David A. Robb

The dynamics of a cracked rotor with an active magnetic bearing

Abstract The dynamic characteristics of a cracked rotor with an active magnetic bearing (AMB) are theoretically analyzed in this paper. The effects of using optimal controller parameters on the dynamic characteristics of the cracked rotor and the effect of a crack on the stability of the active control system are discussed. It is shown that the dynamic characteristics of the cracked rotor with AMBs are clearly more complex than that of the traditional cracked rotor system. Adaptive control with AMBs may hide the fault characteristics of the cracked rotor, rather than helping to diagnose a crack; this will depend on the controller strategy used. It is very difficult to detect a crack in the rotor with an AMB support system when the vibration of the rotor system is fully controlled. Monitoring the super-harmonic components of 2× and 3× revolution in the sub-critical speed region can be used as an index to detect a crack in the rotor with an AMB system. If the effect of the crack is not taken into account at the design stage of the controller, then the rotor–AMB system will lose its stability in some cases when cracks appear.

Laser-based measurement system for measuring the vibration on rotating discs

This paper describes a laser-based system for measuring vibration on rotating discs. The setup allows the user to track an arbitrary periodic path at any multiple of the discs rotational speed. In particular, the laser can be rotationally-locked to the disc and thus track a circle in space allowing continuous measurement of the response at a defined point on the disc for both constant and varying rotation speed. An electro-magnetic non-contacting shaker provides the forcing excitation to the rotating disc. Due to rotation, the vibration spectrum of the rotating disc becomes very complex. In particular, the disc vibrates at several frequencies other than the applied excitation frequency. A stepped sine method has been developed to measure and for parameter identification of the rotating disc vibration properties.

MAGNETORHEOLOGICAL FLUID DAMPERS FOR ROTOR VIBRATION CONTROL

In this paper, magneto-rheological(MR) fluid with a rapid, reversible and dramatic change in its rheological properties by the application of an external magnetic field is used to develop controllable MR fluid dampers for rotating machinery. The basic operation principle of the MR fluid damper is first reviewed, then two MR fluid dampers, which are respectively based on shear operation mode and squeeze film operation mode, are presented. The effectiveness of these MR fluid dampers for attenuating and controlling the vibration of rotor systems are experimentally investigated. The existing problems in the MR fluid dampers are discussed. It is shown that the dynamic characteristics of the MR fluid dampers can be easily controlled by a magnetic coil with a low voltage (less than 50V) and that the MR fluid dampers are very effective for attenuating the vibration of rotor systems. It is possible to give the optimum support damping and stiffness for every vibration mode in rotor systems by controlling the applied current in the MR dampers, if they are not located at the main model nodes of rotor systems.

Active vibration control of rotor system by a magnetorheological fluid damper

A disk-type MR fluid damper based on shear mode has been developed and the experimental results show that the dynamic characteristics of a disk-type MR fluid damper can be controlled by the application of an external magnetic field with a low voltage. In this paper, the effectiveness of on-off control based on feedback of rotational speed, proportional (P) control and proportional-integral (PI) control, both based on feedback of disk vibration eccentricity, on controlling the rotor vibration are experimentally studied. It is shown that the simple on-off control was the most effective method in attaining the required objective, but as expected requires pre-knowledge of the switching speeds and there is a transient response with large amplitude at switching speeds. P and PI control can control the rotor vibrations, but both exhibit significant chattering which can adversely affect the control performance and still require to properly choose the controller parameters.

Effectiveness of a disk-type magnetorheologic fluid damper for rotor system vibration control

A disk-type MR fluid damper based on shear operation mode is presented in this paper. The magnetic field of the disk-type MR fluid damper is analysed by the finite element method. The effect of excitation current in the coil on the magnetic flux density in the axial gaps filled with MR fluid is studied both theoretically and experimentally. Finally, the effectiveness of the disk-type MR fluid damper for attenuating vibration of rotor systems and of a simple open-loop on-off control based on the feedback of rotational speed on controlling vibration of rotor systems are experimentally studied. It is shown that the dynamic characteristics of the disk-type MR fluid damper can be controlled by a simple magnetic coil with a low voltage, and the disk-type MR fluid damper is very effective to attenuate vibration of rotor systems.

The Dynamics of a Cracked Rotor With an Active Magnetic Bearing

The dynamic characteristics of a cracked rotor with an active magnetic bearing (AMB) are theoretically analyzed in this paper. The effects of using optimal controller parameters on the dynamic characteristics of the cracked rotor and the effect of the crack on the stability of the active control system are discussed. It is shown that the dynamic characteristics of the cracked rotor with AMBs are clearly more complex than that of the traditional cracked rotor system. Adaptive control with AMBs may hide the fault characteristics of the cracked rotor, rather than helping to diagnose a crack; this will depend on the controller strategy used. It is very difficult to detect a crack in the AMB-rotor system when the vibration of the rotor system is fully controlled. Only the super-harmonic components of 2X and 3X revolution in the sub-critical speed region can be used as a index to detect a crack in the rotor–AMB system. If the effect of the crack is not considered in designing the controller, then the AMB-rotor system will lose its stability in some cases when cracks appear.© 2001 ASME