JP6715053B2 - Chatter control method for machine tools - Google Patents

Description

The present invention relates to a chatter suppressing method that suppresses chattering due to rotation of a tool of a machine tool or a workpiece.

Generally, in a machine tool, a tool or a workpiece is rotated, and the tool and the workpiece are brought into contact with each other to scrape the workpiece to perform the machining. At this time, vibration is generated due to various factors, and, for example, when the tool is an end mill, periodic vibration is generated by intermittent contact. Further, for example, even a bite causes vibration due to anisotropy of the workpiece and a chuck that supports it. Further, vibration may occur due to imbalance of the tool/workpiece, and depending on the driving method, for example, in the case of gear driving, vibration related to gear meshing may occur.

In response to these vibrations, the natural frequency of the entire system consisting of the workpiece, tool, and machine occurs, and depending on the magnitude of the damping property, vibrations start to amplify, so-called chatter vibration phenomenon occurs. It is known that processing accuracy (particularly surface accuracy) when processing an object deteriorates. Such chatter vibration is roughly classified into forced chatter vibration and self-excited chatter vibration, and it is thought that forced chatter vibration occurs when an excessive external force acts or the frequency of the external force and the resonance frequency of the vibration system are synchronized. Has been. On the other hand, self-excited chatter vibration includes regenerative chatter vibration (replay chatter vibration) and mode-coupling chatter vibration. Regenerative chatter vibration is caused by periodic fluctuation of cutting resistance and periodic fluctuation of cutting thickness. It is considered that the interaction is caused by continuing mutually strengthening cutting (so-called regenerative effect), and chatter vibration of the mode coupling type is such that when the vibration modes in two directions have close resonance frequencies, It is considered to occur in a coupled manner.

On the other hand, a machine tool may be equipped with a function that allows a worker to adjust the rotation speed by multiplying the rotation speed command of a tool or a workpiece, which is called rotation command override or simply override.

Conventionally, among the above chatter vibrations, as a method for avoiding the replay chatter vibration, there is a workaround method of adjusting the rotation speed of a tool or a work piece, and a deep cut can be made while avoiding chatter for each machine/tool. A method of discovering the number of revolutions by experiments has also been proposed, and it is useful for handling tools that rotate at high speed.

For example, in the conventional example described in Non-Patent Document 1, when a machine tool and a tool to be used are designated, focusing on the fact that there is a stable region where chatter does not occur in a range where the spindle speed is extremely high (called a stable pocket). Discloses a technique of predicting a spindle speed at which a stable pocket exists and a possible depth of cut, starting machining based on the prediction, and correcting a deviation from an actually generated prediction while performing machining.

Further, in the conventional example described in Patent Document 2, the vibration generated when the rotation of the tool or the work is started, and the vibration detected from the start of the rotation is the threshold value of the vibration when the machine spindle idles. When it is judged that the vibration exceeds the above range, the vibration is analyzed by Fourier series expansion, the frequency due to the reproduction chatter is calculated, and the reproduction chatter avoidance frequency is obtained by adding the estimated damping ratio to the frequency due to the reproduction chatter to obtain the spindle vibration avoidance frequency. A chattering suppression method for a work machine that adjusts the rotation speed is disclosed.

Improving the efficiency of cutting that can be done up to this point: Understanding and practical use of stable pockets (Special issue: "Chatter vibration" and suppression technology in machining) Tetsutaro Hoshi Mechanical Engineering Mechanical Engineering 61(5),22-29,2013-05 Nikkan Kogyo Publishing Production 1953

JP, 2014-083674, A

However, when adverse conditions of the tool and the work overlap, even if the vibration is small at first, if the work is continued with constant depth of cut/constant speed feed, the component of tens of times the rotation of the tool (or the work) is generated. May cause chatter. In order to deal with such a phenomenon, in the chatter avoidance measure of the conventional example as described above, it is applied to the chatter phenomenon caused by a component of several tens of times of the rotation of the tool (or the workpiece), even if the difference of 10 rotations is several tens. If it is multiplied, it will be a difference of several hundred rotations, which is not realistic. Therefore, the development of a technique for suppressing chatter vibration caused by a component of several tens of times such rotation has been awaited.

The present invention has been made in view of the above-mentioned circumstances, and an object thereof is to provide a technique capable of suppressing chatter vibration caused by a component of several tens of times of rotation as described above.

