Search

Passive rotors are often limited in rotational speed due to bearing constraints, stability and excessive vibration levels. To address the vibration issue, Active Magnetic Bearings (AMBs) levitating the rotor with a magnetic field can be used. They offer a clearance and variable stiffness and damping to the rotor support, which help to mitigate greatly the vibration issue. However, they are also limited at large rotational speed because of the high frequency control force required to levitate the rotor safely. To overcome the frequency limitation, a dual AMBs/internal bending control concept is investigated with associated modelling and control algorithms. This approach is examined in simulation with a 19 kg rotor running up to 10,000 RPM, where three resonance frequencies are present at 2700, 5300, and 9300 RPM, with the first resonant frequency being the most strongly excited. Using internal rotor bending control, a maximum radial displacement of 15 μm for the rotor mid-point is achieved, which gives a reduction in vibration amplitude of 45% compared to the case of no control. Variations of the algorithm are presented and discussed, showing the potential of the proposed approach.

The actuation and energy-harvesting performance of dielectric elastomers are strongly related to their intrinsic electrical and mechanical properties. For future resilient smart transducers, a fast actuation response, efficient energy-harvesting performance, and mechanical robustness are key requirements. In this work, we demonstrate that poly(styrene-butadiene-styrene) (SBS) can be converted into a self-healing dielectric elastomer with high permittivity and low dielectric loss, which can be deformed to large mechanical strains; these are key requirements for actuation and energy-harvesting applications. Using a one-step click reaction at room temperature for 20 min, methyl-3-mercaptopropionate (M3M) was grafted to SBS and reached 95.2% of grafting ratios. The resultant M3M-SBS can be deformed to a high mechanical strain of 1000%, with a relative permittivity of ϵr = 7.5 and a low tan δ= 0.03. When used in a dielectric actuator, it can provide 9.2% strain at an electric field of 39.5 MV m-1 and can also generate an energy density of 11 mJ g-1 from energy harvesting. After being subjected to mechanical damage, the self-healed elastomer can recover 44% of its breakdown strength during energy harvesting. This work demonstrates a facile route to produce self-healing, high permittivity, and low dielectric loss elastomers for both actuation and energy harvesting, which is applicable to a wide range of diene elastomer systems.

Active magnetic bearing systems provide contact-free rotor levitation, which allows near frictionless rotation and high rotational speeds when they operate normally. Under certain operational conditions, a transient fault, overload condition, or disturbance may occur. Touchdown bearings or bushings are therefore implemented in such systems to prevent contact between rotor and stator laminations. If the rotor makes contact with a touchdown bearing, the rotor dynamics may become transient or persistent in contact. Appropriate control strategies through the magnetic bearings to restore contact-free rotor operation may extend the life of touchdown bearings, and minimise operational downtime. To achieve this, an understanding of the contact dynamics is required, together with the relationship between contact and magnetic bearing forces. In this paper, rotor/touchdown bearing contact conditions are investigated experimentally using an active magnetic bearing system with a flexible rotor. Design methodology is presented for a measurement system capable of providing rotor/touchdown bearing contact related data, based on strain measurement. Strain induced contact signals are calibrated against applied magnetic bearing forces. The frequency dependent behaviour of the active magnetic bearing system is considered using evaluated force and phase measurements. The measurement data represent system identification for the potential of active magnetic bearing force-based contact control.

The introduction of active magnetic bearings (AMBs) has enabled turbomachinery to increase power density, controllability, and general resilience to external disturbances. However, because of the limited load capacity of AMBs, the base shock condition that "on-board" machines often encounter may result in contact between the rotor and the touchdown bearings (TDBs), which can seriously damage the machine. A challenge in AMB applications is to alleviate this problem. This study presents a dynamic analysis of a rotor-AMB-TDB system under strong base shocks while the AMBs are operating. Detailed TDB and contact models are presented using Hertzian contact theory. A PD controller was then designed considering system saturation and friction, based on the Coulomb model and the effect of lubrication. The dynamic equations were solved for the dynamic trajectory and FFT spectra, STFT spectra, Poincaré maps and bifurcation diagrams were used for the parametric analysis. The results show that the rotor had three motion modes. System parameters, including unbalance eccentricity, magnetic gap clearance and equivalent stiffness and damping ratio, may lead to complex nonlinear dynamic behavior including periodic, KT-periodic, and quasi-periodic responses and jump phenomenon. Suitable designs that consider these parameters may avoid undesirable rotor dynamic behavior. This study reveals the mechanism for nonlinear response, providing a method for its prediction, and core controller parameter designs for rotor re-levitation.

