A bit Northeast of Sanctuary, at the top of the map, you'll come across a Robotic Disposal Site, that has a deactive Sentry Bot. In a shed, you'll find a terminal and a control holotape for said Sentry Bot. You can then set him to patrol known USAF facilities at a location of your choosing.

I've already cleared Station Revere and Olivia's just got Raiders, but there are three other places on the map you can send him, including Fort Hagen - which my wife informs me I should definitely take him with me there.

He's not a true Companion, but still...something to think about if you need a bit of extra firepower.

Also, near the terminal is Hot Rodder Magazine.

Every time I try to send him anywhere, he says he's mapping a route, but never gets underway. I usually end up activating the self destruct, as he never goes anywhere, and destroying him nets you 2 Fusion Cores from his corpse.