The present inventors have diligently studied from various points of view the possibility of a technique capable of suppressing chatter vibration caused by a component of several tens of rotations as described above, and as a result, with respect to the rotation cycle as described above. If high-order vibration components lead to chatter, it is easier to actually operate the machine and perform an experiment in which the number of revolutions is changed to perform machining, rather than predicting the avoidance number of revolutions by calculation. I found that there is.

Here, the order is intended to be a multiple component such as a certain vibration frequency, and the 1× component is referred to as the 1st component, the 2nd is referred to as the 2nd order, and the 3rd is referred to as the 3rd order, and the high order means a frequency component having a high order.

Generally, a method of slightly changing the rotation speed command is adopted for chatter such that vibration is amplified when machining is continued for a long time. Therefore, what can be considered as a solution is to continuously switch several percent of the spindle rotation command override generated using a pseudo random number at regular time intervals. Actually, the override command in which several% is added to 100% is applied.

For example, a pseudo-random number sequence can be generated by using a linear feedback shift register. One of the random number sequences is used to generate a command sequence in which the override command is 100% when it becomes 0 and 99% when it becomes 1.

However, when I tried to apply the raw pseudo-random number to the spindle override command as it was, low frequency components appeared such that 0 command continued for a long time (100% continued in the actual command), and as expected, several% No switch was made.

Of course, it goes without saying that it is possible to use 2 bits or more as an implementation for assigning an override.

Therefore, the chatter suppressing method for a machine tool of the present invention generates a sequence of 0s and 1s that may have continuous low frequency components of 0 or 1 , and further suppresses low frequency components by using a DC-free coding method. After that, the engine speed command override is generated. That is, instead of using the pseudo-random number sequence as it is, DC-free encoding is performed using an 8-16 modulation table or the like to suppress the long-period component, and then it is used to generate the override command.

In an implementation that allocates 2 bits or more, adjusts so that a constant command is not continuous by passing a constant low-pass filter for a sequence of 0s and 1s where 0s or 1s may have continuous low-frequency components. To do.

Further, the machine tool of the present invention includes at least a rotary shaft, a tool mounted on the rotary shaft, a rotary drive means for the rotary shaft, and a numerical controller for sending a rotational speed command to the rotary drive means. A method for suppressing chatter vibration in a machine tool for machining a workpiece with the tool by causing the rotary shaft drive means to rotate the rotary shaft at a predetermined rotational speed based on the rotational speed command, further comprising a pseudo random number sequence generation unit. , A DC-free modulation unit and an override command generation unit, and the pseudo-random number sequence generation unit generates a sequence of 0s and 1s that may have a low frequency component of 0s or 1s and outputs the sequence to the DCs free modulation unit. transmission, and the DC-free modulation section is transmitted to the override command generating unit to suppress a low-frequency component by using a DC-free encoding method, the override command generating unit based on a signal which suppresses the low frequency components A rotation speed command override is generated, and the rotation speed command override is transmitted to the numerical controller.

According to the present invention, it is possible to suppress chatter vibration caused by a component of several tens of rotations that may occur when a work is continued with constant cutting and constant speed feeding.

It is a graph which shows the example of the signal to which this invention is applied, and its frequency analysis example, (a) shows the example of the signal with which the high-order component was mixed, (b) shows the frequency analysis example, respectively. It is a flow chart which shows a chattering suppression method as a comparative example of the present invention, and is an example which uses a random number sequence as it is. It is an example of a random number sequence showing a problem of the chatter suppression method as a comparative example. It is a figure which shows the structure of the control apparatus in the machine tool to which this invention is applied. It is a flow chart which shows a chattering suppression method of an embodiment of the present invention, and is an example which uses after modulating.

The present inventors have diligently studied from various viewpoints the possibility of a technique capable of suppressing chatter vibration caused by a component of several tens of times of rotation as described above, and as a result, with respect to the rotation cycle as described above. If high-order vibration components lead to chatter, it is easier to actually operate the machine and perform an experiment with slightly changed rotation speed to perform machining than to predict the avoidance rotation speed by calculation. I found that there is. FIG. 1 is a graph showing an example of a signal to which the present invention (and a comparative example) is applied and an example of its frequency analysis. (a) is an example of a signal in which higher-order components are mixed, and (b) is its frequency. Analysis examples are shown respectively. Here, as shown in FIGS. 1(a) and 1(b), the order is intended to be a multiple component such as a certain vibration frequency, and the 1× component is called the 1st component, the 2× is the 2nd, and the 3rd is the 3rd order. Next means a frequency component with a high order.

