|HTN Planning + Constraint Posting | Teamwork Theories | Human-Agent Interaction | Back|
HTN Planning and Constraint Posting
Hierarchical Task Network (HTN) planning is an abstraction-based plan representation that allows an agent
to successively refine planning decisions. As the name itself suggests, HTN planning
is the natural choice for planning performance in hierarchical organisations because
such approach distinguishes actions and goals of different degrees of abstraction and
importance for each level.
We are joining the HTN ideas with an underlying constraint-based representation of plans. In this way, the planning framework can employ powerful problem solvers based on search and constraint reasoning methods, and still retain human intelligibility of the overall process and plans that are created. Constraints have several interesting properties, some of them complementary to the abilities of HTN planning:
|Requirement 1: the temporal planning model must be based on an explicit timeline approach, which must enable the representation of both quantitative and qualitative temporal references as well as relations between them.|
|Requirement 2: the resource planning model must support the tasks of localising services/agents that provide specified capabilities, and also provide information that enables reasoning on such capabilities.|
|Requirement 3: the collaborative model must consider the establishment of commitments to joint activities, enabling consensus on plans or their constituent parts.|
|Requirement 4: the collaborative model must provide ways to the dissemination of information associated with progress, completion and failure of activities.|
|Requirement 5: the collaborative model must underline the idea of mutual support, providing ways to the specification of useful information sharing mechanisms and creation of supportive activities.|
Note that, as discussed in previous works, collaboration between different problem-solving components, such as planning agents, must be designed into systems from the start. It cannot be patched on. In this way, coalition planning processes need to be designed on a collaborative framework that ensures commitments of individual components in carrying out their activities, considering the global coalition objective.
|Requirement 6: the human-agent model must enable the definition of adjustable methods that complement the decision making process of human users.|
|Requirement 7: the human-agent model must provide ways to restrict user options in accordance with the global coalition decisions.|
These requirements can be seen as a problem of finding a suitable level of autonomy to the agents. Depending on this level of autonomy, agents can only carry out user commands or, at the other extreme, replace human reasoning, making all necessary decisions. If the agents' autonomy is adjusted to a correct degree, this will allow them to exploit human abilities to improve their performance, but without becoming overly dependent or intrusive in their human interaction. Research in adjustable autonomy considers this idea, encompassing the strategies by which an agent
selects the appropriate entity such as itself, a human user, or another agent, to make a
decision at key moments when an action is required. These strategies can vary the level of autonomy of agents so that they require a different level of guidance depending on the current situation.
There are two different directions to formulate adjustable autonomy: agent-based and user-based approaches. In the agent-based approach to adjustable autonomy, each agent explicitly reasons by itself about whether and when to transfer decision-making control to another entity. In the agent-based approach each agent explicitly reasons by itself about whether and when to transfer decision-making control to another entity. Differently, the user-based approach to adjustable autonomy explores a human-centred perspective, so that the central issue for this approach is the design of mechanisms by which an user can dynamically modify the scope of autonomy for an agent.
Analysing projects such as TRAINS/TRIPS and O-Plan, we can note that they use the concept of Mixed-Initiative to implement mechanisms to transfer control between agents and humans. Such mechanisms are a key aspect of the adjustable autonomy process so that mixed-initiative interaction can be used to adjust the degree of agents' autonomy. The implementation of adjustable autonomy on the mixed-initiative perspective is a suitable alternative because research in mixed-initiative has also been investigating two fundamental requirements of human-computer interaction, which can be expressed as:
|Requirement 8: the human-agent model must support the definition of mechanisms that intensify the human user control and enable the customisation of solutions.|
|Requirement 9: the human-agent model must support the generation of explanations about autonomous decisions, clarifying the reasons why they were taken.|