Angle Slewing Research Articles

Discrete event simulation of vessel stationkeeping operations in ice-rich waters

This paper describes a high-fidelity numerical model that simulates vessel stationkeeping operations in ice-rich waters. The discrete event simulation engine incorporates several novel features, including new ice floe failure models for bow and midships locations; an ice floe creation strategy that facilitates rafting of ice floes; and a vessel thruster model that takes into account physical limitations such as thruster angle slew rates and propeller ramp rates. It accommodates a wide range of ice field specifications and runs in real-time on a standard desktop personal computer (Intel® Core™ i7 Processor or equivalent). The model has been validated using physical measurements of a generic drillship model in several broken ice conditions; it predicted thruster forces and motions that were comparable to those observed during dynamic positioning operations.

Single-axis attitude control for slew maneuvers with the keep-out zones

The problem of large angle slew maneuvers under the attitude restrictions is considered. The modification of the single axis Lyapunov feedback attitude control is proposed. It “locally” changes the standard attitude controller to accommodate the attitude restrictions avoidance. The problem of the unwanted equilibria appearance is thoroughly investigated. Different keep-out zones intersection cases are considered.

Numerical optimal control for satellite attitude profiles

In this paper we will present a new approach for optimal control of slews and attitude profiles. After the description of a set of the considered Hamiltonian functions and the respective slew maneuvers some analytical consequences of the choices are given. A comparison with the actual operational Euler angle slew is given and shows a close match. The performed numerical investigations of direct solutions help to gain a clearer picture on the underlaying analytical problem. By applying the Pontryagin maximum principle to the Hamiltonian equation, a family of closed dynamics ordinary differential equation for the direct optimal control problem is presented and their solutions and properties are investigated.

AOCS design for the ATHENA X-ray telescope: challenges and solutions

The ATHENA—Advanced Telescope for High-ENergy Astrophysics—mission is currently assessed in a phase A feasibility study as L-class mission in ESA’s Cosmic Vision 2015–2025 plan, with launch foreseen in 2028. Primary mission goal is the mapping of hot gas structures and the determination of their physical properties to search for supermassive black holes. ATHENA is an X-ray telescope with a focal length of 12 m. It has a mass of ~ 7000 kg and it is ~ 15 m high with a diameter of ~ 3 m. The main mass is distributed to the mirror on the one side of the spacecraft and to the science instrument module on the other side of the spacecraft. To achieve its science goals, ATHENA performs a sky survey with precision line-of-sight pointing requirements in the order of arc seconds for absolute pointing and sub-arc seconds for relative pointing in time windows > 1 ks, all at 95% confidence level. That is very demanding for large X-ray telescopes. In addition to the precision pointing requirements, ATHENA cannot violate a certain sun exclusion zone. This is a hard constraint to prevent any stray-light falling onto the instruments, as it would immediately destroy them. The sky survey is defined by an observation plan that is demanding in terms of availability and thus spacecraft agility. The pointing and agility requirements and the fact that ATHENA is a spacecraft with high mass and volume introduce several design challenges for the attitude and orbit control system. This paper presents those challenges, corresponding solutions, and preliminary results, which have been achieved during the phase A study led by Airbus in Friedrichshafen, Germany. The main focus and contribution of this paper are the identification of research and development needs for attitude and orbit control systems to enable the ATHENA mission. In this respect, the ATHENA design challenges are discussed and addressed with the state-of-the-art design methods. This paper concludes with the main identified technology development needs and formulates specific research questions related to practical design problems. In particular, the following attitude and orbit control system design challenges are addressed: autonomous and agile large angle slew manoeuvres with exclusion zones, availability for science observations, precision line-of-sight determination as well as analysis during the design process using the ESA Pointing Error Engineering Tool and pointing control with a hexapod as line-of-sight actuator in the control loop. The last challenge, namely, the hexapod in the control loop, is without precedence in Europe and to the best knowledge of the authors in the world.

