A method using sliding mode and non-linear H_∞ control is presented for an underactuated mechanical system with friction and a force constraint to solve a position regulation problem. A low pass filter is suggested to be used for the observer realization. It was proved that the nonlinear system is globally asymptotically stable and achieves zero steady-state position error even in the presence of certain disturbances, otherwise the controller attenuates others external perturbations and dead zone model discrepancies. The results obtained are illustrate by numerical simulations.