1. Introduction
Recently, research on wireless power transfer (WPT) has been focused on wireless charging of large mobile devices such as electric vehicles, automatic guided vehicles, and robots beyond wireless small devices such as mobile phones, smart watches and wearable devices [
1,
2,
3]. Since it requires thousands of times more power than conventional small devices, high-power WPT has additional requirements for safety during charging as well as the requirements for charging efficiency [
4,
5]. Therefore, technologies such as unwanted emission electromagnetic field reduction, heat control, foreign object detection (FOD), position detection (PD), and human safety have attracted more attention [
6,
7]. In particular, technology that detects metallic foreign objects (MO) such as coins, keys, and foil not only reduces transmission efficiency, but can also cause circuit damage or fire, and even life-threatening if creatures such as cats and dogs get in between. Therefore, technologies such as FOD including metal object detection (MOD) and live object detection (LOD) are defined as very important technologies in the application of the product and are also defined as essential elements that must be observed in industry standardization organizations such as the Wireless Power Consortium (WPC) and Society of Automotive Engineers (SAE) International.
The Qi Wireless Power Transfer System Power Class 0 Specification version 1.2.3 proposes two FOD methods [
8]. The first method monitors the temperature change at the interface surface of the transmitter. Another method monitors power loss between the transmitting interface and the receiving interface. However, in the case of high-power WPT, it is not appropriate to apply the methods proposed by Qi WPT. This is because the power loss caused by a metallic foreign object (MO) is very small compared to the output power in high-power WPT systems, making it difficult to detect. In addition, if there is a separation distance between the transmitter coil and receiver coil unlike the low-power WPC system, the distance between the coils is not constant and the power transfer efficiency varies greatly by misalignment [
9,
10]. Although SAE International’s J2954 wireless power transfer for light-duty plug-in/electrical vehicles and alignment methodology does not propose specific FOD methods, it is regulated to conduct MOD test including coins, paper clips, steel sheet, beverage cans, etc. [
6,
11].
For FOD methods in high-power WPT, wave-based detection methods and field-based detection methods are considered [
6]. Wave-based detection methods require additional sensing devices such as imaging cameras, thermal cameras, or radar sensors to detect the presence of foreign objects. Thus the method has a high cost and is difficult to integrate with the power transfer system. The field-based detection method is a method of detecting a change in a magnetic field generated by the metal objects. To this end, a detection coil other than the transmitter coil and the receiver coil is added, and placed between the transmitter coil and the receiver coil to detect change in the characteristics of the detection coil. However, since the characteristics of the detection coil are also affected by the location of the receiver coil and the surrounding conditions such as temperature, techniques using a balanced coil have recently been introduced.
The balanced coil is also basically located between the transmitter coil and the receiver coil. In addition, a reference coil is added to detect the changes in the characteristics of the balanced coil when metal foreign objects are inserted. The balanced coil and the reference coil are arranged symmetrically with respect to the transmitter coil, so that the magnitude of the magnetic flux induced in the two coils is the same. When a metal object is inserted, the balance of magnetic flux induced in the two coils is broken. Then, the presence of a metal object is detected by detecting that the amount of current flowing through the two coils is different. When there is no metal foreign object, the current does not flow in the coil [
12].
The size of the MO that can be detected depends on the size of the balanced coil, so the balanced coil must be small enough to detect general metal objects. In addition, depending on the shape or arrangement of balanced coils, there may be areas where it is difficult to detect MOs. In other words, when a relatively small MO is inserted or a MO is inserted in an area that is difficult to detect, the change in inductance of the balanced coil is very small and it is difficult to detect it. Therefore, studies have been proposed to amplify a voltage or current generated by the change of inductance with an amplifier, but they have a problem of amplifying noise generated by power transfer signals. Therefore, in this paper, we propose a balanced coil structure with a serial-resonance Maxwell bridge circuit that can detect small MOs in wireless power transfer systems with a certain distance between the transmitter coil and the receiver coil.
3. Results
A wireless power transfer system including balanced coil arrays with serial-resonance Maxwell bridges was constructed and tested as shown in
Figure 8. The AC generator supplies 800 kHz AC to the Maxwell Bridge circuit for MOD. The transmitter generates and passes 70 kHz AC signals up to 2 kW to the power transfer coil that is placed under the balanced coil array.
For the MOD experiment, as shown in
Table 1 and
Figure 9, a 500 Korean Won(KRW) coin, a 10 KRW coin, a clip, and a screw were used.
The transfer coil was configured in a 30 cm × 20 cm rectangular shape and the loop coil of the balanced coil was made of 5 cm × 5 cm. Therefore, as shown in
Figure 10, a two-layer balanced coil array consists of a total of 4 × 6 rectangular loop coils. Here,
Xn means a set of loop coils connected along the
Y axis in the first layer.
