How do automatic doors detect people? Most automatic sliding doors use electronic sensors to recognise movement or presence near an entrance. Microwave radar can detect someone approaching, while infrared sensors help establish whether a person or object remains in the doorway. Together, these technologies allow the doors to open automatically and reduce the risk of closing on someone passing through.
Automatic doors are familiar sights in supermarkets, hospitals, airports, hotels and public buildings. Although they may appear to react instantly when somebody approaches, their operation involves several carefully coordinated components.
The sensor does not usually identify who is approaching. Instead, it detects a change within a monitored area and sends a signal to the door’s electronic controller.
How Automatic Door Sensors Work
An automatic sliding entrance typically combines a detection system, an electronic controller and a motor-driven mechanism.
The basic sequence is straightforward:
- A person approaches the entrance and enters the sensor’s detection area.
- The sensor recognises movement or presence and sends a signal to the controller.
- The controller activates the motor, which moves the door panels along their tracks.
- Sensors continue monitoring the entrance while people pass through.
- When the monitored area is clear and the control conditions allow it, the doors close.
The sensor’s detection area extends beyond the doorway itself. Its shape, size and position depend on the sensor technology and the entrance’s configuration.
Many automatic doors have sensors on both sides, allowing people to approach and leave through the same entrance.
Microwave Radar: Detecting People As They Approach
One common method of detecting approaching pedestrians uses microwave radar.
A microwave sensor transmits low-power electromagnetic waves into a defined area in front of the entrance. These waves reflect from objects and return to the sensor.
When a person moves through the area, the reflected signal changes. Many door sensors use the Doppler effect, in which movement towards or away from the sensor changes the frequency of the returning waves.
The sensor’s electronics interpret this change and determine whether to request that the door open.
Why Microwave Sensors Detect More Than People
Microwave radar does not normally recognise a human being as a particular type of object. It responds to movement that meets its detection criteria.
As a result, automatic doors may also react to:
- Shopping trolleys.
- Wheelchairs and pushchairs.
- Moving luggage.
- Other objects passing through the detection area.
This is useful because an entrance must accommodate more than people walking without assistance.
However, a movement-only radar sensor may no longer respond to someone who stops completely. That is one reason automatic sliding doors commonly use additional presence-detection technology.
Infrared Sensors: Detecting Presence Near The Doorway
Infrared sensors work with electromagnetic radiation beyond the red end of the visible spectrum. Some emit infrared light and measure its reflection, while others detect changes in infrared radiation from their surroundings.
Active Infrared Sensors
Active infrared sensors emit infrared light and measure the light reflected back from surfaces or objects.
In a common automatic-door arrangement, the sensor monitors reflections from the floor within a defined detection area.
When a person, trolley or another object changes the reflected pattern, the sensor recognises that something has entered the monitored space.
Unlike movement-only radar, an active infrared presence sensor can detect someone remaining in a monitored area without continuing to walk.
This makes active infrared particularly useful near the doorway, where a person might stop, hesitate or move slowly.
Passive Infrared Sensors
Passive infrared sensors work differently. They detect changes in infrared radiation emitted by people, animals and surrounding objects rather than transmitting their own infrared light.
Such sensors can detect moving sources of heat, but their operating characteristics differ from those of active infrared presence sensors.
Although passive infrared technology can be used for automatic-door activation, it is not interchangeable with the active infrared safety systems commonly fitted to sliding entrances.
The distinction matters: the word “infrared” does not describe a single detection method.
Why Automatic Doors Use More Than One Sensor
A key distinction in automatic-door technology is the difference between activation detection and safety detection.
Activation detection determines when a door should begin opening.
Safety detection monitors areas where moving door panels could contact people or objects.
For example, somebody might walk towards a supermarket entrance, trigger the radar and then stop briefly in the doorway to adjust a bag.
The radar may no longer detect significant movement, but a properly positioned presence sensor can continue to detect the occupied space and signal the controller accordingly.
Many combined sensors house both microwave radar and active infrared technology in one unit.
Although they share a housing, the two technologies perform different functions.
Infrared Safety Curtains
Some automatic-door sensors project several rows of infrared detection spots across the area near the threshold.
These rows are often called infrared safety curtains.
They monitor defined areas where people might stand or pass while the doors operate.
By combining several monitored rows, the system can cover more of the doorway than a single narrow beam.
The exact arrangement varies between products and installations. Some entrances also require additional protection around the sides of the moving panels.
How Do Automatic Doors Know When To Close?
An automatic sliding door does not simply close a fixed number of seconds after it first detects somebody.
Its controller uses signals from the activation and safety systems, together with programmed operating settings, to decide when closing is appropriate.
A typical sequence involves the door reaching its open position, waiting for an appropriate period and then beginning to close once the monitored areas are clear.
If a relevant safety sensor detects somebody during closing, the controller may stop or reverse the movement, depending on the door’s design and configuration.
The timing and behaviour depend on the equipment, operating mode and applicable safety requirements.
A properly functioning door therefore responds not only to somebody approaching but also to what happens while the entrance is occupied.
Why Do Automatic Doors Sometimes Open When Nobody Is Entering?
An automatic door may occasionally open when somebody walks past rather than towards the entrance.
This usually happens because the person enters the activation sensor’s detection area, even though they do not intend to use the doorway.
The problem is sometimes described as unwanted activation.
For example, a pedestrian walking along a pavement close to a shop entrance might trigger an outward-facing sensor.
Other possible causes include moving objects, unsuitable sensor positioning, environmental disturbances or faults.
Direction-Sensitive Radar
Some microwave sensors can distinguish between approaching and departing movement.
