Paddy Soil Compaction Effect Undergoing Multi-Dimensional Dynamic Load of Combine Harvester Crawler

Abstract

The compaction of soil by agricultural machinery has seriously affected the sustainable development of agriculture. Tracked combine harvesters are widely used around the world; however, frequent rolling causes irreversible compaction of the soil. In this study, a tracked combine harvester traveling test was carried out in order to clarify the mechanism and influence of tracked combine harvester on soil compaction. The effects of multiple rolling on soil compaction at a depth of 0–30 cm were studied when the body traveled at speeds of 0.27 m/s, 0.48 m/s, and 0.95 m/s. The results showed that the dynamic load of the harvester on the soil compaction could penetrate into the soil at least 30 cm, and a non-linear relationship between the soil pressure and the depth was obtained. The peak pressure on the soil was 3.14 to 4.19 times higher than the average pressure, and the response to dynamic load fluctuated significantly in the soil. The self-excited vibration of the combine harvester formed a beating phenomenon in the soil, and the vibration signal was very penetrating in the paddy soil.

1. Introduction

Soil resources, especially farmland soil, are considered non-renewable natural resources all over the world, which is related to the sustainable development of agriculture. Soil porosity, soil aggregate properties, and soil mechanical properties will change after agricultural machinery rolling, ultimately affecting crop yield and soil water storage and carbon sequestration capacity [1,2]. Combine harvesting machinery is developing towards large size, and the huge body will form greater and more frequent loading on the soil. Soil compaction has become a non-negligible problem in agriculture [3]. At present, analysis of the response of soils to compaction by organisms is necessary because almost all croplands worldwide are facing serious problems of subsoil compaction [4,5,6,7].

Many scholars have carried out many studies on soil changes under compaction. Tonoy K [8] synthesize reported data on compaction effects on subsurface processes, including infiltration rate, plant health, root microbiome, and biochemical processes. The results show that compaction could reduce runoff infiltration rate, but adding sand to roadside soil could alleviate the negative impact of compaction. In particular, soil porosity changes after compaction can seriously affect water retention [9,10]. Longkai Yue [11] used the dye tracer method to characterize the characteristics of soybean roots and found that soil compaction reduced macroporosity and increased the heterogeneity of horizontal macropores. Pulido-Moncada et al. [12] found that soil compaction will increase the emission of nitrous oxide (N₂O) from the soil, which will cause the decline of soil fertility and environmental pollution. The effect of compaction on agriculture is negative both in terms of soil physical and chemical quality. The compaction effect of agricultural machinery tracks on soil is manifested in various agricultural productions. Nazari et al. [13] analyzed the effects of logging-related compaction on forest soil microbial biomass carbon (MBC), bulk density, total porosity, and saturated hydraulic conductivity (K-sat). Soil compaction appears to directly affect crop root penetration rates. Jos’e Dorner [14] evaluated the short-term effects of soil compaction on the soil physical quality of an Andosol. Although some studies have explored the positive effect of soil compaction on anti-seepage [15], the negative impact of soil compaction on crops is obviously more significant.

Track shoes are generally considered superior to tires. Servadio et al. [16] performed comparative experiments using tracked and wheeled tractors and analyzed the compaction of the soil [17]. Furthermore, a soil-vehicle interaction dynamics model is established [18]. Long-term controlled trials were able to see the effects of soil and crops under different compaction conditions, but the loading characteristics of the soil were not considered. The soil load caused by agricultural machinery during tillage is huge and intensive, which can cause subsoil degradation [19]. Heavy farmland traffic can alter soil structure at depths greater than 50 cm when wheel loads are high [20]. Conservation tillage and deep tillage appear to be insufficient to meet increasing soil compaction [21,22]. Soil compaction requires more interdisciplinary analysis [23,24,25,26]. However, some study limitations still exist. On the one hand, the influence factors of harvesting mechanical load on soil compaction are unclear. On the other hand, crawler tracks are considered structures capable of alleviating soil compaction, but soil compaction during harvesting is still severe.

