Research On Dynamic Characteristics Of Cracked Flexible Rotor-Active Magnetic Bearing Systems

Compared to the rotor system supported by the traditional bearing such as ball bearings and plain bearings, dynamics characteristics of a cracked flexible rotor system supported by active magnetic bearings are such more complex. Therefore, many problems need to study, such as, the influence of cracks on the stability of the active magnetic bearing controller, the influence of cracks on the dynamic characteristics of the active magnetic bearing-rotor system, the influence of the active magnetic bearing controller on the dynamic characteristics of the cracked rotor, the diagnostic approach of the crack fault in active magnetic bearing-rotor system.The purpose of this paper is to study the dynamic characteristics of the active magnetic bearing-cracked flexible rotor systems, including designing the2DOF PID controller, H∞controller of the active magnetic bearings, building the mathematical model of cracked flexible rotor systems, the dynamic model of the active magnetic bearing-cracked flexible rotor systems and analysing the influence of the parameters of the bearing controller on the dynamic characteristics of the cracked flexible rotor system.2DOF PID controller, which has a performance in the optimal tuning for the disturbance response and the one for the set-point response, is used in the control of the active magnetic bearing-rotor systems. The mathematical model of the2DOF PID controller is built. The control parameters are selected by simulation. The experiments on a experimental platform of an active magnetic bearing-flexible rotor systems validate that the designed2DOF PID controller can easily make the rotor system smoothingly pass through the first flexible critical speed.The H∞controller with good robustness and ability of anti-disturbance is designed. In order to overcome the problem of choosing the weighting functions W1(s)、W2(s) and W3(s), the lumped parameter method is used. The physical parameters of the rotor system are lumped to the position of the active magnetic bearings and the weighting functions W1(s)、W2(s) and W3(s) are chosen and the H∞controller is designed. The experiments on the experimental platform of the active magnetic bearing-flexible rotor systems indicate that the H∞controller can make the rotors smoothingly pass through the first flexible critical speed.Based on the breathing stiffness model of the cracked rotor and finite element method, the equation of the cracked rotor-active magnetic bearing with PD controller is built. The influence of the parameters P and D of the controller on the dynamic characteristics of the cracked flexible rotor system is analysed, theoretically the differences between the cracked rotor system supported respectively by the traditional bearing and the active magnetic bearing are compared. The influence of the bias current Io of the PD controller on the dynamic characteristics of the cracked rotor system is discussed.The equation of the ctive magnetic bearing-cracked flexible rotor with optimum control is built and the simulation is conducted. The influence of the weighting matrixes Q and R in the optimum controller on the dynamic characteristics of cracked rotor system is analysed, the differences between the cracked rotor system supported respectively by the traditional bearings and the active magnetic bearing are compared. The influence of the bias current Io of the optimum controller on the dynamic characteristics of the cracked rotor system is discussed.On the experimental platform of the active magnetic bearing-flexible rotor system. under no changing the fixed conditions of the flexible rotor system, a transverse grooving with the depth of4mm is acquired by wire-electrode cutting. A copper sheet is inset in the transverse grooving and glued one side of the copper sheet to one side of the transverse grooving, so the transverse cracked flexible rotor acquired is similar to a real cracked rotor. Dynamics experiments of cracked flexible rotor that include steady-state response experiments, unbalance response experiments, acceleration response and deceleration response experiments, were done on the active magnetic bearings-cracked flexible rotor systems experimental platform with the unchanged controller parameters of active magnetic bearings used2DOF PID controller and H∞controller. Dynamics characteristics of the tansverse crack of flexible rotor were analyzed when the active magnetic bearings-flexible rotor systems experimental platform used different active magnetic bearings controller. Experimental results show that the depth of crack causes the active magnetic bearings-flexible rotor system instability, and the monitorings of the rotor’s vibration are effective manners to detect the rotor crack faults of the active magnetic bearings-flexible rotor system.