Can SpatialAcceleration be used with Eigen?

Question

I am trying to compute product of \dot{J}\dot{q} in drake, where J is augmented form of angular and linear jacobian. For linear part only, I know that it can be done by CalcBiasTranslationalAcceleration function. The return type is Eigen::Vector3d

 Eigen::Vector3d mdJv_dq = mplant.CalcBiasTranslationalAcceleration(*mContext,
                                  multibody::JacobianWrtVariable::kV,
                                  *mFrame_EE,
                                  Eigen::Vector3d::Zero(),
                                  mplant.world_frame(),
                                  mplant.world_frame()
                                  ); 

However, if I need to calculate both the linear and angular jacobain bias, I have to use SpatialAcceleration MultibodyPlant::CalcBiasSpatialAcceleration() and the function is as follow

multibody::SpatialAcceleration<double> mdJ_Aug_dq_robot1 = mplant.CalcBiasSpatialAcceleration(*mContext,
                                  multibody::JacobianWrtVariable::kQDot,
                                  *mFrame_EE,
                                  Eigen::Vector3d::Zero(),
                                  mplant.world_frame(),
                                  mplant.world_frame()
                                  ); 

Now the return type is spatialacceleration. If we need to use it along with Eigen::Matrix<double, 6,1> like subtracting the two quantities, we get an error as follow

no match for 'operator-' (operand types are 'Eigen::Matrix<double, 6, 1>' and 'drake::multibody::SpatialAcceleration<double>')

I could not find a method to utilize both of them togethor or converting spatialAcceleration quantity to Eigen. Any help and guidance will be appreciated.

Answer 1

You should be able to use get_coeffs() method of SpatialAcceleration to get the Eigen vector out then use normal Eigen operations. doxygen link

Or you can call SpatialAcceleration(my_other_eigen_vector) to lift your Eigen vector up into a SpatialAcceleration.