Optimal LQ feedforward tracking with preview: Practical design for rigid body motion control

For reference-tracking motion control, preview-based linear quadratic (LQ) design methods provide an effective means to balance tracking performance with available actuation capacity. This paper considers a control structure for which the optimal feedforward controller is independent of the feedback controller. In this way, explicit implementation formulas for feedforward controllers are derived that can be applied to a range of rigid-body motion systems. Key aspects of the optimal LQ solutions are identified, particularly how the choice of design weightings affect steady-state error for polynomial tracking. A redesign procedure for finite preview-time is proposed that preserves exact polynomial tracking properties and control bandwidth of the optimal solutions. Comparative experimental results are presented for a motor-driven linear motion stage.     

LQ (optimal) control of hyperbolic PDAEs

The linear quadratic control synthesis for a set of coupled first-order hyperbolic partial differential and algebraic equations is presented by using the infinite-dimensional Hilbert state-space representation of the system and the well-known operator Riccati equation (ORE) method. Solving the algebraic equations and substituting them into the partial differential equations (PDEs) results in a model consisting of a set of pure hyperbolic PDEs. The resulting PDE system involves a hyperbolic operator in which the velocity matrix is spatially varying, non-symmetric, and its eigenvalues are not necessarily negative through of the domain. The C₀-semigroup generation property of such an operator is proven and it is shown that the generated C₀-semigroup is exponentially stable and, consequently, the ORE has a unique and non-negative solution. Conversion of the ORE into a matrix Riccati differential equation allows the use of a numerical scheme to solve the control problem.     

Closed-form solutions to discrete-time LQ optimal control and disturbance attenuation

This paper presents straightforward derivations of closed-form solutions to two discrete-time problems: linear-quadratic optimal control and disturbance attenuation. Then, as the main results, it shows how to reduce directly the closed-form solutions to their recursive forms. These recursive solutions are in the form of Riccati equation and, hence, establish a close relation between the closed-form solutions and the Riccati equations used in both problems.     

非线性离散系统的信息融合最优预见控制

将信息融合思想引入非线性离散系统的预见控制,提出了信息融合最优预见控制,将对控制系统的所有性能要求和系统动力学转化为可融合信息;然后从信息融合估计的角度,使原问题转化为求控制量的"融合估计"问题.推导了基于卡尔曼滤波器的最优预见控制算法,讨论了预见步数的选取问题,并通过对机械手的转移控制仿真研究说明了信息融合最优预见控制算法的有效性.     

Cooperative optimal preview tracking control of continuous-time multi-agent systems

This paper presents the cooperative preview control method for continuous-time multi-agent systems with a time-invariant directed communication topology. First, the cooperative tracking preview control problem is transformed into the optimal regulation problem of an augmented system. Next, by applying the results of the standard optimal preview control of continuous-time linear systems, a controller of the augmented system is obtained. Furthermore, the stabilisability and detectability of the augmented system are studied under the fixed digraph and the prescribed leader. Then, we derive the controller of multi-agent systems with error integral and preview action that can guarantee the achievement of cooperative optimal preview tracking. Finally, the effectiveness of the controller is shown by numerical simulations.     

Maximum likelihood estimators and worst case optimal algorithms for system identification

For system identification problems with stochastic noise, maximum likelihood estimators are frequently used. If noise is deterministic, worst case optimal algorithms should be considered. In this paper we study the following problem: Under what circumstances are maximum likelihood estimators optimal in the worst case?     

Output feedback realization of LQ optimal systems: A general design using data-based synthesis

This paper is an extension of our previous work regarding realization of LQ optimal systems using output feedback in which we showed that the solutions existed when the plant satisfied certain conditions. In this paper, we remove the restrictions set forth in our previous work and present a design which is suitable for any LQ optimal system. This design involves feedback consisting of scalar output, even when a multivariable system is the subject of the design. Moreover, both the discrete-time and the continuous-time formulations of the proposed design are formulated. The present method also combines plant model estimation and control design into a single procedure by using a technique which achieves an output feedback LQ design directly from plant input and output data, thereby eliminating the need for explicit identification of a parametric plant model.     

Active vibration control using optimal LQ tracking system with additional dynamics

This paper focuses on the vibration suppression task using an appropriate controller design for active control of structures with distributed piezoelectric actuators and sensors. The problem arises from the need to control undesired vibrations caused by disturbances or excitations acting upon a structure in an efficient and at the same time a simple way. A special class of disturbances/excitations (periodical, with frequencies equal or near to the eigenfrequencies of the controlled structure) may cause undesired resonant states. In order to reject such disturbances and suppress vibrations in the presence of excitations an optimal LQ controller based on tracking systems with additional dynamics is proposed for the vibration control problem. The controller was tested in the presence of excitations with different frequencies. Controller design is model-based, where for the numeric modelling of the structure the finite element approach was used. Besides, subspace-based model identification was used as well. Controller design, testing and implementation were performed on the funnel-shaped shell structure, the inlet part of the magnetic resonance tomograph. Simulation results as well as the real-time implementation of the controller as a part of the Hardware-in-the-Loop system show considerable vibration suppression in the presence of excitations and confirm the efficiency of the controller.     

