Automatic doors rarely cross anyone’s mind. That's the point. Your arms are full, your thoughts are elsewhere, and the door quietly reacts. It senses the situation like a skilled waiter who refills your drink unprompted. That simple motion hides a sophisticated network of sensors, motors, safety logic, and building codes reacting in milliseconds. The technology dates back to the 1960s, yet today’s versions look nothing like the early prototypes. They’ve become finely tuned, ubiquitous systems that perform so seamlessly they fade into the background. 
The brain of the entire operation is the sensor. Typically, these doors use overhead-mounted microwave sensors or passive infrared detectors, or PIR. PIR sensors detect body heat signatures as people pass. These sensors send out waves and interpret what bounces back. Both technologies have advantages and drawbacks. PIR may be confused by the extreme changes in temperature around the entrance - a cold winter doorway, say, can obscure the thermal difference that a sensor requires. Microwave sensors are more predictable under a variety of conditions, but are sometimes activated by blown debris or a rogue pigeon making a daring life decision. Very busy installations frequently combine the two technologies, making cross-checks on signals prior to commanding the motor to act. The outcome is a minimized amount of false triggers and a reduced number of doors that simply sit there with their mouths open as everybody inside of it freezes. Motor systems have also developed eminently. Early doors relied on basic electromechanical setups with relays and simple timers. The Caesardoor brushless DC motors are used today with variable speed controllers. The door does not slam, but slows down as it nears the complete opening, pauses in the highest position, and then closes at a slow pace. Gone are the abrupt bangs of earlier designs. Software now ensures the door reverses upon encountering resistance. It is not an extravagance. In Europe with EN 16005 and in the US with ANSI/BHMA standards, it is a legal requirement. Any door that closes on a child or a wheelchair user and continues to push is not a door, but a motorized hazard.
The brain of the entire operation is the sensor. Typically, these doors use overhead-mounted microwave sensors or passive infrared detectors, or PIR. PIR sensors detect body heat signatures as people pass. These sensors send out waves and interpret what bounces back. Both technologies have advantages and drawbacks. PIR may be confused by the extreme changes in temperature around the entrance - a cold winter doorway, say, can obscure the thermal difference that a sensor requires. Microwave sensors are more predictable under a variety of conditions, but are sometimes activated by blown debris or a rogue pigeon making a daring life decision. Very busy installations frequently combine the two technologies, making cross-checks on signals prior to commanding the motor to act. The outcome is a minimized amount of false triggers and a reduced number of doors that simply sit there with their mouths open as everybody inside of it freezes. Motor systems have also developed eminently. Early doors relied on basic electromechanical setups with relays and simple timers. The Caesardoor brushless DC motors are used today with variable speed controllers. The door does not slam, but slows down as it nears the complete opening, pauses in the highest position, and then closes at a slow pace. Gone are the abrupt bangs of earlier designs. Software now ensures the door reverses upon encountering resistance. It is not an extravagance. In Europe with EN 16005 and in the US with ANSI/BHMA standards, it is a legal requirement. Any door that closes on a child or a wheelchair user and continues to push is not a door, but a motorized hazard.