William Christopher Evans

Comparing and modeling distributed control strategies for miniature self-assembling robots

We propose two contrasting approaches to the scalable distributed control of a swarm of self-assembling miniaturized robots, specifically the formation of chains of a desired length: (1) a deterministic controller in which robots communicate with each other in order to directly limit the size of each chain, and (2) a probabilistic controller where the average chain size is controlled by the probability a robot will choose to leave its chain. We demonstrate the feasibility of both approaches by implementing them on a real swarm of Alice robots. Using Webots, a realistic simulator for mobile robotics, and macroscopic models based on the Chemical Reaction Network (CRN) framework, we investigate the limitations of the deterministic controller and demonstrate the existence of optimal parameters for the probabilistic controller where exploration and exploitation are well balanced, thus favoring the formation of larger chains.

Top-Down vs. Bottom-Up Model-Based Methodologies for Distributed Control: A Comparative Experimental Study

Model-based synthesis of distributed controllers for multi-robot systems is commonly approached in either a top-down or bottom-up fashion. In this paper, we investigate the experimental challenges of both approaches, with a special emphasis on resource-constrained miniature robots. We make our comparison through a case study in which a group of 2-cm-sized mobile robots screen the environment for undesirable features, and destroy or neutralize them. First, we solve this problem using a top-down approach that relies on a graph-based representation of the system, allowing for direct optimization using numerical techniques (e.g., linear and non-linear convex optimization) under very unrealistic assumptions (e.g., infinite number of robots, perfect localization, global communication, etc.). We show how one can relax these assumptions in the context of resource-constrained robots, and explain the resulting impact on system performance. Second, we solve the same problem using a bottom-up approach, i.e., we build up computationally efficient and accurate models at multiple abstraction levels, and use them to optimize the robots’ controller using evolutionary algorithms. Finally, we outline the differences between the top-down and bottom-up approaches, and experimentally compare their performance.

Distributed Spatiotemporal Suppression for Environmental Data Collection in Real-World Sensor Networks

Environmental processes are often severely oversampled. As sensor networks become more ubiquitous for this purpose, increasing network longevity becomes ever more important. Radio transceivers in particular are a great source of energy consumption, and many networking algorithms have been proposed that seek to minimize their use. Traditionally, such approaches are often data agnostic, i.e., their performance is not dependent on the properties of the data they transport. In this paper we explore algorithms that exploit environmental relationships in order to reduce the amount of transmitted data while maintaining expected levels of accuracy. We employ a realistic testing environment for evaluating the power savings brought by such algorithms, based on Sensorscope, a commercial sensor network product for environmental monitoring. We implement and test a suppression-based data collection algorithm from literature that to our knowledge has never been implemented on a real system, and propose modifications that make it more suitable for real-world conditions. Using a custom extension board developed for in situ power monitoring, we show that while the algorithms greatly reduce the amount of energy spent on transmitting packets, they have no effect on the real systems overall power consumption due to its preexisting network architecture.

Evaluating Efficient Data Collection Algorithms for Environmental Sensor Networks

Although there exists a large body of work on efficient data collection in sensor networks, the vast majority of proposed techniques have not been implemented on real networks or thoroughly studied on real data. As algorithm performance is highly dependent on the characteristics of the data being reported, it is very difficult to make suggestions as to the relative performance of any particular method. In this work we seek to compare and evaluate existing approaches to efficient data gathering in the specific context of environmental monitoring.We examine a choice algorithm that has not, to the best of our knowledge, been thoroughly studied on real data. We detail a number of algorithmic modifications necessary to bring it from theory to reality, and study the algorithm’s performance in simulation using extensive traces from real world sensor network deployments.

TEM cell testing of cable noise reduction techniques from 2 MHz to 200 MHz — Part 1

This paper presents empirical results of cable noise reduction techniques as demonstrated in a TEM cell operating with radiated fields from 2 - 200 MHz. It is the first part of a two-paper series. This first paper discusses cable types and shield connections. In the second paper, the effects of load and source resistances and chassis connections are examined. For each topic, well established theories are compared to data from a real-world physical system. Finally, recommendations for minimizing cable susceptibility (and thus cable emissions) are presented.

Environmental field estimation with hybrid-mobility sensor networks

The remarkable accessibility of modern flying robots makes them an attractive platform for environmental sensing. However, low cost and ease of use are currently incompatible with large payloads, severely limiting the choice of sensor and ultimately modality. This paper describes the design of a system for using a small infrared thermometer to estimate the surface temperature over an area that is large compared to the area measured by the sensor, by mounting it on a flying robot. We leverage a priori knowledge about the spatial statistics of the phenomena under measure in order to plan an informative sampling path, fusing observations by Gaussian process regression. Our approach is designed to be evaluated in an indoor testbed, in which a quadrotor, in cooperation with simulated static sensing nodes, estimates the spatial distribution of surface temperature over a controlled thermal gradient. We perform extensive systematic experimentation both in simulation and our real-world testbed environment, with our algorithm estimating surface temperature to an accuracy of up to 2.1 °C over a 16 m2 area ranging in value from 25-65 °C.

Environmental Sensing using Land-based Spectrally-selective Cameras and a Quadcopter

We investigate the reconstruction of an environmental scalar field using robotic mobility and heterogeneous sensing. Using two land-based, immobile, co-located spectrally selective cameras, and a non-contact infraredbased temperature sensor on a quadcopter, we study the problem of reconstructing the surface temperature of the ground under survey. Both land units — a thermographic camera for low-resolution thermal images and a commercial digital camera for high resolution truecolor images — are mounted on an elevated camera rig. We explore methods for field reconstruction using a combination of the three imaging sensors. First, we show that the quadcopter data is correlated with the synoptic snapshots obtained by the thermal imaging camera. Next, we demonstrate upsampling of the low-resolution thermal camera data with truecolor images. This results in high-resolution reconstruction of the temperature field. Finally, we discuss adaptive sampling techniques that utilize the mobility of the quadcopter to ‘fill the gaps’ in data acquired by the thermal imaging camera. Our work experimentally demonstrates the feasibility of heterogeneous sensing and mobility to effectively reconstruct environmental fields.