FIG. 2 is a flowchart showing a chatter suppressing method as a comparative example of the present invention, which is an example in which a random number sequence is used as it is. As described above, a method of slightly changing the rotation speed command is adopted for chatter in which vibration is amplified when machining is continued for a long time. Therefore, in this comparative example, several percent of the spindle rotation command override generated using a pseudo random number is continuously switched at regular time intervals. Actually, the override command in which several% is added to 100% is applied. For example, a pseudo-random number sequence can be generated by using a linear feedback shift register. A command sequence is generated in which any one bit of this random number sequence is used, and when it is 0, the override command is 100%, and when it is 1, it is 99%.

That is, as shown in FIG. 2, when the flow of the chatter suppression method according to this comparative example starts (S201), pseudo random numbers are generated at regular time intervals, and the random numbers up to that point are updated (S202). For example, a random number sequence 200R shown in FIG. 2 is generated and updated to this random number sequence 200R. Then, in the updated random number sequence 200R, the instruction bit (here, BIT No. 0 is the instruction bit) is confirmed (S203). Then, it is determined whether the instruction bit is 1 (1 or 0) (S204), and if it is 1 (Yes in S204), a 100% override command is generated (S205). The command is displayed (S206) and the process returns to S202 to update the next random number. On the other hand, if the instruction bit is not 1 (No in S204), a 99% override command is generated (S207), this command is displayed (S206), and the process returns to S202 to update to the next random number.

However, as described above, if the raw pseudo random number 200R is continuously applied to the spindle override command as it is, a low frequency component such as 0 command continuously (100% continuously in the actual command) appears for a long time, As expected, a few percent did not switch.

FIG. 3 is an example of a random number sequence showing a problem of the chatter suppression method as a comparative example. As shown in FIG. 3, in this example, when the Fibonacci LFSR (bit number 16) is used for pseudo-random number generation and the least significant bit is used as the instruction bit, there may be a case in which 0 continues for a maximum of 15 times. Even if 1 comes after that, there is a possibility that the continuation of 0 will resume. Therefore, theoretically, such a situation may occur, and it is useful to suppress the long-period component and use it for generating the override command by making some other means.

Therefore, the chatter suppression method according to the embodiment of the present invention does not use the pseudo-random number sequence as it is, but uses the 8-16 modulation table or the like to perform DC-free coding to suppress the long-period component, and then Used to generate the override directive.

FIG. 4 is a diagram showing a configuration of a control device in a machine tool to which the present invention is applied. As shown in FIG. 4, a machine tool (machining control device) 10 to which the present invention is applied has a programmable controller (PLC) 12, an interface unit 14, and various switches/sensors 16. Various input/output signals from various switches/sensors 16 are transmitted to the PLC 12 via the interface unit 14. Further, the machining control device 10 includes a numerical controller (CNC) 18, an input device (keyboard switch) 20 for an operator to input a command such as a numerical value to the CNC 18, and an input (control) set by the command. ) The display 22 which displays a state. Here, the numerical controller (CNC) 18 further includes a tool axis controller 181, and a plurality of axis controllers 182, 183, 184,.... A spindle motor 181a is connected to the tool axis controller 181, and the tool axis controller 181 rotates the spindle motor 181a at a predetermined rotation speed/speed on the basis of a command such as a numerical value of the CNC 18. Correspondingly, a tool is attached to a spindle (not shown) to rotate the tool and process the work. The axis controllers 182, 183, 184,... Correspond to the X-axis, Y-axis, Z-axis, etc., respectively, if the present invention is applied to a machine tool with control of four or more axes. However, they are connected to respective shaft motors and the like (not shown).

