I should have asked earlier, but I assume the 9V battery is also powering the arduino? If so in the real world you want to run a separate wire from the battery ground to the arduino ground and a second (wire from the battery ground to the Motor and Servo (both of which have motors). The current spikes from the inductance and pwm switching of the motors can cause voltage drops (due to wire resistance) in the ground circuit. That in turn will change the level of the analog voltage a bit (which may or may not matter in this case). As always the acid test is to wire it up and try it and see if it works Image may be NSFW.
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. If it works sometimes but not others then the grounds are a thing to look at (but its easiest to wire it correctly the first time Image may be NSFW.
Clik here to view. ). Assuming you are reading analog voltages I believe you also need to connect the arduino 5V wire to the aref pin on the arduino to set the analog reference voltage (I didn't notice that wasn't done the first time).
Peter
