To investigate issues of cooperative behavior, two experimental environments were utilized in this research: a cooperative robot simulator and a heterogeneous team of physical mobile robots.
The purpose of the simulation experiments was three-fold. First, the simulator was used to to debug the intricacies of the ALLIANCE architecture and to test alter-native strategies to its design. This type of debugging and exploration can be quite dicult to perform on physical robots due to the time required to re-download code to each robot, to recharge the robot batteries, to set up the experimental scenarios, and to debug and correct robot mechanical and/or electronic failures. Second, the cooperative robot simulator extended the scope of applications that could be investi-gated by allowing the construction of a wide variety of robots with various mixtures of sensors and eectors that are not currently available in our laboratory. I made every attempt, however, to keep myself honest by assuming only the existence of sensory and eector devices that are available with current robot technology. And third, the speed of the simulator provided the ability to accelerate \real-time", allowing me a more favorable platform for statistical data collection for many types of experiments.
Of course, years of experience in mobile robot development have shown that ap-proaches to robot control which work in simulated robot worlds are often not success-ful when applied to real mobile robot teams due to unrealistic assumptions made in the simulations [Brooks, 1991a]. It is therefore important when developing any robot control paradigm to validate the proposed system on real, physical robots. Thus, I also implemented cooperative tasks on our laboratory's team of heterogeneous mobile robots.
The results of my experiments in both of these domains are reported throughout this report. This chapter describes these two testbeds in some detail.
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Figure 2-1: A typical indoor environment created using the multi-robot simulator.
2.1 The Cooperative Robot Simulator
The original version of the cooperative robot simulator was developed in the MIT Mobot Laboratory by Yasuo Kagawa, a visiting scientist from the Mazda Corporation.
I modied the internal mechanisms of the original version extensively, however, to improve its response time and lower its memory requirements signicantly, and to increase the available sensory and eector models. This simulator, written in object-oriented Allegro Common Lisp version 1.3.2, runs in the Macintosh environment and is designed to simulate indoor oce-type environments. Figure 2-1 shows a typical oce environment which can be created using this simulator. This example is from the janitorial service mission described in chapter 6.
The simulator provides most of the features one would expect in such a cooperative robot simulator: the ability to create a simulated oce environment with obstacles and walls, to dene sensory and eector devices, to dene robots possessing any given combination of sensors and eectors, and the ability for robots to communicate with each other.
2.2. THE POOL OF HETEROGENEOUS ROBOTS 15 The sensors that have been developed are a ranging sensor, an infrared beacon detector, a compass, a microphone, an x;y positioning system, a pyroelectric sensor, and a dust sensor (see [Yamamoto, 1993] for an example of a real-life dust sensor).
Each robot's movement is commanded by velocity and turn angle values; additional eectors implemented are a oor vacuum, a garbage dumper, and a duster arm that can be extended either right or left1. Note that these additional eectors are not modeled mechanically in any way; they merely act as switches to turn some eect on or o, based on the robot's current position and its proximity to obstacles of certain types. Although all of the sensors and eectors can have a variable amount of random noise added to them, a primary disadvantage of this simulator is that no attempt has been made to accurately model their error proles. One must therefore keep this in mind when evaluating behaviors that are generated with the simulator. In my experiments, I typically used values of 20% random noise added to the sensors and eectors.
Obstacle objects can be of two types | convex polyhedral objects and one-dimensional wall-type objects. These objects can possess a number of additional characteristics, such as the ability to emit an IR beacon or sound at a specied inten-sity, or to possess a certain amount of dust, garbage, or heat. A nice feature of the simulator is that objects can be moved around manually during the robot simulation, thus mimicking a dynamic environment.
A major strength of the robot simulator is that the user-written robot control programs are written in the Behavior Language2 [Brooks, 1990a], which is also the programming language used in the real mobile robot experiments. This allows most of the robot control code developed using the simulator to be easily ported to run on an actual mobile robot. The main exception is, of course, the sensor and eector interfaces, which will be dierent on a physical robot.
2.2 The Pool of Heterogeneous Robots
A primary goal of this research is to allow a human system designer to create new teams of cooperative robots by selecting, from a pool of available robots, those robots which have the proper mix of capabilities for the current application. To enable the demonstration of this capability, I have composed a pool of heterogeneous mobile robots from which I can create various teams with diering group capabilities. Shown in gure 2-2, the Mobot Laboratory's pool of heterogeneous robots consists of two
1These eectors were designed for the janitorial service mission, which is described in chapter 6.
2The Behavior Language is a modied and extended version of the subsumption architecture [Brooks, 1986] that facilitates real-time robot control.
types3of mobile robots | three R-2s and one Genghis-II | all of which were designed and built by IS Robotics Corporation located in Somerville, Massachusetts.
It is important to note, however, that even though our laboratory has duplicate copies of the R-2 robot, signicant variations in the sensitivity and accuracy of their sensors and eectors cause them to have quite dierent true capabilities. It is also possible to modify the morphology of an individual robot in several ways, such as installing a gripper attachment, that greatly aects a robot's capabilities. Thus, a heterogeneous robot team can be composed that consists of only the R-2 type of robot, since the robot behavior varies noticeably.
Of course, when working with specic mobile robots, one is limited in the appli-cations that can be demonstrated by the physical limitations of the available robots.
Thus, the physical robot experiments described in this report have been designed specically with the capabilities of the R-2s and Genghis-II in mind. The ALLIANCE architecture, however, is independent of the specic robot platform on which it is im-plemented.
The next two subsections describe the capabilities of these robots, followed by a description of the radio communication and positioning system that allows the robots to send messages to each other and to determine their own current x;y position relative to a global frame of reference.