## 2 Materials and Methods
### Vibration Data Acquisition System  
In this study, the sources of vibration of the onion transplanter were the engine of the vehicle, power transmission, and rotating transplanting parts. A total of four tri-axial accelerometer sensors (model: 356A15, PCB Piezotronics Inc., Depew, NY, USA) were used to measure the vibration levels created from different sources and transmitted all over the onion transplanter and seated operator's body. However, the vibration exposure of the transplanting unit was mainly focused. Location selection for sensor attachment was performed following a review of the literature [16; 32; 44] and the power flow system of the transplanting unit [16; 45]. Two sensors were attached on the base of the rotating hopper to check the vibrational effects on transplanting. One was placed in the conveyor belt as high vibrations spilt the seedlings from the conveyor, and one accelerometer sensor was placed on the floor of the vehicle (underneath the operator seat, as the seat was firmly attached with the vehicle frame, and the transmitted vibration has a significant health effect on the operator's body), to measure the overall vibration level of the onion transplanter. A four  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Mass of Four-Wheel Vehicle (kg)** & **Mass of Vehicle with Transplanter (kg)** \\ \hline Mass of left front wheel & 250 & Mass of left front wheel & 246 \\ Mass of right front wheel & 237 & Mass of right front wheel & 203 \\ Mass of left rear wheel & 32 & Mass of left rear wheel & 411 \\ Mass of right rear wheel & 56 & Mass of right rear wheel & 405 \\ Total mass & 575 & Total mass & 1265 \\ \hline \hline \end{tabular}
\end{table}
Table 1: _Cont._  
Figure 1: A 3-D model of the (**a**) 12-kilowatt self-propelled riding-type automatic onion transplanter, and (**b**) major components of the transplanting part, (**c**) onion seedling tray, and (**d**) transplanting mechanism.  
channel dynamic signal acquisition module (model: NI 9234, National Instruments, Austin, TX, USA) and an NI compactDAQ eight-slot USB chassis (model: NI cDAQ-9178; National Instruments, Austin, TX, USA) were used in this study. A LabVIEW coded program (ver. 2018; National instrument, Austin, TX, USA) was applied to collect the vibration exposures. The sampling frequency was 1000 data/second, and signal block length was 60 s. The detailed specification of the vibration measurement instruments is summarized in Table 2. A schematic diagram of the data acquisition system, a photo of the data acquisition unit, and sensor placements are shown in Figure 2.  
\begin{table}
\begin{tabular}{c c c} \hline
**Item** & **Model** & **Image** & **Specification** \\ \hline Acceleration & 356A15 & Sensitivity (\(\pm\)10\%): 10.2 mV/(ms\({}^{-2}\)) \\  & & Measurement range: \(\pm\)490 ms\({}^{-2}\) \\  & & Frequency range (\(\pm\)10\%): 1.4–6500 Hz \\  & & Resonant frequency: \(\geq\)25 k \\  & & Broadband resolution: 0.002 ms\({}^{-2}\) rms \\  & & Shielded, lightweight, FEP cable \\  & & Four-socket plug, IP68 rated to triple splice \\  & & assembly with (3) 1-foot coaxial cables each \\  & & with a BNC plug (AC) \\ Data acquisition & & Timing accuracy: 50 ppm of sample rate \\  & & Timing resolution: 12.5 ns \\  & & Internal base clocks: 20\(\sim\)100 kHz \\  & & Regeneration: 1.6 MSs\({}^{-1}\) \\  & & NI Instrument \\  & & Professional development system \\  & & (64-bit) for Windows \\  & & NI recommends 1 GB of RAM (min.) \\ \hline \end{tabular}
\end{table}
Table 2: Specifications of the vibration exposure measurement instruments and software.  
Figure 2: Block diagram of the (**a**) data acquisition system, (**b**) photo of the data acquisition unit, and (**c**) sensor placements.