Nicola Bailey

Nonlinear flexure coupling elements for precision control of multibody systems

Conventional multibody systems used in robotics and automated machinery contain bearing components that exhibit complex and uncertain tribological characteristics. These limit fundamentally the precision of the automated motion and also cause wear. Replacing traditional bearing joints with flexure couplings eliminates these tribological effects, together with wear, reducing necessary system maintenance and offering a potential for increased motion precision. A flexure-coupled multibody system is considered and a novel general solution technique is presented. Derivation of a large deflection flexure coupling model is provided and subsequently validated using an experimental facility. A focused study of a unique double-flexure-coupling rigid body system is given; the formulated nonlinear mathematical model can be used for feedforward control. Equivalent control is also applied to a corresponding system with traditional bearing joints. The feasibility of replacing bearing joints by flexure couplings is demonstrated in terms of accurate large displacement control and reduction of high-frequency disturbances.

Control of Flexible Rotor Vibration with Flexibly Mounted Active Magnetic Bearings

Typical rotor/active magnetic bearing (AMB) system layouts involving large, external stator AMBs may be difficult or inconvenient to apply to some rotor systems. Where space in the machine working envelope is at a premium, the space required by traditional AMBs may preclude them from inclusion in the design.

Effect of Uncertainty in External Forcing on a Fluid Lubricated Bearing

Developments in industrial applications motivate improvements in fluid lubricated bearing technology, enabling smaller face clearances and increased rotation rates. Associated film lubrication technology aims to improve efficiency and reliability. A bearing model is developed to evaluate the effect of external, potentially destabilising, random forcing applied to a pair of highly rotating axisymmetric bearing faces, separated by a thin fluid film. Two cases of random external force are examined. A first study considers an imposed random force disturbance constrained to a fixed period, where the average minimum face clearance together with the probability it reaches a specified gap tolerance. More general uncertainties are associated with more complex external forcing and takes the form of a white or coloured noise. In this case the average time for the face clearance to reach a prescribed tolerance is examined.

Active Control of Rotating Machinery Under Rotor-Stator Contact Conditions

A rotor spinning within an active magnetic bearing (AMB) system will normally be levitated and hence operate without rotor-stator contact. External disturbances and inherent unbalance may be compensated with appropriate control to keep rotor deviations within the clearance gap. However, AMBs have limited dynamic load capacity due to magnetic material field saturation. Hence overload conditions may result in rotor-stator contact. A touchdown bearing (TDB) and rotor landing sleeve are usually included to protect the expensive rotor, magnetic bearing and sensor components from damage. Once rotor-TDB contact has been made, rotor dynamic conditions may ensue resulting in persistent rotor bouncing or rubbing limit cycle responses. Prolonged exposure to these severe dynamics will cause TDB degradation and require regular replacement. If possible, a clear aim should be to restore contact-free levitation through available control capability in an efficient manner. This paper is used to guide the control options that are available to restore contact-free levitation. The use of AMB control is appropriate if the required control forces are within saturation limits. It is also possible to actuate TDBs and destabilize persistent rotor dynamic contact conditions. For example, piezo-based actuation offers larger control forces than those from magnetic bearing systems. Hybrid control action involving both types of actuation system has the greatest potential for completely robust restoration of contact-free levitation.