you have searched out the forms of your subjects geometrically - and it seems you can do this with some proficiency and speed - you got a lot done at the zoo. Your work shows lovely depth - you can obviously concieve the 3d forms in your mind as you draw.
As with all high-level critiques, its not a matter of what your doing wrong but what else you could be doing right,,,
At the moment it feels like your works are all 'dummies'. They lack general 'uniqueness' that could be granted by conceptual elements (costume and jewellery, weaponry and so on). Granted these are your studies - and having not seen your finished works this may be moot, but at the moment it just looks a little like your trying to learn the construction of everything and you could be adding a lot of texture and detail to really bring the 'wow' conceptual factor.
by the way, my fave is the female robot character with the half-face mask. I feel like here you have paid attention to the details, rendered with a lot of expression and it generally feels like there is conceptual depth (albeit a cliche general idea, who cares, its a sketch). Reminds me of insane visions' work from about this time last year. Go for that expressive all over tonal work.