# Wireless Control Combining Myoelectric Signal and Human Body Communication for Wearable Robots
## 4 Receiving Electrode Structure  
HBC is roughly divided into two types, the current type and the electric field type. In the current type, a pair of electrodes generates a weak current to flow through the human body, and the signal is transmitted at that current. On the other hand, the electric field type uses changes in the electric field generated on the surface of the human body to transmit a signal. The signal is applied and detected by capacitive coupling with the human body. Different transmission types require different electrode structure.  
In this study, as described in the previous section, the structure of the transmitting electrode is used as the current type to share one of the detection electrodes as the transmitting electrodes. For the receiving electrodes, Figure 9 shows three structures. Structures A and B can be considered current type, and structure C can be considered electric field type.  
In order to quantitatively evaluate the performance of the three types of receiving electrode structures, finite difference time domain (FDTD) simulation using a numerical arm phantom was first performed. The numerical phantom had the same dielectric properties as the gel phantom used in experiment. The length of the phantom was 50 cm or 110 cm. A Gauss pulse with \(-\)10 dB bandwidth of 10 to 60 MHz and peak voltage of 10 V was applied to the transmitting electrode, and the received signal was detected on the receiving electrode. Figure 10 shows the peak-to-peak values of the received voltage. The received voltage of structure B was the highest of the three structures at both distances.  
In addition, Figure 11 shows the simulated electric field distributions near the three receiving electrode structures at 40 MHz, in which the blue parts represent the numerical arm phantom. Figure 11a is a cross-sectional view of the arm phantom, and Figure 11b,c are longitudinal cross-sectional views of the arm phantom. The red arrows indicate the  
Figure 10: Simulated peak-to-peak values of received voltage for the three receiving electrode structures.  
Figure 9: Three receiving electrode structures. The red represents the signal (hot) electrode, and the black represents the ground (cold) electrode. (**a**) Structure A. (**b**) Structure B. (**c**) Structure C.  
direction of the electric field between the electrodes. As can be seen, unlike other structures, the structure A has the worst performance because the electric fields between the receiving electrodes do not point in the same direction and the field levels are also weak.  
Moreover, an experiment was also conducted to verify the simulation result. Figure 12 shows the signal transmission experiment. The three electrode structures were manufactured and connected to the Tx and Rx modules, respectively. The arm was modeled using a gel phantom [22] with a length of 50 cm or 110 cm. The dielectric properties of the phantom were close to the average of the arm [23]. The transmitted signal was an IR-MPPM signal and the received pulse voltage was detected at the AGC output of the Rx module. After the ADC, it was sent to a PC via a USB cable. Figure 13 shows the measured received peak-to-peak voltage. Again, the received voltage of the structure B was the highest at both distances, and the relative magnitude between the three types was the same as in the simulation. The reason why the received voltage at 110 cm was higher than the received voltage at 50 cm is that the AGC operated with different amplification factors at 50 cm and 110 cm.  
Figure 11: Comparison of electric field distributions at 40 MHz for the three receiving electrode structures. (**a**) Structure A. (**b**) Structure B. (**c**) Structure C.  
Figure 12: View of signal transmission experiment for investigating the three electrode structures.  
Figure 13: Measured peak-to-peak values of received voltage for the three receiving electrode structures.