A class of fuzzy observer is designed for the T-S fuzzy system with immeasurable premise variable. The fuzzy
T-S system is changed into the descriptor system. A class of fuzzy observer is proposed for the descriptor system, the control
input variable disappears in the error equation of this kind observer. By Lagrange theorem, the fuzzy observer design problem
is transformed into the problem of solving the linear matrix inequality for the fuzzy system with disturbance in the measure
part. This method is applied for the arc length observing in the gas metal arc welding(GMAW) system. The simulation
results show the validity of this approach.