Process control in resistance spot welding
The process control in resistance spot welding has the task of guiding the welding process in the event of changing influencing variables in such a way that a sufficient connection quality of the weld point is guaranteed. Attempts are made to achieve this by specifically changing the welding parameters with parameter control or in a closed control loop . Possible control variables are the welding current I s , the welding voltage U s , the welding power P s , the welding energy W s or the welding force F s . A regulation of the electrical parameters and the force plays a special role, especially when welding small parts and aluminum, but also when welding parts with coated surfaces, since the resistance ratios between the electrode and the part to be joined and between the parts to be joined change significantly in the course of the process.
Unregulated welding process
( Resistance spot welding process when welding unalloyed steel sheets with a bare surface).
In the course of the welding process, the transition and material resistances R 2 ... 6 , which are decisive for the heating of the welding point, change. After the electrode force has built up and the welding current I s has been switched on, the contact resistances R 2,5,7 decrease by leveling the surface roughness. The beginning of the warming causes the material resistance R 3.5 to rise to a maximum. The material softens, the electrode partially penetrates the material, and the resistance drops. This results in a so-called dynamic resistance curve typical of the material.
If the welding is carried out with approximately the same effective value of the voltage (constant phase control angle for mains frequency welding, constant pulse width for inverter welding), the current flow is established according to Ohm's law . With strongly fluctuating resistance ratios, there is very different heating and thus welded joints of different sizes and strengths.
Stepper function
With spot welding , the electrodes wear out as the number of welds increases, which means that the resistance drops continuously and the weld spots become smaller. To achieve the same spot diameter, the welding current must be regulated to specified target values and increased in a targeted manner. The so-called stepper function can do this. A spot weld counter registers the number of spots welded. If the meter readings are to be specified, the welding current is increased either gradually or continuously. The current setpoints must be determined experimentally. In today's automated systems, the stepper function is combined with cap management (cap milling and changing).
A constant current control is required for the stepper function to work.
Constant current regulation
Keeping the welding current constant is one way of stabilizing the welding process. With constant current regulation, the welding current is measured on the secondary side with the aid of a Rogowski coil and compared with the specified target value. The manipulated variable for constant control is either the firing angle of the thyristors for mains frequency welding or the pulse width for inverter welding. In this way, disturbances such as mains voltage fluctuations, secondary impedance changes or surface changes can be partially recognized and compensated for. In mains frequency applications, the controller works on a period-related basis, the response time of the controller is 20 ms. Inverters react much faster depending on the frequency.
The constant current control is appropriate for welding flat connections with relatively constant contact resistances. In the event of extreme changes in the resistance ratios due to changes in the power line cross-section within a weld (e.g. with small-part and micro-welding, cross-wire welding, projection welding), the constant current control can lead to insufficient welding energy if the resistance drops due to the cross-section widening during the process time or to overheating if the Resistance increases.
Example: When projection welding of weld nuts that do not contact ideally evenly: One-sided contact leads to high resistance with a low current cross-section. The constant current control leads to a welding current that is set for the full welding cross-section. As a result, the material is overheated and spattered. The required welding quality cannot be achieved.
Constant force control
With a constant welding force, a reduction in resistance due to the increase in force associated with thermal expansion is prevented. Furthermore, the follow-up behavior is significantly improved, since the melting of the lenses can lead to a drop in force. The constant force control can basically reduce the welding current, which has a positive effect on the wear of the caps.
Constant voltage regulation
Rapidly changing contact resistances as a result of changes in surface conditions or contact surfaces (hump shapes) or force variations change the heat input while the current remains constant. This can result in splashes or insufficient heating. If the voltage is kept constant, the current can adapt itself to the conditions within certain limits. This type of control is important for cross wire welds and hump-like welds, especially for small parts and micro-welding.
Constant power control
This type of control can be useful to compensate for electrode wear and alloying. Applications can be found particularly in small part and micro welding.
"Adaptive" or "intelligent" controllers
The welding process and the quality are stabilized by regulating the electrical parameters. To this end, various concepts for industrial use have been developed in recent years. The welding current or time are changed according to the current process status with the aim of bringing the quality of the welding point to the desired level.
