This paper presents a method to design a reduced order observer using an invariant manifold approach. The main
advantages of this method are that it enables a systematic design approach, and (unlike most nonlinear observer design
methods), it can be generalized over a larger class of nonlinear systems. The method uses specific mapping functions
in a way that minimises the error dynamics close to zero. Another important aspect is the robustness property which is
due to the manifold attractivity: an important feature when an observer is used in a closed loop control system. A two
degree-of-freedom system is used as an example. The observer design is validated using numerical simulation. Then
experimental validation is carried out using hardware-in-the-loop testing. The proposed observer is then compared with
a very well known nonlinear observer based on the off-line solution of the Riccati equation for systems with Lipschitz
type nonlinearity. In all cases, the performance of the proposed observer is shown to be very high
