UA72571C2 - Method for contact spot microwelding with automatic control - Google Patents

Abstract

The method of contact spot microwelding with automatic control relates to the contact spot microwelding of metallic parts with the wall thickness from several tenths of millimeter and less and it can be used in the instrument-making industry, electronics, during manufacture of many objects of furnishings. In the method performed on the machine with stabilized supply voltage both directly from the network of alternating current and through the frequency converter and number of phases, welding conditions are preliminarily selected to meet the requirements and hereinafter referred to as reference conditions, the voltage on the electrodes is memorized, as well as welding current and available power as function of time, and during subsequent operations of welding the current values of these parameters are compared with reference values, the obtained result is analyzed and on the basis of this analysis control system generates a controlling influence. In case of negative deviation of current within the limits of preliminary set value the energy spent on heating of metal is stabilized with the help of voltage stabilization on the electrodes and increase of time of current flow. In case of positive deviation of current the control system operates in the mode of voltage stabilization on the electrodes. The initial stage of welding operation proceeds at gradually increasing voltage. The improvement of quality of the formation of spot weld is achieved.

Description

The proposed method refers to contact spot microwelding of metal parts with a wall thickness of several tenths of mm or less. Areas of application of microwelding are instrument building, electronics and the production of many household appliances. This method of welding connects parts in the form of sheets, wires and various elements that have undergone stamping or other mechanical processing. Parts can be made of different materials, of the same or different thickness at the joint. Parts are often connected without preliminary preparation of their surfaces or coated with materials with increased electrical resistance. All these features of the application of microwelding complicate its application, especially in the production of products of responsible purpose. Therefore, it is advisable to equip machines for microwelding with automatic control systems that reduce the impact of various types of disturbances and to add energy conversion devices to the machines, which allow to create complex welding cycles necessary for technological understanding. Four main types of such systems are known: - direct supply from the mains through a thyristor contactor, - supply of the welding transformer by the discharge of the capacitor bank, - systems representing a combination of the first two systems, and the first of them is used for heating the parts to be connected, for stabilization of the contact resistance, and the second for the next welding - a system that provides power to the welding transformer with a high-frequency current.

The first three systems do not allow effective control with feedback when the duration of welding is less than 0.1-0.2 seconds. A fourth system with a frequency converter is more suitable for microwelding. This system should be considered as the most promising.

The known methods of contact spot microwelding, which use automatic process control systems, can be divided into three groups: - without feedback - solid control, - stabilization or change according to a defined program of one of the easily measured electrical quantities - the current flowing through the welding contact , or the voltage drop on it, and also, possibly, which are calculated from the first two values - power or resistance, - stabilization or change according to the defined program of thermal expansion, which is heated during welding of the volume of metal.

Despite the abundance of various systems, the use was mainly of rigid control systems without feedback (see the journal "Automatic welding", 1963 Mo5 "Bezinertsionnye schemes of automatic control of contact welding processes" p. 7-10). Such management in the majority of cases is quite acceptable. Many perturbations cause deviations in the dimensions of the joints within rather wide, but acceptable limits, because the number of points is taken with a large margin.

The disadvantage of the first method is the impossibility of interfering with the welding process. Disturbances, the occurrence and effect of which cannot be predicted, are not taken into account here. These include network voltage fluctuations, changes in the active and inductive components of resistance, changes in the shape of the electrodes, compressive forces of the electrodes, and others.

In the course of work carried out on welding machines that use solid control, it was established that the biggest problems arise in micro-welding, when it is impossible to create a margin in terms of the number and size of points, and the implementation of control with feedback is practically not carried out due to the adopted AC power supply system industrial frequency, or supply by discharge of capacitors.

If we have in mind microwelding with high-frequency currents, then stabilization or reproduction of a given program of current or power cannot give a positive result. In this way, it is not possible to "work off" disturbances caused by a change in the condition of the surfaces of electrodes and the product, wear of electrodes and shunting of the welding site. Incomparably better is the system of stabilization of the voltage drop on the electrodes, which refers to the second method of automatic control of spot microwelding, but it also weakens, but does not help to get rid of the influence of fluctuations in the state of the surfaces of the electrodes and the product. In particular, voltage stabilization facilitates the operation of electrodes in the case of unstable surfaces and reduces the possibility of splashes of molten metal. The first experience of creating such about thirty years ago was made in

MVTU named after Bauman V. I. Matveev under the guidance of prof. O.N. Bratkova. They obtained the copyright certificate of the USSR M238701 under the title "Device for contact welding at increased frequency", published on March 10, 1969, bulletin Me10, IPC B23K11/00. This work was not further developed due to the great complexity of the electronic technology of that time and possibly for a number of other reasons. The effectiveness of using high-frequency currents for microwelding was shown on a specific example of connecting a thin wire to a printed circuit board.

The energy stabilization system, like the use of capacitor discharge on a welding machine, does not respond to the volume through which the current flows, to the condition of the surfaces, and therefore is far from ideal.

