Rajiv Kumar Vashisht

Rotors are used to transfer power from one component to the other component in various rotor dynamics applications like steam turbines, aerospace systems, and power generators. In certain applications, the seal is an important and inevitable component of the rotor dynamic system. The linear dynamics of the rotor is converted into nonlinear in the presence of a seal. The presence of a seal, can sometimes, trigger self-excited vibrations in rotor/seal systems. Generally, the amplitude of vibration during such vibrations is several times higher than normal steady-state vibrations. Due to this, other faults like transverse crack initialization and rotor/stator rub get accelerated. The rotating machinery working at high spinning speeds is more prone to this type of instability. Hence, the stability limits of the rotor/seal system must be known in advance for the safe operation of the rotating machinery. The dynamic behavior of such a system qualitatively varies corresponding to different system parameters of the rotating machinery. Cumbersome analytical methods have been used in the past for bifurcation analysis of rotor/seal systems. In the present work, system bifurcations have been investigated using a subharmonic sampling rate strategy. The proposed method is easy to implement and numerically efficient. The effect of various parameters like the spinning speed of the rotor, unstable mass, and seal clearance is investigated for the stability of the rotor/seal system. Experimental testing is done to verify the simulation results.

For certain combinations of cutter spinning speeds and cutting depths in milling operations, self-excited vibrations or chatter of the milling tool are generated. The chatter deteriorates the surface finish of the workpiece and reduces the useful working life of the tool. In the past, extensive work has been reported on chatter detections based on the tool deflection and sound generated during the milling process, which is costly due to the additional sensor and circuitry required. On the other hand, the manual intervention is necessary to interpret the result. In the present research, online chatter detection based on the current signal applied to the ball screw drive (of the CNC machine) has been proposed and evaluated. There is no additional sensor required. Dynamic equations of the process are improved to simulate vibration behaviors of the milling tool during chatter conditions. The sequence of applied control signals for a particular feed rate is decided based on known physical and control parameters of the ball screw drive. The sequence of the applied control signal to the ball screw drive for a particular feed rate can be easily calculated. Hence, costly experimental data are eliminated. Long short-term memory neural networks are trained to detect the chatter based on the simulated sequence of control currents. The trained networks are then used to detect chatter, which shows 98% of accuracy in experiments.

For the reduction of unbalanced vibrations in a multi-input and multi-output flexible rotor system with electromagnetic actuators (EAs), conventional adaptive feedforward controllers (AFFCs) are very sensitive for changes in rotor spin frequencies. Although frequency updating is used in these controllers, a small variation in the rotor spin frequency can completely reduce their effectiveness. An adaptive notch filter is used in this research for the frequency estimation. By using this external frequency estimation, the performance of the conventional AFFCs can be enhanced. During changes in the rotor spin frequency, fundamental harmonics of the flexible rotor are also excited. Their amplitude is much higher compared to steady-state unbalanced vibrations, which can accelerate the wear and tear of components of EAs. By using feedback controllers, the amplitude of these fundamental harmonics can be reduced significantly. In real rotors with flexible bearing supports, any looseness of bolts and presence of transverse cracks can change system parameters significantly. Multiple harmonics are generated corresponding to even single spinning speed of the rotor. Robust stability as well as performance can be achieved in the presence of uncertainty and rotor crack nonlinearities using feedback controllers designed by mu-synthesis. By using the multiharmonic hybrid control, the higher harmonics can be compensated efficiently in case of a crack in rotor systems. The fast Fourier transform of the control signal can indicate the presence of a transverse crack in an online manner. In this way, active vibration control as well as rotor crack fault detection can be done simultaneously.

Due to unbalance present in a rotating machinery, fluctuating stresses are generated leading to the formation of transverse cracks in rotors. The cracks propagate with the passage of time and increase the amplitude of vibration. High vibration amplitudes can give rise to rotor/stator rubs. During the rubbing phase, the crack propagation gets enhanced due to inter-connected nature of these faults. If left unattended, these faults can cause the premature failure of machine components. Hence, there is a need to develop fault detection mechanisms based on the vibration response so that these faults can be diagnosed during initial stages. The effect of gravity and the presence of cracks significantly changes vibration characteristics of the rotor, which is thoroughly investigated in this research for a two-degrees-of-freedom Jeffcott rotor. It has been observed that during rubbing, high harmonics are excited. These harmonics are integer multiple of the rotor spin frequency. Similar type of the response is also observed due to the presence of a transverse rotor crack. It is difficult to distinguish the type of faults based on the steady state dynamic response only. Instead of working only on a steady state vibration response, the transient vibration response during coasting up of the rotor is considered. During coasting up of the rotor, high harmonics are excited for both the crack as well as rotor/stator rubbing. The excitation of higher harmonics starts at much earlier in the spectrogram of the vertical response for the cracked rotor compared to that of rubbing. This fact is used in the development of a fault diagnosis technique based on Short Time Fourier Transform of the vibration response. The proposed technique can efficiently distinguish different types of faults even if multiple faults coexist.

An adaptive hybrid controller is proposed for reducing the unbalanced vibration response of a flexible rotor/active magnetic bearing system. It is observed that conventional adaptive feedforward controller (AFFC) normally used in the active noise control is very sensitive in performance for changes in rotor spin frequencies. Although frequency updating is a part of its architecture, a small practical variation in the rotor spin frequency can reduce its effectiveness drastically. A smart combination of adaptive notch filter and Goertzel filter is proposed for the frequency estimation. During changes of the rotor spin frequency, fundamental harmonics of the flexible rotor are excited. By using hybrid controllers that combine feedback control and AFFC, the amplitude of these fundamental harmonics is reduced significantly. By applying the multi-harmonic hybrid control, the multiple harmonics generated due to coupling misalignment are compensated efficiently. Fourier transform of the control signal is further used to detect the presence of the coupling misalignment.

It is confirmed experimentally that in case of a rotor with crack, multiple harmonics are generated when the rotor revolves at a particular frequency. Only few modeling techniques successfully predict this particular behavior of the cracked rotor. It is observed in this research that modeling cracked rotors using conventional finite element methods cannot predict this particular behavior successfully. A nonlinear dynamic model of the flexible rotor with ball bearings is developed using discrete mass spring damper elements combined with an existing model of the crack to truly predict this confirmed experimental behavior. Certain crack detection techniques based on the steady-state response work well on this basic concept of the multiharmonics generation due to nonlinearities caused by cracks in the rotor. The presence of ball bearings, rotor-coupling misalignment, rotor-stator rub, and rotor bow can also cause significant nonlinearities in the overall system. These additional nonlinearities render these crack detection techniques to lose their effectiveness. Our work justifies through simulations that the Jeffcott rotors are the over simplified version of real-life rotor-bearing systems. Hence, these crack detection techniques cannot be efficiently applied for condition monitoring of real-life rotor-bearing systems. The proposed model also helps to understand that the presence of flexible bearing supports affects the dynamics of the system considerably and negatively affects the effectiveness of these crack detection techniques.