DE2436671A1 - Accurate power control in spot welding - using electronic control circuit to compensate for differences in electrode contact resistance - Google Patents

Abstract

Process is for controlling spot welding machines w.r.t. both time and power output. An optimum build-up curve (the desired curve) is obtd. using a battery to charge a capacitor (I), the voltage across (I) being used as a measure of the power desired; simultaneously, the actual power used is monitored electronically and fed to a second capacitor (II); the voltages on (I) and (II) are compared in a differential amplifier which emits a corresp. voltage driving a controllable resistance, pref. a power transistor, which then controls the phase of the welding machine so the voltage on the capacitor providing the spot welding power matches that on the optimum build-up curve at any moment, and a Schmitt trigger helps to cut off the power when the zero pause on the 50 Hz A.C. mains supply is reached. Irrespective of the large differences which occur in the resistance of the junctions between the welding electrodes and the workpieces, a prescribed amt. of power is delivered.

Description

Method for controlling electrical work in spot welding machines while at the same time taking into account the electrical power supplied to a pivot point Work internals.

The invention is concerned with a method and an arrangement to control the electrical work in point-tail machines, at any point in time the application of further electrical work is taken into account, which amount of work has already been invested in the welding point under consideration.

The technique, two bodies, and in particular sheet metal, with the help to connect by welding points has been known for a long time.

4an often uses two electrodes for spot welding made of electrolytic copper, which are pressed against each other, whereby the one to be welded material is between the electrodes. When administering an electric A current surge to the electrodes causes a current to flow through the electrodes to be welded Media conducted, whereby a voltage is applied to the electrical material resistance of the media falls, which in combination with the flowing current causes a spontaneous rise in temperature has the consequence.

The following noteworthy remarks are also made regarding the state of the art to make: The regulation of the administered electrical power takes place today at the use of a 50 Hz alternating current in such a way that with the help of the so-called Phase view a more or less large part of one or more electrical Half-waves are fed to the welding zone, the phase control method is used in a simple manner to regulate the flowing current. Since the Welding transformers usually only have a certain number of secondary turns anyway to produce the welding current, and in the vast majority of cases the The secondary winding of the welding transformer consists of a single turn the phase control automatically represents a method, the secondary-side Voltage on the welding transformer in a certain amount of time to regulate.

regarding the administration of a desired electrical work Nowadays, use is practically always made of it in a zone of silence, a specific one pre-selectable number of electrical half-waves to administer to the welding zone.

By the two mentioned @ measures, namely by the phase control and through the digital preselection of the electric half-waves to be administered 50 Hz alternating current is now commonly the desired electrical output and the electrical work to be administered is determined in advance.

In addition to the two options mentioned, there is a third Mentioned parameter that is important for the quality of a tail3 point, and although the welding pressure. The tail pressure can generally be preselected on the machine side set and can be considered constant within wide limits. His influence the quality of a welding point remains unacceptedly relevant, as it increases Welding pressure and a constant supply of electrical work result in a reduction in the Welding temperature can be determined, whereas with decreasing welding pressure the welding eating temperature increases sharply.

3 but also methods have become known, which the electrical Work on spot welding machines, taking the welding pressure into account allow taxes. In contrast, however, have all previously known control methods a general idacnt: if one takes into account with the help of a wide variety of Measurement method the electrical work supplied to a welding point and one tries for example, to switch off the welding machine exactly when the electrical work is reached, so looks nan often faced with the difficulty that reaches the optimal amount of electrical work delivered at a time ira, when an electric current often flows at different levels. Figure in the very rarest of skins falls to the ideal value of electrical work Creation of a tail point with the zero crossing of an electrical half-wave through the current zero point supplied by the welding transformer. do However, the vast majority of welding transformers are designed so that the primary side of the welding transformer only when there is an electrical zero crossing Half-wave can be released from the network, as a magnetically charged transformer the effort shows to continue to maintain the flow of current, regardless of Disconnection of the primary side from the network.

The disadvantage of the exact metering of the welding energy while at the same time Consideration of the very different contact resistances to be welded Media is therefore to be sought to a predominant degree in that through an exact recording the performed electrical work can not be guaranteed that the shutdown of the welding transformer in achieving the desired work integral can actually take place and that, moreover, especially when administering a small number of electric half-waves the increase of the work integral a half-wave can possibly be of great importance. If you imagine, for example suggest that the goal of welding technology has recently often been to or 5 half waves of the 50 Hz Metz alternating current produce a weld point it is clearly evident that in the pursuit of the labor integral the Facts can arise that z. B.

after 4.2 periods or half-waves the applied work integral has approximately been reached, but now one in the sense of a correct welding one Timed correct shutdown of the welding machine cannot take place because the electrical The current must only be switched off when the half-wave passes through the gauze can. The semiconductors controlling or interrupting the flow of electricity; such as B. Isnitrone, do not allow an interruption of the current in the ignited state, so that under certain circumstances 20 or 25% more electrical energy is supplied to the welding point was originally estimated, resulting in the weld zone being burned can, whereby there is always a strength impairment.

