Vibration Assessment of a 12-kW Self-Propelled Riding-Type Automatic Onion Transplanter for Transplanting Performance and Operator Comfort

Abstract

Vibration assessment of upland crop machinery under development is essential because high vibrational exposures affect machine efficiency, service life of components, degradation of the working environment, and cause health risks to the operator. It is intensively assessed for automobiles as well as large off-road agricultural vehicles (i.e., tractors). However, it is mostly overlooked in the case of the small or medium riding-type upland utility vehicles. Therefore, the vibration exposures of a 12-kilowatt self-propelled riding-type automatic onion transplanter were measured and evaluated to assess the performance of onion transplantation and the operator’s comfort in this study. Different types of driving surfaces, operating statuses (static and driving), and load conditions were considered to analyze the vibration exposure. The precision of transplantations was evaluated while operating the transplanter on the soil surface with different driving speeds and load conditions. Tri-axial accelerometers and a LabVIEW-coded program were used for data acquisition. The vibrational exposures were evaluated based on ISO standards, and power spectral density (PSD) was estimated to assess the major frequencies. According to the statistical analysis, the daily exposure value (A(8)) and the vibration dose value (VDV) varied from 10 to 15 ms⁻² and 20 to 31 ms^−1.75, respectively, which exceeded the ISO 2631-1 standards (i.e., A(8): 1.15 ms⁻² and VDV: 21 ms^−1.75). The calculated health risk factor (R^A) was moderate. Moreover, a high weighted acceleration (around 8 ms⁻²) was observed on the seedling conveyor belt, which might result in missing seedlings during transplanting. The vibration exposures of the developed onion transplanter need to be minimized following the ISO standards, and vibration reduction would also improve the market competitiveness.

1. Introduction

Onion (Allium cepa L.) is one of the major commercial crops widely cultivated, and used both as a food and for medicinal applications around the world [1]. It is a rich source of several types of phytochemical components such as carbohydrates, proteins, vitamin C, B6, folic acid, sugars (glucose, fructose, galactose, and arabinose), minerals (Ca, Fe, S), flavonoids, antioxidants, and polyphenol components. It is an important element of the Mediterranean diet, promotes health, and prevents several diseases (e.g., cancer, heart disease, obesity, hypercholesterolemia, diabetes, hypertension, and gastrointestinal tracts [2,3,4]. As a food item, onion is usually served cooked, and can also be eaten raw as a vegetable or part of a prepared savory dish. It is also used to make pickles or chutneys. The world cultivation area of onion was extended more than 3 million ha from 1990 to 2019, and world production was around 97 million tons in 2019 [5,6]. However, the cultivation rate of onion is decreasing in many countries, such as Korea (1.59 million tons in 2014 to 1.17 million tons in 2020) [7], due to labor intensiveness, aging of the farmers, decrease in the farming population, high labor cost, and low mechanization rate. The conventional onion cultivation method requires 185–260 man-hr ha⁻¹, which is labor-intensive and also non-efficient [8]. Semi-automatic and automatic transplanters are becoming popular to overcome these drawbacks. However, the onion seedlings are picked and fed manually in semiautomatic transplanters. Transplanting is performed mechanically [9]. Contrarily, a synchronized combination from picking to transplanting is repeated in automatic transplanters. An automatic onion transplanter might be riding type or walking type [10].

Accurate and precise transplanting performance depends on several design and/or operating factors of transplanters, and vibration is one of the major factors among those. Vibration can be defined as a mechanical phenomenon, where oscillations may be periodic or random [11]. It is generally caused by the rapid forward, backward, or rotating movement of mechanical components. The speed, position, and load condition of the component, roughness of driving surface, and operational site affect the vibration level [12,13,14,15]. Severe vibration during field operation of transplanters results in the seedlings dropping down from the conveyer, picking jaws, and dibbling devices. It also hampers the accurate and precise planting of the seedlings. Ultimately, the total yield is reduced. The measurement and evaluation of vibrational exposure of off-road agricultural machinery, especially crop machinery prototypes under development, is essential to reduce dynamic stress, prevent misalignment (belts or shafts), maintain stability, and improve the overall working environment and safety of operation [16]. Severe vibration during vehicle operation also often causes potential health damage to the operators [17,18,19]. Vibration transmits to the operator’s body through the seat, backrest, cabin floor, and steering wheel [20]. The vibrational exposure produced by the engine and other moving components of a vehicle causes not only an uncomfortable feeling for the operator, but also there is evidence of several physiological problems, such as cognitive/motor impairment [21], standing balance [22], and musculoskeletal disorders such as lower-back pain, sciatica [23], neck pain [24], and degenerative changes in the lumbar spine [25].

