Detection by automatic gain control features of gradual cutting tool breakage

• US 4,806,914 A
• Filed: 05/07/1987
• Issued: 02/21/1989
• Est. Priority Date: 05/07/1987
• Status: Expired due to Fees

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Bedard, James F., Wildes, Douglas G., Thomas, Charles E., Harris, Lawson P., Hayashi, Steven R.
• Primary Examiner(s): Orsino, Joseph A.
• Assistant Examiner(s): Hofsass, Jeffery A.
• Application Number:
  US07/046,812
• Time in Patent Office: 656 Days
• Field of Search: 340/679, 340/680, 340/683, 73/104, 73/593, 73/658, 73/660, 364/474, 364/475, 364/508, 330/284
• US Class Current: 340/680
• CPC Class Codes: B23Q 17/0957 Detection of tool breakage detecting failure of drills B23B49/001
• CPC Class Codes: B23Q 17/0971 by measuring mechanical vibrations of parts of the machine arrangements for measuring vibrations B23Q17/12
• CPC Class Codes: G01H 1/16 Amplitude
• CPC Class Codes: G05B 19/4065 Monitoring tool breakage, life or condition
• CPC Class Codes: G05B 2219/37351 Detect vibration, ultrasound

Abstract

A tool break detection system has an automatic gain control to adjust the analog signal channel gain and hold the cutting vibration signal at a desired average level. The AGC time constant is long enough that the detection of abrupt tool breaks and sudden large signal level transistions by the digital signature recognition logic is unaffected. However, the gradual decrease in signal level produced by a crumbly-type break would be removed by AGC action. In a system with a hardware AGC, crossing a high gain threshold resets gain to a low value and the resulting abrupt and persisting change in signal level is detected by the abrupt tool break logic. Another embodiment uses the gain command output by a software AGC and generates a break detected signal directly, without resetting the gain command, as gain rises above a high gain alarm level which is recalculated at the start of each cut.

Claims

1. A machine tool monitor to detect both abrupt and gradual cutting tool breakage comprising:

a vibration sensor to provide a signal representative of vibrations generated by the interaction of a tool and workpiece during the cutting process;

an analog signal processing channel which conditions the cutting vibration signal to a unipolar output voltage in a restricted frequency range and has an attenuator to control gain;

a digital processor comprised of break detection logic to recognize an abrupt tool break vibration signature having a rapid large change in signal level that persists for a given confirmation period, and generate an alarm;

an automatic gain control (AGC) circuit to dynamically adjust the gain of said attenuator and hold said analog output voltage at a chosen average level, with a time constant long enough that said abrupt tool break signature is detected;

means to detect a high gain which results through action of said AGC as a gradually occurring tool break causes the input vibration signal level to slowly decrease; and

means to reset the gain to a low value, resulting in a rapid and persisting change in signal level that is detected by said break detection logic and generates an alarm.

2. The monitor of claim 1 wherein said high gain to trigger reset is selectable and depends on cutting conditions.

3. The monitor of claim 2 wherein said high gain is a given factor times nominal full-cut gain.

4. The monitor of claim 1 wherein said AGC circuit has means to preset gain if said channel output is below a threshold, and said reset means rapidly changes gain to the preset gain.

5. The monitor of claim 1 wherein said attenuator is a digitally programmable multiplying digital-to-analog converter which has binary inputs set by the output of an up/down counter, and said reset means monitors the counter and causes said counter to output a preset gain.

6. A machine tool monitor to detect both abrupt and gradual cutting tool breakage comprising:

a vibration sensor to provide a signal representative of vibrations generated by the interaction of a tool and workpiece during the cutting process;

an analog signal processing channel which conditions the cutting vibration signal to a unipolar output voltage in a restricted frequency range and has a digitally programmable attenuator to control channel gain;

a digital processor to sample and digitize said analog output voltage and analyze signal samples in break detection logic to recognize an abrupt tool break vibration signature having a rapid change in signal level that persists for a given confirmation period, and generate an alarm;

said digital processor further comprising automatic gain control logic to determine a gain command for said attenuator to hold said channel output voltage at a chosen average level, with an AGC time constant long enough that said abrupt tool break signature is detected; and

gradual break logic to monitor said gain command and issue a tool break alarm as said gain rises and crosses a high gain threshold due to a crumbly-type tool break that causes said cutting vibration signal to slowly decrease.

7. The monitor of claim 6 wherein said high gain threshold is user-selectable and depends on cutting conditions.

8. The monitor of claim 7 wherein said high gain threshold is a given factor times nominal full-cut gain.

9. The monitor of claim 6 wherein said gradual break logic is comprised of means to compute said high gain threshold during each cut by capturing the current gain during the cut after gain has stabilized and multiplying by a given ratio.

10. The monitor of claim 9 wherein said current gain is captured at a selected time after said automatic gain control is enabled.

11. The monitor of claim 10 wherein said selected capture time is a multiple of said AGC time constant.

12. The monitor of claim 9 further comprising means to disable, at the end of every cut, comparison of current gain with the computed high gain threshold.

13. The method of detecting cutting tool breakage by monitoring vibrations at the tool-workpiece interface, comprising:

generating a vibration signal and preprocessing the same in an analog signal channel which has a digitally programmable variable gain device to control channel gain;

digitally processing the analog channel output in break detection logic to search for signal patterns indicative of a major tool break and generate an alarm;

automatically controlling the channel gain to hold the analog channel output at a chosen average level, with an AGC (automatic gain control) time constant long enough that abrupt and persisting signal level changes due to tool fracture events are unaffected and trigger said alarm; and

detecting a crumbly tool break that causes said vibration signal to slowly decrease by detecting a preselected high channel gain as AGC action increases the gain.

14. The method of claim 13 wherein a gain command presented to said variable gain device is monitored and a high gain crumbly break alarm is issued upon crossing a user-set high gain threshold.

15. The method of claim 14 wherein said user-set threshold depends on cutting conditions.

16. The method of detecting cutting tool breakage by monitoring vibrations at the tool-workpiece interface, comprising:

generating a vibration signal and preprocessing the same in an analog signal channel which has a digitally programmable variable gain device to control channel gain;

digitally processing the analog channel output in break detection logic to search for signal patterns indicative of a major tool break and generate an abrupt break alarm;

providing an automatic gain control (AGC) system to adjust the gain of said variable gain device and hold the analog channel output at a chosen average level, with an AGC time constant long enough that abrupt and persisting signal level changes due to tool fracture events are unaffected; and

detecting a crumbly tool break that causes said vibration signal to slowly decrease by calculating a high gain alarm level during each cut and generating a crumbly break alarm as AGC action causes the gain to increase and cross said alarm level.

17. The method of claim 16 wherein current gain is captured at a predetermined time after start of the cut and multiplied by a given ratio to calculate said high gain alarm level, and a high gain test to compare gain with said alarm level is enabled.

18. The method of claim 17 wherein said predetermined time is considerably longer than said AGC time constant.

19. The method of claim 17 wherein said high gain test is disabled at a given time after end of the cut.

20. The method of claim 19 wherein said time at which current gain is captured is measured from rise of said analog channel output above a signal level at which said AGC system is enabled, and said high gain test is disabled at a time measured from fall of said analog channel output below the AGC enable signal level.