Output torque modeling of control moment gyros considering rolling element bearing induced disturbances

Control moment gyros (CMG) have been widely used in spacecraft attitude control and large angle slewing maneuvers over the years. Understanding and suppressing high-frequency disturbances in CMG’s output torques is a crucial factor to achieving the desired level of payload performance. Output torque modeling of a single gimbal CMG (SGCMG) with nonlinear rolling bearing supports is conducted in this paper. Taking the installation errors and micro-vibrations of the flywheel into account, three axis output torques of a SGCMG are derived based on Newton-Euler approach and theorem of moment of momentum. Dynamic model is then constructed to obtain the micro-vibration responses of the rotary flywheel. Mass imbalances of the flywheel, flexibility of supporting structures and nonlinearity induced by one pair of angular contact ball bearings are considered in the dynamic model. Especially for the rolling bearing, an improved load distribution analysis is proposed to more accurately obtain the contact deformations and angles between the rolling balls and raceways. Various factors, including the preload condition, surface waviness, Hertz contact and elastohydrodynamic lubrication, are included in the analysis. The bearing restoring forces are then obtained through iteratively solving the load distribution equations at every time step. Dynamic tests on a typical SGCMG supported by angular contact ball bearings are conducted to verify the output torque model. The effects of flywheel dynamic/static eccentricities, inner/outer surface waviness amplitudes, bearing axial preload and installation skew angles on the dynamic output torques are discussed. The obtained results would be useful for the optimal design and vibration control of the SGCMG system.

Dynamic modeling of moment wheel assemblies with nonlinear rolling bearing supports

Moment wheel assemblies (MWA) have been widely used in spacecraft attitude control and large angle slewing maneuvers over the years. Understanding and controlling vibration of MWAs is a crucial factor to achieving the desired level of payload performance. Dynamic modeling of a MWA with nonlinear rolling bearing supports is conducted. An improved load distribution analysis is proposed to more accurately obtain the contact deformations and angles between the rolling balls and raceways. Then, the bearing restoring forces are then obtained through iteratively solving the load distribution equations at every time step. The effects of preload condition, surface waviness, Hertz contact and elastohydrodynamic lubrication could all be reflected in the nonlinear bearing forces. Considering the mass imbalances of the flywheel, flexibility of supporting structures and rolling bearing nonlinearity, the dynamic model of a typical MWA is established based upon the energy theorem. Dynamic tests are conducted to verify the nonlinear dynamic model. The influences of flywheel mass eccentricity and inner/outer waviness amplitudes on the dynamic responses are discussed in detail. The obtained results would be useful for the design and vibration control of the MWA system.

Networked flexible spacecraft attitude maneuver based on adaptive fuzzy sliding mode control

For the attitude stabilization of networked flexible spacecraft during large angle slew maneuver, a novel type of adaptive fuzzy sliding mode control (AFSMC) method for solving the dynamic model with network-induced delay, nonlinear and uncertain parameters is proposed in this paper. A novel and systematic sliding mode control (SMC) scheme, which integrates a time-advanced nonlinear predictor, is proposed to compensate for the network-induced delay and to overcome the negative effect of uncertainties. Then, an adaptive fuzzy system is used to approximate the strong coupling nonlinear dynamics between rigid hub and flexible appendages. Following that, the designed adaptive algorithms are developed in the sense of the Lyapunov stability theorem, so that system-tracking stability can be guaranteed. Finally, simulation results show that, with the application of the proposed method, not only high-precision attitude stabilization of flexible spacecraft is achieved, but also the elastic vibration of flexible spacecraft during maneuver is suppressed effectively, and the system is robust against system uncertainties, network-induced delays and any outer disturbances.

