Abstract: Based on chaos time series and fractal theory, electrode displacement signals are studied in the process of spot welding. Seven states of electrode displacement signals are studied to determine the largest Lyapunov exponents from acquired time series data. According to the calculating results using phase space reconstruction, the largest Lyapunov exponents are positive number in the seven states of electrode displacement signals, and chaos characteristics are discovered in electrode displacement signals of spot welding. Therefore, the result shows a new state in the process of the spot welding and electrode displacement signals should be analysed by chaos method. In order to research electrode displacement signal, box counting dimension is put forward to analyze and calculate chaos characteristics. The experiment and calculation results indicate that the box counting dimension of electrode displacement signals is clearly distinguishable in the welding process with different welding parameters. A new method of judging and forecasting the quality of spot welding is provided.

Key words: Box counting dimension, Chaos time series, Electrode displacement signals, Lyapunov exponents, Spot welding