# Smart Rings vs. Smartwatches: Utilizing Motion Sensors for Gesture Recognition
## 3 Approach and Methodology  
### System Overview  
In the course of this paper, a custom smart ring has been developed (see Figure 1) taking into account well-known challenges and aspects for wearable systems [18]. The ring consists of different modules, which can be stacked onto each other, adding certain capabilities to a microcontroller. The stacking point between modules also serves as a connector between them. All modules were bought from the company TinyCircuits [19] and are designed as extensions for the TinyDuino [19] microcontroller. Each module measures about 20 \(\times\) 20 mm and all required modules stack in height to about 17 mm. The battery has the same form factor as the modules and increases the height to about 20 mm when stacked on top of the modules. The accelerometer board is as close to the bottom as possible to minimize the distance between the finger and the actual sensor. For attaching the stack of modules to the finger, a comfortable elastic ring is used. In addition to the custom hardware developments, software had to be implemented for both, the smart ring as well as the smartwatch. The purpose of both implementations is mainly to read the sensor data and send it to the smartphone. However, other tasks have to be considered too, such as timing the sensor readings, enabling/disabling the sensors, locking/unlocking the activation mechanism and managing power consumption. Regarding the smart ring, an attached force sensitive resistor enables/disables the sensors, but in order to compare different activation methods, the sensors must be able to be triggered externally. Therefore, the smart ring supports remote activation of the sensors via Bluetooth. Additionally, the manual activation mechanism can be locked remotely to prevent false activations while being controlled remotely. Furthermore, the ring periodically sends status updates to the smartphone, which inform its user about the condition of the ring. The purpose of the smartwatch application is similar to the smart ring's: read sensor data and then send them to the smartphone application. The user interface consists of only a single screen. This screen provides a software button as activation mechanism. Figure 2 shows the user interface of the smartwatch application. In contrast to the activation mechanism of the ring, it is not required to keep pressing the button after the initial press. Instead, the button changes its color to the color red, which informs the user that the sensors are enabled. Figure 1: Custom built smart ring with an elastic mounting and a force sensitive resistor on the side of the ring. From bottom to top the modules are the processor board, the sensor module, the Bluetooth module, the USB shield, the prototyping board and the battery. Finally, the two major responsibilities of the smartphone application are (i) recording data and (ii) recognizing gestures using this data. Moreover, it provides information about connected sensor devices and the data they are transmitting. Exemplary screens of this application can be seen in Figure 3.  
Regarding the intended real-time aspect of the proposed system, we specifically aim to meet the requirement that the system is able to compute and return the result (i.e., the recognized gesture) within a maximum delay of 1-2 s (this seems to be an appropriate time-span in terms of user interaction)--thus, speaking of a soft-real-time system (i.e., the missing of a deadline does not fail the system but degrades the usefulness of the result) [20; 21]. The answer of the system could also be defined in terms of response-time, turnaround-time or recognition-time. With our approach and the chosen hardware and software configurations, we achieve this real-time requirement invariably as the results Section 4 shows. The exact details regarding the response-time of the system are out of scope of this paper but we plan to release these findings in a future article. Figure 3: Info screen (**a**), ring monitor screen (**b**), the watch monitor screen (**c**) and the recording screen (**d**).