A flexible plan step execution model for BDI agents

For the past 20 years, BDI (Belief, Desire, Intention) frameworks such as PRS[1], dMARS [2] and JACK [3] have provided, together with Soar [4], the two environments of choice for the development of knowledge rich, industrial strength intelligent agent applications [5]. However, we have observed that while the BDI model of plan execution works well for the tactical reasoning component of such applications, operational reasoning often requires a richer execution model. In this paper, we present an alternative, but complementary model for plan step execution by BDI agents. In the BDI model, plan steps either succeed or fail; if a plan step fails, then the plan fails and reconsideration of the current goal may occur. We have found that this approach is problematic when used for applications where resource contention is a regular occurrence, such as in manufacturing execution [6]. In these situations, it is necessary to review progress after each step, regardless of the step outcome. Our alternative model for plan step execution allows for the explicit modelling of the plan step lifecycle and the utilisation of infrastructure to manage the progression of that lifecycle. The model is realised using the JACKTM Intelligent Agents (JACK) product suite [3] and its feasibility is demonstrated through the development of an execution system for a robotic assembly cell.