Active magnetic bearings (AMBs) have limited dynamic load capacity due to magnetic saturation. Hence, large external disturbances (such as shock loads) may cause contact between the rotor and touchdown bearings (TDBs), which may evolve into complex dynamic behaviour and damage the machine. This paper considers the shock responses of a rotor and viable re-levitation control options when the AMB is still functional. Bi-stable responses and shock-induced persistent forward rubbing were observed in an experimental AMB-flexible rotor facility and its numerical model. The analytical solution for steady synchronous motions with rubbing of a general AMB-flexible rotor system was proposed. The standard control action for a contact-free rotor state would not be appropriate due to phase changes and the displacement amplitude differences in the frequency responses. To destabilise the persistent contact responses and restore contact-free levitation, open-loop phase search based synchronous compensation (PSSC) control and synchronous motion compensation (SMC) control are designed, which are activated when a persistent contact is detected. Stability of the control system and the effectiveness of these two re-levitation control methods are verified by simulation and experimental results. It is also found by comparison that the efficiency of PSSC depends on the phase difference (incorrect phases may degrade rotor response), while the SMC consumes more computing effort.

Purpose The purpose of this paper is to present an optimal posture evaluation model to control the assembly gaps in aircraft wing assembly. The gaps between two mating surfaces should be strictly controlled in precision manufacturing. Oversizing of gaps will decrease the dimensional accuracy and may reduce the fatigue life of a mechanical product. To reduce the gaps and keep them within tolerance, the relative posture (orientation and position) of two components should be optimized in the assembly process. Design/methodology/approach Based on the step alignment strategy, i.e. preliminary alignment and refined alignment, the concept of a small posture transformation (SPT) is introduced. In the preliminary alignment, an initial posture is estimated by a set of auxiliary locating points, with which the components can be quickly aligned near each other. In the refined alignment, the assembly gaps are calculated and the formulation of the gaps with component posture is derived by the SPT. A comprehensive weighted minimization model with gap tolerance constraints is established for redistributing the gaps in multi-regions. Powell-Hestenes-Rockafellar optimization, Singular Value Decomposition and K-Dimensional tree searching are introduced for the solution of the optimal posture for localization. Findings Using the SPT, the trigonometric posture transformation is linearized, which benefits the iterative solution process. Through the constrained model, overall gaps are minimized and excess gaps are controlled within tolerance. Practical implications This method has been tested with simulated model data and real product data, the results of which have shown efficient coordination of mating components. Originality/value This paper proposed an optimal posture evaluation method for minimizing the gaps between mating surfaces through component adjustments. This will promote the assembly automation and variation control in aircraft wing assembly.

This paper deals with optimal controller design for active magnetic bearing (AMB) systems for which nonlinear rotordynamic behavior is evident, and so vibration predicted by operating point linearization differs from that which occurs in actuality. Nonlinear H-infinity control theory is applied with a rotordynamic model involving nonlinear stiffness and/or damping terms. The associated Hamilton-Jacobi-Isaacs (HJI) equation is formulated and solved to obtain a state feedback control law achieving specified vibration attenuation performance in terms of the peak L2 gain of the nonlinear system. The method is applied in case study to a flexible rotor/AMB system that exhibits nonlinear stiffness properties owing to rotor interaction with a clearance bearing. Simulations are performed to quantify RMS vibration due to harmonic disturbances and the results compared with the norm-bound values embedded in the HJI equations. A feedback controller design method is then presented that is similar in approach to the standard loop-shaping/mixed-sensitivity methods used for linear systems, and involves augmenting the system model with weighting transfer functions. Experiments are undertaken to compare controller performance for designs based on nonlinear and linearized models. The results highlight the shortcomings of applying linear optimal control methods with rotor systems exhibiting nonlinear stiffness properties as large amplitude vibration and loss of rotordynamic stability can occur. Application of the described nonlinear H-infinity control method is shown to overcome these problems, albeit at the expense of vibration attenuation performance for operation in linear regimes.

