Building Services/Vertical Transportation/Lift Controls
Early electric lifts were controlled by an attendant and only featured controls to go up and go down. Doors had to be opened manually as well. Nowadays, lift attendants are reserved for only the most exclusive establishments, as the automatic passenger operated lift has all but taken over.
Types of controls
A few lifts still exist with ‘attendant control’ although these are often combined with automatic systems as well. Lifts with attendant control just have a button to go up ad a button to go down. These lifts typically feature sliding gate style doors so that the attendant can see if anyone wants to call the lift although more sophisticated installations feature a series of lamps for each floor indicating that somebody wants the lift to stop there.
The most common type of lift in use today is that with fully automatic controls, that is the lift is operated by the persons who use it. Typically, there is a button at each floor the lift serves that ‘calls’ the lift. The lift control system ‘remembers’ which floors it has to call at and moved the lift car accordingly. There are also buttons in the lift car itself, the idea being that a passenger entering the lift presses the button for the floor they wish to go to. Usually, the buttons that have already been pressed are illuminated but once the lift calls at that floor, the light goes out.
A more complex but very common variant of the above is to have two call buttons on each floor, one or ‘up’ and one for ‘down’, the idea being that the person calling the lift presses the corresponding button for the direction they want to go then the lift car only answers that call if it is going in the right direction. For example a person presses the ‘up’ call button on the 4th floor but the lift is going down from floor 5 to floor 1. Once the lift has answered all the calls going down, it will then answer any going up, including the up call from the 4th floor. The lift typically gives priority to the car buttons over the hall buttons so if somebody already in the lift had pressed the ‘4’ button while it was going down, it would have stopped anyway. The disadvantage with this system is that a lot of people press both up and down buttons with the mistaken idea that they will get to their destination quicker but the lift will answer both calls separately so the person has actually just made things take even longer! Obviously the ‘terminal’ floors at the top and bottom only need one button.
Dumb waiter lifts do not have buttons in the car; typically, each floor has a single call button plus a button for each floor. The operator calls the lift, puts the goods in then presses the relevant button to send the lift to that floor.
Some lifts are fitted with a ‘car preference’ feature that allows the lift to be isolated by an operator using a key that fits into a switch in the lift’s control panel. When the lift is in car preference mode, it will only respond to the buttons inside the lift car and will ignore any calls from floors. This is particularly useful for situations where a lift is used for both goods and passengers and somebody needs to move several lift loads of goods between floors without the interruption of passengers using the lift to go up and down a couple of floors.
As well as these control systems, most modern lifts feature indication systems that inform users of the lift what floor they are at and other such information. Early lifts featured clock style rotating arrows against a semicircular list of floors while for buildings with less floors, lamps were used to light up numbers indicating which floor the lift was at. These indicators were used both inside the lift car and at selected landings (often just the ground floor). Landings and lift cars also often featured illuminated red and green arrows indicating which direction the lift was going, with the general protocol being red for down and green for up.
More modern lift installations typically use LED or LCD display panels, often with animated scrolling up and down arrows to indicate the lift’s status.
For those interested, we will now take a more in-depth look at how lift control systems actually work.
Attendant control lifts are very simple – the up/down buttons are connected to a relay that directly supplies power to the motor.
Automatic lifts use a very complex system to control the lift. We will look at both the traditional way of doing this and the contemporary method.
Automatic lifts were originally controlled using a very complex arrangement of electromagnetic relays and other mechanical apparatus. Any lift control system must know where the lift car is at any time in order to be able to control it and while more modern installations typically use sensors installed in the lift shaft, older installations use an incredibly complex device known as a floor selector. The floor selector is connected via a metal tape and reduction gear to the lift car so when the lift car moves, the ‘head’ of the floor selector does as well. As it moves up and down, the selector head touches various metal contacts that correspond with the floors in the building and cause it to stop if that floor has been requested as a stop.
When somebody presses a call button or button in the car, it causes a relay to close. When the selector head touches the same floor as the relay, if the relay is closed, a circuit is created that causes the motor to slow down and the brake to activate. This is an over simplified account of how these systems work but explains the basics.
Being predominantly mechanical, there are many points of failure, common ones include relay contacts failing, the selector tape breaking and dust or other foreign objects causing disruption to the system. While this technology is over thirty years old, it is still very commonly in use in older buildings. This type of installation uses more electricity, takes longer to install, costs more to maintain and takes up more space than modern equivalents, replacement parts are also hard to come by according to some sources.
The modern replacement for the predominantly mechanical older lift systems are computer controlled systems. These are mainly solid state and rely on microprocessors to carry out the lift operations. These systems can record data about lift usage, employ sophisticated display panels in the lift car and often play recorded messages in the lift car for the benefit of visually impaired persons.
All lift control systems have a range of safety interlocks that prevent the lift from moving while the doors are open and most lifts also feature an infrared beam that prevents the doors from closing when somebody is entering the lift. There are also isolating switches in the machine room that prevent the lift from being used at all.