A dual-wheel multi-mode spacecraft actuator for near-minimum-time large angle slew maneuvers

Abstract A novel implementation of a dual-wheel actuator is presented for a spacecraft with high agility requirements, e.g. where the imaging payload is strapped to the satellite body and the bore sight has to be shifted from one target location to another by slewing the complete satellite in minimum-time. To maximise the torque capability, the actuator can be utilised as a “scissored” type control moment gyro (CMG-mode) with parallel gimbals. A system with 2 single-gimbal CMGs per body axis, can simplify the gimbal steering method with gimbal angle constraints and avoid any control torque singularities prevalent in other CMG configurations. During fine pointing maneuvers the dual-wheel actuator can be used in reaction wheel mode (RW-mode). In this way the unique advantages of both CMG and reaction wheel actuators can be exploited, while the transition between either modes can be done smoothly. In CMG-mode the torque capability can be at least one order of magnitude greater and the peak electrical power requirement will be much lower during the CMG high torque phases of a maneuver, compared to the RW-mode. In RW-mode the torque resolution and wheel induced disturbances can be more conducive for the high stability requirement of an imaging payload. The dual-wheel units can be aligned and parked into two orthogonal axes during reaction wheel use, resulting in multiple redundancy, i.e. any two wheels may fail, while still maintaining full 3-axis control authority. The paper will present the torque, angular momentum and gimbal steering equations for the dual-wheel actuator. Next, a near-minimum-time eigenaxis slew maneuver algorithm is derived for the CMG-mode, where optimal agility is required. Finally, the paper concludes with a simulation of a practical satellite application. The performance of a near-minimum-time eigenaxis slew maneuver in CMG-mode is compared to the results obtained when the maneuver is executed using a pyramid array of four single-gimbal CMGs with similar performance constraints and driven by time optimised, singularity robust steering logic.

Vibration Reduction for Flexible Spacecraft Following Momentum Dumping With/Without Slewing

Vibrations following the e ring of thrusters to dump wheel momentum degrade pointing performance of a reaction-wheel controlled spacecraft. The use of input shaping is suggested to modulate thruster pulses with a set of properly timed impulses to suppress vibrations after dumping momentum. An exact solution is obtained for thruster switching times in the idealized case of no constraints. With realistic constraints on the thruster, permissible attitude error, and elastic dee ection, an optimization problem is solved. The solution is a modie cation ofaconcatenatedsetofpulsesfortheidealizedcase,eachsetdesignedtodumpafractionofthetotalmomentumwith vibration reduction. It is shown that momentum can be dumped during slewing maneuvers along with vibration suppression, resulting in a reduction of slew time with no additional fuel consumption. In this case input shaping is applied to both reaction wheel and thruster commands to minimize residual vibrations. Results of large angle slewing and momentum dumping with and withoutreaction wheel saturation arepresented forthenextgeneration space telescope.

