Lightweight positioning : design and optimization of an actuator with two controlled degrees of freedom

It is known that internal vibrations decrease the performance characteristics and life time of mechanisms, and in some cases they even may lead to mechanical failures. In motion systems used in precision technology (wafer scanners, scanners, pick-and-place machines for production of PCBs, wire-bonders etc.), internal vibrations limit the performance parameters. The vibrations are still a challenge for the generally accepted design approach at present time, which is heading towards higher system accuracy, speed and throughput. Currently, the design approach to precision positioning applications places the dominant vibration frequencies of the mechanical parts several times higher than the required control bandwidth. However, these high mechanical frequencies are reached by constructing the mechanical parts with high stiffness, often at the cost of relatively high mass. To eliminate the negative consequences of the classical methodology, another design philosophy is used in this thesis. A three-disciplinary lightweight positioning approach (control, mechanics and electromechanics) focuses on mass reduction of the moving parts of motion systems. For this purpose, a principle based on over-actuation is used, which allows designing a lighter overall kinematical structure (force-path). In order to evaluate this approach on a general level, benchmarks for classical and lightweight positioning systems are proposed, namely, a so-called stiff beam system and a flexible beam system. The main focus of the thesis is on the design and optimization of a novel Lorentz force actuator for a lightweight positioning system that can also be applied in other precision technology applications. The objective is to reach the maximum mass reduction of the flexible beam system. In order to evaluate and design the novel actuator, a comprehensive static electromagnetic analysis of the actuator is elaborated. The resulting analytical model is based on a magnetic equivalent circuit, which has been identified by means of preliminary finite element calculations. The analytical model plays an essential role in the complete design. It is later used for the optimal dimensioning of the actuator for required performance specifications. Then, a numerical finite element model is built and the results are used to evaluate the accuracy of the analytical model and to identify parasitic forces and torques of the actuator. Another important aspect that determines the operating conditions is the thermal behavior of the actuator. It is also described analytically by a thermal lumped parameter model. The suggested description of the heat transfer captures the static as well as the dynamic behavior. To determine the optimal dimensions of the actuator an optimization approach, which uses the magnetic equivalent circuit and the thermal analytical model, is proposed. In terms of nonlinear programming, the problem statement consists of finding the dimensions of the actuator with minimal mass, where given force and torque are used as constraints. Because of the nonlinear nature of the problem the optimal solution is found numerically. The resulting optimal actuator incorporating two degrees of freedom (DoF) has 22.2% less mass than two equivalent 1-DoF actuators. It may be concluded, based on simulation and measurement results, that the proposed actuator can be analyzed with sufficient accuracy by the presented methods. The invented short-stroke actuator uniquely combines two controlled degrees of freedom: translational and rotational. This combination ensures that the mass of the actuators used in the flexible beam system has been reduced compared to that in the stiff beam system. The actuators support the flexible beam system in a way that introduces less disturbances. Meanwhile, the controllability of higher order vibration modes and, consequently, the global performance are improved. Two lightweight positioning systems were built, one with three 1-DoF actuators and the other with two novel Lorentz force actuators. In both setups the flexible beam has its mass reduced to 38.6% of that of the stiff beam. The total mass of the actuators in both cases is almost the same, but the setup with the innovative actuators allows to control the beam with two forces and two torques, while the setup with three 1-DoF actuators produces only three controlled force

Large space structure damping design

Several FORTRAN subroutines and programs were developed which compute complex eigenvalues of a damped system using different approaches, and which rescale mode shapes to unit generalized mass and make rigid bodies orthogonal to each other. An analytical proof of a Minimum Constrained Frequency Criterion (MCFC) for a single damper is presented. A method to minimize the effect of control spill-over for large space structures is proposed. The characteristic equation of an undamped system with a generalized control law is derived using reanalysis theory. This equation can be implemented in computer programs for efficient eigenvalue analysis or control quasi synthesis. Methods to control vibrations in large space structure are reviewed and analyzed. The resulting prototype, using electromagnetic actuator, is described

