Electric arc welder and method for controlling the welding process of the welder

An electric arc welder for performing a given weld process with a selected A.C. pulse current waveform performed between an electrode and a workpiece, where the current waveform includes a positive segment and a negative segment, with at least one segment including a peak current and background current. The welder comprises: a power source with a controller having a digital processor including a program to calculate the real time power factor of the weld current and weld voltage where the program includes an algorithm to calculate the rms weld voltage, the rms weld current and the average power of said power source; a circuit to multiply the rms current by the rms voltage to produce an rms power level; a circuit to divide the average power by the rms power to create a value representing the actual real time power factor of said power source; and, a circuit to adjust said background current to maintain said power factor at a given level, which is manually adjusted to set the heat of the weld.

The present invention relates to the field of electric arc welding and more particularly to a novel electric arc welder and a system and method for controlling the welding process performed by the welder.

Incorporation by Reference

The invention relates to an electric arc welder for performing a welding process between an electrode and a workpiece wherein the welding process is comprised of a succession of current waveforms. Such current waveforms are created by a number of individual current pulses occurring at a frequency of at least 18 kHz with a magnitude of each of the current pulses being controlled by a wave shaper or waveform generator. In this type of electric arc welder, the waveform generator or wave shaper digitally controls a digital pulse width modulator, usually a program in the controller DSP. The pulse width modulator controls the switching of a high speed switching type power source, such as an inverter. This waveform control technology implemented in an electric arc welder has been pioneered by The Lincoln Electric Company of Cleveland, Ohio and is generally disclosed in Blankenship U.S. Pat. No. 5,278,390. The Blankenship patent is incorporated by reference herein as background illustrating a high speed switching power source, such as a inverter, for controlling a weld process including a series of controlled waveforms determined by the output of a waveform generator or wave shaper.

The invention involves an embedded algorithm for obtaining the root mean square of either the welding current or the welding voltage, as well as average power. The concept of an embedded system programming of the type used in the present invention is generally disclosed in an article by Jack W. Crinshaw entitledEmbedded Systems Programming(Integer Square Root) This article published in February 1998 is incorporated by reference herein as illustrating the background technology used in the digital signal programmer of a standard controller associated with an electric arc welder. Also incorporated by reference herein is an article entitledElectrical Measurements and Heat Input Calculations for GMAW-P Processdated November 2001.

BACKGROUND OF THE INVENTION

As illustrated in prior patents and literature, electric arc welding has heretofore used the average weld voltage and the average weld current for controlling the operation of the power source in the welder. The digital controller includes a digital signal processor (DSP) for controlling a waveform generator or wave shaper that directs the operation of the normal pulse width modulator. This device creates the waveforms successively used by the welder to perform the welding process. Welders regulate the output current or voltage to an average value such as an average weld current by a feedback loop. For a constant voltage process that is welding in the “spray” region, the average current is an accurate gage of the welding process. However, in pulse welding, the average current and average voltage do not accurately reflect the result of the welding process including the deposition rate, heat zone and penetration. This is explained by a example of an ideal pulse welding process, such as one having 500 amperes for 25% of the time and 100 amperes of background current for 75% of the time has an output current of 200 amperes. However, the average current of the welding process merely indicates the deposition rate and does not reflect the true heat input to the welding operation. Consequently, when the welding process is controlled by a series of repetitive waveforms, such as A.C. welding or pulse welding, average current values can not control the heat input. Recently, the welding processes have become quite complex and now often involve a number of successive waveforms, such as A.C. current and pulse current, so the old technology of feedback control for the welding process is not completely accurate and requires a substantial amount of on-site manipulation by a person knowledgeable in welding, especially a person knowledgeable in the new waveform welding procedure using a welder, such as shown in Blankenship U.S. Pat. No. 5,278,390. With the advent of pulse welding using waveform generators and high speed switching power sources, such as inverters, the obtained weld heat has been adjusted by trial and error. Too much heat causes metal to burn through, especially in thin metal welding. Thus, the welding engineer modulates the average current and average voltage to provide the heat input to the welding process to a level so that burn through is theoretically eliminated. This procedure was applicable, however, only for a pure spray type welding process. This procedure of controlling the heat by the average current and average voltage was not applicable to the new generation of electric arc welders where waveforms are changed to control the welding process. This is the new waveform control technology to which the present invention is directed. The old technology used for non-waveform welding is inapplicable to controlling heat in a controlled waveform type welder. The heat is not known by merely reading the voltage and current when the new waveform type arc welders are employed. Consequently, the welding engineer when using waveform control technology changed the base frequency during pulse welding while maintaining a constant or set average voltage. Using this approach of frequency adjustment of a pulse welding procedure while maintaining a constant voltage, the heat could be adjusted by a trial and error technique. When this trial and error procedure was used to modify the waveforms in a new waveform welder, the heat could, indeed, be controlled; however, it was not precise and involves substantial technical knowledge combined with the trial and error procedures.