An entrance can therefore be configured to respond primarily to people moving towards the doors rather than everyone passing nearby.
This can reduce unnecessary opening cycles and limit the exchange of heated or cooled air between a building and the outside environment.
The benefit is not merely convenience. Correctly configured detection can support the building’s energy efficiency.
Why Can Automatic Doors Struggle With Certain Conditions?
Sensor performance depends on more than whether a person is present.
Several environmental and installation factors can affect detection.
Floor Surfaces And Reflections
Active infrared systems depend partly on the light reflected from their monitored surroundings.
Dark flooring, highly reflective materials or changes to the floor’s appearance can affect the reflected signal.
Manufacturers therefore provide adjustment procedures to accommodate different installation environments.
Rain, Snow And Moving Objects
Depending on the sensor type, rain, snow, moving decorations, vegetation or other environmental disturbances may contribute to unwanted activation or reduced detection reliability.
Sensors intended for exposed entrances must be selected and configured for their operating conditions.
Sensor Positioning
A sensor aimed too far away may detect pedestrians who have no intention of entering.
A sensor positioned incorrectly may also leave part of the intended detection area inadequately covered.
Installation height, detection angle, surrounding structures and the direction from which people approach all influence the appropriate configuration.
This is why sensor alignment is an important part of professional installation and maintenance.
Can Automatic Doors Detect Someone Standing Still?
The answer depends on the detection technology.
A movement-only microwave radar sensor generally relies on changes caused by motion. A stationary person may therefore stop producing the signal needed for continued activation.
An active infrared presence sensor, by contrast, can recognise an object remaining within its monitored area.
However, presence detection is limited to the area actually covered by the sensor, and its operation depends on the equipment and settings.
An automatic door should never be assumed to detect every person or object in every position.
Do Automatic Doors Use Cameras?
Not all automatic doors use cameras.
Conventional microwave and infrared sensors can detect movement or presence without capturing ordinary photographic images of pedestrians.
Some automatic-door systems may use image-based or vision detection, but this is a separate technology and should not be assumed to exist in every entrance.
Similarly, a sensor that uses infrared light is not necessarily a thermal imaging camera.
The important distinction is between detecting movement or presence and recording identifiable visual information.
How Are Automatic Doors Designed For Safety?
Automatic doors must balance convenient access with the hazards created by moving panels.
Depending on the door type and installation, safety measures may include presence detection, protected movement areas, controlled opening and closing speeds, force limitation and monitoring of safety devices.
In Europe, EN 16005 establishes safety requirements and test methods for power-operated pedestrian doors. The British version is BS EN 16005, while other regions have their own applicable standards and requirements.
The design of an entrance also matters. A hospital entrance, a busy shopping centre and an emergency escape route may require different arrangements.
Safety is therefore not determined by the sensor alone. It depends on the complete door system, its installation, commissioning, maintenance and the conditions in which people use it.
If an automatic door repeatedly closes unexpectedly, fails to detect pedestrians or behaves erratically, the responsible building operator should arrange inspection by a suitably competent technician. Visitors should not attempt to adjust or bypass safety sensors.
Frequently Asked Questions
Can automatic doors detect children and wheelchairs?
Suitable automatic-door systems are designed to accommodate a range of users, including children and wheelchair users. However, effective detection depends on the sensor’s coverage, configuration and the overall door installation. The presence of a sensor alone does not guarantee detection in every circumstance.
Are automatic-door sensors always above the entrance?
Many sensors are mounted above the doorway, but additional detection devices may be positioned elsewhere around the entrance. Their locations depend on the door design and the areas requiring monitoring.
Do automatic doors use lasers?
Most familiar automatic sliding entrances use technologies such as microwave radar and infrared detection. Certain systems use other optical technologies, but a conventional infrared presence sensor should not automatically be described as a laser scanner.
Why do automatic doors sometimes reopen while closing?
A safety sensor may detect somebody or something within the protected area and cause the controller to interrupt or reverse closing. Other control or fault conditions can also influence the door’s behaviour.
Can automatic doors operate during a power failure?
Their behaviour depends on the door’s design, backup power, manual-release arrangements and intended use. Some systems include provisions for emergency operation or escape, but these arrangements are not identical across installations.
Understanding The Technology Behind An Automatic Entrance
Automatic doors illustrate how different sensing technologies can work together to solve an everyday problem.
Microwave radar can recognise somebody approaching, while active infrared sensors help monitor whether the doorway remains occupied. The controller interprets these signals and operates the motorised door mechanism according to its programmed settings.
The result is an entrance that can respond to people without requiring them to touch a handle or press a button.
Understanding the distinction between movement detection, presence detection and safety monitoring also explains why automatic doors sometimes behave unexpectedly and why careful installation matters as much as the electronics themselves.
BEA Sensors Europe — IXIO-D Tutorial
This manufacturer tutorial demonstrates the adjustment of infrared safety fields and microwave radar detection areas in an automatic sliding-door sensor. It provides a useful practical illustration of the technologies described in this article.
Watch the official tutorial on YouTube
Sources And Further Reading
- OPTEX — Technological Information: Automatic Door Sensors. Explains active infrared, passive infrared and microwave detection principles.
- BEA Sensors — IXIO-DT1 Motion And Presence Sensor. Describes combined microwave activation and active infrared presence detection.
- Ditec — Opening And Safety Sensors. Covers directional radar, infrared detection and energy-saving operation.
- BEA Sensors — Sliding Door Sensor Systems. Explains activation, presence sensing and the protection of pedestrian entrances.
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