This paper deals with the modeling and nonlinear control of an ER (electrorheological) actuator consisting of a double-rod cylinder and four ER valves in a full-bridge configuration. Basically, we have to face two difficulties within the controller design: First of all, the ER effect is inherently nonlinear and secondly, the ER full-bridge provides more control inputs than necessary for solving the primary control task. We will show that these additional degrees-of-freedom can be exploited to circumvent undesirable operation and to optimize the overall closed-loop performance. Furthermore, the nonlinearities of the mathematical model will be systematically included in the controller design. Measurement results performed on an experimental test-stand will demonstrate the feasibility of the proposed strategy.