LQ optimal control for a class of pulse width modulated systems

We consider linear quadratic optimal control for a class of pulse width modulated systems. The problem is motivated from a practical application—digital control of switching power converters. The control synthesis problem is posed based on a sampled data model of the original switching dynamics and a linear quadratic criterion that takes the intersampling behavior into account.     

The generalized continuous algebraic Riccati equation and impulse-free continuous-time LQ optimal control

The purpose of this paper is to investigate the role that the so-called constrained generalized Riccati equation plays within the context of continuous-time singular linear–quadratic (LQ) optimal control. This equation has been defined following the analogy with the discrete-time setting. However, while in the discrete-time case the connections between this equation and the linear–quadratic optimal control problem has been thoroughly investigated, to date very little is known on these connections in the continuous-time setting. This note addresses this point. We show, in particular, that when the continuous-time constrained generalized Riccati equation admits a solution, the corresponding linear–quadratic problem admits an impulse-free optimal control. We also address the corresponding infinite-horizon LQ problem for which we establish a similar result under the additional constraint that there exists a control input for which the cost index is finite.     

Robust optimal parametric LQ control with a guaranteed cost bound and applications

For an optimal parametric linear quadratic (LQ) control problem, a design objective is to determine a controller of constrained structure such that the closed-loop system is asymptotically stable and an associated performance measure is optimized. In the presence of system uncertainty, the system via a parametric LQ design is further required to be robust in terms of maintaining the closed-loop stability with a guaranteed cost bound. This problem is referred to as ‘robust optimal parametric LQ control with a guaranteed cost bound’ and is addressed in this work. A new design method is proposed to find an optimal controller for simultaneously guaranteeing robust stability and performance over a specified range of parameter variations. The results presented generalize some previous work in this area. A versatile numerical algorithm is also given for computing the robust optimal gains. The usefulness of the design method is demonstrated by numerical examples and a design of the robust control of a VTOL helicopter.     

A new calculation method for the worst case tolerance analysis and synthesis in stack-type assemblies

Productivity and industrial product quality improvements entail a rational tolerancing process to be applied as early as product design. Once functional conditions are defined, an optimal specification for each component in a mechanical system is to be developed. Despite numerous studies in this area, the problem is still far from solved. It may be decomposed into two stages: development of specifications based on standards, or qualitative synthesis, and calculation of tolerances. To the extent that these two sets of problems are related, we propose to address them in parallel. In this paper, we present an original method that enables us to solve these two problems for the case of serial assembly (stacking) without clearances. This method is based on the use of influence coefficients to obtain the relationship between the functional tolerance and the tolerances associated with the geometry of the mechanism’s interface surfaces. We will describe a calculation algorithm that helps obtain influence coefficients solely from the assembly’s geometric definition. Then, we will show that under our working hypothesis, this relationship is piecewise linear.     

Stochastic optimal control of vehicle suspension with preview on an irregular surface

An active suspension system with preview under stochastic optimal control of a vehicle travelling on an irregular surface is discussed. The vehicle is modelled as a mass-damper-spring system with two degrees of freedom and the dynamic behaviour of the irregular surface is described by a first-order system subject to a gaussian white random input. The acceleration of the vehicle and the irregular quantity of the surface ahead of the vehicle are in a noisy environment measured using sensors mounted on the vehicle. The state variables of the vehicle dynamics and the dynamic behaviour of the irregular surface are estimated using the Kalman filter. The stochastic optimal control is determined by minimizing the given performance index, and is composed of the weighted linear combination of the estimated state variables of the vehicle dynamics and the dynamic behaviour of the irregular surface. A numerical example shows the effectiveness of the proposed active suspension system with preview.     

Cooperative global optimal preview tracking control of linear multi-agent systems: an internal model approach

This paper investigates the cooperative global optimal preview tracking problem of linear multi-agent systems under the assumption that the output of a leader is a previewable periodic signal and the topology graph contains a directed spanning tree. First, a type of distributed internal model is introduced, and the cooperative preview tracking problem is converted to a global optimal regulation problem of an augmented system. Second, an optimal controller, which can guarantee the asymptotic stability of the augmented system, is obtained by means of the standard linear quadratic optimal preview control theory. Third, on the basis of proving the existence conditions of the controller, sufficient conditions are given for the original problem to be solvable, meanwhile a cooperative global optimal controller with error integral and preview compensation is derived. Finally, the validity of theoretical results is demonstrated by a numerical simulation.     

