טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentAriel Netanel
SubjectAuto-Resonance Based Model Identification of Rotating
Structures
DepartmentDepartment of Design and Manufacturing Management
Supervisors Professor Izhak Bucher
Dr. Eyal Setter
Full Thesis textFull thesis text - English Version


Abstract

Rotating structures exhibit speed dependent natural frequencies and mode shapes that play an important role in the overall dynamics. Accurate experimental identification of these phenomena is of great importance for validating uncertainties in numerical models and for detecting potentially dangerous asynchronous frequencies, often obscured by the imbalance related synchronous vibrations. As the speed dependent natural frequencies cannot be assessed experimentally without actually rotating the structure at the vicinity of these speeds, the task of exciting and fast measuring asynchronous frequencies during rotation without risking the integrity of the machine becomes a great challenge.

The present work proposes an automatic and efficient method to excite a rotating structure at a selected mode shape, while controlling the vibration amplitude, such that a non-destructive test takes place. This stands in contrast with existing experimental methods based on measuring the system's response to imbalance (no external excitation), without the ability to control the excited mode shape and vibration intensity. The disadvantages of these methods are limited ability to detect the system frequencies variation with the rotation speed and risking the system as a result of lack of control over the excitation magnitude.

Automatic excitation of marginally stable vibration occurs upon introducing a phase shifting filter and a nonlinear feedback element. A digital signal processor carries out the latter, therefore the vibration levels are fully controllable. This is accomplished by injecting a precise amount of excitation energy to a particular mode shape. Real-time filtering must be employed to remove the synchronous vibration, which arises mainly due to inherent imbalance, for allowing the desired auto-resonated asynchronous modes of vibration to emerge.

Theoretical analysis, based on the describing function method and modal filtering, is carried out and verified by numerical simulations. Finally, some experimental results are carried out, analyzed and compared to the numerical and theoretical results.  The experimental system exhibits different modes of vibration that are excited selectively, at any desired speed of rotation and at any desired magnitude.

This approach effectively reconstructs the Campbell diagram with only basic knowledge of the system's modal behavior. It is also shown that one can switch, in situ while rotating the system, between modes of vibration in the presence of large imbalance forces.