There is a distinct advantage in pulse welding. This welding process lowers the heat into the joint for the same wire feed speed as a “spray” or “globular” weld process. Thus, a lower heat setting can be set at the factory. The welder had a knob to adjust the nominal frequency, for the purpose indicated above. This change in base frequency did adjust the heat at the welding operation. This resulted in a slight change in the power factor of the welding process through the trial and error method when knowing that the average voltage times average current multiplied by the power factor equals the input heat. Thus, by using a knob to change the base frequency, the power factor was changed to determine heat. However, neither the factory nor the welding engineer at the welding site had the capabilities of directly controlling the power factor. Computation of actual power factor on the fly was not realized in prior control systems and method used for electric arc welders even of the type that used a waveform or wave shape control of the welding process. Consequently, with the introduction of the new waveform welding pioneered by The Lincoln Electric Company, there is a need to control the welding parameters to a value that accurately reflects the heat content. Only in this manner can weld parameters be used in a closed loop feedback system, or otherwise, to control the penetration and heat separately in a weld process using generated waveforms.

SUMMARY OF THE INVENTION

With the advent of the new wave shapes developed for electric arc welding, the present invention provides a control of the welding parameters to accurately reflect the heating content without use of trial and error procedures or the need for on site welding engineers to modulate and control the welding process. The invention is in welding with a series of generated waveforms, such as A.C. welding or A.C. welding.

In order to produce a stable weld while continuously feeding wire into the weld puddle, there are primarily two factors that must be balanced. First, the amount of weld metal wire and its material properties determine how much current is needed to melt the wire. Second, the amount of heat determines the heat affected zone or penetration of the welding process. In the past, an operator dialed in a voltage and wire feed speed and manually adjusted the electric stickout to control the amount of heat put into the weld. Welding literature typically claims that the pulse welding process lowers the current for the same deposition rate of a “spray” procedure. This is technically accurate. The average current is, indeed, much less than the average current of an equivalent “spray” procedure when using “pulse” welding. However, the rms currents of both procedures are about the same. The present invention involves the use of rms current for the feedback loop control of the welding process. Thus, the invention involves the use of rms current and rms voltage for controlling the welding process, especially when using a series of generated pulse waves, such as in A.C. welding and “pulse” welding using the technology described in Blankenship U.S. Pat. No. 5,278,390. By using the rms current and rms voltage, a more accurate control of the waveform type welding process is maintained. In accordance with the invention, the rms value and the average value of current and voltage can be used for feedback control. In this aspect of the invention, a first constant is multiplied by the rms value and a second constant is multiplied by the average value of the parameter. These two constants total one, so the constituent of root mean square in the feedback control is adjusted with respect to the constituent of average in the feedback control. These constants preferably total one. In practice, the rms constant is substantially greater than the average value constant so that normally the rms value is predominate over the average value. It has been found that the rms value more accurately reflects the heating value of the welding process.

In accordance with the invention, the feedback control of the electric arc welder maintains the rms voltage and rms currents constant, while adjusting the calculated real time power factor. This procedure of adjusting the power factor adjusts the heat input to the weld procedure to a desired level.

In the present invention the term “power factor” relates to the power factor of the welding process. This is a parameter obtained by using the present invention through the digital signal processor (DSP) of a welder having an embedded algorithm for calculating the root mean square of both current and voltage. The actual power factor is generated for a closed loop feedback system so that the welding power factor is adjusted to change the average power and, thus, the heat of the welding operation. Consequently, another aspect of the invention is maintaining the rms current constant while adjusting the power factor to change the heat at the welding process. When this is done in a waveform type welder wherein the waveform is created by a number of current pulses occurring at a frequency of at least 18 kHz with a magnitude of each pulse controlled by a wave shaper, the shape of the waveform in the welding process is modified to adjust the power factor. In this aspect of the invention, the current remains constant. This could not be accomplished in other types of welders, nor in waveform control welders, without use of the present invention.

The present invention relates to a control of an electric arc welder of the type wherein a pulse width modulator, normally in the DSP, controls the current waveform constituting the welding process. By using the present invention, the rms current and rms voltage is obtained for the purpose of combining with the average current and average voltage to produce, not only the average power, but also the actual real time power factor. Consequently, the actual power factor can be adjusted, the actual rms current can be adjusted, or the actual rms voltage can be adjusted. In all of these embodiments, the adjustment of the constructed or calculated parameters modifies the waveform to control the welding process accurately in the areas of penetration and heat input. By having the capabilities of the present invention, power factor manipulation adjusts the heat input of the welding process. In accordance with an aspect of the invention, the feedback of current and voltage is a combination of the rms value and the average value in a method or system where the rms value predominates.

The primary aspect of the present invention is the use of the novel control arrangement in an A.C. pulse welding process using waveform technology involving a wave shaper controlling a pulse width modulator. This type of welding process includes waveform with a positive segment and a negative segment wherein one of the segments has a background current which is lower than the peak current. This pulse is, thus, truncated with a peak current portion normally having a leading edge and trailing edge and a magnitude and a background current with a magnitude and length. A circuit to adjust either the background current or the peak current portion of the pulse is employed to maintain the power factor at a given level. Preferably, the background current magnitude or length is adjusted to maintain the given power factor level. The “given level” is adjusted to change the heat of the welding process. Consequently, the A.C. pulse welding process to which the invention is particularly applicable utilizes an adjustment of the background current portion to change the power factor and, thus, control the heat of the welding process.

