## 4 Discussion  
In this study, the vibrational exposure of a 12-kilowatt self-propelled riding-type automatic onion transplanter was measured and evaluated to assess the effects on the operator's comfort and precision of transplanting. The results showed that operating status (static and driving) and load condition (unloaded and loaded) of the developed onion transplanter prototype had a significant effect on operator comfort on different driving surfaces. The vibrational exposures also exceeded the exposure limit value and health risk factor according to ISO 20816-1 and ISO 2631-5, respectively. Usually, the vibrational exposures of riding-type agricultural vehicles are higher than walking-type agricultural vehicles (i.e., two-wheel tractor or power tiller, transplanter, and mower) during field operation [62, 63]. The vibrational exposures of off-road vehicles (i.e., tractors) were notably different based on road types and forward speeds [60]. Crossing the ISO exposure limits could affect the tractor operator's health adversely. Moreover, wheel tire properties (i.e., stiffness, tire width, types, and weight) of the agricultural vehicle also affect vibrational exposure [64, 65, 66]. Over-inflated tires bounce more, and sidewall stiffness, tread rigidity, and less tire-to-road contact area trigger the vibrations. In this study, solid rubber wheel tires were used in the onion transplanter whose stiffness and tread rigidity were very high, and the tire-to-road contact area was very low. This might be a reason for the high vibrational exposure of the onion transplanter. Different types of tires with high ground-contact areas could be tested to minimize the vibration condition. [PERSON] and [PERSON] [67] observed that high wheel weight or loaded condition makes the tire stiffer and increases the tire-road contact region, which reduced the vibration. [PERSON] et al. [68] suggested a well-distributed mass with tire inflation pressure at the desired forward speed to increase operator comfort for agricultural tractors. A significant effect of load condition (weight of the operator and extra carrying load) on vibrational exposures and transplanting operation was also observed in this study. A lower vibrational exposure was observed during the loaded condition in both cases. Among several vibration-control techniques, improvement of the suspension system and installation of an isolation system are commonly applied. [PERSON] et al. [56, 57] emphasized operator seat suspension, vehicle cab suspension, and vehicle axle suspension to minimize the operator exposure to vibration of agricultural vehicles, especially self-propelled sprayers, agricultural tractors, and all-terrain vehicles (ATVs). [PERSON] et al. [43] installed a two-stage vibration isolation system on the engine of a rice transplanter and observed a significant change in the stiffness of the whole system (42.3% reduction in vibration) and avoiding the occurrence of coupling resonance. The observed vibrational exposures in this study can also be minimized and the comfort of the operator seat can be increased by installing a double-layer vibration isolation system and improving the shock-absorbing suspension system of the onion transplanter. Moreover, an ergonomically designed handle with shock-absorbing isolation can minimize vibration transmitted through the arms, which causes vascular damage and leads to 'dead finger' disease [69]. [PERSON] and [PERSON] [61] evaluated the effect of isolators and observed a more than 50% reduction in vibrational exposure transmitted through the operator's hands. Further to operator exposure to vibration, vibrational effects on the transplanting operation were evaluated in this experiment. A high weighted acceleration was observed on the seedling conveyor, which affected the normal transmission of the onion seedling by expelling the seedlings from the conveyor belt (as shown in Figure 5).