[ImpedanceTask] Fix use of targetSurface and implement targetFrame/targetFrameVelocity

[arntanguy]

We noticed with @Hugo-L3174 that it is possible to call ImpedanceTask::targetSurface but that the implementation is wrong. In the current implementation calling the parent's TransformTask::refVelB and TransformTask::target are explicitly disallowed as the impedance task needs to keep track of the desired target (that is modified through the impedance equation to provide a compliance pose/velocity/acceleration that is then given as the TransformTask's target). However since ImpedanceTask::targetSurface is not hidden it is possible to directly modify the target of the underlying TransformTask without modifying the impedance target.

To fix this I propose:

• To make TransformTask::target virtual, which allows ImpedanceTask to override it to keep track of the desired target pose
• To add a virtual TransformTask::targetVel function that allows setting the velocity in world frame. This is both for convenience and to mimic the existing ImpedanceTask::targetVel
• For convenience I also added TransformTask::targetFrame and TransformTask::targetFrameVelocity functions. They work as expected for the ImpedanceTask as they simply call the newly introduced virtual functions.

If we wish to be a bit more comprehensive, we could also do the same for refVelB which is currently still hidden in ImpedanceTask.

[mmurooka]

@arntanguy @gergondet
Thanks for the update. Just one comment.

The target method above is private, so it is meaningless if it is not used in this class (and in fact this overridden target method is not used in the class). Isn't the intention to make it a public method?

[mmurooka]

Oh, sorry. I understand that this is virtual, so it has a meaning when called from inside TransformTask. I understand.

[arntanguy]

Indeed, I in fact considered making it public anyways for consistency between TransformTask and ImpedanceTask, but since we already had ImpedanceTask::targetPose being widely used I left it private. Plus having to explicitly use targetPose highlights that the impedance task target is different from the underlying TransformTask.

[mmurooka]

I found one side effect of this PR. Before this PR was merged, the following two log entries represented different poses; the former is the target pose given by the user, the latter is the command pose modified by the impedance control. After this PR, they represent the same pose, i.e., the target pose given by the user, and we no longer see the modified command pose in the log.

mc_rtc/src/mc_tasks/ImpedanceTask.cpp

Line 211 in afc9934

MC_RTC_LOG_HELPER(name_ + "_targetPose", targetPoseW_);

mc_rtc/src/mc_tasks/TransformTask.cpp

Line 200 in afc9934

logger.addLogEntry(name_ + "_target_pose", this, [this]() { return target(); });