In this study, the influence of walking speed and rolling time on soil compaction and the vibration compaction of paddy soil under different working conditions and depths were analyzed by designing the crawler combine harvester stroke test, collecting the dynamic signals of soil pressure at different positions and the vibration signal of the harvester. The purpose of this study was to analyze the influencing factors of crawler combine harvester load on soil compaction, reveal the compaction mechanism of soil during track driving, and provide a theoretical basis for slowing down soil compaction caused by harvesting machinery.

2. Materials and Methods

2.1. Test Equipment and Parameters

The vehicle used in the test was a Jiangsu World Ryzen 4LZ—7.0EN(Q) tracked combined harvester with a mass of 4030 kg. The effect of the harvester on the soil can be characterized by the collected parameters. Vehicle vibrations and soil pressure at different depths were measured. Vibration signals was measured using a three-way accelerometer. Soil pressure was measured using pressure sensors, all the signals were connected and processed using DH5902(N) dynamic signal acquisition instrument. The signal acquisition equipment used is shown in Figure 1 and Figure 2.

The three-way accelerometer is a piezoelectric sensor, and the 1A312E three-way accelerometer of Jiangsu Taizhou Donghua Testing Company is selected. The three-way sensor is suitable for the JB/T9822-2018 piezoelectric accelerometer standard and JJG233-2008 piezoelectric accelerometer verification regulation. All sensors passed the frequency of the 10–10,000 Hz phase amplitude test. The sensitivity of the channels of the three-way acceleration sensor is calibrated separately. The main parameters of the three-way acceleration sensor are shown in

Table 1. The main parameters of the three-way acceleration sensor.

Environmental Parameters	Value	Physical Parameters	Value	Electrical Parameters	Value
Operating Temperature	−40–120 °C	Sensitive components	Ceramics	Working voltage	18–30 vdc
Shock Limit	5000 g	Weight	13 g	Working current	2–20 ma
Base Strain Sensitivity	−0.02 mg/με	Shell material	Titanium alloy	Dc Bias Voltage	10 ± 2 vdc
Transient Temperature Response	−0.8 mg/°C	Output Method	1/4–28 four pin	Output Signal	>5 vp
Electromagnetic Sensitivity	−0.1 g/t	Dimensions	16.5 × 16.5 × 16.5 mm	Output Impedance	<100 ω

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The BW-type Earth Pressure Sensor of Jiangsu Liyang Jincheng Testing Instrument Factory is selected. Due to the high soil shaping and weak soil fluidity in the deep layers of paddy soil during the harvest period, the pressure of the harvester on the soil can be better measured by using the earth pressure sensor. The main parameters of the pressure sensor are shown in

Table 2. BW-type Pressure Sensor Technical Indicators.

Item Number	Parameter Type	Unit	Value
1	Range	kPa	20
2	Diameter	mm	28
3	Thickness	mm	7
4	Resolution	kPa	0.05
5	Accuracy Error	/	≤0.3% F·S
6	Bridge Mode	/	Full bridge
7	Overload Capacity	/	150%
8	Bridge Resistance	Ω	350

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The combined harvester with a 6–10 kg feeding volume and is widely used in Asia because of its advantages of being lightweight, easy to carry, and low cost. The combined harvester used in the experiment is 4LZ-7.0EN (Made by World Group, Zhenjiang, China). The picture of the harvester is shown in Figure 2. The rubber track was used in the undercarriage system, which enables the harvester to adapt to the wet and soft paddy soil. Some structural parameters of 4LZ-7.0EN are shown in

Table 3. 4LZ-7.0 EN Combine Harvester Structural Parameters.

Item Number	Parameter Type	Unit	Value
1	Power	hp	125
2	Weight	kg	1800
3	Feeding Volume	kg/s	7
4	Working Width	cm	220
5	Track Ground Length	cm	1800
6	Track Shoe Width	mm	500
7	Number of Track Shoe Sections	/	53

.

Both the three-way acceleration sensor and the pressure sensor use the DH5902(N) acquisition system of Jiangsu Taizhou Donghua Testing Company to realize dynamic measurement. The DHDAS acquisition and analysis system of Donghua Testing Company was used to set up different test channels. The parameter settings are shown in

Table 4. DHDAS Acquisition System Sensor Channel Setting.

