Comparing your curves to @Mervious ones you may want to reverse the polarities there as it seems curves start with negative before going positive when the magnet passes thoI put together a H Bridge type circuit to reverse the current running through the coil, accounting for the positive to negative pole change of the magnet as it passes. Here's a quick demo of the coil output.
First attachment shows running in positive direction for one second, then off, then negative direction for one second, then off and repeat.
Second attachment shows this applied to the setup mode. Its interval and pulse width is for a 15kph constant speed. For half the pulse width the coil is positive, then for the second half the coil is negative, representing the passing magnet. The amplitude appears to be changing simply because phone sensor isn't fast enough to pick up the true readings at this rate.
Is this what you're thinking?
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