In order to maintain proper transplanting and minimize adverse health effects on the operator caused by vibration, an assessment of vibrational exposure and safety management must be carried out. ISO 20816-1 [26], ISO 5349-1 [27], and ISO 2631-5 [28] standards were widely used to evaluate the vibrational exposure of mechanical components and human exposure to whole-body vibration (WBV), especially for vibration containing multiple shocks, respectively. The procedures declared in ISO 2631-1 were used in the European Directive 2002/44/EC [29,30] for ensuring the safety of workers. ISO 2631-5 was issued to address human exposure to WBV containing multiple shocks [31]. The methods of this standard can also address the cumulative exposure impact over time on the health of the operator’s lumbar spine. Most recently, ISO 2631-5:2018 was published, emphasizing two exposure regimes: severe conditions and less-severe conditions. However, the exposure limits mentioned in ISO 2631-5:2004 remained the same in the new ISO 2631-5:2018. According to ISO 2631-1, and Directive 2002/44/EC, the daily (8-h) equivalent exposure action value and exposure limit value for the whole body are 0.5 and 1.15 ms⁻², and for hand–arm vibration are 2.5 and 5.0 ms⁻², respectively. Furthermore, ISO 2631-5 scaled the probability of adverse health effects due to WBV (daily compressive dose (SA) of the lumbar spine) into three categories, namely, low (<0.8 MPa), moderate (0.8 to 1.2 MPa), and high (>1.2 MPa) [16,18]. Moreover, attached or hitched implements also play one of the major roles in vibration development. For example, the usual range of WBV of tractors with implements is 0.5~1.49 ms⁻² [32,33,34]. According to ISO 20816-1, the vibration severity of class II medium machines is categorized into four classes: good (0.28–1.12 mms⁻¹), satisfactory (1.8–2.8 mms⁻¹), unsatisfactory (4.5–7.1 mms⁻¹), and above this limit is unacceptable [26,35].

Analysis of vibration exposure of an up-land crop transplanter is essential for ensuring proper transplanting and minimizing the adverse health effects on the operator caused by driving surface roughness, forward speed, operating status, and load conditions. The considered onion transplanter of this study was under development, and consisted of an operator-driven four-wheel vehicle and a mounted onion transplanter. The selected vehicle for this onion transplanter is usually used as a rice transplanter, where the mud (soft soil) acts as padding and minimizes the vibrational exposure significantly. Although several studies were conducted to evaluate the vibrational effects on machine performance and the operator’s health in automobiles and large off-road vehicles such as tractors [6,12,15,16,36], few studies have investigated the vibrational exposure of upland crop transplanters, especially riding-type [37,38,39,40,41], which might adversely affect the performance of crop production as well as endanger the health of the operator. Regarding these issues, the objective of this study was to measure and evaluate vibrational effects of a 12-kilowatt self-propelled riding-type automatic onion transplanter for the operator’s comfort and transplanting performance.

2. Materials and Methods

2.1. Overview of the Onion Transplanter Prototype under Development

A 3-D model of the 12-kilowatt self-propelled riding-type automatic onion transplanter is shown in Figure 1a. It consists of two major parts, the operator-driven four-wheel vehicle and onion transplanter. The physical dimensions of the onion transplanter are summarized in

Table 1. Specification of the 12-kilowatt self-propelled riding-type automatic onion transplanter under development.

Specification of Four-Wheel Vehicle		Specification of Onion Transplanter
Length, width, height (mm)	2602, 1716, 1648	Length, width, height (mm)	875, 1626, 2036
Ground clearance (mm)	363	Ground clearance (mm)	260
Wheelbase (mm)	1196	Number of rows	6
Wheel track (mm)	1200	Row spacing (mm)	200
Front wheel radius (mm)	650	Hill spacing (mm)	174
Rear wheel radius (mm)	950	Power (rpm)	74
Transmission level	HST	Transplanting speed (ms−1)	0.24
Maximum power (kW/rpm)	16.2/3400	Transplanting mechanism	Mechanical
Mass of Four-Wheel Vehicle (kg)		Mass of Vehicle with Transplanter (kg)
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

