Professor Andrew Fleming

Due to the hysteresis exhibited by piezoelectric actuators, many nanopositioning devices require sensor-based closed-loop control. Although closed-loop control can be effective at eliminating nonlinearity at low speeds, the bandwidth compared to open-loop is severely reduced. In addition, sensor-induced noise can significantly degrade the achievable resolution.

Due to their high stiffness, compact dimensions, and extremely high positioning resolution, piezoelectric actuators are used exclusively in the vast majority of nanopositioning systems. This chapter introduces piezoelectric actuators and describes their electromechanical properties, with a focus on those that are relevant in nanopositioning applications.

The term nanopositioner is used generally to describe a wide variety of mechanical positioning devices with resolution in the nanometer range. Typically, a nanopositioner comprises a moving platform suspended by a number of compliant mechanisms or flexures (see example in Fig. 1. 1). The flexures may provide a preloading force on the actuator and guide the motion of the stage.

Position sensors with nanometer resolution are a key component of many precision imaging and fabrication machines.

Unlike feedback control, which reacts to the measured tracking error, feedforward control compensates or anticipates for poor performance. A feedforward controller does this by exploiting some information about the system, and thus a well-designed feedforward controller requires sufficient knowledge of the plant dynamics and nonlinearities.

As discussed in Chap. 1, the foremost speed limitation in nanopositioning systems arised from the first mechanical resonance mode.

The speed of an electromechanical scanner is limited by its first resonance frequency. To maximize scan speed, input signals are required that contain negligible frequency components near, or above the first resonance frequency. Such signals are usually obtained by low-pass filtering the desired scan trajectory.

Mechanical and electrical noise in nanopositioning systems is unavoidable and dictates the maximum positioning resolution. The major sources of noise include sensor noise, amplifier noise, and external disturbances. In this chapter, these noise sources are discussed and their influence on positioning resolution is evaluated.

Up to this point, nanopositioning controllers have used displacement sensors as the feedback variable. The major drawbacks of typical displacement sensors are the limited bandwidth associated measurement noise.

Due to their high stiffness, small dimensions and low mass, piezoelectric stack actuators are capable of developing large displacements with bandwidths of greater than 100 kHz. However, due to their large electrical capacitance, the associated driving amplifier is usually limited in bandwidth to a few kHz.

The dynamic performance of a nanopositioning system depends on its mechanical resonance frequency, damping, the type of controller used, sensor bandwidth, and associated data acquisition hardware. Recently, speed has become a critical issue in many nanopositioning applications, such as video-rate AFM and high-throughput nanomanufacturing.

Feedback control is the most commonly used technique for eliminating positioning errors in nanopositioning systems. This chapter provides an overview of feedback control techniques with an experimental comparison of integral control, inversion-based control, and IRC damping control. When the reference trajectory is periodic, repetitive control (RC) can significantly improve the tracking performance of a feedback loop. The RC approach is introduced for nanopositioning.

This chapter focuses on the fundamentals of hysteresis, including modeling and compensation.

Piezoelectric benders are widely used in industrial applications due to their low-cost and compact size. However, due to the large relative size and cost of displacement sensors, bender actuators are often operated in open-loop or with feed-forward control, which can limit positioning accuracy to 20% of full-scale. To improve the positioning accuracy of piezoelectric benders, this article proposes integrating resistive strain gauges into the electrode surface by chemical etching or laser ablation. These strain sensors are then used to measure and control the tip displacement. The proposed sensors are shown to suffer from significant cross-coupling between the actuator voltage and measured signal; however, this can be mitigated by judicious choice of the sensor location and actuator driving scheme. In addition to position sensing, a method is also presented for simultaneous estimation of the contact force between the actuator tip and load. The proposed methods are validated experimentally by controlling the tip position of a piezoelectric bender while simultaneously estimating the force applied to a reference load cell.

This article derives design guidelines for integrating strain gauges into the electrodes of piezoelectric bending actuators. The proposed sensor can estimate the actuator tip displacement in response to an applied voltage and an external applied tip force. The actuator load force is also estimated with an accuracy of 8% full scale by approximating the actuator response with a linear model. The applications of this work include micro-grippers and pneumatic valves, which both require the measurement of deflection and load force. At present, this is achieved by external sensors. However, this work shows that these functions can be integrated into the actuator electrodes.

This article describes a monolithic nanopositioner constructed from in-plane bending actuators which provide greater deflection than previously reported extension actuators, at the expense of stiffness and resonance frequency. The proposed actuators are demonstrated by constructing an XY nanopositioning stage with a serial kinematic design. Analytical modeling and finite-element-analysis accurately predicts the experimental performance of the nanopositioner. A 10µm range is achieved in the X and Y axes with an applied voltage of +/-200 V. The first resonance mode occurs at 250 Hz in the Z axis. The stage is demonstrated for atomic force microscopy imaging.

