Factors influencing the welding current control accuracy of intermediate frequency spot welder

In the welding process, because the change of resistance will lead to the change of welding current, the welding current needs to be adjusted in time. At present, the commonly used methods include dynamic resistance method and constant current control method, etc., whose purpose is to keep the welding current constant through control measures. Because the dynamic resistance is difficult to measure, the control operation is difficult to implement.

Therefore, Xiaobian adopts constant current control method to discuss, and first analyzes the reasons leading to low welding current control accuracy. The current control of the intermediate frequency spot welding machine, using the regulation of the thyristor conduction Angle to control the welding current, China uses 50Hz alternating current, the period is 20ms, each cycle has two half waves, each half wave is 10ms, that is, the regulation of the thyristor conduction Angle can only be adjusted every 10ms. In terms of digital control, the beat time is 10ms.

This 10ms is the problem: the beat time is too long. Since the resistance of the object to be welded will change with the increase of temperature, 10ms time is enough to produce a considerable amount of change. The conduction Angle calculated at the starting time of 10ms is no longer suitable for the state after the change of resistance, so the welding current will certainly produce a large error. After the closed-loop control is adopted, the conduction Angle of the next beat can be adjusted according to the welding current returned by the feedback, but the same problem will still occur in the next beat, and the output current of the controller will always deviate greatly from the given value.

From the above analysis, it can be seen that too long beat time is the main reason leading to large welding current error. In the welding process, if the resistance change can be predicted in advance, and the influencing factors are taken into account when calculating the on-angle, a more reasonable on-angle can be obtained, so that the welding current is closer to the given value. Based on this, the feedforward control is added on the basis of conventional control, and the feedforward control algorithm is mainly to predict the current change caused by the resistance change. Thus the purpose of precise control of welding current is realized.