Feasibility of using giant magnetostrictive material (GMM) based actuators in active control of journal bearing system

Abstract

A novel idea of using giant magnetostrictive material (GMM or Terfenol-D) based actuators for the implementation of active vibration control on a journal bearing system is introduced in this research. Journal bearings have relatively longer service life and higher load capacity than other types of bearings. They are particularly suitable for long-term and heavy-duty usages. However, the main drawbacks of journal bearings include low accuracy in positioning and stability because of the clearance between the journal shaft and the bearing ring for oil flow and lubrication. Measures to carry out vibration and position control are thus taken to enhance the precision and broaden their applications under various speeds of rotation. Active vibration control of rotor-bearing system has aroused great interest in the world for years. Being an important engineering problem, vibration suppression on rotating machinery has been achieved through various types of vibration damping devices with different control techniques. They mainly make use of electro-magnets, hydraulic systems or smart materials such as piezo-ceramics. However, these active devices either suffer from low load capacity, slow response or limited actuating force and stroke. To achieve active vibration control of a typical journal bearing system, it requires a means which can provide significant strength to work against with the bearing load, a relatively large actuating stroke of being comparative to the journal bearing clearance, and fast response to cope with the high speed rotation of the bearing. The giant magnetostrictive material seems to be a good candidate to serve the purpose. The project is to examine the feasibility of using giant magnetostrictive materials to act as an active control element for journal bearings. GMM is one of the solid-state smart materials that can be triggered to elongate along the direction of a given magnetic field strength. The change in length is due to the domain movement in the level of GMM crystalline structure under desirable magnetic stimulus, which is referred to as magnetostrictive effect. GMM is characterized with its high actuating and dynamic force, long stroke of actuation, and fast response. All of these make it outweigh the other active elements as suggested in the past for active journal bearing applications. As the first step to start with this research, some basic tests on the characterization of GMMs were carried out. Based on the results, two identical controllable GMM actuators were designed and fabricated. Literature on design considerations of GMM actuators had been referred to. Two GMM actuators being used simultaneously were for two-dimensional vibration control on a rotor-bearing system. The self-fabricated GMM actuators were characterized independently on a MTS machine, such that the key properties such as the magnetostrictive strain, force and response time were obtained. Having completed the characterization tests of GMM actuators, a GMM journal bearing was designed. The open-loop function of GMM journal-bearing system was determined as the control rule in the control system. Self-tuning P control under PID algorithm was implemented using LabVIEW software platform as an active control algorithm for the GMM journal bearing. The gain value P of the system under PID control was automatically tuned by itself according to the rotational speeds. The performances of the system running at various journal shaft rotational speeds under active control were investigated. The stability of the GMM-controlled journal bearing system was confirmed. Furthermore, both auto-centering ability and the efficiency of the journal bearing system under active control by GMM actuators were investigated under various steady-state journal shaft rotational speeds. The concept of using GMM actuators for active control of journal bearings is proven. It is demonstrated that GMM actuators have stable performance and efficient actuating stroke under both static and alternating excitation for the application of journal bearings. The results show that the vibration suppression for speeds up to 350 rpm is about 40%, which is significant considering that the control is not yet optimized. In conclusion, this novel concept of using GMM actuators for the active journal bearing control is proven to be feasible and GMM is shown to be an excellent material with promising performance for driving function. The immediate future work includes the system optimization and further development for high-speed conditions. By then, active vibration suppression for the rotor-machinery in heavy industry can be realized. Keywords: Giant magnetostrictive material, Frequency response, Journal bearing, Active vibration control