Channel Number	Sensor Number	Parameter Type	Range	Sensitivity
A11-01	A4182	Pressure	585 kPa	0.0342 mV/kPa
A11-02	A4183	Pressure	667 kPa	0.0300 mV/kPa
A11-03	A4184	Pressure	712 kPa	0.0281 mV/kPa
A11-04	A4185	Pressure	528 kPa	0.0379 mV/kPa
A11-05	A4186	Pressure	683 kPa	0.0293 mV/kPa
A11-06	A4187	Pressure	519 kPa	0.0385 mV/kPa
A11-07	C200106075-X	Acceleration	485 m/s2	1.031 mV/m/s2
A11-08	C200106075-Y	Acceleration	481 m/s2	1.039 mV/m/s2
A11-09	C200106075-Z	Acceleration	482 m/s2	1.038 mV/m/s2
A11-10	C200106080-X	Acceleration	495 m/s2	1.010 mV/m/s2
A11-11	C200106080-Y	Acceleration	509 m/s2	0.982 mV/m/s2
A11-12	C200106080-Z	Acceleration	497 m/s2	1.007 mV/m/s2
A11-13	C200106081-X	Acceleration	483 m/s2	1.035 mV/m/s2
A11-14	C200106081-Y	Acceleration	503 m/s2	0.994 mV/m/s2
A11-15	C200106081-Z	Acceleration	521 m/s2	0.960 mV/m/s2

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Parameter type, sensor sensitivity, sensor type, and measuring range are the main parameters that need to be set. All parameters are set and calibrated before the test. The vertical load on the soil and the Z-direction vibration of the tracked combine harvester were analyzed emphatically during the test. Therefore, the analysis channels are mainly developed for the nine channels A11-01 to A11-06, A11-09, A11-12 and A11-15.

2.2. Experimental Paddy Field Conditions

The test site was located in Zhenjiang, China (119.64° E, 32.10° N). The test date is 15 November 2022, and the temperature was 6.2–15.2 °C. The air humidity was 28.5%. There was no precipitation, and the measured soil moisture was 31.8%. The soil type was alluvial soil, and the contents of clay, silt, and sand are 47%, 22%, and 31%, respectively. Although the soil above the 30 cm depth is considered as topsoil, different composition and physical properties was still observed in the soil profile. As is shown in Figure 3. Differences in soil color can be observed at different depths. The color of the upper layer of 0–15 cm is darker, the color of the 20 cm depth is dark black, and the color of the soil below 25 cm is light yellow. In addition, there are obvious differences in soil compaction with the change of depth. The soil quality of the 0–15 cm soil cultivation layer is relatively loose, and the soil is easy to fall off the wall. The plow bottom layer with a depth of 15–30 cm is very compacted, and the penetration resistance is relatively large.

The color difference is mainly due to the action of microorganisms in the soil, rice roots, and chemical fertilizers. The difference in components directly caused different characteristics of the soil at a depth of 0–35 cm. Therefore, in order to ensure that the test measurement cover all depths of soil, the sensor was buried at a depth of 10 cm, 20 cm, and 30 cm, which is convenient for the linear analysis of the load, so as to analyze the soil bearing characteristics of three different physical properties in the soil layer.

2.3. Design of Harvester Travel Test

The combined harvester traveling test was designed to measure the compaction of paddy soil during the harvester walking process. Firstly, a soil pit with a length of 1 m and a depth of 35 cm was dug along the driving direction of the harvester in the field. Then, dig out horizontal slots at depths of 10 cm, 20 cm, and 30 cm from the ground to ensure that the pressure sensor can be completely horizontally placed inside the soil layer. The locations of the sensors are shown in Figure 4 and Figure 5. Two sensors are arranged horizontally at the same depth, and a total of 2 × 3 pressure sensors are arranged at three depths.

The embedding of the sensor requires that the load plane of the sensor, that is, the two circular planes of the sensor, be perpendicular to the load direction. After the pressure sensor is buried, the combined harvester is controlled to pass over the sensors at a certain speed. The amount of soil subsidence is measured, and the measured sensor signal is saved after the harvester runs over the sensor.