. The onion transplanting process is divided into three major steps (mechanisms): seedling picking (extraction mechanism), seedling supplying (conveyor mechanism), and seedling planting (transplanting mechanism). At first, onion cell trays were placed into the seedling trays. The onion seedlings are extracted from the growing cell tray using the pushing rods and placed in the conveyor. Under the conveyor mechanism, seedlings were conveyed into the planting hopper, and seedlings were transplanted in six rows through three separated units of seedling planting mechanisms. All these units are synchronized together in such a way that each unit can supply 12 seedlings per cycle to the conveyor mechanism. Contrarily, each conveyor mechanism receives 6 seedlings per cycle, which are fed into the hopper and transplanted in the soil bed through the rotary planting mechanism. Twelve axillary pressing wheels (two for each row) press the soil finally to maintain the proper uprightness of seedlings and low damage to the mulch film. As momentum wheel, rotating devices, and powertrain devices are considered as the main sources of vibration, these devices along with the mounted and thin plate parts would be highly affected and damaged by vibration [16,42,43].

2.2. 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-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. Specifications of the vibration exposure measurement instruments and software.

Item	Model	Image	Specification
Acceleration sensor	356A15		Sensitivity (±10%): 10.2 mV/(ms−2) Measurement range: ±490 ms−2 Frequency range (±10%): 1.4–6500 Hz Resonant frequency: ≥25 k Broadband resolution: 0.002 ms−2 rms
Connector cable	C4P5M3BP		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 device	NI cDAQ 9188		Timing accuracy: 50 ppm of sample rate Timing resolution: 12.5 ns Internal base clocks: 20~100 kHz Regeneration: 1.6 MSs−1
Software	LabVIEW 2020		NI Instrument Professional development system (64-bit) for Windows NI recommends 1 GB of RAM (min.)

. A schematic diagram of the data acquisition system, a photo of the data acquisition unit, and sensor placements are shown in Figure 2.

2.3. Vibration Test Conditions

The vibration exposure of the onion transplanter was evaluated for different driving surface types, operating status, load conditions, and driving speeds. The overall test plan is summarized in

Table 3. Vibration exposure test conditions by driving surface, operating status, load condition, and driving speed.

Condition	Driving Surface	Operating Status	Load Condition	Driving Speed
Levels	Soil surface	Static	Unloaded	Low
	Unpaved road	Dynamic	Loaded	High
	Asphalt road

. The vibrational exposure of the vehicle was evaluated under static and driving conditions on soil, unpaved, and asphalt road with and without loads (seedling trays: 40 kg), as shown in Figure 3. The vibrational effects on transplanting were evaluated under driving conditions on the soil surface with different driving speeds and load conditions. A total of 16 tests (12 tests for vehicle vibration and 4 tests for transplanting) were conducted to evaluate the vibration exposure. For each test, vibration levels were measured concurrently in three translational axes (X-longitudinal, Y-transversal, and Z-vertical) upon the surface of the onion transplanter. Three replications were applied. The field tests were performed in the farm field of the onion research center, Republic of Korea (35°33′08.0″ N and 128°28′31.8″ E, Daeji-myeon, Changnyeong-gun, Gyeongsangnam-do, Korea) to evaluate the vibration exposures. The length and width of the field were 30 and 40 m, respectively. The plant-to-plant distance was 13 cm, and 6 onion seedlings were planted at a time.

2.4. Procedures of Vibration Evaluation

Vibration values were measured considering different factors as mentioned in Table 3, and evaluations were performed based on ISO 20816-1 and ISO 2631-5 standards. The vibration exposure of the transplanting section (mechanical components safety) and vehicle frame (operator comfort) were evaluated in this study for the overall improvement in the working environment of the developed transplanter.

2.4.1. Measurement of Vibration at Transplanting Section

The vibration sources of the transplanting section were different rotating shafts, power transmission line, and dibbling components. The vibration levels of the transplanter section, specifically, the conveyer belt and dibbling device, were assessed separately. The acceleration magnitudes were assessed based on the ISO 20816-1 [26,46,47] standard. In this study, the average weighted acceleration (A_S) was calculated using Equation (1), which is the sum of three translational axes values.

$$A_{W,axis}={\left[{{\displaystyle \int }}_0^T\left[a_{W,axis}{\left(t\right)}^{2}dt\right]\right]}^{1/2}$$

$$A_S=\sqrt{{\left(A_{w,x}\right)}^2+{\left(A_{w,y}\right)}^2+{\left(A_{w,z}\right)}^2}$$

(1)

