Breakage determination device and computer-readable storage medium

Abstract

This breakage determination device: obtains samples of the value of the load applied to a shaft of a machine tool; determines whether the samples were detected during a cutting time period or detected during a non-cutting time period; calculates amounts of change between samples during the cutting time period; excludes one or more of the amounts of change; calculates a probability density function from the remaining amounts of change; and determines breakage of a tool on the basis of the probability of occurrence of the excluded one or more amounts of change in the probability density function.

Description

Breakage detection device and computer-readable storage medium

[0001] The present disclosure relates to a breakage determination device and a computer-readable storage medium.

[0002] Conventionally, in a device for detecting or predicting breakage of a tool used in a machine tool, there is a technology in which a feature amount of load during a cutting time period is multiplied by a coefficient to calculate a fluctuating threshold value and determine breakage. For example, Patent Document 1 discloses such a technology.

[0003] Japanese Patent Application Laid-Open No. 2004-130407

[0004] The technique of Patent Document 1 requires trial machining, so breakage cannot be determined from the first machining.

[0005] In the field of numerical control devices, it is desirable to make breakage detection more efficient.

[0006] The breakage detection device includes a data acquisition unit that acquires the value of the load applied to the axis of the machine tool as a sample; a determination unit that determines whether the sample acquired by the data acquisition unit is a sample detected during a cutting time period or a sample detected during a non-cutting time period; a change amount calculation unit that calculates the amount of change in the sample during the cutting time period; a probability calculation unit that excludes one or more of the change amounts and calculates a probability density function from the remaining change amounts; and a breakage detection unit that determines tool breakage based on the probability of the occurrence of the excluded change amount in the probability density function.

[0007] 1 is a block diagram of a breakage determination device. FIG. 2 is a graph showing changes in commands during cutting. FIG. 3 is a graph of samples in normal times. FIG. 4 is a graph of samples when the tool breaks during cutting. FIG. 5 is a graph showing the relationship between a probability density function and an excluded amount of change. FIG. 6 is a graph of samples in normal times and a table showing values ​​in a 3σ interval when one point is excluded. FIG. 7 is a graph of samples when the tool breaks during cutting and a table showing values ​​in a 3σ interval when one point is excluded. FIG. 8 is a flowchart explaining the operation of the breakage determination device. FIG. 9 is a block diagram of a modified example of the breakage determination device. FIG. 10 is a hardware configuration diagram of the breakage determination device.

[0008] The breakage detection device will be described below. The breakage detection device is realized by an information processing device such as a numerical control device or a PC (personal computer).

[0009] 1 is a block diagram of a breakage determination device 100. The breakage determination device 100 includes a data acquisition unit 10, a determination unit 11, a change amount calculation unit 12, a probability calculation unit 13, and a breakage detection unit 14.

[0010] The data acquisition unit 10 acquires at least one of the loads of the spindle and the feed axis. Hereinafter, the load value will be referred to as a sample. The judgment unit 11 judges whether the sample acquired by the data acquisition unit 10 was detected during a cutting time period or a non-cutting time period. A cutting time period is a time period during which the tool is cutting the workpiece. A non-cutting time period is a time period during which the tool is not cutting the workpiece. One method of judgment is to detect a signal. For example, during a cutting time period, a "cutting in progress" command signal is output from the numerical control device. Figure 2 shows an example of a "cutting in progress" command signal. The upper graph in Figure 2 shows the load value, and the lower graph in Figure 2 shows the "cutting in progress" command signal. When the numerical control device analyzes a machining program and executes a cutting command such as G01, the "cutting in progress" command signal turns "ON." This signal can be used to determine whether the time period is a cutting time period or a non-cutting time period.

[0011] The change amount calculation unit 12 calculates the amount of change in the samples acquired during the cutting time period. The amount of change in the samples is the increase or decrease in the value of the samples. The change amount calculation unit 12 calculates the difference between the samples by subtracting the value of the samples at a certain point in time from the value of the samples before or after that point. Figure 3 is a graph showing the change in the samples under normal conditions (when the tool is not broken). Under normal conditions, the values ​​of the samples during the cutting time period are approximately constant. The amount of change in the samples also does not fluctuate greatly.

