David A. Schoenwald

Decentralized control of cooperative robotic vehicles: theory and application

Describes how decentralized control theory can be used to analyze the control of multiple cooperative robotic vehicles. Models of cooperation are discussed and related to the input/output reachability, structural observability, and controllability of the entire system. Whereas decentralized control research in the past has concentrated on using decentralized controllers to partition complex physically interconnected systems, this work uses decentralized methods to connect otherwise independent nontouching robotic vehicles so that they behave in a stable, coordinated fashion. A vector Liapunov method is used to prove stability of two examples: the controlled motion of multiple vehicles along a line and the controlled motion of multiple vehicles in formation. Also presented are three applications of this theory: controlling a formation, guarding a perimeter, and surrounding a facility.

Decentralized Control of Cooperative Robotic Vehicles

This paper describes how decentralized control theory can be used to control multiple cooperative robotic vehicles. Models of cooperation are discussed and related to the input/output reachability and structural observability and controllability of the entire system. Whereas decentralized control research in the past has concentrated on using decentralized controllers to partition complex physically interconnected systems, this work uses decentralized methods to connect otherwise independent non-touching robotic vehicles so that they behave in a stable, coordinated fashion. A vector Liapunov method is used to prove stability of a single example: the controlled motion of multiple vehicles along a line. The results of this stability analysis have been implemented on two applications: a robotic perimeter surveillance system and self-healing minefield.

Aspen-EE: An Agent-Based Model of Infrastructure Interdependency

This report describes the features of Aspen-EE (Electricity Enhancement), a new model for simulating the interdependent effects of market decisions and disruptions in the electric power system on other critical infrastructures in the US economy. Aspen-EE extends and modifies the capabilities of Aspen, an agent-based model previously developed by Sandia National Laboratories. Aspen-EE was tested on a series of scenarios in which the rules governing electric power trades were changed. Analysis of the scenario results indicates that the power generation company agents will adjust the quantity of power bid into each market as a function of the market rules. Results indicate that when two power markets are faced with identical economic circumstances, the traditionally higher-priced market sees its market clearing price decline, while the traditionally lower-priced market sees a relative increase in market clearing price. These results indicate that Aspen-EE is predicting power market trends that are consistent with expected economic behavior.

PV output smoothing with energy storage

This paper describes a simple algorithm designed to reduce the variability of photovoltaic (PV) power output by using an energy storage device. A full-scale implementation was deployed in an actual PV-Energy demonstration project, in partnership with a utility and a battery manufacturer. The paper describes simulation tests as well as field results. In addition to demonstrating implementation of smoothing controls, this work also served to verify the models, identify best parameter sets for utility operations, and study the operation of an advanced energy storage system under partial state of charge and rapid, irregular charge/discharge cycling.

Decentralized control of a collective of autonomous robotic vehicles

In this paper, the performance of a group of autonomous vehicles tracking a prescribed goal is analyzed. The vehicles are considered to be ground-based unmanned robots acting as a group to maintain an unbroken communication network in a building or some other region. Vehicle interactions are modeled as a chain of interconnected systems. The stability of the entire collective as well as individual vehicles is studied using largescale systems theory. Stability can be controlled via two key parameters: vehicle speed constant (maximum vehicle speed times sample time) and vehicle interaction gain. In addition to the stability analysis, simulation of a group of vehicles in a building with walls, doors, and other obstacles is studied with respect to maintaining a communication network among the vehicles at all times.