This paper presents a new ceiling-mounted workshop Measurement Positioning System (C-wMPS) compensating for many deficiencies shown by conventional metrology systems, especially on the possibility of task-oriented designing for coverage ability, measurement accuracy and efficiency. A hybrid calibration system consisting of a high-precision coordinate control field and standard lengths is developed and implemented for the C-wMPS, which can be designed concretely to provide both traceability and the ability of local accuracy enhancement. Layout optimization using a genetic algorithm based on grids is applied to design an appropriate layout of the system, therefore promotes the system's performance and reduce cost. An experiment carried out at the Guidance, Navigation and Control laboratory (GNC Lab, 40 × 30 × 12 m) validates the prominent characteristic of C-wMPS and the fitness of the new calibration system and layout optimization method.

The interaction of a polymer-steel spur gear pair is naturally prone to cause wear of the softer polymer material. Experimental equipment was therefore designed and constructed to run a polymer-steel spur gear pair with sufficiently accurate instrumentation such that the load and speed could be measured under a range of operating conditions. Plastic deformation features on the polymer teeth surface were observed through scanning electron microscopy and these are considered to be the primary sources of polymer gear wear during service. The wear mechanism is discussed and models are presented to describe the process by which it is created. This is due to the sliding/rolling under conforming contact conditions that are intrinsic to the involute spur gear pair. An iterative model based on the nonlinear properties of the polymer is presented, which shows how the material is deformed permanently and progressively under each load cycle. A second model also shows how the wear deformation features are created. Worn gear teeth were inspected and the quantity of material worn correlates well with the predicted wear volume.

Dielectric elastomers are of interest for actuator applications due to their large actuation strain, high bandwidth, high energy density, and their flexible nature. If future dielectric elastomers are to be used reliably in applications that include soft robotics, medical devices, artificial muscles, and electronic skins, there is a need to design devices that are tolerant to electrical and mechanical damage. In this paper, the first report of self‐healing of both electrical breakdown and mechanical damage in dielectric actuators using a thermoplastic methyl thioglycolate–modified styrene–butadiene–styrene (MGSBS) elastomer is provided. The self‐healing functions are examined from the material to device level by detailed examination of the healing process, and characterization of electrical properties and actuator response before and after healing. It is demonstrated that after dielectric breakdown, the initial dielectric strength can be recovered by up to 67%, and after mechanical damage, a 39% recovery can be achieved with no degradation of the strain–voltage response of the actuators. The elastomer can also heal a combination of mechanical and electrical failures. This work provides a route to create robust and damage tolerant dielectric elastomers for soft robotic and other applications related to actuator and energy‐harvesting systems.

Indoor localization is a key enabling technology for mobile robot navigation in industrial manufacturing. As a distributed metrology system based on multi-station intersection measurement, the workshop measurement positioning system (wMPS) is gaining increasing attention in mobile robot localization. In this paper, a new, to the best of our knowledge, wMPS-based resection localization method is proposed using a single onmidirectional transmitter mounted on a mobile robot with scanning photoelectric receivers distributed in the work space. Compared to the traditional method that requires multiple stationary transmitters, our new method provides higher flexibility and cost-effectiveness. The position and orientation of the mobile robot are then iteratively optimized with respect to the constraint equations. In order to obtain the optimal solution rapidly, two methods of initial value determination are presented for different numbers of effective receivers. The propagation of the localization uncertainty is also investigated using Monte-Carlo simulations. Moreover, two experiments of automated guided vehicle localization are conducted, and the results demonstrate the high accuracy of the proposed method.