The invention is primarily applicable for use in an electric arc welder of the type having a pulse shaper or waveform generator to control the shape of the waveform in the welding process. This type of welder has a digitized internal program functioning as a pulse width modulator wherein the current waveform is controlled by the waveform generator or wave shaper as a series of current pulses. The duty cycle of these high speed pulses determines the magnitude of the current at any given position in the constructed waveform of the weld process. This type of welder has a high speed switching power source, such as an inverter. The invention involves the combination of this particular type of power source and implementation of the program and algorithm to form the functions set forth above.

In accordance with the invention, there is provided an electric arc welder for performing a given weld process with a selected waveform performed between an electrode and a workpiece. This type of welder generates the waveforms and includes a controller with a digital signal processor. The sensor reads the instantaneous weld current and a circuit converts the instantaneous current into a digital representation of the level of the instantaneous current. The digital processor has a program circuit or other program routine to periodically read and square the digital representation at a given rate. A register in the processor sums a number of squared digital representations to create a summed value. An embedded algorithm in the processor periodically divides the summed value by a number N, which is the number of samples obtained during the sampling process of the waveform. The quotient provided by dividing the summed value by the number of samples is then directed to the algorithm for taking the square root of the quotient to thereby digitally construct an rms signal representing the root mean square of the weld current. This same procedure is used for obtaining the root mean square or rms signal representing the weld voltage. Consequently, the initial aspect of the invention is the use in a waveform welder, a real time signal indicative of the root mean square of the weld current primarily, but also the weld voltage. These signals have not heretofore been obtainable in an arc welder of the type to which the present invention is directed.

As previously stated, the present invention is directed to an electric arc welder of a specific type wherein a waveform is generated by a waveform generator or wave shaper. Consequently, another aspect of the present invention is the provision of an electric arc welder as defined above wherein the waveform is created by a number of current pulses occurring at a frequency of at least 18 kHz, with a magnitude of each pulse controlled by a wave shaper or waveform generator. The “switching frequency” is the frequency of the pulse width modulator controlling the switching frequency of the power source. This frequency is normally substantially greater than 18 kHz and preferably in the range of 40 kHz.

The invention, as defined above, has a sampling rate for the sensed current and/or voltage. In accordance with another aspect of the present invention, this sampling rate is less than 40 kHz or in another aspect it is in the general range of 5 kHz to 100 kHz. In practice, the sampling rate provides a sample each 0.10 ms. It is anticipated that this rate should have a time as low as 0.025 ms.

In an aspect of the invention, the average power is obtained together with the rms current and the rms voltage. A circuit divides the average power by the rms power to create a signal or level representing the actual real time power factor of the power source. This power factor is compared with the desired power factor to create a corrective value for the wave shaper whereby the actual real time power factor is held at the desired power factor. This maintains a constant power factor. As explained before, by maintaining a constant power factor with a constant rms current, any tolerances in the welding process are overcome so that the welder will operate identically at the site as it did when set up by the manufacturer. This aspect of the invention is primarily employed for pulse welding and changes the shape of the pulse to obtain the desired constant power factor without changing the root mean square current of the welding process.

In accordance with another aspect of the invention relating to the obtained power factor level, the power factor is adjustable to change the heat of the welding process, especially when using the invention for pulse welding. The waveform generator or wave shaper controls the shape of the waveform to adjust the power factor to maintain it constant or to adjust it for the purposes of controlling heat. When this adjustment is employed, the rms current is maintained constant. Thus, the power factor is adjusted without adjusting or changing the actual current. The rms current determines the melting rate of the metal.

In accordance with another aspect of the present invention there is provided a method of controlling an electric arc welder, of the type defined above, which method comprises calculating the actual power factor of the power source using the rms current and the rms voltage. A desired power factor is then selected for the power source and an error signal is obtained by comparing the actual power factor of the power source to the desired power factor of the power source. This is accomplished by adjusting the waveform by the error signal whereby the actual power factor is held at the desired power factor.

The primary object of the present invention is the provision of an electric arc welder for performing A.C. pulse welding using a waveform generator or wave shaper, whereby the heat of the process is controlled by changing the background current of either the negative or positive pulse of the waveform.

In accordance with another object of the present invention is provision of a welder, as defined above which welder adjusts the peak portion of one pulse in the A.C. pulse welding method to control the desired heat or power factor of the welding process.

Yet another object of the present invention is the provision of an electric arc welder, as defined above, which welder utilizes an A.C. pulse welding waveform and adjusts the power factor to control the heat of the welding operation.

These and other objects and advantages will become apparent from the following description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference toFIG. 1, electric arc welder10is shown in block diagram form. A three-phase rectifier12provides power to high speed switching-type power supply14across a DC link in the form of input leads16,18. In a preferred embodiment, high speed switching-type power supply14is an inverter, such as a Power Wave welding power supply available from Lincoln Electric Company of Cleveland, Ohio. However, a high speed switching chopper or other high speed switching-type power supply can also be employed. High speed switching-type power supply14performs a preselected welding process. In accordance with present welding technology, high speed switching-type power supply14preferably switches at about 18 kHz or higher, and more preferably at 40 kHz or higher. High speed switching-type power supply14energizes welding circuit20that includes inductor22and electrode24forming an arc gap with workpiece26during performance of the welding operation. Typically, electrode24is a forward advancing welding wire from a supply spool. The welding wire is driven toward workpiece26at a selected wire speed during performance of the welding operation.

