[Algo] Fix computation of potential energy

bindings/python/algorithm/expose-energy.cpp

@@ -32,7 +32,7 @@ namespace se3
                        "Update kinematics (bool)"),
               "Computes the kinematic energy of the model for the "
               "given joint configuration and velocity and stores the result "
-              " in data.kinetic_energy. By default, the kinematics of model is updated.");
+              "in data.kinetic_energy. By default, the first order kinematic quantities of the model are updated.");
       
       bp::def("potentialEnergy",potentialEnergy,
               bp::args("Model","Data",
@@ -40,7 +40,7 @@ namespace se3
                        "Update kinematics (bool)"),
               "Computes the potential energy of the model for the "
               "given the joint configuration and stores the result "
-              " in data.potential_energy. By default, the kinematics of model is updated.");
+              "in data.potential_energy. By default, the first order kinematic quantities of the model are updated.");
     }
     
   } // namespace python

src/algorithm/energy.hpp

@@ -99,7 +99,7 @@ namespace se3
     for(Model::JointIndex i=1;i<(Model::JointIndex)(model.njoints);++i)
     {
       com_global = data.oMi[i].translation() + data.oMi[i].rotation() * model.inertias[i].lever();
-      data.potential_energy += model.inertias[i].mass() * com_global.dot(g);
+      data.potential_energy -= model.inertias[i].mass() * com_global.dot(g);
     }
     
     return data.potential_energy;