Soft pneumatic actuators are usually fabricated using molding and casting techniques with silicone rubbers, which requires intensive manual labor and limits repeatability and design flexibility for complex geometries. This article presents the design and direct 3D-printing of novel omnidirectional soft pneumatic actuators using stereolithography (SLA) with an elastic resin and fused deposition modeling (FDM) with a thermoplastic polyurethane (TPU). The actuator is modeled and optimized for bending performance using the finite element method along with a hyperelastic material model that is based on experimental uniaxial tensile data. The designs inspired by fast pneumatic network actuators (PneuNets) allow for multimodal actuation including bending, extension and contraction motions under positive, negative or differential pressures. The predicted results from the finite element method are compared with the experimental results for a range of actuation configurations. These novel omnidirectional actuators have significant potential in applications such as pipe inspection and biomedical devices.

Pressure control plays a major role in the overall performance of fluid-driven soft robots. Due to the increasing demand for higher speed actuation and precision, a need exists for a practical design methodology that converts actuator performance specifications to a set of minimum pneumatic specifications, such as receiver volume and pressure, and valve conductance. This article presents a systematic parameter selection approach for pneumatic soft robotic systems by taking into consideration the desired closed-loop pressure responses. The two controllers under evaluation here are the PI controller with anti-windup and the on-off controller with hysteresis. Simulations are developed within Simscape Fluids to evaluate the effect of physical components and controller parameters on the actuator performance. The proposed parameter selection procedures are then applied on three soft actuators and their closed-loop pressure responses are experimentally evaluated. The measured pressure responses are in close agreement with the simulations and satisfy the rise time specifications.

The article describes the design and modeling of a five-axis monolithic nanopositioning stage constructed from a bimorph piezoelectric sheet. Six-axis motion is also possible but requires 16 amplifier channels rather than 8. The nanopositioner is ultra low profile with a thickness of 1 mm. Analytical modeling and finite-element-analysis accurately predict the experimental performance. The stage was conservatively driven with 33% of the maximum voltage, which resulted in an X and Y travel range of 6.22 µm and 5.27 µm respectively; a Z travel range of 26.5 µm; and a rotational motion of 600 µrad and 884 µrad about the X and Y axis respectively. The first resonance frequency occurs at 883 Hz in the Z axis. Experimental atomic force microscopy is performed using the proposed device as a sample scanner.

In this article, boundary element method simulations are used to optimise the geometry of silver and gold nanocone probes to maximise the localised electric field enhancement and tune the near-field resonance wavelength. These objectives are expected to maximise the sensitivity of tip-enhanced Raman microscopes. Similar studies have used limited parameter sets or used a performance metric other than localised electric field enhancement. In this article, the optical responses for a range of nanocone geometries are simulated for excitation wavelengths ranging from 400 to 1000 nm. Performance is evaluated by measuring the electric field enhancement at the sample surface with a resonant illumination wavelength. These results are then used to determine empirical models and derive optimal nanocone geometries for a particular illumination wavelength and tip material. This article concludes that gold nanocones are expected to provide similar performance to silver nanocones at red and near-infrared wavelengths, which is consistent with other results in the literature. In this article, 633 nm is determined to be the shortest usable illumination wavelength for gold nanocones. Below this limit, silver nanocones will provide superior enhancement. The use of gold nanocone probes is expected to dramatically improve probe lifetime, which is currently measured in hours for silver coated probes. Furthermore, the elimination of passivation coatings is expected to enable smaller probe radii and improved topographical resolution.

This paper describes the design of a charge drive for reducing the hysteresis exhibited by a piezoelectric bimorph bender. Existing charge drive circuits cannot be directly applied to bimorph benders since they share a common electrode. In this paper, a new charge drive circuit and electrical configuration are implemented that allows commonly available piezoelectric bimorphs to be linearized. This circuit consists of four major components, including, a high voltage amplifier, a differential amplifier, a piezoelectric load, and a PI feedback controller. An isolation amplifier was used to achieve a differential amplifier with a high common-mode rejection ratio. The charge drive was tested by driving a series poled three layer bimorph bender. The experiment showed a significant improvement to the hysteresis of the bender when compared to a typical voltage drive. This paper has identified an alternative Feedback method to improve the ac hysteresis performance of a piezoelectric bender by using a charge drive.

This article presents a high-speed single- and dual-stage vertical positioners for applications in optical systems. Each positioner employs a unique end-constraint method with orthogonal flexures to preload a piezoelectric stack actuator. This end-constraint method also significantly increases the first mechanical resonance frequency. The single-stage positioner has a displacement range of 7.6 µm and a first resonance frequency of 46.8 kHz. The dual-stage design consists of a long-range slow-stage and a short-range fast-stage. An inertial counterbalance technique was implemented on the fast-stage to cancel inertial forces resulting from high-speed motion. The dual-stage positioner has a combined travel range of approximately 10 µm and a first evident resonance frequency of 130 kHz.

In 1959, the integrated circuit (IC) was invented simultaneously by Jack Kilby of Texas Instruments and Robert Noyce of Shockley Semiconductor [Ki lby, 2000]. This development has been considered one of mankind's most significant innovations.