2.4.2. Measurement of Vibration for Operator Comfort

Driving surface roughness and velocity of the vehicle greatly affect the intensity of vibration. The International Roughness Index (IRI), a well-recognized standard to quantify road surface roughness, is used throughout the world to describe the roughness of driving surfaces [48]. Based on the IRI parameters and previous studies [44,49,50], the roughness of the selected surfaces or roads was also calculated in this study to classify the road conditions. Equation (2) represents the relationship between the road roughness, speeds, and WBV in the vertical direction (Z-axis).

$$\frac{rm{s}_z}{IRI}=0.16{\left(\frac{v}{80}\right)}^{1/2}$$

(2)

According to ISO 2631-1 standards, there are two methods, daily exposure value (A(8)) and the Vibration Dose Value method (VDV), to evaluate vibration exposure using the effective value of weighted acceleration. The A(8) index is measured considering the weighted rms acceleration, where one quarter of rms of the weighted acceleration is used to calculate the VDV instead of the half power. Both methods assess the vibrational exposure to an operator’s body for an 8-h work cycle, and the application of frequency weighting filters for the obtained raw data is required.

Table 4. ISOs for evaluating operator exposure to vibration, and their relevant equations.

ISO	Equations for Vibration Evaluation
2631-1	Daily exposure value A(8)		Vibration Dose Value (VDV)
	$rm{s}_{ws}={\left[\frac{1}{T}{{\displaystyle \int }}_0^T{a}_w^2\left(t\right)dt\right]}^{1/2}$	(3)	$vd{v}_{ws}={\left[{{\displaystyle \int }}_0^T{a}_w^4\left(t\right)dt\right]}^{1/4}$	(6)
	$A{\left(8\right)}_s=K_{s}rm{s}_{ws}\sqrt{\frac{T_{\exp }}{T_0}}$	(4)	$vd{v}_s=K_{s}vd{v}_{ws}\sqrt{\frac{T_{\exp }}{T_{meas}}}$	(7)
	$A\left(8\right)=\max \left[A{\left(8\right)}_x,A{\left(8\right)}_y,A{\left(8\right)}_z\right]$	(5)	$VDV\left(8\right)=\max \left[VD{V}_x,VD{V}_y,VD{V}_z\right]$	(8)
2631-5	Daily compressive dose value		Risk factor
	$S^A={\left({\displaystyle {\displaystyle \sum }_i}{\left(\frac{C_{dyn,i}}{B}\right)}^6\right)}^{1/6}$	(9)	$R^A={\left({\displaystyle {\displaystyle \sum }_{m=1}^n}{\left(\frac{S_d^A{N}_m{}^{1/6}}{S_{ui}^A-S_{stat,i}^A}\right)}^6\right)}^{1/6}$	(11)
	$S_d^A={\left({\displaystyle {\displaystyle \sum }_i}{S}_j^{A6}\frac{t_{dj}}{t_{mj}}\right)}^{1/6}$	(10)	$S_{stat,i}^A=6.765 \mathrm{MPa}-0.067 \mathrm{MPa}$ $\times \left(b+i\right)$	(12)

shows the Equations (3)–(8) of the A(8) and VDV indices.

The ISO 2631-5 standard emphasizes the daily compressive dose value ($S_d^A$) and risk factor ($R^A$) to evaluate the operator exposure to vibration. $S_d^A$ (specifically, internal spinal force) is measured through two methods: the severe-condition regime and the less-severe-condition regime. Here, the regime indicates the place where the operator remains seated under all exposures. The possible ways of acceleration transmission to the operator’s body are the seat, backrest, feet, and hands. The compressive dose value depends on the body mass, posture, and mass index of the operator, and the three directional raw acceleration values were used to determine the less-severe condition. On the other hand, the risk factor is estimated (Equations (9)–(12)) considering the equivalent daily compressive dose of the lumbar spine (T12/L1, L1/L2, L2/L3, L3/L4, L4/L5, L5/S1), exposure days per year, number of years, lumbar spine strength based on age, and the mean value of the compressive–decompressive force, following the methods mentioned by [48]. It is important to mention that the risk factor is not the only indication of the probability of failure. Here, R^A = 1 indicates that the dynamic load of the shock reached the same order of magnitude as the ultimate strength that the vertebra is capable of resisting. In this study, the age of the onion transplanter operator was 40 years and he worked an average of 8 h daily for the previous 10 years. The posture of the operator during transplanting operation was considered as defined in the ISO standard.

2.5. Frequency Analysis

The common vibrational exposure of some off-road agricultural vehicles with or without implement is summarized in

Table 5. Common vibrational exposure of agricultural vehicle during on-farm operation and a safe exposure zone.

