## 2 Materials and Methods
### Field Verification Experiment  
To verify the drag reduction performance of the bionic shovel, a field performance verification was conducted. The experimental field was the potato planting field at Heilongjiang Agricultural Machinery Engineering Research Institute. To match the existing potato digger, the digging shovel needed to be redesigned. The potato excavation device used for the field trial was a 4 MLS-2.2 single-row potato excavator, and the working width was 800 mm. One device was equipped with three shovels and each shovel measured 400 mm \(\times\) 253 mm \(\times\) 8 mm. The 2D design drawing and its installation style are shown in Figure 8.  
The O-P-S was manufactured by a CNC machine tool and its material was 40 MnB or 45 steel. The bionic shovel was obtained as follows. (1) A flat shovel of the same size with the O-P-S was set as the substrate, which was machined by a steel plate. In the area where the bionic structural plate is placed on the substrate steel plate, a flat-bottomed groove measuring 305 mm \(\times\) 253 mm \(\times\) 3 mm was cut by a CNC machine. (2) A bionic structural sheet (305 mm \(\times\) 253 mm \(\times\) 6.5 mm) with a plurality of pangolin scale structures was obtained by 3D printing. (3) The steel substrate plate and the bionic structural sheet were joined by bolts and cementing. This process is shown in Figure 9.  
\begin{table}
\begin{tabular}{c c c} \hline
**Property** & **Value** & **Source** \\ \hline Soil compaction (PSI) & soil bin test & field performance Field verification test & \\ \cline{2-3} Soil compaction (PSI) & \(38\pm 5\) & \(86\pm 5\) & Measured by hand-held soil compactness meter (Field Scout SC-900, Spectrum Technologies, USA) \\ \hline Soil moisture content (\%) & 25\(\sim\)30 & 40\(\sim\)50 & Measured by hand-held soil moisture meter (Field Scout TDR-300, Spectrum Technologies, USA) \\ \hline Tillage depth (cm) & \(20\pm 2\) & \(20\pm 2\) & Measured by ruler \\ \hline Working speed (Km/h) & 3 (or 0.84 m/s) & 6.5 (or 1.80 m/s) & Operator setting \\ \hline \end{tabular}
\end{table}
Table 3: Parameters for test.  
Figure 8: Two-dimensional design and the installation style for the shovel under field testing.  
To reduce the test error, the testing requirements for the conditions of the field ridge were as follows: the soil compactness measurements of the two ridges were similar and so were the soil moisture contents; the ridge shapes of the two ridges were similar to each other. The length of each ridge in the field was approximately 70.00 m. One ridge was the test site of the O-P-S and the other ridge was the test site of the S-U-S-_7 lh3.5_. The soil compaction was measured by using a handheld soil compactness meter (Field Scout SC-900, Spectrum Technologies, Aurora, Illinois, USA), and the soil moisture content was measured with a handheld soil moisture meter (Field Scout TDR-300, Spectrum Technologies, Aurora, Illinois, USA). The field performance test parameters are listed in Table 4. The test system for the field performance test is shown in Figure 10.  
\begin{table}
\begin{tabular}{r c c c} \hline
**Index** & **Total Force/N** & **Draft Force/N** & **Compressive Force/N** \\ \hline O-P-S & 2930.81 & \(-\)2275.92 & 2873.20 \\ \hline
1 & 5961.87 & \(-\)3830.45 & 6095.00 \\
2 & 2851.99 & \(-\)2362.28 & 2817.46 \\
3 & 4618.04 & \(-\)2881.89 & 4396.93 \\
4 & 3141.86 & \(-\)2354.79 & 3016.76 \\
5 & 5603.76 & \(-\)4201.23 & 5372.67 \\
6 & 4142.48 & \(-\)3323.35 & 4137.31 \\
7 & 4007.75 & \(-\)3148.3 & 4051.08 \\ S-U-S & 8 & 2938.37 & \(-\)2090.84 & 2897.27 \\
9 & 7360.12 & \(-\)5040.24 & 7123.69 \\
10 & 4630.84 & \(-\)3148.74 & 4691.66 \\
11 & 6158.36 & \(-\)4083.71 & 5985.47 \\
12 & 3441.39 & \(-\)2976.00 & 3243.01 \\
13 & 5772.89 & \(-\)4009.56 & 5677.79 \\
14 & 2159.02 & \(-\)1728.91 & 2100.03 \\
15 & 5512.85 & \(-\)3888.72 & 5486.83 \\ \hline \end{tabular}
\end{table}
Table 4: Simulation results of the bionic digging shovel (S-U-S) and ordinary plane shovel (O-P-S).  
Figure 9: Shovel samples for field testing.