Turbomachinery shaft seals suffer from rubs caused by thermal growth, assembly misalignment and rotor dynamic vibration at engine start-up and shut-down. Rubs are detrimental to performance, leading to a decrease in overall efficiency and costly corrective maintenance. In recent years, compliant seals have been developed, allowing for variable clearances and a reduced frequency of seal rubs. The design goal for compliant seals is therefore, to maintain a tight clearance between rotating and non-rotating parts, throughout the transient conditions experienced in engines. This paper presents the design of a new high-speed rotating test facility developed for the performance characterisation of turbine shaft seals. The rig features a 254 mm diameter rotor, capable of rotating at speeds of up to 15,000 rpm (equivalent to rotor surface speeds up to 200 m/s). The maximum pressure difference across a seal is 3.5 bar. In the first experimental campaign, the performance of a labyrinth seal was investigated. The rotordynamic coefficients of the seal were calculated by exciting the casing with an electromagnetic shaker. The leakage performance, direct and cross-coupled seal stiffnesses and effective damping coefficients are determined.

This paper demonstrates vibration reduction in a hollow rotating shaft by means of internal-stator active magnetic actuators, which are resiliently mounted. This problem requires further consideration over and above classic rotor/magnetic bearing systems on account of the flexible behavior of the magnetic actuator support structure. This paper presents an experimental facility conforming to the proposed topology, with a particular focus on the control problem such a system presents. The unique challenges are discussed, and a solution is presented in the form of H $_\infty$ -based control. Ultimately, experimental results demonstrate the system to be capable of substantial rotor vibration suppression, including while passing the rotor's first critical speed, which was not obtainable with simpler classical control techniques. This means the top achievable rotor speed was increased from approximately 3000 r/min without magnetic actuator vibration suppression to over 9000 r/min with vibration suppression active. At the rotor critical speed, the magnetic actuators affect a reduction in rotor vibration amplitude of over 70% compared to the rotor supported purely on mechanical bearings, while simultaneously avoiding excessive excitation of the flexible active magnetic actuator support structure.

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.

This paper presents a review of techniques to mitigate jamming in Electromechanical Actuators (EMA) for safety critical applications in aerospace. Published progress to date is evaluated, with the remaining challenges highlighted. Through the use of Hierarchical Process Modelling (HPM), two key approaches to mitigate jamming were identified: (1) Fault Diagnostics (FD) and (2) Fault tolerant design. The development of a fault tolerant EMA system is currently at an early stage for implementation within safety critical systems due to the increased complexity of such systems (for example the anti-jamming system may require FD functionality itself). Challenges also exist for FD approaches particularly in achieving a robust means of fault detection. It is proposed that a hybrid FD approach, using a combination of model based and data-driven techniques to predict the onset of jamming, would be beneficial in capturing the discrepancies between the predicted and observed behaviour used to isolate and identify faults. Furthermore, several aspects of modelling and of data-driven methodologies for FD in the literature omit potentially important behaviours, and recommendations are made to improve upon this. For example, the simulation of faults in test stand analysis and the fidelity modelling of the motor and mechanical components are key areas to develop.

Six-degree-of-freedom (6D) sensors enhance the measurement capability of traditional three-degree-of-freedom (3D) laser trackers. However, the classical 6D measurement techniques still have shortcomings in actual use, such as the problem of line of sight and relatively low data acquisition rate. The proposed approach by integrating an Inertial Measurement Unit (IMU) with a 6D sensor unit of a laser tracker is effective to overcome these limitations. The error is corrected by the combination of a Kalman filter and a backward smoothing algorithm. The Kalman filter only works when the 6D sensor's data is being sent through, while the backward smoothing algorithm works during the whole process. The experiments are performed to compare the error in three positions and three rotational orientations between the proposed method and the Kalman filter and evaluate the effects of different rates and IMU frequencies on the algorithm. The simulations are also performed to estimate the maximum outage time. The results verify that the proposed method can solve the problem of line of sight and low data acquisition rate effectively.

As fuel efficiency becomes a greater concern in commercial aircraft design and operation it is important to gain a deeper understanding of energy use throughout an aircraft’s systems. To meet this need, a new methodology is being developed that proposes the use of exergy as a metric for mapping the energy flows throughout an aircraft at any required level of detail over a given flight. Exergy allows direct comparison of differing energy types and clearer interpretation of losses and availability. Time-variant application of exergy to vehicles, however, presents complexities, such as large data volumes, temporary exergy storage, exergy exchange between system and mass, and an inconsistent reference state. A software tool prototype is introduced to allow the management of an aircraft exergy map. To describe its use an unmanned aerial vehicle is evaluated. A turbojet study isolates the reference state problem and illustrates the need to use a moving, aircraft-fixed reference state. However, it is shown that ...

