Machine tool control device

Abstract

A machine tool control device according to one aspect of the present invention, with which a machine tool driving a main shaft by means of an induction motor is controlled according to a machining program, is equipped with: a load estimation unit which, by pre-calling the machining program, estimates whether the load state of the main shaft is idling operation without any load induced by the machining or actual operation including a load induced by the machining; and a current control unit which controls a d-axis current and a q-axis current of the main shaft currents supplied to the induction motor such that the speed of the main shaft corresponds to a speed according to the machining program, and which sets the d-axis current to a higher value when the load state is estimated to be actual operation than when the load state is estimated to be idling operation.

Description

TECHNICAL AREA The present invention relates to a machine tool control device. BACKGROUND OF THE TECHNOLOGY Induction motors are used as various drive sources. The speed of an induction motor is controlled by supplying current from an inverter to the induction motor (see, for example, patent document 1). Specific methods for controlling the speed of an induction motor include, for example, V / f control and vector control. Bibliography Patent documents Patent document 1: Japanese unexamined patent application, publication no. 2018-57161 REVELATION OF THE INVENTION Problems to be solved by the invention In a machine tool, the motor speed must be precisely controlled. However, the load on the machine tool spindle changes abruptly depending on the machining state, for example, whether a tool is machining a workpiece or not. When the machine tool transitions from an idle state, where no tool is in contact with a workpiece, to an actual operating state, where the tool is machining the workpiece, the output torque of an induction motor needs to increase significantly. Since the secondary magnetic flux of the induction motor takes time to build up, the deviation from the target speed can increase if the control is based on feedback from the speed sensor. Therefore, a technique that enables precise control of the spindle speed of a spindle driven by an induction motor is desirable. Means to solve the problems A machine tool control device according to one aspect of the present disclosure is a machine tool control device for controlling a machine tool which drives a spindle according to a machining program by means of an induction motor, wherein the machine tool control device comprises: a load estimation unit which, by pre-calling the machining program, estimates whether a load state of the spindle is an idle operation without any load caused by the machining or an actual operation with a load caused by the machining;and a current control unit that controls a d-axis current and a q-axis current of a spindle current supplied to the induction motor so that a spindle speed corresponds to a speed according to the machining program, and that sets the d-axis current to a higher value when the load condition is assessed as actual operation than when the load condition is assessed as idle operation. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a block diagram representing a configuration of a machine tool including a machine tool control device according to an embodiment of the present disclosure; Fig. 2 is a diagram representing a constant torque curve in a dq current coordinate system of an induction motor; and Fig. 3 is a time diagram representing changes in a load on a spindle, a setpoint of a current control unit and a state of the induction motor. PREFERRED FORM OF EXECUTION OF THE INVENTION In the following, embodiments of the present disclosure are described with reference to the drawings. Fig. 1 shows a block diagram representing a configuration of a machine tool 1 according to a first embodiment of the present disclosure. The machine tool 1 comprises a machine tool control device 10, a spindle 20 that rotates a workpiece or a tool, an induction motor 30 that drives the spindle 20, a spindle amplifier 40 that supplies a spindle current to the induction motor 30 according to a command from the machine tool control device 10, a positioning mechanism 50 that defines the relative positions of the workpiece and the tool, a plurality of servo motors 60 that drive the positioning mechanism, and a plurality of servo amplifiers 70 that each supply current to the servo motors 60 according to a command from the machine tool control device 10. Since the induction motor 30, the spindle amplifier 40, the positioning mechanism 50, the servo motors 60, and the servo amplifiers 70 are known configurations in the machine tool 1, a detailed description of them is omitted. The machine tool control device 10 is an embodiment of the machine tool control device according to the present invention. The machine tool control device 10 comprises, for example, a memory, a processor, an input / output interface, and the like, and can be implemented by one or more computer devices that execute a corresponding control program. The machine tool control device 10 is typically configured as a numerical control device. The machine tool control device 10 comprises a machining program storage unit 11, a load estimation unit 12, a current control unit 13, and a positioning control unit 14. It should be noted that these components are functional categories of the machine tool control device 10 and may not be clearly distinguishable from one another with respect to their physical and program configurations. The machining program storage unit 11 stores a machining program that describes a relative movement path of the tool in relation to the workpiece in order to machine the workpiece and obtain a desired product, in a predetermined language, such as G-code. The load estimation unit 12 estimates the load state of the spindle 20 by pre-retrieving the machining program. More precisely, the load estimation unit 12 estimates whether the load state of the spindle 20, and thus of the induction motor 30, during a given operation is idling without a machining-induced load or actual operation including a machining-induced load, by reading and analyzing the machining program before the workpiece and tool actually move relative to each other. Preferably, the load estimation unit 12 predicts the magnitude of the load during actual operation. In this way, the current control unit 13 can adjust the spindle current more appropriately according to the predicted load magnitude. The load estimation unit 12 can determine whether a machining-induced load is present or not, depending on the type of program instruction in the machining program, without checking whether the machining is actually being performed. In other words, the load estimation unit 12 can be configured to distinguish whether a set of instructions in the machining program is used to move the workpiece and tool relative to each other for machining, or to move the workpiece and tool to positions where machining begins, or the like, and estimates whether the operation in response to the instruction set is an idle operation or an actual operation. The current control unit 13 controls the spindle current supplied to the induction motor 30 so that the spindle speed corresponds to the speed specified in the machining program. More precisely, based on a measured value of the spindle speed 20, the current control unit 13 determines both the d-axis current, which represents a component forming the magnetic flux, and the q-axis current, which represents a torque-generating component, and inputs a d-axis current setpoint and a q-axis current setpoint to the spindle amplifier 40, which supplies the spindle current