The current guided to the piezo actuator and the voltage which is consequently established thereon is used for calculating with the help of a non-linear actuator model, the characteristics of the longitudinal variations (s) and the force (F) exerted by the actuator (F), and variables therefrom or the derived variables (dF/dt) therefrom determine the beginning of the opening (tA) of a servovalve and the duration of injection (D).