Method and apparatus for state space trajectory control of uncertain dynamical systems

Methods, systems and computer readable media are presented for computing a guidance control policy to transition an uncertain dynamical system from an initial state to a final state, in which a set of points are computed to provide discreet and accurate representation of uncertainty, and in which a guidance control policy is computed based on a set of equations involving the initial state, the final state, state variables, control variables, and parameters, as well as designated parameters of interest, a set of constraints corresponding to state and control variables, a performance metric, statistical distribution types corresponding to the parameters of interest, statistical moments individually corresponding to the parameters of interest, and weighting values corresponding to the parameters of interest. A guidance control policy which defines control variables for transitioning from the initial state to the final state which is robust to the considered system uncertainty is computed according to the computed set of points and the performance metric.

BACKGROUND

Design and implementation of non-linear guidance and control policies for dynamical systems that are robust to uncertainties in the initial and final state conditions, environmental parameters (e.g. pressure, temperature, obstacles, keep out zones, etc.), system parameters, path or route constraints and/or exogenous disturbances has previously been difficult due to ever present limitations on computational resources. Conventionally, a guidance and control policy is determined by optimizing a performance index, such as minimum effort, or minimum transition time assuming nominal initial conditions, system parameters and environmental conditions. A control policy designed in this manner is then typically subjected to a post-design Monte-Carlo analysis by selecting different possible operating conditions (e.g. initial conditions, parameter variations, etc.) to verify its effectiveness over the anticipated range of system uncertainties. A post-design Monte-Carlo analysis tests the ability of a nominal control policy to transition a perturbed dynamical system to the desired final state. A post-design Monte-Carlo analysis, however, is costly in terms of computation time. Moreover, the successful outcome of a Monte Carlo analysis depends on the properties of an initial nominal control policy and associated nominal system trajectory that is created previously, without regard to robustness. This cut and dry approach (i.e. design followed by post-design analysis and iteration) can lead to poor overall performance in the presence of uncertainties, and/or the need for multiple iterations before converging on a guidance and control policy having the desired behavior in the presence of uncertainties. Additionally, a guidance and control policy developed in this way may be overly conservative and prevent an otherwise high-performance dynamic system from being utilized to its full potential. Thus, to meet a given performance objective, the dynamical system may need to be overdesigned, which increases cost. Accordingly, improved techniques and apparatus are desirable for determining and implementing a guidance and control policy to transition a given dynamical system from an initial state to a desired final state in the presence of constraints and uncertainties regarding the initial and final states, the system parameters, environmental parameters and/or other disturbances.

SUMMARY OF DISCLOSURE

Various details of the present disclosure are hereinafter summarized to facilitate a basic understanding, where this summary is not an extensive overview of the disclosure, and is intended neither to identify certain elements of the disclosure, nor to delineate the scope thereof. Rather, the primary purpose of this summary is to present some concepts of this disclosure and the exemplary embodiments provided herein, in a simplified form prior to a more detailed description that is presented hereinafter. The present disclosure relates to guidance and control policy determination for transitioning a controlled system from an initial state to a desired final state in the presence of uncertainties and disturbances in the controlled system as well as in the environment, and includes a mechanism for including the influence of operational constraints on the system parameters and state/control trajectories (path) as part of the guidance and control policy determination.

In accordance with one or more aspects of the present disclosure, methods and systems are provided for guidance and control policy computation by which a controlled system can be transitioned from an initial state through a state space to a final state. The method includes characterizing a problem space through a defined set of equations such as differential equations involving initial and final states, state variables, control variables, and parameters, as well as defining a set of constraints individually corresponding to at least some of the state, parameter and/or control variables and defining a performance metric that enables the performance of the controlled system to be characterized (e.g. time, energy, range, etc.). The method further involves use of one or more processors to compute a set of points providing a discreet representation of system, parametric and/or environmental uncertainty at any instant in time as the controlled system is transitioned through the state space from the initial state to the final state. Any collection of points may be employed, and are chosen based on problem requirements. For example, the points could correspond to upper and lower bounds of a selected parameters space. They may also be computed based at least partially on a set of parameters of interest selected from the parameters of the problem space, and a selected set of statistical distribution types (e.g., uniform, Gaussian, Poisson, Beta, frequency, binomial, etc.), and on statistical moments and weighting values individually corresponding to the parameters of interest. (Individual parameters may have different statistical characterizations, e.g. one parameter Gaussian, and another Uniform.) The processor is further used to compute the guidance and control policy according to the time variation in the controlled set of computed points and the performance metric, for example, to minimize the error in reaching a desired final state that is caused by the uncertainty in the parameters of interest. In one example, the control policy may be computed based upon a statistical measure of the uncertainty in the final state such as minimizing the trace of a function defining the covariance at the final state, or minimizing an integral of the trace of the state error covariance. In certain implementations, moreover, the set of points is a non-arbitrary set of points that are computed by using the processor to solve a constrained non-linear programming problem resulting from the application of the method of the present disclosure. In certain embodiments, the set of points is computed at least partially according to one or more bounds or keep-out zones and/or one or more slack variables corresponding to the statistical moments.

Further aspects of the disclosure relate to a non-transitory computer readable medium with computer executable instructions for computing a guidance control policy, and a system to transition a controlled system through a state space from an initial state to a final state. The system includes an electronic memory and one or more processors operatively coupled with the memory. The processor(s) is/are programmed or configured to compute a set of points enabling computation of a discreet representation of a performance metric related to transitioning the controlled system through the state space from the initial state to the final state, to compute a guidance control policy according to a computed set of points and a performance metric, and to provide a guidance control policy to a command a dynamical system.

According to an exemplary embodiment of this disclosure, provided is a computer-implemented method for generating and executing a guidance control policy operatively associated with transitioning a dynamically controlled system operatively associated with controlling an object and respective trajectory through a state space from an initial state to a final state, the method comprising: generating a set of equations which characterize the state space (and the state space dynamics), the set of equations characterizing the initial state, the final state, one or more state variables, one or more control variables, and a plurality of parameters associated with the dynamically controlled system; generating a set of constraints, each constraint corresponding to one or more single states and single control variables; generating a performance metric, the performance metric measuring performance including the operatively associated object of the guidance control policy to transition the dynamically controlled system through the state space from the initial state to the final state; generating a set of multidimensional HS (Hyper-Pseudospectral) points providing a discrete representation of uncertainty associated with a variation of the plurality of parameters in transitioning the dynamically controlled system through the state space from the initial state to the final state, the dimensionality of the HS points corresponding to a set of parameters of interest selected from the plurality of parameters and the set of multidimensional HS points being computed at least partially according to: selecting the set of parameters of interest from the plurality of parameters of the state space dynamics, selecting a set of statistical distribution types, each statistical distribution type corresponding to a variation of a respective individual parameter of interest, selecting a set of statistical moments, each statistical moment corresponding to a respective statistical distribution type of the individual parameter of interest, selecting a set of moment constraint equations, each corresponding to a statistical moment of interest, and selecting a set of weighting values, each weighting value corresponding to a respective individual parameter of interest, and computing the set of multidimensional HS points as a function of the set of parameters of interest, the set of statistical distribution types, the set of statistical moments, and the set of weighting values; computing the guidance control policy defining the control variables and their time histories and generating a control vector for transitioning the dynamically controlled system through the state space from the initial state to the final state according to the computed set of multidimensional HS points and the performance metric; and controlling the dynamically controlled system in accordance with the guidance control policy and generated control vector to transition the dynamically controlled system and operatively associated object trajectory and respective trajectory through the state space from the initial state to the final state.

