Design of a Wireless Monitoring System for Vibration Characteristics of the Wheeled Tractor at Idle Speeds

: To enhance the precision and efficiency of the tractor-failure-rate and equipment-quality inspection, the present study introduces a wireless rapid detection method for assessing tractor quality. For this study, which was based on the symmetrical structural characteristics of tractors, we designed a magnetic suction accelerometer. The test system was composed of a wireless router, a magnetic suction accelerometer sensor, a data-acquisition terminal, and other components. This test system aimed to test the equipment quality of the tractor at idle speed before leaving the factory. The experiment found that the vibration characteristics of the tractor had a symmetrical pattern on the left and right sides of the front and rear axle at idle. When the idle speed of the tractor was 800 r/min and 1000 r/min, the predominant vibration direction of both sides of the front axle of the tractor was the Y direction, while the predominant vibration direction of the rear axle was the Z direction. The experimental results showed that the proposed wireless rapid detection method of tractor quality and the designed acceleration sensor had good testing accuracy. The present study could provide a novel rapid detection method for the failure detection of power machinery in the agricultural field and for inspection before leaving the factory. The implementation of the method could improve the detection efficiency, and reduce the detection cost and the incidence of failure during actual use.


Introduction
Tractors, as one of the key power systems in agricultural engineering, can be used as traction machinery to pull various types of agricultural machinery, such as rotary tillers, planishing mills, riding machines, and seeders.[1][2][3][4] Therefore, the performance of tractors can seriously affect operational efficiency.With the centralization of land and the increasing number of small cooperative societies established in China, there is a growing demand for enhanced work efficiency from tractors.This trend has led to an increased demand for higher horsepower tractors in agricultural engineering to improve work efficiency.The number of large horsepower tractors in China had reached 4.98 million by the end of 2021 [5].The number of large-and medium-sized horsepower tractors in China in 2021 was five times that of 2000, reflecting the substantial growth in the agricultural machinery sector.
Vibration is an inevitable phenomenon in large-and medium-sized horsepower tractors during operation.However, the vibration of tractors seriously affects the comfort and health of drivers, with the natural frequencies of the human trunk and lumbar vertebrae being 4-8 Hz and 4-5 Hz, respectively [6][7][8][9].During busy seasons, the tractor will be used for more than 10 h [10].When this frequency coincides with the vibration frequency of the tractor, it can have a detrimental impact on the physical health of the driver [11,12].The vibration of a tractor not only affects the health of the driver, but it also affects the overall structural strength, stability, and efficiency of the tractor during the operation process [13,14].For example, tractor vibration can exacerbate the welding deformation and fracture of welded structures, such as frames [8].It will increase the possibility of tractor work failure, and reduce the operation efficiency of the tractor.Meanwhile, the tractor maintenance cost will also increase.Tractor vibration also affects the operational stability, performance, and efficiency of agricultural machinery and equipment [8,9,[15][16][17], potentially resulting in reduced grain yield.Additionally, the vibration of the tractor will have an impact on the soil health [18].Addressing these issues is critical to ensuring not only the safety and comfort of tractor operators, but also the efficiency and sustainability of agricultural practices.
Vibration and sound are ubiquitous phenomena in the working process of power machinery.Although this can affect operational efficiency and the health of the driver, these characteristics can also serve as indicators of the machinery's operational status [19][20][21][22][23][24][25].For example, Delgado-Arredondo et al. [19] determined the functionality of a starter by analyzing the vibration and sound during the working operation.Wang et al. [20] utilized vibration signals from an internal combustion engine to monitor the internal combustion engine operational status.Li et al. [24] obtained the accuracy of tractor engine operating state information by analyzing the vibration signals at various measurement points on the tractor.Furthermore, Qi et al. [25] designed a new energy tractor engine intelligent fault measuring system for big data, which could improve the real-time accuracy of the system.
With the rapidly increasing quality requirements for large-and medium-sized horsepower tractors, a troubleshooting method with advantages such as online access, being fast, and not requiring tractor disassembly, has gained interest and support.Before leaving the factory, the tractor must undergo a rigorous and extensive quality inspection to ascertain compliance with factory specifications.However, the existing vibration detection method is to manually listen for abnormal sounds during the operation of the tractor, which necessitates highly skilled technicians.Meanwhile, the distribution and the structure of the tractor can be considered to be approximately symmetrical, and the idle state is a common operational phase that can be used to characterize the assembly quality characteristics of the tractor.Therefore, the present study developed a wireless vibration sensor for the structural characteristics of the tractor to monitor the assembly quality of the tractor at the time of leaving the factory.By analyzing the vibration of the tractor, it could be determined whether there was a problem with the assembly quality of the tractor, such as the omission of parts or poor-quality parts.The present study could provide a rapid and efficient method for tractor quality inspection before leaving the factory.

