NUMERICAL CONTROL FOR MACHINE TOOL

Abstract

Numerical control (100, 200) for controlling a machine tool (150) which includes a spindle motor (125) consisting of an induction motor, wherein the numerical control (100, 200) comprises: a storage unit (101) that stores a maximum acceleration at which the spindle motor (125) can operate when a strength of the magnetic flux of the spindle motor (125) reaches its maximum; a unit (102) for determining a strength of magnetic flux, which determines a current strength of the magnetic flux of the spindle motor (125); and a unit (103) for changing an acceleration which changes an acceleration of a position command based on a maximum acceleration of the spindle motor (125) stored in the storage unit (101) according to a strength of magnetic flux determined by the unit (102) for determining a strength of magnetic flux at the start of movement of the spindle motor (125) when the spindle motor (125) is operated by means of position control using a position command, where the unit (103) for changing an acceleration is the acceleration A of the position command according to equation (1) below A = Amax × α × Φ and in particular according to equation (2) below A = Amax × Φ / Φ max based on a maximum acceleration Amax of the spindle motor (125), a strength Φ of the magnetic flux at the beginning of the movement of the spindle motor (125), a maximum strength Φmax of the magnetic flux of the spindle motor (125) and an arbitrary proportional constant α.

Description

Field of invention

[0001] The present invention relates to a numerical control of a machine tool which includes a spindle motor consisting of an induction motor. Related technology

[0002] When a workpiece is machined using a machine tool that has a spindle and a feed axis, the movement of the spindle is relatively often coupled to the movement of the feed axis. In this case, with a typical numerical control system controlling a machine tool, either the feed axis is controlled using a characteristic value or a measured value related to the spindle, or the spindle is controlled using a characteristic value or a measured value related to the feed axis.

[0003] Patent document 1, for example, discloses a method for controlling a feed rate using data that is corrected by changing the speed of a spindle motor to enable precise machining even when the actual speed of the spindle motor used as a spindle motor changes abruptly.

[0004] Patent document 2 discloses a method by which a feed rate of a machining feed device is controlled such that a load current value of a motor during machining is as high as or lower than a maximum load current value corresponding to a desired flatness in order to machine a machining object, such as a semiconductor wafer, to a desired flatness using a machining operation.

[0005] Patent document 3 discloses a method by which an excitation current of a static pressure magnetic composite bearing is detected in order to detect a machining state of a mold.

[0006] Patent document 4 discloses a control system for controlling the synchronized operation of a spindle and feed axis. A spindle axis control section comprises a start-up motion control section for accelerating a spindle axis with maximum capacity from a starting position; a maximum acceleration determination section for determining a maximum acceleration of the spindle axis; a remaining rotation rate determination section for determining a remaining rotation rate of the spindle axis; a current speed determination section for determining a current speed of the spindle axis; a deceleration motion control section for decelerating the spindle axis to reach an intermediate speed after acceleration; a positioning motion control section for decelerating the spindle axis to reach the target position after reaching the intermediate speed; and a flux content prediction section for predicting an actual motor flux content at the intermediate speed.and a delay determination part for determining a delay for a positioning operation based on the maximum acceleration, the motor flux content command value, and the motor flux content.; Patent document 1: Unexamined Japanese patent application, publication no. H05-69275 Patent document 2: Unexamined Japanese patent application, publication no. 2013-56392 Patent document 3: Unexamined Japanese patent application, publication no. 2000-263377 Patent document 4: DE 10 2017 000 473 A1 SUMMARY OF THE INVENTION

[0007] An induction motor is sometimes used as a spindle motor in a machine tool. An induction motor works by inducing an excitation current in a stator coil, generating a rotating magnetic field that induces a current in a rotor. This causes the rotor to rotate with the electromagnetic force, following the rotation of the magnetic field. A disadvantage of the induction motor is the generation of heat when an excitation current flows into the stator coil. To mitigate this, a method can be used to reduce the magnetic flux or excitation current of the rotating magnetic field when the load on the induction motor is low and no high torque is required. This method is commonly used when an induction motor is controlled by speed control based on a speed command.