Vibration Energy Harvesting for Wireless Sensors

Kinetic energy harvesters are a viable means of supplying low-power autonomous electronic systems for the remote sensing of operations. In this Special Issue, through twelve diverse contributions, some of the contemporary challenges, solutions and insights around the outlined issues are captured describing a variety of energy harvesting sources, as well as the need to create numerical and experimental evidence based around them. The breadth and interdisciplinarity of the sector are clearly observed, providing the basis for the development of new sensors, methods of measurement, and importantly, for their potential applications in a wide range of technical sectors

Limited Bandwidth Wireless Communication Strategies for Structural Control of Seismically Excited Shear Structures

Structural control is used to mitigate unwanted vibrations in structures when large excitations occur, such as high winds and earthquakes. To increase reliability and controllability in structural control applications, engineers are making use of semi-active control devices. Semi-active control gives engineers greater control authority over structural response versus passive controllers, but are less expensive and more reliable than active devices. However, the large numbers of actuators required for semi-active structural control networks introduce more cabling within control systems leading to increased cost. Researchers are exploring the use of wireless technology for structural control to cut down on the installation cost associated with cabling. However wireless communication latency (time delays in data transmissions) can be a barrier to full acceptance of wireless technology for structural control. As the number of sensors in a control network grows, it becomes increasingly difficult to transmit all sensor data during a single control step over the fixed wireless bandwidth. Because control force calculations rely on accurate state measurements or estimates, the use of strategic bandwidth allocation becomes more necessary to provide good control performance. The traditional method for speeding up the control step in larger wireless networks is to spatially decentralize the network into multiple subnetworks, sacrificing communication for speed. This dissertation seeks to provide an additional approach to address the issue of communication latency that may be an alternative, or even a supplement, to spatial decentralization of the control network. The proposed approach is to use temporal decentralization, or the decentralization of the control network over time, as opposed to space/location. Temporal decentralization is first presented with a means of selecting and evaluating different communication group sizes and wireless unit combinations for staggered temporal group communication that still provide highly accurate state estimates. It is found that, in staggered communication schemes, state estimation and control performance are affected by the network topology used at each time step with some sensor combinations providing more useful information than others. Sensor placement theory is used to form sensor groups that provide consistently high-quality output information to the network during each time step, but still utilize all sensors. If the demand for sensors to communicate data outweighs the available bandwidth, traditional temporal and spatial approaches are no longer feasible. This dissertation examines and validates a dynamic approach for bandwidth allocation relying on an extended, autonomous and controller-aware, carrier sense multiple access with collision detection (CSMA/CD) protocol. Stochastic parameters are derived to strategically alter back-off times in the CSMA/CD algorithm based on nodal observability and output estimation error. Inspired by data fusion approaches, this second study presents two different methods for neighborhood state estimation using a dynamic form of measurement-only fusion. To validate these wireless structural control approaches, a small-scale experimental semi-active structural control testbed is developed that captures the important attributes of a full-scale structure

Extended analytical charge modeling for permanent-magnet based devices : practical application to the interactions in a vibration isolation system