[0012] FIG. 4 shows the changes in samples when a tool breaks during cutting. For the sake of explanation, FIG. 4 shows a small number of samples (5 points). The actual number of samples is assumed to be sufficiently large. The change amount calculation unit 12 calculates the change amounts of the samples. The change amount from sample 1 to sample 2 is "0", the change amount from sample 2 to sample 3 is "+0.2", the change amount from sample 3 to sample 4 is "-1.5", and the change amount from sample 4 to sample 5 is "-0.1". The change amount calculation unit 12 calculates a sufficient number of change amounts.

[0013] The probability calculation unit 13 calculates a probability density function of the change amount. The probability calculation unit 13 performs preprocessing. In the preprocessing, some of the change amounts are excluded from the samples. The probability calculation unit 13 excludes values ​​with large absolute values. In other words, values ​​with large changes are excluded regardless of whether they are positive or negative. The number of values ​​to be excluded may be one or more. The exclusion criteria may be the order of the magnitude of the change or a threshold value. The breakage detection unit 14 calculates the probability of occurrence of the excluded value using the probability density function of the change amount of the sample. If the probability of occurrence of the excluded value is sufficiently small, the breakage detection unit 14 determines that the tool has broken. In other words, if the probability of occurrence of the excluded value is sufficiently small in the probability density function calculated from the remaining change amounts after excluding some of the change amounts, the breakage detection unit 14 determines that the tool has broken. FIG. 5 is a graph showing the relationship between the probability density function calculated from the remaining change amounts after excluding some of the change amounts and the probability of occurrence of the excluded change amounts. In FIG. 5, the normal distribution on the right is the probability density function, and the points on the left are the excluded change amounts. In the example of FIG. 5, the probability of the occurrence of the excluded variation is extremely close to "0", so it is determined that the tool is broken.

[0014] Figure 6 shows samples under normal conditions (when not broken). The values ​​of Samples 1 to 6 show almost no change. Specifically, the change from Sample 1 to Sample 2 is "0.036926," the change from Sample 2 to Sample 3 is "0.173767," the change from Sample 3 to Sample 4 is "0.014832," the change from Sample 4 to Sample 5 is "0.046814," the change from Sample 5 to Sample 6 is "0.006226," and the change from Sample 6 to Sample 7 is "0.031311."

[0015] FIG. 6 lists the "3σ intervals" when each change amount is excluded. When the change amount from sample 1 to sample 2 is excluded, "μ-3σ" is "-0.1094" and "μ+3σ" is "0.218577". When the change amount from sample 2 to sample 3 is excluded, "μ-3σ" is "-0.05415" and "μ+3σ" is "0.108589". When the change amount from sample 3 to sample 4 is excluded, "μ-3σ" is "-0.11801" and "μ+3σ" is "0.236033". When the change amount from sample 4 to sample 5 is excluded, "μ-3σ" is "-0.10553" and "μ+3σ" is "0.21075". When the change amount from sample 5 to sample 6 is excluded, "μ-3σ" is "-0.12136" and "μ+3σ" is "0.242822". When the change amount from sample 6 to sample 7 is excluded, "μ-3σ" is "-0.11159" and "μ+3σ" is "0.223017".

[0016] Figure 7 shows samples taken when a tool breaks during cutting. The values ​​of Sample 1 to Sample 6 change rapidly. Specifically, the change from Sample 1 to Sample 2 is "0.032484," the change from Sample 2 to Sample 3 is "0.128205," the change from Sample 3 to Sample 4 is "0.124071," the change from Sample 4 to Sample 5 is "0.197852," the change from Sample 5 to Sample 6 is "3.245687," and the change from Sample 6 to Sample 7 is "15.30834."

[0017] FIG. 7 lists the "3σ intervals" when each amount of change is excluded. When the amount of change from sample 1 to sample 2 is excluded, "μ-3σ" is "-7.68314" and "μ+3σ" is "15.2848". When the amount of change from sample 2 to sample 3 is excluded, "μ-3σ" is "-7.64643" and "μ+3σ" is "15.2098". When the amount of change from sample 3 to sample 4 is excluded, "μ-3σ" is "-7.64801" and "μ+3σ" is "15.21304". When the amount of change from sample 4 to sample 5 is excluded, "μ-3σ" is "-7.61971" and "μ+3σ" is "15.15523". When the change amount from sample 5 to sample 6 is excluded, "μ-3σ" is "-6.44736" and "μ+3σ" is "12.76374". When the change amount from sample 6 to sample 7 is excluded, "μ-3σ" is "-1.51215" and "μ+3σ" is "3.003473".