Ym refers to the set of loop coils connected along the
X axis in the second layer. And (
xn,
ym) means the location where
Xn and
Ym intersect. Since the loop coil is arranged one space apart, the loop coil at the intersection is connected to only one of the
Xn and
Ym. The following Equations (15) and (16) summarize the loop coils connected to
Xn and
Ym for each layer.
As shown in Equations (15) and (16), Xn consists of two loop coils and Ym consists of three loop coils. Therefore, the coil inductance of the balanced coil array of the first and second layers is different. The inductance of X and Y is 40 μH and 60 μH, respectively. The 1000 pF and 680 pF capacitors were used, respectively, to achieve resonance at 800 kHz with the balanced coil.
The quality factor (Q) of the serial resonant circuit was simulated as shown in
Figure 11 to determine the resistance
R1 and
R2 of the proposed Maxwell bridge. The smaller the resistors, the larger the Q. However, if the resistor is too small, the current flows excessively and it takes a long time for the circuit to stabilize, so it is not suitable for a circuit that measures impedance. If the resistor is too large, the current flows less, so it makes difficult to detect the impedance change by a foreign metal object. Therefore, the following simulations and experiments were repeated to determine the magnitude of
R1 and
R2.
The serial-resonance Maxwell bridge circuit has a
VAB of zero when the Equation (10) is satisfied. However, there are errors in the circuit components.
Figure 12a shows the
VAB when the values of
R1 and
R2 do not match. When the resistors,
R1 and
R2, are selected as 100 Ω and the resistors have a tolerance of 5%,
VAB is about 120 mV at the maximum. Similarly, when the capacitors,
C1 and
C2, have a tolerance of 5%,
VAB is about 800 mV at the maximum as shown in
Figure 12b.
When the capacitor changes, the resonance frequency of the circuit changes, and when the resistor changes, the bandwidth of Q changes. If the resistors are 10 Ω, the bandwidth of Q becomes narrower than when the resistors are 100 Ω and VAB is about 2 V for a 5% tolerance change of the capacitor. Conversely, if the resistors are 200 Ω, the bandwidth of Q becomes wider, and VAB is about 450 mV. That is, when two capacitors are mismatched, if the resistors are small, VAB increases, and if the resistors are large, VAB decreases. Therefore, it is preferable that the resistors are appropriately large. However, when the equivalent capacitor slightly changes due to a small MO, a change of VAB must occurs, so it is preferable that the resistors are appropriately small. Therefore, in this paper, R1 and R2 are determined as 100 Ω.
In the absence of MO, VAB can be made to zero by adjusting the resistors. In this paper, the resistors were not fine-tuned to remove the offset. Instead, the offset does not affect the MOD by adjusting the threshold to determine MO.
Figure 10b shows the balanced coil set whose balance is broken according to the position of the MO. When a MO is at position (
x1,
y1), the MO is above coil set
X1, not
Y1. Only the inductance of
X1 is changed by MO. Since
X1 constitutes a balanced coil with
X6, the balance between
X1 and
X6 is broken, and
VAB for the balanced coil B{
X1:
X6}, B(
X1,
X6), changes. Conversely, if B(
X3,
X4) changes, according to Equation (15), it can be seen that MO is placed on one of the loop coils of (
x3,
y1), (
x3,
y3), (
x4,
y2) and (
x4,
y4), which belong to
X3 or
X4. If a MO is at (
x1,
y2), the balance of B{
Y2:
Y3} is broken, so B(
Y2,
Y3) changes. Similarly, if B(
Y1,
Y4) changes, according to Equation (16), it can be seen that MO is placed on one of the loop coils of (
x2,
y1), (
x4,
y1), (
x6,
y1) and (
x1,
y4), which belong to
Y1 or
Y4. The voltage for each position in
Figure 10a is a value measured when 500 KWN coin is placed in the center of each position and is about 700–1200 mV
zero-to-peak.
Figure 13 shows the voltage difference between when Maxwell bridge is used and when not used for MOD.
Figure 13a shows the case where a 500 KWN coin is at (
x1,
y1). The actual voltage difference between the coil
X1 and
X6 is about 0.5 V. However, as in this paper, if the voltage difference is measured using the Maxwell bridge, the phase difference is also reflected, so it is measured as about 2.2 V peak-to-peak. In other words, the voltage difference occurs about four times more than when the Maxwell bridge circuit is not used.
