Experimental Nonlinear Dynamics of a Closed Loop Electro-Mechanical Valve Actuator (EMVA)

Summary. Electromechanical Valve Actuators (EMVA) are fundamental components of camless engines. This work is concerned with the experimental validation of the bifurcation behaviour of an EMVA controlled via a so-called "key-on" feedback controller. The experimental results are validated via an appropriate mathematical model and the existence of limit cycles proved analytically by using a describing function approach. The numerical and theoretical analysis are shown to capture the observed experimental behaviour. Introduction and motivation Double magnet Electromechanical Valve Actuators (EMVA) (7) are essential components in camless engines (14). They consist of an armature linked to the valve, mounted between two symmetric high-stiffness springs. The valve is operated by means of a magnetic force produced by two magnetic coils so as to be open or closed. In so doing, it is essential for a feedback control action to be present in order to ensure acceptable performance. As shown in (5), to design such a controller it is essential to keep into account the many nonlinear terms affecting the valve operation such as the presence of friction, magnetic forces and the nonlinear behaviour of the springs (10). Here we consider the mathematical model of the valve developed in (5) where a nonlinear Stribeck friction model (2) is used. The proposed control is a switched control action aimed at compensating friction so as to move the valve during the key-on maneuver away from its stable equilibrium position at rest (9). When the controller operates the EMVA, the rigid body (armature and valve body) alternately sticks and slips with the constraints. This is caused by the fact that friction var ies as a function of velocity, i.e., loosely speaking, in certai n velocity range, the friction force is larger at rest than du ring motion. Note that during operation, high and varying temperatures can produce important variations in the parameters of the EMVA, that can affect strongly the controller performance. Therefore, the study of such parameters variation and their effect on system behaviour becomes a key point during controller design in order to tune the controller and increase the robustness range where the system performance is acceptable. In this paper, we validate experimentally the nonlinear behavior of the controlled valve proposing bifurcation analysis as an invaluable tool to tune the controller gains. We also show that the coexistence of limit cycles detected experimental ly can be accounted for by using a describing function approach (3, 13).