Controller30controls high speed switching-type power supply14during the welding operation. In accordance with the present welding technology, controller30is a digital device including waveform generator32that outputs power level waveform34represented by a line that is the input to pulse width modulator36. Pulse width modulator36produces pulse train38(represented by a line) of pulses with pulse widths corresponding to the power level of waveform34. In other words, waveform34is converted into pulse width modulated pulse train signal38by pulse width modulator36. Pulse width modulator36produces pulses of controlled width at a frequency preferably above 18 kHz, and more preferably about 40 kHz or higher, which is the input to high speed switching-type power supply14. The power supply switching is controlled by pulse-width modulated pulse train38to energize welding circuit20in accordance with power level waveform34.

Waveform34implements a desired welding process. Typically, a welding process is made up of a waveform train of repeating waveforms. For pulse welding, power level waveform34has a preselected wave shape for generating a welding process pulse. The average power or true heat produced in the welding process implemented by waveform34over a time interval [T1, T2] is given by:Pavg=1T2-T1⁢∫T1T2⁢v⁡(t)·i⁡(t)⁢ⅆt,(1)
where Pavgis the average power, v(t) is the instantaneous voltage, i(t) is the instantaneous welding current, v(t)·i(t) is the instantaneous power, and T1and T2are the starting and ending time points of the time interval, respectively, of the integration. In the case of a substantially periodic waveform, the average power can be expressed in terms of root-mean-square (rms) voltage and rms current according to:
Pavg=Vrms·Irms·PF(2),
where the rms voltage, Vrms, and rms current, Irms, are given by:Vrms=∫T1T2⁢[v⁡(t)]2⁢ⅆtT2-T1,Irms=∫T1T2⁢[i⁡(t)]2T2-T1,(3)
respectively, and PF is the power factor. In computing the average power and the rms current and voltage values for waveform34that implements pulse welding, the time interval [T1, T2] preferably corresponds to one pulse or a plurality of pulses. In waveform-controlled welding, the pulse time interval may vary for successive pulses. Hence, in the described preferred embodiment, the starting and stopping times T1and T2are extracted from waveform34as event signals T determined from a characteristic feature of waveform34.

Equation (3) can be rewritten to define the power factor PF according to:PF=PavgVrms·Irms.(4)
There is in general a close relationship for substantially any waveform34between the rms voltage and current values and the average power.

In contrast, the average voltage, Vavg, and average current, Iavg, given by:Vavg=∫T1T2⁢v⁡(t)⁢ⅆtT2-T1,Iavg=∫T1T2⁢i⁡(t)⁢ⅆtT2-T1,(5)
have a close relationship with the average power only for certain waveforms, such as are used in constant-voltage “spray” type welding. However, if, for example, the waveform includes a stepped pulse that is 500 amperes for 25% of the time and 100 amperes for 75% of the time, the rms value is 265 amperes, while the average value is 200 amperes. In this case, the rms values provide a more accurate true heat value.

With continuing reference toFIG. 1, controller30of electric arc welder10implements an exemplary pulse welding process in which the magnitude of waveform34is controlled using an rms current40that is calculated from an instantaneous welding current Ia42measured across shunt44. In the constant current welding process shown inFIG. 1, rms current40is compared with set rms current46by digital error amplifier48to produce error signal50that controls an amplitude of waveform34to maintain a constant rms current. Similarly, for a constant voltage welding process, control is suitably based on an rms voltage calculated from instantaneous welding voltage Va52measured across the weld by voltmeter54.

With reference toFIG. 2, computation of the rms current from instantaneous welding current Ia42includes processing with analog-to-digital converter56to produce digitized instantaneous current58, which is the input to digital signal processing block60. Digital signal processing block60performs the current squared integration of Equation (3) digitally as a Riemann sum, dividing the current into time intervals Δt defined by oscillator62for the summing. The digitizing interval Δt for the Riemann sum is suitably about 0.1 milliseconds to provide adequate samples for each pulse or repetition of waveform34. Sample-and-hold circuit64holds the digitized current for the period Δt, and squaring processor66computes the square of the held current value.

In order to enable continuous summation of rms current in parallel with related processing such as the computation of the square-root operation of Equation (3), the summing preferably employs two alternating storage buffers, namely first buffer70(identified as Buffer A), and second buffer72(identified as Buffer B). Values are stored in the active buffer at intervals76,78that are preferably in a range of about 0.025-0.100 milliseconds. When first buffer70is active, switch80transfers values at time intervals Δt to first buffer70, which accumulates the current-squared values and also maintains a sample count N of a number of accumulated current samples. As a background process during accumulation in first buffer70, the contents of second buffer72are processed by division processor82to divide by the number of samples N, and by square-root processor84to complete computation of the root-mean-square calculation of Equation (3).

At a selected event signal T generated by a characteristic of waveform34, the operation of buffers70,72switches. Second accumulator72is cleared, and switch80subsequently transfers current-squared samples into second accumulator72. As a background process during accumulation in second buffer72, the contents of first buffer70are processed by division processor86to divide by the number of samples N, and by square-root processor88to complete computation of the root-mean-square calculation of Equation (3).

