Many model based control methods exist in the literature. Producing a sufficient system model can be cumbersome and a new non-model based method for control of nonlinear systems with input/output maps exhibiting sigmoid function properties is therefore proposed. The method utilizes an excitation signal together with Fourier analysis to generate a feedback signal and simulations have shown that different system gains and time constants does not change the global equilibrium/operating point. An evaporator in a refrigeration system was used as example in the simulations, however, it is anticipated that the method is applicable in a wide variety of systems satisfying the sigmoid function properties.