Is the differential drive wheeled mobile robot underactuated or not?

Question

I have read about underactuated robotic but I still need to know about differential drive wheeled mobile robot with ordinary wheels, is it underactuated or not? In many articles there is written in title "underacutated mobile robot" and in others only "mobile robot". Does the word "underactuated" has any sense for this kind of robots?

Answer 1

There is an ambiguity in that matter in mobile robotics, which leads to confusions.

As usual the problem lies in the definiotion of "the underactuation" and "degrees of freedom".

The following definitions comes from mechanics and phisics:

• degrees of freedom - the dimension of a configuration space $X$, i.e. $\dim (X)=n$,
• underactuation - the property of the system that has less controls than degrees of freedom, i.e. $n>m$.

Using the above definions, most of kinematic equations of mobile robots are underactuated (moreover the fact that they are driftless makes them nonholonomic)

However I have read an article in the field of mobile robotics (I cannot find it now), where thouse definitions were quite different and the authors claimed that the system (the differential-driven robot) is fully actuated since it has 2 controls and it can move it two directions only. To me, it is confusing and incorrect.