Paul Anthony Kawka

Closed-loop control over wireless networks

This article includes an implementation of an 802.11b wireless control loop with high communication rates and a timing scheme that uses clock driven sensing and actuation with event-driven control. The timing scheme was implemented using standard PCs and an 802.11b peer-to-peer wireless network. The control scheme was scaled up to a multimode multiplant arrangement with coordination among loops. To examine the wireless control loop performance, we use the rotating base variation of the linear inverted pendulum. Using the wireless control loop, we can examine a number of the characteristics of UDP data transmission over 802.11b.

Stability and feedback control of wireless networked systems

This study analyzes data losses and their effect on wireless networked system stability/performance using a simple communication scheme and a stochastic 2-state Markov network model. An architecture utilizing event-driven control and clock-driven, co-located sensing and actuation serves as the base framework. Data losses are treated in a switched system scheme where two discrete dynamics are available. When the network transmits data successfully, the closed loop dynamics are active; when packets are dropped, zero control is applied and open loop dynamics exist. Using analysis tools for stochastic stability of Markov jump linear systems, operating conditions in a probability space can be identified for given plant dynamics that ensure second moment stability (SMS). Additionally, switched system H/sub /spl infin// norms can be used to identify regions of network operation in the probability space with expected levels of performance. Results from these tools and experiments involving wireless hardware suggest a scheme for decentralized controller variation that rebalances network loading for the control loops when communication disturbances occur in the network. This scheme varies the sampling period of the individual loops based on network condition measurements, system stability/performance requirements, and computation/bandwidth limits of the hardware.

Stability and performance of packet-based feedback control over a Markov channel

This paper examines a packet-based control strategy to improve the stability and performance of a networked control system with transmission losses that follow a stochastic 2-state Markov model. Typically, in a discrete time network control framework, a single control value is transmitted during each sample period. In a packet-based environment, however, there is no penalty for sending additional information in each transmission up to the fixed minimum packet size. When an accurate model of the plant dynamics is available, estimated control values can be included in each packet and applied when transmission delays are longer than the sample period or dropouts occur. The effects of data losses are analyzed using a discrete dynamics switched system framework. When the network transmits data successfully, standard closed loop dynamics exist using the latest feedback measurements. When packets are dropped, an alternate open loop set of dynamics are active. These dynamics incorporate the estimated control values that are applied when information is lost or delayed. Using analysis tools for stochastic stability of Markov jump linear systems, network operating conditions can be identified for given plant dynamics that ensure second moment stability (SMS) for zero, TV, or infinitely many estimated control values per packet. Additionally, a bounded real lemma for stochastic switched system Hinfin norms enables comparison of the expected disturbance to output performance for different numbers of control estimates. Analysis and experiments using an inverted pendulum show that this system obtains the greatest incremental stability and performance benefit from the first estimated control value

Robust Wireless Servo Control Using a Discrete-Time Uncertain Markovian Jump Linear Model

This paper proposes a robust control synthesis technique for wireless servo applications modeled as a uncertain discrete-time Markovian jump linear system. It is desired to find mean square stabilizing state feedback controllers with upper bounded quadratic cost when the transition probabilities of the Markov chain describing the network conditions, the state space dynamics, and the initial condition are unknown but belong to known convex polytopic sets. Under appropriate assumptions, these parametric uncertainties can be examined simultaneously, and controllers can be synthesized using linear matrix inequalities. The LMIs utilize extended parameters to reduce conservativeness due to initial condition and plant uncertainty. The proposed method is used to design and demonstrate a robust wireless servo controller for an inverted pendulum system.

An Analysis Framework for Evaluating Dropout Compensation Strategies in Wireless Servo Systems

This paper presents an approach for analyzing the performance of dropout compensation strategies for linear servo control systems operating over communication channels with losses. A loss of communication causes the normal control action to be replaced by an appropriately designed dropout compensation action. A linear matrix inequality based approach is given for examining H ∞ and/or H 2 performance of wireless feedback systems using dropout compensation where we assume a two-state Markov model for the communication network. To illustrate the analysis method, we introduce two specific data dropout compensation schemes: zero order hold and estimation. These two schemes are compared in simulation and experiment to validate the effectiveness of the performance analysis.

Robust wireless servo control using a discrete-time uncertain Markovian jump linear model

This paper proposes a robust control synthesis technique for wireless servo applications modeled as a uncertain discrete-time Markovian jump linear system. It is desired to find mean square stabilizing state feedback controllers with upper bounded quadratic cost when the transition probabilities of the Markov chain describing the network conditions, the state space dynamics, and the initial condition are unknown but belong to known convex polytopic sets. Under appropriate assumptions, these parametric uncertainties can be examined simultaneously, and controllers can be synthesized using linear matrix inequalities. The LMIs utilize extended parameters to reduce conservativeness due to initial condition and plant uncertainty. The proposed method is used to design and demonstrate a robust wireless servo controller for an inverted pendulum system.

UDP Network Communications for Wireless Control With Loop Coordination

With the advent of Bluetooth and wireless 802.11 Ethernet protocols having transmission speeds up to 54 Mbps, wireless communication for closed-loop control is becoming more and more achievable. Some researchers have utilized Bluetooth networks for wireless control, resulting in successful stabilization of an unstable plant with a network controller. Previously, the authors of this paper developed a novel event-based control with time-based sensing and actuation communication method using 11 Mbps wireless Ethernet with the user datagram protocol (UDP). Near real-time control of an unstable Furuta pendulum with up to 250 Hz closed loop bandwidth was obtained using off the shelf hardware, Matlab, and Windows 2000 operating systems. The present work extends that communication scheme to two independent wireless loops that share a mutual goal, making additional communication between the two controllers advantageous. The communication framework for the coupled control in this ad hoc peer-to-peer network is presented along with some practical limitations. Data from a physical system implementing this framework demonstrates its effectiveness in application. The test plant couples a simple light tracking plant with a Furuta pendulum and a shared goal of maintaining line of sight (LOS) under normal conditions as well as reestablishing LOS in the case of lost contact due to sudden obstacles.Copyright

Discussion on: “Development and Experimental Verification of a Mobile Client-Centric Networked Controlled System”

The paper by A. Tzes, G. Nikolakopoulos and I. Koutroulis examines mobile client-centric network control motivated by the potential use of General Purpose Radio Service (GPRS) over mobile phone infrastructures for control applications. The authors in this work begin to address two key requirements for control utilizing GPRS, namely the need for clientcentric communication and tolerance of variable and large periods of delay which is the primary contribution. Linear matrix inequality (LMI) techniques for closed-loop system stability analysis with delay are presented along with simulation, experimental tracking and network performance results showing their applicability. Additionally, the experiments demonstrate one of the first tests of a GPRS network for a simulated control application.