What is the working principle of automatic opening and closing glass doors at the entrance of large shopping malls and hotels?

The infrared signal collector can get the abnormal changes around it (that is, there are people or no people), and then transmit the signal to the judgment institution. Single chip microcomputer is used for general electric doors, and PLC is used for factory automation equipment. Then the single chip microcomputer (or PLC) judges whether it is necessary to open the door, and if so, it sends a door opening signal to the actuator, that is, the motor. Stop when the door touches the limit switch when opening or closing.