to the induction motor 30.As a method to bring the spindle speed into line with a setpoint, it is possible to use a method in which the output torque is changed by adjusting at least one of the following quantities: slip frequency, normal of the spindle current, and d-axis current and q-axis current of the spindle current. The current control unit 13 sets the d-axis current of the spindle current to a higher value when the load condition is assessed as actual operation than when the load condition is assessed as no-load operation, or preferably sets it to a higher value when the predicted load increases. Conversely, the current control unit 13 sets the d-axis current of the spindle current to a lower value when the load condition is assessed as no-load operation than when the load condition is assessed as actual operation. In this way, it is possible to generate sufficient secondary magnetic flux and produce a load-balanced torque during actual operation, while simultaneously reducing the current consumption due to the d-axis current during no-load operation, in which the load torque is low.The current control unit 13 can first define the value of the d-axis current for each machining process and then switch to a known control system, such as an MTPA controller. It is possible to limit spindle speed 20 errors that can occur due to fluctuations in the load torque simply by defining an initial value of the d-axis current based on the result of the pre-flush machining program. Therefore, it is possible to further reduce power consumption by switching to a control system that also optimizes the d-axis current value. The current control unit 13 can change the control procedures depending on whether the load condition is considered actual operation or idle operation. The current control unit 13 preferably performs a control such that both the d-axis current and the q-axis current are equal to or less than a minimum value Imin (within the area indicated by the dashed line in Fig. 2) of the current standard on a constant torque curve, which corresponds to the frictional torque of the spindle 20 during no-load operation, when the load condition is considered to be no-load operation. For this purpose, the current control unit 13 can be configured to prestore the minimum value Imin of the current on the constant torque curve during no-load operation. Furthermore, the current control unit 13 can be configured to set the d-axis current to a predefined value to satisfy the aforementioned condition and adjust the q-axis current by means of the aforementioned speed control.Although the frictional torque of the spindle 20 changes depending on the rotational speed, the change in frictional torque within an actually used rotational speed range is sufficiently small compared to the torque caused by the machining process. Therefore, the current control unit 13 can be configured to output a constant d-axis current regardless of the rotational speed of the spindle 20. The current control unit 13 can control the spindle current such that the slip frequency of the induction motor 30 corresponds to the inverse of a second-order time constant of the induction motor 30 when the load condition is considered to be no-load operation. Furthermore, the current control unit 13 can regulate the d-axis current and the q-axis current to equal values ​​when the load condition is considered to be no-load operation (point A in Fig. 2). It should be noted that "corresponds to the inverse" means that a deviation from the inverse is equal to or less than 10% of the inverse, and preferably equal to or less than 5% of the inverse, and that "equal values" means that a deviation between them is equal to or less than 10% of an average value, and preferably equal to or less than 5% of the average value. Since the magnetic flux is sufficiently reduced during no-load operation, voltage saturation need not be considered, and since the output torque is also sufficiently low, current limiting does not need to be considered either. Therefore, it is possible to calculate the minimum spindle current for a required torque using the formulas (1) and (2) below. Here, a torque setpoint is defined as T* [Nm], the d-axis current as I1d [A], the q-axis current setpoint as I1q [A], the spindle current norm as I [A], the number of pole pairs as Pn, the mutual inductance between a stator winding and a rotor winding as M [H], the secondary self-inductance as L2 [H], the secondary resistance as R2 [Ω], and the slip frequency as ws [rad / s]. [Formula 1] [Formula 2] If the d-axis current I1d = I cosθ and the q-axis current I1q = I sinθ, the spindle current I can be minimized at θ = π / 4. In this case, the d-axis current setpoint I*1d and the q-axis current setpoint I*1q can be represented by the following formulas (3) and (4). [Formula 3] [Formula 4] Furthermore, the slip frequency ωs can be represented by the following formula (5). [Formula 5] Substituting formulas (3) and (4) into formula (5) yields the following formula (6), and it can be determined that the slip frequency ωs is the reciprocal of a second-order time constant of the induction motor 30. [Formula 6] In a case where the load condition is assessed as no-load operation, it is therefore possible to reduce the power consumption of the induction motor 30 if the values ​​of the d-axis current I1d and the slip frequency ωs of the induction motor 30 are each set to a pre-stored optimal value and the spindle speed is maintained at a setpoint by fine-tuning the q-axis current I1q. In a case where the load condition is assessed as no-load operation, it is also possible to reduce the power consumption of the induction motor 30 by first setting the d-axis current and the q-axis current to the same pre-stored optimal values ​​and then maintaining the spindle speed at the setpoint by smoothly adjusting the d-axis current and the q-axis current. In a case where a transition from idle operation to actual operation is anticipated, the current control unit 13 preferably sets the d-axis current to a value greater than the value during idle operation before the transition to actual operation occurs. This makes it possible to increase the secondary magnetic flux before the transition to actual operation, to rapidly increase the output torque when the load condition transitions to actual operation and the load torque increases, and thereby to dampen fluctuations in spindle speed due to the increase in load torque. To reliably dampen the speed fluctuations, the d-axis current set to the higher value before the transition to actual operation is maintained at least until the load condition transitions to actual operation.On the other hand, in a case where a transition from actual operation to no-load operation is expected, there is no need to change the secondary magnetic flux in advance, and it is therefore only necessary to set the value of the d-axis current to the value described above at the moment when the transition to no-load operation is expected. For example, in Fig. 2, the spindle current is at point A during no-load operation and at point B during actual operation, and operation is carried out with the spindle current at point C, while the d-axis current is increased in advance in the case of the transition from no-load operation to actual operation, whereas in the case of the transition from actual operation to no-load operation, the spindle current changes directly from point B to point A. Although the value of the d-axis current rises or falls essentially without delay in response to the setpoint value of the d-axis current, the secondary magnetic flux of the induction motor 30 has a relatively large time constant, so there is a delay in its rise and fall. Since the torque that the induction motor 30 can deliver is limited by