which section the current position of the nut part 8a enters every 50 ms, and for each section, calculates the total amount of calorific value generated by the movement of the nut part 8a according to the data of the feed speed and the formula (1) (S3) . After a certain period of time (6400 ms) has elapsed, the CPU 51 calculates the calorific value in the nut portion movement interval based on the ratio η _N of the total calorific value Q _TTL obtained by adding together the calorific value in each interval and the calorific value in the nut portion movement interval. Q _N . The CPU 51 calculates the probability of existence of the nut portion in each section based on the total calorific value Q _TTL and the average calorific value in each section. The CPU 51 allocates the product value of the nut portion moving section calorific value Q _N and the existence probability as the calorific value generated by nut movement to each section divided in step S1 ( S4 ).

The CPU 51 uses the current flowing through the servo motor and the rotational speed of the motor to obtain the saturation temperature from Equation (2). The CPU 51 calculates the increase in temperature of the servo motor main body based on the saturation temperature and equations (3) and (4) ( S5 ). The CPU 51 calculates the input heat transferred to the section adjacent to the servomotor, that is, the heat generated by the front bearing, using the temperature rise of the servomotor body and the temperature of the end of the screw shaft according to equation (5) ( S6 ).

The CPU 51 obtains the temperature distribution in each section using the calorific value for each section obtained in steps S4 and S6 and the unsteady-state equation (6) ( S7 ). The CPU 51 calculates the amount of thermal displacement in each section using the temperature distribution obtained in step S7 using Equation (8) ( S8 ). CPU51 calculates the thermal displacement amount with respect to the reference position memorize|stored in step S2, that is, the correction amount used for process control (S9). When the correction mode is ON, the CPU 51 transmits a feed amount signal corresponding to the correction amount obtained in step S9 to the axis control unit 61a (S10). Once the CPU 51 completes the processing up to step S10, it returns to step S3, and continues calculation periodically (every 6400 ms).

The CPU 51 that executes step S3 corresponds to a first calorific value calculation unit. The CPU 51 executing step S5 corresponds to a temperature detection unit. The CPU 51 that executes step S6 corresponds to a second calorific value calculation unit. The CPU 51 that executes step S7 corresponds to a temperature distribution calculation unit. The CPU 51 executing step S8 corresponds to a thermal displacement calculation unit. The CPU 51 executing step S9 corresponds to a correction amount calculation unit.

Example 2

Other embodiments will be described below. The difference from Example 1 above is that the temperature rise of the servo motor is calculated using a temperature sensor attached to the servo motor and a room temperature sensor for measuring room temperature instead of using the current and rotation speed.

The temperature Θ _MO of the servo motor body detected by a temperature sensor (not shown) separately provided on the X-axis motor 71 and the room temperature sensor (not shown) provided on the machining center M to detect the temperature of the outside air are detected. Arrived Θatm is sent toward CPU51. The CPU 51 uses the following equation to obtain the temperature rise Θ of the servo motor body.

Θ＝Θ _MO —Θatm

The heat generated by the front bearing part Q _F is generated by the input heat generated by the temperature rise of the servo motor. Therefore, the CPU 51 uses the following equation to obtain the heat generation Q _F of the front bearing portion from the temperature rise Θ of the servo motor body.

Q _F ＝K ₅ (Θ—Θ _S )

_K5 is a coefficient, and _ΘS is the temperature rise at the end of the screw shaft from time t=0. By using the calculation described above, the detection accuracy of the input heat transferred to the end of the screw shaft obtained by the thermal displacement correction algorithm becomes better. Therefore, in Embodiment 2, the accuracy of thermal displacement correction can be further improved. The temperature sensor and the room temperature sensor correspond to a temperature detection unit.

Next, a modified example in which the above-described embodiment is partially modified will be described. In the above-mentioned first embodiment, an example was described in which the temperature rise Θ of the servo motor main body was obtained using equations (3) and (4). The temperature rise Θ of the servo motor body can also be solved using the following equation from a discretized first-order delay system.