A variable that reflects the process state and can be used to control the process is the dynamic resistance R s ( t ). On the basis of the measurement of current and voltage, the resistance is calculated as their quotient after the formation of the effective values. A control signal for the welding current is generated from this. The patent does not describe how the control signal is obtained.
Another concept is an extension of the constant current regulator (KSR). There are I s ( t ) and U s ( t ) measured on the welding gun. The current resistance or power is calculated from this. Depending on the resistance curve or the power, the current setpoint of the KSR is changed and, if necessary, the welding time is also extended.
A third concept is based on the principle of reference welding. Here too, the course of the dynamic resistance is used to characterize the process. So-called reference welds are first carried out, during which the welding current is regulated to the set value with KSR. If the welding time, welding current and electrode force are optimally selected, a good weld connection is achieved without interference. The welding current and electrode voltage data are saved. After switching to the "Control" operating mode, all subsequent welds are controlled in such a way that they largely correspond to the saved MASTER reference data.
A fourth concept was developed on the technical basis of recording the process voltage directly between the cap surfaces via virtual measuring lines.
The measurement points for the voltage located inside the welding gun are transformed via a "virtual machine" (according to P. Puschner, 1992) to the contact points between the caps and the component. This enables the current process parameters impedance, power and energy to be determined in a 50 µs cycle. Instead of the known process controls according to a current / time program, the energy dependent on the total sheet thickness is introduced. For this purpose, the current total sheet thickness is determined for each individual point using an integrated, precise measuring system after the clamps have been closed. The method is based on a calorimetric model which, based on the required lens volume and thus the geometric expansion of the lens, calculates the required energy input, including the heat dissipation into the water-cooled electrodes and the surrounding component, and uses it as a reference variable. From this, regardless of process-related impedance changes during joining, but also due to the respective material combination, the safe required energy input follows, which in turn represents a significant advance in quality assurance from point to point.
The process is to be explained using the process control image for resistance spot welding with a “virtual machine” . A rapidly controllable power source (1) is connected to a welding gun (3) via the power cable (2). At the end of the two tong arms are the electrodes, on the surfaces of which the current process voltage is to be recorded. Measurement lines lead from measurement points for voltage detection on the clamping devices of the arms in the pliers via a measurement processing unit (9) to the control unit (12) (virtual machine), which transforms the real measurement lines onto the cap surfaces and thus generates virtual measurement lines (8). The instantaneous process impedance is calculated from the voltage signal (10) (process voltage) generated in this way and the current signal (11). Depending on the total sheet thickness measured, the control unit receives, via the sensor system (13) for the various process phases, reference values from a database (14) for the energy to be introduced and the currently active static and dynamic generator behavior of an electrical source virtually formed on the cap surfaces. Depending on the requirements of the process, the effective source property is adapted to the process in a 50 µs time frame. The ability to adjust the generator properties during different process phases means that spatter formation can be largely avoided and, as a rule, completely prevented. The power unit is influenced via the control unit. [6]
credentials
- ↑ M. Krause: Resistance pressure welding . DVS- Verlag, Düsseldorf 1993, ISBN 3-87155-531-2 .
- ↑ a b Patent DE4317557C1 : Device for welding current control during spot welding with a measuring transducer that works according to the effective value method for quick detection of the actual value of the resistance between the electrodes. Inventor: St. Köbler.
- ↑ Th. Jansen, J. Eggers, R. Bothfeld: GeniusMFI IQR - a new inverter power supply with adaptive regulation system to assure the quality for resistance spot welding. IIW -Document III-1480-08, 2008.
- ^ Matuschek Messtechnik GmbH: MASTER control method
- ↑ a b P. Puschner: Spot welding with a "virtual machine". In: welding and cutting. 57, H. 1-2, 2005.
- ↑ Patent DE10334478B4 : Method and device for resistance welding. Inventors: G. Kölzer, P. Puschner, D. Regner, W. Riether.