Tractors have been one of the core pieces of equipment in the rapid transition towards electrification and intelligence in modern agriculture. Electric tractors can be the major direction in recent years, due to their zero emissions, high energy efficiency, and high precision. The distributed drive system with wheel-side motors can be used to further enhance the adaptability of electric tractors in complex terrain, such as high efficiency and precise control. Permanent magnet synchronous motors (known for their high-power density and reliability) can serve as the drive system. However, the cooperative control of dual wheel-side motors cannot fully meet the various agronomic requirements under complex terrain. Existing disturbances (like sudden soil variations and uneven loads) can easily lead to uneven ploughing depth and trajectory deviation, which seriously constrain the operational quality and efficiency. In this study, a coordinated drive control was proposed for the dual wheel-side motors of electric tractors using the load feedforward torque difference compensation with nonlinear predictive cooperative control (FTC-NPCC). Response speed and anti-interference were improved under variable working conditions. Current control was used as the motor torque equation. The intermediate conversion of voltage commands was eliminated to directly generate q axis current reference values, which were closely related to torque requirements. Thereby, the torque pulsations were effectively suppressed from the voltage error accumulation in the conventional system. The disturbance resistance was enhanced in the sliding mode load torque observer, where the feedforward compensation of disturbances was embedded in the obtained load torque into the control system. A feedforward compensation term was used to offset the load disturbances. The amplitude of the discontinuous terms was significantly reduced in the sliding mode control, thereby effectively suppressing chattering for the system's robustness. The control input was optimized in real time to adjust the q axis current reference value for the response speed. A nonlinear prediction cooperative control architecture was designed using torque difference compensation. Optimal torque difference commands were directly generated to effectively enhance the synchronization accuracy and disturbance rejection. An experimental platform was constructed to validate the effectiveness of the control strategy under multiple operations. Three control strategies were evaluated during the straight-line and curve driving tests: Strategy 1 was the PI-based cross-coupling control (PI-CCC), Strategy 2 was the load feedforward nonlinear predictive current-based cross-coupling control (FNPC-CCC), and Strategy 3 was the FTC-NPCC. The experimental results indicate that the FNPC-CCC reduced the synchronization error fluctuation from 3.47 to 2.37 r/min under straight-line driving with variable load conditions, which was reduced by 31.7%, compared with the PI-CCC. The synchronization error settling time was shortened from 4.3 to 3.1 s, which was reduced by 27.9%. Compared with the PI-CCC, the FTC-NPCC reduced the synchronization error fluctuation from 3.47 to 1.11 r/min, with a reduction of 68%; The synchronization error settling time was shortened from 4.3 s to 2.0 s, with a reduction of 53.4%. Under curve driving with variable load conditions, the FNPC-CCC reduced the synchronization error fluctuation from 6.11 to 2.61 r/min, with a reduction of 57.2%; The synchronization error settling time was shortened from 3.8 s to 3.2 s, with a reduction of 15.8%. The FTC-NPCC reduced the synchronization error fluctuation from 6.11 to 2.18 r/min, with the reduction of 64.3%; The synchronization error settling time was shortened from 3.8 s to 2.2 s, with the reduction of 42.1%. In conclusion, the FTC-NPCC significantly enhanced the cooperative control precision and disturbance resistance of the dual wheel-side motor system. The finding can provide an effective control approach for the high-precision and high-stability operation of electric tractors in complex field environments.

