Abdul-Hadi G. Abulrub

Multiple Sliding and Rolling Contact Dynamics for a Flexible Rotor/Magnetic Bearing System

Active magnetic bearings (AMBs) offer contact-free and frictionless support of rotating machinery. However, because of their limited force capacity, they have to incorporate retainer bearings to protect the rotor and stator laminations against high-amplitude vibration levels. Efficient modeling of contact dynamics is important for the design of adaptive controllers to prevent contact. If, however, contact does occur, it is necessary to recover the rotor position with minimum damage and without shutting down the system. This paper utilizes constrained Lagrangian equations of motion to develop a computationally efficient method to model contact dynamics. The method does not require a direct physical modeling of contact forces, although the contact forces are automatically evaluated from the constraint conditions, and it can be applied to multicontact cases. Furthermore, the technique is capable of detecting and simulating the destructive backward whirl rolling motion. A model reduction technique is introduced to improve the computational efficiency. This is demonstrated by comparing numerical predictions with experimental results, obtained for a 2-m-long flexible rotor supported by two magnetic bearings

Virtual reality in engineering education: The future of creative learning

Virtual reality has achieved an adequate level of development for it to be considered in innovative applications such as education, training, and research in higher education. Virtual reality offers both opportunities and challenges for the educational sector. One of the challenges of virtual reality technology is the costs associated which have been unaffordable for educational institutes. However, in recent years, computer hardware and software development has made it more feasible to incorporate virtual reality technology into future teaching strategies. Despite the cost challenges, educational benefits of implementing virtual reality remain compelling. This paper explains virtual reality principle and describes the interactive educational environment developed at WMG, the University of Warwick. It also discusses the benefits of using state-of-the-art 3D photorealistic interactive and immersive virtual environment for engineering undergraduates and postgraduate teaching, learning and training.

ADAPTIVE CONTROL OF ACTIVE MAGNETIC BEARINGS TO PREVENT ROTOR-BEARING CONTACT

Active Magnetic Bearing (AMB) systems offer various advantages over conventional bearings but due to their limited force capacity, with high levels of vibrations the rotor may come into contact with retainer bearings. Under conventional PID control, when a rotor comes into contact with its retainer bearings it remains in contact, until the rotor is run down and the system shut down. This may not be acceptable in some applications, such as aerospace and automotive applications. In this paper, a recursive open-loop adaptive control (ROLAC) algorithm is presented, as an extension of the existing open loop adaptive controller (OLAC), that updates the control force amplitude and phase at each sampling period for rapid response to changes in external excitations. The effectiveness of the algorithm in counteracting a sudden change of rotor unbalance is demonstrated by simulation and experimental results. The experimental system consists of a flexible 2 m long rotor with a mass of 100 kg supported by two radial active magnetic bearings. A simulation model of the system, including the contact dynamics, was used to assess the feasibility of the suggested controller before applying it to the experimental system. Depending on excitation levels, it is shown that the proposed controller is fast enough to prevent contact in most cases. If contact does occur the impact is minimized, and the method is able to recover the rotor position quickly. The proposed controller is implemented in real time and applied to the experimental system. It is shown that the controller works efficiently as predicted by the simulation studies.Copyright

EFFECTIVE MODEL REDUCTION FOR MAGNETICALLY LEVITATED FLEXIBLE ROTORS INCLUDING CONTACT DYNAMICS

Reduced order models are required for controller design purposes to allow practical realization and implementation in real-time applications. In the rotor dynamics field, the controllers associated with magnetic bearings are classic examples of such a requirement. A technique is introduced to reduce the order of a finite element rotor model to one involving only the required rectilinear displacements in sensor and controller nodal planes. The reduced order model is shown to predict a number of natural frequencies and responses to step changes in unbalance with considerable accuracy. A simulation of the rotor making contact with a rigid retainer bearing is also established using a constrained Lagrangian multiplier. Again, the differences between the full and reduced order model rotor orbits are shown to be small.Copyright

Performance Assessment of a Multi-Frequency Controller Applied to a Flexible Rotor Magnetic Bearing System - Contact Dynamics