Common Vibration Levels a		Safe Exposure Range b
Vehicle/Implement	WBV (ms−2)	A(8) (ms−2)	RA (ms−2)
Self-propelled sprayer	0.53–0.69	Exposure Action Value: 0.50 Exposure Limit Value: 1.15	Low: < 0.8 Medium: 0.8 < RA < 1.2 High: > 1.2
Tractor-mounted sprayer	0.5–0.74
Tractor-mounted plough	0.73–0.89
Tractor–trailer transport	1.05–1.32
Tractor-mounted cultivator	1.2–1.49
All-terrain vehicles	0.85–1.39
Tow tractor	0.2–1.50
Backhoe loader	0.2–1.65

^a [56,57,58], ^b [44].

. Sometimes, unconsidered factors also affect vibration exposure, which might not be associated harmonically. This type of phenomenon cannot be anticipated and accurately predicted in advance. Therefore, some intensive analysis, i.e., power spectral density (PSD) is required to understand the reason for the repetition of the random vibration, and its effect on the targeted operation. In this regard, data transformation (time domain to frequency domain) is essential [51,52]. In this study, Fast Fourier Transform (FFT) was used for this purpose. FFT is a computational method which converts a signal from its original domain (time or space) to a frequency domain and vice versa [53]. Sometimes, FFT cannot describe the non-stationery signals. A power spectral density (PSD) is widely used in research in those situations. PSD helps understanding of the distribution pattern or frequency variations of the time signal energy with differing spectral resolutions [54]. In this study, PSD was obtained through Welch’s overlapped segments averaging estimator [55], using windows of 1000 data with an overlap of 600 data on a segment of 1000 discrete Fourier transform (DFT) points extracted from the middle of each record. Then, each PSD was normalized by its median to reduce the effect of different offsets in the recordings. Equal numbers of data were considered before and after the central data point of the recorded acceleration. The used formula of DFT and PSD are mentioned in Equations (13) and (14), respectively. Notations, definitions, and relevant units of all variables used in this manuscript to evaluate the vibration exposure were listed in the nomenclature section.

$$X\left(f\right)=\frac{1}{N}{\displaystyle \sum }_{n=0}^{N-1}x\left[N\right]e^{-i2\pi \frac{f}{N}},f=0,1,2,\dots ,N-1$$

(13)

$$PSD=\frac{a_{RMS}{}^2}{\Delta f}$$

(14)

2.6. Statistical Analysis

The statistical analysis of this study was performed using the Minitab 19.0 statistical package (ver. 2019, Minitab, Rd State College, PA, USA). The raw data were pre-processed at first. The first and third quartile, interquartile range, upper bound, and lower bound were calculated to remove the noise and outliers. Then, the average and standard deviation of the considered parameters were calculated. The overall vibrational exposure was analyzed using multi-factor ANOVA. The significance of differences between mean values was determined using Tukey’s two-way ANOVA at a confidence level of 95%. PSD analysis was performed based on Welch’s method using MATLAB R2015a software (ver. 8.5, The MathWorks, Natick, MA, USA).

3. Results

3.1. ANOVA of the Vibration Sources for Transplanting Performance and Operator Comfort

The effects of vibrational exposure on the operator’s body and transplanting were analyzed based on multi-factor ANOVA. During the ANOVA, three different driving surfaces, two operating statuses, and two load conditions were selected to evaluate operator comfort or health hazard. Similarly, two load conditions and two driving speeds were considered for transplanting operation evaluation. The results of the ANOVA, as shown in

Table 6. Multi-factor ANOVA test showing the individual effects of the considered factors (i.e., driving surface, operational status, and load conditions) on transplanting performance and operator comfort.

Source Variation	DF	Adj SS	Adj MS	F-Value	p-Value
Transplanting performance
Load condition	1	21.773	21.772	45.070	<0.001
Driving speed	1	11.749	11.749	24.320	<0.001
Error	9	4.348	0.483
Operator comfort
Driving surface	2	3.642	1.821	1.670	>0.05
Operating status	1	3.343	3.343	3.070	<0.05
Load condition	1	14.423	14.423	13.260	<0.001
Error	31	33.722	1.087

DF: degrees of freedom, SS: sum of squares, MS: mean squares.

, indicate that the operating status (p ≤ 0.05) and load condition (p ≤ 0.001) had significant impacts on operator comfort. Contrastingly, the load condition and driving speed of the onion transplanter had significant impacts (p ≤ 0.001) on the precision of transplanting. Although driving surface is a crucial factor which can affect operator comfort negatively, the obtained ANOVA results indicates that the driving surfaces of this study did not influence the comfort of the operator significantly.

