CHAPTER
5
RESULTS AND DISCUSSION
In this section, the experimental characterization of the SCG prototype will be described.
5.1 Fabrication results
From the results of the analytical model, the pulling of the cable produces both a bending and a shrinkage of the nger. During a grasping, the latter translates into a reduction of the contact area of the gripper ngers with an increasing tension. So it is expected that the shrinkage leads the ngers to slide over the object surface. This is not desirable from the grasping stability point of view, which is proportional to the contact surface between the gripper and the object. To mitigate these consequences, two variants of the basic prototype have been then introduced, that, via two dierent solutions, attempt to solve this problem. The rst variant embeds ngernails, which in the human hand provide stability during precision grasping, positioned at the tip and on the nger dorsal side to locally
(a) (b) (c)
Figure 5.1: The realized prototypes: (a) the base prototype, (b) gripper with ngernails and (c) gripper with a milled layer on the nipper ventral surface.
5.2. Testing plan Chapter 5. Results and discussion increase stiness (Figure 5.1b). The second variant presents a milled layer on the nger ventral surface, to try to limit the slippage due to the shrinkage augmenting the grip (Figure 5.1c).
Figure 5.2: Characterization particulars.
5.2 Testing plan
The test bench described in 4.3 has been realized in order to provide the device characterization. In gure Figure 5.2, three particulars taken from the characteri-zation phase show the grippers adapting to the shape of the balloon object. In the second picture the nails force the ngertips to a pinch pose, like in human's hand, while in the third picture, the additional layer to prevent the slippage. A custom sensing system has been speci cally realized to indirectly acquire the grasping force: a ST Microelectronics LP S331AP MEMS pressure sensor soldered on a ST iN EM O board has been embedded inside a silicone casted balloon. Once the balloon was created, the sensor was inserted inside the base of the sphere with the sensitive surface facing upward, and the openings were sealed to ensure that the amount of air inside the chamber remains constant in time (Figure 5.3a).
5.3 Preliminary experimental results
Grasping force measurements have been performed to derive the relationship between the cables tension and the grasping force exerted by the gripper in the act of grabbing the sensorized silicone balloon. The characterization aims to highlight any changes due to implementations with respect to the standard condition. In Figure 5.3b the preliminary results of the prototypes characterization are shown. The scatter data have been tted with a quadratic polynomial curve. It can be seen how the implementations led the modi ed grippers to better performances. The high rmse for the tting of the red scatter data (gripper variant with milled layers characterization) can be simply justi ed: this improvement fails in the case
5.3. Preliminary experimental results Chapter 5. Results and discussion of a polished surface object, because in that case, the contact area is reduced, and it produces the opposite to the desired eect. Instead, it is expected that for the case of tissues, which are characterized by jagged surfaces, this kind of implementation could lead eectively to a better grip during grasping.
5.3. Preliminary experimental results Chapter 5. Results and discussion
(a)
(b)
Figure 5.3: (a) the custom sensing system and (b) the scatter plot, with quadratic polynomial tting with rmse: 24.3607(blue), 38.0094(green), 60.1830(red).
