stepper motor as an odometer with ARDUINO

I made a common circuit of driving a stepper motor. Using an arduino, an H L298 bridge. I used two pitch motors, one of them will be as an actuator motor and another as a sensor motor. My motors are unipolar type, permanent magnet at 7.5 degrees. When I connected the wire of a coil and the reference in the center tape motor sensor on the oscilloscope, I saw that they are acting like photo.
Why are there two peaks up and down?

Hi Andre-I'm not sure you're seeing the actual waveform going to the coils. What is your supply voltage?  Since you'ere using an L298 I would guess that it's higher than 5 volts, but from the attached picture I only see narrow spikes that peak just below 5 volts. What does the waveform look like on the two inputs of the L298?  The output should look much the same, only at a higher voltage. Is the oscilloscope connected to the sensor motor and not the motor being driven by the L298?-Rick

Andre...I am assuming that your are looking at one of the coils on your sensor motor. When you say "Odometer" are you driving a mechanical counter?  (BTW Why do you need a sensor motor, a stepper motor gives very precise position control and all you'd need is maybe a optical sensor for the Zero position).  
In that case your waveform makes sense....stepper motors make a "step" movement very quickly and are then more or less stationary till the next step.  So what you're seeing are spikes induced by the movements of the magnet in your sensor stepper motor - first positive and then negative.  This makes even more sense when you see that the intermediate spikes are of less amplitude than the higher ones - the higher ones occur when the magnet is right on the winding that you are looking at, the lower spikes are when it is on an adjacent winding.I may be completely off track here, but from your explanation of what you're doing, the above makes sense.Let me know if I am missing anything or have misinterpreted your question?Cheers // David