# Design and Testing of Two Haptic Devices Based on Reconfigurable 2R Joints
## 6 Discussion on Haptic Device Test Results
### Control Mode Comparison and Evaluation  
The statistical analysis indicated (see Table 2) that there was a significant difference in the performance of the haptic devices in velocity and position control modes. As the difference was significant, it is possible to use the means of the data samples directly. It can be seen from Tables A1 and A2 of Appendix A that the means of the data samples that corresponded to the velocity control mode were significantly smaller. Thus, the haptic device demonstrated better performance in the velocity mode. The survey conducted among the operators after the tests also indicated that the velocity control mode was preferable in most of the cases. The position control mode was preferred by the participants only in 17.6% cases for the active 2R spherical mechanism-based joint and in 5.3% cases for the differential gear-based design joint. Most of the written commentaries considered the velocity mode to be more convenient; however, some of the testers referred to the position control mode as \"more intuitive\". There are two main reasons why velocity control mode provided better performance than position control mode. The first is the resolution of the handle. As in the position mode, the angle of the handle inclination directly defines the position of the end-effector, even a small motion of the handle will affect the mechanism. From the hardware point of view there is no restriction on the possible resolution of the haptic device; however, there is such a limitation on the side of the operator. The human senses are not able to detect the small changes in the handle angles that can be sensed by the haptic device. This was especially important in the positions that are close to the workspace borders, as the force feedback was affecting the operators' control on the device handle. The resisting force caused the small displacement of the handle and the mechanism reacted to it. Meanwhile the operator tried to adjust the handle by moving it further, which in its turn caused a \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mode** & **Velocity Control** & \multicolumn{2}{c}{**Position Control**} \\ \hline
**Task** & **3** & **4** & **3** & **4** \\ \hline Significant difference & No & No & Yes & Yes \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Results of the statistical analysis of the two joints performance in tasks 2–3. stronger force response from the haptic device and the cycle repeated itself. This resulted in the oscillations of the end-effector and confusion of the operator. In addition, in the position mode, the velocity is not limited and the operator can reach the border of the workspace very quickly, experiencing a strong force response right away, without the phase of a smooth increase. In the velocity control mode, the maximum velocity of the end-effector was limited to 5 cm/s, which gave the operator (and the system) time to react. When the end-effector of the mechanism approached the border, the transition of the force that resisted the operator was smooth and predictable. This made operation in the velocity control mode clearer for the majority of testers. The second reason is the presence of clearances in the mechanism, due to the manufacturing technology that was used for the prototype. The position control mode is very sensitive to these clearances; however, in the velocity control mode, the difference between two velocities of the end-effector equals 10\({}^{\circ}\) of handle inclination, which is sufficient to compensate for any possible clearance. This problem can be eliminated by changing the joint manufacturing technology; however, it is caused by the first and the main problem of the excessive sensitivity of the haptic device in the position control mode. The sensitivity problem of the position mode, unfortunately, does not have a simple solution. In order to make the small changes in the handle inclination angle more significant for the operator, it is possible to increase the length of the handle.