According to another exemplary embodiment of this disclosure, provided is a non-transitory computer readable medium with computer executable instructions for generating and executing a guidance control policy operatively associated with transitioning a dynamically controlled system operatively associated with controlling an object and respective trajectory through a state space from an initial state to a final state, comprising computer executable instructions to execute the method comprising: generating a set of equations which characterize the state space and state space dynamics, the set of equations characterizing the initial state, the final state, one or more state variables, one or more control variables, and a plurality of parameters associated with the dynamically controlled system; generating a set of constraints, each constraint corresponding to one or more single states and single control variables; generating a performance metric, the performance metric measuring performance including the operatively associated object of the guidance control policy to transition the dynamically controlled system through the state space from the initial state to the final state; generating a set of multidimensional HS (Hyper-Pseudospectral) points providing a discrete representation of uncertainty associated with a variation of the plurality of parameters in transitioning the dynamically controlled system through the state space from the initial state to the final state, the dimensionality of the HS points corresponding to a set of parameters of interest selected from the plurality of parameters and the set of multidimensional HS points being computed at least partially according to: selecting the set of parameters of interest from the plurality of parameters of the state space, selecting a set of statistical distribution types, each statistical distribution type corresponding to a variation of a respective individual parameter of interest, selecting a set of statistical moments, each statistical moment corresponding to a respective statistical distribution type of the individual parameter of interest, selecting a set of moment constraint equations, each corresponding to a statistical moment of interest, and computing the set of multidimensional HS points as a function of the set of parameters of interest, the set of statistical distribution types, the set of statistical moments, and the set of weighting values; computing the guidance control policy defining the control variables and their time histories and generating a control vector for transitioning the dynamically controlled system through the state space from the initial state to the final state according to the computed set of multidimensional HS points and the performance metric; and controlling the dynamically controlled system in accordance with the guidance control policy and generated control vector to transition the dynamically controlled system and operatively associated object and respective trajectory through the state space from the initial state to the final state.

According to another exemplary embodiment of this disclosure, provided is a system for generating and executing a guidance control policy operatively associated with transitioning a dynamically controlled system operatively associated with controlling an object and respective trajectory through a state space from an initial state to a final state, comprising: at least one electronic memory; and at least one processor operatively coupled with the at least one electronic memory and programmed to: generate a set of equations which characterize the state space (and state space dynamics), the set of equations characterizing the initial state, the final state, one or more state variables, one or more control variables, and a plurality of parameters associated with the dynamically controlled system; generate a set of constraints, each constraint corresponding to one or more single states and single control variables; generate a performance metric, the performance metric measuring performance including the operatively associated object of the guidance control policy to transition the dynamically controlled system through the state space from the initial state to the final state; generate a set of multidimensional HS (Hyper-Pseudospectral) points providing a discrete representation of uncertainty associated with a variation of the plurality of parameters in transitioning the dynamically controlled system through the state space from the initial state to the final state, the dimensionality of the HS points corresponding to a set of parameters of interest selected from the plurality of parameters and the set of multidimensional HS points being computed at least partially according to: selecting the set of parameters of interest from the plurality of parameters of the state space, selecting a set of statistical distribution types, each statistical distribution type corresponding to a variation of a respective individual parameter of interest, selecting a set of statistical moments, each statistical moment corresponding to a respective statistical distribution type of the individual parameter of interest, selecting a set of moment constraint equations, each corresponding to a statistical moment of interest, and computing the set of multidimensional HS points as a function of the set of parameters of interest, the set of statistical distribution types, the set of statistical moments, and the set of weighting values; compute the guidance control policy defining the control variables and their time histories and generating a control vector for transitioning the dynamically controlled system through the state space from the initial state to the final state according to the computed set of multidimensional HS points and the performance metric; and control the dynamically controlled system in accordance with the guidance control policy and generated control vector to transition the dynamically controlled system and operatively associated object and respective trajectory through the state space from the initial state to the final state.

DETAILED DESCRIPTION OF THE DISCLOSURE

One or more exemplary embodiments or implementations are set forth in conjunction with the drawings, where like reference numerals refer to like elements throughout, and where the various features are not necessarily drawn to scale. It will be understood that many additional changes in the details, materials, procedures and arrangement of parts, which have been herein described and illustrated to explain the nature of the disclosed exemplary embodiments, may be made by those skilled in the art within the principal and scope of the disclosure as expressed in the appended claims. Techniques and apparatus are disclosed for control of uncertain dynamical systems, which may be employed in a variety of applications including without limitation launch vehicle guidance systems, missile guidance systems, spacecraft attitude control systems, weapons systems, robot manipulator control systems, etc. The disclosure in certain implementations provides a specific collection of points intended to sample the domain(s) of uncertainty that need not be in a compact domain, obtained with generalized discretization methods, which in one embodiment leads to the so-called “sigma” points associated with the well-known unscented transformation, with the objective of representing the time-behavior of a collection of perturbed dynamical systems. Here, the notion of a collection (ensemble) of perturbed dynamical systems is utilized to represent the behavior of a nominal controlled system whose performance may be influenced by the presence of uncertainties. The disclosure in certain implementations also provides a specific collection of discrete sampling points referred to herein as hyper-pseudospectral or HS points to parameterize and characterize the uncertainties in the domain(s) of interest. The usage of HS points enables higher-order statistics to be accounted for by the ensemble of dynamical system models and thereby higher-order statistics may be incorporated as part of the control policy computation. The discrete sigma and/or HS points, are used to form a specific collection of copies of dynamical system models that together encode the behavior of the nominal dynamical system over the entire range of system uncertainty considered. The dynamical system models obtained by way of the sigma and/or HS points may then be used to facilitate the computation of a robust guidance control policy for an uncertain system that can be implemented either in open-loop fashion or may be provided as input(s) to an inner, closed-loop, feedback system. Techniques are thus provided for computing a guidance and control policy that defines control variables and their time histories for transitioning the controlled system through the state space from an initial state to a desired final state with robust performance against uncertainties about the nominal system as encoded via a set of points selected via an unscented transform or the HS points whose computation is described herein. Using this approach, the set of all possible initial states and/or parameters within the bounds specified for a particular problem scenario may be sampled in a deterministic fashion by a comparatively small number of sigma and/or HS points and, utilizing the sigma and/or HS points, a control policy is computed in certain embodiments which drives the controlled system to the desired terminal or final state, while maximizing a robustness index or minimizing a performance metric that, via the sampling property of the sigma and/or HS points, considers the influence of the specified uncertainty on the performance of the nominal dynamical system, where minimizing the performance metric reduces, for example, the terminal error at the final state. In certain embodiments, for example, a control objective can be to minimize the volume of a one-sigma uncertainty ellipsoid at the final state, which can represent a location or orientation in three dimensions. In this regard, the sigma and/or HS points are utilized to facilitate the reconstruction of the statistics (mean, standard deviation, etc.) of the terminal state by propagating the solved control policy though the collection of dynamical models derived from the deterministic sigma point and/or HS point samples. The statistics of the terminal states may also be determined as part of a collocation process if this type of approach is utilized in the computation of the robust guidance policy. Note that arbitrary sample points may also be utilized in certain embodiments of the present disclosure. Proper reconstruction of the statistics on the terminal state may, however, require a large number of arbitrary sample points (generally greater than 1000). Thus, while arbitrary sample points may be used as part of the process for control policy computation described herein, the implementation of the process may be extremely inefficient. A small number of samples, such as upper and lower bounds on parameters can be efficient, but not statistically meaningful; while a large sample, such as can be obtained using a Monte-Carlo selection, is statistically meaningful, but extremely inefficient. The method of the present disclosure may be used to generate a small number of samples that are statistically meaningful, thereby advantageously improving efficiency and accuracy.

While described herein in the context of controlling a system, such as a missile, launch vehicle or spacecraft, for physical transition from an initial position to a desired final or target position in three dimensional space, the disclosed concepts and systems can be utilized in association with transitioning any dynamical system, e.g. biological, chemical, electrical, mechanical, etc., through any desired state space from an initial state to a final state, and can be used in conjunction with any other relevant optimization criteria, such as minimizing energy consumption, minimizing transition time, etc.

FIGS. 1 and 2provide schematic diagrams2illustrating transition of a controlled system, such as a missile in one example, from an initial or starting state4in a three dimensional state space to a final or terminal (e.g. target) state8in the presence of system and environmental uncertainty. As seen inFIG. 1, an ellipsoid6shows an example of uncertainty in the state space regarding the actual initial or starting state4, and likewise an ellipsoid10illustrates exemplary uncertainty in the terminal state8. In addition,FIG. 1shows two examples of potential “keep-out” regions12and14in the state space in which it is desired to avoid having the controlled system transition through the keep-out areas. In the example of missile guidance control, for instance, the keep-out areas12and14may represent locations of friendly vessels, with the target state8representing an enemy craft, and the initial or starting state4representing the assumed current position of a firing vessel. Thus, the keep-out areas12and14serve to represent uncertain trajectory or path constraints in the state space between the initial state4and the final state8.FIG. 1further illustrates three examples of possible trajectories16,18and20beginning at different points within the uncertainty region6surrounding the initial state4, and terminating at different locations within the uncertainty region10surrounding the target final state8.

