说明：  基于BP神经网络的中PID控制，把被控对象的模型，现在变为二阶传递函数：G(s)=1/(0.003s^2+0.067s) ，想仿真此对象的阶跃跟踪的效果
(Based on BP neural network PID control, the model of the controlled object, now turned into second order transfer function: G (s) = 1 / (0.003 s ^ 2 + 0.003 s), to the simulation step tracking effect of the object.)