MACHINE PROTECTION SYSTEM

• US 3,836,834 A
• Filed: 11/13/1973
• Issued: 09/17/1974
• Est. Priority Date: 11/13/1973
• Status: Expired due to Term

• Current Assignee: Leonard A. Abbatiello, Paul C. Turner, Robert E. Hewgley Jr
• Original Assignee: Leonard A. Abbatiello, Paul C. Turner, Robert E. Hewgley Jr
• Inventors: Abbatiello, Leonard A., Hewgley, Robert E. Jr., Turner, Paul C.
• Primary Examiner(s): Dobeck, B.
• Application Number:
  US05/415,460
• Time in Patent Office: 308 Days
• Field of Search: 318/488,39,563,571 235/151.11
• US Class Current: 318/563
• CPC Class Codes: G05B 19/4065 Monitoring tool breakage, life or condition
• CPC Class Codes: G05B 2219/37245 Breakage tool, failure

Abstract

A protection system has been provided for preventing damage to a numerically controlled machining device in the event of a wreck. The protection system consists of a strain-gage-instrumented tool holder or tool holder support; amplifying; filtering; and comparing circuitry; and quick acting machine shutdown circuitry. The instrumented tool holder senses the cutting and side forces by means of strain gages mounted on the tool holder. The system initiates a shutdown whenever (1) an instantaneous force exceeds a preselected maximum (absolute failure), or (2) an average force suddenly changes its level by a preselected factor (dynamic error). Although the system does not prevent a wreck, the effects of the wreck are restricted to limited part damage rather than damage to the machine.

Claims

1. In combination with a numerical controlled machining system wherein a cutting tool carried by a tool holder which is automatically positioned by means of the application of drive commands to at least one prime mover, a protection system, comprising:

a cutting force sensing means mounted with said tool holder for sensing forces induced in said holder from the cutting force applied to said cutting tool and providing an electrical signal at an output proportional to said cutting force;

a feed force sensing means mounted with said tool holder for sensing forces induced in said holder from the feed force applied to said cutting tool and providing an electrical signal at an output proportional to said feed force;

a feed force comparator means connected to receive the feed force signal for comparing said feed force with a feed force reference signal level and producing a signal at an output thereof when said feed force signal exceeds said feed force reference signal level;

a summing amplifier having a summing input and an output, said summing input connected to receive the cutting force signal from said cutting force sensing means and the output of said feed force comparator to provide an instantaneous output signal normally proportional to said cutting force;

a filter connected to the output of said summing amplifier for averaging the cutting force signal output of said summing amplifier over a time period selected by the time constant of said filter;

a dynamic force comparator means responsive to the averaged signal output of said filter and the instantaneous signal output of said summing amplifier for producing a shutdown signal at an output thereof when said instantaneous signal level exceeds a preselected multiple of said signal level;

an absolute force comparator means connected to the output of said summing amplifier for comparing the signal level at the output of said summing amplifier with a maximum absolute force reference signal level and providing a shutdown signal at an output thereof when the absolute force signal level exceeds said maximum absolute force reference signal level; and

an output circuit means responsive to a shutdown signal from said dynamic force comparator means or said absolute force comparator means for interrupting the drive commands to said prime mover.

2. The combination as set forth in claim 1 wherein said dynamic force comparator means includes a comparator amplifier having one input connected to the output of said filter, an instantaneous force signal amplifier connected to the output of said summing amplifier for providing an amplifier signal at an output thereof proportional to the instantaneous force applied to said cutting tool, and a variable resistance means connected between the output of said instantaneous force signal amplifier and a second input of said comparator for selecting the level at which said dynamic force comparator triggers to produce a shutdown signal.

3. The combination as set forth in claim 2 wherein said output circuit means includes a shutdown control circuit means coupled to said prime mover for interrupting said drive commands when a shutdown signal is applied to an input thereof, an OR gate having a first and second input aNd an output, said first input being connected to the output of said absolute force comparator means;

an AND gate having first and second inputs and an output, said output connected to said second input of said OR gate and said first input connected to the output of said dynamic force comparator means; and

a logic circuit means connected between the output of said filter and said second input of said AND gate for disabling said AND gate to block the passage of shutdown signals when said averaged cutting force signal at the output of said filter is at a zero level.

4. The combination as set forth in claim 1 wherein said feed force sensing means and said feed force comparator means comprise a first strain gage bridge network including strain gages mounted on opposite surfaces of said tool holder to sense the magnitude and direction of forces induced in said tool holder from feed forces applied to said cutting tool so that said bridge network produces a feed force output signal having a magnitude and a polarity corresponding to the direction of said feed force, a first comparator coupled to said bridge network and having a negative reference voltage level for producing an output signal to said summing amplifier when said feed force signal exceeds said negative reference voltage level indicating an excessive feed force in one direction and a second comparator coupled to said bridge network and having a positive reference voltage level for producing an output signal to said summing amplifier when said feed force signal exceeds said positive reference voltage level indicating an excessive feed force in the direction opposite said one direction.

5. The combination as set forth in claim 4 wherein said cutting force sensing means comprises a second strain gage bridge network including further strain gages mounted on opposite surfaces of said tool holder at right angles to the surfaces on which said strain gages are mounted to sense said feed forces, said second bridge network having an output coupled to the input of said summing amplifier.