Arhu should have the controller, and should give it to you if you ask him about the robot. You may be able to pickpocket it, if he still has it, but I'm not sure what skill/character level and dexterity are required.
There are 3 levers in the cave which (when all pulled) can discharge the robot's main attack, or there are other approaches to handle the fight (a recent Steam discussion: Sparkmaster quest ), or you can use invisibility to bypass the fight, and come back to it later. There may be other areas you can explore, as well, before heading through that cave.