Nonlinear Predictive Control of Spacecraft

A new approach for the control of a spacecraft with large angle maneuvers is presented. This new approach is based on a nonlinear predictive control scheme which determines the required torque input so that the predicted responses match the desired trajectories. This is accomplished by minimizing the norm-squared local errors between the predicted and desired quantities. Formulations are presented which use either attitude and rate tracking or attitude-tracking solely. The robustness of the new controller with respect to large system uncertainties is also demonstrated. Finally, simulations results are shown which use the new control strategy to stabilize the motion of the Microwave Anisotropy Probe spacecraft. Introduction The control of spacecraft for large angle slewing maneuvers poses a difficult problem. Some of these difficulties include: the governing equations have highly nonlinear characteristics, control rate and saturation constraints and limits, and incomplete state knowledge due to sensor failure or omission. The control of spacecraft with large angle slews can be accomplished by either open-loop or closed-loop schemes. Open-loop schemes usually require a predetermined pointing maneuver and are typically determined using optimal control techniques, which involve the solution of a two-point boundary value problem (e.g., see the time optimal maneuver problem [1]). Also, open-loop schemes are sensitive to spacecraft parameter uncertainties and unexpected disturbances [2]. Closed-loop systems can account for parameter uncertainties and disturbances, and thus provide a more robust design methodology. In recent years, much effort has been devoted to the closed-loop design of spacecraft with large angle slews. Wie and Barber [3] derive a number of simple control schemes using quaternion and angular velocity (rate) feedback. Asymptotic stability is also shown by Copyright c 1997 by John L. Crassidis. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. using a Lyapunov function analysis for all cases. Tsiotras [4] expands upon these formulations by deriving simple control laws based on both a Gibbs vector parameterization and a modified Rodrigues parameterization each with rate feedback (for a complete survey of attitude parameterizations, see [5]). Lyapunov functions are shown for all the controllers developed in [4] as well. Other full state feedback techniques have been developed which are based on sliding mode (variable structure) control, which uses a feedback linearizing technique and an additional term aimed at dealing with model uncertainty [6]. This type of control has been successfully applied for large angle maneuvers using a Gibbs vector parameterization [7], a quaternion parameterization [8], and a modified Rodrigues parameterization [9]. Another robust control scheme using a nonlinear H∞ control methodology has been developed by Kang [10]. This scheme involves the solution of Hamilton-Jacobi-Isaacs inequalities, which essentially determines feedback gains for the full state feedback control problem so that the spacecraft is stabilized in the presence of uncertainties and disturbances. Another class of controllers involves adaptive techniques, which update the model during operation based on measured performances (e.g., see [6]). An adaptive scheme which estimates for external torques by means of tracking a Lyapunov function has been developed by Schaub et. al. [11]. This method has been shown to be very robust in the presence of spacecraft modeling errors. The aforementioned techniques all utilize full state knowledge (i.e., attitude and rate feedback). The problem of controlling a spacecraft without full state feedback becomes increasingly complex. The basic approaches used to solve this problem can be divided into methods which estimate for the unmeasured states using a filter algorithm, or methods which develop control laws directly from output feedback. Filtering methods, such as the extended Kalman filter, have been successfully applied on numerous spacecraft systems without the use of rate-integrating gyro measurements (e.g., see [12]-[14]). An advantage of these methods is that the attitude may be estimated by using only one set of vector attitude measurements (such as magnetometer measurements). However, these methods are usually

Large angle slew maneuvers with autonomous sun vector avoidance

References 1 Jamark, B., Turn of an Theory and Applications of Nonlinear Control Systems, North-Holland, New York, 1986, pp. 53-64. 2 Clements, J. C, Minimum-Time Turn Trajectories to Fly-toPoints, Optimal Control Applications & Methods, Vol. 11, No. 1, 1990, pp. 39-50. 3 Hedrick, and Bryson, A. E., Minimum Time Turns for a Supersonic Airplane at Constant Altitude, Journal of Aircraft, 1971, pp. 182-187. 4 Clements, J. C., Real-Time Optimal Control of Aircraft Turn Trajectories, Journal of Guidance, Control, and Dynamics, Vol. 16, No. 2, 1993, pp. 385-387. 5 Bryson, A. E., and Ho, Y.-C., Applied Optimal Control, Hemisphere, Washington, DC, 1975.

Optimal feedback gains for three-dimensional large angle slewing of spacecraft

Optimal feedback gains for three-dimensional large angle slewing of spacecraft

A Controller Design for Multi-body Large Angle Maneuvers

Active large angle slewing maneuvers of a multi-body flexible dynamic system are investigated. An appropriate state variable transformation and a feedback linearization technique are employed to transform the dynamics of the nonlinear system to a new state that is more amenable to control design procedures. Closed-loop feedback algorithms are implemented to perform slewing maneuvers, while simultaneously suppressing flexural vibrations of the system. Stability of this class of nonlinear systems is also investigated, whereby a sufficient condition for asymptotic stability of the system is established. Numerical examples are presented to demonstrate the proposed active control algorithms.

Recent advances in structural dynamics of large space structures

Recent progress in the area of structural dynamics of large space structures is reviewed. Topics include system identification, large angle slewing of flexible structures, definition of scaling limitations in structural models, and recent results on a tension-stabilized antenna concept known as the hoop-column. Increasingly complex laboratory experiments guide most of the activities leading to realistic technological developments. Theoretical progress in system identification based on system realization theory resulting in unification of several methods is reviewed. Experimental results from implementation of a theoretical large-angle slewing control approach are shown. Status and results of the development of a research computer program for analysis of the transient dynamics of large angle motion of flexible structures are presented. Correlation of results from analysis and vibration tests of the hoop-column antenna concept is summarized.