Nominal trajectory16represents the state space trajectory of the nominal dynamical system for the given guidance control policy. Trajectories18and20, on the other hand, represent other possible off nominal trajectories that may be obtained when the same guidance control policy as was used to generate nominal trajectory16is applied under uncertain conditions as represented by ellipse6.FIG. 1thus illustrates the effects of initial state and/or system and/or environmental uncertainties on the ability of a missile or other dynamical system to reach the desired terminal state8using a given control policy. In the presence of uncertainty, a trajectory originating from anywhere in ellipse6(including the nominal initial state4) will reach a terminal state that is contained within terminal ellipse10, which includes that desired end target state8. In many applications it is desired to reduce the size of ellipse10(computed based on statistical metrics or other means) as much as possible within the operational constraints of the dynamical system.

FIG. 2further illustrates the influence of system uncertainty on the performance of a dynamical system as the exemplary errors associated with two nominal feasible trajectories, wherein a first trajectory22using a given control policy may transition the system from the initial state4to the target state8, whereas an associated perturbed trajectory26does not reach the target state8due to the effects of the system uncertainty. Likewise, another possible nominal feasible trajectory24(using a different control policy than the one used to produce trajectory22) is illustrated, along with an associated perturbed trajectory28(considering the same uncertainty as in perturbed trajectory26) which again misses the mark. Thus, while there may be many different feasible control policies that can guide a dynamical system to a desired target, the error between the desired end state and the actual terminal condition (the terminal error) can differ when any of these control policies are applied under off nominal (uncertain) conditions. It is highly desirable, therefore to develop a process by which the best feasible control policy can be identified where the usage of the term best refers to the control policy which minimizes the deleterious effects of uncertain conditions as much as possible with respect to given metrics (e.g. mean and/or standard deviation of the terminal error).

Referring also toFIGS. 3, and 15, the present disclosure advantageously provides for computation of a control policy that reduces target-miss errors arising from off-nominal conditions. Among all feasible guidance and control trajectories, the present techniques advantageously facilitate computationally efficient determination of a feasible guidance control trajectory which is robust with respect to uncertainties, relative to an ensemble-wide performance metric appropriate to the mission. Certain embodiments, moreover, mitigate or avoid linearization of the plant model to leverage the presence of advantageous non-linear coupling, although the process can also be applied to linear plants and non-linear plants that have been linearized. The present disclosure further facilitates the compact characterization of system and environmental uncertainties through use of hyper-pseudospectral (HS) sample points that are generated by a numerical method which allows the generation of a collection of HS points subject to the mission requirements as specified by the user. Certain embodiments of this approach take into account statistical “moment matching”, but advantageously enable the moment matching to be carried out only for those moments of interest to the user of the process, and only at the desired degree of relative accuracy. This distinguishes the properties of the HS points described herein from so-called sigma points. The HS point generation concept, moreover, may advantageously relax the standard moment constraints, making them part of a cost index to be minimized. In this regard, the cost index can also include other components completely unrelated to statistical analyses, but relevant in the context of guidance and control. Using the presently disclosed techniques, the computation of the HS points can be completed in a way that weights and/or de-weights the contribution of higher-order statistical moments, and constraints may be added in which the HS points are selected in such a way that certain classes, regions, or numerical values will be restricted by definition of one or more bounds or keep-out zones. For example, it may be known that a certain parameter may take on positive or negative values but never be precisely zero. In this case, selecting a HS point at zero would not be consistent with the system under consideration. The presently disclosed techniques enable a constraint to be added as part of the HS point computation process that prevents a HS point from being selected at zero. The HS points, moreover, can be computed for both bounded and unbounded statistical distribution domains, including but not limited to Gaussian, uniform, Gamma, Beta or other statistical distributions or any combination thereof. For example, if a parameter that describes the plant model has a hard stop, the disclosed approach can incorporate this as a constraint, achieving the best possible selection of points within the scope of such a constraint.

FIG. 3illustrates an exemplary method or process30for computing a guidance control policy including computation of HS points at38, andFIG. 4illustrates an exemplary process38for HS point computation in accordance with one or more aspects of the present disclosure. Although the illustrated processes or methods30,38and other methods of the present disclosure are depicted and described in the form of a series of acts or events, it will be appreciated that the various methods of the disclosure are not limited by the illustrated ordering of such acts or events except as specifically set forth herein. Except as specifically provided hereinafter, some acts or events may occur in different order and/or concurrently with other acts or events apart from those illustrated and described herein, and not all illustrated steps may be required to implement a process or method in accordance with the present disclosure. The illustrated methods may be implemented in hardware, processor-executed software, or combinations thereof, in order to provide various functions as described herein, and possible embodiments or implementations include non-transitory computer readable mediums having computer-executable instructions for performing the illustrated and described methods. For instance, the processes30,38may be implemented in whole, or in part, in the shipboard processor system illustrated and described below in connection withFIG. 15, and the processor152inFIG. 15may be programmed with corresponding computer-executable instructions for implementing the described methods.

FIG. 15illustrates an exemplary shipboard system150having a processor152operatively connected with an electronic memory154, a GPS or similar system156, a user interface158, and a weapon guidance system160forming a system to perform guidance of a missile or other projectile from a current location164(e.g., as determined by the GPS or similar system156) to a selected target location162. Thus, the system inFIG. 15provides guidance control by computation of a guidance control policy170using the processor152to transition the controlled system (missile) through a state space from an initial state to a final state as described by the selected target location162. In the illustrated example, the processor152operates according to computer executable instructions to implement the functionality set forth herein, which instructions may be stored in the electronic memory154or other non-transitory memory or other computer readable medium(s) operatively associated with the processor152, and uses the memory154for storing various parameters, data, etc. For example, the illustrated example stores a problem space definition172, constraints174and a determined performance metric176in the memory154, with the processor152implementing an HS point computation component166to use the data172,174and176along with data or values178to compute a set of HS points180.

The processor152further implements a guidance control policy computation component168, as further described with reference toFIG. 3below, which uses the current location164and selected target location162and the HS points180along with the problem space172, constraints174and performance metric176to compute a guidance control policy170which is then provided to the weapon guidance system160for open or closed-loop control of the missile which is then translated to the target. As further seen inFIG. 15, moreover, the memory154further stores a set of uncertain parameters “p”, including parameters of interest182(which may include some or even all parameters associated with the problem space172), as well as one or more statistical distribution types 184, one or more statistical moments186, one or more weighting values188, and/or slack variable bounds190associated with a corresponding one of the parameters of interest182.

Additionally, the memory154stores optional HS point keep-out zones192and a desired cost function194used for computing the HS points180by the component166.

HS computation process38inFIG. 4advantageously facilitates judicious sampling of neighboring states and parameters from a set of all possible states/parameter values that can be used to determine a robust guidance and control policy170, with respect to the uncertainty in the system and/or environment as embodied by the set “p” of uncertain parameters182. Moreover, the guidance and control policy170provides a single control trajectory that can be applied to the dynamical system defined in the problem space172regardless of the specific values of the parameters “p” and further a single control trajectory that may be optimized relative to a performance index measured with respect to the uncertainty embodied in set “p”. Computation of the performance index associated with the methods of the present disclosure, moreover, is facilitated using a novel procedure for generating HS points. The HS points180in this regard, provide a computationally efficient means for discrete representation of the uncertainty in the parameters “p”, which include any uncertain parameters pertaining to the system and/or environment and which may additionally include uncertainty (6inFIG. 1) in the initial and/or final states (4and8inFIGS. 1 and 2above, corresponding to the current location164and the selected target location162inFIG. 15) associated to the problem172, wherein disturbances can also be included by considering their effect as equivalent perturbations in the initial state4. The guidance control policy computation method30inFIG. 3begins at32with definition of the problem space172, which provides a mathematical definition of the problem, including in one possible embodiment, a set of differential equations that define a dynamical model of the system to be controlled. The problem space172and other data described herein can be computed via the processor152and/or may be user defined via the user interface158. In certain cases, such as a fairly well-defined weapons guidance system implementation as seen inFIG. 15, the problem space172may be preprogrammed (e.g., stored in the memory154) into the system for repeated usage. In a typical example, this definition is given by a set of linear and/or nonlinear ordinary differential equations (ODEs) that involve both state variables, control variables and parameters. In the illustrated example, for instance, the equations may characterize the problem space in any suitable fashion, such as to characterize operation of a missile in traveling from a launch location to a target destination. This may include equations involving the characterization of the current location164, such as accuracy parameters related to operation of the GPS-equipped or similar navigation system156, as well as those of the final state (selected target location162). In addition, state variables “X” and parameters “p” may form part of the problem space characterization172relating to operational characteristics of the missile itself, such as the amount of mass as a function of time and distance traveled considering fuel expenditure, aerodynamics, propulsion system performance, etc., as well as equations defining the medium through which the missile is traveling (e.g., air pressure, wind factors, etc.) and the like. In addition, the problem space equations172may further characterize the control variables, such as thrust and/or control surface deflections, used to launch and potentially guide the missile from the current location164to the target162.

