Method and apparatus for predicting and controlling the quality of a resistance spot weld

• US 4,493,965 A
• Filed: 05/25/1983
• Issued: 01/15/1985
• Est. Priority Date: 05/25/1983
• Status: Expired due to Fees

• Current Assignee: General Motors Corporation (General Motors Company)
• Original Assignee: General Motors Corporation (General Motors Company)
• Inventors: Cleveland, Dixon, Nagel, Gregory L., Houchens, Albert F., Decina, Basil A. Jr., Havens, James R., Watney, Dale K.
• Primary Examiner(s): Shaw, Clifford C.
• Application Number:
  US06/498,061
• Time in Patent Office: 601 Days
• Field of Search: 219/110, 219/109, 219/108, 219/117.1
• US Class Current: 219/109
• CPC Class Codes: B23K 11/256 the measured parameter being the inter-electrode electrical resistance

Abstract

During a weld period the weld parameters are monitored and data representing the weld resistance curve and the power curve are acquired and stored for analysis by a suitably programmed computer. The derivative of R, R is calculated and stored and the function is divided by power P to obtain R/P. The resistance curve is searched to obtain the maximum value R_m which occurs during the heating phase and the function R/P is searched prior to the time of R_m to find the maximum of that curve representing the highest rate of resistance increase. Then the R/P curve is searched subsequent to its maximum to determine when the function reaches a specified percentage of the maximum. That value occurs at the knee of the resistance curve and approximates the onset of melting in the weld.

A prediction of whether a weld is a nugget or a sticker is made by calculating the ratio of weld energy after the onset of melting to the total weld energy, the ratio of the resistance drop after the resistance peak to the peak resistance, and then a weighted sum of the energy ratio and resistance drop ratio.

An edge weld geometry is detected by calculating from the resistance and power curves the weld energy during the period of expulsion, if any, divided by the cumulative energy between the onset of melting and the end of expulsion, a measure of expulsion intensity based on the maximum degree of resistance inflection, and then a weighted sum of the energy value and the inflection value.

Claims

1. A method of predicting the quality of a resistance spot weld comprising the steps of:

measuring the weld resistance and power,

determining from the resistance characteristic and the electrical power the onset of melting,

deriving from the measured power the total energy put into the weld and the energy put into the weld after the onset of melting and

assessing the degree of weld growth by determining the ratio of energy put into the weld after the onset of melting to the total energy and comparing the ratio to an empirical standard ratio representing a good weld whereby the quality of the weld is predicted as good when the measured ratio equals or exceeds the standard ratio.

2. A method of predicting the quality of a resistance spot weld comprising the steps of:

measuring the weld resistance and power,

determining from the resistance characteristic and the electrical power the onset of melting,

deriving from the measured power the total energy put into the weld and the energy put into the weld after the onset of melting,

calculating the ratio of energy put into the weld after the onset of melting to the total energy,

searching the measured resistance value to find the resistance peak and the resistance drop following the peak,

calculating the ratio of the resistance drop to the resistance peak, and

assessing the degree of weld growth by calculating a weighted sum of the energy ratio and the resistance ratio wherein the sum is a measure of weld quality.

3. A method of predicting the quality of a resistance spot weld comprising the steps of:

measuring the weld resistance and power,

determining from the resistance characteristic and the electrical power the onset of melting,

deriving from the measured power the total energy put into the weld and the energy put into the weld after the onset of melting,

calculating %E, which is the ratio of energy put into the weld after the onset of melting to the total cumulative energy,

searching the measured resistance value to find the resistance peak and the resistance drop following the peak,

calculating %R_drop, which is the ratio of the resistance drop to the resistance peak, and

determining the degree of weld growth by the model Y=A₀ +A₁ %E+A₂ %R_drop where A₀, A₁ and A₂ are empirically derived coefficients and Y is a weld quality predictor, wherein when Y is greater than zero the weld is predicted to be good.

4. A method of predicting the quality of a resistance spot weld comprising the steps of:

measuring the weld resistance and power,

determining from the resistance characteristic and the electrical power the onset of melting,

deriving from the measured power the total energy put into the weld and the energy put into the weld after the onset of melting, and

determining weld quality by calculating the amount of energy put into the weld after the onset of melting as a function of the total weld energy and comparing the calculated result to a specified value representing a good weld whereby the weld quality is predicted as good when the calculated result equals or exceeds the specified value.

5. A method of predicting the quality of a resistance spot weld made by applying a plurality of weld current pulses separated by cooling periods, comprising the steps of:

measuring the weld resistance and power,

determining from the resistance and the power the onset of melting,

deriving an energy parameter by calculating the amount of energy put into the weld after the onset of melting as a function of the total weld energy,

searching the measured resistance value to find the maximum resistance peak and any local resistance peaks occurring during subsequent current pulses,

determining the sum of the resistance drops within individual pulses following the said maximum and local peaks,

deriving a resistance drop parameter by calculating the ratio of the sum of the resistance drops to the resistance peak, and

determining weld quality by calculating a weighted sum of the energy parameter and the resistance drop parameter.

6. A method of controlling the quality of a resistance spot weld comprising the steps of:

applying weld current to a weld zone,

measuring the weld resistance and power,

determining from the resistance characteristic and the electrical power the onset of melting,

deriving from the measured power the total energy put into the weld and the energy put into the weld after the onset of melting,

assessing the degree of weld growth by determining the ratio of energy put into the weld after the onset of melting to the total energy and comparing the energy ratio to an empirical standard ratio representing a good weld whereby the quality of the weld is predicted as good when the measured ratio equals or exceeds the standard ratio, and

terminating the weld current when the energy ratio reaches a value sufficient to assure a good weld.