© 2016 Elsevier B.V.This article presents a novel vertical positioning stage with a dual-sensor arrangement suitable for scanning probe microscopy. The stage has a travel range of 8.4¿µm and a first resonance frequency of 24¿kHz in the direction of travel. The sensor arrangement consists of an integrated piezoelectric force sensor and laminated piezoresistive strain sensor. The piezoelectric force sensor exhibits extremely low noise and introduces a zero into the dynamics which allows the use of integral force feedback. This control method provides excellent damping performance and guaranteed stability. The piezoresistive sensor is used for tracking control with an analog PI controller which is shown to be an approximate inverse of the damped system. The resulting closed-loop system has a bandwidth is 11.4¿kHz and 6s-resolution of 3.6¿nm, which is ideal for nanopositioning and atomic force microscopy (AFM) applications. The proposed vertical stage is used to replace the vertical axis of a commercial AFM. Scans are performed in constant-force contact mode with a tip velocity of 0.2¿mm/s, 1¿mm/s and 2¿mm/s. The recorded images contain negligible artefacts due to insufficient vertical bandwidth.

Repetitive control (RC) achieves tracking and rejection of periodic exogenous signals by incorporating a model of a periodic signal in the feedback path. To improve the performance, an inverse plant response filter (IPRF) is used. To improve robustness, the periodic signal model is bandwidth-limited. This limitation is largely dependent on the accuracy of the IPRF. A new method is presented for synthesizing the IPRF for discrete-time RC. The method produces filters in a simpler and more consistent manner than existing best-practice methods available in the literature, as the only variable involved is the selection of a windowing function. It is also more efficient in terms of memory and computational complexity than existing methods. Experimental results for a nanopositioning stage show that the proposed method yields the same or better tracking performance compared to existing methods.

This article describes a position sensitive interferometer with closed-loop control of the reference mirror. A calibrated nanopositioner is used to lock the interferometer phase to the most sensitive point in the interferogram. In this configuration, large low-frequency movements of the sensor mirror can be detected from the control signal applied to the nanopositioner and high-frequency short-range signals can be measured directly from the photodiode. It is demonstrated that these two signals are complementary and can be summed to find the total displacement. The resulting interferometer has a number of desirable characteristics: it is optically simple, does not require polarization or modulation to detect the direction of motion, does not require fringe-counting or interpolation electronics, and has a bandwidth equal to that of the photodiode. Experimental results demonstrate the frequency response analysis of a high-speed positioning stage. The proposed instrument is ideal for measuring the frequency response of nanopositioners, electro-optical components, MEMs devices, ultrasonic devices, and sensors such as surface acoustic wave detectors.

Positive velocity and position feedback (PVPF) is a widely used control scheme in lightly damped resonant systems with collocated sensor actuator pairs. The popularity of PVPF is due to the ability to achieve a chosen damping ratio by repositioning the poles of the system. The addition of a tracking controller, to reduce the effects of inherent nonlinearities, causes the poles to deviate from the intended location and can be a detriment to the damping achieved. By designing the PVPF and tracking controllers simultaneously, the optimal damping and tracking can be achieved. Simulations show full damping of the first resonance mode and significantly higher bandwidth than that achieved using the traditional PVPF design method, allowing for high-speed scanning with accurate tracking. Experimental results are also provided to verify performance in implementation.

This paper explores the possibility of piezoelectric actuators with integrated high-voltage power electronics. Such devices dramatically simplify the application of piezoelectric actuators since the power electronics are already optimized for the voltage range, capacitance, and power dissipation of the actuator. The foremost consideration is the thermal impedance of the actuator and heat dissipation. Analytical and finite-element methods are described for predicting the thermal impedance of a piezoelectric bender. The predictions are compared experimentally using thermal imaging on a piezoelectric bender with laminated miniature power electronics.

This article describes an improvement to integral resonance damping control (IRC) for reference tracking applications, such as scanning probe microscopy and nanofabrication. It is demonstrated that IRC control introduces a low-frequency pole into the tracking loop that is detrimental for performance. In this work, the location of this pole is found analytically using Cardano¿s method then compensated by parameterizing the tracking controller accordingly. This approach maximizes the closed-loop bandwidth while being robust to changes in the resonance frequencies. The refined IRC controller is comprehensively compared to other low-order methods in a practical environment.

Abstract This paper proposes an improvement to Integral Force Feedback (IFF), which is a popular method for active vibration control of structures and mechanical systems. Benefits of IFF include robustness, guaranteed stability and simplicity. However, the maximum damping performance is dependent on the stiffness of the system; hence, some systems cannot be adequately controlled. In this paper, an improvement to the classical force feedback control scheme is proposed. The improved method achieves arbitrary damping for any mechanical system by introducing a feed-through term. The proposed improvement is experimentally demonstrated by actively damping an objective lens assembly for a high-speed confocal microscope.

This paper describes a new vibration damping technique based on Integral Force Feedback (IFF). Classical IFF utilizes a force sensor and integral controller to damp the resonance modes of a mechanical system. However, the maximum modal damping depends on the frequency difference between the system's poles and zeros. If the frequency difference is small, the achievable modal damping may be severely limited. The proposed technique allows an arbitrary damping ratio to be achieved by introducing an additional feed-through term to the control system. This results in an extra degree of freedom that allows the position of the zeros to be modified and the maximum modal damping to be increased. The second contribution of this paper is a structured PI tracking controller that is parameterized to cancel the additional pole introduced by integral force feedback. The parameterized controller has only one tuning parameter and does not suffer from reduced phase margin. The proposed techniques are demonstrated on a piezoelectric objective lens positioner. The results show exceptional tracking and damping performance while maintaining insensitivity to changes in resonance frequency. The maximum bandwidth achievable with a commercial PID controller is 26.1 Hz. In contrast, with the proposed damping and tracking controller, the bandwidth is increased to 255 Hz. © 2014 Elsevier Ltd. All rights reserved.