FIG. 7shows a simplified block diagram of digital signal processing block60, which omits the details of the alternating summation buffers70,72and related switching circuitry that are shown in detail in FIG.2. InFIG. 7, current-squaring block66, switch80, and alternating summation blocks70,72are represented by a single summation block100that sums current-squared samples between occurrences of the event signal T triggered by a characteristic of waveform34, and also maintains the count N of the number of accumulated samples. Division background processes82,86ofFIG. 2are represented by a single normalization background process102in FIG.7. Square-root background processes84,88ofFIG. 2are represented by a single square root background process104in FIG.7.

With reference toFIG. 8, it will be appreciated that digital signal processing block60shown in FIG.2and represented in simplified form inFIG. 7is readily adapted to perform rms voltage calculations, by replacing measured instantaneous current Ia42with instantaneous voltage Va52measured by voltmeter54of FIG.1.FIG. 8shows rms voltage digital signal processing block60′ in a simplified form analogous to the simplified form of FIG.7. The digitized voltage is processed by sample-and-hold circuit64′ which holds the digitized voltage for the period Δt. Voltage-squared summation block100′ sums voltage-squared samples and maintains a count N of the number of accumulated voltage samples. Preferably, summation block100′ uses alternating summation buffers analogous to buffers70,72shown for the current-squared summation in FIG.2. Normalization background process102′ divides the voltage-squared sample sum by the number of samples N. Square root background process104′ takes the square root to complete implementation of the rms voltage Vrmsmathematically shown in Equation (3).

With reference toFIG. 9, it will be appreciated that digital signal processing block60shown in FIG.2and represented in simplified form inFIG. 7is similarly readily adapted to perform average power calculations, by inputting both measured instantaneous current Ia42and measured instantaneous voltage Va52.FIG. 9shows average power digital signal processing block60″ in a simplified form analogous to the simplified form of FIG.7. Sample-and-hold circuits64,64′ which hold the digitized current and voltage, respectively, for the period Δt, are accessed by current-times-voltage summation block100″ which sums current-times-voltage samples and maintains a count N of the number of accumulated current-times-voltage samples. Preferably, summation block100″ uses alternating summation buffers analogous to buffers70,72shown for the current-squared summation in FIG.2. Normalization background process102″ divides the current-times-voltage sample sum by the number of samples N to produce the average power Pavgshown mathematically in Equation (1).

Digital signal processing blocks60,60′,60″ compute the rms current, the rms voltage, and the average power as Riemann sums.FIG. 4shows exemplary current waveform120that is digitally sampled. Each digital sample is represented by a rectangular sample bar122of time duration Δt and height corresponding to the digitized value of current waveform120held by sample-and-hold circuit64at the time interval Δt.

Digital signal processing blocks60,60′,60″ are optionally implemented as separate processing pathways that execute in parallel. However, in a preferred embodiment digital signal processing blocks60,60′,60″ use some common digital signal processing blocks into which the sampled voltage and current signals are time-domain multiplexed. Such a multiplexing approach reduces the amount of circuitry required. Each summation (voltage-squared, current-squared, and voltage-times-current) has its own alternating summation buffer set (for example, summation buffer set70,72for summing current-squared values as shown in FIG.2).

With reference toFIG. 2A, a suitable process cycling for the time-domain multiplexing is shown. The process cycling employs four cycles130,132,134,136each occupying one-fourth of the sampling period Δt. For the exemplary Δt equal 0.1 millisecond, each of the four cycles130,132,134,136occupies 0.025 milliseconds. During first cycle130, the voltage Vaand current Iaare digitized and sample/held. During second cycle132, the current-squared is computed and added to the current-squared summation. During third cycle134, the voltage-squared is computed and added to the voltage-squared summation. During fourth cycle136, a check is performed to determine whether an event signal T has been detected, and the sample count is incremented. Moreover, throughout the cycling other processing, such as computation of the square roots of values stored in the inactive summation buffers, can be performed as background processes. Similarly, digital signal processing welding control operations, such as waveform shaping described by Blankenship U.S. Pat. No. 5,278,390, can be performed as background control processes during the cycling.

With reference toFIGS. 2 and 2A, and with further reference to FIG.3A andFIG. 6, the cycling as applied to the current-squared calculation is described.FIG. 3Aillustrates current waveform34extending between first event signal T1and second event signal T2. Event signals T1, T2are are suitably generated by a circuit controlled by waveform34. InFIG. 3A, the circuit generates event signal T1responsive to onset of the rising edge of current pulse140, and the circuit generates event signal T2responsive to onset of the rising edge of current pulse142. Thus, there is a current pulse between each two successive event signals T. Rather than detecting the rising edge, the event signals can instead be generated by detecting another characteristic of the pulse, such as the falling edge of the current pulse.