The disclosed procedures encapsulate concepts of optimal robust control to design feasible guidance and control strategies that can be employed in the presence of statistical and other system disturbances, which can reduce or minimize the errors associated with these disturbances. An example of the problem space definition at32inFIG. 3involves providing a dynamical model for the missile system to be controlled as a set of K differential flows governed by a common mapping f:N→N, and indexed by a collection of system parameter vectors, {right arrow over (p)}k. In the missile example, each differential flow represents the time-evolution of the dynamical model for the missile system for different initial conditions and/or values of the system parameters. In one possible implementation, the problem space definition172is a set of state space equations of the following form:
{dot over ({right arrow over (x)})}k={right arrow over (f)}({right arrow over (x)}k,{right arrow over (u)},t;{right arrow over (p)}k),k=0, . . . ,Kwhere{right arrow over (x)}k(t)εNxis the vector of system states over time, associated to {right arrow over (p)}k{right arrow over (u)}(t)εNuis the vector of system controls over time{right arrow over (p)}kεNpis a set of system parameters

For each k, k=0, . . . , K, the system evolves through time in a manner dependent on the explicit value of the k-th parameter vector, {right arrow over (p)}k, subject to a common control policy, {right arrow over (u)}(t). This information may be assembled as:

One differential flow described in the equation above corresponds to the behavior of the nominal system, wherein the nominal parameters for the system, {right arrow over (p)}0are used. For k=0, where the value of k depends on the distribution of its points for the nominal state, one example assumes a non-empty manifold of admissible guidance policies that meet all desired path and terminal constraints,

𝕌0={u→⁡(t,x→;p→o),t∈[totf]}.
Each element of the control policy0drives the missile to a terminal constraint {right arrow over (e)}({right arrow over (x)}0,{right arrow over (x)}f,t0,tf;{right arrow over (p)}0)={right arrow over (0)} at some final or terminal time tfwhich may be fixed, free, or bounded, and where the initial time tomay, but need not, be fixed. The vectors of system parameters, {right arrow over (p)}k, k=0, . . . , K, are general, and can include, but are not limited to, items such as initial states, final states, initial and final times, environmental parameters, etc.

The differential flows, Fk(t), t=1, . . . , K describe the behavior of the dynamical system for off nominal parameter vectors, {right arrow over (p)}k, k=1, . . . , K, and may be assembled as

At34inFIG. 3, bounds are established for each of the system variables. These bounds in certain embodiments may represent practical limitations of the dynamical system or constraints imposed by the operator of the dynamical system. All of the variables that define the dynamical system are constrained to be in a general set, which in principle can be either finite or (mathematically) infinite. For example, the constraint definition at34may include defining bounds on the allowed set of initial conditions 4, final conditions 8, system parameters “p”, and/or state/control constraints, referred to as en-route or path constraints174. In one possible non-limiting embodiment, a nominal initial state4may be assumed, with an associated set of parameters, for example|xi(t0)≦bi, i=1, . . . , N, with bounds biand |pk|≦B, k=1, . . . , K, having a (component-wise) bound B. In certain implementations, these bounds can be set at32large enough so as to be non-binding and thus not limiting in the selection of the HS points. It is also preferable that all trajectory bounds be identified and set at32, where the path constraints may restrict one or both of the possible state values X and/or the possible controls U, and are preferably also identified early in the process.

At36, the objective index or performance metric176(“J”) is defined or otherwise established for the guidance and control policy170. In one embodiment, the performance metric176takes into account the evolution of all the differential flows. One non-limiting example will seek to reduce or minimize the stochastic uncertainty of the terminal state (i.e. the deviation from the selected target location162inFIG. 15and point8inFIGS. 1 and 2), for example by computing the trace of the terminal covariance. That is, obtain a guidance and control policy170that minimizes J=trace((tf)), where(t)=cov(x(t)) is the time varying covariance of the nominal state. In another possible embodiment, the user may seek to minimize the integrated trace of the covariance, i.e. j=∫ trace((t))dt. The former metric176favors minimizing terminal system uncertainty, whereas the latter attempts to minimize uncertainty over the entire trajectory.

For both of the exemplary performance metrics176described above the continuous covariance of the nominal state is computed, by the methods and procedures of this disclosure, as the covariance of the discrete data in {right arrow over (P)}(t) as described by the HS points at each time instant of interest as(t)≡cov({right arrow over (P)}(t)). By utilizing the covariance, the exemplary performance metric is seen to employ a 2ndorder statistical measure of the state variations due to uncertainty. Other choices, including those utilizing higher-order statistics can be selected to meet any suitable mission requirements for a given application. In addition, non-statistical metrics may be utilized. In one example, a performance index which minimizes a measure of terminal spread from the nominal target is given by:

At38inFIG. 3, a set of HS points180is computed via the HS point computation component166implemented by the processor152inFIG. 15, where the computed HS points180are consistent with the operator's understanding of the statistical properties of the uncertain parameters of the initial states, etc. as discussed further below in connection withFIG. 4. It is noted that any suitable set of points180may be used at38(including HS points and/or sigma points and/or arbitrary points or any combination thereof), whereinFIG. 4illustrates a preferred implementation using HS points180. In addition, the selection of the points180at38may be made in view of the desired performance index determined at36.

With the points determined at38, a guidance and control policy170is computed at40inFIG. 3, for instance, via the guidance control policy computation component168implemented using the processor152inFIG. 15. In this regard, the computed control policy170in certain implementations preferably optimizes the performance index over the entire set of differential flows as determined by the selected values of nominal and HS points determined at38, relative to the performance index J 176 chosen at36. In certain implementations, the guidance control policy may be determined analytically (when possible) or numerically, for example, via the processor152. Any appropriate numerical technique can be used to solve this problem, including without limitation Shooting/Multiple-shooting, Genetic algorithms, Multi-objective programming, Dynamic programming, Co-location methods, and/or the use of direct pseudospectral methods. In any numerical implementation, however, it is desirable to reduce the total number of differential flows that are considered in order to improve the efficiency of the computation. This aspect is facilitated, in part, through the use of HS points which provide a process for sampling the domain of uncertainty in a manner which gives the desired accuracy of statistics with the smallest number of sample points. In addition, the process38ofFIG. 4for computation of the HS points enables the user of the process the ability to trade statistical accuracy against the desired number of HS points for any given problem.