3.2. Evaluation of Vibrational Exposure on Transplanting

The effects of vibrational exposure on onion seedling transplanting were evaluated by measuring the vibration level on the seedling conveyor belt, as high vibrations spilt the seedlings from the conveyor track, and two different dibbling parts (base of rotating hopper) because the precision and missing of transplanting depends on these parts. Figure 4 shows the measured vibrational exposure at the mentioned locations under the loaded and unloaded conditions and high and low driving speeds during onion seedling transplanting. A high vibrational exposure (3–8 ms⁻²) was observed at the conveyor belt under both load conditions and driving speeds, especially Z-axis directional vibration (6–8 ms⁻²), which caused onion seedling expulsion from the conveyor belt as shown in Figure 5. However, the overall vibrational exposure was reduced (0.5–2 ms⁻²) due to load addition and in the low-driving-speed condition. The recorded vibration levels of the dibbling parts were very reasonable compared to the conveyor belt. Except for split seedlings, no missed transplanting was observed due to the vibrational exposure of the dibbling devices during the field test.

3.3. Evaluation of Operator Comfort

Driving surface roughness is one of the major factors affecting vibration exposure significantly. According to Equation (1) and other obtained data, the roughness of the soil surface, unpaved road, and asphalt road were 1.11, 1.15, and 1.19 mkm⁻¹, respectively. The standard United States IRI thresholds range from 1.5 to 2.68 mkm⁻¹ for all kinds of roads, and IRI < 1.5 mkm⁻¹ is considered as a good road segment [59,60]. The calculated IRI of this study was lower than the standard range, which indicates a good condition of the driving surfaces. As the tests were conducted in the machinery testing beds of the TYM Tractors (TYM Tractors, Co., Ltd., Iksan, Republic of Korea), the homogenous condition of the driving surfaces of this study might be the possible reason behind this low roughness. Figure 6 shows the maximum level of vibration measured at the operator’s seat frame under different driving surfaces, operating statuses, and load conditions regarding the three translational axes (X, Y, and Z). Vibration exposures at the Z-axis were very low (0.15–0.20 ms⁻²) compared to the X-axis (8–12 ms⁻²) and Y-axis (5–8 ms⁻²) for all conditions, due to the absence of a suspension system. More specifically, the selected vehicle is manufactured as a rice transplanter, where a suspension system is not essential due to the soft driving surface (saturated soil). Generally, vibrational magnitudes vary depending on vehicle types, suspension systems, and driving surface. During driving on the soil surface, the vibration level was higher in the dynamic condition compared to the static condition. Some upland crop machinery, such as hand tractors, also produce similar vibration exposure. Tewari and Dewangan [61] observed 6.7–7.8 ms⁻² and 5.1–6.2 ms⁻² weighted vibration during rota-tilling and rota-puddling, respectively, at forward speed of operation from 0.30 to 0.63 ms⁻¹. An opposite scenario was observed during driving on the unpaved and asphalt roads for both unloaded and loaded conditions. Moreover, a small (1–2 ms⁻²) vibration difference was observed for 40 kg load difference (unloaded and loaded conditions) for all kinds of driving surfaces and operating statuses.

Table 7. Risk Factor (R^A) estimation according to ISO 2634-5.

