The XFEL (X-Ray Free Electron Laser) with its beam energy up to 20GeV will be a new milestone for accelerator based experimental opportunities. The radiation wavelength of 6 to 0.085 nm in pulses of 100 fs, allows scientist for example to study dynamical processes like chemical reactions in atomic level. To generate this brilliant laser, a high accurate accelerator is essential. The demand for stability and precision of the accelerating voltage, high degree of automation of operation, built-in diagnostics, and flexibility to change controller characteristics, favours the implementation of digital controllers. This device must be designed to satisfy all the above requirements. Extensive plant modeling is a prerequisite for the controller design. Therefore a pilot project test facility is already installed for testing and verifying new developments in design. The developed plant model will include cavity, klystron, field detectors and actuators for control. The identification of types and magnitudes of field perturbations, for instance Lorentz force detuning and beam loading are vertices to define optimality criteria for the controller. To ensure long term stability, proposals for beam based feedback need to be found.