[0008] However, if, for example, a spindle motor and a feed axis motor are operated synchronously, the induction motor, which is the spindle motor, is often controlled by position control based on a position command. In this case, the control mode of the induction motor is switched from speed control to position control. In this case, if, as described above, the control mode is switched from speed control to position control to accelerate or decelerate the induction motor in a state where torque is reduced due to a weaker magnetic flux, the induction motor, being a spindle motor, may not respond to the position command, as it is not possible to instantly accelerate or decelerate the induction motor with maximum torque (acceleration).This can lead to an increase in deviation from a position command, spindle overshoot, and excessive machining errors. Furthermore, the accuracy of the synchronization between the spindle motor and the feed axis motor may be affected.

[0009] Patent documents 1 and 2 do not aim to limit heat generation or improve the stability of position control based on a position command. Furthermore, in patent document 3, the measured excitation current value is used solely to determine the processing status.

[0010] Therefore, one object of the present invention is to create a numerical control system for a machine tool that can limit heat generation and achieve stable position control.

[0011] (A) A numerical control (e.g., a numerical control 100, 200 described below) for machine tools according to the present invention is a numerical control for controlling a machine tool (e.g., a machine tool 150 described below) which includes a spindle motor (e.g., an induction motor 125 described below) consisting of an induction motor, wherein the numerical control includes a storage unit (e.g., a storage unit 101 described below) which stores a maximum acceleration at which the spindle motor can operate when a magnetic flux strength of the spindle motor reaches its maximum, a magnetic flux determination unit (e.g., a magnetic flux determination unit 102 described below) which determines a current magnetic flux strength of the spindle motor, and a variable acceleration unit (e.g.,a unit 103 described below for changing an acceleration) which changes an acceleration of a position command based on a maximum acceleration of the spindle motor stored in the memory unit according to a strength of magnetic flux determined by the unit for determining a strength of magnetic flux at the start of movement of the spindle motor when the spindle motor is operated by means of position control using a position command.

[0012] (B) The numerical control according to (A) can further include a position command generation unit (e.g. a position command generation unit 104 described below) which generates a position command for a plurality of motors (e.g. an induction motor 125 and a servo motor 145 as described below) for synchronous operation based on the acceleration of the spindle motor position command modified by the unit for changing an acceleration, when the plurality of motors including the spindle motor are operated synchronously by means of position control.

[0013] (C) In the numerical control according to (A) or (B), the unit for changing an acceleration can change the acceleration of the position command such that the acceleration of the position command is weaker the lower the strength of the magnetic flux at the start of movement of the spindle motor.

[0014] (D) In ​​the numerical control according to (C) the unit to change an acceleration changes the acceleration A of the position command according to the following equation (1) based on a maximum acceleration Amax of the spindle motor, a strength Φ of the magnetic flux at the start of movement of the spindle motor and an arbitrary proportional constant α. A=Amax×α×Φ

[0015] (D) In ​​the numerical control according to (C) the unit for changing an acceleration can change the acceleration A of the position command according to the following equation (2) based on a maximum acceleration Amax of the spindle motor, a maximum strength Φmax of the magnetic flux of the spindle motor and a strength Φ of the magnetic flux at the start of movement of the spindle motor. A=Amax×Φ / Φmax

[0016] (F) In the numerical control according to (A) to (E) the unit for determining a strength of magnetic flux can estimate a current strength of magnetic flux of the spindle motor and determines the estimated strength of magnetic flux as the current strength of magnetic flux.

[0017] (G) The numerical control according to (A) to (F) may further include a ratio calculation unit (e.g., a ratio calculation unit 105 described below) which receives an actual magnetic flux strength of the spindle motor determined by the magnetic flux determination unit and calculates a magnetic flux strength ratio such that the actual magnetic flux strength of the spindle motor is a ratio of the maximum magnetic flux strength of the spindle motor, wherein the acceleration change unit modifies the acceleration of the position command based on the maximum acceleration of the spindle motor according to the magnetic flux strength ratio calculated by the ratio calculation unit.

