Popular Mechanics - USA (2019-09)

(Antfer) #1
This is why your typical ele-
vator bay has call buttons for
up and down: so the car that’s
already heading skyward can
stop to collect anybody who’s
going up. This policy doesn’t
account for most of the factors
mentioned earlier, but it’s not
a bad place to start. The eleva-
tor algorithm is easy to follow
and fairly energy-efficient,
and everyone gets an elevator
within one round-trip. (The
very same algorithm controls
the read-and-write head on
many hard drives.)
Small office and apartment
buildings, which don’t need to
squeeze every bit of efficiency
out of their elevators, tend to
use this simple approach. In
larger buildings, though, col-
lective control starts to cause
problems. The elevator ser-
vices the middle f loors each
time it passes by, but it’s never
going to stop by the basement
on its way to f loor 7. So the wait
at the very top and very bot-
tom, the areas most in need of
elevators, can be a nightmare.
More importantly, large
buildings usually have banks

of elevators, not just one.
If each follows the elevator
algorithm, then under heavy
traffic, the elevators start
leapfrogging each other a
few f loors at a time. And they
bunch up in the middle of the
building, potentially even
serving the same calls twice.
To handle these larger set-
ups, engineers developed a
slew of tricks. Just having the
lifts talking to each other goes
a long way. If Car 1 is headed
up, Car 2 can instead handle
a lobby request. Furthermore,
lifts can be assigned to spe-
cific clusters of f loors. You
also may have seen eleva-
tors hanging out in a lobby,
doors wide open. This is the
parking strategy, where idle
elevators return to a com-
monly requested floor. Thanks
to traffic prediction and real-
time monitoring, the elevators
can switch between strategies
to adapt to the morning or
close-of-business rush.

Does it serve the


person who’s been


waiting the longest?


Or always go to the


closest call?


68 September 2019

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