The process38ofFIG. 4for computing HS points advantageously provides a novel computational technique that is an improvement over the prior art, in the following ways: applies to arbitrary distributions, relaxes the moment-matching concept and uses this relaxation as one of the factors that is used to select the HS points, allows user selection of the desired number of HS points, allows HS point keep-out zones, i.e. regions in state-space that will not be allowed to contain HS-points, and is entirely numerical in nature. In this regard, other approaches have used so-called sigma points (a particular subset of general HS points) to enhance the effectiveness of navigation filters, wherein the sigma points are employed to better predict the evolution of uncertainties in the future (at the next sampled observation). In contrast, the present disclosure, and exemplary embodiments provided herein, advantageously applies the concept of HS points to the problem of guidance and control or other state space transition problems in general. In this regard, navigation problems typically involve asking the question “Where am I?” In this context, uncertainty regarding the location of a vehicle, vessel, etc. is of primary concern, whereas guidance and control problems ask the question “How do I get there?” Thus, guidance and control problems seek to find the path and associated control commands that must be followed in order to transition the controlled system from the initial state to the desired final state in a way that reduces the error between the actual terminal point and the desired target. Previous guidance and control techniques are based on developing a control policy for a nominal plant comprising only nominal parameters and initial conditions, etc., wherein the effect of system uncertainty is evaluated (if at all) after the fact by the application of Monte-Carlo or similar methods to test the efficacy the proposed guidance policy. Other so-called robust control approaches apply linearization techniques in an attempt to generate more robust guidance commands, using the concept of desensitization, in which uncertainties are linearized over the trajectory, and certain analyses employ some metric of the covariance as a measure of trajectory robustness. However, this approach may fail to perform as expected for the real nonlinear system, or may result in a conservative control policy in which some of the available performance envelope of the system cannot be utilized. In contrast, the present disclosure advantageously addresses the deleterious effects of uncertainty directly as part of the guidance and control design process by coupling the concept of HS points with the concept of ensemble control in the form of a compact collection of properly selected differential flows, and thus provides a novel, non-linear method for creating robust guidance and control commands170for uncertain dynamical systems, whether the illustrated missile navigation system or any transition of a dynamical system from an initial state4to a final state8.

As seen inFIG. 4, one implementation of the HS point determination process38involves determining the number of parameters of interest at41(e.g., the parameters of interest182inFIG. 15below). At42, for each parameter of interest182identified at41, a statistical distribution type is assigned (assumed or otherwise determined), for example, a uniform distribution, a Gaussian distribution, a Poisson distribution, a frequency distribution, or a binomial distribution in certain embodiments. At43, certain embodiments further allow determination or setting of particular statistical moments to be considered for each of the identified parameters of interest182. At44inFIG. 4, moreover, one or more weighting factors can be set for individual statistical moments so as to emphasize their influence when evaluating the performance metric J36. The number of HS points to be considered is determined at45. One or more bounds or keep-out zones192may be set at46inFIG. 4for the set of HS points180. At47, at least one bound is optionally set on at least one slack variable for a corresponding statistical moment186. A desired cost function194is determined at48for computing the HS points180, and the processor152is used at49to solve a constrained non-linear programming problem to compute the HS points180at least partially according to the slack variable bound, the bounds or keep-out zones192for the HS points180, and the cost function194.

As seen inFIG. 4, once the desired objectives and constraints have been established in the process30ofFIG. 3, sampling the space of uncertain parameters is done by the determination of a finite set of HS points. To illustrate the process, consider the case of a Gaussian Density function with mean and covariance zero and unity, respectively, and assuming f:RN→R is a mapping from the a real N-dimensional space to the real line, the Gaussian expectation of this function is given by

E⁡[f⁡(x)]=∫RN⁢f⁡(x)⁢p⁡(x;0,1)⁢⁢ⅆx
where p(x;0,1) is the N-dimensional Gauss distribution density function. Expanding in a Taylor series about the mean (which is 0), gives the expectation of the function as:

E⁡[f⁡(x)]=f⁡(0)+∑n=1∞⁢∑α=n⁢∂α⁢f⁡(0)⁢E⁡[xa]/α!,where⁢⁢α=(α1,…⁢,αn),α=∑i=1N⁢αi,is⁢⁢a⁢⁢multi⁢-⁢index,where⁢⁢xα=x1α1⁢…⁢⁢xNaN⁢⁢α!=α1!⁢⁢…⁢⁢αn!,and⁢⁢∂α⁢=∂α⁢f∂a1⁢…⁢∂αN(1)
Evaluating this expectation operator by quadrature, it can be assumed that there exists a finite set of positive real weights wi, and points, xiεRNsuch that:

E⁡[f⁡(x)]=f⁡(0)⁢(∑⁢w)+∑j=1∞⁢⁢∂α⁢f⁡(0)⁢(∑i⁢⁢wi⁢xiα)α!.(3)
The following then provides the moment constraint equations whose embodiment may be described by lower and upper bounds on the weighted sum of monomials, given by:

-Lα≤∑i⁢wi⁢xiα-E⁡[xα]≤Uα,and-Lwi≤wi≤Uwi,with⁢⁢⁢Lα,Uα,Lwi,Uwi≥0(4)
The lower and upper bounds may both be set to zero for all moments up to a desired order (precise moment matching), and higher order moments of interest may be either ignored, or the next higher order may be placed into a performance index to be minimized. In certain embodiments, the bounds may be chosen by taking into account different mission requirements. For example, if some of the moments are known precisely, then the bounds can be set to zero. The box-bounds on those moments which are less well understood can be loosened in certain implementations. As is known, a moment is a quantitative measure of the shape of a set of points, wherein the first moment is the mean, the second moment can be seen as a measure of the width of a set of points in one dimension, or in higher dimensions measures the shape of a cloud of points as it could be fit by an ellipsoid. Other moments describe other aspects of a distribution, for instance, skewing of the distribution from the mean, where a given distribution may be characterized by a number of features (e.g., mean, variance, skewness, etc.) related to the moments of a probability distribution for a random variable.

In certain embodiments, non-linear programming concepts can be used to calculate a given finite number of HS points, including all moments of interest, via the HS point computation component166implemented via the processor152inFIG. 15. In one implementation, a cost function J 194 (FIG. 15) may be used for determining the HS points180, which cost function194is given by the following equation:

An example application is illustrated below, in the context of the classic Zermelo problem, to illustrate some key points related to the implementation of the presently disclosed techniques without the need to describe the complicated equations of motion associated with the exemplary missile guidance problem discussed above. In Zermelo's problem, it is desired to find how to navigate on a body of water by choosing a path to get from an initial location (initial state) to a final destination (terminal state) in the least possible time. The equations of motion for the dynamical system that describes the exact problem are given by:

In this case, the state variables of the dynamical system are “x” and “y”, the constrained control variables represent the cosine ux, and sine uy, of an angle, and the symbol “p” in this example is as an uncertain constant used to describe the strength of the current of the body of water. In the exemplary problem description current flows in the direction of the x-coordinate and has a strength that is proportional to the cube of the distance from the shore described by the line where y=0.

Beginning at an initial position or ‘state’ [x0, y0] with the objective to transition the boat to an origin [0, 0] (final state), a control policy170can be computed by assuming the nominal value of p and the nominal initial condition. The control policy for moving the boat may complete the transition from the initial state to the final state in minimum time, or with minimum effort in two possible non-limiting examples. However, deriving a guidance control policy170in this manner (specifically by selecting only the nominal value for p and only the nominal initial condition) may fail to achieve the final desired state in the presence of uncertainty in the value of p and/or in the values of the initial conditions. The present disclosure provides techniques for computing a guidance control policy170which is most robust with respect to this uncertainty according to the defined performance index, and thus will reduce the errors caused by uncertainty. To illustrate, the HS points in three dimensions are used, assuming that both the initial condition and the parameter p are uncertain. In this case, it is assumed without loss of generality, that the errors are described by a Gaussian probability distribution with a specified mean and variance. The nominal parameters and their uncertainties are seen in Table 1 below, and can be construed as being rather large, for example for cases of fog with GPS malfunctions.

A collection of HS points in three dimensions which match moments up to order 3, was obtained by the process38ofFIG. 4and their values can be seen in Table 3 below. These HS points may be employed in the Zermelo problem to generate six discrete values for each uncertain parameter (two (2) initial conditions Xo and Yo and the parameter p describing the current strength) which enable the statistics on the terminal state to be computed via a discrete collection of seven differential flows, one flow describing the nominal system and 6 flows describing the evolution of the off nominal system equations for the 6 sets of uncertain parameters. The raw HS points (Table 3) are first transformed into the used HS points. This is accomplished in this example by multiplying the matrix of raw HS points by one of the square-roots of the assumed covariance of the parameters of interest. There are many possible choices of square root for a positive definite symmetric matrix, such as a covariance. In the case of an uncorrelated Gaussian system, the initial covariance is diagonal, with the squares of the 1-sigma uncertainty values along the diagonal; in this instance, a natural square root is the diagonal matrix with the diagonal entries composed of the 1-sigma uncertainties.

and
HSij=theithcomponent of thejthcomputed HS point  (9)
Other transformations of the HS points are possible. In the current example, assuming that the parameter covariance matrix is uncorrelated, the covariance square root is computed as the diagonal matrix composed of the one-sigma values given in Table 1. The values of the used HS points as computed based on the raw HS points (Table 3) are displayed in Table 2 below.