[0018] According to the present invention, it is possible to create a numerical control system for a machine tool that can limit heat generation and achieve stable position control. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic representation of the present invention. Fig. Figure 2 is a scheme representing a configuration of a control system according to a first embodiment of the present invention. Fig. Figure 3 is a scheme representing a function of the control system according to the first embodiment of the present invention. Fig. Figure 4 is an illustrative scheme of a method for estimating the strength of a magnetic flux according to the present invention. Fig. Figure 5 is a scheme that represents an example of a position command for position control according to the present invention. Fig. Figure 6 is a scheme representing a configuration of a control system according to a second embodiment of the present invention. Fig. Figure 7 is a scheme that represents a function of the control system according to the second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION First embodiment

[0019] In the following, a first embodiment of the present invention is described with reference to Fig. Items 1 to 5 are described in detail. Fig. Figure 1 is a schematic representation of a basic principle of the present invention. A numerical control system 50 comprises a numerical controller 51, a spindle control unit 52, a feed axis control unit 53, an induction motor 54 as a spindle motor, and a servo motor 55 as a feed axis drive motor. Furthermore, the numerical controller 51 controls the induction motor 54 by means of the spindle control unit 52 and controls the servo motor 55 by means of the feed axis control unit 53.

[0020] Although an induction motor is often controlled by speed control based on a speed command, when the induction motor 54 (the spindle motor) and the servo motor 55 (the feed axis motor) operate synchronously, the control mode of the induction motor is switched from speed control to position control based on a position command. In this case, if the induction motor 54 is accelerated or decelerated by switching its control mode from speed control to position control in a state where the magnetic flux of the induction motor 54 is weak, the induction motor may not respond to the position command, resulting in spindle overshoot and excessive machining errors. Furthermore, the accuracy of the synchronization between the induction motor 54 and the servo motor 55 may be compromised.The spindle control unit 52 determines the strength of the magnetic flux of the induction motor 54. The spindle control unit 52 transmits this strength of the magnetic flux, or, in another example, the ratio of a current strength of the magnetic flux to the maximum strength of the magnetic flux of the induction motor 54 (hereinafter also referred to as a "ratio of magnetic flux strength"), to the numerical control 51.

[0021] The numerical control unit 51 modifies the acceleration of the position command of the induction motor 54, taking into account the magnetic flux strength or the magnetic flux ratio, and transmits a position command generated based on the modified acceleration to the spindle control unit 52. Furthermore, the numerical control unit 51 also generates a position command for the servo motor 55 based on the acceleration of the position command of the induction motor 54 and transmits the position command to the feed axis control unit 53. The spindle control unit 52 and the feed axis control unit 53 control the induction motor 54 and the servo motor 55 according to the position commands received from the numerical control unit 51.

[0022] Although the magnetic flux strength or the ratio of the magnetic flux strength, as described above, is transmitted from the spindle control unit 52 to the numerical controller 51, the spindle control unit 52 and the numerical controller 51 actually perform bidirectional communication in each predetermined cycle. This allows the magnetic flux strength or the ratio of the magnetic flux strength to be supplied from the spindle control unit 52 to the numerical controller 51 during this communication.

[0023] Fig. Figure 2 presents an example of the configuration of a numerical control system 10 according to the first embodiment. The numerical control system 10 includes a numerical controller 100 and a machine tool 150. The machine tool 150 comprises a spindle 115 to which a workpiece 110 is attached, a speed-changing gearbox 120, an induction motor 125 as the spindle motor, which rotates the spindle 115 with the aid of the speed-changing gearbox 120, an amplifier 130 that outputs a drive current for the induction motor 125, a cutting tool 135 that cuts the workpiece 110, a ball screw 140 that moves the cutting tool 135 in an axial direction, a servo motor 145 that rotates the ball screw 140 as a feed axis drive motor, and an amplifier 148 that outputs a drive current for the servo motor 145. The motor is not limited to a motor whose shaft rotates, but can be a linear motor.The amplifier 130 in . Fig. 2 corresponds to the spindle control unit 52 in Fig. 1. The amplifier 148 in Fig. 2 corresponds to the feed axis control unit 53 in Fig. 1.