L _Tn =K ₆ ·ω+K ₇ ·i ² ...(9)

Θ _n ＝(1—K ₈ )Θ _n-1 +L _Tn

K ₆ , K ₇ , and K ₈ are constants inherent to the servo motor.

When the temperature rise of the servo motor is solved using only the rotational speed, Equation (2) becomes as follows.

L _1a =K ₉ ·ω

Alternatively, when the temperature rise of the servo motor is calculated using only the drive current value, the expression (2) becomes as follows.

L _1a =K ₁₀ ·i ²

_K9 and _K10 are constants inherent to the servo motor.

In addition, when the temperature rise of the servo motor body is solved using only the rotational speed based on the discretized first-order delay system, equation (9) becomes as follows.

L _Tn = K ₁₁ ·ω

Alternatively, when it is solved using only the drive current value, Equation (9) becomes as follows.

L _Tn =K ₁₂ ·i ²

_K11 and _K12 are constants inherent to the servo motor.

Claims (4)

1. the thermal walking modification method of a lathe,

Described lathe comprises:

Have sending to of lead screw shaft and nut drive with ball screw framework,

According to process data calculate described lead screw shaft to the feed control appliance of the feed of described nut,

Drive the rotation of described lead screw shaft servo motor and

Control the speed control apparatus of the rotary speed of described servo motor according to described process data,

The thermal walking modification method of described lathe is characterised in that, comprising:

Find the solution the first step of first caloric value on described lead screw shaft, produce because of moving of described nut according to described rotary speed;

The temperature of described servo motor risen to detect and rise according to described temperature find the solution second step of second caloric value of transmitting from described servo motor towards described lead screw shaft;

Come computing described lead screw shaft to be cut apart along its length the third step of the Temperature Distribution in a plurality of intervals that form according to described first caloric value and described second caloric value;

Come the 4th step of the described a plurality of intervals of computing heat displacement amount separately according to described Temperature Distribution; And

Come the 5th step of the correction of the described process data of computing according to described heat displacement amount.

2. the thermal walking modification method of lathe as claimed in claim 1 is characterized in that, the described temperature that detects described servo motor according at least one side in the driving current value of described rotary speed and described servo motor rises.

3. the thermal walking correcting device of a lathe,

Described lathe comprises:

Have sending to of lead screw shaft and nut drive with ball screw framework,

According to process data calculate described lead screw shaft to the feed control appliance of the feed of described nut,

Drive the rotation of described lead screw shaft servo motor and

Control the speed control apparatus of the rotary speed of described servo motor according to described process data,

The thermal walking correcting device of described lathe is characterised in that, comprising:

Speed detection equipment, this speed detection equipment detects described rotary speed;

Temperature detecting part, this temperature detecting part detects the temperature rising of described servo motor;

The first caloric value operational part, this first caloric value operational part comes move first caloric value that on described lead screw shaft produce of computing because of described nut according to by the detected described rotary speed of described speed detection equipment;

The second caloric value operational part, this second caloric value operational part is according to rising second caloric value of coming computing to transmit towards described lead screw shaft from described servo motor by the detected described temperature of described temperature detecting part;

The Temperature Distribution operational part, this Temperature Distribution operational part is according to described first caloric value that is calculated by the described first caloric value operational part and described second caloric value that is calculated by the described second caloric value operational part, comes computing described lead screw shaft to be cut apart along its length and the Temperature Distribution in a plurality of intervals of forming;

Thermal displacement quantity computing, this thermal displacement quantity computing is come the described a plurality of intervals of computing heat displacement amount separately according to the described Temperature Distribution that is calculated by described Temperature Distribution operational part; And

The correction operational part, this correction operational part is according to the described heat displacement amount that is calculated by described thermal displacement quantity computing, the correction of coming the computing process data.

4. the thermal walking correcting device of lathe as claimed in claim 3 is characterized in that, described temperature detecting part detects described temperature according at least one side in the driving current value of described rotary speed and described servo motor and rises.

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