How do automatic doors work?

With more and more automatic doors are applied to our daily life in apartments, homes, shopping malls, hospitals, metro stations and airports etc. People are getting familiar with automatic door.
Well still there a lot of people wondering how do automatic doors work?

Automatic doors are very wide range including a lot of types of doors and gates opened by the approaching sensors without manual push or pull to open it.

The sensor will “sense” when someone approaching the door or the gate, then the sensor(considering it’s the eye of human) giving signal to the control unit( head), the control unit (head) then sending opening request to the drive unit( arms/leg) then push or pull the door to open.

Then the automatic door will hold open for a while(mostly this period is adjustable) to allow people to get fully access of the door.

The door then closed by the order given by the control unit(head) after a set period of time.