[0024] The numerical control 100 controls the induction motor 125 and the servo motor 145 such that the cutting tool 135, which moves with the rotation of the ball screw 140, performs the desired cutting operation with respect to the workpiece 110, which is attached to the spindle 115, which rotates with the rotation of the induction motor 125. That is, the numerical control 100 includes a memory unit 101, a unit 102 for determining the strength of the magnetic flux, a unit 103 for changing the acceleration, and a unit 104 for generating a position command. Although not shown in the drawing, the numerical control 100 can include the same components and functions as a conventional numerical control.

[0025] The memory unit 101 stores a maximum acceleration Amax (parameter) at which the induction motor 125 can operate when the magnetic flux strength of the induction motor 125 reaches its maximum. Furthermore, the memory unit 101 stores a time constant for changes in the magnetic flux strength of the induction motor 125. The memory unit 101 can store one or a plurality of time constants, which are defined depending on the magnetic flux strength at the start of operation.

[0026] Unit 102 for determining the strength of a magnetic flux refers to an actual strength Φ of the magnetic flux (at the start of movement) of the induction motor 125 from the amplifier 130. A method for determining (estimating) information about the strength of the magnetic flux in unit 102 for determining the strength of a magnetic flux is described below with reference to Fig. 4 described. 4.

[0027] When the induction motor 125 is operated by means of position control based on the position command, the unit 103 for changing an acceleration modifies the acceleration A of the position command based on the maximum acceleration Amax of the induction motor 125 stored in the storage unit 101, according to the current strength Φ of the magnetic flux (at the start of movement) of the induction motor 125 determined by the unit 102 for determining the strength of the magnetic flux. That is, the unit 103 for changing an acceleration modifies the acceleration A of the position command according to the following equation (1) based on the maximum acceleration Amax of the induction motor 125, the current strength Φ of the magnetic flux (at the start of movement) of the induction motor 125, and an arbitrary proportional constant α. A=Amax×α×Φ

[0028] When the induction motor 125 and the servo motor 145 are operated synchronously by means of position control, the unit 104 for generating a position command generates the position commands of the induction motor 125 and the servo motor 145 for synchronous operation based on the acceleration A of the position command of the induction motor 125, which is modified by the unit 103 for changing an acceleration. That is, the unit 104 for generating a position command generates the position command of the induction motor 125 based on the acceleration A of the position command of the induction motor 125 from the unit 103 for changing an acceleration, as well as a total amount S of the movement and a maximum speed Fmax for the induction motor 125, which are obtained from an processing program P.Furthermore, unit 104 generates the position command for servomotor 145 based on the acceleration A of the position command for induction motor 125 from unit 103 for changing an acceleration, as well as a total amount of movement and a maximum speed for servomotor 145, which are obtained from the machining program P. A method for generating the position command in unit 104 is described below with reference to... Fig. 5 described.

[0029] Fig. 3 represents a function of the numerical control system 10 according to the first embodiment. In Fig. 3 A functional sequence is considered in which the induction motor 125 and the servo motor 145 are operated synchronously by means of position control and the induction motor 125 is operated by switching the control mode from speed control based on a speed command to position control based on a position command.

[0030] In step S11, the unit 102, to determine the strength of the magnetic flux, obtains the current strength Φ of the magnetic flux (at the start of movement) of the induction motor 125 from the amplifier 130.

[0031] Unit 102 for determining the strength of the magnetic flux can obtain the current strength Φ of the magnetic flux (at the start of movement) of the induction motor 125 directly from the induction motor 125 or it can detect the current strength Φ of the magnetic flux. Alternatively, the current strength Φ of the magnetic flux (at the start of movement) can be estimated, for example, according to the strength of an excitation current of the induction motor 125 or the like. An example of an estimation method is given with reference to Fig. 4 described.