To the described shortcomings and disadvantages in the practice of A method according to the invention is to avoid spot welding methods customary today and an arrangement for controlling the optimal labor investment in a spot weld applied, in which by a comparison of a target curve determined as optimal an in-target control of the electrical work based on a so-called actual curve is carried out, at the same time by two measures to be described the creation of the target curve is taken into account that on the one hand the shutdown of the welding machine both after reaching the optimal work integral takes place as is also done at a point in time in which exactly or almost there is exactly one zero crossing of the electrical mains alternating current. The mentioned target and actual curves are used as electrical charging functions understood from DC voltages to integration capacities, the amount of an the integration capacities pending electrical voltage in each case a measure for represents the desired electrical work up to a point in time (Target curve) or a measure for the amount already administered up to the point in time under consideration represents electrical work (actual curve).

According to the invention, a stabilized direct current source is used via a permanently adjustable resistance of the first the target curve bringing about integration capacitance is supplied with a track voltage, which carries out a linear charge over time on this capacitance, with the slope the voltage curve over time represents an angle which is ideal Increase curve of the real electrical work performed at the welding point reflects. In addition, with the help of electronic detection elements, such as described in more detail below, care taken) that on a second integration capacity a voltage curve arises over time, as it is the real integral of U.I.dt is equivalent to. The permanent comparison of the rise of both voltages increases the integration capacities allows a differential amplifier to be used to determine whether the actual curve the target curve has always followed or whether an adaptation of the actual curve to the target curve has to be done. By means of the comparison measures described below of the two curves mentioned and the resulting control interventions is closed recognize that the actual curve is adapted to the target curve at any time and in addition, through the appropriate choice of the length of the target curve on the time axis always an exact shutdown; of the electric current in the zero crossing made possible will.

In detail, the invention is based on a simple one Example explained in more detail without limiting the number of possible designs. For this purpose, the individual elements and modes of operation are explained for a better understanding also shown in FIG. In Figure I, the number 1 is the welding transformer and with number 2 the element switching this transformer, e.g. B. a semiconductor, shown. With number 3 are the two welding electrodes and with number 4 that Sciweißgut indicated. With Zifer 5 a semiconductor is shown, which measures the magnetic field around one of the two welding electrodes. As a painter a so-called Hall generator is preferably used. The mall generator for its part if the resistor 6 is fed directly from the secondary mounting of the welding electrodes, whereby the operating current required for the Hall generator @ is ensured. Do the amplification of the voltage of the product of welding voltage emitted by the Hall generator and welding current in an amplifier 7 causes a rectification to rectifier 8, the resulting DC voltage via an adjustable resistor U of the fixed loading capacity 10 is supplied.

parallel to the processes described so far, which are already in the Prin ip are known, a further capacitor 11 is charged according to the invention Via a permanently adjustable resistor 12 with a DC voltage from a Battery 18. The increase in the voltage across the capacitance shows a slope, like it is necessary in the sense of an ideal supply of apleits to the weld metal, if dapei the increase in the first integration capacitance 10 is taken into account.

According to the invention, the one at the Ladeka @ acity 10 and the one at the Charge capacity 11 at any point in time there is a DC voltage in the differential amplifier 14 compared. Care is now taken that after empirical determination of the The ideal course of a target curve at the integration capacitance 11 actually crosses over A voltage curve occurs over time which meets the conditions described below is fair, the voltages across the capacitance are compared at the same time 10 and capacity 11. This comparison shows to what extent the actually electrical work carried out on the welding point up to a given time has reached, exceeded or undershot the work integral considered at the point. So it becomes the determination from one half-wave to the other half-wave whether up to the point in time considered in the sense of ideal work tracking to Much or too little work has been done.

The DC voltage output by the differential amplifier 14 controls now in turn a semiconductor, preferably a power transistor 15, the is in turn switched into the phase shift control circuit, so that with the laid status of the power semiconductor at the same time the phase connection control is changed from a verbafolten half-wave to the next so that the voltage of the actual curve developing at the charging capacitor 13 is largely the on the charging capacitor 11 adapts predetermined target curve.

On this journey, the welding work carried out will be controlled in a targeted manner initiates> while at the same time the time monitoring described in Fiur II is also carried out.

In FIG. II, the number 1 denotes the charging capacitor shown in FIG 11 reproduced. The voltage building up on this charging capacitor 11 has To fulfill two functions in terms of target control of electrical work: First of all, care is taken to ensure that the slope of the charging curve on the charging capacitor 1 a readjustment of the actual curve on capacitor 10 Figure I is carried out.

Second, according to the invention, additionally with the aid of a Threshold switch, primarily a Schmitt trigger 2, when a certain voltage, the further supply of electric current to the welding point is prevented. For this purpose, an electrical quick switch 3, preferably with the help of a so-called reed relay, a switching process is triggered, which one Intervention at switch 2 of Figure I causes, and interrupts the welding current. Since, however, the welding current is switched off at one point in time will must if at the same time. a Hull passage of the 50 ^ -Tz-I mains alternating current can be recorded is, according to the invention, the charging time at the integration capacity is 1 to for switching the Schmitt trigger 2 in Figure II so dimensioned that precisely then a The power supply is switched off when a JJull due to the 50 Hz alternating current coincides with the switching time of the Schmitt trigger 2. In achieving at this point in time, on the one hand, care is taken to ensure that with the help of the im In connection with the arrangement described in FIG. I, readjustment of the actual curve to the Soll-iMrve and that at the same time as that described in Figure II Arrangement of the timely switch-off at the zero crossing or shortly before the zero crossing a half-wave takes place. The Recrel potentiometer 4 in Figure II enables the shiftability the switch-off time, whereas the control potentiometer 12 in Figure I for the Specification of the target curve is responsible.