As seen in the graph50ofFIG. 5, the nominal boat (origin) is properly driven to the target zero location via the minimum time trajectory51as a consequence of applying the minimum time control trajectories uxuyseen in the graph60ofFIG. 6. The off-nominal trajectories52inFIG. 5are given by the differential flows corresponding to the HS points using the same control policy given inFIG. 6. Since the control policy was derived based on the nominal parameters only, it is seen that the trajectories of the boat diverge significantly from the desired terminal point (0, 0) as a consequence of the domain of parameter uncertainty as embodied by the HS points.

The concepts of the present disclosure may be employed to advantageously reduce or minimize the uncertainty associated with such terminal dispersions, i.e. the dispersion of the final states inFIG. 5. Using this as the objective of our desired control policy, and applying the method of the present disclosure, a significantly different set of “boat” trajectories is achieved, as seen in the graph70ofFIG. 7. The new control policy obtained through the application of the disclosure is seen in the graph80ofFIG. 8. The control policy is determined by minimizing the trace of the terminal uncertainty covariance matrix, which can be computed from the HS point weights as follows:

M=E⁡(x→k⁡(tf),k=1,…⁢,Np)≡∑k=1Np⁢⁢wk⁢x→k⁡(tf)(11)
For example, with respect to the one-sigma uncertainty ellipses portrayed in the graph90ofFIG. 9, the 1-sigma uncertainty ellipse associated with the policy that minimizes the trace of the terminal uncertainty covariance matrix (computed by applying the method of the present disclosure) has a semi-major axis significantly smaller than the uncertainty ellipse associated with the control policy computed using the minimum time metric and only the nominal values for the initial conditions and parameter. The trace of the covariance can be measured during the boat trajectory, where the graph100inFIG. 10illustrates this measure for both the Minimum Time (conventional) and Minimum Uncertainty (using the method of the present disclosure) control policies. In this case, it is seen that the Minimum Time covariance trace diverges dramatically from the Minimum Uncertainty covariance trace exactly when the Minimum Uncertainty controls “flip”, as can be seen in the graph80ofFIG. 8. The control policy provided using the concepts of the present disclosure generates this flip precisely at the point in time that minimizes the terminal uncertainty. Thus, considering the effects of the uncertainty as part of the computation of the control policy using the approach described herein has the desired effect of reducing the uncertainty in the terminal state.

Another example application is illustrated below in the context of a spacecraft attitude control problem, to illustrate some additional key points related to the implementation of the presently disclosed techniques. In particular this example focuses on the effectiveness of the present disclosure in reducing terminal uncertainty associated with a large angle rigid body slew maneuver. The equations of motion for the dynamical system that describes the exact problem are given by:

In this case, the seven (7) state variables of the dynamical system are the body rotation

rate,w→=[wxwywz]T,
and the body attitude unit quaternion,

q→=[q1q2q3q4]T.
The control vector

u→=[uxuyuz]T
is bounded component wise by unity. The inertia tensor is assumed to be diagonal, scaled to unity:

I=[Ix000Iy000Iz]=[100010001],
with the three diagonal elements being the selected as the constant parameters of the problem and, with each constant parameter having a 1 sigma uncertainty equal to 0.01. In this example problem, the uncertain parameters are the diagonal elements of the inertia tensor. Beginning at an initial rest state, w(to)=[0,0,0]T, q(to)=[0,0,0,1]Tthe objective is to transition, (or slew) the rigid body to a final attitude, also at rest, with terminal conditions given by w(tf)=[0,0,0]T, q(tf)=[0.7233170.439680−0.3604230.391904]T. A control policy170can be computed to achieve the objective in minimum time, or minimum effort, or some other non-limiting metric. However, deriving a guidance control policy170in this manner may fail to achieve the final desired state in the presence of uncertainty. The present disclosure provides techniques for computing a guidance control policy170which is most robust with respect to the considered uncertainty in the spacecraft inertia tensor, and thus will reduce the errors caused by this uncertainty. To illustrate, HS points in three dimensions which match up to the 5thorder are used. Again, it is assumed without loss of generality, that the errors are described by a Gaussian probability distribution. The normalized values of the order 5 HS points can be seen in Table 4 below.

In addition to the nominal case, the HS points give rise to 13 differential flows that can be used to describe the influence of the parametric uncertainty on the behavior of the rigid body system and hence allow statistics on the terminal performance to be evaluated. The concepts of the present disclosure may therefore be employed to advantageously reduce or minimize the dispersion of the final states. This is demonstrated graphically inFIGS. 16-23which illustrate the pairwise covariance between the system states i.e. between the rotation rates, and between the quaternion components with one feasible minimum time guidance policy170, denoted as “Feas1” versus a guidance policy170employing the methods and techniques of the present disclosure to minimize the terminal uncertainty, denoted as “MinU”. The error ellipse for the “Feas1” solution in these figures is displayed with dash-dot ellipse. The error ellipse for the “MinU’ is displayed by the dotted ellipse. It can be seen that both the rate and attitude error uncertainty is significantly improved by the methods and techniques employed by the present disclosure. It is also worthwhile to note that although only a small set of 14 differential flows were used in the computation of the guidance control policy, the statistics on the terminal states as computed based on the final states of each of the 13 off-nominal differential flows are in agreement with a Monte Carlo analysis in which more than 1000 randomly selected points was used to evaluate the performance of the computed guidance control policy. This aspect illustrates the advantageous use of a comparatively small set of HS points as obtained by the methods of the present invention to accurately model the statistics at the terminal state.

Referring also to the graphs110and120inFIGS. 11 and 12, the present disclosure provides techniques that may facilitate systematic creation of HS points180, weighing the choice by an objective or cost function194which is tailored to the application at hand. In the example below, HS points in three dimensions may be generated using the procedure described in the following. The slack-bounds on the moment constraint objectives are set to zero up to and including the order of interest, for example order 3 (110inFIG. 11) or order 5 (120inFIG. 12). Such a setting on the moment constraint objectives ensures precise matching of the statistical moments up to and including the specified order. In this example, an additional objective, (which is now the only binding one) minimizes the average size of the squared lengths of the HS points. The final constrained objective function in this example, denoted as Javg, is expressed as:

In certain non-limiting examples, (such as the present one), minimizing the average size of the squared lengths of the HS points makes the values of the components of the HS points more desirable from the point of view of numerical stability. Similar objectives might include minimizing the maximum magnitude of the computed HS points. As seen inFIG. 11, HS points for three dimensions (assuming a Gaussian distribution along each dimension) match all moments up to and including order 3, and have minimum average length. Matching up to order three is imposed by solving the associated non-linear programming problem. In this case, the resulting HS points form an octahedron in 3-dimensions. The HS points, their weights and their lengths are shown in Table 3 below, where the length of each HS point is the square root of 3 and the weights are all equal to the numerical value ⅙.

As shown inFIG. 12, HS points that match all the statistical moments up to the order 5 may be generated in another example using the same kind of objective and constraints as described above. Further, the average length of the points is minimized using the process of the present disclosure. One of the HS points lies at the origin, [0, 0, 0] and the remaining 12 form a regular Icosahedron. As shown in Table 4 below, all of the non-zero HS points have exactly the same length, i.e., 2.23607, which in this case is the square root of 5, and the same weight which is equal to the numerical value 1/20. The weight corresponding to the HS point at the origin has a numerical value equal to 2/5.