The chatter suppressing method of the present invention is a method of generating a pseudo random number sequence, further suppressing a low frequency component by using a DC-free encoding method or the like, and then generating a rotation speed command override. The machining control device 10 is a machine tool that employs such a chatter suppression method. That is, the PLC 12 of the machine tool (machining control device) 10 includes a pseudo random number sequence generation unit 121, a DC free modulation unit 122, and an override command generation unit 123. The pseudo random number sequence generation unit 121 generates a pseudo random number sequence and transmits it to the DC free modulation unit 122, and the DC free modulation unit 122 further suppresses the low frequency component by using a DC free encoding method or the like to issue an override command. It is transmitted to the generation unit 123. The override command generation unit 123 generates a rotation speed command override based on the signal in which the low frequency component is suppressed, and transmits the rotation speed command override to the numerical controller (CNC) 18. In FIG. 4, the chatter suppression program may not be in the PLC 12, may be on the CNC 18 side, or may be another unit. Further, the pseudo random number sequence generation unit 121 and the DC-free modulation unit 122 may be outside the PLC 12. Furthermore, the pseudo random number sequence generation unit 121, the DC-free modulation unit 122, the override command generation unit 123, and the like may be used as separate peripheral devices of the machine tool 10.

FIG. 5 is a flow chart showing the chatter suppressing method according to the embodiment of the present invention, which is an example of using by modulating. That is, as shown in FIGS. 4 and 5, in the PLC 12 of the machine tool (machining control device) 10, when the flow of the chatter suppression method of the present embodiment starts (S301), the pseudo random number sequence generation unit 121 causes the pseudo random number sequence generation unit 121 to perform a predetermined period of time. Then, eight pseudo random number sequences are generated (S302), and the instruction bits of each of the eight pseudo random number sequences are confirmed (S303). Then, eight designated bit strings consisting of these eight designated bits are generated (S304), the DC-free modulator 122 performs DC-free coding using the 8-16 modulation table, and the FIFO of the buffer memory (not shown). The data is stored in the area (first-in first-out buffer memory) (S305). Subsequently, one bit is acquired from the FIFO buffer (S306), and it is determined whether the acquired bit is 1 (1 or 0) (S307) and when it is 1 (Yes in S307). , 100% override command is generated (S308), and this command is displayed (S309). Then, it is determined whether or not there is a remainder in the instruction sequence (S310), and if there is a remainder (Yes in S310), the process returns to S306 to obtain the next 1 bit from the FIFO buffer, and the processing in S307 and subsequent steps. To continue. When there is no remaining (No in S310), the process returns to S302, and the next eight instruction columns are generated and updated. On the other hand, if the acquired bit is not 1 in S307 (No in S307), a 99% override command is generated (S311), this command is displayed (S309), and the process returns to S302 to return to the next random number. To update.

In the embodiment described above, one bit is used as the directive bit of the pseudo-random number for assigning the override, but it goes without saying that two or more bits can be used for assigning the override as an implementation. However, in an implementation in which 2 bits or more are used for assigning an override, it is effective to adjust a random number sequence so that certain commands do not continue by passing through a certain low-pass blocking filter.

In the embodiment described above, the present invention is applied to a method of generating a pseudo-random number sequence, further suppressing a low frequency component by using a DC-free encoding method, and then generating a rotation speed command override. Of course, it is not limited. The present invention may use a modulation method other than the DC-free coding method as long as the low frequency component can be suppressed.

10 machine tool (processing control device), 12 programmable controller (PLC),
14 interface units, 16 switches/sensors,
18 numerical controller (CNC), 20 input device (keyboard switch),
22 indicator, 181 tool axis controller, 182, 183, 184 axis controller,
181a spindle motor, 121 pseudo random number sequence generation unit, 122 DC free modulation unit,
123 Override command generator

Claims (2)

A feature is that a sequence of 0s and 1s that may have a low frequency component in which 0 or 1 is continuous is generated, and a low frequency component is suppressed using a DC-free encoding method, and then a rotation speed command override is generated. Chatter control method for machine tools. At least a rotary shaft, a tool attached to the rotary shaft, a rotary drive means for the rotary shaft, and a numerical controller for sending a rotational speed command to the rotary drive means, and the rotary shaft based on the rotational speed command. A method for suppressing chatter vibration in a machine tool for machining a workpiece by using the tool by rotating the rotary shaft by a predetermined number of revolutions, further comprising a pseudo random number sequence generation unit, a DC free modulation unit, and an override command. comprising a generator, the pseudo random number sequence generation unit may transmit generate a 0 to consist of 1 sequence which may have a low-frequency component of 0 or 1 is continuous with the DC-free modulation unit, the DC-free modulation section using a DC-free coding scheme to suppress the low frequency components transmitted to the override command generating unit, the override command generating unit generates a rotational speed command override based on the signal which suppresses the low frequency components, the A machine tool characterized by transmitting a rotation speed command override to the numerical control unit.

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