## 3 System Architecture and Methodology
### Wearable Hand Device  
To construct a wearable device that can fit most users, some ergonomic analyses have been carried out in the planning stage. First, the average dimension of the adult hand is obtained, shown in Table 1. There is a slight variation in the measurement between the hand sizes of adult males and females. The construction material of gloves is determined in the next step. It would be made of elastic material to adopt the variation. The sensing device is based on Table 2. In order to fulfill the requirements concluded in the ergonomic analysis, two kinds of sensors are chosen. They are accelerometers with gyroscopes and flex sensors. The accelerometer measures linear acceleration in mV/g along one or several axes, while the gyroscope measures angular velocity in mV/deg/s. Through inputting the data from the accelerometer and gyroscope into the set location in the MR-based training platform, the virtual hands' moving direction and destination can be fully represented by virtual hands. Figure 3 shows the proposed wearable hand device. A coin-shaped vibrator on each fingertip stimulates the feedback of interacting with different objects while using the gloves. To make the user experience closer to reality, a wide range of vibration strength and lasting time could be set. A clicker is located on the left side of the second joint of the index finger. It is used to act as a confirm key. Before the construction of the glove, circuitry should be tested to ensure that there are no potential connection mistakes and component failures. If the step is not implemented before the construction of the glove, it will be difficult to distinguish between a connection mistake and a component failure, resulting in a waste of time and effort. The ESP32 datasheet provides information about the function of each pin. Pin 3V3, IO26, and GND are used for the button. Pin 3V3, GND, IO22, and IO21 are used for the MPU. Pin 3V3, IO2, and GND are used for the flex sensor. Pin IO4, 3V3, and GND are used for the heartbeat sensor. A flex sensor detects the movement of fingers by changing the numerical value of resistance. The sensor has a resistance of around 7000 to 13,000 ohms when in a flat situation. During bending fingers, the resistance value would increase--at least two times of flat resistance value when a 180-degree pinch bend is obtained. The proportion of the bend level and the resistance value can be found through calculation. Then, the fingers' level can be estimated by analyzing the reading shown on the Arduino platform. A flex sensor is located above each of the fingers. An accelerometer with gyroscope and microcontroller is placed in the gloves' middle. All the flex sensors and accelerometers with a gyroscope are connected to the microcontroller by wire. To achieve the target of a wireless wearable, a battery box \begin{table}
\begin{tabular}{c c c} \hline \hline
**Gender** & **Average Length** & **Average Breadth** \\ \hline Male & 18.9 cm & 8.4 cm \\ Female & 17.2 cm & 7.4 cm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The average dimension of the adult hands. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sensor Type** & **Usage** & **Relationship with Project** \\ \hline Temperature sensors & \(\bullet\) & Detect a person’s skin temperature [103] & Track trainer’s skin temperature \\ \hline Position sensors & \(\bullet\) & Detect linear position. & Track the movement of fingers, such as bending \\  & \(\bullet\) & Detect rotary position. & Track trainer’s gesture during work (formal gesture or not) \\ \hline Motion sensors [104] & \(\bullet\) & Detect gesture or hand motion & Track trainer’s applied force to the glove \\ \hline Force sensors [104] & \(\bullet\) & Detect the level of force used by the trainer’s hands & Track trainer’s applied force to the glove \\ \hline Accelerometer with gyroscope [105] & \(\bullet\) & Detect linear movement and orientation of the gloves & Track the position of the whole hand \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Sensing device analysis.