Here's the plot showing angle change over time. I put sin on the one winding and cos on the other, and motor spins as expected, but also a little trembling is noticeable. It is as if on every step needle slowed down a little bit. The effect is, that the movement is not smooth as I like it to be. I tried to modify my sin and cos tables, but with no luck. So I used some rotary encoder and measured the angle over time in hope that this information will allow me to somehow modify my arrays and correct the error (this non-linearity is caused by the shape of the rotor and stator I believe).