## 5 Discussion and Conclusions
As shown in the two experiments of Section 3, the measured torque errors were not negligible. The major cause of this problem was the backlash of the gear train inside the driving motor. It is reasonable to regard this phenomenon as a result of the actuator performance rather than the torque sensor. Figure 13 is the simulation result to show the inaccuracy of the measured torque when an input torque is applied to the torque sensor, by rotating a distal weight in one direction using an actuator with a huge reducer backlash. When a maximum input torque of 117.2 mNm is applied as in Experiment 2, a 10\({}^{\circ}\) sampling interval and 40\({}^{\circ}\) actuator backlash leads to a resultant torque RMSE of 27.88 mNm, exaggeratedly. More practically, in the case of 0.01\({}^{\circ}\) sampling interval and 1.9\({}^{\circ}\) actuator backlash, the resultant torque RMSE 1.3753 mNm is substantially similar to the first RMSE measured in the experiment (1.2454, 1.2226 and 1.2444 mNm). Therefore, it can be said that the error in Section 3 was mostly caused by the actuator backlash. As evidence, it was confirmed that when the rotation angle range was limited to 1-179\({}^{\circ}\), the measurement error dramatically decreased to less than 30% of the previous value. In conclusion, it is not desirable to generate the sine-type torque using an actuator using a gear reducer as in this study, and it is recommended to replace the gear, for example with a harmonic drive reducer that can ignore the backlash of the reducer. Alternatively, it is also desirable to use this motor-equipped torque sensor within a limited range where the direction of torque does not change. Another limitation of this study is that the dynamic characteristics of the proposed torque sensor were not experimentally quantified. However, the natural frequency of the torque sensor was calculated through the kinematic calculation, and the result made it possible to approximate the bandwidth of this sensor. As described previously, the purpose of this study is to propose a new torque sensor structure, and to predict and evaluate its basic performance. Therefore, additional measurements of the dynamic properties and practical application methods of this sensor are left as future works. In addition, it was  
Figure 13: The effect of actuator backlash on output torque in continuous torque measurement: 10\({}^{\circ}\) sampling interval, 40\({}^{\circ}\) actuator backlash and 117.2 mNm maximum input torque. Resultant torque root mean square errors (RMSE) is 27.88 mNm. The \(x\)-axis is (**a**) the input rotation angle and (**b**) the converted torque. not clearly revealed through this study that the measurement sensitivity can be adjusted by changing the length of the moment arm. If the equipped motor is removed and only the performance of the torque sensor is measured, the adjustability of the sensitivity will inevitably become remarkable, but it is regrettable that this characteristic has not been completely proven due to the limitations of the backlash and coarse motion control of the actuator with the current experimental methods. Nonetheless, there are some ways to improve the performance of this torque sensor through component replacement and design modifications. First, if the currently used planetary gear head is replaced with a harmonic drive, the angle of the rotary shaft becomes accurate, and the hysteresis of the measurement can be dramatically reduced. Although torsional wind-up phenomenon can cause inaccurate torque measurements, it will generally be better to secure positional accuracy by deceleration by harmonic drive rather than gear, unless the measured torque is at an extremely low level. Furthermore, replacing the rod ends with other components of less friction and less clearance will also work. A reinforced design to minimize strain on the lever and base arms is also a good way to improve the measurement accuracy for high torque. Lastly, utilizing another load cell with high performance will understandably improve the measurement accuracy.