force_constraint.cc

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#include <towr/constraints/force_constraint.h>

#include <towr/variables/variable_names.h>

namespace towr {


ForceConstraint::ForceConstraint (const HeightMap::Ptr& terrain,
                                  double force_limit,
                                  EE ee)
    :ifopt::ConstraintSet(kSpecifyLater, "Force-Constraint-" + id::EEForceNodes(ee))
{
  terrain_ = terrain;
  fn_max_  = force_limit;
  mu_      = terrain->GetFrictionCoeff();
  ee_      = ee;

  n_constraints_per_node_ = 1 + 2*k2D; // positive normal force + 4 friction pyramid constraints
}

void
ForceConstraint::InitVariableDependedQuantities (const VariablesPtr& x)
{
  ee_force_  = x->GetComponent<PhaseNodes>(id::EEForceNodes(ee_));
  ee_motion_ = x->GetComponent<PhaseNodes>(id::EEMotionNodes(ee_));

  pure_stance_force_node_ids_ = ee_force_->GetIndicesOfNonConstantNodes();

  int constraint_count = pure_stance_force_node_ids_.size()*n_constraints_per_node_;
  SetRows(constraint_count);
}

Eigen::VectorXd
ForceConstraint::GetValues () const
{
  VectorXd g(GetRows());

  int row=0;
  auto force_nodes = ee_force_->GetNodes();
  for (int f_node_id : pure_stance_force_node_ids_) {

    int phase  = ee_force_->GetPhase(f_node_id);
    Vector3d p = ee_motion_->GetValueAtStartOfPhase(phase); // doesn't change during stance phase
    Vector3d n = terrain_->GetNormalizedBasis(HeightMap::Normal, p.x(), p.y());
    Vector3d f = force_nodes.at(f_node_id).p();

    // unilateral force
    g(row++) = f.transpose() * n; // >0 (unilateral forces)

    // frictional pyramid
    Vector3d t1 = terrain_->GetNormalizedBasis(HeightMap::Tangent1, p.x(), p.y());
    g(row++) = f.transpose() * (t1 - mu_*n); // t1 < mu*n
    g(row++) = f.transpose() * (t1 + mu_*n); // t1 > -mu*n

    Vector3d t2 = terrain_->GetNormalizedBasis(HeightMap::Tangent2, p.x(), p.y());
    g(row++) = f.transpose() * (t2 - mu_*n); // t2 < mu*n
    g(row++) = f.transpose() * (t2 + mu_*n); // t2 > -mu*n
  }

  return g;
}

ForceConstraint::VecBound
ForceConstraint::GetBounds () const
{
  VecBound bounds;

  for (int f_node_id : pure_stance_force_node_ids_) {
    bounds.push_back(ifopt::Bounds(0.0, fn_max_)); // unilateral forces
    bounds.push_back(ifopt::BoundSmallerZero); // f_t1 <  mu*n
    bounds.push_back(ifopt::BoundGreaterZero); // f_t1 > -mu*n
    bounds.push_back(ifopt::BoundSmallerZero); // f_t2 <  mu*n
    bounds.push_back(ifopt::BoundGreaterZero); // f_t2 > -mu*n
  }

  return bounds;
}

void
ForceConstraint::FillJacobianBlock (std::string var_set,
                                    Jacobian& jac) const
{
  if (var_set == ee_force_->GetName()) {

    int row = 0;
    for (int f_node_id : pure_stance_force_node_ids_) {

      // unilateral force
      int phase   = ee_force_->GetPhase(f_node_id);
      Vector3d p  = ee_motion_->GetValueAtStartOfPhase(phase); // doesn't change during phase
      Vector3d n  = terrain_->GetNormalizedBasis(HeightMap::Normal,   p.x(), p.y());
      Vector3d t1 = terrain_->GetNormalizedBasis(HeightMap::Tangent1, p.x(), p.y());
      Vector3d t2 = terrain_->GetNormalizedBasis(HeightMap::Tangent2, p.x(), p.y());

      for (auto dim : {X,Y,Z}) {
        int idx = ee_force_->Index(f_node_id, kPos, dim);

        int row_reset=row;

        jac.coeffRef(row_reset++, idx) = n(dim);              // unilateral force
        jac.coeffRef(row_reset++, idx) = t1(dim)-mu_*n(dim);  // f_t1 <  mu*n
        jac.coeffRef(row_reset++, idx) = t1(dim)+mu_*n(dim);  // f_t1 > -mu*n
        jac.coeffRef(row_reset++, idx) = t2(dim)-mu_*n(dim);  // f_t2 <  mu*n
        jac.coeffRef(row_reset++, idx) = t2(dim)+mu_*n(dim);  // f_t2 > -mu*n
      }

      row += n_constraints_per_node_;
    }
  }


  if (var_set == ee_motion_->GetName()) {

    int row = 0;
    auto force_nodes = ee_force_->GetNodes();
    for (int f_node_id : pure_stance_force_node_ids_) {

      int phase  = ee_force_->GetPhase(f_node_id);
      int ee_node_id = ee_motion_->GetNodeIDAtStartOfPhase(phase);

      Vector3d p = ee_motion_->GetValueAtStartOfPhase(phase); // doesn't change during pahse
      Vector3d f = force_nodes.at(f_node_id).p();

      for (auto dim : {X_,Y_}) {

        Vector3d dn  = terrain_->GetDerivativeOfNormalizedBasisWrt(HeightMap::Normal, dim, p.x(), p.y());
        Vector3d dt1 = terrain_->GetDerivativeOfNormalizedBasisWrt(HeightMap::Tangent1, dim, p.x(), p.y());
        Vector3d dt2 = terrain_->GetDerivativeOfNormalizedBasisWrt(HeightMap::Tangent2, dim, p.x(), p.y());

        int idx = ee_motion_->Index(ee_node_id, kPos, dim);
        int row_reset=row;

        // unilateral force
        jac.coeffRef(row_reset++, idx) = f.transpose()*dn;

        // friction force tangent 1 derivative
        jac.coeffRef(row_reset++, idx) = f.transpose()*(dt1-mu_*dn);
        jac.coeffRef(row_reset++, idx) = f.transpose()*(dt1+mu_*dn);

        // friction force tangent 2 derivative
        jac.coeffRef(row_reset++, idx) = f.transpose()*(dt2-mu_*dn);
        jac.coeffRef(row_reset++, idx) = f.transpose()*(dt2+mu_*dn);
      }

      row += n_constraints_per_node_;
    }
  }
}

} /* namespace towr */