The signal of the harvester passing the sensor for the first time is saved as the first set of data. Then, control the harvester to reciprocate at the same speed, perform four rollings at the same speed, and save four sets of data. In order to ensure that the initial state of the soil is consistent during each set of tests, the interval between each set of tests is 30 min so that the soil has sufficient rebound time [27]. Control the combine harvester to repeat the above test process at low speed, medium speed, and high speed and finally collect 4 × 3 groups of test data.

2.4. Test Design of Harvester Vibratory Compaction

There is obvious self-excited vibration in the operation process of the tracked combine harvester, which vibrates and compacts the soil. Therefore, the influence of self-excited vibration on soil compaction was analyzed by measuring the vibration signal of the combined harvester and soil ballast. The test first buried the sensor on the harvesting path, and the burial method and sensor arrangement position are the same as those in Figure 4. Move the harvester forward at a normal harvesting speed and harvest the rice. When the harvester reaches above the sensor, stop moving forward. Place the three acceleration sensors in turn on the chassis frame of the combine harvester, the 12 o’clock position of the bearing wheel, and under the track. The test process and sensor placement are shown in Figure 6 and Figure 7.

Start the combine harvester again, and keep it for 2 min under the two conditions of only engine start, threshing, and harvesting. After all the working conditions are collected, close the combine harvester and save the collected data.

3. Results and Discussion

3.1. Harvester Travel Pressure Load Collection

The pressure sensor was used to test and collect the loading conditions of the soil at depths of 10 cm, 20 cm, and 30 cm. The test considered the three factors of rolling speed, rolling times, and rolling depth and collected a total of 72 sets of test data under different conditions. First, the crawler combines advances at a low speed (0.27 m/s), and the pressure signals of six measuring points are obtained, as shown in Figure 8.

The peak stresses on the soil at the two measuring points at a depth of 10 cm are 101.135 kPa and 84.892 kPa. The difference between the two measurement points is small, and the values of the two measurement points can be considered to be valid. Sensors 1 and 4 are buried relatively close to the surface, and the time-domain waveform of the pressure can show the dynamic bearing characteristics of the soil after crawler compaction compared with the deep sensor. Therefore, the waveforms of the two groups of curves a and b in Figure 7 are selected to reflect the load disturbance of the harvester on the soil during the travel process. The waveforms of the two groups of curves a and b have a common feature, that is, two obvious load increase areas appear around 10 s and 17 s. The sensor signals at 20 cm and 30 cm also exhibit similar waveform patterns. In order to clarify the information on the six measuring points, the signal information in Figure 7 is extracted, and the specific values are shown in

Table 5. Soil dynamic load parameters at low speed.

Measuring Point	Depth (cm)	The First Peak (kPa)	The Second Peak (kPa)	Average (kPa)	Residual load (kPa)	Kurtosis Value	Effectiveness
1	10	101.135	59.390	32.457	13.445	−0.240	*
2	20	18.639	/	2.601	1.42	10.317
3	30	2.995	/	0.346	0.17	13.565
4	10	84.892	40.223	20.277	10.480	1.203	*
5	20	46.884	21.214	7.984	0.533	1.921	*
6	30	35.728	12.140	7.086	0.295	2.583	*

“*” means that the set of data is valid and can be used for load law analysis.

.

As shown in Table 5, the average load values of points 1 and 4 are 32.457 kPa and 20.277 kPa, respectively, while the maximum values are 101.135 kPa and 84.892 kPa, respectively. The ratios of the maximum load to the average load of the two measuring points are 3.12 and 4.19, respectively. This shows that the vertical load of crawler combine harvesting is very unevenly distributed in the longitudinal direction of the crawler during the marching process, which is negative for reducing soil rolling.

Due to the viscoplasticity of the soil, plastic deformation and internal load will be formed inside the soil after being rolled, and the earth pressure sensor actually measures the pressure of the soil above the measuring point under the dynamic load of the body, which can not fully reflect the stress distribution of the combine harvester body on the soil in the longitudinal direction. Therefore, there is a deviation between the pressure values of 101.135 kPa and 84.892 kPa at a depth of 10 cm and the peak static load of the combine harvester on the soil, and it is necessary to further design a repeated rolling test to find out the law.