This paper presents a novel topology for enhanced vibration sensing in which wireless MEMS accelerometers embedded within a hollow rotor measure vibration in a synchronously rotating frame of reference. Theoretical relations between rotor-embedded accelerometer signals and the vibration of the rotor in an inertial reference frame are derived. It is thereby shown that functionality as a virtual stator-mounted displacement transducer can be achieved through appropriate signal processing. Experimental tests on a prototype rotor confirm that both magnitude and phase information of synchronous vibration can be measured directly without additional stator-mounted key-phasor sensors. Displacement amplitudes calculated from accelerometer signals will become erroneous at low rotational speeds due to accelerometer zero-g offsets, hence a corrective procedure is introduced. Impact tests are also undertaken to examine the ability of the internal accelerometers to measure transient vibration. A further capability is demonstrated, whereby the accelerometer signals are used to measure rotational speed of the rotor by analysing the signal component due to gravity. The study highlights the extended functionality afforded by internal accelerometers and demonstrates the feasibility of internal sensor topologies, which can provide improved observability of rotor vibration at externally inaccessible rotor locations.

A new methodology to predict the transient operational temperature of a polymer–steel gear pair under loaded running is presented. For the involute gear form, rolling and sliding leads to a loss of gear efficiency and generation of heat in the contact zone. The power dissipated is used to set the conditions for a series of rod on disc experiments. The rod-on-disc data are processed in a time averaging procedure, which allows prediction of the complete gear temperature. This is assessed with analytical and finite element models to validate the predicted temperature rise against the experimental data. The significance is that the experimental procedures may be used to assess gear thermal performance without testing full gear pairs.

A topology composed of a hollow-shaft rotor on rolling element bearings coupled with internal-stator magnetic bearings mounted on independently supported flexible shafts is considered for the purposes of vibration reduction. Details of a bespoke test rig constructed to assess this topology are presented. The rig makes use of custom-designed internal-stator magnetic bearings fabricated from Soft Magnetic Composite material. Results from standard numerical analysis of the rig are given to demonstrate the expected behaviour of the system, with a particular focus on its potential to reduce synchronous vibration seen in the rotor while passing critical speeds. These behaviours are then confirmed via a series of experimental tests. These results show that the proposed topology can be used effectively to alter the system vibration behaviour.

A novel rotor topology is proposed with the goal of reducing vibration in high speed rotor systems. Reduction in vibration at critical speeds is targeted. In essence, the novel topology consists of a hollow tube rotor coupled to a secondary non-rotating shaft located concentrically within the primary rotor. The two shafts are linked via a number of actively managed magnetic couplings. The topology is provided, along with an analysis comparing the out-of-balance vibration response of the proposed rotor to a comparable simple passive rotor. Designs for a test rig based on this topology are presented, together with an outline of anticipated testing and development of the idea. Special consideration is given to the design of the magnetic bearings, which form the system couplings. An internal-stator homopolar bearing design fabricated from soft magnetic composite is presented. The qualities of soft magnetic composite as a bearing core material are shown to be comparable with a standard laminated core with the added advantage of minimal eddy current losses within the primary rotor.

This paper describes a method of uncertainty evaluation for axi-symmetric measurement machines. Specialized measuring machines for the inspection of axisymmetric components enable the measurement of properties such as roundness (radial runout), axial runout and coning. These machines typically consist of a rotary table and a number of contact measurement probes located on slideways. Sources of uncertainty include the probe calibration process, probe repeatability, probe alignment, geometric errors in the rotary table, the dimensional stability of the structure holding the probes and form errors in the reference hemisphere which is used to calibrate the system. The generic method is described and an evaluation of an industrial machine is described as a worked example. Expanded uncertainties, at 95% confidence, were then calculated for the measurement of; radial runout (1.2 μm with a plunger probe or 1.7 μm with a lever probe); axial runout (1.2 μm with a plunger probe or 1.5 μm with a lever probe); and coning/swash (0.44 arcseconds with a plunger probe or 0.60 arcseconds with a lever probe).

