How Do Automatic Doors Detect People?

Microwave radar, infrared sensing and safety beams tell a controller when to open and when not to close

The short answer

The apparently simple response depends on layered sensing. Opening early, staying open while someone occupies the threshold and closing promptly without trapping anyone are separate engineering tasks.

Microwave radar detects changing reflections

A transmitter sends low-power radio waves into a zone. Motion changes the frequency or phase of returned energy through the Doppler effect, indicating an approaching target.

Active infrared maps a detection zone

Infrared emitters project light onto the floor or doorway and receivers monitor the return. A body or object changes reflected intensity and pattern.

Presence differs from motion

A motion-only sensor can lose a stationary person. Threshold safety sensors are designed to keep detecting occupancy even after movement stops.

Controllers combine several inputs

Logic evaluates approach sensors, safety curtains, push plates, access control, fire signals and door position before choosing a motor command.

Electric operators control acceleration

Motors and drive systems open panels smoothly, limit force and slow near the end of travel. Encoders or switches report position.

Safety beams protect the opening

Photoelectric beams and infrared curtains span hazardous areas. Interrupted light prevents closing or triggers reversal.

Sensor geometry prevents blind spots

Mounting height, angle, pattern width and reflections from floors or metal frames determine coverage. Commissioning adjusts the field to the real doorway.

Algorithms filter environmental noise

Rain, moving signs, cross traffic, sunlight and vibration can resemble useful signals. Timing and pattern rules reduce unwanted openings without ignoring people.

Standards define safe behaviour

Accessibility and door-safety standards govern activation zones, opening time, closing force, emergency operation and routine inspection.

Common misconceptions

Most automatic doors do not identify faces or continuously film visitors. A radar or infrared return can be enough. However, some modern entrances integrate cameras or access systems, so the exact equipment must be assessed rather than assumed.

How scientists know

Engineers map detection fields with test targets, measure force and timing, inject electrical faults and repeat approaches at different speeds and angles. Safety testing includes stationary occupants, mobility aids, children-sized targets and loss of power.

Why do doors open when nobody is there?

Moving objects, reflections, weather or an overly wide detection field can create a trigger.

Why do I need to move sometimes?

A motion sensor may not retain a stationary target; separate presence sensing should protect the threshold.

Can doors detect wheelchairs?

Properly designed and adjusted systems use coverage and activation timing intended for varied users and mobility aids.

What happens during a power failure?

Designs vary; doors may release for manual use, open on backup power or follow fire-system requirements.

Why are there several sensor boxes?

Approach activation and threshold safety require different fields and often independent devices.