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The discontinuous control signal in a sliding mode controller causes significant problems of chattering. A smooth control signal is important if the controller is to suppress torsional vibrations in drillstrings. This is mandatory to avoid placing the expensive top-drive under loads of oscillatory torque; maintaining its integrity and minimizing maintenance costs. A chattering free sliding-mode controller optimized using a Particle Swarm Optimization (PSO) routine is proposed to avoid bit sticking phenomena and in addition, provide a smooth control signal to the top drive system. In this work, a discontinuous lumped-parameter torsional model of four degrees of freedom is considered, and the bit-rock contact is taken to be a variation of the Stribeck friction together with the dry friction model.