FUZZY LOGIC CONTROL OF A DRILL-STRING T. G. Ritto M. Gruzman R. Sampaio H. I. Weber thiagoritto@gmail.com maurgruzman@hotmail.com rsampaio@puc-rio.br hans@puc-rio.br PUC-Rio, Rua Marquês de São Vicente 225, 22453-900, RJ, Brasil Abstract. This work is concerned with the dynamics of a drill-string, which is a slender structure used to search for oil and gas. The system of equations obtained from the continuous model of the column is discretized by means of the Finite Element Method and reduced using a basis of normal modes of the prestressed configuration. Coupled axial and torsional vibrations are taken into account, and a bit-rock interaction that describes how the bit penetrates the rock is applied. A DC motor is connected to the top of the drill-string in order to provide the torque that rotates the system. The motor is controlled by the armature voltage with saturation limits and the adequate voltage is obtained by a Fuzzy Logic Controller. The inputs for the controller are the stress measured at some point of the drill-string and its derivative with respect to time. The output of the controller is an increment (or decrement) to the actual voltage provided to the motor. This control approach assures that the drill-string perforates the rock with a high rate of penetration and, simultaneously, that the stress in the drill-string is kept at an acceptable level, contributing to increase its lifetime. Keywords: nonlinear dynamics, drill-string control, fuzzy logic control