Institute for Systems Research
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Item Stabilization of LTI Systems with Communication Constraints(2001) Hristu, Dimitrios; ISR; CDCSSThis work is aimed at exploring the interaction of communication andcontrol in systems whose sensors and actuators are distributed across ashared network. Examples of such systems include groups of autonomousvehicles, MEMS arrays and other network-controlled systems. Wegeneralize recent results concerning the stabilization of LTI systemsunder limited communication. We seek a stabilizing static outputfeedbackcontroller whose communication with the underlying plant follows a givenperiodic pattern. We present an algorithm that allows us to pass to atime-invariant formulation of the problem and use simulated annealing tosearch for stabilizing feedback gains.Item The MDLe Engine -- A Software Tool for Hybrid Motion Control(2000) Hristu, Dimitrios; Krishnaprasad, Perinkulam S.; Andersson, Sean B.; Zhang, F.; Sodre, P.; D'Anna, L.; ISR; CDCSSOne of the important but often overlooked practical challenges in motion control for robotics and other autonomous machines has to do with the implementation of theoretical tools into software that will allow the system to interact effectively with the physical world. More often than not motion control programs are machine-specific and not reusable, even when the underlying algorithm does not require any changes.The work on Motion Description Languages (MDL) has been an effort to formalize a general-purpose robot programming language that allows one to incorporate both switching logic and differential equations. Extended MDL (MDLe) is a device-independent programming language for hybrid motion control, accommodating hybrid controllers, multi-robot interactions and robot-to-robot communications.
The purpose of this paper is to describe the "MDLe engine," a software tool that implements the MDLe language.
We have designed a basic compiler/software foundation for writing MDLe code. We provide a brief description of the MDLe syntax, implementation architecture, and functionality. Sample programs are presented together with the results of their execution on a set of physical and simulated mobile robots.
Item Robot Formations: Learning Minimum-Length Paths on Uneven Terrain(2000) Hristu, Dimitrios; ISR; CDCSSWe discuss a prototypeproblem involving terrain exploration and learning by formations ofautonomous vehicles. We investigate an algorithm forcoordinating multiple robots whose task is to find the shortest pathbetween a fixed pair of start and target locations, without access toa "global" map containing those locations.Odometry information alone isnot sufficient for minimizing path length if the terrain is uneven orif it includes obstacles. We generalize existing results on a simplecontrol law, also known as "local pursuit," which is appropriate inthe context of formations and which requires limited interactionbetween vehicles.
Our algorithm is iterative and converges to alocally optimal path. We include simulations and experimentsillustrating the performance of the proposed strategy.
The research and scientific content in this material has been published in the IEEE Mediterranean Conference on Control and Automation, July 2000. Item Experimenting with Hybrid Control(2000) Brockett, Roger W.; Hristu, Dimitrios; ISR; CDCSSThere 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. Item Generalized Inverses for Finite-Horizon Tracking(2000) Hristu, Dimitrios; ISR; CDCSSControl 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. Item The Dynamics of a Forced Sphere-Plate Mechanical System(2000) Hristu, Dimitrios; ISR; CDCSSWe 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. Item The Performance of a Deformable-Membrane Tactile Sensor: Basic Results on Geometrically-Defined Tasks(1999) Hristu, Dimitrios; Ferrier, Nicola J.; Brockett, Roger W.; ISR; CDCSSThe limitations of rigid fingertips in the precise andalgorithmic study of manipulation have been discussed in many works,some dating back more than a decade. Despite that fact, much of thework in dexterous manipulation has continued to use the"point-contact" model for finger-object interactions. In fact, mostexsisting tactile sensing technologies are not adaptable todeformable fingertips.In this work we report on experimentalresults obtained with a deformable tactile sensor whose properties arewell-suited to manipulation. The results presented here show that thesensor described provides a rich set of tactile data.
Item Stabilization of LTI Systems with Communication Constraints(1999) Hristu, Dimitrios; ISR; CDCSSThis 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.