FIG. 13illustrates a graph130showing HS Points in two dimensions for order 7 moment matching assuming a uniform probability distribution along each dimension, and the graph140inFIG. 14shows HS Points in two dimensions for order 9 moment matching assuming a uniform probability distribution along each dimension. The method of the present disclosure for computation of HS points is applicable to any statistical distribution, and thus is not limited to Gaussian or Uniform distributions as outlined in the examples herein. The method of the present disclosure additionally applies to distributions with both bounded and unbounded domains. For example, using the same objective that has been described above, and using the method of this disclosure, both 7thorder and 9thorder HS point sets, (on two variables) are displayed inFIG. 13andFIG. 14, respectively. The 7thorder system of HS points ofFIG. 13precisely matches all statistical moments up to and including order seven using only 12 HS points, whereas the 9thorder system inFIG. 14precisely matches all statistical moments up to and including order 9 using 18 HS points. Thus, statistics having accuracy up to the 7thorder may be computed by sampling the domain of uncertainty using only 12 points in the case ofFIG. 13. Referring again to the guidance control policy computation40, the set of 12 HS points given inFIG. 13can be employed to generate 12 differential flows (in addition to the nominal flow) that describe the behavior of a dynamical system in the state space given uniform parametric uncertainty in two dimensions. Moreover, since the statistics given by the HS points given inFIG. 13match moments up to the 7thorder, the performance index36utilized as part of the guidance control policy generation can utilize any or all combinations of statistical measures up to the 7thorder moment. This aspect is a distinguishing feature of the method of the present disclosure for robust state space control of uncertain dynamical systems. A similar discussion also holds for the order 9 HS points given inFIG. 14. Using the methods of the present disclosure, the computation of the HS points takes into account the creation of the HS points subject to problem specific objectives and constraints, and the illustrated HS points are related to a particular instantiation of the disclosed concepts and are constructed only as illustrative examples.

FIGS. 25 and 26illustrate examples of robust trajectory determination200and HS point computation220, for the described Zermelo problem, according to the present disclosure. Application of the method of the present disclosure is similar to the exemplary implementation shown inFIGS. 25 and 26for other general problems in which it is desired to transition an uncertain dynamical system from an initial state through a state space to a final state while minimizing or reducing the effects of uncertainty. Moreover, the equations and computations illustrated inFIGS. 25 and 26may be implemented using any suitable processor or processors in certain implementations, such as processor152inFIG. 15in one non-limiting example. The trajectory determination200ofFIG. 25includes identification at210of system dynamics which are provided to a robust guidance control policy computation component250. In addition, Gauss HS points and weights are determined via component220and provided to the guidance control policy computation component250. A collection of dynamical system models is constructed via component230in which the collection of dynamical system models includes at least one dynamical system model corresponding to the nominal parameter values and/or initial conditions or at least one dynamical system model corresponding to the uncertain parameters as represented by samples of the considered domain of parameter uncertainty. Initial and final conditions, box bounds, and path constraints are determined via component240, with the results being provided to the robust guidance control policy computation component250.FIG. 26illustrates further details of one implementation of the HS point computation220, in which the HS point computation is initialized via component222, with the initialized components being provided to an HS point computation component228. Component224provides for initialization of moments, and bounds and constraints are initialized via component226, with the results being provided to component228for computation of the requested exemplary HS points.

FIGS. 27-33illustrate an exemplary application of the present disclosure to provide agile maneuvering of an imaging spacecraft2810controlled by a plurality of three or more control moment gyroscopes (CMGs)2712wherein each CMG2712has a gimbal axis pointing in a different direction.FIG. 27illustrates an exemplary diagram of a control moment gyroscope (CMG)2712. InFIG. 27, a CMG2712includes a rotor2714as well as a rotor housing2716, a gimbal axis2718and a spin axis2720about which rotor2714rotates to store angular momentum. In operation, the angle of the gimbal of CMG2712is changed by rotating spin axis2720about gimbal axis2718to produce a rotational gimbal angle2722. Changing the value of rotational gimbal angle2722changes the direction of the momentum vector H for a given CMG2712and this causes a torque to be applied to rotate a spacecraft.FIG. 28, illustrates three or more CMGs2712arranged so that the gimbal axis2718of each CMG2712points in a different direction to control the attitude angles of a spacecraft2810.

FIG. 29illustrates a block diagram of a conventional feedback system2900for controlling the attitude of spacecraft, such as spacecraft2810. Exemplary feedback system2900is comprised of an attitude estimator, a feedback law and a CMG steering law. Compensation of the gyroscopic terms is not shown but may be introduced as part of the control loop in a straightforward fashion by one skilled in the art. A feedback law provides a torque command, τc, in the spacecraft body frame to drive the error between the commanded attitude and rate (qc(t) and ωc(t)) and the estimated attitude and rate ({circumflex over (q)} and {circumflex over (ω)}) to zero. Feedforward acceleration commands, αc(t), may also be introduced as shown inFIG. 29. The role of the steering law is to allocate the commanded torque, τc, amongst the individual CMGs2712in terms of a set of gimbal rate commands, {dot over (δ)}c. A key difficulty in utilizing a feedback system like the one shown inFIG. 29for controlling the attitude of an imaging spacecraft is that the CMG steering law may become singular for some orientations of the CMG gimbals. In the singular condition, it is impossible to produce torque in the commanded direction, leading to attitude errors. If spacecraft2810is an imaging satellite such attitude errors may prevent the system from acquiring the desired image.

In an effort to overcome the problem of CMG singularities, feedback system2900can be rearranged as shown inFIG. 30(3000). Gimbal rate feedforward commands, {dot over (δ)}c(t), may be used to drive the CMG gimbals directly without utilizing the feedback control system. Nominal gimbal rate commands, {dot over (δ)}c(t), are naturally obtained in the current state of the art by the computation of a standard guidance control policy. Guidance control policies obtained by existing approaches are, however, not amenable to operational implementation due to system uncertainties and other non-idealities such as modeling errors. In this context the open-loop solutions available using current approaches cannot be adequately implemented due to inaccuracies in the system model. Therefore, in a practical application, a guidance policy must be implemented using a feedback controller. Implementing a guidance policy using a feedback architecture is straight forward as guidance trajectories for the attitude and rate (qc(t) and ωc(t)) inFIG. 30are also available in addition to gimbal rate commands, {dot over (δ)}c(t), as part of the solution to a guidance policy computation problem.

Despite the apparent simplicity of the control loop inFIG. 30, the feedback implementation of a guidance policy can fail due to corrective actions, {dot over (δ)}fb, (resulting from the feedback steering law) that are incompatible with available guidance control commands. The failure is manifest in this exemplary problem as a reduction in the CMG similarity index, possibly to zero, from the expected nominal value. The CMG singularity index is one measure of the control authority of the CMG array. This problem can be resolved using the method of the present invention to obtain a new and different guidance control policy that specifically desensitizes the variation in the CMG singularity index to large perturbations in the orientations of the gimbals that may occur due to feedback. Using the method of the present invention, feedback authority can be explicitly reserved to counteract the effects of system uncertainties so that the new guidance control policy can be successfully implemented in a feedback setting.

FIG. 31illustrates a block diagram of one embodiment in which the method of the present invention is utilized to generate a guidance control policy for an exemplary CMG spacecraft, such as spacecraft2810. InFIG. 31the computation of a guidance control policy31150is done on board spacecraft2810. In other embodiments, computation of a guidance control policy can be done as part of a process instantiated on a ground based system3160. Referring toFIG. 31, the desired attitude of spacecraft2810may be given at instants in time as determined by a target selector3161. Target selector3161uses an internal algorithm to determine the ideal utilization of the imaging sensor of spacecraft2810. Desired attitude targets3162and their time tags are communicated to spacecraft2810over an appropriate communications interface3148and3128. After being received at interface3128on spacecraft2810, desired attitude targets and their corresponding time stamps are stored in a command buffer3142for subsequent execution by attitude control system3130.

When a desired attitude target3146becomes active (as determined in one instantiation by comparing an attitude target time tag with a clock operated on board spacecraft2810), an attitude target is read by the guidance control policy computation module31150. The current location and attitude of spacecraft2810and other information related to the state of spacecraft2810is measured by sensors3124and made available to both the attitude controller3132which may implement a version of the control block diagram given inFIG. 30, and the guidance control computation block31160of the present invention. In order to determine an appropriate guidance command trajectory31162to achieve a desired attitude target the guidance control policy computation block31160makes use of problem specific information stored in a problem space module31154, a constraints module31153, and a performance metric module31152.

For a rapid imaging satellite, it is advantageous to reduce the total maneuver time required to transition the imaging sensor of satellite2810from the current attitude as determined by sensors3124and the desired attitude3146at the current time. In this context, an appropriate performance metric31152is to minimize the final time of the maneuver
J=tf−to(20)
where in equation (20), tfis a final clock time of a maneuver and tois an initial clock time.