The hip joint is subjected to cyclic loading and motion during activities of daily living and this can induce micromotions at the bone-implant interface of cementless total hip replacements. Initial stability has been identified as a crucial factor to achieve osseointegration and long-term survival. Whilst fixation of femoral stems achieves good clinical results, the fixation of acetabular components remains a challenge. In vitro methods assessing cup stability keep the hip joint in a fixed position, overlooking the effect of hip motion. The effect of hip motion on cup micromotion using a hip motion simulator replicating hip flexion-extension and a six degrees of freedom measurement system was investigated. The results show an increase in cup micromotion under dynamic hip motion compared to Static Flexion. This highlights the need to incorporate hip motion and measure all degrees of freedom when assessing cup micromotion. In addition, comparison of two press-fit acetabular cups with different surface coatings suggested similar stability between the two cups. This new method provides a basis for a more representative protocol for future pre-clinical evaluation of different cup designs.

This paper considers a control system design for a rotor–magnetic bearing system that integrates a number of fault-tolerant control methods. A survey is undertaken of possible system failure modes which are classified according to whether they are internal or external to the magnetic bearing control system. Improved tolerance to specific external faults is achieved through multivariable controller design with H ∞ optimised disturbance rejection criteria. Tolerance to internal faults requires the integration of additional control sub-systems, including a fault detection algorithm and a supervisory algorithm to reconfigure control on occurrence of a fault. Experimental results obtained from a flexible rotor system are used to demonstrate the effectiveness of the control implementations.

This paper considers the derivation and application of closed loop vibration controllers that are designed for the attenuation of multiple discrete frequency rotor vibration components. The complex amplitudes of each vibration component are evaluated in real-time and used for dynamic feedback control with frequency matched control signals. Multi-input, multi-output system gain matrices are derived from on-line identification routines, performed under a finite set of different operating conditions. Controller gain matrices are synthesised using linear matrix inequality existence conditions for closed loop system stability and attenuation performance. Thus, the controllers can be designed with a degree of robustness to changing/non-linear dynamics. Implementation and testing is undertaken on a flexible rotor test rig with magnetic bearings. The controllers are shown to simultaneously attenuate vibration due to direct rotor disturbance forces and other indirectly forced non-synchronous frequency components over a range of operating conditions.

A novel calibration method is presented for a sensor fusion system in large-scale metrology, which improves the calibration efficiency and reliability. The attitude sensor is composed of a pinhole prism, a converging lens, an area-array camera and a biaxial inclinometer. A mathematical model is established to determine its three-dimensional attitude relative to a cooperative total station by using two vector observations from the imaging system and the inclinometer. The measurement model developed has two aspects to be calibrated: the intrinsic parameters of the imaging model; and the transformation matrix between the camera and the inclinometer. An integrated calibration method using a three-axis rotary table and a total station is then proposed. A single mounting position of the attitude sensor on the rotary table is sufficient to solve for all parameters of the measurement model. A correction technique for the reference laser beam of the total station removes the need for accurate positioning of the sensor on the rotary table. Calibration measurements are made at multiple angular positions of the rotary table in order to determine all the unknown parameters in the model. Experimental verification has verified the practicality and accuracy of this calibration method. Results show that the mean deviations of attitude angles using the proposed method are less than 0.01°.

It is now common practice to supplement a magnetic bearing with a touchdown bearing to protect the rotor and stator components. Rotor/touchdown bearing contact may arise from rotor drop, caused by power loss or emergency shutdown. This paper considers the control options that are viable when the magnetic bearing is still functional should contact arise from intermittent faults or overload conditions. The problem is that bi-stable rotor responses are possible, with and without contact. If rotor contact should become persistent, the desirable course of action is to destabilize the rotor response and induce a return to contact-free levitation. In order to achieve this, it is appropriate to gain an understanding of the rotor dynamic behavior. This is determined from analytical and simulated results to reveal suitable control actions. These may be applied through the magnetic bearing control system, or by activating the touchdown bearing through a separate control loop. The issue is that standard control action for a contact-free rotor state will not be appropriate for a rotor in persistent contact since the basic plants to be controlled are significantly different. The required control action should be activated only when contact is detected. The results demonstrate that appropriately phased synchronous forcing could destabilize synchronous forward rub responses. Alternatively, small whirl motions of a touchdown bearing could also be beneficial without disturbing the main magnetic bearing control loop.

