DE4133263A1 - Operating a resistance welding machine - by ensuring symmetric magnetisation in the transformer core - Google Patents

Abstract

Method of operating a resistance welding machine involves the use of mains fed direct current rectifier (1), an inverter (3), a welding transformer (4), measuring devices (5, 6) for measuring the instantaneous load current and its corresp. voltage in each half wave in the prim. circuit; and computer (12) for calculating the energy content in each half wave and a regulator (13) connected to the computer and A/C rectifier for regulating the following oppositely polarised half wave such that it has the same energy content. ADVANTAGE - Ensures symmetric magnetisation in the transformer core by rapidly changing the current and voltage to enable equal energy contents in the positive and negative cycles of the waves.

Description

The invention relates to a method and a device for operating a resistance welding machine with a Medium frequency welding transformer. Such sweat Transformers have the advantage that they are the same Performance considerably lighter than a mains frequency transform mator are, therefore advantageously directly with the Connect welding gun and when using flexible fer manufacturing systems, for example industrial robots, directly Arranged by the robot arm and so easily into any welding po let sition bring. Instead of a flexible, water resistant filled cooking power line is only to one light connecting line significantly lower cross cut from the robot arm.

To be a medium frequency transformer with a current To supply the tuned frequency, the mains current initially one, two or three phases in one controlled Rectifier converted. The direct current can be direct or a ge via a buffer or capacitor bank controlled inverters.

In one of the German published patent application 36 18 348 known device for operating a resistor welding machine serves as an inverter leading pulse-controlled inverter, which consists of a rectifier ter powered battery with the highest possible frequency of pulses about 500 Hz. Instead of batteries or accumulators gates can also be in the known device use large electrolytic capacitors if that Welding machine with very short switch-on and long ones Break times works. Charging current, load current and battery  voltage are monitored by a controller and according to the load or energy requirement through control of the rectifier and inverter changed.

The disadvantages of the known device lie in the brisk technical part, in particular in the regulation of Welding current and in the magnetization of sweat transformers.

An important requirement when dining on Transforma is by inverters or AC power controllers their symmetrical magnetization by the imprinted Tension. It is necessary that the nega tives and the positive half waves each the same Have energy content, otherwise the transformer too with smaller differences magnetized asymmetrically becomes. If a difference persists, a will always be large Major part of the energy supplied for magnetic reversal spent because the permeability of the iron core returned goes.

Due to the magnetic losses, the Iron core of the transformer more and more, whereby the Permeability also deteriorated. This feedback kung causes the efficiency of the transforma to decrease tors up to the area in which it is unipolar saturated and therefore almost no more energy can be transmitted and instead most of the energy supplied as reactive power between supply and transformer commutes, with a part being converted into heat.

These conditions occur in welding transformers equal to normally used transformers due to the extreme switch-on and operating conditions essential earlier, because the secun därkreis is short-circuited at the moment of switching on and  Welding transformers without economic reasons towards the saturation limit.

This disadvantage manifests itself in welding transformato with a short duty cycle and clock rate can only be here losses and thus by a worse we degree of efficiency, since the welding transformer is among these Operating conditions in the power pauses magnetically can and becomes magnetically neutral again. At Welding transformers with longer duty cycles and / or high clock rate, however, very quickly occur high current consumption and heating up to saturation, so that continuous operation of asymmetrically magnetized and saturated transformers is not possible.

An asymmetrical magnetization and saturation of the Welding transformer of the device according to the German Laid-open application 36 18 348 can be described with benen circuit not avoid, so that this be knew the resistance welding machine described Disadvantages occur.

In "Welding and Cutting 33", 1981, Issue 8, pages 368 to 371, a resistance welding machine is described, where the asymmetries in sweat described above transformer through an output series resonant circuit a resonant circuit capacitor and the control inductance of the welding transformer can be avoided. The swing circuit is fed by the inverter. Disadvantageous is with this resistance welding machine that a too additional capacitor in the output series resonant circuit is required to transmit the entire welding energy and therefore must be dimensioned accordingly large. Of others are an additional medium frequency separation trans formator and in the secondary circuit of the welding transformer  a high-current rectifier in the center circuit is required such.