the secondary magnetic flux, a delay in the fall of the secondary magnetic flux does not cause a problem, whereas a delay in the rise of the secondary magnetic flux can lead to a lack of output torque.The current control unit 13 in the present embodiment increases the secondary magnetic flux in advance by setting the d-axis current to a value greater than the value during idle operation before the transition from idle operation to actual operation takes place, and it is thus possible to instantaneously increase the output torque at the time of the start of actual operation. Fig. 3 shows the changes in the load on the spindle 20, a setpoint of the current control unit 13, and a state of the induction motor 30. More precisely, Fig. 3 shows a time diagram depicting temporal changes in the load state derived from the machining process, the expected load magnitude, the setpoints for the d-axis current and the q-axis current, the slip frequency, and the number of magnetic fluxes in the secondary magnetic flux. As described above, the d-axis current is set to a low value simultaneously with the start of actual operation during the transition from actual operation to no-load operation. Consequently, due to frequency maintenance control, the q-axis current decreases essentially instantaneously with respect to a decrease in load torque, and the slip frequency increases essentially instantaneously, while the secondary magnetic flux decreases with a delay relative to a decrease in the d-axis current. During the transition from no-load operation to actual operation, the d-axis current is set to a higher value before the start of actual operation. Consequently, due to frequency maintenance control, the slip frequency decreases essentially instantaneously with respect to the decrease in the d-axis current, and the secondary magnetic flux increases with a delay relative to the increase in the d-axis current.If the number of secondary magnetic fluxes is sufficiently large at the start of actual operation, the q-axis current and slip frequency can increase essentially without delay. Although the d-axis current should be set to a higher value during actual operation because the expected load in the drawing is greater, the d-axis current can initially be set to a value that is somewhat high and can be further optimized after the start of actual operation by switching to a known control method, such as MTPA control. A lower limit of the lead-up time from the time at which the current control unit 13 sets the d-axis current to a value greater than the value during idle operation, until the time at which the transition to actual operation is expected to occur, is preferably four times, and more preferably six times, the time constant of the secondary magnetic flux of the induction motor 30. Conversely, the upper limit of the lead-up time is preferably ten times, and more preferably eight times, the time constant of the secondary magnetic flux of the induction motor 30. It is possible to sufficiently increase the secondary magnetic flux before the transition to actual operation by setting the lead-up time to a value equal to or greater than the lower limit. Furthermore, it is possible to limit an increase in current consumption during idle operation by setting the lead-up time to a value equal to or less than the upper limit. The positioning control unit 14 has a known configuration to output a setpoint that determines a drive current for driving the servomotors 60, so that the relative positions of the workpiece and the tool can be defined according to the machining program. As described above, the machine tool control device 10, according to the present embodiment, sets the d-axis current and the q-axis current to small values ​​when the load state of the spindle 20 is assessed as idle operation, thereby limiting the power consumption of the induction motor 30. Furthermore, the machine tool control device 10 increases the secondary magnetic flux in advance by increasing the d-axis current before the transition from idle to actual operation, thus preventing a torque deficiency at the start of actual operation. Further remarks on the foregoing embodiments and their modifications are disclosed below. (Additional Note 1) A machine tool control device (10) for controlling a machine tool (1) that drives a spindle (20) by means of an induction motor (30) according to a machining program comprises: a load estimation unit (12) that estimates, by pre-calling the machining program, whether a load state of the spindle (20) is idle operation without any load induced by machining or actual operation including a load induced by machining; and a current control unit (13) that controls a current of the spindle (20) supplied to the induction motor (30) such that a spindle speed (20) corresponds to a speed according to the machining program, and that sets a d-axis current of the current of the spindle (20) to a higher value when the load state is estimated as actual operation than when the load state is estimated as idle operation. (Additional Note 2) In the machine tool control device (10) according to the additional note (1), the current control unit (13) can, in a case where the load condition is considered to be idle operation, control both the d-axis current and the q-axis current so that they are equal to or less than a minimum value of a standard of the current on a constant torque curve corresponding to the frictional torque of the spindle (20) during idle operation. (Additional Note 3) In the machine tool control device (10) according to the additional note (1) or (2), the current control unit (13) can, in a case where the load condition is considered to be idle operation, control the current of the spindle (20) such that a slip frequency of the induction motor (30) corresponds to the reciprocal of a second-order time constant of the induction motor (30). (Additional note 4) In the machine tool control device (10) according to the additional note (1) or (2), the current control unit (13) can regulate the d-axis current and the q-axis current to the same values ​​in a case where the load condition is considered to be idle operation. (Additional note 5) In the machine tool control device (10) according to one of the additional notes (1) to (4), the current control unit (13) can, in a case where a transition from idle operation to actual operation is expected, set the d-axis current to a value greater than a value during idle operation before the transition to actual operation takes place. (Additional Note 6) In the machine tool control device (10) according to the additional note (5), a lead time from a time at which the d-axis current is set to a value greater than the value during idle operation until a time at which the transition to actual operation is expected to take place may be equal to or greater than four times and equal to or less than ten times a secondary magnetic flux time constant of the induction motor (30). Although the present disclosure has been described in detail above, it is not limited to the individual embodiments mentioned above. Various additions, replacements, modifications, partial deletions, and the like may be made to these embodiments without departing from the spirit of the present disclosure or from the spirit of the present disclosure as derived from the content described in the claims and their equivalents. EXPLANATION OF THE REFERENCE SYMBOLS 1 Machine tool 10 Machine tool control device 11 Machining program storage unit 12 Load estimation unit 13 Current control unit 14 Positioning control unit 20 Spindle 30 Induction motor 40 Spindle amplifier 50 Positioning mechanism 60 Servo motor 70 Servo amplifier QUOTES INCLUDED IN THE DESCRIPTION This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature JP 2018-57161