Vibrations associated with external disturbances and rotor faults, such as cracks, usually exhibit themselves as harmonic components of the synchronous frequency. The essence of the multi-frequency form of the synchronous recursive open loop adaptive controller (ROLAC) is that it can minimise a number of vibration components simultaneously, for example, synchronous rotor vibration at frequency Ω, its harmonics and sub-harmonics within any defined range. This requires on-line identification of the speed dependent partial receptance matrix by using a multifrequency test signal incorporating all of the pertinent harmonic components. The question arises: What is the degradation in performance if the rotor comes into contact with its retainer bearing? This may arise when a magnetically levitated rotor is installed on a movable base frame, for example on board a ship or an aircraft. A simulation study is described to examine this question. It comprises a flexible rotor supported by two active magnetic bearings. Eight displacement transducers are positioned along the rotor. A local PID controller is provided for each axis of the magnetic bearings to ensure stability and alignment of the rotor at a central position. An outer ROLAC loop is incorporated at each bearing to control the vibrations at discrete frequencies of 0.5Ω, Ω, 2Ω and 3Ω. In addition, a multi-frequency disturbance was applied to the rotor causing contact with its retainer bearing. The performance of the multi-frequency ROLAC is assessed in preventing contact, or recovering the rotor position if contact occurs.

Experiments on the transient performance of an adaptive multi-objective controller for rotating machinery

This paper describes an experimental assessment of the transient performance of a multi-objective adaptive approach to the control of flexible rotors. This is applicable to any arrangement of controllable bearings or actuators. In the case reported here, the rotor is supported by active magnetic bearings. The theory underlying the controller is outlined. The objectives include minimization of the forces transmitted to the base while restricting rotor vibrations to a user-defined limit. A third objective is to prevent rotor contact with the auxiliary bearings, which are used to protect the active elements. These objectives are met by a two-stage weighting strategy followed by the adaptive control of two parameters that automatically and continuously adjust the weightings of individual objective functions to satisfy user-defined performance criteria.

Virtual reality for visualisation of complex macrostructure

Virtual reality systems involve the integration of both hardware and software components. The hardware enables users to navigate, interact and become immersed in the virtual environment while underlying support software creates the virtual environment and integrates the hardware into a working system. Although virtual reality is widely used for product design visualisation in engineering, there exist a number of possible avenues where virtual reality can be very effective.

A multi-objective adaptive controller for magnetic bearing systems

The paper considers three issues in flexible rotor and magnetic bearing systems, namely control of rotor vibration, control of transmitted forces, and prevention of rotor contact with auxiliary bearings. An adaptive multi-objective optimization method is developed to tackle these issues simultaneously using a modified recursive open loop adaptive controller. The proposed method involves automatic tuning of the weighting parameters in accordance with performance specifications. A two-stage weighting strategy is implemented involving base weightings, calculated from a singular value decomposition of the system’s receptance matrices, and two adjustable weighting parameters to shift the balance between the three objective functions. The receptance matrices are functions of rotational speed and they are estimated in situ. The whole process does not require prior knowledge of the system parameters. Real-time implementation of the proposed controller is explained and tested by using an experimental flexible rotor magnetic bearing system. The rotor displacements were measured relative to the base frame using four pairs of eddy current displacement transducers. System stability is ensured through local PID controllers. The proposed adaptive controller is implemented in parallel and the effectiveness of the weighting parameters in changing the balance between the transmitted forces and rotor vibrations is demonstrated experimentally.Copyright

Experiments on the Transient Performance of an Adaptive Multi-Objective Controller for Rotating Machinery

The paper describes an experimental assessment of the transient performance of a multi-objective adaptive approach to the control of flexible rotors previously presented by the authors. The approach is applicable to any form of controllable bearings or actuators. In the case reported here, the rotor is supported by active magnetic bearings (AMBs). The theory underlying the controller is outlined: the objectives include minimization of the forces transmitted to the base while restricting rotor vibrations to a user-defined limit. A third objective is to prevent rotor contact with the auxiliary bearings used to protect the active elements. These three objectives are met by a two-stage weighting strategy followed by the adaptive control of two parameters that automatically and continuously adjust the weightings of individual objective functions to satisfy user defined performance criteria.