Driving Surface	Operating Status	Load Conditions	RA Factor for Lumbar Spines According to ISO 2634-5
			T12/L1	L1/L2	L2/L3	L3/L4	L4/L5	L5/S1	RA
Soil surface	Static	Unloaded	0.70 ± 0.16	0.71 ± 0.11	0.72 ± 0.08	0.74 ± 0.13	0.72 ± 0.09	0.77 ± 0.02	0.77
		Loaded	0.78 ± 0.07	0.82 ± 0.16	0.85 ± 0.12	0.87 ± 0.17	0.85 ± 0.15	0.77 ± 0.07	0.87
	Dynamic	Unloaded	0.75 ± 0.12	0.77 ± 0.10	0.80 ± 0.11	0.82 ± 0.13	0.81 ± 0.12	0.74 ± 0.07	0.82
		Loaded	0.76 ± 0.09	0.75 ± 0.05	0.73 ± 0.07	0.73 ± 0.06	0.73 ± 0.08	0.70 ± 0.05	0.76
Unpaved road	Static	Unloaded	0.79 ± 0.05	0.79 ± 0.05	0.81 ± 0.10	0.82 ± 0.12	0.81 ± 0.10	0.75 ± 0.10	0.82
		Loaded	0.76 ± 0.08	0.76 ± 0.09	0.77 ± 0.06	0.79 ± 0.09	0.78 ± 0.08	0.72 ± 0.08	0.79
	Dynamic	Unloaded	0.85 ± 0.16	0.83 ± 0.15	0.82 ± 0.15	0.83 ± 0.10	0.81 ± 0.11	0.87 ± 0.07	0.85
		Loaded	0.80 ± 0.13	0.78 ± 0.08	0.78 ± 0.05	0.78 ± 0.08	0.77 ± 0.08	0.74 ± 0.09	0.80
Asphalt road	Static	Unloaded	0.65 ± 0.09	0.63 ± 0.06	0.62 ± 0.05	0.53 ± 0.05	0.61 ± 0.05	0.57 ± 0.04	0.65
		Loaded	0.63 ± 0.05	0.69 ± 0.03	0.71 ± 0.09	0.65 ± 0.03	0.67 ± 0.05	0.62 ± 0.03	0.71
	Dynamic	Unloaded	0.67 ± 0.09	0.73 ± 0.10	0.70 ± 0.03	0.69 ± 0.03	0.67 ± 0.01	0.55 ± 0.01	0.73
		Loaded	0.68 ± 0.11	0.69 ± 0.08	0.72 ± 0.11	0.68 ± 0.01	0.70 ± 0.07	0.56 ± 0.01	0.72

shows the evaluation of operator exposure based on ISO 2634-5 standards. Compared to the limits mentioned in the ISO (low (R^A < 0.8), moderate (0.8 < R^A < 1.2), and high (R^A > 1.2)), the operator will face high health risk in all types of operations, especially on irregular surfaces (i.e., soil surface and unpaved road), as the minimum observed R^A was 1.36.

3.4. Overall Vibrational Exposure Assessment

Daily exposure value (A(8)) indicates an average vibrational exposure adjusted to represent an 8-h in a day, where vibration dose value (VDV) represents a cumulative exposure of vibration over the working day. A comparison of the calculated vibrational exposure based on different ISO models for different driving surfaces, operating statuses, load conditions, and driving speeds is summarized in

Table 8. Mean of measured vibrational exposure on different driving surfaces for different operational status, load conditions, and driving speeds.

Target	ISO Standards		Operating Status	Load Condition	Driving Surface
					Soil Surface	Unpaved Road	Asphalt Road
Operator exposure to vibration	ISO 2631-1	A(8) (ms−2)	Static	Unloaded	12.27 b	15.87 a	14.23 b
				Loaded	11.57 a	13.77 a	13.11 a
			Dynamic	Unloaded	15.31 a	11.77 b	12.39 b
				Loaded	14.05 a	10.43 b	10.68 b
		VDV (ms−4)	Static	Unloaded	25.10 c	31.09 a	27.36 b
				Loaded	19.61 b	26.77 a	25.55 a
			Dynamic	Unloaded	28.23 a	23.19 b	25.12 b
				Loaded	21.34 a	20.57 a	20.91 a
	ISO 2631-5	RA	Static	Unloaded	1.54 a	1.62 a	1.55 a
				Loaded	1.67 a	1.59 a	1.51 a
			Dynamic	Unloaded	1.62 a	1.65 a	1.53 a
				Loaded	1.56 a	1.60 a	1.52 a
Transplanting section	ISO 20816-1	AS (ms−2)	Driving speed	Load condition	Conveyor	Dibbling part-1	Dibbling part-2
			High (0.24 ms−1)	Unloaded	11.85 a	3.18 b	3.35 b
				Loaded	8.30 a	1.87 b	1.67 b
			Low (0.12 ms−1)	Unloaded	9.02 a	1.33 b	1.81 b
				Loaded	7.18 a	0.78 b	0.84 b

a, b, c Different letters in the same row indicate a significant difference (p ≤ 0.05).

. For each measured vibration, standard ANOVA tests were performed on the driving surfaces for the same operating status and load condition (same row). According to the statistical analysis, significant differences were found among the driving surfaces, except under static–loaded conditions of A(8) exposures, and under dynamic–loaded conditions of VDV exposures. According to ISO 2631-1, there is a high health-risk possibility for all considered factors, and no significant difference was observed. For the transplanting section, a high vibrational exposure was recorded on the seedling conveyor compared to the dibbling part, which was also significantly different for all the considered factors.