During the time interval between event signal T1and event signal T2, current-squared samples are accumulated in summation buffer70, as indicated inFIG. 3Aby the notation “Adding to Buffer A”. Each occurrence of second cycle132ofFIG. 2Aadds another current-squared sample to buffer70. Although not shown inFIGS. 2,3A, or6, voltage-squared samples and average power samples are preferably being accumulated in their respective buffers during the other cycles of the four-cycle process of FIG.2A. Detection of event signal T2is indicated by detection block150of FIG.6. Responsive to detection150, buffers70,72are switched so that buffer72is used to accumulate current-squared samples of next pulse142of waveform34, while buffer70in which the current-squared samples of pulse140are accumulated is shifted152into the background. In background processing, the current-squared sum is divided154by the number of samples N and the square-root is taken156to complete the rms algorithm. The computed rms current value for pulse140is written158to a register for use in welding process control.

With reference toFIG. 5, a suitable method for generating event signals T is described. A field programmable gate array (FPGA) includes cycle counter state machine170that updates two-bit counter172. State machine170is configured to increment two-bit counter172each time the state changes. Each change of state corresponds to an occurrence of event signal T. In the digital signal processing (DSP), two-bit comparator174compares the value of two-bit counter172with previous counter value register176during fourth cycle136ofFIG. 2A. Achange in the value of two-bit counter172indicated by the comparison corresponds to an occurrence of event signal T. Responsive to event signal T, digital gate178loads the new value of two-bit counter172into previous counter value register176. In this arrangement, the value stored in two-bit counter172is not significant; rather, a change in the counter value is detected.

With continuing reference to FIG.5and with further reference toFIG. 5A, the polarity of waveform34along with an auxiliary “Misc2” signal are input to state machine170through“OR” gate174. This arrangement enables the FPGA to generate event signals T for pulse welding and for a.c. welding. In the case of a.c. welding, Misc2 is set to zero so that the polarity signal feeds through to cycle counter state machine170. For pulse welding, Misc2 is set to one when the arc is shorted, and zero when the arc is not shorted.FIG. 5Ashows a graph of pulse current180and the value of Misc2182when pulse welding is used instead of A.C. welding.

With continuing reference to FIG.5and with further reference toFIG. 3, events initiated by an occurrence of event signal T are described. At fourth cycle136ofFIG. 2A, the digital signal processing performs a check190to see if an occurrence of event signal T has been detected. This is done by comparing the current value of two-bit counter172with stored counter value176using two-bit comparator174. If no change in counter value has occurred, the digital signal processing continues to loop through the four states130,132,134,136of FIG.2A. However, if check190detects an occurrence of event signal T, the rms value is computed192as set forth in Equation (3) and in accordance withFIGS. 2 and 7. Computation192is a background digital signal process. Additionally, a buffer switch194is performed so that whichever buffer (buffer A70or buffer B72) had been active is switched to the background, and whichever buffer (buffer B72or buffer A70) had been the background buffer is made the active accumulation buffer.

Exemplary digital signal processing circuitry and associated FPGA circuitry for substantially real-time computation of rms voltage Vrms, rms current Irms, and average power Pavghave been described with reference toFIGS. 1-9. The described digital signal processing circuitry implements Equations (1) and (3) using Riemann sums, and is exemplary only. Those skilled in the art can readily modify the illustrated digital circuitry or substitute other digital circuitry to perform these computations or substantial equivalents thereof. The illustrated circuitry provides certain features that may be optionally omitted or modified. For example, separate and independent digital signal processing pathways can be provided for computing each of the rms voltage Vrms, rms current Irms, and average power Pavgvalues. In this arrangement, time-domain multiplexing aspects of the circuitry can be omitted. Rather than having two alternating accumulators, a single accumulator can be employed in conjunction with a storage register that stores the previous sum for background normalization/square root processing. Moreover, if the digital signal processing is sufficiently fast or if parallel processing is employed, the temporary storage may be omitted entirely, and the normalization/square root processing performed substantially in real time for intervals between successive event signals T. Still further, a trapezoidal or otherwise-shaped integral element can be substituted for rectangular sample bars122of the Riemann sum illustrated in FIG.4. Those skilled in the art can make other modifications to the exemplary digital signal processing and FPGA circuitry illustrated herein for implementing Equations (1) and (3) as digital circuitry.

With reference toFIG. 10, digital signal processing block200computes the power factor (PF) in accordance with Equation (4) from the rms voltage Vrms, rms current Irms, and average power Pavgvalues. The denominator of Equation (4) is computed using multiplier202acting on the rms current Irmsand rms voltage Vrmsoutput by digital signal processing blocks60,60′ ofFIGS. 7 and 8, respectively. The average power Pavgoutput by digital signal processing bloc60″ ofFIG. 9is divided by this denominator using division block204to compute the power factor PF.

With continuing reference to FIG.10and with further reference toFIG. 11, electric arc welder10ofFIG. 1is readily adapted to implement a constant power factor control of the weld process in pulse welding. Controller30′ is a modified version of controller30of FIG.1. Digital error amplifier48′ produces error signal50′ based on the power factor PF. Digital error amplifier48′ compares the power factor PF output by digital signal processing block200(shown in detail inFIG. 10) with PF set value46′. Waveform generator32′ modifies selected waveform shape210based on error signal50′ as described in Blankenship U.S. Pat. No. 5,278,390 which is incorporated by reference herein.

