THE HIDDEN SCIENCE OF ELEVATORS

The hidden science of elevators is something you never think about, unless you’re an elevator engineer. The exception is when you walk to the elevator, you hit the Up button, and you wait. And you wait. And…you wait. Then you curse elevators. Or, in the case of a small group of businessmen in Buenos Aires several years ago, who walked up to the elevator, still avidly engaged in a group conversation. One of them pressed the “DOWN” button, as they continued with their conversation. The bell rang, the doors opened, and nearest businessman turned and took one step into the elevator. Only it wasn’t there! I assume he thought about elevators, for a short time at least.

     Every time we press the button, we give elevators a computational challenge: the hidden science of elevators. The elevator control system must decide which car to send for you, and when. It must choose whether to go up from the fifth floor to collect those people on the seventh before coming down to the lobby to answer your call. It must consider who’s been waiting longer, and which of the many choices is the most efficient, and least painful for everybody. Elevator traffic is an elaborate, delicate dance, and once you see the steps, you can’t help but applaud the engineers who choreograph it all.

     Elevators weren’t always so complex. The earliest electric elevators were controlled by human operators. An attendant standing inside would drive the lift up and down with a throttle, stopping wherever he saw a waiting passenger. But humans proved to be clumsy, expensive, and prone to strikes. So, by the 1950s, electrical switches took over, but that required engineers to spell out the operational rules.         

     The simplest method was like taking a bus. You waited for the 3:10 car up to the 10th floor, and then found your way from there. This, of course, was grossly inefficient. During busy times of day, the elevators would waste everyone’s time sitting still until their scheduled departure time.

     By 1965, engineers settled on the model we all know and love: Passengers push buttons to call elevators, and the elevators respond. But here it gets tricky. As requests from different parts of a building pile up, how does an elevator decide where to go?

     Further complications 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 seriously piss off passengers, like bypassing someone’s desired floor without stopping. A good system will balance all these goals and worries.

     The earliest and simplest reasonable approach to elevator dispatching is still surprisingly common. Its known as “collective control,” or simply “the elevator algorithm,” and it consists of two rules:

1. As long as there’s someone inside or ahead of the elevator who wants to go in the current direction, keep heading in that direction.
2. 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.

     This is why your typical elevator bay has call buttons for up and down. However, in larger buildings, collective control starts to cause problems. The elevator will stop at the middle floors each time it passes by, but it’s never going to stop by the basement on its way to floor 7. So the wait at the very top and very bottom 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 floors at a time. And they bunch up in the middle of the building.

     In the 1970s, a flurry of new algorithms hit the shafts. One strategy, still popular today, is called “estimated time of arrival control.” Basically, the computer considers all cars moving toward a call and assigns the one it thinks will get there the fastest, minimize journey time, use the least energy, or whatever else the designers prioritized.

     The apex of the computerized control is destination dispatch, which you can experience if you visit skyscrapers built or modernized since the 1990s. In these buildings, rather than simply pressing up or down, you enter on the keypad what floor you want to go to, and it tells you which elevator will come to take you there. Because they know exactly where you’re going, these systems can bunch people together who are headed to the same floor, turning each elevator into an express train. You might have to wait longer for a lift, but you will get there faster. It gets so complicated that engineers are faced with the problem of choosing the best algorithm.

     One of the most successful approaches has been to let a computer decide. Using machine learning techniques, engineers can specify what success looks like, then let the elevator controller experiment on its own, in simulation. The system inspects the state of each simulated elevator and the parameters of each outstanding request, decides what to do, and measures the results. The software eventually learns a policy for each combination of factors. With these more sophisticated policies, even the people who built the software often don’t know why it’s doing what it’s doing.

     Now, is that scary, or what? The hidden science of elevators just became worrying.

     I have a friend who is deathly afraid of elevators and refuses to use them, even if they only take you up and down between two floors. After researching this article, particularly the latest evolution, I might accord him more respect.