3.5 Coordinating Joint Activity

Communication and a meta-planner operator jointly form the mechanism whereby joint activities are coordinated in MOVERS-WORLD. The actors do not have sufficiently powerful models of each other to coordinate principally by observation [37] or plan recognition [38]. When cooperation is first established during communication, the actors must determine how they will coordinate the cooperative activity. Sometimes, nothing need be done - for example, if two lifters are both adjacent to a ready-to-be-lifted box and both are ready to lift it. More often, though, the requester will have to idle for one or several rounds - for example, if the requestee lifter is not currently ready to lift the box. Another common situation is when a hand-truck operator idles after a lifter agrees to load a box onto a hand-truck. Idling is presently implemented by adding a WAIT to the beginning of her plan. While the WAIT is at the beginning of her plan, an actor is waiting for one of two events to occur: communication indicating that joint action can occur (e.g., the other lifter now indicates she is ready to act) or the completion of her request (e.g. the box appears on the hand-truck). If an actor is idle too long, she will become frustrated and inquire about the status of her request, possibly discovering the other actor has opted out.

Wait operators are never generated directly by the planner, but they can become part of the actor's plan in two ways (see Table 1). Either an actor is explicitly told to wait during a dialog or an actor adds a wait operator during offline learning (see Section 4.4).

 

Table 1: Kinds of wait operators.

Told to Wait	Learn to Wait
WAIT	WAIT-FOR-REQUEST
	WAIT-IMPLICIT

 

There are two variants of the WAIT operator contained in coordinated procedures. They are functionally equivalent to WAIT, but there are semantic differences which are relevant during communication. The first variant, WAIT-FOR-REQUEST (abbreviated as WAIT-FOR), is introduced whenever an actor agreed to a request during a previous activity. It acts as a place-holder to represent when the actor expects a request to be made; during communication, this must be treated differently than WAIT, which is only present when the actor has an explicit agreement. The second variant, WAIT-IMPLICIT (WAIT-IMP for short), is an optimization. A WAIT-IMP replaces a SIGN operator which would be requesting a service to be performed. For example, with time, the hand-truck operator can learn when to expect the lifter to load the hand-truck without explicitly being told to do so. Thus, a WAIT-IMP is an optimistic plan modification: the actor expects a service by another actor without having to ask for it.

The responses actors give during communication depend, in part, on how their current plan relates to the incoming request. Since WAIT operators represent past and potential agreements (important relationships between plans and requests), responses are often strongly influenced by their presence.