  In various applications in the field of control engineering the estimation of
the state variables of dynamic systems in the presence of unknown inputs plays
an important role. Existing methods require the so-called observer matching
condition to be satisfied, rely on the boundedness of the state variables or
exhibit an increased observer order of twice the plant order. In this article,
a novel observer normal form for strongly observable linear time-invariant
multivariable systems is proposed. In contrast to classical normal forms, the
proposed approach also takes the unknown inputs into account. The proposed
observer normal form allows for the straightforward construction of a
higher-order sliding mode observer, which ensures global convergence of the
estimation error within finite time even in the presence of unknown bounded
inputs. Its application is not restricted to systems which satisfy the
aforementioned limitations of already existing unknown input observers. The
proposed approach can be exploited for the reconstruction of unknown inputs
with bounded derivative and robust state-feedback control, which is shown by
means of a tutorial example. Numerical simulations confirm the effectiveness of
the presented work.
