## 3 System Architecture for Hand Therapy
### Wearable and Master Controller  
Figure 2(a) shows the wearable device which covers a part of the forearm and fingers. This device is made up of two parts: the main part as shown in the Figure 2(b) and the thumb part as shown in the Figure 2(c). The thumb part is attached to a servo motor which is used to connect both the parts for opening and closing of the wearable. This helps the users to do hand exercises and also practice grasping and gripping of objects and displacement of objects in their process of rehabilitation. For this purpose, we have introduced the multiple control methods.  
The proposed wearable can be used for both the right and left hand, which can fit an average human hand. It is fabricated using polypropylene material to make it lightweight at 270 g. For motion transmission from the servo motor, two spur gears in a 1:1 ratio are used so that the torque transfer is the same. Each spur gear has fifteen teeth. The master controller is a NodeMCU, which is used to control the servo motor via the commands from the smartphone app or IoT-based dashboard and get the sensor feed. It is also used to wirelessly connect to the cloud, where the sensor data along with the key parameters, indicators, and control commands can be stored and monitored by therapists. NodeMCU is a low-cost microcontroller unit (MCU) which uses MCU ESP8266 with integrated Bluetooth and Wi-Fi facilities and provides one of the best platforms for cheaper IoT application development. It is similar to an Arduino board but is lesser in size compared with the Arduino. The commands from the user while operating the wearable device are sent to  
Figure 3: Wearable Device Model (**a**) shows the wearable device worn over the hand and part of the forearm. It has two parts; (**b**) the part covering the forearm and fingers excluding the thumb, (**c**) the part covering the thumb attached to a servo motor. Parts shown in (**a**,**b**) are attached via servo motor.  
Figure 2: The wearable device system. Both a mobile gateway (smartphone) and a fixed gateway using NodeMCU, which is a microcontroller unit, can connect to the IoT-based cloud for data storage and analysis.  
the NodeMCU via the Bluetooth wireless communication interface. The NodeMCU, along with current sensors, makes the electronics part of the wearable.  
The wearable uses a current sensor to measure currents drawn by the servo motor when the user operates the device. The power supply is managed via two sources: Switched-mode Power Supply (SMPS) and rechargeable batteries. SMPS is useful when the user is at any facility during the therapy and batteries are handy when the user is travelling and has time for therapy. Apart from sending data, key parameters, indicators, and control commands to the cloud, the wearable system via servo motor can move the thumb of the user to perform open/close actions of the hand. Based on open/close actions, there are three key parameters that are monitored, including the count, width, and speed. The indicators refer to the grasp and grip indicators which are obtained from current sensor data. An ultra-torque (35 kg-cm) metal gear servo that weighs only 64 g is used for the wearable to keep the overall weight of the wearable device as low as possible and provide enough torque for the user to open/close the hand, grasp and grip objects, etc. The device operates in the range of 6-8.4 V and consumes 3.2-3.5 A, stall current at lock. The current sensor ACS70331 can provide a base sensitivity of 800 mV/A at a supply voltage of 3.3 V/5 V.