Fig 1.
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.
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.
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.
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.
Table 1.
The statically determined minimum required cable forces to hold the “mechanical egg” (Fslip) and its maximum allowed cable forces (Fbreak) for the two object’s breaking force settings derived for the prehensor’s two spring stiffnesses.
Fig 5.
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.
Table 2.
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.
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.
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.