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, 14 (11), e0225263
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Perception and Control of Low Cable Operation Forces in Voluntary Closing Body-Powered Upper-Limb Prostheses

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Perception and Control of Low Cable Operation Forces in Voluntary Closing Body-Powered Upper-Limb Prostheses

Mona Hichert et al. PLoS One.

Abstract

Operating a body-powered prosthesis can be painful and tiring due to high cable operation forces, illustrating that low cable operation forces are a desirable design property for body-powered prostheses. However, lower operation forces might negatively affect controllability and force perception, which is plausible but not known. This study aims to quantify the accuracy of cable force perception and control for body-powered prostheses in a low cable operation force range by utilizing isometric and dynamic force reproduction experiments. Twenty-five subjects with trans-radial absence conducted two force reproduction tasks; first an isometric task of reproducing 10, 15, 20, 25, 30 or 40 N and second a force reproduction task of 10 and 20 N, for cable excursions of 10, 20, 40, 60 and 80 mm. Task performance was quantified by the force reproduction error and the variability in the generated force. The results of the isometric experiment demonstrated that increasing force levels enlarge the force variability, but do not influence the force reproduction error for the tested force range. The second experiment showed that increased cable excursions resulted in a decreased force reproduction error, for both tested force levels, whereas the force variability remained unchanged. In conclusion, the design recommendations for voluntary closing body-powered prostheses suggested by this study are to minimize cable operation forces: this does not affect force reproduction error but does reduce force variability. Furthermore, increased cable excursions facilitate users with additional information to meet a target force more accurately.

Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Measurement set-up.
The measurement set-up for the force reproduction task (A) and maximum force measurements (B) consisted of a “figure-of –nine” harness (a) and thermoplastic shell (b) which are connected through a Bowden cable (c) running through a cable housing (d). In the maximum force measurements setup (B) the cable excursion is disabled. Here the cable (c) is interrupted by a force sensor at the subject’s back (e). In the force reproduction task setup the cable is interrupted by two force sensors (e & f), which measure the cable forces before (Fback) and after (Farm) the cable housing respectively. In this figure a threaded rod (g) is illustrated leading to disabled cable excursions. The threaded rod is interchangeable with springs of different stiffness, which resulted in different cable force-excursion characteristics. A displacement sensor is recording cable excursions (h).
Fig 2
Fig 2. Body-map.
The Body-Map was coloured by one subject indicating pain (red) at the back of the left elbow, irritation (orange) in the right arm pit and touchiness (green) on a stripe of his back.
Fig 3
Fig 3. Experimental procedure.
The flowchart of the experimental procedure illustrates the chronologic order of maximum force measurements (rounded rectangles), body-map questionnaires (circles), force reproduction experiments with and without cable excursion (rectangles) with alternating force reproduction trials (squares) and Nasa-TLX questionnaires (triangles).
Fig 4
Fig 4. Experimental procedure of ‘no cable excursion’ trials.
Flowchart illustrating the experimental procedure of the six ‘no cable excursion’ trials as shown in Fig 3. After practicing the force reproduction task at 22 N (F0), six force levels (10, 15, 20, 25, 30 and 40 N) were examined during 11 alternating visual and blind blocks. The force reproduction task at each force level (squares) was followed by a Nasa-TLX questionnaire (triangle). The order of force levels (F1 to F6) was counterbalanced over the subjects. The outer (purple) bars indicate the target force; the inner (blue) bar indicates the measured force.
Fig 5
Fig 5. Raw data ‘cable excursion’ trials.
The raw data of the first 30 seconds of a typical trial, condition 20 N – 10 mm, represents the target force of 20 N, the approximate 10 mm cable excursion measured by the displacement sensor and the two cable forces measured at the arm (Farm) and the back (Fback) of the subject. Visual blocks (V1, V2) are alternating with blind blocks (B1, B2).
Fig 6
Fig 6. Force reproduction error ‘no cable excursion’ trials.
The force reproduction error for the ‘no cable excursion’ trials shows no significant differences between the tested conditions of target forces between 10 and 40 N. The bars indicate the group’s average and the whiskers the standard deviation.
Fig 7
Fig 7. Force variability ‘no cable excursion’ trials.
The force variability increases with increasing target force for the ‘no cable excursion’ trials. The bars indicate the group’s average and the whiskers the standard deviation.
Fig 8
Fig 8. Force reproduction error ‘cable excursion’ trials.
The force reproduction error decreases with increasing cable excursion for the ‘cable excursion’ trials for both target forces of 10 and 20 N. The force reproduction error does not differ between force levels. The zero line indicates when the target force is met. A negative force reproduction error indicates a lower reproduced force than target force. The bars indicate the group’s average and the whiskers the standard deviation.
Fig 9
Fig 9. Force variability ‘cable excursion’ trials.
The force variability remains constant with increasing cable excursion for ‘cable excursion’ trials. The force variability is lower for 10 N target force than for 20 N. The bars indicate the group’s average and the whiskers the standard deviation.

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References

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Publication types

Grant support

This study was financially supported by Fonds NutsOhra, grant number 1101-049 (https://www.fnozorgvoorkansen.nl/) to DHP and MH. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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