# Wireless Control Combining Myoelectric Signal and Human Body Communication for Wearable Robots
## 5 Performance Evaluation  
This section first reports the packet detection rate (PDR) and BER measurement results for communication performance evaluation. The measurement was conducted in a normal room using a gel phantom simulating a human arm. Table 3 shows the measurement conditions, and Figure 14 shows the conceptual diagram of the PDR and BER measurements. All the three receiving electrode structures were used for comparison. The transmission distance on the arm gel phantom was changed to 20 cm, 50 cm, 80 cm, and 110 cm, respectively. Since the myoelectric signal is detected at the arm and the length of a human arm usually does not exceed 100 cm, the length of the arm phantom was set to 110 cm and the verification experiment was limited to 110 cm. The packets transmitted from the Tx module were \(10^{5}\) packets corresponding to \(3.6\times 10^{6}\) bits. The received signals were demodulated by the Rx module and the demodulated information bits were sent to a PC via a USB cable to calculate the PDR and BER.  
Figure 15 shows the PDR and BER measurement results as a function of transmission distance. The Rx module consists of an AGC. The longer the transmission distance, the higher the gain of the amplifier and the more noise power it produces. Therefore, the longer distance corresponds to larger noise power and lower signal to noise power ratio (SNR). Since it is difficult to obtain the SNR by separating the signal power and noise power in the Rx module, the PDR and BER performances are shown versus the transmission distance. It is found that the PDR is the same for the three receiving electrode structures, but the BER of the structures B and C is better than the structure A. This is consistent with the result in the previous section. At all distances investigated, the PDR achieves 100% and the BER is less than \(10^{-6}\) for the three electrode structures. Such PDR and BER levels are well acceptable at the physical layer of wireless communication. However, if we assume to transmit a brain wave signal from the head to the hand, the transmission distance can be longer than 110 cm, and the PDR will deteriorate according to our preliminary measurement result [24]. In addition, the main noise in the measurement is Gaussian thermal noise from the amplifier. In actual use, impulse noise such as electrostatic discharge from the human body itself may also occur [25]. In that case, achieving a PDR of 100% is not easy because the peak voltage of impulse noise is too large.  
Moreover, an experiment to operate a wearable robot hand was conducted using the developed HBC modules for wireless control. As discussed, for the receiving electrode structure, any of the three structures can be used, but the structure B provides the best performance. In this experiment, however, the structure A was used to consider the worst case. Figure 16 shows the experimental view in a normal room. The myoelectric signal was detected from the human subject's shallow finger flexors and sent to the robot hand for wireless control. The human subject was asked to perform two actions. One is to hold the hand for five seconds, and the other is to open the hand for five seconds. The two actions were alternately repeated five times to see whether the robot hand acts according to the human subject's hand actions. Table 4 summarizes the success rate of wireless control of  
Figure 14: Block diagram of PDR and BER measurements.  
\begin{table}
\begin{tabular}{c c} \hline \hline Transmission distances & 20 cm, 50 cm, 80 cm, 110 cm \\ Transmitted packets & \(10^{5}\) packets \\ Transmitted bits & \(3.6\times 10^{6}\) bits \\ \hline \hline \end{tabular}
\end{table}
Table 3: PDR and BER measurement conditions.  
the wearable robot hand using the developed HBC module. The result shows a success rate of 100%, which demonstrates that the developed Tx and Rx modules are suitable for controlling the robot hand using myoelectric signals.