The elevator control system consists of all components supposed to manage the operation of the equipment, such as car speed and direction, door closing, acoustic and light signals.
It includes the controller (relay or microprocessor based), buttons, proximity sensors, limit and safety switches etc.
Early elevators had a manual control system. They required an operator which drove the car by holding two buttons (up and down). Later an automatic leveling system was designed by a famous elevator company: once the operator released the control button, the elevator reduced the speed and stopped at the nearest floor.
The first automatic elevator control system was installed after the first global war and worked by relay logic; this technology is still now used on old equipments.
The movement of the car is controlled through a direction selector on each floor. This device consists of an exchange (SPDT) switch, whose central pin is connected to the contact of the floor request relay. So, when a floor button is pressed, a direction contactor (up or down) is enabled, depending on the position of the related selector.
In some cases direction selectors are mounted in the shaft on each floor and turned by a car shape. Otherwise they are mounted in the machine room and turned by mechanical gears. However the basic concept is the same.
Basically there are two kinds of elevator control system: single and collective operation.
The elevator carries out one floor request at once, so when the car is moving all request buttons are disabled.
The control system can remember multiple requests. On most collective operation elevators there are two call button at each floor. This is because the controller carries out all requests in one direction, than reverses. So, if you are waiting for the elevator and you press, for example, the down call button, the car will pick up you while going down.
Requests management system
Multiple elevators share the exterior requests. So, when a call button is pressed from a floor, the request is sent to the nearest elevator. On relay based control systems this feature is managed by a set of resistors. To understand the working principle, take a look at this simplified schematic. Assume that the first elevator is on the first floor, while the second one is on the 4th floor. If you call the elevator from the 3rd floor, the 3E relay turns on. Both the relays A and B are supplied, but the second one prevails because its voltage is a bit higher. So the request is sent to the elevator B.
Microprocessor control systems
They began to be installed around 1980. Unlike relay based controllers, they have few mechanical parts, so they require less maintenance. Besides they consume less electricity. The position of the elevator is usually detected by proximity sensors, mounted on the top of the car.
On modern elevators, Programmable Logic Controllers (PLC) are often used instead of dedicated boards. The operation of a PLC based control system can be changed without modifying the hardware.
Another advantage of microprocessor technology is the possibility of using bus communication, which reduces the cost of wiring, especially on big buildings.