GB2068147A

GB2068147A - Welding current control in resistance welding

Info

Publication number: GB2068147A
Application number: GB8101257A
Authority: GB
Original assignee: Vyskumny Ustav Zvaracsky VUZ
Current assignee: Vyskumny Ustav Zvaracsky VUZ
Priority date: 1980-01-28
Filing date: 1981-01-15
Publication date: 1981-08-05
Also published as: CS211021B1; HU189489B; GB2068147B; IT1135092B; CH650185A5; FR2474367A1; FR2474367B3; DD158166A3; DE3048488C2; JPS56128685A; DE3048488A1; IT8119217A0; SU1316769A1

Abstract

During resistance welding the welding current is caused to rise gradually to a maximum value at which it is held constant. The weld resistance value is determined by a generator (11) connected, through a differentiator (12), and a comparator (13), having a reference signal input (19), to an input (20) of a trigger (14). The output of the trigger is connected to a stopping input (23) of a ramp generator (15) which, through electronic control means (16), varies the welding process (17). A command (26) for starting (24) the ramp in the electronic control means is also connected to a clearing input (22) of the trigger, through a delay circuit (18), to a clock input (21) of the trigger. <IMAGE>

Description

SPECIFICATION A method of welding current control in resistance welding, particularly spot welding, and a control system therefore The invention relates to a method of welding current control in resistance welding, particularly spot welding, and a control system therefor.

In the practice of resistance welding, particularly spot welding, a high and uniform quality has to be secured under conditions of considerable variation of technological conditions which may affect the welding process and thus joint-quality as well. The recent practice is based on a fixed setting transformer and phase output control at the beginning of the welding process in order to keep approximately constant welding current during the welding. This practice is unreliable due to mains voltage fluctuations, wear of welding electrodes, changes of the surface conditions of a welded sheet and other variations affecting the welding current. Under these circumstances the welding current can drop to such a low value that a weld of poor quality is produced or it is not produced at all.

On the other hand control systems for the compensation of voltage drop in the mains are known. However, their application does not prevent welding current drops caused by other effects.

There also exist control systems for the regulation of welding current to a constant value, or other values, e.g., stress on the weld or input supplied into the weld. These solutions exhibit one common drawback, namely, they do not adjust to the varying conditions of joint production as they are affected by the welded parts proper, geometry of the welded joint, its surface conditions, and the condition of the electrodes, and require resetting at each change of welded thicknesses, material, number of sheets, surface finish, etc.

There are also known control systems which compare some of energy values of welding, such as current, voltage, or input, with preset limit values, and according to a preset program they adjust the conditions for the next weld production on finding of an unacceptable value measured during weld production.

These are also unsuitable because the control adjustment does not affect the weld in which the change was detected, but just the next one for which the conditions could change again. Moreover, these systems are hard to set up. The optimum setting values are achieved experimentally, especially for each technological case.

Furthermore, another control system is known which utilizes a ramp rise of welding current, further current rise ceasing when the thermal expansion (temperature) of a weld given by its time derivation reaches a preset value and further welding being performed at the achieved value of welding current. The drawback of this otherwise perfect method, lies in the measurement of the mechanical value-thermal expansion. This causes certain problems with some welding machines. Moreover, it necessitates a special sensor for converting the mechanical value (thermal expansion) to an electrical signal.

The invention eliminates to a great extent the drawbacks of the above mentioned methods. According to the invention the method of welding current control in resistance welding, particularly spot welding, with a ramp rise of the welding current is characterized by stopping the rise of welding current when the electrical resistance of the weld reaches a local maximum value or approaches this maximum value so that the welding process continues at the value of welding current achieved at the moment of stopping its rise.

The system of welding current control in accordance with the invention facilitates the performance of the method in accordance with the invention includes source of "weld resistance" signals which source is connected through a differentiator and a comparator provided with a reference input to the signal input of a trigger device. The output of the trigger device is connected to the stopping input of a ramp generator which is connected to an electric control means for controlling the welding process parameters including a setting input for the welding process. The command signal for starting the ramp of the electronic control means is connected to a resetting input of the trigger, to a starting input of the ramp generator and in a parallel manner through a delay circuit having an input for setting the delay to a clock input of the trigger.

This control system in accordance with the invention is advantageous because it facilitates balancing of random and disturbing external effects such as shunting of current through an adjacent weld or through incidental contact of parts being welded which could otherwise lead to a reduction in the current flowing through the weld area and thus to poor quality of the joint being made. Moreover, the method in accordance with the invention facilitates adjustment of the welding current to situations occurring due to the designer's intentions, such as different thicknesses of welded sheets, various numbers of sheets changing in the course of welding operations on one weidment. The operative's task in setting the welding conditions becomes easier.After a certain initial setting, the control system itself adjusts the welding current to suit the varying technological conditions.

