SENSORLESS PRESSURE CHANGE DETECTION FOR SERVO GUN

• US 20180029155A1
• Filed: 07/29/2016
• Published: 02/01/2018
• Est. Priority Date: 07/29/2016
• Status: Active Grant

• Current Assignee: Fanuc America Corporation (Fanuc Corporation)
• Original Assignee: Fanuc America Corporation (Fanuc Corporation)
• Inventors: Garza, Frank
• Granted Patent: US 10,639,739 B2
• Time in Patent Office: Days
• CPC Class Codes: B23K 11/11 Spot welding
• CPC Class Codes: B23K 11/115 by means of two electrodes placed opposite one another on both sides of the welded parts
• CPC Class Codes: B23K 11/255 the measured parameter being a force B23K11/253 takes precedence
• CPC Class Codes: B23K 31/125 Weld quality monitoring
• CPC Class Codes: G01L 25/003 for measuring torque
• CPC Class Codes: G01L 5/0066 Calibration arrangements
• CPC Class Codes: G01L 5/0076 Force sensors associated with manufacturing machines G01L5/0066, G01L5/0071 and B23Q17/09 take precedence; for the specific machine or operation involved see relevant class, e.g. B21 - B42

Abstract

A method and an apparatus detects pressure changes at servo gun tips of a robotic welding system having a servo gun with a movable tip and an opposed fixed tip configured to weld a part. The method and apparatus observe a tip deflection value and convert the value to a current pressure value using a pressure estimator. The current pressure value is compared to a benchmark pressure value to detect any difference.

Claims

1. A method for detecting pressure changes at servo gun tips of a robotic welding system having a servo gun with a movable tip and an opposed fixed tip configured to weld a part comprising the steps of:

storing benchmark values for a spring constant, a pressure estimator, an efficiency and an inertia/friction during noload operation;

operating the servo gun to apply a first calibrated force at the gun tips and measuring a resulting first deflection value at the gun tips;

operating the servo gun to apply a second calibrated force at the gun tips and measuring a resulting second deflection value at the gun tips;

using the measured first and second deflection values and the pressure estimator to calculate current values for the spring constant, a first gun tip pressure, a second gun tip pressure, the efficiency and the inertia/friction during noload operation;

comparing each of the current values with a corresponding one of the benchmark values; and

generating an error indication for any of the current values that differs from the corresponding benchmark value by a predetermined amount.

2. The method according to claim 1 including selecting a standard force value;

calculating a motor torque command based upon the standard force value and a motor torque constant for a motor to actuate the servo gun with a corresponding calibrated torque value;

determining a calculated force value from the calibrated torque value and a pressure calibration table.

3. The method according to claim 2 including forming the pressure estimator by plotting two points of calibrated pressure values and corresponding measured deflection values of the gun tips, drawing a line between the points, and converting the line to an equation for determining a new calibrated force value from a new measured deflection value.

4. The method according to claim 1 including if the spring constant current value is different from the spring constant benchmark value generating an error indication.

5. The method according to claim 1 including if the first pressure current value is different from the first pressure benchmark value generating an error indication.

6. The method according to claim 5 including only generating the error indication if the difference is greater than ten percent.

7. The method according to claim 1 including if the second pressure current value is different from the second pressure benchmark value generating an error indication.

8. The method according to claim 7 including only generating the error indication if the difference is greater than ten percent.

9. The method according to claim 1 including if the efficiency current value is different from the efficiency benchmark value generating an error indication.

10. The method according to claim 9 including only generating the error indication if the difference is less than or greater than a predetermined range of a ratio of the efficiency current value to the efficiency benchmark value.

11. The method according to claim 10 wherein the range is 65% to 135%.

12. The method according to claim 1 including only generating the error indication if the Inertia/friction current value is different from the inertia/friction benchmark value.

13. The method according to claim 12 including only generating the error indication if the difference is greater than thirty percent.

14. The method according to claim 1 including performing tip wear measurements on the gun tips and only performing the steps if the tip wear measurements meet predetermined wear schedule conditions.

15. The method according to claim 1 including generating the error indication from a computer controlling the servo gun in response to any of the current values differing from a corresponding one of the benchmark values.

16. An apparatus for detecting pressure changes in a robotic welding system comprising:

a servo gun with a movable tip opposed by a fixed tip;

a motor coupled to the movable tip for moving the movable tip toward and away from the fixed tip;

a computer connected to the motor for actuating the motor to move the movable tip; and

a pressure test computer program executed by the computer causing

the motor to move the movable tip to apply a first pressure at the fixed tip,

the motor then move the movable tip to apply a second pressure at the fixed tip,

the computer to store tip deflection values for the first and second pressures,

the computer to calculate and store values for a spring constant, a pressure force estimator, efficiency and inertia/friction during noload operation from the stored deflection values, and

if the pressure test is a first pressure test, designate the stored values as benchmark values.

17. The apparatus according to claim 15 wherein if the pressure test is not the first pressure test, designate the stored values as current values.

18. The apparatus according to claim 17 wherein the computer executes the pressure test computer program to generate an error indication in response to any of the current values differing from a corresponding one of the benchmark values.

19. A method for detecting pressure changes at servo gun tips of a robotic welding system having a servo gun with a movable tip and an opposed fixed tip configured to weld a part comprising the steps of:

closing the movable tip against the fixed tip to apply a pressure at the tips;

measure a deflection value of the fixed tip from a reference point corresponding to zero pressure at the tips;

converting the deflection value to a calibrated pressure value using a pressure estimator;

comparing the calibrated pressure value to a benchmark pressure value; and

generating an error indication if a difference between the compared values exceeds a predetermined amount.

20. The method according to claim 19 including generating the pressure estimator by plotting two points of calibrated pressure values and corresponding measured deflection values of the tips, drawing a line between the points, and converting the line to an equation for determining a new calibrated force value from a new measured deflection value.