Minimum-time trajectory control of a two-link flexible robotic manipulator

An analysis is made of the experimental results of a minimum-time trajectory control scheme for a two-link flexible robot. An offline optimization routine determines a minimum-time, straight-line tip trajectory which stays within the torque constraints of the motors and ends with no vibrational transients. An efficient finite-element model is used in the optimization to approximate the flexible arm dynamics. The control strategy described is used to determine the feedback gains for the position, velocity, and strain gage signals from a quadratic cost criterion based on the finite-element model linearized about the straight-line tip trajectory. These feedback signals are added to the modeled torque values obtained from the optimization routine and used to control the robot arm actuators. The results indicate that this combination of model-based and error-driven control strategies, achieves a closer tracking of the desired trajectory and a better handling of modeling errors than either strategy alone.<<ETX>>

Damping of inter-area oscillations using energy storage

Low frequency inter-area oscillations have been identified as a significant problem in utility systems due to the potential for system damage and the resulting restrictions on power transmission over select lines. Previous research has identified real power injection by energy storage based damping control nodes as a promising approach to mitigate inter-area oscillations. In this paper, a candidate energy storage system based on UltraCapacitor technology is evaluated for damping control applications in the Western Electric Coordinating Council (WECC), and an analytical method for ensuring proper stability margins is also presented for inclusion in a future supervisory control algorithm. Dynamic simulations of the WECC were performed to validate the expected system performance. Finally, the Nyquist stability criteria was employed to derive safe operating regions in the gain, time delay space for a simple two-area system to provide guaranteed margins of stability.

Protocol for Uniformly Measuring and Expressing the Performance of Energy Storage Systems

The Protocol for Uniformly Measuring and Expressing the Performance of Energy Storage Systems (PNNL-22010) was first issued in November 2012 as a first step toward providing a foundational basis for developing an initial standard for the uniform measurement and expression of energy storage system (ESS) performance. Its subsequent use in the field and review by the protocol working group and most importantly the users’ subgroup and the thermal subgroup has led to the fundamental modifications reflected in this update of the 2012 Protocol. As an update of the 2012 Protocol, this document (the June 2014 Protocol) is intended to supersede its predecessor and be used as the basis for measuring and expressing ESS performance. The foreword provides general and specific details about what additions, revisions, and enhancements have been made to the 2012 Protocol and the rationale for them in arriving at the June 2014 Protocol.

Creating dynamic equivalent PV circuit models with impedance spectroscopy for arc fault modeling

Article 690.11 in the 2011 National Electrical Code® (NEC®) requires new photovoltaic (PV) systems on or penetrating a building to include a listed arc fault protection device. Currently there is little experimental or empirical research into the behavior of the arcing frequencies through PV components despite the potential for modules and other PV components to filter or attenuate arcing signatures that could render the arc detector ineffective. To model AC arcing signal propagation along PV strings, the well-studied DC diode models were found to inadequately capture the behavior of high frequency arcing signals. Instead dynamic equivalent circuit models of PV modules were required to describe the impedance for alternating currents in modules. The nonlinearities present in PV cells resulting from irradiance, temperature, frequency, and bias voltage variations make modeling these systems challenging. Linearized dynamic equivalent circuits were created for multiple PV module manufacturers and module technologies. The equivalent resistances and capacitances for the modules were determined using impedance spectroscopy with no bias voltage and no irradiance. The equivalent circuit model was employed to evaluate modules having irradiance conditions that could not be measured directly with the instrumentation. Although there was a wide range of circuit component values, the complex impedance model does not predict filtering of arc fault frequencies in PV strings for any irradiance level. Experimental results with no irradiance agree with the model and show nearly no attenuation for 1 Hz to 100 kHz input frequencies.

Stability Analysis of Decentralized Cooperative Controls

This paper describes how large-scale decentralized control theory may be used to analyze the stability of multiple cooperative robotic vehicles. Models of cooperation are discussed from a decentralized control system point of view. Whereas decentralized control research in the past has concentrated on using decentralized controllers to partition complex physically interconnected systems, this work uses decentralized methods to connect otherwise independent non-touching robotic vehicles so that they behave in a stable, coordinated fashion. A vector Liapunov method is used to prove stability of two examples: the controlled motion of multiple vehicles along a line and the controlled motion of multiple vehicles in a plane.