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Sense - Plan - Act

Timelines are the first pillar of T-REX. Ubiquitous, embedded planning is the second.

Goal-directed Feedback Control

We think of a control loop for an agent as having a goal to accomplish, in the face of feedback from the environment. T-REX calls such a control loop a Teleo-Reactor reflecting its goal-directed (teleo) and reactive nature. A Teleo-Reactor encapsulates three components of control:

The concept of control as a continual process of Sense, Plan, Act is not new. It made its debut with Shakey back in the 60s, but fell out of favor for a variety of reasons, including the fact that planning was slow, and thus the robot was not very reactive. This evolution of robot control architectures is described here. Computers and planning technologies have come along way since then, making planning much faster, and more sophisticated.

The Deliberative Reactor

T-REX includes a particular kind of Teleo-Reactor that is implemented with an embedded temporal planning system (EUROPA). We call this a Deliberative Reactor since it utilizes a deliberative planner (i.e. one that can look ahead in formulating a plan) at the core of its control loop. The figure below depicts the internal structure of a Deliberative Reactor .

deliberative_reactor.png

The Plan Database is the store for reactor state in the form of timelines and tokens as described here. This database actively propagates rules and constraints associated with a partial plan. The roles of sensing, planning and acting are played by the Synchronizer, the Planner, and the Dispatcher respectively. Keep in mind the notion of a timeline in the context of execution described earlier. Observations are input to the reactor from the external world, expressing new values arising on timelines. They are converted into tokens and inserted into the relevant timeline by the Synchronizer. Goals are desired values for timelines and again are expressed as tokens in the plan database. The Planner tracks goal requests, and monitors the database for new flaws. It will activate whenever the plan is incomplete, and resolve the matter. The Dispatcher is responsible for communicating planned timeline values to the responsible actors external to the reactor. If the timeline is internal, there is nothing to dispatch. The Synchronizer publishes observations for any timelines internal to the reactor.

Benefits of the Deliberative Reactor

The Deliberative Reactor is something of a wonder widget, since this basic building block can be applied in a wide variety of scenarios, customized by the set of timelines instantiated in the database, the time horizon over which it must plan, the model it uses, and on perhaps specialized plug-ins for the planner or the plan database as permitted by the EUROPA libraries modular design. This offers a great degree of software re-use in building a controller. In addition to the obvious advantages of software re-use, the planning centric control model implemented in the Deliberative Reactor offers a number of benefits:

Drawbacks

There are drawbacks. Chief among them are:

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2023-10-28 13:08