A computational method for simultaneous LQ optimal control designvia piecewise constant output feedback

The paper is concerned with simultaneous linear-quadratic (LQ) optimal control design for a set of LTI systems via piecewise constant output feedback. First, the discrete-time simultaneous LQ optimal control design problem is reduced to solving a set of coupled matrix inequalities and an iterative LMI algorithm is presented to compute the feedback gain. Then, simultaneous stabilization and simultaneous LQ optimal control design of a set of LTI continuous-time systems are considered via periodic piecewise constant feedback gain. It is shown that the design of a periodic piecewise constant feedback gain simultaneously minimizing a set of given continuous-time performance indexes can be reduced to that of a constant feedback gain minimizing a set of equivalent discrete-time performance indexes. Explicit formulas for computing the equivalent discrete-time systems and performance indexes are derived. Examples are used to demonstrate the effectiveness of the proposed method.     

An accurate worst case timing analysis for RISC processors

An accurate and safe estimation of a task's worst case execution time (WCET) is crucial for reasoning about the timing properties of real time systems. In RISC processors, the execution time of a program construct (e.g., a statement) is affected by various factors such as cache hits/misses and pipeline hazards, and these factors impose serious problems in analyzing the WCETs of tasks. To analyze the timing effects of RISC's pipelined execution and cache memory, we propose extensions to the original timing schema where the timing information associated with each program construct is a simple time bound. In our approach, associated with each program construct is worst case timing abstraction, (WCTA), which contains detailed timing information of every execution path that might be the worst case execution path of the program construct. This extension leads to a revised timing schema that is similar to the original timing schema except that concatenation and pruning operations on WCTAs are newly defined to replace the add and max operations on time bounds in the original timing schema. Our revised timing schema accurately accounts for the timing effects of pipelined execution and cache memory not only within but also across program constructs. The paper also reports on preliminary results of WCET analysis for a RISC processor. Our results show that tight WCET bounds (within a maximum of about 30% overestimation) can be obtained by using the revised timing schema approach     

Optimal tracking performance: preview control and exponentialsignals

We study tracking performance limitation problems. Two issues are addressed concerning how earlier results developed elsewhere may be extended to more general classes of reference signals, and how tracking performance may be further improved beyond that offered by feedback control. Toward these issues we consider exponentially increasing reference inputs and examine the use of preview control for tracking. Using an optimal interpolation approach, we develop analytical expressions and a conceptual insight which will aid in the understanding of these issues. To this effect, we derive explicit expressions for the optimal tracking error, either as exact solutions or bounds. It is found that for the exponential signals the earlier results can be directly extended, and similar conclusive statements can be drawn. It is also shown that, in general, preview can be used to advantage for improving tracking performance, especially in dealing with the effect resulted from plant nonminimum phase zeros     

Infinite-dimensional LQ optimal control of a dimethyl ether (DME) catalytic distillation column

This contribution addresses the development of a linear quadratic (LQ) regulator in order to control the concentration profiles along a catalytic distillation column, which is modelled by a set of coupled hyperbolic partial differential and algebraic equations (PDAEs). The proposed method is based on an infinite-dimensional state-space representation of the PDAE system which is generated by a transport operator. The presence of the algebraic equations, makes the velocity matrix in the transport operator, spatially varying, non-diagonal, and not necessarily negative through of the domain. The optimal control problem is treated using operator Riccati equation (ORE) approach. The existence and uniqueness of the non-negative solution to the ORE are shown and the ORE is converted into a matrix Riccati differential equation which allows the use of a numerical scheme to solve the control problem. The result is then extended to design an optimal proportional plus integral controller which can reject the effect of load losses. The performance of the designed control policy is assessed through a numerical study.     

Deadbeat control system with time-varying uncertainty: Minimization of worst case steady-state tracking error

This paper poses and solves a new problem of the synthesis of a controller that minimizes the worst case steady-state controlled error in the presence of time-varying unstructured uncertainty. We present an easy, straightforward design method for obtaining a controller that minimizes the worst case steady-state controlled error under the assumption that the plant is strictly proper. The detailed contributions are as follows. First, we derive a new, simple expression for calculating the worst steady-state error, which gives useful and interesting suggestions about the controller design. We then show that the synthesis problem is reduced to a simple and tractable l1 -norm minimization problem. Therefore, this reduction provides a feasible method for solving the design problem of a controller that minimizes the worst case steadystate error. Second, a deadbeat tracking control problem is considered and a straightforward design method is presented for obtaining a deadbeat controller with given settling steps both to guarantee robust stability and to minimize the worst case steady-state error. The proposed controller is easily obtained by solving a simple linear programming problem. Finally, we show that the proposed controller minimizes the maximum error bound of the controlled output to persistent bounded disturbance.