EQ-501
Sunx infrared transceiver
Comes with transmitter and receiver built into a single unit.  Adjustable range 0.2 to 2.5 meter.  Relay contact output.  20 msec response time.

Detecting presence of pedestrian, animal, other vehicle or any other object that might be blocking the pathway   Avoid arm to accidentally fall onto the pedestrian or vehicle - additional safety measure.