Ankle frame for wrench distribution

[Saeed-Mansouri]

Hi Dr. Caron,

Based on what you stated in the paper, the foot contact wrench is expressed in the sole and ankle frames in the wrench distribution QP problem. In the LIPM walking controller, you just added some offsets in the sagittal and lateral planes to the foot center position for calculating the position of the ankle frame origin. It means that the ankle offset in the z-direction is omitted in the LIPM codes. I like to know why you didn't consider the z-offset?

  Eigen::Vector3d anklePos(const Sole & sole) const
  {
    if(surfaceName == "LeftFootCenter")
    {
      return p() + sole.leftAnkleOffset.x() * sagittal() + sole.leftAnkleOffset.y() * lateral();
    }
    else if(surfaceName == "RightFootCenter")
    {
      return p() + sole.leftAnkleOffset.x() * sagittal() - sole.leftAnkleOffset.y() * lateral();
    }
    else
    {
      LOG_ERROR("Cannot compute anklePos for surface " << surfaceName);
      return p();
    }
  }

[stephane-caron]

This point is explained briefly in the paper, where the frame is defined as:

[...] in the sole center frame lc and ankle frame la (sole frame closest to the ankle joint)

We call this frame "ankle" for short, but it is rather the projection of the ankle joint frame onto the sole, along the z-axis of the sole frame (hence the zero offset along the z-axis of the sole frame).