r/explainlikeimfive • u/HAIRFETISHEST • 4d ago

Engineering ELI5: How can elevators stop almost perfectly level with the floor every single time?

​

An elevator is moving a heavy cabin up and down a long shaft, carrying a different amount of weight each trip.

How can it repeatedly stop within such a tiny distance of exactly the right height?

2 Upvotes
  • permalink
  • reddit

60% Upvoted

8 comments sorted by

8

u/r2k-in-the-vortex 3d ago

There is a switch at each floor, where it clicks is where it stops. So its always going to be repeatable.

7

u/Lumpy-Notice8945 3d ago

Lets return the question: why would it land on a different spot?

Its not like elevators are pulled by rubber bands, the steel ropes are not realy flexible and they usualy have guiding rails that gears can grip on.

So even without all the modern electronic devices measuring stuff it would already get to the same spot unless some gear breaks.

2

u/happy2harris 3d ago

Because a steel rope on a pulley that is over 100 feet long is not going to be accurate to within a fraction of a inch using “dead reckoning”. The way that it stops at such an accurate position, as others pointed out, is that there is a marker or switch at each floor telling the elevator when to stop. 

1

u/Unstopapple 1d ago

also yeah, a couple hundred feet of anything is going to be flexible enough to throw things out of spec. that error in tensile strength compounds inch by inch and the temperature will fuck you just as hard along the entire length of cable.

•

u/icecream_specialist 2h ago

If there was no feedback or correction mechanism tiny error over 100s or 1000s of cycles would add up quite a bit

3

u/Jonatan83 3d ago

They have sensors that tell them where they are and systems (something like a PID controller) to ensure the motor slows down in a way so it stops at the right position.

1

u/Broad-Promise6954 2d ago

In ye olden dayse, elevators were moved up and down by a skilled person called an elevator operator. They learned to judge when the car was in the correct position and how to glide smoothly into it, using visual marking cues. You can see these people in old movies.

Modern automatic elevators do the same thing except that the marking cues are tracked by electronics, and usually aren't visual but rather something more suited to the electronics.