M.Sc Thesis

M.Sc StudentBitansky Avi
SubjectUsing Multiple Sensing to Overcome Structural Bending Models
in Flight Control
DepartmentDepartment of Aerospace Engineering
Supervisors PROF. Yoseph Ben-Asher
PROF. George Hexner
Full Thesis textFull thesis text - English Version


For over half a century aerospace engineering has been challenged by the need to control flexible systems with structural elastic bending modes.

The presence of elastic modes can introduce destructive instability to an aerospace system. The deflection of the control surfaces or other external forces may excite these oscillatory structural modes. The sensors measure these vibrations, and introduce them into the flight control. In an unstable case, the control may cause a diverging affect which can be destructive to the system.

The most common and straight-forward method of dealing with these elastic modes is the use of notch filters. The major downside to using filters is the added lag to the phase at low frequencies. This added lag degrades flight performance and reduces stability margins. In cases where stability margins are minimal, this may become crucial.

This research examines a solution to the problem based on fusing data from multiple sensors. The measurements from these sensors are fused to create a measurement nearly without the elastic modes and without adding lag common to filtering. The data fusion algorithm uses an elastic model of the system, and by using the data from n sensors along the body, an output of the rigid body motion cleaned of the first n-1 bending modes is produced. This research describes an implementation of an algorithm using multiple rate gyros which was found in the literature survey. Another variation using accelerometers is suggested.

The goal of this research is to show the feasibility of implementing this method on a designed test case and with real hardware. A test case of a long flexible missile is analyzed. The performance of the implemented algorithm on the test case is reviewed. A sensitivity study is performed. A lab experiment was designed and conducted allowing for a performance analysis of the algorithm on real hardware, specifically, simple, off the shelf gyros. The results of the simulated test case and of the lab experiment showed that good attenuation of the first and second elastic mode can be achieved with no added lag. The conclusion from this work is that this method can potentially simplify the problem described above of the interaction of structural elasticity and flight control.