1. Why the Door Closes When You Stop Moving
Most sliding automatic doors have a sensor on top. When this sensor detects movement from a passerby, the door opens. If it detects nothing for a while, it begins to close. If it detects movement again while closing, it opens back up [1]. Because of this, if you stand still in the open doorway, the sensor cannot find your movement. The door then starts to close, which can lead to being caught in it. The Consumer Affairs Agency’s accident information database lists 165 such accidents between September 2009 and the end of February 2018. This number includes cases where the facts were not fully confirmed [1]. However, it is not true that the door will always close if you stand still. This depends on the specific type and model of the sensor.
2. Why Snow, Insects, and Flags Can Make the Door Open
A major manufacturer’s frequently asked questions list several causes for malfunctions. These include snow, insects, puddles, potted plants, and flags swaying in the wind. Temperature differences between indoors and outdoors caused by air conditioning can also be a reason [2]. Note that this list applies to specific models and may not apply to all automatic doors. Looking at this list, it seems the sensors described by this manufacturer do not distinguish between "a person" and "not a person." Instead, they detect that something has changed within a fixed area. It is like a cat waking up because a blanket moved slightly near it. However, the sensor is not thinking like a cat.
3. How Infrared Sensors Detect People
Infrared is a type of light that we cannot see. Manufacturers explain that there are two main ways automatic door sensors use it [2]. The first method sends near-infrared light toward the floor. It looks for the difference between the reflection from the floor and the reflection from an object entering the detection area. The second method looks for the difference in temperature between the floor and the object. Each method has weaknesses. The reflection-difference method may struggle with low objects or dark clothing, as the difference in reflection becomes smaller. The temperature-difference method can have trouble on hot days when the air temperature is close to body temperature, making the difference smaller and harder to detect [2]. The manufacturer states on its website that it developed and productized the world’s first infrared sensor for automatic doors in 1980 [3]. English Wikipedia also notes that the first automatic door using an infrared sensor appeared in 1980 [4]. Since both sources claim this was a "world first," it is best to consider the timing as around 1980.
4. How Microwave Sensors Use the Doppler Effect to Detect Motion
Another method uses radio waves called microwaves. The door sends out radio waves and examines the waves that bounce back from moving objects. When waves hit a moving object, the frequency (the fineness of the wave) of the returning waves changes. This is the same Doppler effect that makes a siren sound higher when an ambulance approaches and lower when it moves away. Patent documents describe a method that mixes the radio waves sent by the sensor with the received waves to find moving objects [5]. The manufacturer explains that sensors using this method operate stably even at low temperatures of minus 35 degrees Celsius [3]. However, not all automatic doors use this method.
5. The History of Automatic Doors: From Ancient Greek Temples to Mat Sensors
It is said that Hero of Alexandria, an engineer from ancient Greece (around the 1st or 2nd century), described a mechanism to automatically open temple doors [6]. The fire on an altar heated the air in a closed container. The pushed-out water added weight, pulling a rope to open the door. This was not a mechanism to detect people. It is also unclear if it was actually built and worked. Still, the wish for a door to open automatically when someone approaches seems to have existed since that time. English Wikipedia states that Horton and Hewitt invented a sliding door that opened when a mat was stepped on in 1954 [4]. After the mat, methods that detect from above using infrared or microwaves became widespread.
6. Why Doors Have a Time Delay and Extra Safety Sensors
There is a time delay between when the sensor detects someone and when the door fully opens. If you run through or enter from an angle, you might hit the door before it opens enough [1]. Doors with auxiliary photoelectric sensors will move the door in the opposite direction to stop it from closing if a person is in the path of the moving part while it is closing [1]. However, the gap between the door and the side wall is outside the range where the sensor can detect people. Considering that the door is a large, heavy machine, approaching it at a walking speed seems to be the most reliable way to use it for both the door and the person. Safety Note: There is a time delay between detection and opening. Running through or entering at an angle can cause you to hit the door before it is fully open [1].
7. Can Automatic Doors Tell Who Wants to Enter?
A study in 2014 stated that conventional automatic doors have difficulty distinguishing between people who want to pass through and those who are just walking past. As a result, they sometimes open when not needed [7]. This is because the methods we have looked at detect that "something changed," but they do not know the person’s intent. Research at the University of Electro-Communications has reported attempts to use a 3D laser scanner to measure human movement in detail and determine if there is an intent to pass through [7]. It is interesting to wonder if a day will come when doors can read "wanting to pass through" from human movement.
8. How to Observe an Automatic Door's Sensor Yourself
If you want to investigate further, try recalling where the sensor was located on an automatic door you use often. If you cannot remember, make it a goal to look next time you pass through. When you go out, try to find the sensor window on top of the door at a supermarket or similar entrance. Walk through after the door is fully open, and try to recall where the opening started. This alone might help you notice differences in the methods. Safety Note: Do not stand still directly under the door to test it. This is dangerous and could lead to being trapped [1].