703.1.2 Control Implementation
3.4 Discussion and Conclusions
The results of the user study demonstrate that the hand-held force-amplifying device can reduce the threshold at which users could perceive contact force of the tool tip. The force- amplifying device, when trialled with force amplification active, improves human force perception compared to the two other cases: use of the device without force amplification and the control case in which users handled a lightweight syringe.
When the device was trialled with force amplification, the median force detection threshold for all of the users was reduced by a factor of 5 when compared to the case when the device was trialled without force amplification. The minimum reported force detection threshold was comparatively reduced by 59% and the maximum reported threshold was also comparatively reduced by 71% when force amplification was active. In comparison to the control case, when the device was trialled with force amplification, the median force detection threshold for all of the users was reduced by a factor of 3.6. There was a 40.3% increase in the median force detection threshold between the case of the device being trialled
without force amplification and the control case. Mann-Whitney U statistical comparison
tests were performed on all of the force detection thresholds measured for the three cases considered in this user study. The force detection thresholds that were observed when the device was trialled with force amplification were significantly reduced in comparison to when the device was trialled without force amplification (p<0.001) and in comparison to the control case (p<0.001). There was no statistically significant difference between the observed force detection thresholds for the case of the device being trialled without force amplification and the control case (p=0.4734). The force amplifying device was also demonstrated to improve the consistency with which a minimum force detection threshold can be perceived: the interquartile range of force detection threshold for the with force amplification case was 34mN compared to 236mN and 156mN in the without force amplification and control cases, respectively.
The reduction in the median force detection threshold observed in the case with force amplification compared to the other cases was lower than the pre-set scaling factor of 15. It is hypothesised that experimental errors are a significant cause of this. The force detection thresholds observed across the entire study would have been higher than the actual perceived
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force threshold as a result of the time delay between the user perceiving tool contact and physically depressing the push button. During this latent period, the force exerted on to the low stiffness spring would have increased as the platform would have translated and compressed the spring further. It is likely that this error would have differed from user to user but would remain consistent across the different cases examined. This error will have therefore disproportionately increased the observed force detection thresholds for the case
with force amplification relative to the without force amplification and control cases.
The mechanical response of the force-amplifying device when used with force amplification will have introduced some additional latency between the tool tip making contact and the user perceiving contact. Inertial, gravitational and frictional loads experienced by the slider/roller mechanism and tool shaft of the force-amplifying device would contribute to this mechanical response time and will have artificially increased the observed force detection thresholds due to the spring compression effect previously cited. Anecdotally, Stetten et al also observed that the perceived force amplification was consistently lower than the actual magnification in their psychophysical studies and they cite mechanical factors, including as weight a probably cause of this. The loss of performance due to additional weight and other contaminating forces is in accordance with Weber’s law. Another possible explanation for the discrepancy between the pre-set scaling factor and observed device performance is due to the different tactile stimulation modes through which the user would perceive tool contact. When the users trialled the force-amplifying device with force amplification they would experience a compressive tactile sensation at the tip of their index finger. When handling the syringe, the tactile stimulation occurs in a shearing mode and is distributed across all points of grasping contact.
A further source of error is the erroneous physiological motion of the device caused by the user, including tremor and hand drift, would have resulted in a change of deflection in the spring and therefore give an error (±) in the force detection threshold measurements. Attempts at mitigating these errors were made by actuating the platform with a low velocity ramp and through use of a low-stiffness spring. The time delay error previously mentioned will have varied from user to user and added further uncertainties in to the results. These errors are systematic across the entire study and do not bias the results. Nonetheless, for all
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of the reasons previously cited, it is probable that the force-amplifying device improves tool contact perception to a greater extent than has been concluded in this experiment.
This chapter has presented an ungrounded hand-held force-amplifying augmenting device designed to improve user perception of tool contact forces by providing tactile feedback to the user’s fingertip. The bench tests reported in this chapter demonstrate the device’s ability to intrinsically amplify force, allowing scaling factors of up to 15, greater than that achieved by comparable devices being developed by researchers [144][145]. The user study confirmed that user perception of tool contact is unequivocally improved; the study also demonstrated improvements in the consistency with which tool contact can be perceived by users. The experiments performed nonetheless highlight a number of improvements that are subsequently addressed in Chapters 4 and 6.
The experiments performed highlight intrinsic limitations of the developed device. A significant limitation was the force sensor which has a limited resolution in the sub-threshold range for human tactile perception. Whilst scaling factors of 15 were reported, the device was primarily evaluated at force ranges at the upper limits of microsurgery where this type of augmenting device is likely to be at its most useful. The integration of high resolution, mirco-scale force sensors in to hand-held mechatronic surgical tools is thus investigated in Chapter 4. The work in this chapter also highlights the importance of minimising weight and optimising device ergonomics which could serve to degrade rather than improve user performance. This point is especially true in the targeting of delicate microsurgical interventions. The user study also highlights the potential importance of analysing aspects such as the device bandwidth which are also considered in Chapter 4.