where *b*_{d} indicates the equivalent torsional damping, *K*_{t} is the equivalent torsional stiffness, *V* stands for the supply voltage, *x* is the plunger displacement, *r* indicates the radius of the pinion, *C*_{1} and *C*_{2} are the reluctances of the magnetic path without air gap and that of the air gap, respectively, *F*_{m} is the motive force, *N* stands for the number of coils, *i* indicates the applied current, *g*_{m} is the nominal airgap, *J* indicates the polar moment of inertia of the valve's disk, and *R* is the electrical resistance of coil.