Design Principle of the Wireless Measuring System
The purpose of the study was to determine the assembly quality of the tractor by measuring the vibration of the tractor in the idle state.However, vibration testing using wired vibration sensors can easily confuse the testing site and consume a lot of time.It also increases the test cost and reduces work efficiency.Therefore, the present study proposes an innovative detection method that utilizes a wireless rapid detection method for assessing the vibration characteristics of the tractor at idle speed.The wireless measuring system for the vibration state of the tractor under idle speed is shown in Figure 1.The measuring system is composed of the tractor, wireless vibration sensors, wireless routers, and data collection terminals.The wireless vibration sensor can measure and collect the vibration data of the tractor in the idling state in real-time, and transmit the vibration data to a data-acquisition terminal.Moreover, tractors with symmetrical structures produce similar vibration characteristics.Hence, the vibration sensor designed in the present study can be placed on both sides of the tractor to determine if any discrepancies in vibration occur during the transmission process.This bilateral sensor placement will ensure a comprehensive assessment of the tractor's vibration symmetry, providing a more accurate evaluation of its assembly integrity and overall performance.

Design Principle of the Wireless Vibration Sensor
According to the requirements that the tractor should be tested quickly and accurately before leaving the factory, a wireless vibration sensor should be characterized by wireless transmission, magnetic suction, and self-powered operation.Furthermore, the vibration sensor designed for collecting vibration data should have a data conversion function and be able to convert the vibration acceleration generated by the tractor in an idle state into velocity and displacement through calculus processing.This conversion capability is crucial for a better understanding of the tractor's vibration behavior, which can significantly contribute to the assessment of its assembly quality.
Based on the above requirements and references [26], the present research team collaborated with Shanghai Zhendi Testing Technology Co., Ltd.(Shanghai, China) to jointly design and develop a wireless vibration sensor.The wireless vibration sensor is shown in Figure 2. The designed vibration sensor consists of an STM32F103 microcontroller-based control unit, MEMS chip-based acquisition unit, Bluetooth communication module-based wireless transmission unit, and other components.Signal processing is primarily handled by the STM32 and its peripheral circuits, which serve as the processing unit for the threeaxis sensor and also act as the primary control unit.The processed data are transmitted wirelessly via the Bluetooth module, which is the central component of the wireless data transmission unit.The sensor operates on an internal 3.7 V lithium battery and features a Micro-USB interface for convenient charging.Additionally, it includes essential circuits for voltage regulation, buck-boost conversion, protection, and voltage detection to ensure reliable and safe operation.The designed vibration sensor is capable of not only collecting vibration data, but also realizing spectrum analysis and statistical functions on vibration data.Moreover, the data collected by the sensor could be wirelessly transmitted, enhancing the efficiency and convenience of the testing process.This innovative approach streamlined the data collection and analysis The sensor could collect vibration acceleration signals, including vibration speed signals, and vibration displacement signals.It was essential for the test system to be able to measure vibration velocity, which was used to determine the vibration severity.The measurement range of vibration acceleration of the sensor was 0.1 mm/s-200 mm/s.The accuracy of the sensor was ±5%.The response range of the vibration acceleration frequency of the sensor was 10 Hz-5 kHz.The calibration of the sensor was carried out by the Wuxi Institute of Metrology and Testing, Wuxi, China.