The emerging production of ultrathin graphite material is applied to thermal management in a numerical comparison of aluminum and graphite-based plate-fin heat exchangers. Considering anisotropic thermal conductivity in which out-of-plane transport is about two orders of magnitude lower than in-plane values, the ultrathin graphite-based solution outperforms aluminum by rejecting up to 20% more heat on a volumetric basis. Thermal and hydraulic performance is characterized for both solutions over a range of airflow rates in a notional water/air device. Laminar through fully turbulent regimes are considered. Steady and unsteady three-dimensional (3-D) conjugate simulations reveal a faster equilibration rate for the ultrathin graphite-based solution, minimizing thermal lag that must be accounted for in on-demand electronics cooling. Fin optimization studies predict equivalent conductance with graphite at one-tenth the thickness of aluminum. The combination of improved heat rejection, rapid response rate, and low material density make an ultrathin graphite-based solution uniquely suited to aerospace thermal management. © 2012 Taylor and Francis Group, LLC.

This paper describes the design and flight implementation of time-optimal attitude maneuvers performed onboard NASA's Transition Region and Coronal Explorer spacecraft. Minimum-time reorientation maneuvers have obvious applications for improving the agility of spacecraft systems, yet this type of capability has never before been demonstrated in flight due to the lack of reliable algorithms for generating practical optimal control solutions suitable for flight implementation. Constrained time-optimal maneuvering of a rigid body is studied first, in order to demonstrate the potential for enhancing the performance of the Transition Region and Coronal Explorer spacecraft. Issues related to the experimental flight implementation of time-optimal maneuvers onboard Transition Region and Coronal Explorer are discussed. A description of an optimal control problem that includes practical constraints such as the nonlinear reaction wheel torque-momentum envelope and rate gyro saturation limits is given. The problem is solved using the pseudospectral optimal control theory implemented in the MATLAB® software DIDO. Flight results, presented for a typical large-angle time-optimal reorientation maneuver, show that the maneuvers can be implemented without any modification of the existing spacecraft attitude control system. A clear improvement in spacecraft maneuver performance as compared with conventional eigenaxis maneuvering is demonstrated. Copyright © 2011 by Mark Karpenko, Sagar Bhatt, Nazareth Bedrossian, Andy Fleming, and I. Michael Ross.

This paper examines the performance of the pseudospectral optimal control scheme for closed-loop time-optimal attitude maneuvering of the NPSAT1 spacecraft, a magnetically actuated spacecraft designed and built at the Naval Postgraduate School. The closed-loop control is devised and implemented using the notion of Carathéodory-p solutions: repeated computation and update of the complete open-loop control solution in real-time. The performance of the pseudospectral feedback-control scheme is compared to a standard state feedback-control technique. It is shown that the use of standard state feedback control leads to significantly slower convergence time and may lead to substantially lower performance metrics. The substantial performance gains when using closed-loop optimal control are attributed to the optimal scheme's ability to exploit the full maneuverability envelope of the spacecraft by applying bang-bang controls in all three directions. In contrast, traditional gain-based feedback control laws substantially limit the performance of the vehicle to well below its physical capabilities. The feasibility of each open-loop optimal control solution is verified by numerical propagation while Pontryagin's necessary conditions for optimality are used to verify the solution's optimality.

An experiment has been developed to examine the behavior of a titanium-water loop heat pipe under standard and elevated acceleration fields. The loop heat pipe was mounted on a 2.44 meter-diameter centrifuge table on edge with heat applied to the evaporator via a mica heater and heat rejected using a high-temperature polyalphaolefin oil coolant loop. The loop heat pipe was tested under the following parametric ranges: heat load at the evaporator 100 = Qin = 600 W, heat load at the compensation chamber 0 = Qcc = 50 W, radial acceleration 0 = ar = 10 g. For stationary operation, the evaporative heat transfer coefficient decreased monotonically with heat load whereas the thermal resistance decreased to a minimum then increased. Heat input to the compensation chamber was found to increase the evaporative heat transfer coefficient and decrease the thermal resistance for Qin = 500 W. Transient periodic flow reversal in the loop heat pipe was found for some cases, which was likely due to vapor bubble formation in the primary wick. Operation in an elevated acceleration environment revealed that dry out was dependent on both Qin and ar, and the ability for the loop heat pipe to reprime after an acceleration event that induced dry out was influenced by the evaporator temperature. The evaporative heat transfer coefficient and thermal resistance were found not to be significantly dependent on radial acceleration. However, the evaporator wall superheat was found to increase slightly with radial acceleration at high heat loads.

