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. 2017 Jan 18;12(1):e0169996.
doi: 10.1371/journal.pone.0169996. eCollection 2017.

High Cable Forces Deteriorate Pinch Force Control in Voluntary-Closing Body-Powered Prostheses

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Free PMC article

High Cable Forces Deteriorate Pinch Force Control in Voluntary-Closing Body-Powered Prostheses

Mona Hichert et al. PLoS One. .
Free PMC article

Abstract

Background: It is generally asserted that reliable and intuitive control of upper-limb prostheses requires adequate feedback of prosthetic finger positions and pinch forces applied to objects. Body-powered prostheses (BPPs) provide the user with direct proprioceptive feedback. Currently available BPPs often require high cable operation forces, which complicates control of the forces at the terminal device. The aim of this study is to quantify the influence of high cable forces on object manipulation with voluntary-closing prostheses.

Method: Able-bodied male subjects were fitted with a bypass-prosthesis with low and high cable force settings for the prehensor. Subjects were requested to grasp and transfer a collapsible object as fast as they could without dropping or breaking it. The object had a low and a high breaking force setting.

Results: Subjects conducted significantly more successful manipulations with the low cable force setting, both for the low (33% more) and high (50%) object's breaking force. The time to complete the task was not different between settings during successful manipulation trials.

Conclusion: High cable forces lead to reduced pinch force control during object manipulation. This implies that low cable operation forces should be a key design requirement for voluntary-closing BPPs.

Conflict of interest statement

The authors have declared that no competing interests exist. Dick H. Plettenburg holds stocks of Delft Prosthetics BV, Delft, The Netherlands, which produces hand prostheses for the Dutch market. Delft Prosthetics BV did not have any involvement in the study design, data collection, data analysis and interpretation, nor in the writing and submission of this article. This does not alter our adherence to PLOS ONE policies on sharing data and materials.

Figures

Fig 1
Fig 1. Apparatus.
Side-view (a) and back-view (b) of one subject wearing the custom-made bypass-prosthesis. The prehensor (1) is connected to the fitting. The prehensor (1) was connected via a Bowden cable (3) to the “figure-of-nine” harness (5). The Bowden cable forces were measured before and after the outer cable housing with force sensor 1 (2) and force sensor 2 (4).
Fig 2
Fig 2. Prehensor.
TRS hook with the internal torsion spring replaced by external linear springs in the high force setting (3 x 1.7 N/mm springs); 2 x 0.11 N/mm springs were used for the low force setting.
Fig 3
Fig 3. Test object.
The “mechanical egg’s” breaking mechanism [5] is shown in the left picture (a) and the experimental setup is shown to the right (b).
Fig 4
Fig 4. Cable to pinch force.
The cable force to pinch force relationship is shown when the TRS hook is fully closed and when the test object is held utilizing the prehensor’s low spring stiffness setting. The force at which the object slips out of the prehensor (Fslip), and the forces at which the “mechanical egg” breaks span the operating window in which the test object can be manipulated, for both the low (F1,break) and high (F2,break) breaking force settings. Note that the cable force at which the TRS hook starts to build up a pinch force on the test object is an estimation, since it was not experimentally determined. As a consequence the pinch force values are not representative.
Fig 5
Fig 5. Visualization of one trial.
The subject hits the self-timer button A to start the time measurement, moves 29 cm to grasp the object at the lower area B, then moves the object 29 cm to the higher target area C. After releasing the object, the subject needs to hit the self-timer to stop the time measurement.
Fig 6
Fig 6. Number of unsuccessful trials.
The number of unsuccessful trials out of 25 trials per condition are indicated by “x” per subject (N = 11), averaged over all subjects (“o”) with the 95% confidence intervals (whiskers). The results are compared for the high (left) versus the low (right) breaking force setting of the test object as well as the low (0.22 N/mm) versus high (5.1 N/mm) spring stiffness of the prehensor.
Fig 7
Fig 7. Task completion time.
The time to complete the experimental task was determined by the average of all successful trials per condition per subject (N = 11) indicated by “x”. The error bars represent the average of all subjects (“o“) with the 95% confidence intervals (whiskers). High (left) versus low (right)) breaking force setting of the test object as well as the low (0.22 N/mm) versus high (5.1 N/mm) spring stiffness of the prehensor were compared.

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Grant support

The work presented in this article is part of ongoing research on physiological prosthesis control systems at Delft University of Technology, made possible by the sponsorship of Fonds NutsOHRA (grant number 1101-049; www.fondsnutsohra.nl). Mona Hichert’s visit at the University of New Brunswick was financially supported by ISPO—The Netherlands (www.ispo.nl).

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