Test Details and Methods
The designed vibration sensor shown in Figure 2 was used in the tractor vibration test.The test location was Luoyang, China.The tractor was the newly assembled tractor (model size: LX2204) of China Yto Group Corporation (Luoyang, China).The tractor was equipped with an engine power of 162 kW, and the speed range of the tractor during the idle stage was 700 r/min to 1200 r/min.
It is well known that the engine acts as the primary source of vibration excitation in a tractor.Conventional-wheel tractors are not equipped with spring shock absorbers or suspension systems.Consequently, the vibration from the engine is transmitted to the various components by a rigid connection.There are two vibration transmission routes: forward transmission to the front axle and backward transmission to the rear axle, as shown in Figure 3.The front and rear axles of the tractor are the terminals of the tractor vibration transmission, and can indicate whether there is a defect in the vibration transmission route.It is important to note that the vibration characteristics of the tractor can be reduced by tires.The structure and mass of the tractor are symmetrical to the axis between the two wheels.To verify the effectiveness of the proposed vibration detection method and test the performance of the developed wireless vibration acceleration sensor, the wireless vibration acceleration sensor is symmetrically distributed on the tractor component surface.The symmetrical distribution is essential for obtaining accurate and reliable vibration data, which can identify any anomalies in the vibration transmission routes.Due to the symmetrical structure of the tractor, the wireless sensor was placed symmetrically in the middle position between the front and rear axles of the tractor, as shown in Figure 4.The orientation of the vibration sensor's data collection was defined as follows: The X-axis direction of the vibration sensor was the forward movement direction of the tractor.The Y-axis direction was oriented to the left side of the driver's position when seated in the tractor.The Z-axis direction was the forward direction of the vertical tractor, capturing vertical vibrations.The tractor was parked at the designed testing station, as shown in Figure 5a.The vibration sensor and the PC were connected wirelessly through the wireless router, as shown in Figure 5b.The wireless vibration sensor was carefully placed in the intended test location, as shown in Figure 5c.When the tractor engine speed in the idling state stabilized at 800 r/min or 1000 r/min, the initial data of the vibration sensor were set to zero by the acquisition terminal software.The vibration data acquisition on the vibration characteristics of the tractor in the idling state were collected.The software was responsible for acquiring and storing the collected data.The vibration test result of the tractor in an idling state was be displayed on the collected software, as shown in Figure 5d.The visual of the test results facilitated a clear understanding of the vibration behavior of the tractor, enabling the identification of any issues that might affect its performance.When the vibration characteristic of the tractor at 800 r/min idle speed was tested, the tractor idle speed was accelerated to 1000 r/min.After the engine speed was stabilized, the vibration characteristics of the tractor were collected at an idling speed of 1000 r/min.

Test Index
According to the literature [27,28], the working vibration amplitude of machinery, such as a tractor, can be represented by the vibration severity.The tractor is classed as agricultural power machinery, so the vibration amplitude of the tractor can be expressed by the vibration severity.Vibration intensity is a comprehensive characteristic quantity that can adeptly reflect the vibrational state of a machine in a succinct, pragmatic, and efficacious way.Therefore, vibration severity was used to characterize the vibration characteristics of the tractor at idle speed during the present study.According to the literature [27], the vibration severity can be calculated by Formula (1): where rms v is vibration severity (mm/s); vx, vy, and vz are the vibration velocities in X, Y, and Z directions (mm/s); and Nx, Ny, Nz are the test points in the X, Y, and Z directions.

Results
Based on the vibration test procedures and details, wireless vibration sensors were arranged on the tractor axle as shown in Figure 4.The vibration waves at the left and right test points of the front and rear axles of the tractor at different idle speeds are shown in Figures 6 and 7.During the test, the vibration collection time was 400 ms.As can be seen from Figure 8, the difference in vibration intensity between the left and right sides of the front and rear axles of the tractor in the idle state was small.The total vibration intensity of the front axle of the tractor at an idle speed of 800 r/min was greater than that of the front axle of the tractor at an idle speed of 1000 r/min, and the vibration intensity of the front axle in the X and Y directions was greater than that of the rear axle in the X and Y directions.The vibration intensity of the front axle in the Z direction was greater than that of the rear axle in the Z direction, although the difference between the vibration intensity of the front axle and the rear axle in the Z direction was relatively small.Furthermore, when the idle speed of the tractor was set at 800 r/min, the vibration intensity of the left side of the front axle in the X, Y, and Z directions was greater than that of the right side of the front axle in the X, Y, and Z directions.These results are crucial for understanding the vibration distribution of the tractor's frame and can be instrumental in diagnosing any potential mechanical issues.
When the idling speed of the tractor was 1000 r/min, the vibration intensity of the left side of the front and rear axles of the tractor in the X and Y directions was greater than the vibration intensity of the right side of the front and rear axles in the X and Y directions.This was opposite to the change in vibration intensity of the left and right sides of the front axle in the Z direction.
Based on the test results shown in Figure 8, the main vibration direction of the tractor in the idle state was determined based on the vibration intensity in different directions at different idle speeds.The results are shown in Figure 9.It can be seen that regardless of the idle speed of the tractor engine setting at 800 r/min or 1000 r/min, the main vibration direction on both sides of the front axle was in the Y direction, and the main vibration direction on the rear axle was in the Z direction.

