# Development of Power-Assist Device for a Manual Wheelchair Using Cycloidal Reducer
## 4 Discussions  
In Section 2, the new design method for a cycloidal gear was explained, and in Section 3, the designed cycloidal gear was fabricated and installed on a manual wheelchair, and a performance test was conducted to confirm the possibility of using it as a PAD. In several previous studies on cycloid teeth, the profile of the gear was determined according to the diameter of the cycloidal pin roller and the distance from the center of the reducer to its center. However, the cycloidal gear design method proposed in this study can design a different tooth profile while maintaining the size of the cycloidal gear by changing only the TTR in the range of 0.4 to 0.6. This design method can determine the size of the gear by the  
Figure 11: Maximum speed test and consumption test of the wheelchair. Figure 10: Slope climbing test for the wheelchair. module and number of teeth, and in particular, if the tooth width ratio is 0.5, it can have a tooth profile very similar to the involute tooth profile. When designing an internal and external involute gear pair with a difference of one in the number of teeth, a large pressure angle (45\({}^{\circ}\)) and addendum truncation are required [14; 22]. In general, as the pressure angle increases, the noise increases and the efficiency decreases, so a reducer with an involute gear has a higher noise than a reducer with a cycloidal gear. In the lab, two types of reducers with involute and cycloidal gear were produced. As a result of comparing the noise, the noise in the involute gear was in the range of 65 to 73 dB, and the noise in the cycloidal gear was 58 to 62 dB. The tooth size of the harmonic drive is smaller than that of the cycloidal gear due to a difference of two in the number of teeth between the housing gear and cycloidal disk, and these teeth are easily damaged by shock load. Also, the planetary reducers require a multi-stage structure to apply to PAD, as a result, the size of the reducer is larger than that of the cycloidal reducer. Due to these characteristics of the cycloidal reducer, it can be easily applied to the PAD. As the use time of the cycloidal reducer with pin roller increases, the possibility of vibration due to wear of the pin roller shaft increases [23]. However, this problem does not appear in the cycloidal gear proposed in this study because the housing gear and cycloidal disk are directly engaged. In addition, since the housing gear and cycloidal disk have a single contact structure, assembly and clearance management are easy. PADs should enable a user to move a manual wheelchair and should be user-friendly for inexperienced users. To satisfy these requirements, we proposed an assist device for which a small-sized reducer and a thin cycloidal reducer are required. The joystick was adopted as a wheelchair controller because of its prolonged use in wheelchairs, and it is easy to use and learn. Existing cycloidal reducers are expensive and unsuitable for wheelchairs; therefore, we developed a reducer that has a simple structure and requires a small number of parts to manufacture. This design lowers the production cost of the cycloidal reducer and improves its ease of assembly. The proposed reducer can be mounted on any manual wheelchair because of its thinness and small size, and it can provide convenience for active users who travel long distances. The manual wheel can be easily released from the power unit by pressing a button at the end of the shaft, allowing the wheelchair to be loaded into a vehicle. The maximum speed of the PAD was set considering the rigidity of the wheelchair frame and user safety. When the maximum speed is exceeded, the wheelchair frame may be damaged by impact with the ground or other obstacles, and there is a risk of tipping when the wheelchair turns rapidly at a high speed. PADs must have sufficient distance to travel after fully charging the battery to support the daily activities of the user.