Typical optimal feedback controls are nonsmooth functions. Nonsmooth controls raise fundamental theoretical problems on the existence and uniqueness of state trajectories. Many of these problems are frequently addressed in control applications through the concept of a Filippov solution. In recent years, the simpler concept of a p solution has emerged as a practical and powerful means to address these theoretical issues. In this paper, we advance the notion of Carathéodory-p solutions that stem from the equivalence between closed-loop and feedback trajectories. In recognizing that feedback controls are not necessarily closed-form expressions, we develop a sampling theorem that indicates that the Lipschitz constant of the dynamics is a fundamental sampling frequency. These ideas lead to a new set of foundations for achieving feedback wherein optimality principles are interwoven to achieve stability and system performance, whereas the computation of optimal controls is at the level of first principles. We demonstrate these principles by way of pseudospectral methods because these techniques can generate Carathéodory-p solutions at a sufficiently fast sampling rate even when implemented in a MATLAB® environment running on legacy computer hardware. To facilitate an exposition of the proposed ideas to a wide audience, we introduce the core principles only and relegate the intricate details to numerous recent references. These principles are then applied to generate pseudospectral feedback controls for the slew maneuvering of NPSAT1, a spacecraft conceived, designed, and built at the Naval Postgraduate School and scheduled to be launched in fall 2007.

A homodyne path stabilised Michelson based interferometer displacement sensor was developed. This sensor achieved a noise floor of 100 fm/vHz, for frequencies higher than 100 KHz. A prototype AFM that integrated this sensor was developed. Using tapping mode, topography maps of an AFM test grid were produced.

Repetitive control (RC) is used to track and reject periodic exogenous signals by including a model of a periodic signal in the feedback path. The performance of RC can be improved by also including an. The accuracy of this filter is the main limitation to the RC bandwidth. The bandwidth is typically limited with a robustness filter - often a low-pass filter which attenuates the model at high-frequencies where the model-mismatch often occurs. In this paper, two robustness filter designs are compared. The first design is a brick-wall low-pass filter commonly used in the literature. The second design is based on the uncertainty between the inverse plant response filter and the measured response of the system. Experimental results demonstrate that the proposed method outperforms the traditional brick-wall filter approach.

This article describes a position sensitive interferometer with closed-loop control of the reference mirror. A calibrated nanopositioner is used to lock the interferometer phase to the most sensitive point in the interferogram. In this configuration, large low-frequency movements of the sensor mirror can be detected from the control signal applied to the nanopositioner and high-frequency short-range signals can be measured directly from the photodiode. It is demonstrated that these two signals are complementary and can be summed to find the total displacement. The resulting interferometer has a number of desirable characteristics, it is optically simple, does not require polarization or modulation to detect the direction of motion, does not require fringe-counting or interpolation electronics, and has an effectively unlimited bandwidth. Experimental results demonstrate the frequency response analysis of a high-speed positioning stage. The proposed instrument is ideal for measuring the frequency response of nanopositioners, electro-optical components, MEMs devices, Ultrasonic devices, and sensors such as surface acoustic wave detectors.

This article identifies the design considerations for a two degree of freedom (DoF) miniature robotic leg utilizing piezoelectric bimorph actuators with a specific focus on the resonance modes of the system. An analytical model was developed using three independent lumped mass models with superposition for tuning the resonance frequencies and optimizing the performance of the leg. The model was verified both experimentally and using finite element analysis (FEA).

Numerous scientific and official documents stress that energy efficiency improvements can play a key role in both assuring a sustainable energy future and mitigating climate change through socio-economic development. Last World Summit in Durban concluded that changing unsustainable patterns of energy use is a key area for global action to ensure the survival of our planet. This paper proposes a new Concept of five (5) Essentials of Application Engineering, (5 EAE), which would enable designers, manufacturers, and end-users to consistently design and evaluate any design or retrofits of power converters and/or industrial system drives. Five (5) essentials of application engineering have already been successful in designing and manufacturing integrated industrial system drives (IISD), and has created new basis-of-design for drives in mining and heavy industries in South Africa. 5 EAE, as a concept, produces several collateral benefits: 1) An increased technical and economic performance of processes; 2) The defusing of incipient energy and economic crisis; 3) An improvement of environmental conditions; and 4) The creation of jobs in industries. Consequently, 5 EAE can be used as a model for improving a company's corporate policies and/or utilities, and government energy policies. © 2012 IEEE.

Fast evolving techniques for macroscale graphene and ultrathin graphite material production are promising for applications of graphene-based materials in thermal management. A numerical comparison of aluminium and graphene-based plate-fin heat exchangers is conducted. Anisotropic thermal conductivity of graphene-based solution shows an improvement of up to twenty percent in heat rejection over the aluminium design. Thermal and hydraulic performance is characterized for both designs over a range of air flow rates in both laminar and turbulent regimes. Steady and unsteady 3-D conjugate simulations reveal a faster equilibration rate for the graphene-based solution, minimizing thermal lag that must be accounted for in on-demand electronics cooling. The combination of improved heat rejection, rapid response rate, and low material density make a graphene-based solution uniquely suited to aerospace thermal management. Copyright © 2011 by ASME.

