The use of Variable Valve Actuation (VVA) systems offers many advantages in terms of increased engine power, reduced fuel consumption and pollutant emissions, accomplishing a significant improvement of the global efficiency of the engine. In the last decade different devices have been proposed to implement advanced and innovative VVA managements on four-stroke engines. ElectroMechanical Valve Actuator (EMVA) formed by two opposite magnets and two balanced springs seem to be a very promising solution among several camless actuation systems. This type of valve actuator is characterized by highly nonlinear and strongly coupled dynamics which makes very difficult to govern engine valve motion during the last part of the closing and opening strokes, where an unstable behavior is exhibited.
In this regard the control problem of the EMVA is tackled in this paper. A predictive electromagnetic model based on a hybrid analytical-FEM approach is used to decouple and linearize the nonlinear cross-coupled electric, magnetic and mechanical dynamics of the actuator. The model takes into account self and mutual magnetic effects (inductances, back-EMFs, magnetomotive forces) over linear and saturation regions of the materials and for all engine valve positions. The proposed EMVA control scheme results in two nested state feedback controllers: the inner one regulates the currents of two coils, and the outer one controls the engine valve position through the magnetomotive force. An inversion algorithm transforms and splits the control force produced by the outer controller into the desired coil currents to be tracked by the inner controllers. Particular attention is paid in the design of a desired valve motion trajectory addressing a soft landing task. The control problem related to the first lift manoeuvre of the engine valve is also tackled exploiting the mechanical resonance of the actuator. Simulation results are presented to show the performances of the proposed decoupling control strategy. Eventually, the power required for EMVA operations is evaluated for different coil current control strategies.