Finite element analysis of maneuvering spacecraft truss structures

A finite element modeling and solution technique capable of determining the time response of flexible spacecraft truss structures undergoing large angle slew maneuvers has been developed. The elastic deformations of the structure are coupled with large nonsteady translational and rotational motions with respect to an inertial reference frame. The governing equations of motion of the system are derived using momentum conservation principles and the principle of virtual work. The finite element approximation is applied to the equations of motion and the resulting set of nonlinear second order matrix differential equations is solved timewise by an iterative direct numerical integration scheme based on the trapezoidal rule. The solution technique is tested on both planar and three-dimensional maneuvering spacecraft truss structures.

Experiments for Control of Large Space Structures

Recent progress in the area of control experiments for large space structures is reviewed. Topics include large angle slewing of a flexible panel and the Spacecraft Control Laboratory Experiment (SCOLE). In large angle slewing, experiments on a flexible solar panel model are discussed. Experimental results for slewing control in air and in vacuum are presented to illustrate the effect of air drag on the controller design. In SCOLE, the design of laboratory apparatus will be discussed. The status of research activities oriented primarily to the improvement of analysis capabilities through testing is presented

Configuration tradeoffs for the space infrared telescope facility pointing control system

Candidate pointing control configurations for a Space Infrared Telescope Facility are examined in terms of fine guidance pointing and large angle slewing accuracies. In particular, the fundamental tradeoff is between a strategy that uses body pointing only, where the entire spacecraft is controlled to null pointing error, or body pointing plus image motion compensation, where a movable secondary mirror is employed to remove high-frequency disturbances and therefore mitigate excitation of flexible body modes. Using known characteristics of available attitude control sensors and actuators, a linear covariance analysis is used to compute rms pointing error, while a time domain simulation is used to compute transient response to specific disturbance torques. It is found that body pointing alone can provide excellent fine pointing accuracy but limits slewing performance. Both slewing performance and disturbance rejection may be significantly improved by a steerable secondary mirror, but the noise level of the attitude sensor under study limits control bandwidth and thus restricts the potential advantages of this approach. It is recommended that improved sensor designs be pursued while enhancing slewing performance by appropriate combination of command profile and control compensation.

Observer-based Attitude Control In The Sliding Mode

With the increasing interest in small, relatively inexpensive satellites, envisaged to be produced on a large scale, there is a need to minimise development costs by embodying as many common elements as possible. The research presented here is aimed at the production of a 'universal' attitude controller in partial fulfilment of this need. Continuous momentum exchange torque actuators are assumed at present. Such a controller must maintain a specified dynamic response to changing attitude demands through a wide range of spacecraft dynamics parameters and in the presence of external disturbance torques. The sliding mode approach is pursued in view of its relative simplicity and effectiveness, especially when dealing with the continuous non-linearities of a) the kinematic differential equations for large angle slewing manoeuvres and b) the spacecraft dynamics when the angular velocities are sufficiently high for gyroscopic inter-axis coupling to be significant. Since the pointing accuracy of any attitude control system is ultimately limited by the accuracy of the attitude measurement system, a minimal requirement is on-board instrumentation for high precision measurement of attitude angles. To avoid the necessity of additional instrumentation for the measurement of other state variables such as angular accelerations, the control system is observer based. The sliding mode controller embodies two novel features: a) control smoothing integrators to eliminate actuator drive chatter and b) block control principle to facilitate straightforward design despite the non-linearities. The control system may be designed simply by eigenvalue assignment to yield a specified closed-loop dynamic performance. Three possible special observers Transactions on the Built Environment vol 19, © 1996 WIT Press, www.witpress.com, ISSN 1743-3509 640 Structures in Space are investigated which are intended to be insensitive to spacecraft dynamics modelling errors, to be compatible with the control law The control system performance is demonstrated by simulation, including sensitivity to measurement noise.