The Ups and Downs of Making Elevators Go
►http://online.wsj.com/article/SB10001424127887324469304578143200385871618.html
As a mathematician steeped in the theories of vertical transportation at Otis Elevator Co., Ms. Christy, 55, has spent a quarter-century developing systems that make elevators run as perfectly as possible—which means getting most riders into a car in less than 20 seconds. “Traditionally, the wait time is the most important factor,” she says. “The thing people hate the most is waiting.”
Here is a typical problem: A passenger on the sixth floor wants to descend. The closest car is on the seventh floor, but it already has three riders and has made two stops. Is it the right choice to make that car stop again? That would be the best result for the sixth-floor passenger, but it would make the other people’s rides longer.
For Ms. Christy, these are mathematical problems with no one optimum solution. In the real world, there are so many parameters and combinations that everything changes as soon as the next rider presses a button. In a building with six elevators and 10 people trying to move between floors, there are over 60 million possible combinations—too many, she says, for the elevator’s computer to process in split seconds.
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