Another disadvantage is that it is not possible to To regulate the welding current periodically in order to create a peri to achieve electrode modulation of the welding current like it required when welding coated materials is. Also short, steep and high-energy welding pulses cannot be generated in a controlled manner, as they are for Welding workpieces with a larger thickness ratio nis are required because the welding current setpoint at higher welding currents is only reached after 50 ns. That also changes the short dead time after reaching the current Setpoint nothing because the current after a preset is ramped up. Finally, the broadcasts Output series resonant circuit only certain frequencies without Damping.

The invention is therefore based on the object, a Ver drive and a device for operating a contra stationary welding machine with a mains-powered control rectifier, a controlled inverter and to create a welding transformer where one asymmetrical magnetization prevented or in the shortest possible time balanced and a performance control is achieved in a simple manner.

Based on this task, a ver drive the aforementioned type of eyes according to the invention current value of the load current of each half-wave in the primary circuit of the welding transformer and synchronously with that voltage measured, the energy content of each half wave determined and the following, opposite polarity Half wave on with regard to a symmetrical magnet welding transformer using the same energy stop regulated.  

In this way, the transformer core can be adjusted as required keep the period magnetically neutral and still possibly also the load current for each individual period accordingly control an adjustable setpoint.

In addition, the entry of the transformer core in Preventing the saturation limit can come from the current rate of increase of the load current for each individual Half wave reaching the sweat saturation limit transformers determined, this half-wave switched off and the following, half-wave with opposite polarity the same energy content can be switched on.

The energy content of the half-waves can be advantageous usually over the duty cycle of the respective bridges regulate branches of the inverter.

A con capacitor battery during welding when switched off serve rectifier, the welding performance constant when the capacitor voltage drops an adjustable setpoint can be regulated. The con The capacitor battery can then be controlled by the DC refilled during the breaks in welding will.

The regulation of the energy content of each half-wave of the load currents for each individual period and switching the Half waves corresponding to the rate of current rise can be carried out in real time and is particularly advantageous reached when the instantaneous values of the load current and the voltage digita in equally long time intervals lized, fed to a digital signal processor, the Energy content calculated for each time interval, the on added up and compared with a setpoint  and as a controlled variable for switching on the respective Bridge branches of the inverter for the calculated Serving time.

Digitization can currently be used switchable time intervals from 50 to 200 ns pass. With the method according to the invention is always ensured that the welding transformer in each loading drive case is magnetized symmetrically, resulting in a ho efficiency of the resistance welding machine resul animals. Furthermore, the welding transformer is on due to the ongoing real-time measurement of the supplied elec saturation never unforeseen drove. This leads to optimal use of the egg lowering. By feeding the inverter from one disconnected capacitor during welding battery will calculate the falling voltage when calculating the welding performance in real time and con kept at the setpoint.

Since there is a separate electricity target for each individual period value, it is possible to adjust the welding current modulate the course according to the requirements.

The welding sequence can be up to the maximum transferable Increase the frequency of the welding transformer. The reach frequencies can go up to 2 kHz, so that the Welding transformer very small and easy to build.

Overall, the control system automatically adapts to the respective welding transformer, the location freely select the limit values within this adjustment leaves.

The inventive device for operating a Wi derstandsschweißmaschine has a measuring device for  Instantaneous value of the load current of each half-wave in the primary circuit of the welding transformer, a measuring device for the corresponding instantaneous value of the tension of each half wave in the primary circuit of the welding transformer, one Calculator to calculate the energy content of each half-wave and one connected to the computer and the inverter those controllers for regulating the following, opposite polarized half-wave on the same energy content.

A digital computer is preferably used, since a such a computer when it clocked at 20 MHz Ana log / digital converters for the measured instantaneous values of the load current and voltage related Calculate and add up the instantaneous values of the energy content in real time and via one with the Computer and controller connected to the inverter with Um setting the digital control signals of the computer in Steuer signals the inverter can operate accordingly.

The computer can consist of a signal processor, with a fast static RAM (Random Access Memory) for buffering the measured values, the calculation results and the operand is connected. Furthermore, with a fast bipolar PROM to the signal processor (Programmable Read Only Memory) to save the sequence and control program.

