本文利用分片光滑的Lyapunov函数来进行扩张状态观测器的误差分析，指出了非线性扩张状态观测器有更好的估计精度能力，给出了为提高估计精度扩张状态观测器的参数所满足的条件．

The piece-wise smoothy Lyapunov function is used for the error analysis of the second order ESO. It is indicated that the nonlinear ESO has ability to get better estimation precision. For the better estimation precision the conditions that should be satisfied by the parameters of ESO are obtained.