This paper describes the design and flight implementation of off-eigenaxis time-optimal reorientation maneuvers for the NASA Transition Region and Coronal Explorer (TRACE) spacecraft. Time-optimal reorientation maneuvers have obvious application for improving the maneuver performance of spacecraft systems. Yet, this type of maneuvering capability has never before been demonstrated in flight. Constrained time-optimal maneuvering of a rigid body is studied first in order to demonstrate the potential for enhancing the performance of the TRACE spacecraft. Issues related to the experimental flight implementation of time-optimal maneuvers on board TRACE are discussed. A description of an optimal control problem formulation found to be suitable for reaction wheel spacecraft maneuver design is given. The optimizationmodel is solved using the pseudospectral optimal controlmethod and includes practical constraints such as the nonlinear reaction wheel torque-momentum envelope and rate gyro saturation limits. Flight results, presented for a typical large angle time-optimal reorientation maneuver, show that time-optimal maneuvers can be implemented without any modification of the existing spacecraft attitude control system and demonstrate a clear improvement in spacecraft maneuver performance as compared to conventional eigenaxis maneuvering.

This paper presents the application of pseudospectral optimal control techniques for solving time-optimal reorientation maneuvers and their implementation on the NASA space telescope TRACE. A time-optimal reorientation maneuver is designed by first deriving a constrained optimal control problem formulation from models of the TRACE spacecraft dynamics and reaction wheel-based attitude control system. A detailed flight simulation model of the TRACE spacecraft is constructed to assist in this process. The resulting optimal control problem is then solved using fast spectral algorithms. Both simulation results and telemetry data obtained from a flight experiment show that the designed timeoptimal maneuver adheres to the specified constraints, including the nonlinear reaction wheel torque-momentum envelope and other limitations imposed by on board sensors. Moreover, the time-optimal maneuver was implemented on orbit without the need to modify the existing spacecraft attitude control system, making the approach suitable for application to other spacecraft already on orbit. © 2011 by M. Karpenko, S. Bhatt, N. Bedrossian, A. Fleming, I.M. Ross. Published by the American Institute of Aeronautics and Astronautics, Inc.

We address the time-optimal reorientation of a spacecraft with control moment gyros (CMGs) as the torque generating devices. The rigid-body solutions are inapplicable for a CMG-driven spacecraft due to the strong coupling between the actuator dynamics and the rigid-body equations. No assumptions are made with regards to the eigenaxis being the optimal axis for slewing. In sharp contrast to the rigid-body system, singular arcs occur quite naturally in the CMG-driven system. Solutions are computed using recent results from optimal control theory and pseudospectral methods. Our computed solutions are practically implementable in that we incorporate limits on the gimbal accelerations. The state space of the resulting optimal control problem is in R15. Necessary conditions for this optimal control problem are derived and the extremality of the computed solution is demonstrated by way of Pontryagin's Principle. A key discovery of our analysis is the strong connection between CMG sizing and jerk limits.

An experiment has been developed to examine the behavior of a titanium-water loop heat pipe (LHP) under standard and elevated acceleration fields. The LHP was mounted on a 2.44 m diameter centrifuge table on edge with heat applied to the evaporator via a mica heater and heat rejected using a high-temperature polyalphaolefin oil (PAO) coolant loop. The LHP was tested under the following parametric ranges: heat load at the evaporator, 100 = Qin = 600 W; heat load at the compensation chamber, 0 = Q cc = 50 W; radial acceleration, 0 = ar = 10 g. For stationary operation, the evaporative heat transfer coefficient decreased monotonically with heat load while the thermal resistance decreased to a minimum then increased. Heat input to the compensation chamber was found to increase the evaporative heat transfer coefficient and decrease the thermal resistance for Qin = 500 W. Transient periodic flow reversal in the LHP was found for some cases, which was likely due to vapor bubble formation in the primary wick. Operation in an elevated acceleration environment revealed that dry-out was dependent on both Qin and ar, and the ability for the LHP to reprime after an acceleration event that induced dry-out was influenced by the evaporator temperature. The evaporative heat transfer coefficient and thermal resistance were found not to be significantly dependent on radial acceleration. However, the evaporator wall superheat was found to increase slightly with radial acceleration at high heat loads.

The Centrifuge Table Test Facility in the Propulsion Directorate of the Air Force Research Laboratory at Wright-Patterson Air Force Base has a long history of providing critical knowledge of the response of two-phase heat transfer systems to changing accelerations in aerospace applications. This paper will discuss the centrifuge table facility, a review of literature produced using the facility, the importance of two-phase fluid systems subjected to varying accelerations, and the capabilities and limitations of the centrifuge table. The test bed consists of a 2.44 m diameter horizontal rotating table driven by a 20 hp DC electric motor. The test bed can deliver the following to devices mounted to the rotating table: Conditioned DC electrical power through three separate power supplies, 120 VAC power, temperature-controlled ethylene glycol coolant, and electrical signals for analog or digital control. In addition, electrical signals are collected from instruments on the table and stored in a data acquisition computer. The table is capable of radial acceleration up to a r = 12 g, with a maximum onset of ¿r = 10 g/s, inducing a tangential acceleration. The table rotation can be varied manually using a potentiometer, or controlled digitally using a signal generator in the data acquisition and control system. The acceleration field is measured using an orthogonal triaxial accelerometer.

