The Circlelock Sensors

Form:
Sensor controlled.  The passage is blocked should two people enter.
Position:
Sensor checks the position of the feet.  The surface area available offers room for only one pair of feet.
Lodging:
Sensor checks the outline of the walls.  Should the walls curve outwards, the passage will be blocked.
Weight:
Sensor checks the weight of the user.  This takes into account the weight of the entire door assembly cage.