Swarm Robot Project Simulator

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(Recording a movie)

Revision as of 22:06, 24 May 2009

The simulator is a part of the Swarm Robot Project that attempts to model the robots using MATLAB. It is built around a modular control structure so that it can be easily adapted to run other algorithms. Features include: asynchronous update cycles, a packet model, lossy communication, and flexible rendering options.

Contents

Getting started

To run the simulator:

  1. Download the zip file and extract the folder.
  2. Open simulator.m and press Run.
  3. If prompted, select "Change Directory".

What you're seeing

This version implements the formation control algorithm. The swarm will start centered around a random location and then converge on the goal moments (shown as the stationary green ellipse). The user may click a location on the grid to restart the simulation with the robots centered around that point.

Constants

The constants at the top of the M-file (denoted by all-caps) give the user a great deal of control over the performance and rendering of the simulation without having to understand the code.

See the inline comments for details.

Control structure

Control structure flowchart. Blue boxes represent functions that are defined in their own files.

The control structure represents the decentralized code that runs on the individual e-pucks. It is written modularly and without extraneous dependencies so it can be readily adapted to other algorithms.

The whole control structure is called from updateBotComputations. Information about neighbors flows to and from the consensus estimator via the packet model.

Collision avoidance

The velocity output from the Controller is put through a collision avoidance algorithm. It is not shown in the flowchart because the algorithm has access to the positions of all the agents (information not known to any single agent).

The present algorithm does not work very well.

Environment

The environment encapsulates the control structure, simulating wireless communications, passage of time, and the motion of the robots.

Time model

The simulation advances in timesteps; equally spaced spans of time. Timesteps are the unit used in specifying the distance between discrete events (ie. updating robot calculations or rendering).

The main loop runs once per timestep. It moves the robots along their chosen velocity vectors every timestep and makes calls to time-dependent update functions as needed.

Data model

Since all state data in the simulation can be associated with a robot, the only stored variable is a vector of N robots called bots. Each bot is individually represented as a structure, with the following fields:

  • bots vector of N robots
    • state information the robot knows about itself
      • p position (arranged randomly inside a square of side length SIDE, centered about a specified point)
      • angle rotational orientation
    • packets vector of N received packets (packet i of bot i is that robot's own estimate and does not age like the rest)
      • age timesteps since received
      • v PI decision variable
      • w PI estimator state
    • u velocity
    • lastUpdate timestep of last update
    • waitTime timesteps between previous update and next update

Packet model

Consensus information is transferred between the e-pucks using wireless communications. The simulator transfers packets between robots on an asynchronous basis and includes the possibility of packet loss as a function of distance. Each robot's packets vector contains the most recently received packet from each of its neighbors. A robot will transmit its own consensus information just before its control structure is executed.

Packet loss is modeled by a cumulative Rice function, as advised by problem of Wi-Fi radio-fading simulation: Solution and applications. As distance between agents increases, the probability of receiving a packet decreases.

Visualizing packet loss

Packet loss can be easily visualized by setting the constant DRAW_CONNECTIONS to true.

Between two agents...

  • a black line means at least one agent has a fresh packet from the other.
  • a red line means neither has fresh data, but at least one is using stale data.
  • no line means neither agent is using data from the other because the last communicated packet is older than PACKET_AGE_LIMIT.

Recording a movie

To record a run of the simulation, set MOVIE_FRAME_COUNT to the desired movie length, in frames. The movie will be saved as "movie.avi" in the same directory as the simulator M-file.

Movies are recorded as uncompressed AVIs because MATLAB does a poor job of compression. See Swarm Robot Project Documentation#Making Videos with Overlays.

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