Power spectral density analysis is the easiest way to understand the patterns of any frequency. Figure 7 shows the PSD of the measured vibrational exposure on the operator seat for different driving surfaces under the unloaded operating condition. The PSDs regarding different axes are marked in different colors. The X-axis represents the frequency in Hz (0~12,000). The Y-axis represents the normalized amplitude of the PSD on a logarithmic scale (dB/Hz). Although the operator exposure to whole-body vibration for all the treatments exceeded the exposure limit value (>1.15 ms⁻² according to ISO 2631-1), PSD analysis was performed for three among the twelve treatments considering the operating status (driving on different surfaces). The X- and Y- translational axes had almost similar patterns due to the similar vibrational exposures of the selected treatments. The Z-axis had a distinctive pattern with a gradual downward slope indicating low power at all frequencies, as no distinct resonance frequencies were observed for the Z-axis during data acquisition.

Similarly, Figure 8 shows the PSD of the measured vibrational exposure on the conveyor belt for the high and low transplanting speeds under the unloaded condition. During the PSD analysis, only the vibrational exposure of the conveyor belt was considered, as the vibration of the other two considered points (dibbling parts 1 and 2) were under the exposure limit value (<1.15 ms⁻²). The legends and x- and y-axes represent this in Figure 7. The three translational axes (X, Y, and Z) are all near each other, begin to fan out, but gradually decrease, indicating a decrease in the power of the vibration.

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. Swami and Pandey [67] observed that high wheel weight or loaded condition makes the tire stiffer and increases the tire–road contact region, which reduced the vibration. Cutini 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. Scarlett 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). Jin 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]. Tewari and Dewangan [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). As the transplanting section is mounted by the four-wheel vehicle, the high vibration of the vehicle is transmitted to the transplanting section through three-point hitch joints. On the other hand, the dibbling devices kept contact with the soil surface through additional supporting wheels, which reduced the transmitted vibrational exposure significantly. The effects of transmitted vibration through a three-point hitch were also observed by Chowdhury et al. [16] and Jang et al. [70] during radish and Chinese cabbage collecting operations, respectively. They suggested minimization of the vibration exposures of conveyor belts for smooth crop harvest, collection, and delivery, as well as increasing component durability. As the onion transplanter is attached to the vehicle through a three-point hitch, the weight balance of components, proper fitting and adjustment of the power drive components, and the reduction in vibrational exposures of the vehicle can improve the acceleration condition of the onion conveyor belt. The suggested design considerations are summarized in

Table 9. Suggested design considerations to reduce the vibration exposure of the onion transplanter for the operator’s comfort and transplanting performance.

Topic	Design Points	Related References
Operator comfort	Improvement of the damping and suspension system of the operator seat, foam firmness, vehicle cab, and vehicle axle.	[56,57,71,72,73]
	Installation of a single- or multi-stage vibration isolation system.	[43,74,75]
	Enhancement of the tire-to-road contact area.	[64,65,66]
	Well-balanced loads, correct fitting of spare parts, and proper adjustments of suspensions.	[67,68]
	Ergonomically designed handle along with shock-absorbing isolation.	[61,69,76]
	Vibration absorbing wheel structure	[77]
Transplanting performance	Weight balance of components, proper fitting, and adjustment of the power drive components.	[42,78]
	Vibration reduction in the self-propelled vehicle.	[16,70]

.

5. Conclusions

The present study focused on the evaluation of vibrational exposures of a 12-kilowatt self-propelled riding-type automatic onion transplanter in terms of the transplanting performance and the operator’s comfort. The vibrational exposures were measured for different driving surfaces, operating status, load conditions, and operating speeds, and evaluated based on ISO 2631-1, ISO 2631-5, and ISO 20816-1 standards. PSD analysis was also performed to understand the patterns of the major frequencies. The major findings are as follows:

• Operator exposure to vibration is significantly affected by operating status and load conditions of the transplanter. However, vibration for all the considered conditions exceeded the exposure limit value (>1.15 ms⁻² and >21 ms^−1.75) and health risk factor (>1.12) of the ISO standards.
• The acceleration of the conveyor belt also crossed the exposure limits, which will affect the precision transplanting operation.
• Several design considerations were suggested to reduce the vibrational exposure, such as improvement of the suspension and damping system, installation of the vibration isolation system, load balance, and ergonomically designed handle with isolation for WBV, and proper fitting and adjustment of the power drive components for transplanting unit. Implementation of these design considerations and following the ISO standards during machine development would aid manufacturers to minimize vibrational exposures and improve the overall working environment.