Recent advances in optimal control theory and computation are used to develop minimum-time solutions for the rest-to-rest reorientation of a generic rigid body. It is shown that the differences in the geometry of the inertia ellipsoid and the control space lead to counterintuitive noneigenaxis maneuvers. The optimality of the open-loop solutions are verified by an application of Pontryagin's principle as a necessary condition and not as a problem-solving tool. It is shown that the nonsmoothness of the lower Hamiltonian compounds the curse of dimensionality associated in solving the Hamilton-Jacobi-Bellman equations for feedback solutions. These difficulties are circumvented by generating Carathéodory-jr trajectories, which are based on the fundamental notion that a closed loop does not necessarily imply closed-form solutions. While demonstrating the successful implementation of the proposed method for practical applications, these closed-loop results reveal yet another counterintuitive phenomenon: a suggestion that parameter uncertainties may aid the optimality of the maneuver rather than hinder it. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.

Recent advances in optimal control theory and computation are used to develop minimumtime solutions for the rest-to-rest reorientation of a generic rigid-body. It is shown that the differences in the geometry of the inertia ellipsoid and the control space lead to counterintuitive non-eigenaxis maneuvers. The optimality of the open-loop solutions are verified by an application of Pontryagin's Principle as a necessary condition and not as a problemsolving tool. It is shown that the nonsmoothness of the lower Hamiltonian compounds the curse of dimensionality associated in solving the Hamilton-Jacobi-Bellman equations for feedback solutions. These difficulties are circumvented by generating arathéodory-p trajectories which are based on the fundamental notion that closed-loop does not necessarily imply closed-form solutions. While demonstrating the successful implementation of the proposed method for practical applications, these closed-loop results reveal yet another counter-intuitive phenomenon: a suggestion that parameter uncertainties may aid the optimality of the maneuver rather than hinder it. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.

This paper presents a simple second-order controller that damps the resonance typical of piezoelectric nanopositioners and delivers good tracking performance. This method employs the Integral Resonant Control scheme (IRC) for damping the dominant resonant mode of the piezoelectric nanopositioner and uses an integral controller to achieve tracking performance. As disturbance rejection is a main concern in nanopositioning applications, the control scheme is tested for its disturbance rejection performance. It is seen that the control scheme has good disturbance rejection characteristics deeming it suitable for nanopositioning applications. To test the tracking performance, the system is made to track a 20 Hz triangular input at various integral gains. It is shown that improved tracking performance can be achieved at high gains with only a slight degradation in disturbance rejection performance at high frequencies.

Since the inception of optimal control theory, minimum-time solutions have been sought for various problems as they provide the foundations for understanding the dynamics of the control system. In space applications, rapid maneuvering capabilities are inherently vital to both civilian and military needs. In this paper, we address the time-optimal reorientation of a spacecraft with control moment gyros (CMGs) as the torque generating devices. No assumptions are made with regards to the eigenaxis being the optimal axis for slewing. In addition to its attitude and attitude rates, the state-space of a CMG-driven spacecraft includes the gimbal positions. Taking gimbal rates as the control variables, the coupled non-Eulerian dynamics are fully exploited to formulate and solve the true minimum-time attitude maneuvering problem. Solutions are obtained using recent results from optimal control theory and pseudospectral methods. In sharp contrast to the rigid-body system, singular arcs occur quite naturally in the CMG-driven system. The extremality of the solution is demonstrated by way of Pontryagin's Principle.

Minimum-time solutions are developed for the rest-to-rest reorientation of an asymmetric rigid-body. The optimality of the open-loop solutions are demonstrated by application of Pontryagin's Minimum Principle. Bellman's theory is used to further demonstrate optimality while extending open-loop theory to real-time application. The open-loop time optimal control is, next, used to construct the closed-loop Carathéodory-p control solution for a similar maneuver. Closed-loop results presented for the system with and without parameter uncertainties verify the successful implementation of the method in practical applications. © 2008 by A. Fleming, P. Sekhavat, I.M. Ross.

Shen and Tsiotras considered the case where the axisymmetric rigid body was subject to only two control torques which spanned the plane perpendicular to the axis of symmetry. They used a cascaded computational scheme which involved both direct and indirect methods of optimization. Additionally, their method required initial costate guesses which further complicated numerical results. They concluded that two torques were sufficient to achieve a time-optimal maneuver. In this work the Legendre Pseudospectral method will be used to solve the two-torque problem with only a two-point guess to demonstrate the method's superiority. Additionally, it will be demonstrated that while reorientation of the spin axis is possible with two control torques spanning the plane perpendicular to the spin axis, the addition of a third control torque about the axis of symmetry further reduces the objective function. It will be shown that this new solution has significantly different characteristics form the previously published work. © 2008 by Andrew Fleming.

Recent advances in aircraft technology have raised much concern over the manner in which aircraft thermal management is carried out. These advances range from the incorporation of high-power electronics to transporting thermal loads at high temperatures. These types of technological advances have brought about a necessity for new aircraft thermal management architectures in order to maintain reasonable cost, size, weight, and power requirements of the overall system. The objective of this study is to address the requirements and performance aspects of existing system configurations in an effort to identify inefficiencies and highlight potential areas for improvement. As a result of this study, a new aircraft thermal management architecture, which can best be described as a vapor-compression thermal bus, is proposed as a replacement for existing technology. This paper will present the findings of a thermodynamic analysis that compares the requirements and performance aspects of existing architectures to those of the vapor-compression thermal bus. Copyright © 2008 SAE International.

