What are the methods for detecting foreign objects during power transmission?
Nov 25, 2021| In the process of power transmission, there are many ways to detect foreign objects:
1. In low-power applications, directly detect the surface temperature. This method is rough and straightforward, but the foreign body may have risen to a certain temperature when it is detected, and the foreign body is a single-point heat source, and the temperature of the transmitting end is often also single-point detection. It is difficult to ensure that the temperature threshold is reasonable and accurate. So it is not the best way. (Difficulty: easy)
2. Judging by the efficiency or the power difference between transmission and reception. The receiving end will tell the transmitting end how much power it received, and the transmitting end will also detect the input power at the input. In this way, the efficiency or power difference can be calculated. If the efficiency is too low, or the power difference is too large, it is considered that there may be metal foreign objects between emission and reception, because the metal absorbs part of the loss. However, when the coupling between the receiving end and the transmitting end is not very good, the efficiency will be relatively low, and the power difference will be relatively large, so this method will cause a relatively large conflict between the accuracy of FOD and the degree of freedom of charging. (Difficulty: Medium)
3. Establish an accurate power loss model. Figure 4 shows the power transmission model, taken from WPC protocol Part1&2, Figure 47. Let's take a look at the power loss.
The losses at the transmitting end include: driving loss of the power full bridge, conduction loss of the power full bridge, ESR loss of the resonant capacitor, ACR loss of the inductor, eddy current loss of the magnetic material and metal of the transmitting end, and other circuit losses of the transmitting end. Here the sum of them is set to Pptloss. The transmitter can calculate the Pptloss under different couplings by detecting the average current at the input, the transient current in the resonant cavity, the voltage between the capacitor and the inductor, and so on.
