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Control Systems, IEEE

Issue 1 • Date Feb. 2000

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Displaying Results 1 - 4 of 4
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  • Fragility and robustness: a case study on paper machine headbox control

    Page(s): 13 - 22
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (524 KB)  

    We discuss the fragility of controllers against small inaccuracies of controller parameters. A useful way to gain insight into controller fragility is to consider the fragility of poles and zeros. Certain controller design methods and controller parametrizations are especially vulnerable to fragility problems. It is hoped that the controversy and debate around the claimed fragility of designs obtained by modern robust and optimal control theories calls attention to the relevance of the work on fixed wordlength effects in controller implementation. A case study on a paper machine headbox control problem is included to illustrate that if a good robust control design method is used, together with a reasonable controller realization (parametrization), it should be possible to combat controller fragility in a fairly satisfactory, although indirect, manner. However, it would certainly be interesting to study whether controller fragility could be directly addressed within some generalized robust control design theory View full abstract»

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  • Control of constrained nonlinear systems: a case study

    Page(s): 23 - 32
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    Presents a Lyapunov function approach to the control of nonlinear systems that are subject to pointwise-in-time constraints on state and control. This approach is applied to an electromechanical system that serves as a prototype for the first mode of an electrostatically shaped membrane. Electrostatically shaped membranes have been proposed as mirrors and antennas since the early 1960s because they can be used as lightweight reflectors for radar, radio, and optics applications. Lightweight reflectors are in demand, for example, in spacecraft applications where launch weight is a significant constraint. A thin, electrically conducting membrane is formed into a desired shape by electrostatic forces that are controlled by varying the electrical potential between the membrane and an electrode mounted below it. Because the membrane is under lateral in-plane tension and a uniform normal stress due to the electrostatic potential, it assumes a paraboloidal shape for optics applications. Since the focal length can be varied by changing the gap distance, electrostatically controlled membranes are particularly suitable for adaptive optics applications. Small focal lengths needed for many applications can be achieved if the gap distance between the membrane and the fixed plate is made sufficiently small View full abstract»

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  • Fault-tolerant control in dynamic systems: application to a winding machine

    Page(s): 33 - 49
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (852 KB)  

    The general fault-tolerant control method described in the article addresses actuator and sensor faults, which often affect highly automated systems. These faults correspond to a loss of actuator effectiveness or fault sensor measurements. After describing these faults, a fault estimation and compensation method was proposed. In addition to providing information to operators concerning the system operating conditions, the fault diagnosis module is especially important in fault-tolerant control systems where one needs to know exactly which element is faulty to react safely. The method's abilities to compensate for such faults are illustrated by applying it to a winding machine, which represents a subsystem of many industrial systems. The results show that once the fault is detected and isolated, it is easy to reduce its effect on the system, and process control is resumed with degraded performances close to nominal ones. Thus, stopping the system immediately can be avoided. However, the limits of this method are reached when there is the complete loss of an actuator. In this case, only a hardware redundancy is effective and could ensure performance reliability. The method proposed here assumes the availability of the state variables for measurement View full abstract»

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Richard D. Braatz
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