Suppression of rotary unbalance spin-up vibration using passive and semi-active vibration absorbers.

摘要

Rotating machine unbalance can be the source of unwanted vibrations in many mechanical systems. One problematic form of unbalance force is generated by machine start-up or shut-down. Start-up/shut-down unbalance induces a harmonic excitation force with a varying frequency that is directly proportional to rotor speed, and an amplitude that is proportional to the square of the frequency. When the frequency of a start-up/shut-down unbalance approaches or passes a system resonant frequency, large amplitude vibrations can occur. These vibrations generate greater than normal system operating forces, which could accelerate cumulative part wear and damage internal components. Such degradation could significantly reduce a mechanical system's life.; This thesis presents a passive and a semi-active control method for the suppression of vibrations caused by unbalance spin-up excitation (focusing on the start-up scenario) in a mechanical structure. Both methods employ dynamic vibration absorbers (DVAs).; A passive control method using dual mechanical DVAs has been previously proposed (Bursal, 1995). As a basis for design, Bursal (1995) made a conjecture that shaping the system's frequency response function (FRF) by using DVAs to provide a low, flattened, FRF curve over the excitation spin-up frequency range, would minimize the structural vibration response for an unbalance spin-up event. Although the method has been shown to be effective for a few sets of conditions, the conjecture of using steady-state-based FRF-Shaping to suppress transient responses (spin-up unbalance generates a transient response) has not been substantiated.; This thesis validates the dual absorber design conjecture and provides additional information regarding the optimal design of such a system. The parametric studies compare the performances between an optimal design (minimum peak response) and the FRF-Shaping design. It is shown that the performances of the two designs are similar for very slow spin-up rates. As the spin-up rate increases, differences grow progressively larger. At very high spin-up rates, the maximum vibration amplitude of the optimal system could be approximately three-quarters of that of the FRF-Shaping design.; Piezoelectric absorbers with inductive-resistive shunts are evaluated to determine their viability for dual passive absorber applications. Experimental investigations are conducted to validate general findings from the model-based simulation study of dual absorbers. The physical structures of the mechanical and piezoelectric absorber systems are found to be fundamentally different. However, despite more complex basic parameter definitions and a less straightforward design process, the piezoelectric absorbers are found to provide vibration suppression comparable to that of the mechanical absorbers.; Finally, semi-active, variable stiffness control of a single DVA is examined. An existing open-loop control scheme (Walsh and Lamancusa, 1992) prescribes a multiple-stepped, optimal absorber stiffness profile. The optimal stiffness profile is determined using model-based simulation and a multivariable feasible direction search. The complexity of the scheme and the associated computation cost prompted a search for a simpler control method. From insights gained in the parametric study of the dual passive absorbers in this thesis, a new, single variable stiffness control law is developed. This law, which requires the definition of only a single variable and a simple numerical line search to determine the optimal parameter, is found to be highly effective across a broad range of spin-up rates. The reduced computational effort and simplification of implementation make the new scheme an attractive alternative for suppressing spin-up vibration with semi-active DVA control.