To ensure that the problem is feasible for implementation by attitude controller3132, N copies of a model of the spacecraft dynamics are needed to define the problem space stored in module31154. One copy of a representative model is given as

x.=[12⁢(q4⁢ω-ω×q)-12⁢ωT⁢qJ-1⁢{-ω×(J⁢⁢ω+Rp⁢∑i=1n⁢⁢hi)-Rp⁢A⁡(Δ)⁢u}u](21)
where x is a vector of system states, q is a vector of quaternions, w is a vector of angular rates and J is a spacecraft inertia tensor. The elements of vector h are the individual momenta of CMGs3112projected into the body-fixed frame of spacecraft3110. A rotation matrix Rp can be used as part of the projection mapping and A(D) is the CMG Jacobian matrix, which depends on the instantaneous values of gimbal angles3122. Vector u is the control vector of individual gimbal rates.

In the absence of uncertainty in the dynamical model of spacecraft2810, δfb(t)=0 inFIG. 30when a guidance policy computed based on equation (21) is used as an input to attitude controller3132. In a practical setting, it is possible for {dot over (δ)}fb(t)>>0 due to uncertainties. This discrepancy can lead to singularities due to a reduction in the control authority of a CMG array. To accommodate uncertainties via a feedback mechanism such as the one disclosed inFIG. 30, it is necessary to ensure that a margin for feedback is always retained. This can be done in one embodiment by adjusting a guidance control policy to ensure that the CMG singularity index always lies above a threshold, Smin, determined by an operator of spacecraft2810. The CMG singularity index for an array of n CMGs is given by

S⁡(Δ)=det⁡[A⁡(Δ)⁢AT⁡(Δ)]⁢⁢where⁢⁢Δ⁡(t)=[δ1⁡(t),δ2⁡(t),…⁢,δn⁡(t)]T.(22)
The precise nature of {dot over (δ)}fb(t) is impossible to know in advance but can be easily approximated. For example, in one embodiment it is possible to model the value of the instantaneous gimbal angle of each CMG using a Normal distribution,(μi,σ2) where μiis the nominal (mean) value of the gimbal angle3122for a CMG3112and σ is the standard deviation from the mean (the statistical off-nominal perturbation induced by the feedback controller) that is presumed by an operator of spacecraft3110. At each instant in time, statistics on S(Δ,t) can be computed as a multi-dimensional Riemann-Stieltjes integral. For example, the expected value of S(Δ,t) may be computed as

E⁡[S⁡(Δ,t)]=∫∫…⁢∫S⁡(δ1⁡(t),δ2⁡(t),…⁢,δn⁡(t))⁢ⅆΦ⁡(δ1⁡(t)-μ1⁡(t)σ2)⁢ⅆΦ⁡(δ2⁡(t)-μ2⁡(t)σ2)⁢⁢…⁢⁢ⅆΦ⁡(δn⁡(t)-μn⁡(t)σ2)(23)
To ensure some margin for feedback (and thus avoid the possibility of encountering singularities) an advantageous operational constraint (which may be stored in constraint module31153is
E[S(Δ,t)]−3E[(S(Δ,t)−E(S(Δ,t))2]≧Smin(24)
It is apparent that evaluation of equation (24) requires computation of higher-order expectations. Nonetheless, a guidance control policy that satisfies equation (24) ensures that the probability that the value of S(Δ,t) is greater than the operator selected threshold, Smin, 99% of the time. This is because the left hand side of equation (24) represents the mean value of the CMG singularity index minus 3 times the standard deviation of the CMG singularity index. Thus, the reliability of the feedback controller can be improved to nearly 100% by computing a guidance control policy that satisfies equation (24). Using the method of the present invention, efficient evaluation of equation (24) can be accomplished by evaluating HS points in module31151as per the disclosed procedure. As illustrated in Table 5, 5-th order HS points are sufficient in this example application because the first two central moments (necessary for accurate computation of equation 1624 have the correct values when compared to a benchmark Monte Carlo simulation. Because there are 23 5-th order HS points, N=23 copies of the dynamic equations given in equation (21) are needed to formulate the problem space31154of the exemplary guidance control policy computation problem.

One formulation of an appropriate guidance policy computation problem is given below in equation (17). This is one example of a problem that may be stored or constructed in the problem space module31154to enable rapid slewing of a spacecraft2810. Upon computation of a guidance control policy that satisfies all of the appropriate constraints using block31160, a guidance command trajectory31162is then provided to attitude controller3132at the attitude controller sample rate, which in turn determines a vector of gimbal angle commands3134for commanding CMGs3112based on the guidance command trajectory and measurements of the state of spacecraft3110as measured by sensors3124.

PRS⁡(Xo,Xf):{MinimizeJ⁡[X⁡(·),u⁡(·),t]=tf-t0Subject⁢⁢tox^1=f⁡(x1,u,t)x^2=f⁡(x2,u,t)⋮x^N=f⁡(xN,u,t)x10=[q⁡(t0),ω⁡(t0),Δ1⁡(t0)]Tx20=[q⁡(t0),ω⁡(t0),Δ2⁡(t0)]T⋮xN0=[q⁡(t0),ω⁡(t0),ΔN⁡(t0)]Tx1f=[q⁡(tf),ω⁡(tf),Δ1⁡(tf)]TΔt⁡(t0)~N⁡(Δnom⁡(t0),σ2⁢I4×4)Smin≤E⁡[S⁡(Δ,t)]-3⁢E[(S(Δ,t)-E[S⁡(Δ,t))2]h⁡(x,u,t)≤0(25)
To illustrate the advantageous nature of the method of the present invention, the block diagram ofFIG. 31was implemented on a commercially available spacecraft simulator. An experiment was implemented to demonstrate agile CMG maneuvers for a point-to-point image collection problem.

A schematic illustrating an exemplary path of a satellite boresight (projected over the Earth) for successful collection of four exemplary images is shown inFIG. 32. InFIG. 32, each image collection region is denoted by a shaded area. In order to acquire the imagery product, it is necessary for a satellite boresight to traverse through the center of each collection region as shown by the solid red lines. Note that it in this representation, it is not necessary that the boresight traces extend beyond the shaded collection regions (as is shown for clarity of the illustration). Due to the relative motion between the Earth and a satellite moving along its ground track, performing an imaging task requires careful coordination of the satellite attitude and rate so that specified tolerances on the attitude and rate tracking error during imaging can be satisfied. Meeting these stringent requirements during imaging cannot be done in the open loop and is handled by the feedback controller. In order to facilitate rapid image acquisition it is ideal that the boundary conditions on each point-to-point maneuver are consistent with the attitude and rate conditions required for imaging each region. By designing the appropriate non-rest maneuvers using the method of the present invention, the transition between the maneuvering and imaging operations can be handled in a seamless fashion.

A sequence of agile maneuvers was designed using the method of the present invention wherein the problem formulation for guidance policy computation is given by equation (25). It was desired to maintain the total variation of the CMG singularity index above a threshold, Smin=0.45. The standard deviation of each CMG gimbal angle from nominal was assumed to be σ=10°. A guidance control policy computed by the method of the present invention will therefore provide gimbal angle trajectories where probability of S(Δ)<Smin, due to feedback perturbations, will be small (<1%). The guidance control policy computed by the method of the present invention provides a statistically guaranteed margin for feedback so that singularities can be avoided even in the presence of system and model uncertainty.

The experimental implementation of a guidance control policy obtained using the method of the present invention is shown inFIGS. 33A and 33B. Referring toFIG. 33A, the satellite simulator is able to acquire all 4 of the requested images as evidenced by the path of the boresight trace, which passes directly through the center of each image collection region within the error tolerances required for imaging. The time history of the CMG singularity index measured during the experiment is observed to lie well within the required 3σ envelope as shown inFIG. 33B. A guidance control policy obtained using the method of the present invention has thus provided a solution in which the chance of encountering a singularity is small so that feedback authority can be maintained despite the presence of system and other uncertainties.

It will be understood that many additional changes in the details, materials, procedures and arrangement of parts, which have been herein described and illustrated to explain the nature of the disclosure, may be made by those skilled in the art within the principal and scope of the disclosure as expressed in the appended claims. The present disclosure can thus be modified and applied by anyone of ordinary skill in the art to develop a guidance control policy for other applications.