3.2. Analysis of Soil Pressure on Harvester Rolling Times

In order to further prove the load distribution law of the gravity of the fuselage of the combine harvester in the forward direction, rolling was carried out four times in the traveling test. Among them, the 1st and 3rd times are for the forward rolling of the harvester, and the 2nd and 4th are for the backward rolling of the harvester. A total of 4 × 6 groups of maximum pressure values of all measuring points were extracted, and the results are shown in

Table 6. Peak soil load with multiple compactions at low speed.

Rolling Times	Peak Soil Load (kPa)
	Point 1–10 cm	Point 2–20 cm	Point 3–30 cm	Point 4–10 cm	Point 5–20 cm	Point 6–30 cm
1	101.135	18.639	2.995	84.892	46.884	35.728
2	95.033	8.353	3.983	34.503	22.834	24.737
3	86.689	8.697	8.735	31.634	21.222	18.512
4	71.787	8.569	12.340	39.488	22.295	23.919

.

As shown in Table 6, after the first rolling of the same depth measurement point, the peak value has a significant difference, and this difference is still maintained in the subsequent rolling process. The peak value of measuring point 2 is lower than that of measuring point 5 by 12.525 kPa~28.245 kPa. The peak value of measuring point 3 is lower than that of measuring point 6 by 11.579 kPa~32.733 kPa. The reason for the discrepancy is that the load wheel section failed to pass completely over the pressure sensor; that is, the buried position of sensors 2 and 3 is slightly deviated from the traveling track because the pressure value is smaller. The values of sensors 5 and 6 are considered to represent the real values of the soil rolling by the crawler combine harvester.

The load data of measuring points 4, 5, and 6 show relatively consistent regularity. Therefore, we chose to analyze these three measuring points from the rolling times and rolling directions. The trend lines of the peak values of these three measuring points are shown in Figure 9.

Under the first rolling and subsequent rolling of the harvester, the soil load value showed a significant difference, and the difference between measuring points 4 and 5 was more obvious. However, repeated rolling 2 to 4 times did not show significant differences. The peak load of the four rolling compactions generally shows a decreasing trend as the rolling times increase. The pressure loads all reached the maximum the first time, and the load values fluctuated within a small range in the last three tests. The fluctuation difference is compared with the average load of the last three times, and the ratios are 22.3%, 7.3% and 27.8%, respectively. In comparison, the relationship between the soil load at a depth of 30 cm and the number of rolling times was not significant.

Field soil is considered to be a special material with both elasticity and plasticity. After being loaded, the soil interior will undergo both elastic deformation and plastic deformation. Soil subsidence is the appearance of plastic deformation, while elastic deformation shows that the relationship between soil pressure and external load presents a linear relationship. If the pressure value of the measuring point does not change significantly, it means that the soil basically no longer has plastic deformation. If the pressure value continues to decrease with the increase in rolling times in the test, it is considered that plastic deformation always exists.

Via the analysis of the four peak pressures, it was found that the plastic deformation of the soil under the first rolling was the most obvious at the depths of 10 cm and 20 cm. The pressure values of the three depth measuring points have a downward trend from the second to the third time, but the trend is small. The load increased again from the third to the fourth time, but the difference was small. It is believed that the subsequent three rollings were dominated by elastic deformation.

3.3. Analysis of Soil Pressure at Different Depths

The compaction results of the harvester at different depths need to be analyzed. During the experiment, a total of 3 × 4 rolling was completed. The pressure peaks of measuring points 1–6 were counted, and the change curve was obtained, as shown in Figure 10.

Each set of line charts contains the values of the four rollings. After the track-type combine has rolled over the soil, the soil pressure decreases with depth. At the same measuring point at the same travel speed, the numerical coincidence of the four times of rolling is relatively high. In addition, the pressure peak at the depth of 10 cm in the soil is generally larger than that at the depth of 20 cm–30 cm. In the case of high-speed rolling at measuring points 1–3, the pressure value of the 30 cm depth is slightly higher than the pressure value of 20 cm in the four times of crawler rolling.