[0032] The magnetic flux strength Φ of the induction motor is proportional to the product of the excitation current (Id) required to generate the magnetic flux and the mutual inductance M of the induction motor. Although the numerical control of an induction motor changes the excitation current according to a target magnetic flux strength, the actual magnetic flux responds to a primary delay with a time constant τ(s) with respect to a change in the excitation current. If a constant excitation current Id is continuously applied starting from a state in which the excitation current Id = 0 and the magnetic flux strength Φ = 0, the magnetic flux strength Φ(t) will change after the elapsed time t(s), as shown in Fig. 4 shown, estimated using the following equation (3). Φ(t)=M×ld×(1−exp(−t / τ(s)))

[0033] In the actual calculation, a command value of an excitation current or a feedback value is used as the excitation current Id, and an output determined by applying a primary low-pass filter with time constant τ to M×Id, determined in each control cycle, is used as an estimated value of the magnetic flux. The unit 102 for determining a magnetic flux strength estimates the magnetic flux strength of the induction motor 125 and determines the estimated magnetic flux strength as the current magnetic flux strength (at the start of movement).

[0034] Subsequently, in step S12, the acceleration-modifying unit 103 changes the acceleration A of the position command based on the maximum acceleration Amax of the induction motor 125 stored in the storage unit 101, according to the current magnetic flux strength Φ (at the start of movement) of the induction motor 125 determined by the magnetic flux strength 102. That is, the acceleration-modifying unit 103 changes the acceleration A of the position command according to equation (1) based on the maximum acceleration Amax of the induction motor 125, the current magnetic flux strength Φ (at the start of movement) of the induction motor 125, and an arbitrary proportional constant α.Thus, unit 103, to change an acceleration, modifies the acceleration A of the position command such that the acceleration A of the position command is weaker the lower the current strength Φ of the magnetic flux (at the start of movement) of the induction motor 125. That is, the acceleration A is increased when the current strength Φ of the magnetic flux (at the start of movement) is large, while the acceleration A is decreased when the current strength Φ of the magnetic flux (at the start of movement) is small.

[0035] Subsequently, in step S12, unit 104 generates a position command for synchronization between the induction motor 125 and the servo motor 145, based on the acceleration A of the induction motor 125 position command, which is modified by unit 103 for changing an acceleration. That is, unit 104 generates a position command for the induction motor 125 based on the acceleration A of the induction motor 125 position command from unit 103 for changing an acceleration, as well as a total amount S of movement and a maximum speed Fmax for the induction motor 125, which are obtained from the processing program P.Furthermore, unit 104 generates a position command for servomotor 145 based on the acceleration A of the position command for induction motor 125 from unit 103. This command changes the acceleration, the total amount of movement, and the maximum speed of servomotor 145, all derived from the machining program P. An example of a method for generating the position command is given with reference to... Fig. 5 described.

[0036] Fig. Figure 5 is a schematic representation illustrating an example of a position command. Fig. 5 represents one position command per unit of time on a vertical axis, and one time on a horizontal axis. The unit 104 for generating a position command, as shown in Fig. 5 represented by a continuous line, a position command such that the maximum value of the position command per unit of time is the maximum velocity Fmax, a change in the position command per unit of time (i.e. a slope of an increase or a decrease) is the acceleration A, and the area is the total amount S of the motion.

[0037] Fig. Figure 5 represents a position command generated based on the maximum acceleration Amax (see dashed line). It can be seen that in the present embodiment, the slope of an increase or decrease in the position command per unit of time (i.e., an acceleration of the position command) decreases.

[0038] Unit 104, for generating a position command, outputs the generated position command of the induction motor 125 to the amplifier 130 and outputs the generated position command of the servo motor 145 to the amplifier 148. Since the feed position of the ball screw 140, which is a feed axis, is the product of the rotation position of the servo motor 145 and the pitch width of the ball screw 140, the feed position of the feed axis changes when the rotation position of the servo motor 145 changes. Advantages of the first embodiment

[0039] In the numerical control system 10 according to the first embodiment, if position control is carried out on the basis of a position command by changing the acceleration of the position command of the induction motor 125 according to the strength of the magnetic flux of the induction motor 125, it is possible to limit heat generation in a low load state of the induction motor 125 and to realize stable position control when accelerating or decelerating the induction motor 125.