This thesis researches the analytical surface charge modeling technique which provides a fast, mesh-free and accurate description of complex unbound electromagnetic problems. To date, it has scarcely been used to design passive and active permanent-magnet devices, since ready-to-use equations were still limited to a few domain areas. Although publications available in the literature have demonstrated the surface-charge modeling potential, they have only scratched the surface of its application domain. The research that is presented in this thesis proposes ready-to-use novel analytical equations for force, stiffness and torque. The analytical force equations for cuboidal permanent magnets are now applicable to any magnetization vector combination and any relative position. Symbolically derived stiffness equations directly provide the analytical 3 £ 3 stiffness matrix solution. Furthermore, analytical torque equations are introduced that allow for an arbitrary reference point, hence a direct torque calculation on any assembly of cuboidal permanent magnets. Some topics, such as the analytical calculation of the force and torque for rotated magnets and extensions to the field description of unconventionally shaped magnets, are outside the scope of this thesis are recommended for further research. A worldwide first permanent-magnet-based, high-force and low-stiffness vibration isolation system has been researched and developed using this advanced modeling technique. This one-of-a-kind 6-DoF vibration isolation system consumes a minimal amount of energy (Ç 1W) and exploits its electromagnetic nature by maximizing the isolation bandwidth (> 700Hz). The resulting system has its resonance > 1Hz with a -2dB per decade acceleration slope. It behaves near-linear throughout its entire 6-DoF working range, which allows for uncomplicated control structures. Its position accuracy is around 4mum, which is in close proximity to the sensor’s theoretical noise level of 1mum. The extensively researched passive (no energy consumption) permanent-magnet based gravity compensator forms the magnetic heart of this vibration isolation system. It combines a 7.1kN vertical force with <10kN/m stiffness in all six degrees of freedom. These contradictory requirements are extremely challenging and require the extensive research into gravity compensator topologies that is presented in this thesis. The resulting cross-shaped topology with vertical airgaps has been filed as a European patent. Experiments have illustrated the influence of the ambient temperature on the magnetic behavior, 1.7h/K or 12N/K, respectively. The gravity compensator has two integrated voice coil actuators that are designed to exhibit a high force and low power consumption (a steepness of 625N2/W and a force constant of 31N/A) within the given current and voltage constraints. Three of these vibration isolators, each with a passive 6-DoF gravity compensator and integrated 2-DoF actuation, are able to stabilize the six degrees of freedom. The experimental results demonstrate the feasibility of passive magnet-based gravity compensation for an advanced, high-force vibration isolation system. Its modular topology enables an easy force and stiffness scaling. Overall, the research presented in this thesis shows the high potential of this new class of electromagnetic devices for vibration isolation purposes or other applications that are demanding in terms of force, stiffness and energy consumption. As for any new class of devices, there are still some topics that require further study before this design can be implemented in the next generation of vibration isolation systems. Examples of these topics are the tunability of the gravity compensator’s force and a reduction of magnetic flux leakage

Advanced suspension system using magnetorheological technology for vehicle vibration control

In the past forty years, the concept of controllable vehicle suspension has attracted extensive attention. Since high price of an active suspension system and deficiencies on a passive suspension, researchers pay a lot attention to semi-active suspension. Magneto-rheological fluid (MRF) is always an ideal material of semi-active structure. Thanks to its outstanding features like large yield stress, fast response time, low energy consumption and significant rheological effect. MR damper gradually becomes a preferred component of semi-active suspension for improving the riding performance of vehicle. However, because of the inherent nonlinear nature of MR damper, one of the challenging aspects of utilizing MR dampers to achieve high levels of performance is the development of an appropriate control strategy that can take advantage of the unique characteristics of MR dampers. This is why this project has studied semi-active MR control technology of vehicle suspensions to improve their performance. Focusing on MR semi-active suspension, the aim of this thesis sought to develop system structure and semi-active control strategy to give a vehicle opportunity to have a better performance on riding comfort. The issues of vibration control of the vehicle suspension were systematically analysed in this project. As a part of this research, a quarter-car test rig was built; the models of suspension and MR damper were established; the optimization work of mechanical structure and controller parameters was conducted to further improve the system performance; an optimized MR damper (OMRD) for a vehicle suspension was designed, fabricated, and tested. To utilize OMRD to achieve higher level of performance, an appropriate semi-active control algorithm, state observer-based Takagi-Sugeno fuzzy controller (SOTSFC), was designed for the semi-active suspension system, and its feasibility was verified through an experiment. Several tests were conducted on the quarter-car suspension to investigate the real effect of this semiactive control by changing suspension damping. In order to further enhance the vibration reduction performance of the vehicle, a fullsize variable stiffness and variable damping (VSVD) suspension was further designed, fabricated, and tested in this project. The suspension can be easily installed into a vehicle suspension system without any change to the original configuration. A new 3- degree of freedom (DOF) phenomenological model to further accurately describe the dynamic characteristic of the VSVD suspension was also presented. Based on a simple on-off controller, the performance of the variable stiffness and damping suspension was verified numerically. In addition, an innovative TS fuzzy modelling based VSVD controller was designed. The TS fuzzy modelling controller includes a skyhook damping control module and a state observer based stiffness control module which considering road dominant frequency in real-time. The performance evaluation of the VSVD control algorithm was based on the quarter-car test rig which equipping the VSVD suspension. The experiment results showed that this strategy increases riding comfort effectively, especially under off-road working condition. The semi-active control system developed in this thesis can be adapted and used on a vehicle suspension in order to better control vibration