Research on the attitude control of spacecraft using control moment gyros (CMGs) has been largely dominated by singularity avoidance. In this paper, we show that the singularity problem disappears if the control is properly identified in terms of the actuator inputs rather than the torques on the spacecraft body. Under this formulation, the spacecraft can now be properly steered. Although any optimality criterion may be used for steering, we choose time-optimality as it a fundamental problem in both control theory and spacecraft attitude maneuvering. Results for a small spacecraft based on the parameters of NPSAT1 demonstrate the details of our approach.

Suppose optimal open-loop controls could be computed in real time. This implies optimal feedback control. These controls are, typically, nonsmooth, Nonsmooth controls raise fundamental theoretical problems on the existence and uniqueness of feedback solutions. A simple, yet powerful, approach to address these theoretical issues is the concept of a p-solution that is closely linked to the practical implementation of zero-order-hold control sampling. In other words, even traditional feedback controls involve open-loop controls through the process of sampling. In this paper, we advance the notion of Carathéodory-p solutions that use the sampling intervals to compute optimal open-loop controls. A sampling theorem is developed which indicates that the Lipschitz constant of the dynamics is a fundamental sampling frequency. This places computation at the level of first principles in describing the foundations for achieving feedback. We obtain these controls by way of pseudospectral (PS) methods as these techniques can generate optimal open-loop controls within fractions of a second even when implemented in a MATLAB© environment running on legacy computer hardware. In order to facilitate an exposition of the proposed ideas to a wide audience, we introduce the core principles only while relegating the Intricate details to numerous recent references. These principles are then applied to generate PS feedback controls for the slew maneuvering of NFSAT1, a spacecraft conceived, designed and built at the Naval Postgraduate School and scheduled to be launched in Fall 2007.

The objective of this paper is to describe the design of an experiment that will examine the effects of elevated acceleration environments on a high-temperature, titanium-water loop heat pipe for actuator cooling. An experimental test setup has been designed for mounting a loop heat pipe on an 8-ft-diameter centrifuge table, which is capable of radial accelerations of up to 12-g's. A high-temperature PAO loop will interface the condenser of the loop heat pipe to simulate the rejection of the transported heat to an elevated temperature. In addition to LHP experimentation, a mathematical model has been developed for aerodynamic heating of highspeed aircraft. A flat plate at zero-incidence, used to model an aircraft wing, was subjected to sub- and supersonic flow to examine whether heat will be rejected or absorbed. The results of this analysis will be used to determine the condenser conditions of the loop heat pipe during centrifuge testing. Copyright © 2006 SAE International.

The effects of elevated acceleration fields on spray cooling heat transfer are discussed in this paper. Spray cooling has proven to be one of the most efficient methods of heat removal. This technology is being transitioned into more advanced applications, such as fighter aircraft that must withstand a wide range of variable acceleration-induced body forces. Heat transfer associated with closed-loop spray cooling will be affected by acceleration body forces, the extent of which is not yet known. To test these various effects, an eight-foot-diameter centrifuge table will be outfitted with a spray cooling system to test for the effects associated with elevated gravity. Copyright © 2006 SAE International.

Magnetic actuators have been used for momentum dumping and attitude control in many spacecraft. Of late, magnetic actuators have also been proposed as a sole means for attitude control, particularly for small inexpensive spacecraft. In this paper, we investigate the use of magnetic actuators for the time-optimal slew maneuver using a spectral algorithm implemented in the reusable software package, DIDO. The parameters of the example problem correspond to that of NPSATI, a small satellite being built at the Naval Postgraduate School, and due to launch in January 2006. Numerical experiments reveal that real-time optimal solutions can be easily obtained.

NPSAT1 is a small satellite being built at the Naval Postgraduate School, and due to launch in January 2006. It uses magnetic actuators and a pitch momentum wheel for attitude control. In this paper, a novel time-optimal sampled-data feedback control algorithm is introduced for closed-loop control of NPSAT1 in the presence of disturbances. The feedback law is not analytically explicit; rather, it is obtained by a rapid re-computation of the open-loop time-optimal control at each update instant. The implementation of the proposed controller is based on a shrinking horizon approach and does not require any advance knowledge of the computation time. Preground-test simulations show that the proposed control scheme performs well in the presence of parameter uncertainties and external disturbance torques. © 2005 IEEE.

A typical passive mechanical isolation system utilizes a mass-spring-damper as a mechanical filter. Active isolation control systems typically include an electromagnetic transducer to develop the required control forces. In this paper, the technique of sensor-less active shunt control is applied to a mechanical isolation system. An electrical impedance is designed and connected to an electromagnetic transducer with a view to minimizing structural vibration. Standard control tools can be applied to design the required shunt impedance. The technique requires no additional feedback sensors. Vibration of an experimental isolation apparatus is heavily attenuated by the application of an active shunt impedance.