Distributed Control for Cyber-Physical Systems
Abstract
Networked Cyber-Physical Systems (CPS) are fundamentallyconstrained by the tight coupling and closed-loop control and actuationof physical processes. To address actuation in such closed-loop wirelesscontrol systems there is a strong need to re-think the communication architecturesand protocols for maintaining stability and performance in thepresence of disturbances to the network, environment and overall systemobjectives. We review the current state of network control efforts forCPS and present two complementary approaches for robust, optimal andcomposable control over networks. We first introduce a computer systemsapproach with Embedded Virtual Machines (EVM), a programmingabstraction where controller tasks, with their control and timing properties,are maintained across physical node boundaries. Controller functionalityis decoupled from the physical substrate and is capable of runtime migrationto the most competent set of physical controllers to maintain stabilityin the presence of changes to nodes, links and network topology.
We then view the problem from a control theoretic perspective todeliver fully distributed control over networks with Wireless Control Networks(WCN). As opposed to traditional networked control schemes wherethe nodes simply route information to and from a dedicated controller, ourapproach treats the network itself as the controller. In other words, thecomputation of the control law is done in a fully distributed way inside thenetwork. In this approach, at each time-step, each node updates its internalstate to be a linear combination of the states of the nodes in its neighborhood.This causes the entire network to behave as a linear dynamicalsystem, with sparsity constraints imposed by the network topology. Thiseliminates the need for routing between “sensor → channel → dedicatedcontroller/estimator → channel → actuator”, allows for simple transmissionscheduling, is operational on resource constrained low-power nodesand allows for composition of additional control loops and plants. Wedemonstrate the potential of such distributed controllers to be robust to ahigh degree of link failures and to maintain stability even in cases of nodefailures.
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