High cable forces deteriorate pinch force control in voluntary-closing body-powered prostheses

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. 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. 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. In 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

Feedback In Voluntary Closing Arm Prostheses

The purpose of this study was to find an optimal operation force, at which the prosthetic user receives the best force feedback during comfortable prosthesis operation. Three performance factors were introduced: reproducibility, stability, and repeatability

Apparatus.

<p>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).</p

Task completion time.

<p>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.</p

Test object.

<p>The “mechanical egg’s” breaking mechanism [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0169996#pone.0169996.ref005" target="_blank">5</a>] is shown in the left picture (a) and the experimental setup is shown to the right (b).</p

The statically determined minimum required cable forces to hold the “mechanical egg” (F_slip) and its maximum allowed cable forces (F_break) for the two object’s breaking force settings derived for the prehensor’s two spring stiffnesses.

<p>The statically determined minimum required cable forces to hold the “mechanical egg” (F_slip) and its maximum allowed cable forces (F_break) for the two object’s breaking force settings derived for the prehensor’s two spring stiffnesses.</p

Cable to pinch force.

<p>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 (F_slip), 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 (F_1,break) and high (F_2,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.</p

Number of unsuccessful trials.

<p>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.</p

The peak forces for successful trials measured at the forearm (2 in Fig 1) and at the back (4 in Fig 1) of the subject and averaged over all subjects per condition.

<p>The peak forces for successful trials measured at the forearm (2 in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0169996#pone.0169996.g001" target="_blank">Fig 1</a>) and at the back (4 in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0169996#pone.0169996.g001" target="_blank">Fig 1</a>) of the subject and averaged over all subjects per condition.</p