With the help of the two mentioned potentiometers 12 in Figure 1 and 4 in Figure II are the level elements used in today's welding machines largely substituted, namely the control elements phase control and digital period specification.

The inventive advantage of the method described and the Arrangement is that regardless of the very un different contact resistances between welding electrodes and weld metal at any time a preselected electrical Work is forced, whereby by the measure of the specification of a target curve also the time of the correct switch-off in the sense of optimally performed welding work is ensured.

Claims (6)

PATENT CLAIMS Method for controlling spot welding machines, in which a influencing time and also the amount of work done the welding machine is effected, d u r c h e k e n n n z e i c h -n e t that once a rise curve determined to be optimal on one of a battery A resistor-fed charging capacitor forms a so-called target charging curve is, where the at the poles of the charging capacitor to each considered arbitrarily Point in time standing voltage a measure for the up to this point in time as optimal represents considered labor investment in the welding point and being at the same time on a second loading link that works in the same way that actually invested electrical work is detected with the help of electronic means and this second Charging capacitor is supplied, after which with the help of a differential amplifier, which at any point in time the voltages on the charging capacitors compares a voltage is created, which is the difference between the voltage on the so-called target charging capacitor and represented on the actual charging capacitor, with the help of which a controllable resistor, preferably a power semiconductor is operated, which in turn controls the phase control the welding machine is influenced at any point in time in the sense that the Phase control applied electrical energy to the welding point like this it is regulated that the voltage at the actual charging capacitor is the same as the voltage at the target charging capacitor fully or as closely as possible, which results in a synchronism of the 2 curves is, at the same time by the choice of the slope of the voltage curve on the target charging capacitor the supply of further electrical energy with the help of a Schmitt trigger is then prevented when the optimal power integral at a point in time administered became, as there is a straight or almost straight zero crossing of the 50 Hz mains alternating current is reached, so that the optimal or by the claimed follow-up control near optimal electrical work was being administered at a time when a It is possible to switch off the machine by appropriately selecting the duration of the target curve is, i.e. at the zero crossing of the 50 Hz mains alternating current. 2. Arrangement according to claim 1, d a d u r c h g e -k e n n z e i c h n e t that the measurement of the electrical work applied to the weld is carried out Multipliers are made, which is the product of the applied to the welding electrodes applied electrical voltage and the electrical input flowing to the welding point Electricity is formed with the help of networks or with the help of semiconductor multipliers whereupon the electrical voltage representing the rodut is applied to an integration capacitance is integrated and at this always the instantaneous value, which is up to any Represents the time of the work invested in the welding zone. 3. The method according to claim i and 2, d a d u r c h g e -k e n n z e i c h n e t that with the help of a stabilized voltage source via a controllable one or permanently adjustable resistance of a fixed capacitance one determined to be ideal Amount of current is supplied, so that the respective voltage on this capacitor increases has assumed a value at every observed point in time, which is the ideal value of the so-called Corresponds to the target curve at this point in time. 4. The method and arrangement according to one of the preceding claims, d a d u r c h e k e n n -z e i c h n e t that with the help of a differential amplifier a permanent comparison between the voltages on the two capacitors mentioned is carried out, the voltage output by the differential amplifier being a Semiconductor, preferably a power transistor, controls, which as a controllable Resistance influences the phase control of the welding machine in the sense that that the actual curve, i.e. the representation of the actually applied electrical work, exactly or almost exactly the course of the voltage behavior of the target curve adapts so that at any time, but at least after the administration of a half-wave or several parts of it, a sufficiently good target control of the weld made was so that cessation of the supply of further energy at a point in time is controlled by the target curve, nothing stands in the way, since the target curve switches off at a point in time through a controllable resistor because a zero crossing of the 50 Hz mains alternating current is recorded. 5. The method and arrangement according to one of the preceding claims, d a d u r c h e k e n n z e i o h -n e t that the previously used in welding controls The usual so-called power specification for the phase control through the setting a controllable resistor in front of the capacitance to form the target curve so that the actual charging curve is based on the slope of the target charging curve and thus the media used up to now for influencing the phase control angle be substituted by said resistor. 6. The method and arrangement according to claim 1, d a d u r c h g e k e n n z e i c h n e t that the timed shutdown in the zero crossing of the 50-Iiz mains alternating current the effect is that, with the help of a potentiometer, the switching point a so-called Schmitt trigger occurs when a zero crossing at 50-EIz mains alternating current is recorded and at the same time also the optimal The amount of electrical work to be done was supplied to the weld point to be created. L e r s e i t e