Wednesday, June 18, 2008

"May I ask you, inquired the elevator in its sweetest, most reasonable voice, if you've considered all the possibilities that down might offer you?"

levators are fascinating things. I say that despite being trapped in one for forty-five minutes last week (no lie — myself and five other strangers found ourselves stuck 14 floors up, making nervous jokes during a rush hour malfunction).

What I often ponder is the ideal algorithm for running one, or more critically, a set of elevators. Poking around I’ve found that my initial hunch was correct — there’s what’s referred to as the elevator algorithm that dictates that an elevator will move in its current direction, stopping only to let people on or off, until it has no more calls in that direction.

At that point it can either sit there and wait (which is probably more energy and cost efficient) or try to go to a more useful floor (the lobby when people are expected to arrive, or the top floor when they’re expected to leave).

Some other interesting factoids:

  • The elevator algorithm is also used for hard disk access, to optimize the motion of the arm when dealing with read/write requests.
  • In areas where there are a lot of Jews, you will often find Sabbath elevators, which operate in accordance with some Orthodox and conservative rabbinic prohibitions. Wild!
  • Some modern elevators (including, apparently, the one in the Adelaide office of my company) require users to select their desired floor from a central console. They are then told which numbered elevator to get on. Inside the elevator, there are no buttons to push. This is apparently much more efficient but has some human-factors drawbacks:

    • The console doesn’t recognize when a group of people is too large to fit in a single elevator.

    • A single person requesting an elevator multiple times might end up with multiple elevators dispatched to retrieve her/him.

    • People not knowing the system often get on an elevator and end up being taken for a ride!

What other heuristics could you use if you had to program a set of elevators?

Another thing to ponder: how could you determine the finer points of the algorithm used in a given office building, just by calling and riding the elevators? It would seem to require an accomplice at the very least.


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