Discussion
It is well known that the different components of the tractor are rigidly connected, and the engine is the main vibration source of the tractor.The major components in the vibration transmission route of the tractor, such as the front axle, engine, gearbox, differential box, and rear axle, are all rigidly connected and fixed to the main frame structure.Meanwhile, the tractor has an approximately symmetrical structure, so the left and right vibration characteristics of the front and rear axles should be similar when the tractor is under idling conditions.Based on the test results, it could be seen that the left and right sides of the tractor had good symmetry in the main vibration direction.
The engine of the tractor is a compression ignition engine, which generates the main force in the Z direction.The main force is also the driving force that drives the tractor.Moreover, the reciprocating motion components of the engine can also generate forces in the Y direction.Based on the structure of the tractor, the front axle is closer to the engine than the rear axle and the front axle bears more engine weight than the rear axle.Furthermore, the pneumatic tires, have a damping effect that helps reduce the vibration intensity in the Z direction [13,29].Consequently, the fluctuation of the air pressure inside the tires due to vibration can increase the Y direction vibration of the front axle of the tractor, as can be seen in Figures 8 and 9.The vibration generated by the tractor engine is transmitted along the vibration transmission route 2 shown in Figure 3.The engine, transmission, differential, and rear axle are rigidly connected and the cab situated over the rear axle is also rigidly connected to the frame.This will limit the transmission of tractor vibration along transmission route 2. Hence, the vibration intensity of the rear axle test locations will be reduced, especially in the X and Y directions [30].The vibration intensity of the rear axle test points in the X and Y directions was significantly lower than that of the front axle test points in the X and Y directions.The main vibration direction of the rear axle test point was the Z direction.Therefore, the main vibration directions of the front and rear axle test points of the tractor were inconsistent, as can be seen from Figures 8 and 9.
When the tractor was at high idle speed or low idle speed, respectively, the main vibration direction for the front and rear axle test points remained constant.However, the vibration intensity of the tractor at an idle speed of 1000 r/min was lower than that at an idle speed of 800 r/min, as shown in Figure 8.This reduction in vibration intensity may be attributed to the working principle of the compression ignition engine of the tractor, which can encounter issues such as unstable engine operation and inadequate combustion at lower speeds.Unnecessary vibration will be caused.
Combined with the test results, it can be seen that the proposed wireless vibration testing method can be utilized to effectively and accurately assess the vibration characteristics of the tractor in the idle state.The proposed wireless vibration testing method can be used to accurately determine the main direction of vibration generated by the tractor under idle conditions.In turn, it can be used to detect any faults or assembly defects in the tractor.

Conclusions
To improve the efficiency of detection of tractor faults as well as the quality of equipment before leaving the factory, the present study proposed a wireless vibration detection method based on the symmetrical characteristics of the tractor structure.A magnetic suction wireless vibration acceleration sensor was designed.Repeated tests were performed on newly assembled tractors to assess the effectiveness of the method.When the idle speed of the tractor was 800 r/min and 1000 r/min, the left and right sides of the front and rear axles of the tractor had similar symmetry.It was also found that the main vibration direction of the front axle was in the Y direction and the main vibration direction on the rear axle was in the Z direction.The experiment showed that the tractor wireless vibration detection method proposed in the present study had good accuracy and efficiency.
During the experiment, some uncertain factors could have affected the results.Factors such as the consistency of the engine and gearbox batches, the experience of workers in the manual assembly process, and the shaking of the three-point suspension, may all have exerted an uncertain impact on the test results.The present study could provide a new method for the rapid detection of tractor faults or tractor assembly faults before leaving the factory, improve the efficiency of tractor fault detection, and reduce the cost of offline detection.Compared with manual test judgment, this method would improve the test efficiency and accuracy.In the future, wireless detection could be combined with cloud technology and large database technology to detect the vibration characteristics of each tractor, and could be expanded to include many functions, such as reminding the driver to maintain the tractor to reduce the incidence of tractor failure.

Figure 1 .
Figure 1.Design principle of the wireless measuring system for vibration test.

Figure 4 .
Figure 4.The distribution positions of vibration sensors.

Figure 5 .
Figure 5.The procedure for the vibration test.

Figure 6 .
Figure 6.Vibration waves on the left and right sides of the front axle when idling at 800 rpm.

Figure 7 .
Figure 7. Vibration waves on the left and right sides of the front axle when idling at 1000 rpm.The vibration test data collected from model LX2204 tractor at idle speeds of 800 r/min and 1000 r/min were converted and processed using Equation (1).The vibration intensity at different test positions of the tractor was calculated.Figure8shows the results of repeated vibration tests of the tractor at different idle speeds, providing a visual representation of vibration intensity at each test location.

Figure 8 .
Figure 8. Vibration test results of tractor in idle state.

Figure 9 .
Figure 9.The proportion of vibration intensity in different directions at idle speeds.