stepper motor block specification

Hi, everyone, I am building a model with a stepper motor in it. I connect a torque sensor with this motor, and I found out that the torque oscillating between +/- of a certain value, it doesn't make sense to me because of the negative value. plus, the model of stepper motor didn't mention the type ,say bipolar or unipolar? please help me . thank you.

Jingbo