Resonance tuning

A wireless power transfer device, comprising:

• power transfer terminals for coupling to a power transfer resonator external to the device, the resonator including:
• a resonator power transfer coil, and
• a resonator tuning capacitor with an adjustable tuning capacitance C_A,
• the power transfer resonator having a self-resonant frequency controllable based on adjusting C_A;
• a variable converter circuit to generate a variable alternating current voltage Vac and adjustable tuning signals to adjust adjustable tuning capacitance C_A, output through the power transfer terminals, corresponding to the controllable self-resonant frequency of the power transfer resonator;
• the variable converter circuit operable, when the power transfer device is magnetically coupled through the power transfer resonator to a power receiver device with a power receiver resonator, to:
• output through the power transfer terminals an initial Vac corresponding to an initial self-resonant frequency fA for the power transfer resonator, based on an initial capacitance value C_A for the adjustable tuning capacitor;
• iteratively adjust Vac to control the self-resonant frequency fA for the power transfer resonator, including iteratively adjusting capacitance C_A for the resonator tuning capacitor, based on iteratively estimating a self-resonant frequency fB of the power receiver device, by:
• sweeping the frequency of Vac in discrete frequency steps, and in each frequency step, monitoring a circuit parameter of the variable converter circuit,
• identifying a first local maximum and a second local maximum for the values of the circuit parameter corresponding to two resonant modes of the power transfer resonator and the power receiver resonator,
• ascertaining a first frequency and a second frequency corresponding to the first and second local maxima, respectively, and
• estimating a self-resonant frequency fB_est of the power receiver device based on the first and second frequencies;
• the variable converter circuit operable, when (fA−fB_est) is different than a threshold value, to:
• adjust the tuning capacitance C_A so that fA is substantially equal to fB_est;
• and
• iteratively perform a next estimation for a next estimated self-resonant frequency fB_est.