Principle and Application of the Contactless Load Detection Based on the Amplitude Decay Rate in a Transient Process

For maximizing the power transfer in an inductive power transfer system, the operating frequency of the system is normally designed to be the same as the resonant frequency of the secondary power pick-up. But the load deviation reflected from secondary side to primary side will result in the frequency mismatch issue. Therefore, the contactless load detection technique is significant to ensure the power transfer efficiency and capacity. The emerging technique taking advantages of the amplitude decay rate in a transient process is promising. The robustness and practicability can get enhanced and the complicated steady circuit model is avoided. However, the accuracy for the small resistance application is still a challenge even with sophisticated compensation algorithms. This paper classified the system detectable region through the root locus method. The transient process in different subregions is excavated mathematically. It is revealed that the variation of the system from the second order to fourth order is the key issue for the detection error in the small load resistance application. The analysis brings out a novel automatic contactless load detection method to extend the detection range. Both simulation and experimental results verified the effectiveness of the proposed theory and detection method.