Moreover, the method and control system are advantageous because the welding current correction takes place during, actually at the beginning of, that welding process in which the change of conditions requiring that correction, is determined.

The control system in accordance with the invention does not require any special sensor or converter and it can be used for stationary resistance welding machines and also for spot welding guns.

One example of the application of the invention is shown in the accompanying diagrammatic drawings in which Figure 1 depicts a typical weld resistance curve and a curve showing the increase of weld nugget diameter, both for a constant setting of the welding current; Figure -2 compares the curves for weld resistance and increase of weld nugget diameter for a rising, i.e. ramp welding current, Figure 3 elucidates the operation principle of the method in accordance with the invention, i.e. how the signal for stopping the welding current rise is derived from the resistance curve or from its derivative; and Figure 4 shows an example of a control system in accordance with the invention.

On a constant setting of welding current, a typical course of resistance course R of the weld with time is shown by the curve 1. The curve 2 depicts the simultaneous increase of the weld nugget diameter D. It is seen that approximately at the moment tmax, when the weld resistance R attains a local maximum value Rmax, the molten weld nugget begins to form, its diameter D growing with time t during the welding.

If we let the welding current I rise steeply during welding, see the curve 3, then the weld resistance R, curve 4, reaches again a local maximum Rmax in the course of time.

This maximum approximately coincides with the beginning of molten weld nugget formation, but its diameter D, the curve 5, rapidly increases with the increasing value of the welding current I. The rate of increase in diameter D is substantially higher when compared with the case of constant setting of welding current I, so that in the mean time it leads to energetic vaporisation (oversaturation) of the weld and spatter of molten metal of the weld.

It has been proved experimentally that the more rapidly the welding current I rises, the earlier the local maximum value Rmax of the weld resistance is attained and the earlier the molten weld nugget begins to grow.

Moreover, it has been found that due to a change of external conditions as e.g. an increase in thickness, or number of sheets being welded or an increase in the electrode diameter (e.g. as the result of their wear), the maximum Rmax is attained later, i.e., at higher values of the welding current I. If from the adequate values la corresponding to the attainment of Rmax at the times tmax, the ratio l/Dc is formed, which is called the intensity of the welding process and it is made certain that it is the invariant of the welding process for one kind of material, where Dc is the diameter of welding electrodes. This ratio varies with different thicknesses and number of welded sheets, as well as for different electrode diameters; it is constant at constant conditions.It follows that the time tmax at which the maximum weld resistance value Rmax is attained, represents the welding process invariant which shows that the value of welding current Ii, adequate for energy demands of the joint just being welded, has been achieved.

The control system in accordance with the invention operates on the described principle.

The welding current I is allowed to rise at a rate according to the curve 6, and the weld resistance R, curve 7, is measured and simultaneously its derivative dR/dt, curve 8, is determined. At the time tmax at which the resistance curve R reaches its local maximum Rmax, the curve 8 of the derivative dR/dt passes through zero. This state is detected, and using the signal obtained therefrom, further growth of welding current I is stopped at value Ii. Welding is then completed at this constant value of the welding current, see curve 9 Fig. 3. For technical reasons relating to the design of the electronic parts it can be advantageous to detect the value 10 of the derivative dR/dt as it approaches zero, so that the signal for stopping further rise of the welding current I is given at a time interval At before the weld resistance maximum Rmax is achieved.The time interval At can be comparable to or less than one half period of the mains frequency 50 Hz, i.e., 0.01 s in the case of a commercial welding equipment.

A method of welding current control in resistance welding, particularly spot welding, in accordance with the invention is facilitated by a control system in accordance with the invention, see Fig. 4, in which a source 11 of weld resistance value signal is connected through a differentiator 12 and a comparator 13, which has a reference input 19, to the signal input 20 of a trigger 14. The output of the trigger 14 is connected to a setting input 23 of a ramp generator 15which is connected through an electronic control means for 16 controlling the welding process parameters, which means has a setting input 25, to the welding process 17. The command output 26 for starting the output ramp of the electronic control means 16 is connected to a clearing (resetting) input 22 of the trigger 14, a starting input 24 of the ramp generator 15, and in parallel through a delay circuit 18, which has an input 27 for setting the delay, to a clock input 21 of the trigger 14.

