I work in a hotel and ride elevators upwards of 50 times a day. Never once has the bottom of the elevator been even a fraction of an inch above or below the floor where the elevator begins. Assuming there is a minimal amount of slippage in the cabling, how do the elevators move the exact same distance every single time? Is there some sort of cable/feet math going on where each floor is x amount of motor turns?
In: Engineering
The elevator car is controlled by a computer that gets signals from sensors in fixed locations up the entire height of the shaft. They tell the computer where the elevator car is.
The computer can operate the motor and wind the cable up or down to position the car precisely at the correct level. It does not know exactly how many times to go around to achieve the desired result – it will simply wind up the cable until the sensors in the shaft tell it to slow down, and then stop entirely, at the right place.
If you pay attention, you can sometimes feel the car almost coming coming to a stop, and then slowly being moved a small amount more before the door opens. This is the fine movement that allows the level to match the floor perfectly.
Using sensors in fixed positions on the shaft means that it does not matter if the cable stretches over time – the motor will still move the car to the correct position.
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