Modeling and Micro-Radian Precision Pointing of a Flexible Manipulator With the Existence of Static Friction

Controlling flexible structures has been considered as a well-developed topic in the control field. In this topic, vibration suppression and line-of-sight (LOS) pointing are two of the main research issues. Unfortunately, among all works reported, only a few have really achieved high levels of pointing precision. This is mainly because friction, especially static friction, is hard to deal with. In this research, the LOS control of the flexible beam of a standard hub-beam system is studied. The precision required is 1 s^-1. Instead of treating friction as an external noise and trying to compensate for it, the friction phenomenon is represented by a linear spring and is included as part of the system model when the hub sticks. This is, in fact, a good approximation of the hysteretic friction behavior in the presliding phase if the spring stiffness is appropriately chosen. This modification greatly enhances the fidelity of the system model. What is more important is that it makes active controls still available in the stick phase to facilitate the fine tuning of the pointing error. Based on this modified model, it is then not hard to synthesize traditional proportional-integral-derivative and linear quadratic Gaussian controls to get LOS pointing with microradian precision.