The disadvantage of the second method of microwelding is that all control systems, in which the regulated value is the two main electrical quantities or other quantities obtained from the first two by calculation, only indirectly reflect the process of the formation of the welded joint, react only to part of the disturbances.

There is positive experience in the development and application of devices for operational assessment of the quality of a welded joint, assessed by the diameter of the cast core of the point. For this, statistical models and artificial neural networks are used. Works in this promising direction were carried out in the Special Economic Zone named after E.O. N. V. Paton of the National Academy of Sciences of Ukraine

Podolai and his employees under the leadership of Acad. B.E. Paton (see the journal "Automatic welding", "Mathematical modeling of welding processes for the creation of systems for forecasting the quality of joints and optimal control" 1971. Me7, p. 1-5; "Estimation of the quality of contact spot welding with the help of neural networks" 1998. M12, pp. 3-10, etc.) In the future, there is a possibility of creating "intelligent" automatic regulation systems built on the same or similar principles.

At one time, great hopes were attached to the use of the effect of thermal expansion of metal in the place of heating. Thermal expansion directly reflects the thermal state of the metal and in this connection it can be considered the most suitable as an adjustable value. Institute of electric welding named after E.O. Paton National Academy of Sciences

Equipment samples were created at the NIAT of Ukraine and Moscow, the tests of which in spot welding gave good results (see the magazine "Automatic welding", 1951, Me5 "Regulation of the process of spot welding by the size of the movement of the electric welding machine!" p. 18-29).

The disadvantages of this method are the reasons why these works did not receive further development - the complexity of implementation, as well as the negative impact of metal splashes on the operation of this type of system.

During microwelding, the impact of disturbances is more pronounced due to the advantage in the overall contact resistance of unstable transient electrode-part and part-part resistances. In this regard, it is especially important to create systems of effectively operating control systems with a short duration of welding. The creation of such not too expensive and reliable systems is possible on the basis of modern power electronics and microelectronics.

The main part of the power system is a converter consisting of: a mains voltage rectifier, a smoothing filter, an inverter on transistors and a control system with feedback. The inverter must work in pulse-width mode. Due to the fact that the thickness of parts during microwelding can vary widely, the current frequency and welding time must change.

Perhaps the other is simpler, when the inverter works with a constant frequency, and the regulation is carried out by a transistor regulator included between the rectifier and the inverter. At the same time, the regulator implements the specified current limitation, thus fulfilling the protective function of the converter. In general, the option with a transistor regulator is better for a three-phase power supply of the machine. The system ensures electromagnetic compatibility of the machine's power supply with other energy receivers.

As a prototype of the claimed method, the method protected by the patent ЕРО947279 "ВНевивиапсе мжеиаипд демисе", МКИ В2ЗК11/25, the applicant Maiszspec Mezziespik SMVN (CE), published on 10.06.1999, was adopted.

A known method of contact spot microwelding with automatic control, carried out on a machine with a stabilized supply voltage both directly from the alternating current network and through a frequency converter and the number of phases, in which the welding mode that meets the requirements and is referred to as the reference, memorized in advance is selected measure the voltage on the electrodes, the welding current, and during welding, measuring devices continuously measure the dynamics of changes in these parameters. If deviations from the specified curve occur, the automatic control system adjusts the current source and the compression force of the electrodes so that the measured curves coincide with the specified ones as much as possible.

The disadvantage of this method is that in the case of a negative deviation of the welding current curve from the specified one, the control system increases only the primary current and voltage, although it is possible to increase the time of the process, thus stabilizing the energy invested in welding; also, the system does not take into account cases when the negative deviation of the current from the set point is too large, which can be caused, for example, by wear of electrodes or other reasons. In this case, the system will probably try to increase the welding current, perhaps without reaching the required value and the welding point will be recognized as poor quality.

The basis of the invention is the task of improving the above-mentioned method of contact spot microwelding with the duration of the process up to a hundredth of a second.

The significant differences of the proposed method are as follows: - the stabilization of transient contact-part and part-part resistances occurs with a smooth rise of the voltage on the electrodes from zero to the reference value by means of improving the controller program that controls the process contained in the control scheme; - remember the voltage on the electrodes, the welding current and the useful power as a function of time; - with the measured negative deviation of the current, the system stabilizes, in addition to the voltage, the energy spent on heating the metal by increasing the current flow time; - the user receives a sound or other signal indicating the need to replace or clean the electrodes, or to clean the surface of the product being welded, with the simultaneous termination of the heating process, if the negative deviation of the current in absolute value exceeds the preset value; - the entire device is greatly simplified.

For optimal regulation of the welding process, a control method is used, in which the main values: welding current strength, voltage on the electrodes, welding time are continuously fixed at regular small intervals by measuring devices. If the deviation of the welding current curve from the recommended one occurs, the monitoring and/or regulating equipment adjusts the voltage on the electrodes and/or the welding time so that the value of the energy spent on welding coincides with the value recommended as much as possible.

