# Wireless Control Combining Myoelectric Signal and Human Body Communication for Wearable Robots
## 1 Introduction  
Wearable robots are attracting a great deal of attention in today's aging society with a declining birthrate. Controlling wearable robots according to the human will is expected in fields such as industrial robots and disaster relief robots. A wearable robot hand can replace real human hand to perform more complex and precise tasks. It can also be expected to work as a third human hand. For patients with severe disabilities in limb movement or language function, a wearable robot hand such as electric artificial hand is also very useful if it can be controlled by human will. The most popular method to control a wearable robot hand is to use myoelectric signals. It usually consists of a sensor unit which detects the myoelectric signals, a controller unit which analyzes the signals and generates a pulse width modulation signal necessary for motor control, and a motor drive unit to drive the robot hand [1, 2, 3, 4, 5, 6, 7]. A distance usually exists between the myoelectric signal sensor unit and the motor drive unit, so that some wires have to be used for signal connection between the two units. These wires not only cause inconvenience during use, but also often act as antennas to cause electromagnetic compatibility (EMC) problems [8, 9]. Therefore, it is expected to replace the wires with wireless technology [10, 11]. Figure 1 illustrates the concept of a wearable robot hand controlled using myoelectric signals. Both the sensor unit and the motor drive unit are on or near the human body. Detection of myoelectric signals in the sensor unit typically uses multiple pairs of electrodes that come into contact with the skin surface of the arm. Technologies available for wireless transmission between the sensor unit and the motor drive unit include Bluetooth, WiFi, and ZigBee at 2.4 GHz band, ultra wide band (UWB) technology using 3.1-10.6 GHz [12], and human body communication (HBC) [13, 14]. The technologies are often used in wireless body area netweoks (WBANs) [15]. WBAN is a network which requires a number of sensors to be connected. To manage the transmission and reception of signals from many sensors, complex medium access control (MAC) layer protocols are required [16, 17, 18, 19, 20]. However, the purpose in this study is to wirelessly connect a myoelectric signal sensor and a robothand controller. This is a point-to-point communication and does not require network protocols. As analyzed in [10], for frequencies above hundreds of MHz, the mechanism of wireless transmission on the human body is based on radiated field components. However, HBC typically uses a few MHz to a few tens of MHz. Since the human body looks like a conductive medium at these frequencies, the mechanism of wireless transmission on the human body is based on electric field coupling (or capacitive coupling). It is a different mechanism of transmission from 2.4 GHz band or UWB. Instead of wireless transmission at 2.4 GHz band or UWB, which uses an antenna to radiate radio-frequency signals, HBC uses a transmitting electrode to apply signal to the human body and a receiving electrode to detect the signal from the human body according to its electric field coupling mechanism. Table 1 compares the superiority and inferiority of the three technologies for wireless transmission on the human body. Bluetooth is used as a representative of the 2.4 GHz band. As can be seen, HBC shows four advantages over Bluetooth and UWB: (1) HBC has a much smaller transmission loss than Bluetooth and UWB due to the frequency-dependent lossy dielectric properties of the human body [10]. (2) HBC does not radiate radio-frequency signal to the outside, so, it has higher security than Bluetooth and UWB. The signal transmission in HBC is mainly along the body surface due to its electric field coupling mechanism. (3) Because of the wide bandwidth, HBC can provide faster transmission speeds than Bluetooth, although it is not as fast as UWB.