[0040] This means that a case is considered in which a method in which a magnetic flux or an excitation current of a rotating magnetic field is attenuated when a load on the induction motor 125 is low and no high torque is required, is used in a mode in which the induction motor 125 is controlled by means of speed control based on a speed command in order to avoid a disadvantage in that heat is generated when an excitation current flows into a stator coil of the induction motor 125.

[0041] When the spindle motor and the feed axis motor are operating synchronously, the control mode of the induction motor 125, which is the spindle motor, is switched from speed control to position control based on a position command. In this case, if, as described above, the control mode is switched from speed control to position control to accelerate or decelerate the induction motor 125 in a state where torque is reduced due to a weaker magnetic flux of the induction motor 125, since it is not possible to immediately accelerate or decelerate the induction motor 125 with maximum torque (maximum acceleration), the induction motor 125 may not follow the position command.This can lead to an increase in the deviation with respect to a position command, resulting in spindle overshoot and excessive machining errors. However, in the present embodiment, control is implemented by reducing the acceleration A of the position command of the induction motor 125 when the current strength Φ of the magnetic flux (at the start of movement) of the induction motor 125 is low. This allows the induction motor 125 to follow the position command, and the deviation with respect to the position command is reduced. This prevents spindle overshoot and excessive increases in machining errors, thus achieving stable position control.

[0042] Furthermore, in the present embodiment, since the acceleration of the position command of the servomotor 145 is also generated on the basis of the acceleration of the speed command of the induction motor 125, it is possible to prevent impairment of the accuracy of synchronization between the spindle motor and the feed axis motor.

[0043] In general, when position control based on a position command is used as a control mode of the induction motor, the maximum acceleration at which the induction motor can operate at maximum magnetic flux strength can be determined as the acceleration of the position command of the induction motor after waiting until the magnetic flux strength of the induction motor reaches its maximum. However, in the present embodiment, when synchronous operation of the spindle motor and the feed axis motor is performed (i.e., when the control mode of the induction motor 125, which is the spindle motor, is switched from speed control to position control), it is possible to issue the position command immediately and control the induction motor 125 without waiting until the magnetic flux strength of the induction motor reaches its maximum. Second embodiment

[0044] A second embodiment of the present invention is described below with reference to Fig. 6 and Fig. 7 described. Fig. Figure 6 shows an example of the configuration of a numerical control system 20 according to a second embodiment of the present invention. The same components as those of the numerical control system 10 according to the first embodiment are identified by the same reference numerals, and their description is omitted.

[0045] In contrast to the control system 10 according to the first embodiment, a control system 20 includes a numerical control 200. The numerical control 200, although not shown in the drawing, can include the same components and functions as those of a conventional numerical control.

[0046] In contrast to the numerical control 100 of the first embodiment, the numerical control 200 further includes a unit 105 for calculating a ratio. The unit 105 for calculating a ratio calculates, for example, the ratio (a ratio Φ / Φmax of the magnetic flux strength) of the current magnetic flux strength Φ, determined by the unit 102 for determining the magnetic flux strength, to the maximum magnetic flux strength Φmax of the induction motor 125 stored in the storage unit 101. The unit 105 for calculating a ratio transmits the calculated ratio Φ / Φmax of the magnetic flux strength to the unit 103 for changing an acceleration.

