This report investigates how a force controller to a servo actuator system consisting of an asymmetric cylinder and a symmetrical proportional valve may be designed. A non linear model of the system is derived and validated by comparing the simulated behaviour to the actual system behaviour. Using linear control theory, a PI controller is designed achieving satisfactory force tracking performance when given a sinusoidal force reference. To improve the controller, it is extended by employing a valve compensator. Best flow estimation performance is found when compensating with respect to one chamber pressure instead of both chamber pressures. The error was reduced when employing the compensator, and similar behaviour was achieved when lowering the supply pressure using a compensator while the error was increased without a compensator. It was found that the valve signal or force reference may be used to govern the compensator provided that the valve dynamic is sufficiently fast. Similar force tracking performance is achieved when using a valve matched to the cylinder. The chamber pressure gradients are seen to be smoother, and a lower supply pressure is required.