Existence of discrete-time LQG-controllers

Existence results for the LQG-controller are investigated. An infimal Riccati equation based controller may potentially give closed loop eigenvalues on the unit circle. Assuming left and right invertibility it is shown that there exists an optimal controller if and only if the Riccati equation based controller stabilizes the closed loop system after removal of all its unobservable and uncontrollable modes. Furthermore this reduced controller is the optimal controller, and its transfer function is unique. This existence condition is a considerable simplification of the more general geometric condition recently derived by Trentelman and Stoorvogel.     

Controlled Massive Encapsulation via Tandem Step Emulsification in Glass

Controlled encapsulation is important in pharmaceutics, agriculture, food products, and in emerging materials applications. Microfluidics offers a compelling approach to create controlled emulsions and microcapsules for these applications, but upscaling of this technology for the robust encapsulation of chemically diverse active ingredients is not yet demonstrated. Here, it is shown that microfluidic step emulsification can be exploited in upscaled glass devices to robustly produce monodisperse double emulsions and functional microcapsules in tandem at high throughput rates. The effect of geometrical parameters of the devices and the operating flow rates on the morphology, dimensions, and structure of monodisperse double emulsions is investigated and quantified using simple quantitative models. Using such double emulsions as templates, mechanoresponsive microcapsules that can be embedded in a soft matrix to generate damage‐reporting polymer parts that change color in areas subjected to excessive mechanical loads are created. Thanks to the chemical versatility and mechanical robustness of glass, this platform should enable the high‐throughput encapsulation of a wide variety of chemicals while providing the exquisite control achievable through microfluidics.     

Dynamic start-up optimization of a plate reactor with uncertainties

In this paper, start-up of a plate reactor with an exothermic reaction is considered. Dynamic optimization is used to obtain start-up trajectories, while a feedback control system is designed for on-line control. The problem is challenging, since the process model is highly nonlinear, and subject to uncertainty. Limited bandwidth of the feedback control system requires that the optimization gives optimal solutions with low sensitivity to uncertainty. Special attention is given to the problem of formulating an optimal control problem based on physical insight to reduce the sensitivity of the solution, thus increasing the overall robustness of the closed loop system. The resulting solutions are evaluated in Monte Carlo simulations. The study shows that introducing concentration constraints on the injected chemical in combination with high frequency penalties on the control signals lead to less sensitive optimal solutions, thus yielding a more robust start-up. Automatic computer tools that greatly simplifies the task of formulating complex dynamic optimization problems have been used to obtain the start-up trajectories and are briefly presented.     

Numerical solution of AS + SA + Q = 0

A survey of techniques to solve A^TS + SA + Q = 0 is presented, and nine algorithms are coded and tested on a batch of examples. Which algorithm to be recommended depends mainly on the order of the system.     

Comments on `Conditions for stable zeros of sampled systems' by M.Ishitobi

The commenter argues that the result of the above-titled work (see ibid., vol.37, no.10, p.1558-1561, Oct. 1992) is incorrect. It is pointed out that when sampling a continuous-time system G(s ) using zero-order hold, the zeros of the resulting discrete-time system H(z) become complicated functions of the sampling interval T. The system G(s) has unstable continuous-time zeros, s=0.1±i. The zeros of the corresponding sampled system start for small T from a double zero at z=1 as exp(T(0.1±i )), i.e., on the unstable side. For T>1.067 . . . the zeros become stable. The criterion function of the above-titled work, F(T)=G*(jω_s/2)= H(-1)T/2, is, however, positive for all T, indicating only stable zeros. The zero-locus crosses the unit circle at complex values     

Dyspepsia in general practice: history and symptoms in relation to Helicobacter pylori serum antibodies

The stereoselectivity of Baker's yeast reduction of prochiral alpha-oxygenated 2-propanones has been studied by varying the substrate structure. The 1-hydroxy-3-methoxy-3-propanone 1a was reduced to the corresponding alcohol (R)-2a with 88% enantiomeric excess. Replacing the hydroxy group in 1a with phenoxy or benzyloxy (1b and 1c) gave the alcohols (S)-2b and (S)-2c with 53 and 32% ee, respectively. Reduction of the methyl ketone 1d gave the alcohol (S)-2d with 91% ee. Attempts to improve the enantioselectivity of the reduction of 1c by lowering the substrate concentration or addition of selective reductase inhibitors had only small effect on the enantioselectivity.     

Zeros of sampled systems

The zeros of the discrete-time system obtained when sampling a continuous time system are explored. Theorems for the limiting zeros for large and small sampling periods are given. Conditions which guarantee that the sampled system has stable zeros are also presented.     

A new design of constrained controllers for linear systems

A new method is presented to find stabilizing saturated linear state feedback controllers for linear continuous-time and discrete-time systems. A controller of this type was satisfactorily tested on board a submarine as a depth regulator.     

On the notion of strong stabilizability

A system is called strongly stabilizable (SS) if it can be stabilized using a stable controller. Observer state feedback can be used to design a controller of the same order as the system, but the designs may lead to unstable controllers. The implementation of an unstable controller should be done with care and with a well-designed safety net. One implementation could use a stable observer fed with actual input in addition to the measured output. Several examples are discussed, and how unstable controllers might be required for good closed-loop performance, even if they are not required for stability is shown     

A feedback scheduler for real-time controller tasks

The problem studied in this paper is how to distribute computing resources over a set of real-time control loops in order to optimize the total control performance. Two subproblems are investigated: how the control performance depends on the sampling interval, and how a recursive resource allocation optimization routine can be designed. Linear quadratic cost functions are used as performance indicators. Expressions for calculating their dependence on the sampling interval are given. An optimization routine, called a feedback scheduler, that uses these expressions is designed.