Consider a solenoid valve where input voltage e(t) creates current i(t) in the circuit that produces magnetic force, proportional to the current as
f_(m)(t)=K_(m)l(t). This magnetic force actuates the valve plug with mass m that is connected to the valve body via spring-damper system, k and b.
Therefore the displacement of the valve plug x(t) driven by this magnetic force controls the valve opening to adjust the flow rate q(t) that is also
directly proportional to the plug displacement x(t) as q(t)=K_(v)x(t).
ae(t) to current i(t),
and relating current i(t) to magnetic force f_(m)(t),
and relating magnetic force f_(m)(t) to plug displacement x(t),
and relating to plug displacement x(t) to flow rate q(t).
be(t) to the flow rate q(t),
ce(t) to output flow rate q(t).