Initial stability is an essential prerequisite to achieve osseointegration of press-fit acetabular cups in total hip replacements.Most in vitro methods that assess cup stability do not reproduce physiological loading conditions and use simplifiedacetabular models with a spherical cavity. The aim of this study was to investigate the effect of bone density andacetabular geometry on cup stability using a novel method for measuring acetabular cup micromotion. A press-fit cupwas inserted into Sawbones foam blocks having different densities to simulate normal and osteoporotic bone variationsand different acetabular geometries. The stability of the cup was assessed in two ways: (a) measurement of micromotionof the cup in 6 degrees of freedom under physiological loading and (b) uniaxial push-out tests. The results indicate thatchanges in bone substrate density and acetabular geometry affect the stability of press-fit acetabular cups. They also suggestthat cups implanted into weaker, for example, osteoporotic, bone are subjected to higher levels of micromotionand are therefore more prone to loosening. The decrease in stability of the cup in the physiological model suggests thatusing simplified spherical cavities to model the acetabulum over-estimates the initial stability of press-fit cups. This noveltesting method should provide the basis for a more representative protocol for future pre-clinical evaluation of newacetabular cup designs.

Synchronous vibration is the most common form of vibration in rotorbearing systems. Open-loop controllers, in conjunction with magnetic bearings, have been shown to be very effective in controlling synchronous vibration in flexible rotor-bearing systems. A well-known open-loop controller, called a mini-least-squares controller, minimises the measured rotor vibration in a least-squares sense. In this paper, it is shown how convex programming can be used to design mini-major-axis controllers, which minimise the largest measured rotor displacement. A numerical example illustrates the potential improvement in performance of mini-major-axis controllers compared to mini-least-squares controllers.

Understanding the uncertainty of dimensional measurements for large products such as aircraft, spacecraft and wind turbines is fundamental to improving efficiency in these products. Much work has been done to ascertain the uncertainty associated with the main types of instruments used, based on laser tracking and photogrammetry, and the propagation of this uncertainty through networked measurements. Unfortunately this is not sufficient to understand the combined uncertainty of industrial measurements, which include secondary tooling and datum structures used to locate the coordinate frame. This paper presents for the first time a complete evaluation of the uncertainty of large scale industrial measurement processes. Generic analysis and design rules are proven through uncertainty evaluation and optimization for the measurement of a large aero gas turbine engine. This shows how the instrument uncertainty can be considered to be negligible. Before optimization the dominant source of uncertainty was the tooling design, after optimization the dominant source was thermal expansion of the engine; meaning that no further improvement can be made without measurement in a temperature controlled environment. These results will have a significant impact on the ability of aircraft and wind turbines to improve efficiency and therefore reduce carbon emissions, as well as the improved reliability of these products.

There are machine operating regimes in which rotor/stator interactions may lead to problematic rotor dynamic behavior. For example, dynamic heat sources arising from seals, bearings and other rubbing stator components may cause rotor thermal bend instability. In active magnetic bearing (AMB) systems, the rotor may experience forward and backward whirl rubs with touchdown bearings (TDBs). In abnormal cases, rotor transient and bounce interactions with such bearings may involve highly localized and short duration contacts. This paper discusses certain contact phenomena that may occur in passive and active systems. For example, the rub induced spiral behavior arises from a combination of unbalance and a thermal input that moves slowly around the rotor, typically in passive rotor-bearing systems. However, the instability can be regarded as if arising from a closed-loop feedback system. Hence it is possible to analyze the phenomenon using techniques that have been developed for active control systems. Rotors levitated by AMBs are truly active, but there are fundamental issues that may arise when contact with TDBs occurs. AMB control and contact interactions are discussed together with the benefits for making the TDB an active element. The reason for this lies in the potential ability to control the contact dynamics and associated mechanical and thermal stresses. A prototype system is described.

