Current regulation of solenoids is well-known and standard in any proportional electro-hydraulic valve. The goal is to provide a wide band transfer function from the current reference to the measured current, thus making the solenoid a fast and ideal force actuator for the position control within the limits of the power supplier. The latter is a Pulse Width Modulation (PWM) amplifier fixing the voltage limit and the Nyquist frequency of the regulator. Typical analogue regulators include three main terms: a feedforward channel, a proportional feedback channel and the electromotive force compensation fed by an estimate of the plunger velocity. The latter may be replaced by an integrative feedback. Here the problem is faced through a model based design (Embedded Model Control), based on a wide-band embedded model of the solenoids which includes also the effect of the eddy currents. To this end the model must be identified. The Embedded Model includes a disturbance dynamics capable of completing and correcting the electromotive contribution with parametric uncertainty, variability and state dependence. The embedded model fed by the measured current and the supplied voltage becomes a state estimator of the controllable and disturbance dynamics. The control law combines reference generator, state feedback and disturbance rejection to dispatch PWM with the appropriate duty cycle. Modelling, identification and control design are outlined together with regulator performance
