title: Can SpatialAcceleration be used with Eigen?
body: <p>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</p>
  <pre><code> Eigen::Vector3d mdJv_dq = mplant.CalcBiasTranslationalAcceleration(*mContext,
                                    multibody::JacobianWrtVariable::kV,
                                    *mFrame_EE,
                                    Eigen::Vector3d::Zero(),
                                    mplant.world_frame(),
                                    mplant.world_frame()
                                    ); 
  </code></pre>
  <p>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</p>
  <pre><code>multibody::SpatialAcceleration&lt;double&gt; mdJ_Aug_dq_robot1 = mplant.CalcBiasSpatialAcceleration(*mContext,
                                    multibody::JacobianWrtVariable::kQDot,
                                    *mFrame_EE,
                                    Eigen::Vector3d::Zero(),
                                    mplant.world_frame(),
                                    mplant.world_frame()
                                    ); 
  </code></pre>
  <p>Now the return type is spatialacceleration. If we need to use it along with Eigen::Matrix&lt;double, 6,1&gt; like subtracting the two quantities, we get an error as follow</p>
  <pre><code>no match for 'operator-' (operand types are 'Eigen::Matrix&lt;double, 6, 1&gt;' and 'drake::multibody::SpatialAcceleration&lt;double&gt;')
  </code></pre>
  <p>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.</p>