Solution 4: Blackboard Architecture

Mobile Robot

Solution 4: Blackboard Architecture

This figure describes a blackboard architecture for mobile robots. This paradigm was used in the NAVLAB project, as part of the CODGER system [Shafer86].

< A Blackboard Architecture

The "whiteboard" architecture, as it is named in [Shafer86], works with abstractions reminiscent of those encountered in the layered architecture. The components of CODGER are:

The "captain", the overall supervisor.
The "map navigator", the high level path planner.
The "lookout", a module that monitors the environment for landmarks.
The "pilot", the low level path planner and motor controller.
The perception subsystem, the modules that accept the raw input from multiple sensors and integrate it into a coherent interpretation.

(R1) The components (including the modules inside the perception subsystem) communicate via the characteristic central database of the blackboard systems. Modules indicate their interest in certain types of information. The database returns them such data either immediately or when some other module inserts them into the database.

For instance, the lookout may watch for certain geographic features; the database informs it when the perception subsystem stores images matching the description.

One difficulty with the CODGER architecture is that all control flow has to be coerced to fit the database mechanism, even under circumstances where direct interaction between components would be more natural.

(R2) The blackboard is also the means for resolving conflicts or uncertainties in the robot's world view. For instance, the lookout's landmark detections provide a reality check for the distance estimation by dead-reckoning, both stored in the database. The modules responsible for the uncertainty resolution register with the database to obtain the necessary data.

The main example of this activity is sensor fusion, performed by the perception subsystem to reconcile the input from its diverse sensors.

(R3) The communication via the database is similar to the communication via TCA's central message server. The exception mechanism, wiretapping and monitoring - guarantors of reaction speed, safety, and reliability - can be implemented in CODGER by defining separate modules that watch the database for the tell-tale signs of unexpected occurrences or the beginnings of troublesome situations. TCA's safety mechanism of double-checking messages through wiretaps cannot be fully duplicated because it may be too late to prevent an action once it manifests itself in the database. (TCA holds the message while the wiretap processes it.)

(R4) As with TCA, the blackboard architecture offers support for concurrency and decouples senders from receivers, thus gaining flexibility for maintenance.

In summary, the blackboard architecture is capable of modeling the cooperation of tasks, both for coordination and uncertainty resolution in a very flexible manner, thanks to an implicit invocation mechanism based on the contents of the database. These features are only slightly less powerful than TCA's equivalent capabilities.

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Updated Halloween 95 by Mary Shaw
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