With continuing reference to FIG.10and with further reference toFIG. 12, electric arc welder10ofFIG. 1is similarly readily adapted to implement a constant current welding process in which heat input to the weld is controlled by adjusting the power factor PF. Controller30″ is a modified version of controller30of FIG.1. The rms current40is compared with set rms current46by digital error amplifier48to produce current error signal50as in FIG.1. Additionally, a second digital error amplifier220produces power factor error signal222by comparing the power factor PF output by digital signal processing block200(shown in detail inFIG. 10) with adjustable welding heat set value224. Waveform generator32″ modifies selected waveform shape210based on error signals50,222as described in Blankenship U.S. Pat. No. 5,278,390.

With reference returning to FIG.11and with further reference toFIG. 13, in digital error amplifier48′ the power factor error signal optionally incorporates digital filtering. As shown inFIG. 13, digital error amplifier48′ includes difference operator232that computes difference signal234which is proportional to a difference between the computed power factor and power factor set value46′. Difference value234is input into digital filter236which generates control signal50′ for adjusting the waveform shape in accordance with the method described in Blankenship U.S. Pat. No. 5,278,390. In one suitable embodiment, digital filter236is an infinite impulse response filter. The digital filter can be used to amplify the signal, smooth the signal, remove high frequency signal components, or otherwise adjust the control signal.

With reference toFIG. 14, a digital error amplifier240for constant voltage control is shown. Digital error amplifier240includes difference operator242that computes difference signal E(n)246given by:
E(n)=Vset−(a·Vavg+b·Vrms)  (6),
where Vsetis a set voltage value, Vavgis an average voltage value computed in accordance with Equation (5), a is an average voltage weighting factor implemented by multiplier250, Vrms, is the rms voltage of Equation (3) that is output by digital signal processing block60′ ofFIG. 8, and b is an rms voltage weighting factor implemented by multiplier252. It will be recognized that difference signal E(n)246can be biased by adjusting the weighting factors a and b toward average voltage control, rms voltage control, or a selected weighted combination of average voltage and rms voltage control. Because the rms voltage is typically a better measure of the true heat input to the weld by the welding process, the rms weight b is preferably greater than the average weight a, that is, b>a. Moreover, the sum of the weighting factors is preferably unity, that is, a+b=1. Optionally, difference signal E(n)246is processed by digital filter254, such as an infinite impulse response filter, to amplify, smooth, or otherwise manipulate difference signal E(n)246to produce control signal256for adjusting the waveform shape in accordance with the method described in Blankenship U.S. Pat. No. 5,278,390

With reference toFIG. 15, a digital error amplifier260for constant current control is shown. Digital error amplifier260includes difference operator262that computes difference signal E(n)266given by:
E(n)=Iset−(a·Iavg+b·Irms)  (7),
where Isetis a set current value, Iavgis an average current value computed in accordance with Equation (5), a is an average current weighting factor implemented by multiplier270, Irmsis the rms current of Equation (3) that is output by digital signal processing block60ofFIG. 7, and b is an rms current weighting factor implemented by multiplier272. It will be recognized that difference signal E(n)266can be biased by adjusting the weighting factors a and b toward average current control, rms current control, or a selected weighted combination of average current and rms current control. Because the rms current is typically a better measure of the true heat input to the weld by the welding process, the rms weight b is preferably greater than the average weight a, that is, b>a. Moreover, the sum of the weighting factors is preferably unity, that is, a+b=1. Optionally, difference signal E(n)266is processed by digital filter274, such as an infinite impulse response filter, to amplify, smooth, or otherwise manipulate difference signal E(n)266to produce control signal276for adjusting the waveform shape in accordance with the method described in Blankenship U.S. Pat. No. 5,278,390

With reference toFIG. 15A, an exemplary waveform shape adjustment in accordance with the waveform shape adjustment method of Blankenship U.S. Pat. No. 5,278,390 is illustrated. Two waveforms280,282are shown in solid and dashed lines, respectively. For b=1 and a=0 in Equation (6) or Equation (7) (for voltage control or current control, respectively), waveforms280,282have equal rms values. However, the average value is generally different for waveforms280,282. Compared with waveform280, waveform282has a reduced voltage or current background magnitude and an increased voltage or current magnitude in the pulse.

Moreover, it will be appreciated that the pulse repetition period of waveforms280,282may be different. This difference in repetition period is accounted for in the digital signal processing by performing the Riemann sums of Equations (1), (3), and (5) over intervals between successive event signals T, instead of performing the Riemann summing over time intervals of fixed length. Generating event signals T at a rising pulse edge or other identifiable characteristic of the waveform allows the summation interval to track the repetition period of the waveform as the repetition period is adjusted by the waveform shaping.

With reference toFIG. 16, two digital error amplifiers300,302compute current and voltage error signals for use in a constant current, constant voltage welding process control. Digital error amplifier300includes difference operator310, weighting factors a312and b314, and digital filter316. Digital error amplifier300has the same voltage inputs and general circuit topology as amplifier240ofFIG. 14; however, digital error amplifier300produces control signal318for controlling wire feed speed during the welding process. With increasing output of amplifier300the wire feed speed should be decreased, while with decreasing output of amplifier300the wire feed speed should be increased. Digital amplifier302includes difference operator330, weighting factors c332and d334, and digital filter336. Digital error amplifier302has the same current inputs and general circuit topology as amplifier260ofFIG. 15, and produces control output338for adjusting the waveform shape in accordance with the method described in Blankenship U.S. Pat. No. 5,278,390. Hence, the waveform shape and the wire feed speed are simultaneously controlled using digital error amplifiers300,302to keep both voltage and current constant.