[0003]

Claims

A machine tool control device for controlling a machine tool that drives a spindle according to a machining program by means of an induction motor, the machine tool control device comprising: a load estimation unit that estimates, by pre-calling the machining program, whether the spindle is in an idle state without a machining-induced load or in an actual operating state including a machining-induced load; and a current control unit that controls a d-axis current and a q-axis current of a spindle current supplied to the induction motor such that a spindle speed corresponds to a speed according to the machining program, and that sets the d-axis current to a higher value when the load state is estimated to be actual operation than when the load state is estimated to be idle operation. The machine tool control device according to claim 1, wherein, in a case where the load condition is considered to be idle operation, the current control unit controls the d-axis current and the q-axis current respectively such that they are equal to or less than a minimum value of a standard of a current on a constant torque curve corresponding to a frictional torque of the spindle during idle operation. The machine tool control device according to claim 1 or 2, wherein in a case where the load condition is considered to be idle operation, the current control unit controls the spindle current such that a slip frequency of the induction motor corresponds to the reciprocal of a second-order time constant of the induction motor. The machine tool control device according to claim 1 or 2, wherein in a case where the load condition is assessed as idle operation, the current control unit regulates the d-axis current and the q-axis current to equal values. The machine tool control device according to one of claims 1 to 4, wherein in a case where a transition from idle operation to actual operation is expected, the current control unit sets the d-axis current to a value that is greater than a value during idle operation before the transition to actual operation takes place. The machine tool control device according to claim 5, wherein a lead time from a time at which the d-axis current is set to a higher value than the value during idle operation, until a time at which the transition to actual operation is expected to take place, is equal to or greater than four times and equal to or less than ten times a secondary magnetic flux time constant of the induction motor.

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