To ensure that the instantaneous values of the load current and the associated voltage correctly assigned and processed, the analog / digital converter connected to the signal processor via a control line be and be synchronized by this.

For a fast and correct signal transfer between the signal processor, the RAM as well as the PROM and the reg with regard to the setpoints, limit values, inputs and  Switch-off signals and logging of the welding par meter can be between the signal processor and the controller a central computer connected to the PROM and the RAM be switched.

In addition to the inverter, the controller can also be used with the Rectifiers connected to one between the Rectifiers and the switched condensers sator battery controlled during welding interruptions load and an emergency shutdown if To carry out dangers.

The invention is based on a in the drawing tion shown embodiment of the closer he purifies. Show in the drawing

Fig. 1 is a circuit diagram of the Vorrich device according to the invention and

Fig. 2 shows a current rise curve and a voltage curve, which are measured in real time at intervals of 200 ns.

A controlled rectifier 1 is connected to three phases L1, L2, L3 of a network. The generated DC voltage serves to charge an existing capacitor battery 2 made of electrolytic capacitors. The capacitor battery 2 is connected to a controlled inverter 3 , which in turn supplies a primary circuit 15 of a welding transformer 4 with medium-frequency current of a frequency of up to 2 kHz. In the primary circuit 15 , a current transformer 5 with largely delay-free detection of the load current of the welding transformer by means of Hall elements is arranged in its air gap. The measurement of the primary voltage on the welding transformer 4 is carried out by means of an optoelectronic voltmeter 6 , which is galvanically separated from the primary circuit 15 of the welding transformer 4 . The secondary circuit 16 of the welding transformer 4 forms the welding circuit.

Two functionally equivalent analog / digital converters 7 , 8 convert the instantaneously measured instantaneous values of the voltage and the current into a digital value with a resolution of nine bits. The analog / digital converters 7 , 8 are clocked synchronously at 20 MHz by a signal processor 9 via a control line 14 .

The signal processor 9 serves to carry out all mathematical operations and size comparisons including the decisions provided in the program. With the signal processor 9 , a fast static RAM 10 is connected, which is used for the intermediate storage of the measured values, computing results and operants. Furthermore, a fast bipolar PROM 11 is connected to the signal processor 9 , in which the sequence and control program is stored.

A central computer 12 is connected to the signal processor 9 as well as the RAM 10 and the PROM 11 ; It carries out the entire signal transfer for the setpoint, limit value, switch-on and switch-off signals and the logging of the welding parameters. The central computer 12 acts on a controller 13 , which serves as an auxiliary control to convert the signals received by the central computer 12 into signals which can be implemented by the rectifier 1 and the inverter 3 . In the controller 13 , the hardware signal linkage is thus taken before; the potentials required for this are implemented and / or galvanically isolated.

As shown in FIG. 2, the instantaneous values of the current and the voltage are detected at a distance of 200 ns, converted into digital quantities with equally long time intervals by means of the analog / digital converters 7 , 8 and fed to the signal processor 9 . The signal processor 9 calculates the instantaneous values of the energy content of a half wave and adds them up. The intermediate values, measured values, calculation results and operants are stored in RAM 10 . Using the sequence and control programs stored in the PROM 11 , the signal processor 9 calculates the energy content which the next, oppositely polarized half-wave must contain. The target value for the welding energy is used as the basic value for the entire calculation process. The respective bridge branches of the inverter are then switched on for the calculated time period via the central computer 12 and the control device 13 . The signal processor 9 also recognizes by evaluating the current rise rate di / dt the entry into the saturation limit of the welding transformer core and causes the current half wave to be switched off. The current and voltage values remain practically constant between the measuring intervals due to the inductive component of the welding transformer.

The inverter 3 is thus the final control element of the rule. It draws the energy from the capacitor bank 2 during welding, forms it taking into account the correction values calculated by the signal processor 9 and feeds it to the welding transformer 4 . During welding, the rectifier 1 is switched off, so that the capacitor bank 2 is disconnected from the mains during this time. The falling voltage of the capacitor bank 2 is taken into account in the calculation of the welding power in real time and kept constant at the setpoint. During the welding interruptions, the controlled rectifier 1 recharges the capacitor bank 2 .

