# In-Field Load Acquisitions on a Variable Chamber Round
## 2 Research Methodology
### Calibration  
Figure 4a presents a schematisation of the adopted calibration procedure along with the signal acquired at the L hub carrier during one of the three monotonic load ramps executed. Specifically, two digital scales and an overhead crane to lift the baler were used. Initially, the full-bridge channels were balanced (i.e., zeroed) with no load applied to the hub carriers (i.e., the baler was fully lifted by the overhead crane and without the wheels attached to the hub carriers), and then three experimental points were acquired for each monotonic load ramp: _(i)_ the baler was fully lifted by the overhead crane (the strain gauge bridges returned \(\epsilon_{1}\!>\!0\) due to the wheel weight (see Figure 1), digital scales returned 0 kN); _(ii)_ the baler was partially lifted by the overhead crane (the strain gauge bridges returned \(\epsilon_{1}\!=\!0\), digital scales returned the wheel weight equal to 1.24 kN); and _(iii)_ the baler was not supported by the overhead crane (the strain gauge bridges returned \(\epsilon_{1}\!<\!0\) (see Figure 1), left and right digital scales returned 14.2 kN and 9.10 kN, respectively). Figure 4b,c report the calibration curves obtained for the L and R hub carriers, respectively. Furthermore, in Figure 4b, c, the relationship between measured strains and the corresponding computed loads is highlighted: negative \(\epsilon_{1}\) strains correspond to positive loads. Therefore, vertical loads, F\({}_{v}\), are considered positive when acting in the positive z direction, as illustrated in Figure 1. Finally, it is worth mentioning that the sensitivity of the strain gauge bridges to potential lateral or longitudinal loads on the L and R hub carriers was not evaluated, as these were considered negligible owing to flatness of the test field, the pre-planned in-field load acquisition maneuvers, and the use of an unbraked baler. Figure 5 reports the experimental setup used for calibrating the dynamometric towing pin used to measure the horizontal loads F\({}_{h}\)(t) on the round baler's drawbar. A specially dedicated servo-hydraulic test bench was used to obtain the relationship between the measured strains provided by the upper and lower half bridges (i.e., \(\epsilon_{\text{IP}}\) and \(\epsilon_{\text{low}}\), respectively, see Figure 3b) and the load applied to the dynamometric towing pin. To this end, a 70 kN EnginLAB servo-hydraulic actuator equipped with a 100 kN Aep TC4 AMP load cell and a closed-loop digital controller EnginLAB RTC9000 were used. The role of the dynamometric towing pin stiffness during calibration was extensively investigated in the discussion accompanying the next Figures 6 and 7. Figure 6 shows the relationships obtained between the strains provided by the upper and lower half bridges and the load F\({}_{\rm exp}\) applied to the dynamometric towing pin by  
Figure 4: Calibration of the full-bridge channels at the hub carriers: (**a**) data acquisition procedure, (**b**,**c**) experimental sensitivity obtained for the L and R hub carriers, respectively. Figure 5: Servo-hydraulic test bench used for calibrating the dynamometric towing pin: (**a**) side view (**b**) upper view and (**c**) image of the experimental setup. means of the servo-hydraulic actuator. In particular, three pulling configurations (force in the negative x direction in Figure 5a) with \(c=0\) mm, \(c=-2.0\) mm, and \(c=3.8\) mm (\(c\) visible in Figure 3) and three pushing configurations (force in the positive x direction in Figure 5a) with \(c=0\) mm, \(c=-3.8\) mm, and \(c=3.8\) mm were investigated. Specifically, two loading-unloading monotonic ramps were performed for each configuration. It is worth mentioning that the applied loads were such as to obtain the relationship between \(\varepsilon_{\text{tip},\text{low}}\) and F\({}_{\text{exp}}\) across the entire range of strains anticipated during the in-field load acquisitions.