The generality shown in the above line graph can basically satisfy the prediction made based on the stratification of the soil before compaction. First, the pressure value of the soil decreases with depth. This is because the soil, as an elastic-plastic body, can not only transmit the upper layer load but also bear it. When the top of the sensor is dominated by plastic deformation, the pressure value measured by the earth pressure sensor is relatively large. The upper layer of the soil at a depth of 10 cm is a soft tillage layer, the soil porosity is relatively large, and the soil will undergo plastic deformation after being rolled. Therefore, the stress on the sensor is basically consistent with the body load on the track shoes.

The depth of 20 cm is exactly in the transition area between the tillage layer and the bottom layer of the plow, and when the crawler combines harvester is operating, the soil at this depth will undergo both plastic and elastic deformation, and the internal porosity will be further reduced, but the pressure value will be significantly reduced compared to the depth of 10 cm.

The average value of the soil peak pressure at a depth of 30 cm is 13.02 kPa at point 4 and 24.67 kPa at point 6, which is not significantly different from the soil pressure at a depth of 20 cm. In order to further distinguish the pressure values of the soil at these two depths, the pressure time domain curves obtained from the test were compared. Since the test data at the same speed are relatively similar, it is only necessary to analyze the data of the first two rollings. The resulting curve is shown in Figure 11.

The 20 cm depth curve always wraps above the 30 cm depth curve under all conditions. This means that the soil pressure value is lower at deeper locations. The waveforms of pressure values versus time for both curves in the same figure are remarkably similar, which shows that the pressure values at the two depths differ relatively little. However, there is still an overall decrease in pressure with increasing depth.

Combined with the above analysis, the soil at a depth of 0–10 cm responds obviously to the load of the harvester, and the soil is dominated by non-recoverable plastic deformation. The soil pressure value of 10–30 cm depth is significantly smaller than that of 0–10 cm depth. The subsoil has a large penetration resistance at the beginning of rolling, so it is considered that there is elastic deformation during the rolling process. The soil load change at a depth of 20 cm is more sensitive than that at a depth of 30 cm, and it is believed that plastic deformation still exists at 20 cm. The paddy soil at a depth of 0–30 cm is crushed by the harvester, as shown in Figure 12.

The soil is roughly divided into a tillage layer (0–20 cm) and a plow layer (20–30 cm) before rolling. During the rolling process of the harvester, the part of the cultivated layer close to the surface is plastically deformed. Simultaneous elastoplastic deformation occurs in the 10–20 cm depth of the plowed layer. Elastic deformation of the soil occurs at a depth of 20–30 cm. After the combined harvester leaves, the elastically deformed part of the soil will gradually rebound.

Due to the large pressure load, the topsoil exhibits obvious shaping characteristics, while the soil at a depth of 20–30 cm was subjected to less load and was dominated by elastic deformation. Although rice soil is a complex physical research object, the elastic-plastic deformation results show that the soil material is similar to many other single-component materials, especially some metal materials. Macroscopically, the paddy soil can still obtain a relatively uniform mechanical law via experiments, which is conducive to the study of soil materials.

3.4. Analysis of Soil at Different Travel Speeds

The pressure value on the paddy soil is the convolution result of the dynamic load applied by the harvester. The pressure value is related not only to the mass distribution in the forward direction of the harvester but also to the loading time. In order to weaken the difference of soil elastic-plastic deformation caused by the rolling sequence, the third rolling data under the condition of three driving speeds was selected for analysis, and the pressure time domain signal of the effective section was intercepted, and the obtained curves are shown in Figure 13.

In the figure, a total of 3 × 3 groups of soil pressure values are extracted, all of which are the third rolling data under the same conditions. The data obtained by extracting the peak pressure on the soil under different travel speeds are shown in

Table 7. Soil peak pressure at different travel speeds.

Rolling Depth (cm)	Peak Pressure (kPa)
	0.27 m/s	0.48 m/s	0.95 m/s
10	86.689	72.421	71.504
20	8.697	9.258	13.745
30	8.735	15.124	19.662

.

Combining the pressure curve and the extracted peak pressure, the pressure value of the soil at the same measuring point does not change much at different speeds. The maximum pressure range of 10 cm deep soil is 71.504 kPa~86.689 kPa. The peak pressure range of 20 cm and 30 cm depth soil is 8.697 kPa~19.662 kPa, and the pressure value is obviously smaller.