The device for implementing the method of contact spot microwelding consists of: - a control circuit containing a control controller with a built-in high-frequency pulse generator, a generator of control pulses of power transistors, a panel with a keyboard for entering parameters, a graphic display; - power sources of the control circuit; - the inverter, which is a block of power bipolar transistors with an isolated gate; - welding transformer; - welding circuit with electrodes; - voltage sensor on the electrodes; - welding current sensor.

The implementation of the proposed method is carried out by a device whose block diagram is shown in Fig. 1.

Figure 1 shows: 1 - control circuit 2 - input voltage regulator.

C - block of power transistors (inverter) 4 - welding transformer - welding circuit

6 - electrode 7 - voltage sensor 8 - current sensor 9 - permanent memory device - operational memory device.

The method is carried out as follows.

The user, guided by his own experience, selects the welding mode that ensures the required quality of the welded joint. At the same time, they use the data contained in the non-volatile memory device 9. The control system remembers this mode as a reference mode. For the welding operation, carefully prepared parts are required, which are clamped in the electrodes 6. The power circuit of the inverter 3, the welding transformer 4 and the welding circuit 5 are included.

The initial stage of the microwelding process proceeds with a gradually increasing voltage, which is provided by the input voltage regulator 2 according to the signals from the control circuit 1. At the same time, transient supports in the contact zone stabilize or completely disappear due to heating and deformation of the microprotrusions on the contacting parts, thinning and partially diffusing in the main megal surface films. In order to achieve stabilization of transient resistances, the initial stage of the welding process must be long enough.

Next, after the voltage has risen to the reference value, the control system monitors the welding current with a fairly high frequency using the current sensor 8, which is connected between the input voltage regulator 2 and the inverter 3, the voltage on the electrodes b using the voltage sensor 7, as well as the corresponding this power value, compares the instantaneous values of the welding current with its reference value. These data are entered into the operational memory device 10. The operating mode of the system is determined by the sign of the deviation of the current from its reference value at the end of the first stage.

There are three options for continuing the welding process: - the deviation from the reference value is close to zero - in this case, the system works in the mode of stabilizing the voltage drop on the electrodes until the end of welding. - If the current deviation is positive, the system operates in the secondary voltage reduction mode until the current instantaneous values of the welding current become equal to the reference values. - If the current deviation is negative, this indicates an increased resistance of the welding contact.

It is advisable that the system also works in voltage stabilization mode, but the lack of energy should be compensated by increasing the time of current flow. For this, it is necessary to stop the welding process when the energy reaches the value corresponding to the reference mode. However, if the negative deviation of the current exceeds a predetermined acceptable control threshold, the heating process is stopped and, at the same time, the user receives an audible or other signal indicating the need to replace or clean the surface of the product being welded.

The proposed method has the following advantages compared to the prototype: an improved controller program that controls the process - the initial stage of the microwelding process, which proceeds at a gradually increasing voltage to stabilize the contact resistance, is introduced; the system stabilizes, in addition to the voltage, the energy spent on heating the metal by increasing the current flow time; special signal devices are introduced to notify the user of the need to replace electrodes, malfunctions in the equipment, etc., the whole device necessary for the implementation of the proposed method is simplified. Thanks to these positive qualities of the proposed method, a positive effect in the process of forming a welded spot is achieved. implementation of the method is possible on the basis of modern microelectronics and does not require large costs, which allows this method of microwelding to be used in industry, in particular, instrument building, electronics, and the production of household appliances.

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Claims (5)

1. The method of contact spot microwelding with automatic control, carried out on a machine with a stabilized supply voltage both directly from the alternating current network and through a frequency converter and the number of phases, in which the welding mode that meets the requirements and is referred to as the reference, is memorized in advance the voltage on the electrodes, the welding current and the useful power are measured as a function of time, and during subsequent weldings, the current values of these values are compared with the reference values, the obtained result is analyzed, and on the basis of this analysis, a control effect is produced, which differs in that if the value measured during the welding process is negative deviation of the welding current from the reference current in absolute value does not exceed the pre-set limit value, then stabilize the energy spent on heating the metal by stabilizing the voltage on the electrodes and increasing the current flow time. 2. The method according to claim 1, which is distinguished by the fact that the stabilization of transient resistances contact-part and part-part occurs with a smooth rise of the voltage on the electrodes from zero to the reference value. 3. The method according to claim 2, which differs in that the sign of the current deviation from the reference is determined at the end of the voltage rise. 4. The method according to point 3, which differs in that when the measured positive deviation of the welding current value is from the reference value, the energy spent on heating the metal is stabilized by stabilizing the voltage on the electrodes. 5. The method according to claim 4, which is characterized by the fact that if the negative deviation of the welding current in absolute value exceeds a predetermined limit value, the user receives a sound or other signal indicating the need to replace or clean the electrodes or clean the surface of the product being welded with simultaneous termination of the heating process.