angle = theta2 - theta1;
    helical_length = hypot(angle * radius, axis_len);
    thelix = fabs(helical_length / ini_maxvel);
    helix_angle = atan2((radius * angle),axis_len);
    radial_vel = sin(helix_angle)*(helical_length/thelix);
    //find the centripetal acceleration of the helix
    centripetal_acc = radial_vel * radial_vel / radius;
    acc = MIN(acc, centripetal_acc);