The peak soil pressure at 10 cm depth decreased from 86.689 kPa to 71.504 kPa with the increase in velocity. The reason for the pressure drop is believed to be related to the plastic deformation of the upper soil. The pressure value of the surface soil reflects the convolution process of the dynamic driving load. The harvester load is a positive load, and the convolution result is positively correlated with the loading time. The rule shown is that the longer the loading time, the greater the peak value of the superimposed load. Therefore, peak pressure is inversely related to velocity.

The soil pressure peaks at depths of 20 cm and 30 cm increased with increasing loading velocity. Surface soil and deep soil deform in different ways during the loading process. The pressure value change caused by the elastic soil deformation is related to the vertical instantaneous load, and the test phenomenon that the pressure peak increases with the increase in the travel speed of the combine harvester in the test indicates that the soil is subjected to a large vertical load at a higher driving speed.

There is obvious road surface unevenness on the field ground, which makes the crawler combine harvester subject to obvious unevenness excitation during driving, which will form a two-way load excitation between the vehicle body and the ground. Pavement unevenness excitation is directly related to the driving speed of the vehicle; the faster the driving speed, the greater the vertical excitation caused by pavement unevenness. For crawler combines, the faster the driving speed, the greater the instantaneous vertical excitation caused by the unevenness of the field, which causes the elastic deformation of the soil at a depth of 20 cm and 30 cm at the bottom layer, and the instantaneous pressure value measured by the pressure sensor here will also be greater.

The pressure peak of the soil showed different regularities at a depth of 10 cm and deeper; this is considered to be closely related to the different deformation forms of the soil. The soil in the plow layer mainly undergoes plastic deformation, and the travel speed affects the loading time, which in turn affects the pressure value of the soil under plastic deformation. However, the deep soil mainly undergoes elastic deformation, and the speed of travel affects the instantaneous impact load, which in turn affects the pressure value of the soil under elastic deformation.

3.5. Analysis of Spectrum of Self-Excited Vibration Signal

Several sources of vibration exist in a combined harvester, including the threshing drum, cleaning unit, header, and engine. These vibration signals can be transmitted to the track shoes and the soil and have an impact on the soil structure. The three measuring points in the test are on the soil under the chassis frame, bearing wheels, and track shoes. The test set up two working conditions: only engine working and full power working. The time-frequency domain signals collected by the acceleration sensor under the two working conditions are shown in Figure 14.

From the time-domain signal, only when the engine is started the average amplitude of the envelope after signal processing is 2.332 m/s², and when it is working at full power, the average amplitude of the envelope after signal processing is 1.969 m/s². There is no obvious difference in the vibration intensity of the harvester under the two working conditions in the frame position. Therefore, it is considered that the vibratory compaction strength of the soil under the two working conditions is similar.

The difference in the frequency domain between the two conditions can be observed after fast Fourier transformation (FFT). The main frequency is 81.055 Hz, and the amplitude is about 0.451 m/s² when only the engine is working. This is the signal component of the engine to the frame. The main frequency is 80.078 Hz, and the amplitude is 0.524 m/s² when working at full power. Although the main frequency is very close, there are new signals superimposed under full power operation.

3.6. Analysis of Soil Compaction by Self-Excited Vibration

The vibration signal of the harvester will be transmitted not only to the surface soil but also to the interior of the paddy soil. The pressure signals of the soil at different depths were collected and analyzed to clarify the vibration compaction effect of the combined harvester vibration on the field soil.

Based on the buried position of the sensor, the time-domain pressure values of measuring points 1, 2, and 3 at a depth of 10 cm to 30 cm are selected for analysis. The combine rests over the pressure sensor in full power mode to compact the soil. The pressure values of these three measurement points under the corresponding working conditions were collected and intercepted, and the signal was changed by Fourier. The obtained time-frequency domain signal is shown in Figure 15.

First, different temporal signal levels of pressure values at different depths can be observed. The mean pressures at depths of 10 cm, 20 cm, and 30 cm are 10.725 kPa, 4.833 kPa, and 1.72 kPa, respectively, in the time domain signal. The mean value of pressure and soil depth showed a clear change rule. That is, with the increase in soil depth, the average value of soil pressure decreases, which is consistent with the change rule of soil pressure value and depth under the traveling load of the machine body. However, compared with the soil pressure value measured during driving, the pressure value of the soil under the braking state of the harvester is smaller. Therefore, the effect of the load change caused by the self-excited vibration of the body on the soil is not significant compared with the dynamic load during travel.

