# Wireless Control Combining Myoelectric Signal and Human Body Communication for Wearable Robots
## 3 Transmitting Electrode Structure  
Since the transmission of myoelectric signals uses HBC technology, the structure of the transmitting and receiving electrodes plays an important role. This section focuses on sharing the myoelectric signal detection electrode as the transmitting electrode. Figure 6a shows a block diagram of myoelectric signal transmission using HBC for wearable robot control. Figure 6b shows the internal configuration of the myoelectric signal detector consisting of a differential amplifier, a band elimination filter (BEF), a high pass filter (HPF), a low pass filter (LPF), and a variable amplifier. The BEF is used to block 50 or 60 Hz noise from the commercial power supply, and the HPF and LPF are used to remove artifacts and unnecessary frequency components caused by breathing, body movement and others. The voltage level of myoelectric signals ranges from tens of \(\upmu\)V to several mV. In contrast, the voltage level of common mode (CM) noise induced in the human body by an external electromagnetic field such as a commercial power supply or a transmission signal applied to the human body from the Tx module is about 100 to 1000 times that of the myoelectric signals. Therefore, usual detector uses three electrodes consisting of two signal detection electrodes and a ground electrode, and inputs the potential difference between the two detection electrodes to the differential amplifier to detect the myoelectric signal and cancel the CM noise. However, due to the difference in contact impedance between the two detection electrodes with the human body, the CM noise cannot be completely canceled and part of the CM noise is converted to differential mode interference voltage. Since the myoelectric signal is detected and transmitted in different time slots, the myoelectric signal can be detected correctly without being affected by the CM noise generated by signal transmission. This allows one of the two signal detection electrodes to be used as a transmission electrode, i.e., the signal detection electrode and the transmitting electrode can be shared. However, the voltage levels of the transmission signal superimposed on the shared and non-shared detection electrodes will be significantly different. This causes the transmission signal to be converted into an interference voltage without being canceled by the differential amplifier. The interference voltage is band-limited by the filters and amplified by the variable amplifier. Figure 7 shows the time waveform and frequency spectrum of the output from the myoelectric signal detector when one of the two detection electrodes is used as the transmitting electrode. The interference voltage is a pulse repeated at 2 kHz, which is the same as the packet rate of the transmission signal, and prevents the detection of myoelectric signal in the range of 10 to 300 Hz. In order to use one of the two detection electrodes as the transmitting electrode, interference countermeasures are required. We added an LPF with a cut-off frequency of 500 Hz to each of two inputs of the differential amplifier. This cuts off the transmission signal in 10-60 MHz band without interfering with the detection of the myoelectric signals  
Figure 6: Conventional myoelectric signal transmission using HBC. (**a**) Block diagram. (**b**) Internal configuration of myoelectric signal detector. up to 300 Hz. Figure 8 shows the block diagram of the myoelectric signal transmission using HBC for wearable robot control after applying the interference countermeasure. As can be seen from the time waveform and frequency spectrum of the output from the myoelectric signal detector, which is also shown in Figure 7, the interference voltage is reduced by about 50 dB at maximum due to the interference countermeasure. As a result, the detection of myoelectric signals up to 300 Hz is achieved by sharing one of the detection electrodes as the transmission electrode. This transmitting electrode structure contributes to the overall miniaturization of the myoelectric signal detector and the Tx module.