As shown in
Figure 13b, when a 500 KWN coin is at (
x1,
y2), the actual voltage difference is about 0.05 V and the voltage difference using the Maxwell bridge is about 1.2 Vpeak-to-peak. The reason why B(
X1,
X6) and B(
Y2,
Y3) are different for the same MO is that the inductance and capacitance of the balanced coil set B{
X1:
X6} and B{
Y2:
Y3} are different. As shown in Equations (14) and (15), the voltage difference
VAB occurs according to the phase difference between the currents flowing through the two nodes, and the current is determined by resistance
R, inductance
L and capacitance
C in each path. Even with the same MO, the degree of phase change is different because the
R,
L, and
C values of B{
X1:
X6} and B{
Y2:
Y3} are different. Therefore, the larger the phase change by MO, the larger the value of
VAB.
When each MO was placed in the center of (
x1,
y1), the zero-to-peak
VAB in the serial-resonance Maxwell bridge was measured and summarized in
Table 2. When there is no MO,
VAB was 0.197 V. Although it was not possible to compare the components of the MOs, the voltage difference tended to decrease as the cross-sectional area was small. In this experiment, we confirmed that the clips were sufficiently detectable by increasing the voltage difference by approximately 1.5 times compared to when there were not MOs.
To find the hidden area, the position of the 500 KRW coin was moved as shown in
Figure 14 and the voltage difference B(
X1,
X6) and B(
Y1,
Y4) were measured. The measured results are summarized in
Figure 14b and
Table 3. When the coin is in the ‘a’ position, i.e., the coin is at the center of (
x1,
y1), the impedance of
X1 changes. Therefore, as a result of measurement, B(
X1,
X6) is about 1.1 V. As the coin is away from the center of the
X1 coil, the influence of the coin on the loop coil at the (
x1,
y1) position decreases, so the difference between the impedance of
X1 and the impedance of
X6 gradually decreases. That is, B(
X1,
X6) gradually decreases. When there is no overlap with the
X1 coil, i.e., when the MO is in the ‘d’ and ‘e’ positions, the coin hardly affects the impedance of
X1. So, B(
X1,
X6) is almost the same level as when there is no coin. Conversely, when the coin is on the loop coil of (
x1,
y1), it hardly affects the impedance of
Ym. As the coin is placed on the loop coil of (
x2,
y1), the impedance of
Y1 starts to change. Therefore, when the coin is placed in the ‘e’ position, which is the center of (
x2,
y1), the impedance difference between
Y1 and
Y4 becomes the greatest. That is, B(
Y1,
Y4) was greatest when the coin was in the ‘e’ position, and it decreased toward the ‘a’ position.
When there was a MOD on the coil of layer 1, B(Xn, XN−n+1) was increased several hundred mV or more. The changed value that is about 3 to 5 times larger than the value measured when there is no MO. When it was on the coil of layer 2, B(Ym, YM−m+1) was increased relatively small. However, the changed value that is about 3 to 7 times larger than the value measured when there is no MO. Since the loop coils are arranged regularly, the same phenomenon occurs at other locations. Therefore, it is possible to detect MO over the entire area.
The process and results of MOD using the proposed Maxwell bridge with an independent power of 800 kHz are described. However, since this circuit is included in the wireless power transfer system, the following experiment was conducted to find out the effect between the wireless power transfer operation and the MOD operation. As shown in the
Figure 15, there is a balanced coil array between the transmitter and receiver coils. While transmitting the power of 70 kHz, the
VAB of the Maxwell bridge was measured while changing the distance between the transmitter and receiver coils to 3 cm, 5 cm, and 7 cm.
Figure 16 shows the results of measuring
VA,
VB, and
VAB of the proposed Maxwell Bridge circuit at 7 cm distance. When 70 kHz high power is radiated from the transmitter coil to transmit power in the middle of using detecting MOs at 800 kHz, it is also induced in the balanced coil. As a result, it can be seen that the 800 kHz signal has an envelope of 70 kHz.
Table 4 summarizes the measured values of
VAB_peak_to_peak and
VENV_70k at each distance during power transferring. Since the resonance between the transmitter and receiver coils is matched when it is 5 cm or more, the maximum power is transmitted when the distance is 7 cm, and the
VENV_70k is also maximum. However,
VAB_peak_to_peak is almost constant regardless of power transmission as well as distance.
In order to increase the accuracy of
VAB signal, a filter to remove signals in the 70 kHz band can be used.
Figure 17a,b show magnitude in time domain and in frequency domain of
VAB during power transmitting, respectively.
Figure 17c,d show magnitude in time domain and in frequency domain of
VAB after removing 70 kHz band signals, respectively. As shown in
Figure 16, there are 800 kHz band signals and 70 kHz band signals during power transmitting, but the 70 kHz band signals are removed after using the filter. In this paper, however, the slotted Q factor method during power transmitting is used. A slot to detect MOs in the middle of the in-power is added to make it as less sensitive to the power of the transmitter coil as possible.