# Parallel Fin Ray Soft Gripper with Embedded Mechano-Optical Force Sensor
## 4 Characterization and Experimental Results  
The characterization of the mechano-optical force sensor (Figure 6) was conducted using the same dimensions and gyroid infill pattern (15%) as in the final gripper design. The experimental setup included a dial indicator to measure the deformation distance and an Imada SSII-20R dynamometer to apply controlled compression to the sensor's contact \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Parameter** & **TPE Value** & **PLA Value** & **Unit** \\ \hline Nozzle diameter & 0.4 & 0.4 & mm \\ Layer height & 0.2 & 0.2 & mm \\ Infill & 100 & 20 & \% \\ Material temp & 225 & 200 & \({}^{\circ}\)C \\ Build plate temp & 50 & 50 & \({}^{\circ}\)C \\ Print speed & 20 & 50 & mm/s \\ Extrusion multiplier & 115 & 100 & \% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Printing parameters for flexible TPE 70A and PLA filaments. Figure 5: Parallel Fin Ray soft gripper manufacturing process by 3D printing. (**a**) Infill of the parallel Fin Ray soft gripper. (**b**) Detail view of the compression layer gyroid infill. (**c**) Manufacturing process. surface (Figure 6a). After calibrating the force sensor with the dynamometer, multiple load cycles were performed to assess repeatability and hysteresis effects. The results confirmed that deviations remained within acceptable limits, ensuring the reliability of the sensor readings. The sensor exhibited a hysteresis of 11.92%, determined through multiple loading and unloading cycles at a rate of 15 mm min\({}^{-1}\). This value is consistent with the results reported in [49]. In addition, the maximum sample standard deviation was found to be 0.5 for the voltage-deformation measurement (Figure 6c) and 3.3 for the force-deformation measurement (Figure 6d). The sensor demonstrated a sensitivity of 0.004 V 0.16 N, representing the smallest detectable change in force. The force-response behavior of the gyroid-patterned sensor closely aligns with the findings of [49] (Figure 6c,d). The response is divided into two distinct, nearly linear regions, both exhibiting a high coefficient of determination (\(R^{2}\) = 0.96) (Figure 6e,f). In the first region (0-2.5 mm), the gyroid lattice maintains its structural integrity and exhibits elastic resistance, supporting forces up to approximately 24 N while preserving a linear response. However, beyond 2.5 mm compression, the gyroid structure collapses, leading to a shift in mechanical behavior. In this second region (2.5-4 mm), force measurements are primarily dictated by the compression of the deformed TPE layers, enabling the sensor to reach a peak force of approximately 150 N. As shown in Figure 6b, this transition occurs  
Figure 6: Performance of the 3D-printed mechano-optical sensor. (**a**) Close-up view of the dynamometer in contact with the sensor surface during characterization. (**b**) Detail view of the total collapse of the gyroid pattern. (**c**) Voltage vs. compression deformation. (**d**) Force vs. compression deformation curve. (**e**) Linear force response in the first deformation stage (0–2.5 mm). (**f**) Linear force response in the second deformation stage (2.5–4 mm). precisely at 2.5 mm compression due to the complete structural collapse of the gyroid pattern, fundamentally altering the sensor's mechanical response. After characterizing the mechano-optical sensor, the parallel soft gripper was mounted onto the HortiRobot dual-arm robotic system. The HortiRobot experimental platform [51] is based on an ABB YuMiR--IRB 14000, a collaborative robot with two high-dexterity seven-axis arms [52]. To evaluate the sensor's performance, we tested eight different geometries, each measuring 20 \(\times\) 20 \(\times\) 20 mm (Figure 7). The grasping test evaluated sensor response variations when an object was offset from the gripper's center. Each geometry produced distinct force readings, with notable differences in semi-sphere and pyramid-shaped objects.