TARS is a highly-adaptable robot that can operate in many configurations depending on the need at hand. What's the precedent or inspiration?
I found an interview with Nolan talking about the inspiration behind the design on a news website. He says:
You have robots in this movie. What did you hope to achieve with those characters?
I wanted a more realistic approach to what a robot would be. I didn't even call them robots in the script. I referred to them as 'articulated machines' because I wanted my crew and everybody to stop thinking of your standard idea of a robot. I wanted to have a machine in the film that was like a piece of gear — very tough, very resilient — that had been designed for whatever purpose best suited it.
How did you approach the design of TARS? He's different from other robots we've seen in film.
As we pushed the concept further, it became a very minimalist appearance that disguises very complex functionality. My idea was to remove any trace of anthropomorphism, so it doesn't have a face. It doesn't have arms and legs. It does have a voice, and therefore a personality. The great Bill Irwin, who was puppeteering and voicing TARS, was able to give an inanimate, non-human object a personality.
Touching on Xander's comment of the monolith in Space Odessey is a description on another site. Production designer Nathan Crowley says:
Partially inspired by the iconic monolith from 2001, and with a more snarky personality than supercomputer HAL 9000's, TARS started out simply as a block of metal.
Nolan also adds to the 2001 comparison of inspiration towards the robots' design (from the first link):
For their shape, were you inspired by the monolith from 2001: A Space Odyssey?
I think, in its science fiction context, inevitably your mind goes to that — and that's fine by me. Definitely, the spirit of 2001 hangs over the film. It was one of our aspirations to pay homage to that film. It also relates strongly to the architecture of Mies van der Rohe. As we honed in on the idea, I asked my designer (Nathan Crowley), who's a very big fan of modern architecture: What if we designed a robot as if Mie van der Rohe designed a robot? I think he really nailed it.
I just saw the movie last night and immediately thought of this six legged spinning robot in the scene where CASE rescues Dr Brand in the water on Miller's planet.