Among the three levels of time-domain signals, the soil pressure values at 10 cm and 20 cm depths decreased as the loading time continued. The statistics of the pressure center values at the initial stage and the end stage of loading are shown in

Table 8. Variation of soil mean pressure value with loading time.

Soil Depth (cm)	Initial Pressure Value (kPa)	End Pressure Value (kPa)	Pressure Change Value (kPa)
10	10.929	10.434	−0.495
20	5.331	4.562	−0.769
30	1.765	1.978	+0.213

. The initial pressure value is the average value of the time-domain signal of the pressure value collected when the body is fully powered on. The end pressure value is the pressure signal value of the soil at the end of collection.

The average soil pressure at 10 cm and 20 cm depths decreased by 0.495 KPa and 0.769 kPa during loading; however, the soil pressure at the depth of 30 cm increased by 0.213 kPa. Phenomena differences can still be considered as differences in elastoplastic deformation occurring at different depths. After the body load stabilizes, the upper layer of soil is compacted by the harvester and mainly deformed plastically, while the lower layer of soil is slightly deformed and mainly elastically deformed. Next, under the dynamic load formed by self-excited vibration, creep occurs inside the soil. Further compaction of the upper soil prevents the downward transfer of the load, which in turn reduces the soil pressure values at the 10 cm and 20 cm depth points. However, at a depth of 30 cm, the increase in pressure indicates that the relatively stable plow bottom layer of the soil also creeps under the action of vibration, which makes the bottom soil develop towards a trend of lower porosity. From the time domain signal, it could be concluded that the continuous self-excited vibration of the crawler will cause creep to the underlying soil, which will cause the plow layer of the soil plow to move up.

In addition, the time-domain signal curves of the three depths all showed an obvious beating phenomenon. Beat vibration is caused by the superposition of two simple harmonic vibrations with similar frequency and amplitude. Combining the frequency peak lists for the three soil depths, two frequencies, 6.836 Hz and 2.93 Hz, were found in the 10 cm, 20 cm, and 30 cm depths. The difference between the two frequencies is 3.906 Hz, and the amplitude difference is less than 0.1 kPa, which satisfies the conditions for the formation of beat vibration. For paddy soil, the amplitude of the resonance signal formed by the superposition of beat vibration increases significantly, resulting in a more obvious soil dynamic load. Tests have shown that the beating vibration phenomenon can be transmitted via the soil to a depth of 30 cm, which is very unfavorable for soil protection. Even in the transfer process, the main frequency amplitude of 6.836 Hz is only reduced from 0.227 kPa to 0.125 kPa. This shows that the beating vibration formed in the soil has more penetrating power, and the self-excited vibration of the harvester will form a vibration compaction effect on the soil with a depth of more than 30 cm.

4. Conclusions

(1)

The combined dynamic load of the crawler can penetrate at least 30 cm into the soil. Variation characteristics of the pressure value on paddy soil at different depths were found. Soil at depths of 0–10 cm responds significantly to harvester travel loads. The soil pressure at a depth of 10–30 cm is significantly smaller than that at a depth of 0–10 cm. Soil pressure exhibits a non-linear relationship with depth during harvester rolling.

(2)

The travel speed and the number of rolls of the tracked harvester have obvious differences in the degree of compaction of the soil at different depths. The 10 cm soil pressure was negatively correlated with the travel speed, while the 20 cm and 30 cm soil pressure was positively correlated with the travel speed. The first two compactions had a significant difference in soil pressure loads at depths of 20 and 30 cm. As a result, soil compaction can be improved by adjusting the harvester speed and the number of passes.

(3)

The response of the self-excited vibration of the harvester in the soil layer was preliminarily analyzed, and it was found that the penetration of the vibration load was stronger than expected. The soil could filter out high-frequency noise, but under the superposition of low-frequency vibrations, beat vibrations were formed in the soil layer. The self-excited vibration of the harvester would form a vibratory compaction effect on the soil at a depth of more than 30 cm.