In accordance with the invention the control system operates as follows: the signal of the weld resistance measuring source 11 is fed to the differentiator 12, the output of which is connected to the comparator 13. The comparator 13 determines by comparison with the reference signal applied at input 19 the achievment of the local resistance maximum value Rmax and gives a command signal to the trigger 14 which thereupon triggers and by its output, connected to the stopping input 23 of the ramp generator 15, stops the ramp increase of the generator output signal. Thus the welding current rise is also stopped, this being achieved by virtue of the connexion of the output of the ramp generator 15 through the electronic control means 16to the welding process 17. The command output 26 for starting the ramp of the electronic control means 16 is connected to the clearing input 22 of the trigger 14 and the starting input 24 of the ramp generator 15, and due to this connection the trigger 14 is unblocked at starting of the ramp and consequently the welding current rises. In parallel, the command output 26 for starting of the ramp is connected through the delay circuit 18, which has the input 27 for setting a delay, to the clock input 21 of the trigger 14. Thus blocking of the trigger during the first two to five periods is possible.

The method and control system in accordance with the present invention can be used for welding process control of resistance spot welding.

Claims

1. A method of controlling the welding current in a resistance welding process, particularly a spot welding process, in which method the welding current rises gradually; the welding current rise is stopped when the weld resistance reaches a local maximum value or a value closely approaching this maximum value; and the welding process continues at the value of welding current achieved at the moment its rise was stopped.

2. A control system for carrying out the method of Claim 1, in which system a weld resistance value generator is connected through a differentiator and a comparator, having a reference signal input, to a signal input of a trigger; the output of the trigger is connected to a stopping input of a ramp generator which is connected through electronic control means for varying the welding process parameters, said means having a setting input, to the welding process; the output of the command for starting of the ramp of the electronic control means is connected to a clearing input of the trigger, to a starting input of the ramp generator and in a parallel manner through a delay circuit, having an input for setting the delay, to a clock input of the trigger.

3. A method of controlling a welding current in a resistance welding process, in which method -the welding current is controlled so as initially to increase gradually, -the resistance value of the welding current path in the workpiece being welded is monitored during the initial gradual current increase, -the growth of the welding current is terminated when the said resistance value, after an initial decline, reaches a local maximum value, and -the welding current is maintained after termination of said growth at the value reached at the time of such termination.

4. A method according to Claim 3, wherein the welding current is controlled so as initially to increase gradually at a uniform rate.

5. A method according to Claim 3 or Claim 4, wherein the time derivative of said resistance value is monitored, and said growth of the welding current is terminated when said time derivative falls to zero value.

6. A method according to Claim 5, wherein said growth of the welding current is terminated at a predetermined time interval after said time derivative falls to a predetermined low, near-zero value.

7. A method according to any one of the Claims 1 and 3 to 6, substantially as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.

8. Welding current supply apparatus for carrying out a method according to any preceding claim, including: (a) welding current control means for controlling the growth of welding current so that initially said current increases gradually in a predetermined manner, and (b) auxiliary control means responsive to the electrical resistance of the welding current path in the workpiece being resistancewelded, and operative to supply to said welding current control means a signal for terminating said growth of the welding current when said resistance, after an initial fall, reaches a local maximum value, whereby said current control means is caused to terminate said growth, the welding current thereafter remaining at the value reached at the termination of said growth.

9. Apparatus according to Claim 8, wherein said welding current control means is effective to provide a linear increase of welding current.

10. Apparatus according to Claim 8 or Claim 9, wherein said auxiliary control means includes a differentiating means for providing a control signal dependent on the time derivative of said welding current path resistance, and comparison means for supplying an output signal when said control signal falls to zero value, which output signal constitutes said signal for terminating growth of the welding current.

11. Apparatus according to Claim 8 or Claim 9, wherein said auxiliary control means includes a differentiating means for providing a control signal dependent on the time deriva tive of said welding current path resistance, comparison means for supplying an output signal when said control signal falls to a predetermined low, near-zero value, and delay means responsive to said output signal for providing after a predetermined time delay a delayed output signal, which delayed output signal constitutes said signal for terminating growth of the welding current.

12. Apparatus according to any one of the Claims 8 to 11, wherein said welding current control means includes a pattern signal generator for producing a patterns signal that increases gradually with time, and means for regulating the welding current in accordance with the magnitude of said pattern signal, and wherein said signal for terminating said growth of the welding current is supplied to said pattern signal generator whereby to terminate growth of said pattern signal, said pattern signal thereafter remaining at the value reached at the termination of said growth.

13. Apparatus according Claim 12, wherein said pattern signal generator comprises a ramp generator for producing a pattern signal that increases linearly with time.

14. Apparatus according to Claim 10 or any subsequent claim as dependent thereon, or to Claim 11 or any subsequent claim as dependent thereon, wherein said signal for terminating growth of said welding current is passed to said welding current control means through a trigger device, which trigger device has an associated delay means, controlled by a "start welding current" signal, for unblocking said trigger device after a predetermined time delay.

15. Apparatus according to any one of the Claims 2 and 8 to 14, substantially as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.

16. A workpiece having parts welded together by a resistance-welding method according to any one of the Claims 1 and 3 to 7, or by means of an apparatus according to any one of the Claims 2 and 8 to 15.