## 3 Results  
Figure 11a shows data recorded from one trial of one volunteer. The pinch force, pulling force, and displacement signals were recorded from the customized pinch force load cell, pulling force load cell, and the LVDT sensor, respectively. The linear electric actuator started to pull the pincer object, as indicated by a sudden increase in the pinch and pulling force signals at 3 s. The volunteer exerted pinch force increased as the pulling force increased gradually. From this point, the pincer object totally slipped away from volunteer's fingers, as indicated by a sudden increase in displacement measurements at 20 s. For each trial, it was important to extract the pinch and pulling force values at  
Figure 10: (**a**) Body and upper limb positioning during the experiment; and (**b**) thumb and index finger opposed onto the pincer object. Note that the volunteer should resist the automatic pull applied by the linear actuator system. the slip onset. The slip onset is the point that differentiates between slip and non-slip where the pincer object begins to slip and is the first instance where the displacement started to increase from a relatively flat level. In Figure 11a, it is difficult to determine the slip onset visually due to the small magnitude of the increase in the displacement absolute scale (mm). In addition, the increase in displacement is nonlinear, thus a nonlinear scaling is required. Subsequently, the nonlinear decibel scale (dB) was used such that the logarithm base 10 of the displacement absolute value was multiplied by 20 to provide the necessary scaling. Upon that, the slip onset was precisely determined at 12 s, as shown in Figure 11b. The pinch and pulling force values at the slip onset (12 s) were 15.39 and 7.05 N, respectively. Since the static coefficient of friction of copper alloy is constant, it is expected that the relationship between pinch and pulling force prior to slip onset is linear. Figure 12 shows the pinch-pulling force relationship before and after the slip onset. It was observed that the volunteer initially exerted excessive resistance (indicated by a sudden increase in pulling force), because he had to react and adjust the pinch force according to the perceived automatic pull generated by the linear actuator. There was also some nonlinearity observed at 12 and 14.5 N pinch forces that were probably due to the absence of visual feedback as well as the continuous increase in the amount of automatic pull. Prior to the slip onset, the pinch-force relationship had high linearity of R\({}^{2}\) = 0.8971, while the linearity decreased to R\({}^{2}\) = 0.5845 after the slip onset. This decrease in linearity as due to the inability of the volunteer to exert pinch force according to the continuous increasing of pulling force, which was indicated by the continuous slipping. The COF was computed as the ratio of pulling force to the pinch force at the slip onset, which was equal to 0.45 (7.05/15.39). For each hand, three trials were recorded, and then the pinch and pulling force values were extracted at the slip onsets using the same criteria involved in the above trial. Figure A3a (Appendix A) shows the three trials collected from the right hand of a volunteer. It can be observed that the pinch force signals were close to each other during the three trials, and they had excellent reliability ([PERSON]'s  
Figure 11: (**a**) One trial recorded from one volunteer; and (**b**) displacement in decibel scale. Figure 12: Pinch–pulling force relationship. alpha = 0.989). This was also observed for pulling force signals ([PERSON]'s alpha = 0.969). Based on the displacement measurements after being converted to decibels, the slip onsets were at 17, 14, and 12 s for Trials 1-3, respectively, as shown in Figure A3b. The pinch and pulling force values at the slip onsets as well as the static COFs are summarized in Table 1. The interpretation is that this volunteer could exert an average pinch force of 15.93 N against the maximum automatic pull of 7.65 N without slipping.