1 Semi active suspension control techniques using only one measurement sensor F´ abio Luis Marques dos Santos, 1 Luiz Carlos Sandoval G´ oes, 1 and Alberto Luiz Serpa 2 1 Instituto Tecnol´ ogico de Aernon´ autica 2 Universidade Estadual de Campinas Summary. In this paper two control strategies are addressed - the Linear Quadratic Regulator (LQR) and the Internal Model Control (IMC). These techniques were tested in a system where saturation is present, the semi-active suspension problem. They are then evaluated by some performance indexes that characterize the problem as optimization problems, and solved by the use of some specific tools of the MATLAB software. The main goal is to obtain optimal performance, in order to minimize the sprung mass (chassis) acceleration and to ensure road-holding characteristics, while minimizing the number of feedback sensors used. To make-up for the minimum sensor usage, state observers are designed. A comparison is then made, by means of numerical simulation, using a quarter- car model and real road profiles as well. 1 INTRODUCTION There is great effort nowadays to improve vehicle comfort quality and security. The vehicle suspensions are responsible for the isolation of the passenger compartment, which can be regarded as a comfort objective, while still holding the upper part of the vehicle, that is, counteracting internal and external forces, that is, to keep the vehicle stable. For long there have been studies on both industry and academia, to improve the suspension performance. These studies have led to the active and semi-active suspension systems. To better explain these systems, it can be easier to first introduce the so called passive systems.