[0018] The amount of change from sample 6 to sample 7 exceeds the "3σ interval." The probability that an amount of change outside the range of the "3σ interval" occurs is 0.03%. The breakage detection unit 14 determines that breakage has occurred.

[0019] The operation of the breakage determination device 100 will be described with reference to Figure 8. The numerical control device is started, setup is completed, and machining is started (step S1). The numerical control device moves the axis at rapid traverse and rotates the spindle at the cutting start position. The data acquisition unit 10 acquires the spindle load value as a sample. The judgment unit 11 judges whether the sample acquired by the data acquisition unit 10 is a sample detected during a cutting time period or a sample detected during a non-cutting time period. The data acquisition unit 10 acquires the sample during the cutting time period (step S2).

[0020] The change amount calculation unit 12 calculates the change amount of the samples during the cutting time period (step S3). The probability calculation unit 13 excludes some of the change amounts (step S4). Either one sample or multiple samples may be excluded. Values ​​with large changes are excluded, regardless of whether they are positive or negative.

[0021] The probability calculation unit 13 calculates a probability density function from the remaining amount of change after excluding some of the amounts of change (step S5).The probability calculation unit 13 calculates the probability of the excluded amount of change occurring in the calculated probability density function (step S6).When the probability of the excluded amount of change occurring is sufficiently low, the probability calculation unit 13 determines that the tool has broken (step S7).

[0022] The breakage detection device 100 of this embodiment acquires the current load value as a sample and determines breakage using the acquired value, so trial processing is not required and breakage detection can be made more efficient.

[0023] (Modification) A modified breakage detection device 100 combines two types of detection methods. The breakage detection device 100 of FIG. 9 is configured by adding a second probability calculation unit 15, a feature calculation unit 16, and a second breakage detection unit 17 to the breakage detection device 100 of FIG. 1. The second probability calculation unit 15 calculates a probability density function of samples acquired during non-cutting time periods. The feature calculation unit 16 calculates feature values ​​of samples acquired during cutting time periods. The feature values ​​are representative values ​​of the samples acquired during cutting time periods. Feature values ​​include the mean, median, maximum value, minimum value, and mode. The feature values ​​may also be a moving average, moving median, moving maximum value, moving minimum value, and moving mode. The type of feature value is not limited.

[0024] The second breakage detection unit 17 compares the probability density function of the samples during the non-cutting time period with the feature values ​​of the samples during the cutting time period. If the probability of occurrence of the feature values ​​during the cutting time period is sufficiently small in the probability density function of the samples during the non-cutting time period, the second breakage detection unit 17 detects tool breakage.

[0025] The breakage detection unit 14 determines breakage based on the gradient of the change in the samples. The second breakage detection unit 17 determines breakage based on whether the feature amount (representative value) of the sample during the cutting time period occurs during the non-cutting time period. The modified breakage detection device 100 determines breakage by combining these two methods.

[0026] In this modification, breakage can be determined using both the slope and the sample value, which broadens the range of breakage determination. Also, by providing multiple feature amounts, breakage can be detected under various circumstances.

[0027] The hardware configuration of the breakage detection device 100 to which the present disclosure is applied will be described below. Fig. 10 is a hardware configuration diagram of the breakage detection device 100. As shown in Fig. 10, the breakage detection device 100 includes a CPU 111 that controls the entire breakage detection device 100, a ROM 112 that records programs and data, and a RAM 113 for temporarily expanding data. The CPU 111 reads out a system program recorded in the ROM 112 via a bus and determines whether a breakage has occurred in accordance with the system program.

[0028] The nonvolatile memory 114 is backed up by, for example, a battery (not shown), and the stored state is maintained even when the power to the breakage detection device 100 is turned off. The nonvolatile memory 114 stores various data such as programs read from the external device 120 via the interfaces 115, 118, and 119 and operation inputs input via the input unit 30. The nonvolatile memory 114 may store programs and data for executing the breakage detection device 100 of this embodiment.

[0029] The interface 115 is an interface for connecting the breakage detection device 100 to an external device 120 such as an adapter. Programs, various parameters, etc. are loaded from the external device 120. The interface 118 is an interface for connecting the breakage detection device 100 to a display unit 70 such as a liquid crystal display. The display unit 70 displays various data loaded into memory, data obtained as a result of executing programs, etc. The interface 119 is an interface for connecting the breakage detection device 100 to an input unit 30 such as a keyboard or pointing device. The input unit 30 passes commands, data, etc. based on operations by an operator to the CPU 111 via the interface 119.

