Research On Measurement Of Thermal Expansion Electrode Displacement And Quality Control For Spot Welding

Resistance welding is one of important welding methods with extensive application in automobile industry. Constant current control is usually adopted to ensure the welding quality in production and this method is simple to control and low cost, but the influence cannot be compensated that caused by shunt, loop inductive reactance, and electrode wear, etc. Therefore, welding quality control using electrode displacement of thermal expansion is proposed in this paper. The heating strength of the welding zone and the nugget size can be reflected by the electrode displacement amount, the displacement rate and displacement peak value are taken as characteristics. The heating strength is reflected by the displacement rate and the welding time is reflected by the displacement peak value, thus the welding quality can be ensured by regulating these characteristics.The specific study contents of the proposed quality control method are as follows: Thermal expansion amount is real time detected by grating displacement sensor with quick dynamic response, high resolution and repeatability. The data is acquired and processed by the PIO-D24 board and VB program. The repetitive spot welding experiment is carried on the DN-25 pneumatic control spot welding machine, and the variable regulation of the electrode displacement curve in different welding characteristics is analysed. The variable regulation of the electrode displacement curve is also analysed by simulating some factors such as shunt, electrode wear and loop inductive reactance, etc. The nugget size fluctuation is discussed respectively in the circumstance of with or without the influencing factors. The optimum characteristics of the electrode displacement are established for the special slab thickness of the stainless steel or the low carbon steel. The closed loop control is realized by adopting doubled processor by taking the displacement rate and the displacement peak value as characteristics, in witch 89C51 MCU is taken as the core.The results of displacement measurement and the spot welding experiments indicate that the grating sensing device for measuring electrode displacement is characterized by high sensitivity and resolution. As its small volume and the measurability without reference zero point it is suitable for dynamic measurement. Therefore the measured data obtained from grating displacement sensor is precise and high of repeatability. It is applicable for measuring the expansion displacement repeatedly and realizing the automation for spot welding. The welding experimental results indicate that the proposed method can effectively compensate the influence caused by various factors in which way the welding quality stability can be ensured.