INTELLIGENT CONTROL OF MATERIAL HANDLING SYSTEMS - Environmentally Conscious Materials Handling - page 88

control. The toploader initiates the journey of the tote on the BHS. A destina-
tion (discharger) must be set for the tote. In order to increase capacity, several
dischargers are often allocated to the same flight destination. 1 Therefore, the
toploader agents initiate a negotiation with the possible dischargers to find the
best-suited discharger. The evaluation of the proposals from the dischargers is
not trivially chosen as the lowest offer, but weighted with the current route length
to the dischargers, which the toploader requests from a route agent — a mediator
agent with a global focus on the dynamic route lengths of the BHS.
The toploader can take two different approaches for routing the tote:
1. Routing by static shortest path . After the toploader has decided on the
discharger, it could instruct all diverting elements along the route to direct
that specific tote along the shortest path. Then the agent system would,
in principle, work as the traditional control system by sending all totes
along predefined static shortest routes. 2
2. Routingontheway . Instead of planning the entire route through the BHS,
the toploader could just send the tote to the next decision point along
the shortest route. This is a more dynamical and flexible approach, as
the tote can be rerouted at a decision point if the route conditions have
changed — perhaps another route has become the dynamical shortest one,
or the preferred discharging point have changed.
More formally, the principal tasks of the toploader can be illustrated as the
diagram in Figure 3.13, but it hides the advanced decision logic between the
state changes and message interactions:
• Straight elements . Most of the elements of a BHS are naturally straight
or curved elements (conveyor lanes) that connect the nodes of the routing
graph. Straight or curved elements are not considered as agents in our
current design, because mechanically they will always forward a tote to
the next element if it free; thus, there are no decisions to be made. In
principle, the speed of each element could be adjusted to give a more
smooth flow and avoid queuing, so one could argue that these decisions
should be taken by the element itself. In the current setup, it would generate
an enormous communication overhead, because each element should be
notified individually and the agents should be very responsive to change
the speed in order to gain anything from speed adjustments.
• Diverters . Divert elements (Figure 3.14) become the first natural decision
points on a route. A diverter splits a conveyor lane into two, either a left or
1 Due to the stopping of totes while unloading, the discharger has a lower line capacity than straight
elements.
2 In the researched BHS the decision between the alternative dischargers would also be predefined
in the conventional control. The BHS is built in layers to minimize cost and maximize space
utilization, and alternative dischargers are always split on different layers. The control system
would try to avoid switching layers.