7. A method of controlling the quality of a resistance spot weld comprising the steps of:

applying weld current to a weld zone,

measuring the weld resistance and power,

determining from the resistance characteristic and the electrical power the onset of melting,

deriving from the measured power the total energy put into the weld and the energy put into the weld after the onset of melting,

calculating %E, which is the ratio of energy put into the weld after the onset of melting to the total cumulative energy,

searching the measured resistance value to find the resistance peak and the resistance drop following the peak,

calculating %R_drop, which is the ratio of the resistance drop to the resistance peak,

determining the degree of weld growth by the model Y=A₀ +A₁ %E+A₂ %R_drop where A₀, A₁ and A₂ are empirically derived coefficients and Y is a weld quality predictor, wherein when Y is greater than zero the weld is predicted to be good, and

terminating the weld current when the weld quality predictor Y reaches a determined value greater than zero, whereby it is assured that sufficient energy is applied to produce a good weld.

8. Weld monitoring apparatus for determining whether a resistance spot weld is good, comprising:

means for acquiring data representing the weld resistance and power curves; and

digital computer means for storing the acquired data, said computer means being programmed to;

(a) determine from the resistance characteristic and the electrical power the onset of melting,

(b) derive from the measured power the total energy put into the weld and the energy put into the weld after the onset of melting, and

(c) assess the degree of weld growth by determining the ratio of energy put into the weld after the onset of melting to the total energy and comparing the ratio to an empirical standard ratio representing a good weld whereby the quality of the weld is predicted as good when the measured ratio equals or exceeds the standard ratio.

9. Weld monitoring apparatus for determining whether a resistance spot weld is good, comprising:

means for acquiring data representing the weld resistance and power curves; and

digital computer means for storing the acquired data, said computer means being programmed to;

(a) determine from the resistance characteristic and the electrical power the onset of melting,

(b) derive from the measured power the total energy put into the weld and the energy put into the weld after the onset of melting,

(c) calculate the ratio of energy put into the weld after the onset of melting to the total cumulative energy,

(d) search the measured resistance value to find the resistance peak and the resistance drop following the peak,

(e) calculate the ratio of the resistance drop to the resistance peak, and

(f) assess the degree of weld growth by calculating a weighted sum of the energy ratio and the resistance ratio wherein the sum is a measure of weld quality.

10. Weld monitoring apparatus for determining whether a resistance spot weld is good, comprising:

means for acquiring data representing the weld resistance and power curves, and

digital computer means for storing the acquired data, said computer means being programmed to;

(a) determine from the resistance characteristic and the electrical power the onset of melting,

(b) derive from the measured power the total energy put into the weld and the energy put into the weld after the onset of melting,

(c) calculate %E, which is the ratio of energy put into the weld after the onset of melting to the total cumulative energy,

(d) search the measured resistance value to find the resistance peak and the resistance drop following the peak,

(e) calculate %R_drop, which is the ratio of the resistance drop to the resistance peak, and

(f) determine the degree of weld growth by the model Y=A₀ +A₁ %E+A₂ %R_drop where A₀, A₁ and A₂ are empirically derived coefficients and Y is a weld quality predictor, wherein when Y is greater than zero the weld is predicted to be good.

11. Resistance spot weld apparatus for controlling weld quality comprising:

means for applying weld current to a weld zone,

means for acquiring data representing the weld resistance and power curves, and

digital computer means for storing the acquired data, said computer means being programmed to;

(a) determine from the resistance characteristic and the electrical power the onset of melting,

(b) derive from the measured power the total energy put into the weld and the energy put into the weld after the onset of melting,

(c) calculate the ratio of energy put into the weld after the onset of melting to the total cumulative energy,

(d) search the measured resistance value to find the resistance peak and the resistance drop following the peak,

(e) calculate the ratio of the resistance drop to the resistance peak,

(f) assess the degree of weld growth by calculating a weighted sum of the energy ratio and the resistance ratio wherein the sum is a measure of weld quality, and

(g) issue a signal to the weld current applying means to terminate the weld current when the weighted sum reaches a determined value assuring that a good weld has been made.

12. Resistance spot weld apparatus for controlling weld quality, comprising:

means for applying weld current to a weld zone,

means for acquiring data representing the weld resistance and power curves, and

digital computer means for storing the acquired data, said computer means being programmed to;

(a) determine from the resistance characteristic and the electrical power the onset of melting,

(b) derive from the measured power the total energy put into the weld and the energy put into the weld after the onset of melting,

(c) calculate %E, which is the ratio of energy put into the weld after the onset of melting to the total cumulative energy,

(d) search the measured resistance value to find the resistance peak and the resistance drop following the peak,

(e) calculate %R_drop, which is the ratio of the resistance drop to the resistance peak,

(f) determine the degree of weld growth by the model Y=A₀ +A₁ %E+A₂ %R_drop where A₀, A₁ and A₂ are empirically derived coefficients and Y is a weld quality predictor, wherein when Y is greater than zero the weld is predicted to be good, and

(g) issue a signal to the current applying means to terminate the weld current when the weld quality predictor Y reaches a determined value greater than zero, whereby it is assured that sufficient weld energy is applied to produce a good weld.