Using Multiple Sensing to Overcome Structural Bending Modes in Flight Control

Avi Bitansky
Work towards M.Sc. degree under the supervision of Prof. Ben-Asher and Dr. Hexner
Faculty of Aerospace Engineering
Technion – Israel Institute of Technology

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 control surfaces may excite these oscillatory structural modes as part of their normal function. 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 paper presents a solution to the problem based on multiple sensors. The measurements from these sensors are fused to create a measurement clean of elastic modes 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, an output of the rigid body motion cleaned of the first n-1 bending modes is produced. The n sensors must be placed at different location along the body. This paper describes an algorithm using multiple rate gyros and another using a combination of a rate gyro and accelerometers.
A test case of a long flexible missile is analyzed. The performance of both algorithms on the test case, are reviewed. A sensitivity study is performed. A lab experiment was designed and conducted allowing for a performance analysis of both algorithms on real hardware. The results on the simulated test case and in the lab experiment, showed good attenuation of the first and second elastic mode could be achieved.

The talk will be given in Hebrew

Wed, 13-08-2014, 16:30 (Gathering at 16:00)

Classroom 165, ground floor, Library, Aerospace Eng.

Light refreshments will be served before the lecture

Using Multiple Sensing to Overcome Structural Bending Modes in Flight Control