[0047] Unit 103 for changing acceleration modifies the acceleration A of the position command based on the maximum acceleration Amax of the induction motor 125 according to the ratio Φ / Φmax of the magnetic flux strength calculated by unit 105 for calculating a ratio. That is, unit 103 for changing acceleration modifies the acceleration A of the position command according to the following equation (2) based on the maximum acceleration Amax of the induction motor 125, the maximum magnetic flux strength Φmax of the induction motor 125, and the current magnetic flux strength Φ (at the start of movement) of the induction motor 125. A=Amax×Φ / Φmax

[0048] Fig. 7 represents a function of the numerical control system 20 according to the second embodiment. In Fig. 7 considers a functional sequence in which the induction motor 125 and the servo motor 145 are operated synchronously by means of position control and the induction motor 125 is operated by switching the control mode from speed control based on a speed command to position control based on a position command.

[0049] In step S21, the unit 102 is used to determine the strength of the magnetic flux, similar to the process described above in step S11. Fig. 3, the current strength Φ of the magnetic flux (at the start of movement) of the induction motor 125 from the amplifier 130.

[0050] Subsequently, in step S22, unit 105 calculates the ratio Φ / Φmax (hereinafter also referred to as a “ratio Φ / Φmax of the strength of the magnetic flux”) of the current strength Φ of the magnetic flux determined by unit 102 to determine a strength of the magnetic flux to the maximum strength Φmax of the magnetic flux of the induction motor 125 and transfers the ratio Φ / Φmax of the strength of the magnetic flux to unit 103 for changing an acceleration.

[0051] Then, in step S23, the unit 103 is changed to modify an acceleration, similar to the process described above in step S12. Fig. 3. The acceleration A of the position command is based on the maximum acceleration Amax of the induction motor 125, according to the ratio Φ / Φmax of the magnetic flux strength calculated by the unit 105. That is, the unit 103 for changing an acceleration modifies the acceleration A of the position command according to equation (2) based on the maximum acceleration Amax of the induction motor 125, the maximum magnetic flux strength Φmax of the induction motor 125, and the current magnetic flux strength Φ (at the start of movement) of the induction motor 125. Thus, the unit 103 for changing an acceleration modifies the acceleration A of the position command such that the acceleration A of the position command is weaker the smaller the ratio Φ / Φmax of the current magnetic flux strength (at the start of movement) of the induction motor 125.This means that the acceleration A is increased when the ratio Φ / Φmax of the current strength of the magnetic flux (at the start of motion) is large, while the acceleration A is decreased when the ratio Φ / Φmax of the current strength of the magnetic flux (at the start of motion) is small.

[0052] Subsequently, in step S24, unit 104 generates the position commands for the induction motor 125 and the servo motor 145 based on the acceleration A of the position command for the induction motor 125, which is modified by unit 103 for changing an acceleration, and outputs, similarly to the process described above in step S13, in Fig. 3, the generated position commands to amplifiers 130 and 148. Advantages of the second embodiment

[0053] In the numerical control system 20 according to the second embodiment, by changing the acceleration of the position command according to the strength of the magnetic flux of the induction motor 125, it is possible, similar to the numerical control system 10 according to the first embodiment, to limit heat generation and achieve stable position control. Furthermore, it is possible to prevent impairment of the accuracy of synchronization between the spindle motor and the feed axis motor. Moreover, when synchronous operation of the spindle motor and the feed axis motor is performed (i.e., when the control mode of the induction motor 125, which is the spindle motor, is switched from speed control to position control), it is possible to issue the position command immediately and to control the induction motor 125.

[0054] Although embodiments of the present invention have been described, the present invention is not limited to the embodiments described above. Furthermore, the advantages described in the present embodiment serve only to describe the most desirable advantages provided by the present invention, and the advantages of the present disclosure are not limited to those described in the present embodiment.

[0055] For example, in the embodiments described above, the control method is implemented by the numerical control 100 or 200 using software. When the control method is implemented using software, a program constituting this software is installed on a computer (the numerical control 100 or 200). Furthermore, the program can be recorded on a removable medium and distributed to a user by downloading it to the user's computer over a network. Alternatively, the program can be provided to the user's computer (the numerical control 100 or 200) as a web service over a network without requiring a download.

