improve PI model

代码说明：

  基于改进的PI模型对非线性曲线进行拟合，二次寻优算法进行参数辨识，用逆模型前馈补偿(Based on the improved PI model, the nonlinear curve is fitted, and the second optimization algorithm is used to identify the parameters, and the inverse model feedforward compensation is used.)

文件列表：

improve.m, 2834 , 2018-02-06