V. EFFECTS OF DISTANCE VARIATIONS
It can be seen in equation 16 that Zin and Zout vary
when the distance between the two coils is modified (which
modifies M12). To compensate this effect, it will be necessary
to apply a different impedance in source and load for each
value of distance (separation between drive and load coils).
Therefore, because a variation in distance modifies load and
source impedances, there are two options to maximize the
power transfer efficiency:
If the system has fixed source and load impedances, it is
necessary to modify the distance between transmitter and
receiver.
If the system has a variable distance, we should compensate
this by adapting the impedances in source and
load.
In figure 3 the efficiency of the system for different values
of RS = RL has been superimposed upon the maximum
theoretical efficiency achievable (obtained under variable
distance adaptive impedance matching conditions) and the
maximum efficiency obtained with the Finite Element Field
Solver FEKO. These results were obtained from a two-coil
copper-made RIC link with coil radius a = 0:5m, wire radius
b = 5mm and R1 = R2 = 10
. It can be seen that
the static curves are equal to the maximum efficiency limit
when the distance within the coils forces the input and output
impedance of the system to be the same as the source and load
resistors, this is; when an impedance match occurs. It can be
observed that given a value for the source and load resistors
(ZS = ZL = RS = RL) there is an optimal distance at
which the input and output impedances are perfectly matched
(RL = RS = Rin = Rout). To obtain this distance it is
necessary to find the mutual inductance between coils. In the
quasi-static limit and at large distances (D >> a), a coil
behaves as an electrically small antenna and the magnetic flux
density at the second coil as a result of the first has the form
of a dipole [6]