In the event of dangers, such as load shedding due to an open secondary circuit or contact errors, the rectifier 1 is switched off by the controller 13 .

Claims (17)

1. Method of operating a resistance welding machine with

• - a mains-powered, controlled rectifier,
• - a controlled inverter,
• - a welding transformer in which
• - The instantaneous value of the load current of each half-wave in the primary circuit of the welding transformer and
• - synchronously measured the corresponding voltage in the primary circuit,
• - The energy content of each half-wave is determined and
• - The following, oppositely polarized half wave is regulated to the same energy content for a symmetrical magnetization of the welding transformer.

2. The method according to claim 1, characterized in that that the load current corresponds to every single period is regulated according to an adjustable setpoint. 3. The method according to claim 1 or 2, characterized records that from the rate of current rise of the load current for each individual half-wave Chen the saturation limit of the welding transformer determined, this half-wave switched off and the fol half-wave with the same polarity ben energy content is switched on. 4. The method according to one or more of claims 1 to 3, characterized in that the energy content the half-waves over the duty cycle of the respective Bridge branches of the inverter can be regulated.   5. The method according to one or more of claims 1 to 4, characterized in that the inverter during welding with rectification switched off ter from a capacitor bank with energy will worry. 6. The method according to claim 5, characterized in that the welding performance with sinking capacitor voltage constant to an adjustable target value is regulated. 7. The method according to claim 5 or 6, characterized records that the capacitor bank during the Sweat interruption through the controlled equal is reloaded. 8. The method according to one or more of claims 1 to 7, characterized in that the moment values of the load current and the voltage in equal lengths Digitized time intervals, a digital Signal processor supplied, the energy content for each of the time interval, the individual values dated, compared with a target value and as Control variable for switching on the respective bridges branches of the inverter for the calculated time serve permanently. 9. The method according to claim 8, characterized in that the duration of the time intervals be 50 to 200 ns wearing. 10. Device for operating a resistance welding machine with

• - a mains-powered, controlled rectifier ( 1 ),
• - a controlled inverter ( 3 ),
• - a welding transformer ( 4 ),
• a measuring device ( 5 ) for the instantaneous value of the load current of each half-wave in the primary circuit ( 15 ) of the welding transformer,
• - A measuring device ( 6 ) for the corresponding eyesight of the voltage of each half-wave in the primary circuit of the welding transformer,
• - A calculator ( 9 ) for calculating the energy content of each half-wave and
• - A ver with the computer and the inverter connected controller ( 13 ) for controlling the following, oppositely polarized half-wave on the same energy content for a symmetrical magnetization of the welding transformer.

11. The device according to claim 10, characterized by clocked at 20 MHz analog / digital converter ( 7 , 8 ) for the measured instantaneous values of the load current and voltage, a digital signal processor ( 9 ) for calculating and adding up the instantaneous values of the energy content, one with the signal processor and the inverter ( 3 ) connected controller ( 13 ) with conversion of the digital control signals of the signal processor into control signals for the inverter. 12. The apparatus according to claim 11, characterized by a with the signal processor ( 9 ) connected, fast les static RAM ( 10 ) for temporarily storing the measured values, the calculation results and the operants. 13. The apparatus of claim 11 or 12, characterized by a connected to the signal processor ( 9 ), fast bipolar PROM ( 11 ) for storing the run and control program. 14. The device according to one or more of claims 11 to 13, characterized in that the Ana log / digital converter ( 7 , 8 ) via a control line ( 14 ) connected to the signal processor ( 9 ) and clocked synchronously by the signal processor. 15. The device according to one or more of claims 10 to 14, characterized in that between the signal processor ( 9 ) and the controller ( 13 ) with the PROM ( 11 ) and the RAM ( 10 ) connected central computer ( 12 ) for the signal transfer between the signal processor, the RAM, the PROM and the controller with regard to the setpoints, limit values, switch-on and switch-off signals and logging of the welding parameters. 16. The device according to one or more of claims 10 to 15, characterized by a between the rectifier ( 1 ) and the inverter ( 3 ) switched capacitor battery ( 2 ). 17. The device according to one or more of claims 10 to 16, characterized in that the controller ( 13 ) is connected to the rectifier ( 1 ).