🚀 go-pugleaf

Could somebody post some information or suggest a book on sensors 
for detecting balance, i.e. orientation relative to the horizontal plane?
I'm praticularly interested in knowing what different types are available. 
Are there any without moving parts?

I recently saw a t.v. program which showed a 1 legged hopping robot in
action. How did this sense its balance? Could someone direct me to a 
technical publication on this work?

thanks.