This paper presents a new control scheme for a class of nonlinear systems. In the proposed method, an adaptive neural network observer with Rise feedback controller are applied to realize sensorless control scheme. This observer is tuned online and no exact information of nonlinear term of plant is required. So, this characteristic can compensate mismodeling phenomena. Also, a new controller called Robust Integral of the Sign of the Error (Rise) is considered to realize control purpose. This controller is inspired from 2nd order sliding mode while it can control system with different relative degree. Also, its chattering is acceptable in comparison with sliding mode strategy. This observer based control scheme is considered for modified Duffing chaotic system. The modified Duffing system is derived from Metamorphic shape-changing Underwater autonomous vehicle (MUV). The chaotic behavior of modified Duffing system has a negative impact on MUV performance. Therefore, the controlling of this system can be important. In order to assess the performance of the proposed method, this strategy is compared with observer-based sliding mode control. The comparison results confirm the advantages of proposed method.