Institute for Systems Research

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Author: search.filters.author.Hristu, DimitriosSubject: search.filters.subject.linear systems

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    Experimenting with Hybrid Control
    (2000) Brockett, Roger W.; Hristu, Dimitrios; ISR; CDCSS
    There is a growing realization among educators andemployers that students of automatic control should be encouraged tothink of the subject in broader terms. The systems approach shouldembrace communication requirements, signal processing, data logging,etc. all the way up to and including the level of complexity suggestedby the phrase "enterprise control." Designing a controlexperiment that is illustrative and instructional in this broadersense presents a number of challenges beyond those discussedabove. The systems under consideration must be very flexible. Ofcourse the hardware must continue to be reliable and relatively easyto understand at an intuitive level. They should also reflect thecomplexity of purpose and the possibility of multi-modal operationthat one expects to find in complex systems. With these qualities inmind, we have assembled and extensively exercised an experimentalhybrid control system for use in an instructional/research laboratoryat Harvard. Our goal with this paper is to describe for others thestructure of the system and to present a sample of the experimentsthat were facilitated by it.

    An important feature of the facility we describe is that it uses severaltypes of sensing modalities including position sensing, tactile sensingand more conventional vision sensing. It can interact with objectsof different complexity and is subject to communication constraints arising in a completelynatural and generic way. In constructing it we have used off-the-shelfcomponents wherever possible and made choices with an eye towardflexibility and reliability.

    The research and scientific content in this material has been submitted to the IEEE Control Systems Magazine.
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    Generalized Inverses for Finite-Horizon Tracking
    (2000) Hristu, Dimitrios; ISR; CDCSS
    Control and communication issues aretraditionally "decoupled" in discussions of decision and controlproblems, as this simplifies the analysis and generally works well forclassical models. This fundamental assumption deserves re-examinationas control applications spread into new areas where system complexityis significant. Such areas include the coordinated control of aerialvehicles (UAVs), MEMS devices, multi-joint manipulators and othersettings where many systems must share the attention of adecision-maker. We consider a new class ofsampled-data systems (termed "computer-controlled systems") thatoffer the possibility of jointly optimizing between control andcommunication goals. Computer-controlled LTI systems can be viewed aslinear operators between appropriate inner-product spaces. Thegeneralized inverses of these operators are used to solve a class offinite-horizon tracking problems.

    This work was presented at the IEEE Conf. on Decision and Control, Dec. 1999.
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    The Dynamics of a Forced Sphere-Plate Mechanical System
    (2000) Hristu, Dimitrios; ISR; CDCSS
    We study the dynamics and explore thecontrollability of a family of sphere-plate mechanical systems. Theseare nonholonomic systems with a five-dimensional configuration spaceand three independent velocities. They consist of a sphere rollingin contact with two horizontal plates. Kinematic models ofsphere-plate systems have played an important role in the controlsystems literature addressing the kinematics of rolling bodies, aswell as in discussions of nonholonomic systems. However, kinematicanalysis falls short of allowing one to understand the dynamicbehavior of such systems. In this work we formulate and study adynamic model for a class of sphere-plate systems in order to answerthe question: "Is it possible to impart a net angular momentum to asphere which rolls without slipping between two plates, given thatthe position of the top plate is subject to exogenousforces?"

    The research and scientific content in this material will appearin IEEE Transactions on Automatic Control.
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    Stabilization of LTI Systems with Communication Constraints
    (1999) Hristu, Dimitrios; ISR; CDCSS
    This work is directed towards exploring interactions ofcommunication and control in systems with communication constraints.Examples of such systems include groups of autonomousvehicles, MEMS arrays and systems whose sensors and actuators aredistributed across a network. We extend some recent results involvingthe stabilization of LTI systems under limited communication andaddress a class of feed-forward control problems for the systems ofinterest.