Experimental investigations of the Mach-effect for breakthrough space propulsion

This research was conducted within the framework of the SpaceDrive project funded by the German Aerospace Center to develop propellantless propulsion for interstellar travel. The experiments attempted to measure mass fluctuations predicted by the Mach-effect theory derived from General Relativity and observed through torsion balance measurements by Woodward (2012). The combination of such mass fluctuations with synchronized actuation promises propellantless thrust with a significantly better thrust-to-power ratio than photon sails. Thus, experiments using different electromechanical devices including the piezoelectric Mach-effect thruster as tested by Woodward et al. (2012) were pursued on sensitive thrust balances. The tests were automated, performed in vacuum and included proper electromagnetic shielding, calibrations, and different dummy tests. To obtain appropriate driving conditions for maximum thrust, characterization of the experimental devices involved spectrometry, vibrometry, finite element analysis, and circuit modeling. Driving modes consisted of sweeps, resonance tracking, fixed frequency, and mixed signals. The driving voltage, frequency, stack pre-tension, mounting, and thruster orientation were also varied. Lastly, different amplifier electronics were tested as well, including Woodward’s original equipment. Experiments on the double-pendulum and torsion balances with a resolution of under 10 nN and an accuracy of 88.1 % revealed the presence of force peaks with a maximum amplitude of 100 nN and a drift of up to 500 nN. The forces mainly consisted of switching transients whose signs depended on the device’s orientation. These force transients were also observed in the zero-thrust configurations. No additional thrust was observed above the balance drift, regardless of the driving conditions or devices tested. In addition, finite element and vibrometry analysis revealed that the vibration from the actuator was transmitted to the balance beam. Moreover, simulations using a simple spring-mass model showed that the slower transient effects observed can be reproduced using small amplitude, high-frequency vibrations. Hence, the forces observed can be explained by vibrational artifacts rather than the predicted Mach-effect thrust. Then, centrifugal balance experiments measured the mass of a device subjected to rotation and energy fluctuations, with a precision of up to 10 µg and a high time resolution. The measurements relied on piezoelectric- and strain gauges. Their calibration methods presented limitations in the frequency range of interest, resulting in discrepancies of up to 500 %. However, the tests conducted with capacitive and inductive test devices yielded experimental artifacts about three orders of magnitude below the mass fluctuations of several milligrams predicted by the Mach-effect theory. Although the piezoelectric devices presented more artifacts due to nonlinearity and electromagnetic interaction, all rotation experiments did not show the expected dependence on the rotation frequency. In summary, the search for low thrust and small mass fluctuations consisted of challenging experiments that led to the development of innovative and sensitive instruments, while requiring a careful consideration of experimental artifacts. The results analysis led to the rejection of mass fluctuations and thrusts claimed by Woodward’s Mach-effect theory and experiments. The quest for breakthrough space propulsion must thus continue a different theoretical or experimental path.:List of Figures List of Tables List of Abbreviations List of Variables and Symbols 1. Introduction 1.1 Research Motivation 1.2 Objectives 1.3 Content Overview 1.4 Team Work 2. Literature Review 2.1 Fundamentals of Space Propulsion 2.2 Mach’s Principle 2.3 Woodward’s Mach-effect Theory 2.3.1 Derivation of the Mass Fluctuation Equation 2.3.2 Design of a Mass Fluctuation Thruster 2.4 Woodward-type Experiments 2.5 Force and Transient Mass Measurements 3. Electromechanical Characterization 3.1 Piezoelectric Actuators 3.1.1 Basic Properties 3.1.2 Actuator Design 3.1.3 Mach-effect Thruster Devices 3.1.4 Magnetostrictive Actuator 3.1.5 Numerical Analysis of MET Behavior 3.1.6 Vibrometry Analysis 3.1.7 Impedance Spectroscopy 3.1.8 Circuit Modeling 3.1.9 Predictions 3.2 Electronics 3.2.1 Description 3.2.2 Characterization 3.3 Torsion Balances 3.3.1 Description 3.3.2 Characterization 3.3.3 Simulation 3.4 Double-pendulum Balance 3.4.1 Description 3.4.2 Characterization 3.5 Laboratory Setup 3.5.1 Vacuum Chambers 3.5.2 Software and Test Setup 4. Thrust Balance Experiments 4.1 Torsion Balance I Test Results 4.1.1 Dummy Tests 4.1.2 CU18A 4.1.3 MET03 4.1.4 MET04 4.1.5 Discussion 4.2 Torsion Balance II Test Results 4.2.1 Dummy Tests 4.2.2 MET05 4.2.3 Beam Vibration 4.2.4 Discussion 4.3 Double-pendulum Balance Test Results 4.3.1 Dummy Tests 4.3.2 MET03 4.3.3 Discussion 5. Centrifugal Balance Experiments 5.1 Centrifugal Balance 5.1.1 Description 5.1.2 Centrifugal Devices 5.1.3 Predictions 5.2 Transducer Calibration 5.2.1 Quasi-Static Calibration I 5.2.2 Quasi-Static Calibration II 5.2.3 Dynamic Calibration 5.3 Centrifugal Balance Test Results 5.3.1 Characterization 5.3.2 CD01 5.3.3 CD02 5.3.4 CD03 5.3.5 CD04 5.3.6 CD05 5.4 Discussion & Error Analysis 6 Conclusions 6.1 Research Summary 6.2 Further Research Appendix A Appendix B Bibliograph

