Infrared (Active Infrared Devices)
Active devices that use infrared technology have an infrared diode, which constantly or periodically emits infrared waves. Concurrently there is a receiver, which monitors the reflected wave levels .The absence of a reflected wave or a change in its properties (wavelength or amplitude) will indicate to the receiver that something has changed in the detection zone. In the case of a door sensor this change will cause a relay to trip.
Infrared (Passive Infrared Devices)
Passive Infrared devices monitor a certain area for the presence of infrared rays, which are emitted by humans in the form of body heat.
Microwave devices constantly emit inaudible microwaves. Concurrently, there is a receiver, which monitors the reflected waves, which are reflected from objects. The change of reflected waves' parameters indicate that something or someone has entered the monitored space. This triggers the door to open. Microwave technology is different from ultrasonic one by waves' length. Microwaves are shorter than ultrasonic waves.
Ultrasonic devices constantly emit inaudible sound waves. At the same time the device is scanning for reflected waves, which are reflected at a preset speed. The speed corresponds to the height of the door and if a faster reflected wave is detected it indicates that something or someone has entered the monitored space. This triggers the door to open.
Radio Transmission (Touch-Sensitive Device)
When the device is activated a radio wave is transmitted along a preset path to a nearby receiver. The receiver then activates a control circuit, which carries out an action (e.g. opening the door) based on the presence of the signal or the results of signal analysis.
Automatic doors are powered open and closed, a door fitted with a spring to close it is not an automatic door.
There are three methods by which an automatic door is activated.
- A sensor detects traffic is approaching. Sensors for automatic doors are generally:
- A pressure sensor - a floor mat which reacts to the pressure of someone standing on it.
- An infrared curtain or beam which shines invisible light onto sensors; if someone or something blocks the beam the door can open.
- A motion sensor which uses low-power microwave radar.
- An electronic sensor (e.g. based on infrared or radio waves) can be triggered by something that someone carries, or is installed inside a vehicle. These are popular for garage doors.
- A switch is operated manually, perhaps after security checks. This can be a push button switch or a swipe card.
- The user pushes, or pulls the door, once the door detects the movement it completes the open and close cycle. These are also known as power-assisted doors.
In addition to activate sensors automatic doors are generally fitted with safety sensors. These are usually an infrared curtain or beam, but can be a pressure mat fitted on the swing side of the door. The purpose of the safety sensor is to prevent the door opening or slow its speed if an object is detected in its path whilst opening and to prevent the door closing or reactivate it if an object is detected in its path whilst closing.