Nib asked
How do elevators know where to stop?
An elevator does not look at floor numbers. It counts the turns of its motor like an odometer, uses fixed markers in the shaft to correct the count and to level with each landing, and serves the calls it has stored in the direction it is…
6 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1it counts
It is tempting to think the car looks out at each floor and recognises it. It does not. Most elevators fit a rotary encoder to the traction machine: every small turn sends a pulse, and the controller adds the pulses up to know how far the car has moved, the way a car's odometer counts wheel turns.
The elevator mostly counts pulses from its motor, like an odometer.
a common mix-upThe elevator does not read floor numbers as it passes; it keeps a running count of how far its motor has turned.
2floor table
Before normal service, elevators typically learn their hoistway: the car runs through the shaft to find its total travel and the position of each floor, and stores them. Afterwards the controller compares the running count with its stored table to know which floor it is at, and how far away the next one is.
The elevator learns the shaft once and stores each floor as a count.
What does the controller compare its running count with?
Show the answer
The table of floor positions it stored when it learned the shaft.
3floor markers
The encoder only counts. If a steel rope slips on the traction wheel, the count drifts away from the car's true position. So the controller corrects itself against fixed reference points in the shaft, such as markers at the landings, read by sensors on the car. At each marker the count is reset to what it should be.
Fixed markers in the shaft let a sensor on the car correct the drifting count.
Why can the count drift?
Show the answer
The ropes can slip on the wheel, so the motor's turns no longer match the car's true movement.
4stored calls
Every button press, inside the car or in a hallway, is stored by the controller. In a collective control scheme the car answers the stored calls one after another along its current direction of travel, and only then turns round to serve the calls in the opposite direction. That is why a car going up may skip you if you are below it and wanting to go up.
The controller stores every call and serves them in the car's current direction first.
Why might a rising car pass you by?
Show the answer
It serves calls ahead of it in its direction of travel first, then turns around for the rest.
5the stop
To stop an elevator smoothly and level with a landing, the system must know when to begin the stop. The rough position from the count says when to start slowing. Then sensors on the car read the magnets or other markers beside the landing, and the car keeps moving until the signal says it is level with the floor. Only then do the doors open.
The count says when to start slowing; the landing sensors say when the car is level.
What tells the car it is level with the floor?
Show the answer
Sensors on the car reading markers beside the landing.
6the whole idea
The whole elevator on one page: it counts motor turns like an odometer, corrects the count at fixed markers in the shaft, serves stored calls in its direction of travel, begins slowing when the count says so, levels with the landing using the sensors on the car, and then opens the doors.
Count, correct at markers, serve calls by direction, slow, level, open.
Next time the doors slide open exactly level, remember: the car got there by counting, and then checking its work.