Moving enough mass to make the effect larger than residual forces. The stepper motors were limited in the torque they could apply. So the masses I could use were limited in both weight and acceleration. Coordinating the angular position and motion of the two weights. Tolchin used gears for this purpose.
Stepper motors are precise, but can lose their coordination when maximum torque is exceeded.
Balancing the two weights and getting their paths coplanar. Balancing took some careful adjusting. Aligning the thrust vector with the center of mass. Controlling the angular motion of the weights. Stepper motors allow precise control of rotational position, speed, and acceleration, within their torque limits.