Acoustic tool break detection system and method
First Claim
1. An acoustic monitoring system to detect cutting tool break events while machining a workpiece on a machine tool comprising:
- a broadband vibration sensor which generates an electrical signal representing vibrations at the tool-workpiece interface;
an analog preprocessor including means to discriminate against lower frequency machinery noise and to detect the signal energy in an acoustic frequency band below 100 KHz;
means for sampling the output signal of said analog preprocessor and converting each sample to a digital value;
digital pattern recognition circuitry to detect tool break events that produce gradually occurring, substantial changes in cutting conditions, comprising means for calculating the running mean signal level of N samples, means for determining when the cutting noise and mean signal level drops below a lower break detection threshold which is just above the continuous traverse noise level of said machine tool, and means for generating a tool break alarm after counting a preset number of low level cutting noise samples below the lower threshold.
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Abstract
Substantial cutting condition changes that occur gradually, as opposed to the more usual sudden large change, are detected by setting upper and lower cutting noise mean level thresholds. When the mean cutting noise exceeds the upper threshold or stays below the lower threshold for a preset number of signal samples, a tool break alarm is generated. Techniques are given to reduce false alarms at the start and end of the cut and on runout on initial rough surface cuts. The system comprises an accelerometer or other sensor whose signal is preprocessed to attenuate lower frequency machinery noise and detect the signal energy in a band below 100 KHz, then sampled, and the digitized signal samples analyzed by pattern recognition logic.
41 Citations
10 Claims
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1. An acoustic monitoring system to detect cutting tool break events while machining a workpiece on a machine tool comprising:
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a broadband vibration sensor which generates an electrical signal representing vibrations at the tool-workpiece interface; an analog preprocessor including means to discriminate against lower frequency machinery noise and to detect the signal energy in an acoustic frequency band below 100 KHz; means for sampling the output signal of said analog preprocessor and converting each sample to a digital value; digital pattern recognition circuitry to detect tool break events that produce gradually occurring, substantial changes in cutting conditions, comprising means for calculating the running mean signal level of N samples, means for determining when the cutting noise and mean signal level drops below a lower break detection threshold which is just above the continuous traverse noise level of said machine tool, and means for generating a tool break alarm after counting a preset number of low level cutting noise samples below the lower threshold. - View Dependent Claims (2, 3, 4, 5)
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6. An acoustic monitoring system to detect cutting tool break events while machining a workpiece comprising:
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a broadband accelerometer which is most sensitive to frequencies around a resonant frequency and is positioned on a machine tool to sense vibrations at the tool-workpiece interface and convert these and other vibrations to an electrical signal; analog signal processing means including means for high-pass filtering said vibration signal to discriminate against lower frequency machinery noise, and means to rectify and low-pass filter said signal to detect the signal energy in an acoustic frequency band between 30 KHz and 100 KHz; means for sampling the unipolar output signal of said analog processing means and converting each sample to digital form; a digital processor to detect major cutting condition changes occurring gradually over a period of time that can damage the workpiece or machine tool; said digital processor comprising means for calculating the running mean signal level of a selected number of samples, means for activating the lower threshold test of a threshold crossing detector after the mean signal level rises above a much higher flag-set threshold and remains there for a preset number of consecutive signal samples, said detector determining when the mean signal level crosses lower or upper break detection thresholds where the former is just above the continuous traverse noise level of said machine tool and the latter is just below the system saturation level, and means for testing that the mean signal level persistently remains outside of said thresholds, and generating a tool break alarm. - View Dependent Claims (7)
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8. The method of acoustically detecting tool breakage capable of damaging the workpiece or machine tool comprising:
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sensing vibrations at the tool-workpiece interface and converting these and other machine tool vibrations to an electrical signal; preprocessing said vibration signal to discriminate against lower frequency machinery noise and rectify and low-pass filter said signal to detect the signal energy in an acoustic frequency band below 100 KHz; sampling the processed vibration signal and converting each sample to digital form; and analyzing said signal samples to detect gradually occurring, but substantial, cutting noise changes due to tool break events capable of damage, comprising setting upper and lower cutting noise thresholds, determining when the running mean signal level of N samples drops below said lower threshold, which is just above the continuous traverse noise level of said machine tool, and generating a tool break alarm after a preset number of consecutive signal samples and the cutting noise are outside of said thresholds. - View Dependent Claims (9, 10)
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Specification