[0056] In the embodiments described above, synchronous control of a spindle motor consisting of an induction motor and a feed axis motor consisting of a servo motor has been described. However, the features of the present invention are not limited thereto, but can be applied to the synchronous control of two spindle motors consisting of an induction motor. Furthermore, the features of the present invention can be applied to the synchronous control of a plurality of motors, including a spindle motor or a plurality thereof consisting of one or more induction motors, or a feed axis motor or a plurality thereof consisting of one or more feed axis motors.In this case, the acceleration of the position commands for a multitude of motors can be changed based on the lowest strength of the magnetic flux from the strengths of the magnetic flux of the multitude of spindle motors that consist of an induction motor.

[0057] In the embodiments described above, the acceleration of the position command was changed according to the current strength of the magnetic flux when the control mode of the induction motor was switched from speed control to position control in order to accelerate or decelerate the induction motor in a state where the magnetic flux of the induction motor is weak, as in a spindle motor. However, the features of the present invention can be applied to a case in which the induction motor is accelerated or decelerated in a state where the magnetic flux of the induction motor is weak, when the induction motor is controlled by position control from the outset. EXPLANATION OF REFERENCE SYMBOLS 10, 20 numerical control system 100, 200 numerical control 101 storage unit 102 Unit for determining the strength of magnetic flux 103 Unit for changing an acceleration 104 Unit for generating a position command 105 units for calculating a ratio 125 Induction motor (spindle motor) 145 Servomotor (feed axis motor) 150 machine tool

Claims

[1] Numerical control (100, 200) for controlling a machine tool (150) including a spindle motor (125) consisting of an induction motor, the numerical control (100, 200) comprising: a storage unit (101) that stores a maximum acceleration at which the spindle motor (125) can operate when a strength of the magnetic flux of the spindle motor (125) reaches its maximum; a magnetic flux strength determining unit (102) that determines a current magnetic flux strength of the spindle motor (125); and an acceleration changing unit (103) that changes an acceleration of a position command based on a maximum acceleration of the spindle motor (125) stored in the storage unit (101) according to a magnetic flux intensity determined by the magnetic flux intensity determining unit (102) at the start of movement of the spindle motor (125) when the spindle motor (125) is operated by position control using a position command, wherein the acceleration changing unit (103) changes the acceleration A of the position command according to the equation (1) below A=Amax×α×Φ and in particular according to equation (2) below A=Amax×Φ / Φmax based on a maximum acceleration Amax of the spindle motor (125), a strength Φ of the magnetic flux at the start of the movement of the spindle motor (125), a maximum strength Φmax of the magnetic flux of the spindle motor (125) and an arbitrary proportional constant α. [2] Numerical control (100, 200) according to claim 1, further comprising: a position command generation unit (104) that generates a position command of a plurality of motors (125, 145) for synchronous operation based on the acceleration of the position command of the spindle motor (125) changed by the acceleration changing unit (103) when the plurality of motors (125, 145) including the spindle motor (125) are operated synchronously by means of position control. [3] The numerical controller (100, 200) according to claim 1 or 2, wherein the acceleration changing unit (103) changes the acceleration of the position command such that the smaller the strength of the magnetic flux at the start of movement of the spindle motor (125), the weaker the acceleration of the position command. [4] The numerical controller (100, 200) according to any one of claims 1 to 3, wherein the magnetic flux intensity determining unit (102) estimates a current magnetic flux intensity of the spindle motor (125) and determines the estimated magnetic flux intensity as the current magnetic flux intensity. [5] Numerical control (100, 200) according to one of claims 1 to 4, further comprising: a ratio calculation unit (105) that receives a current magnetic flux intensity of the spindle motor (125) determined by the magnetic flux intensity determination unit (102) and calculates a magnetic flux intensity ratio that is a ratio of the current magnetic flux intensity of the spindle motor (125) to the maximum magnetic flux intensity of the spindle motor (125), wherein the acceleration changing unit (103) changes the acceleration A of the position command based on the maximum acceleration of the spindle motor (125) according to the ratio of the strength of the magnetic flux calculated by the ratio calculating unit (105).

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