FIGS. 17-23disclose the use of the present invention for an A.C. pulse welding operation, wherein the heat of the A.C. pulse welding operation is controlled by changing certain aspects of waveform400, best show inFIGS. 18,19. Referring now toFIG. 17, Power Wave power source14produces a waveform across electrode24and workpiece26through choke22. A voltage in line5L is created across the arc to provide a real time representation of the arc voltage. In a like manner, shunt44produces a voltage in line42which is the instantaneous arc current. As previously described, waveform generator32has an output represented by lead34to control the duty cycle of the pulse width modulator36. The modulator is normally preformed by software and has a pulse rate established by oscillator36a. Of course, a hard wired pulse width modulator is sometimes employed. The digital or analog voltage on line38determines the wave shape of the welding operation waveform performed by the power source. A Power Wave sold by The Lincoln Electric Company of Cleveland, Ohio is the illustrated, preferred power source. This unit is disclosed generally in Blankenship U.S. Pat. No. 5,278,390. The waveform created by generator32has a shape controlled by wave shaper210so the output voltage, digital or analog, on line210adetermines the signal in line34that generates the specific current waveform at the welding operation. As so far described, the technology is explained above and is well known in the art. In accordance with of the invention, digital comparator220, having an output222compares the real time power factor signal represented by the value in line220awith the desired heat to be created as represented by the digital or analog voltage at line224. Thus, output voltage in line222is the voltage indicating the relationship between the real time power factor and the desired heat, which is represented as the desired power factor in line224. In accordance with the invention, an adjusting circuit220bprovides a signal in line222athat is responsible to the different signal in line222. Thus, as the signal in line222varies, the output voltage in line222amodifies the wave shape in wave shaper210to change the shape of the waveform. This action obtains the desired heat as referenced by the manually adjusted voltage in line224. The block diagrams shown inFIG. 17are performed digitally by controller software using standard DPS to perform waveform technology control of the electric arc welder. The voltage on line222amodifies the A.C. pulse waveform structured by wave shaper210to maintain the desired heat based upon a relationship with the real time power factor. To accomplish this objective, various aspects of waveform400are adjusted by circuit220b.

To illustrate various portions of the waveform which are adjusted to control heat, waveform400is shown schematically inFIGS. 18 and 19. Waveform400comprises one of a succession of A.C. pulses including a positive pulse segment402and a negative pulse segment404. In the preferred embodiment, positive pulse segment402is constructed with a peak current portion410and a background portion412with peak level430. The magnitude of the peak current is represented as level418. As shown inFIG. 19, heat adjustment of waveform400is accomplished by changing peak level418, shown as dashed lines402aand represented by c. Adjustment of the magnitude of the peak current is one implementation of the invention, where the shape of the waveform is modified to control heat, based upon the real time power factor of the welder. Height414of background current portion412is indicated as adjustable by dashed lines414a. In a like manner, leading edge416is adjustable to change the heat of the welding operation as indicated by dashed line416a. Magnitude change a of the background current and the change b in the width of background current are the primary adjustments implemented to cause waveform400to create the desired welding heat, while maintaining Irmsconstant. The primary aspect of the invention for modifying peak current portion410is adjustment of peak current magnitude as indicated by c as the distance between line402aand line402. However, peak portion410normally has a leading ramp420and a trailing ramp422as shown in the second occurrence of waveform400. These two ramps are adjustable to change the heat at the welding operation under the control of the real time power factor. As illustrated inFIG. 19, the dimensions a, b, and c as well as the angles of the ramps indicated by d, are adjustable to control heat. Circuits to accomplish these adjustments are illustrated inFIGS. 20-23. In these figures, digital circuit220bcontrols the wave shaper210by the voltage in line222a.FIG. 20illustrates the use of circuit220bto adjust dimension a. The power factor value in line220a(preferably digital) is obtained by dividing the average power in line204aby the product in line204eobtained by multiplying Irmsin line204bby Vrmsin line204cwith circuit204d. Dimension b is adjusted by the circuit shown in FIG.21. Using the circuits shown inFIGS. 20 and 21the magnitude of the background current in portion412is varied so that the power factor signal at line220ais compared with the desired heat represented as a voltage on line224to change the background current. Thus, the background current is adjusted to maintain the desired heat caused by the waveform400. The circuits inFIGS. 22 and 23implement adjustments of the dimensions c, d. This changes the magnitude of the peak current or the angle of one or both ramps220,222. In this manner, the peak current portion of waveform400is adjusted to create the desired heat. Other aspects of the waveform are adjustable to control the desired heat based upon the real time power factor of the welding operation using a circuit as shown inFIGS. 20-23.

The invention has been described with reference to certain preferred embodiments illustrated inFIGS. 17-23. Modifications in these embodiments can be made without departing from the intended spirit and scope of the present invention as defined in the appended claims.