[0030] Although the present disclosure has been described in detail, the present disclosure is not limited to the individual embodiments described above. Various additions, substitutions, modifications, partial deletions, etc. are possible in these embodiments without departing from the gist of the present disclosure or the gist of the present disclosure derived from the claims and their equivalents. Furthermore, these embodiments can also be implemented in combination. For example, in the above-described embodiments, the order of each operation and the order of each process are shown as examples and are not limited to these.

[0031] The following supplementary notes are further disclosed regarding the above-described embodiment and modified examples. (Supplementary Note 1) A breakage determination device (100) includes a data acquisition unit (10) that acquires sample values ​​of loads applied to a machine tool shaft; a determination unit (11) that determines whether the samples acquired by the data acquisition unit (10) are samples detected during a cutting time period or a non-cutting time period; a change amount calculation unit (12) that calculates a change amount in the samples during the cutting time period; a probability calculation unit (13) that excludes one or more of the change amounts and calculates a probability density function from the remaining change amounts; and a breakage detection unit (14) that determines tool breakage based on the probability of occurrence of the excluded change amount in the probability density function. (Supplementary Note 2) The determination unit (11) determines whether the time period is a cutting time period or a non-cutting time period based on a command signal that controls the machine tool. (Supplementary Note 3) The breakage detection unit (14) determines that the tool is broken if the probability of occurrence of the excluded change amount is sufficiently small. (Supplementary Note 4) The probability calculation unit (13) excludes amounts of change with large absolute values. (Supplementary Note 5) The machine tool includes a second probability calculation unit (15) that calculates a probability density function of samples during the non-cutting time period, a feature calculation unit (16) that calculates feature amounts of samples during the cutting time period, and a second breakage detection unit (17) that determines tool breakage based on the probability of occurrence of the feature amount in the probability density function for the non-cutting time period. (Supplementary Note 6) A computer-readable storage medium (112, 113, 114) stores instructions that cause one or more processors (111) to execute processes of acquiring load values ​​applied to a shaft of a machine tool as samples, determining whether the samples are samples detected during a cutting time period or non-cutting time periods, calculating amounts of change of the samples during the cutting time period, excluding one or more of the amounts of change, calculating a probability density function from the remaining amounts of change, and determining tool breakage based on the probability of occurrence of the amount of change excluded in the probability density function.

[0032] REFERENCE SIGNS LIST 100 Breakage determination device 10 Data acquisition unit 11 Determination unit 12 Change amount calculation unit 13 Probability calculation unit 14 Breakage detection unit 111 CPU 112 ROM 113 RAM 114 Non-volatile memory

Claims

1. A breakage determination device comprising: a data acquisition unit that acquires, as a sample, a value of a load applied to a shaft of a machine tool; a determination unit that determines whether the sample acquired by the data acquisition unit is a sample detected during a cutting time period or a sample detected during a non-cutting time period; a change amount calculation unit that calculates a change amount of the samples during the cutting time period; a probability calculation unit that excludes one or more of the change amounts and calculates a probability density function from the remaining change amounts; and a breakage detection unit that determines breakage of a tool based on a probability that the excluded change amount occurs in the probability density function.

2. The breakage determination device according to claim 1, wherein the determination unit determines a cutting time period and a non-cutting time period based on a command signal for controlling the machine tool.

3. The breakage determination device according to claim 1, wherein the breakage detection unit determines that the tool is broken when the probability that the excluded change amount occurs is sufficiently small.

4. The breakage determination device according to claim 1, wherein the probability calculation unit excludes change amounts having large absolute values.

5. The breakage determination device according to claim 1, further comprising: a second probability calculation unit that calculates a probability density function of samples during the non-cutting time period; a feature amount calculation unit that calculates a feature amount of samples during the cutting time period; and a second breakage detection unit that determines breakage of a tool based on a probability that the feature amount occurs in the probability density function of samples during the non-cutting time period.

6. A computer-readable storage medium storing instructions for causing one or more processors to perform a process of acquiring, as a sample, a value of a load applied to a shaft of a machine tool, determining whether the sample is a sample detected during a cutting time period or a sample detected during a non-cutting time period, calculating a change amount of the samples during the cutting time period, excluding one or more of the change amounts, calculating a probability density function from the remaining change amounts, and determining breakage of a tool based on a probability that the excluded change amount occurs in the probability density function.

Images