Auto-guard

An auto-guard was a security device. Aboard Nerva Beacon, a Wirrn Queen was killed when she was attacked by an auto-guard. She managed to disable the device and plant an egg within Dune before dying.

The Fourth Doctor reactivated the auto-guard while trying to turn the oxygen on. By using one of Harry Sullivan's shoes to distract it, he was able to use the cut out and deactivate it. (TV: The Ark in Space)