Make sure you have everything set up right by testing it with a known voltage first. I suggest connecting the +3.3V output of your Arduino to your analog input and verifying the reading (should be 675 or so). If it's around 675 and holding steady(ish) then your software is good and there's something wrong with your sensor or how you have it hooked up.
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The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected