a customized pain index, in
which the computer system
weighs the awfulness of each
kind of delay.
Further complicating mat-
ters are an elevator’s many
constraints. It has physical
limits on its speed, and it has
only a second or two to choose
its next move. It also shouldn’t
do anything that will seri-
ously piss off passengers, like
bypassing someone’s desired
floor without stopping, which
is just asking for a fist in the
control panel. A good system
will balance all these goals and
worries, even when you needed
to be upstairs 10 minutes ago.The Elevator
Algorithm
THE EARLIEST AND sim-
plest reasonable approach to
elevator dispatching is still
surprisingly common. Known
as “collective control,” or sim-
ply “the elevator algorithm,” it
consists of two rules:- As long as there’s some-
one inside or ahead of the
elevator who wants to go in the
current direction, keep head-
ing in that direction. - Once the elevator has
exhausted the requests in
its current direction, switch
directions if there’s a request in
the other direction. Otherwise,
stop and wait for a call.
Even the people who built
the software often don’t know
why it’s doing what it’s doing.
10
sec30
sec60
sec30
sec 1:001:10
TOTAL
TIME
TOTAL
TIME
FIG. 1FIG. 2WAIT TIMEWAIT TIMERIDE TIMERIDE TIMEGETTY IMAGES