The influence upon the temperature distribution within a hydrodynamically lubricated journal bearing of a time varying journal orbit is considered and the oscillatory source distribution arising as a result of unsteady shearing of the lubricant is examined with respect to axes which rotate with the journal. The time dependent thermal bend induced in a rotor by such a process has been evaluated and incorporated into a rotor dynamic model for the assessment of system stability. Techniques have been developed enabling calculations to be carried out on an idealized rotor-bearing configuration and the speed range over which instability occurred was established. The rate of growth of the unstable vibrations together with the rate and direction of ‘spiralling’ of the thermal bend vector were also determined. Features of rotor and bearing design having an influence on such thermally induced instability are assessed.

The nature of differential heating across the journal of a hydrodynamically lubricated bearing has been investigated theoretically and an assessment made of the local thermal bend in the rotor at the bearing location. The energy balance for the system is time dependent and involves a stationary bearing component, an oscillating lubricant film, and a rotating, orbiting section of journal. Thermo-hydrodynamic solutions are developed which allow for temporal and spatial periodic variations of bearing, lubricant and journal temperatures. The influence of a synchronous journal orbit on the temperature distribution throughout the system is discussed. In particular, the relation between any prescribed orbit and the steady state temperature differential across the journal, resulting in steady state rotor bending, is established. The role played by such thermal bending in the onset of unstable synchronous whirl in the rotors of high-speed machines is outlined.

We analyse the novel dynamics arising in a nonlinear rotor dynamic system by investigating the discontinuity-induced bifurcations corresponding to collisions with the rotor housing (touchdown bearing surface interactions). The simplified Föppl/Jeffcott rotor with clearance and mass unbalance is modelled by a two degree of freedom impact–friction oscillator, as appropriate for a rigid rotor levitated by magnetic bearings. Two types of motion observed in experiments are of interest in this paper: no contact and repeated instantaneous contact. We study how these are affected by damping and stiffness present in the system using analytical and numerical piecewise-smooth dynamical systems methods. By studying the impact map, we show that these types of motion arise at a novel non-smooth Hopf-type bifurcation from a boundary equilibrium bifurcation point for certain parameter values. A local analysis of this bifurcation point allows us a complete understanding of this behaviour in a general setting. The analysis identifies criteria for the existence of such smooth and non-smooth bifurcations, which is an essential step towards achieving reliable and robust controllers that can take compensating action.

This paper reports how the stiffness of a Flat Spring can be predicted using nonlinear Finite Element Analysis (FEA). The analysis of a Flat Spring is a nonlinear problem involving contact mechanics, geometric nonlinearity and material property nonlinearity. Research has been focused on improving the accuracy of the model by identifying and exploring the significant assumptions contributing to errors. This paper presents results from some of the models developed using FEA software. The validation process is shown to identify where improvements can be made to the model assumptions to increase the accuracy of prediction. The goal is to achieve an accuracy level of ±10 % as the intention is to replace practical testing with FEA modelling, thereby reducing the product development time and cost. Results from the FEA models are compared with experimental results to validate the accuracy.

Non-linear seal friction can impede the performance of hydraulic actuation systems designed for high precision positioning with favourable dynamic response. Methods for predicting seal friction are required to help develop sealing systems for this type of application. Recent simulation techniques have claimed progress, although have yet to be validated experimentally. A conventional reciprocating rod seal is analysed using established elastohydrodynamic theory and the mixed lubrication Greenwood-Williamson-average Reynolds model. A test rig was used to assess the accuracy of the simulation results for both instroke and outstroke. Inverse hydrodynamic theory is shown to predict a U0.5 power law between rod speed and friction. Comparison with experimental data shows the theory to be qualitatively inaccurate and to predict friction levels an order of magnitude lower than those measured. It was not possible to model the regions very close to the inlet and outlet due to the high pressure gradients at the edges of the contact. The mixed lubrication model produces friction levels within the correct order of magnitude, although incorrectly predicts higher friction during instroke than outstroke. Previous experiments have reported higher friction during instroke than outstroke for rectangular seals, suggesting that the mixed lubrication model used could possibly be suitable for symmetric seals, although not for seal tribology in general.