# Smart Rings vs. Smartwatches: Utilizing Motion Sensors for Gesture Recognition
## 3 Approach and Methodology
### Relevant Gestures  
Since there is no gesture catalog that is universally applicable and intuitive for different users, a set of 12 different gestures is proposed, inspired by [PERSON] et al. [22; 23]. All considered gestures based on direction information implicitly introduce a gesture in the opposite direction. Some gestures are based on rotation movements and others combine rotation information with direction information. Moreover, the gesture catalog contains gestures that produce low as well as high acceleration values and gestures that utilize different dimensions of the sensor reference coordinate system. In order to be able to compare finger-mounted and wrist-mounted sensors, all gestures are performed using the whole hand. Since most users are right-handed and the smartwatch is (usually) worn on the left hand, the smart ring is also worn on the left hand. Therefore, all gestures are performed with the left hand.  
As depicted in Figure 4, both sensor devices provide measurements inside their own reference coordinate system. Both systems are very similar in regard to the axis labeling. However, the systems' axes are not parallel due to the anatomy of the hand and might be different from user to user. The gravity vector always points along the negative Z-axis, which indicates that the participants can either stand or sit when performing gestures.  
Figure 5 shows four directional gestures. Each gesture is performed in the YZ-plane and resembles a quick movement in one of four directions. Therefore, the acceleration on the X-axis is relatively low compared to accelerations in the plane. Due to the lack of rotation movements, the gyroscope measurements contain mainly noise.  
The first two gestures in Figure 6 are very similar to the wake gesture of the Apple Watch and resemble either one or two quick rotation movements around the X-axis. The delay between the rotations of the _Rotate Twice_ gesture might vary between users. The other two gestures are performed in the YZ-plane and resemble a circle which is drawn into the air, either clockwise or counter-clockwise. The _Circle_ gestures are the only gestures that  
Figure 4: Reference coordinate systems of the smartwatch and the smart ring.  
Figure 5: Gestures based on direction information in the YZ-plane.  
smoothly transition between acceleration axes. The measurements of the former gestures are mainly visible on the gyroscope and the latter gestures mainly affect the accelerometer.  
The first two gestures of Figure 7 resemble clap movements, which lead to relatively high accelerations on the Z-axis compared to other gestures. Similar to the _Rotate Twice_ gesture, the time delay between claps might be different from user to user. Furthermore, the initial rotation of the hand, which is necessary to get into a clapping position, affects the gyroscope. The _Checkmark_ gesture contains movements in multiple directions inside the YZ-plane and is the only gesture that abruptly changes acceleration axes. The _Imaginary Button_ resembles a gesture pressing an imaginary button in front of the user. It is the only gesture performed along the X-axis and therefore, transitions the gesture catalog from a 2D to a 3D space.  
It is notable to mention that all persons have performed the gestures with the whole left hand and that the devices have not been worn reversed or generally wrong for the experiments. Since the gestures are performed with the whole hand, the finger on which the ring is worn does not significantly influence the performance of the system (i.e., the coordinate system and thus the movement patterns do not change).