2Guo B Z, Han J Q, Xi F B. Linear trackingdifferentiator and application to online estimation of the frequenc y of a sinusoidal signal with random noise tion[J]. Int J of Systems Science, 2002, 33(5) :
3SuY X, Zheng C H, Mueller P C, et al. A simple improved velocity estimation for low-speed regions based on position measurements only[J]. IEEE Trans on Control Systems Technology, 2006, 14 (5): 937- 942.