Medical robots for MRI guided diagnosis and therapy

Magnetic Resonance Imaging (MRI) provides the capability of imaging tissue with fine resolution and superior soft tissue contrast, when compared with conventional ultrasound and CT imaging, which makes it an important tool for clinicians to perform more accurate diagnosis and image guided therapy. Medical robotic devices combining the high resolution anatomical images with real-time navigation, are ideal for precise and repeatable interventions. Despite these advantages, the MR environment imposes constraints on mechatronic devices operating within it. This thesis presents a study on the design and development of robotic systems for particular MR interventions, in which the issue of testing the MR compatibility of mechatronic components, actuation control, kinematics and workspace analysis, and mechanical and electrical design of the robot have been investigated. Two types of robotic systems have therefore been developed and evaluated along the above aspects. (i) A device for MR guided transrectal prostate biopsy: The system was designed from components which are proven to be MR compatible, actuated by pneumatic motors and ultrasonic motors, and tracked by optical position sensors and ducial markers. Clinical trials have been performed with the device on three patients, and the results reported have demonstrated its capability to perform needle positioning under MR guidance, with a procedure time of around 40mins and with no compromised image quality, which achieved our system speci cations. (ii) Limb positioning devices to facilitate the magic angle effect for diagnosis of tendinous injuries: Two systems were designed particularly for lower and upper limb positioning, which are actuated and tracked by the similar methods as the first device. A group of volunteers were recruited to conduct tests to verify the functionality of the systems. The results demonstrate the clear enhancement of the image quality with an increase in signal intensity up to 24 times in the tendon tissue caused by the magic angle effect, showing the feasibility of the proposed devices to be applied in clinical diagnosis