Under the operation mode of “mobile motorized platform + multiple types of implements” widely adopted in the field of high value-added crop cultivation in greenhouse and other modern facility agriculture, the driving wheels of the mobile platform are very prone to slipping and destabilization due to the complex and variable soil conditions, uneven load distribution, and frequent changes in traction resistance, which seriously affects the operation quality, precision and reliability of the equipment, ultimately leading to reduced crop yields and increased operational costs.In this paper, to address these challenges, a new type of fully power decoupled electric facility agricultural operation platform with enhanced traction performance and energy efficiency is designed, and a model-based predictive algorithm for anti-slip control based on the optimal reference slip rate is proposed, which can effectively inhibit the problems of tire slippage and dragging in plowing operation by dynamically adjusting the driving torque in real time according to the actual soil conditions and operational requirements.In the study, the research process is systematically carried out in several key stages: firstly, a comprehensive dynamic model of the traction operation system of the operation platform is established by considering factors such as wheel-soil interaction mechanics, implement dynamics, and load transfer, to accurately determine the optimal reference slip rate of the operation platform under different soil conditions, including variations in moisture content, compaction, and texture; secondly, to achieve precise slip control, a model-based predictive algorithm anti-slip control with the optimal reference slip rate is proposed with the optimal reference slip rate as the control objective, incorporating real time slip rate estimation, predictive optimization, and feedback correction to ensure robustness under varying operational conditions.Finally, to validate the proposed control strategy, extensive experimental evaluations are conducted by changing the soil compactness and plowing depth in comparison with traditional control methods, and the effectiveness and robustness of the proposed control strategy are verified through quantitative performance metrics such as slip rate reduction, traction efficiency improvement, and operational stability enhancement. The experiments showed that, compared with no anti-slip control and sliding mode anti-slip control, the proposed control strategy demonstrated superior performance under different operational scenarios: it reduced the peak slip rate by 18.75%-73.86% and 7.14%-64.20% under the conditions of 1215 kPa soil tightness (tillage depth of 10 and 15 cm) and 525 kPa soil tightness (tillage depth of 15 cm), respectively, with standard deviation reductions of 29.58%-75.86% and 1.74%-63.75%, indicating significantly improved stability; additionally, the root mean square error is reduced by 19.23%-65.56%, demonstrating higher control accuracy; furthermore, the traction efficiency is improved by 4.47%-35.56% and 9.25%-17.55%, and the standard deviation is reduced by 39.30%-63.77% and 7.34%-50.82%, respectively, confirming enhanced energy utilization and operational consistency.The anti-slip control strategy proposed in this paper has significant advantages in reducing tire slip, enhancing operational traction performance, improving operational stability and straightness of operational trajectory, which not only provides a new method for anti-slip control of fully power decoupled electric facility agricultural operation platform, but also improves the operational quality under complex traction conditions, thereby contributing to the advancement of precision agriculture and sustainable farming practices. The findings of this study offer valuable insights for the design and optimization of intelligent agricultural machinery, paving the way for future research on adaptive control systems in agricultural automation.

The current farming platform has been widely equipped with unmanned four-wheel independent driving and four-wheel independent steering (4WID-4WIS). However, the control system of path tracking is required for high accuracy and sufficient stability under complex working conditions. There were also complex working conditions under crop ridge cultivation, such as Π type target path, curves, initial pose deviation, various soil moisture, and bumpy ground landscapes. In this study, a control strategy of path tracking was proposed using a nonlinear disturbance observer (NOB). A mathematical in-situ steering model was introduced for the relatively low tracking errors in the turn area of the Π type path, compared with the traditional Ackerman steering model. Two steering methods were then used to realize the turn path tracking. Meanwhile, a switch control strategy was designed between yaw angle proportional integral control and pure pursuit control using in-situ steering. Furthermore, the curve and initial pose deviation shared a relatively significant impact on the accuracy of working path tracking. Moreover, the distance traveled by the 4WID-4WIS farming platform was reduced to reach the working path and the maximum lateral deviation. The tracking accuracy of the work paths was improved to design a pure pursuit control using a lookahead distance function, and a fuzzy proportional compensator using the lateral deviation, as well as the curvature of the foresight area in the work path. Besides, the feedforward compensator with NOB was designed to avoid the relatively large yaw speed disturbances from the complex soil moisture, bumpy ground landscapes, kinematic models, and measurement errors. The NOB was also constructed to achieve the precise observation of disturbance for the expected path of farming platforms. The steering compensation angle was then calculated for the feedforward compensator to counteract the disturbance. Finally, the simulation was carried out in the Ubuntu/ROS environment. The NOB strategy of path tracking effectively reduced the distance traveled by farming platforms to reach the working paths, the maximum lateral deviation, and curve tracking errors. The accuracy and stability of path tracking were achieved in the anti-interference performance, where the disturbance momentum was observed accurately. And, the outdoor experiments show that the switch control strategy performed a smaller error of turn tracking on Π type target path, compared with the traditional pure pursuit control. The tracking performance was also effectively improved. The pure pursuit with the look-ahead distance function and fuzzy proportional compensator under different initial pose deviation states reduced the distance traveled by farming platforms to reach the working paths by 32.2%-43.4%. The maximum lateral deviation, mean absolute errors of the whole line and curved area were reduced by 0-42.4%, 27.7%-49.5%, and 33.7%-39.5%, respectively, indicating the high accuracy of working paths tracking. The NOB-based feedforward compensator was reduced by 6.25% mean absolute error in the steady area under hard slate condition, 33.3% under grassland condition, and 41.7% under farmland condition. This control strategy of path-tracking effectively improved the system's robustness and path-tracking accuracy. The finding can also provide innovative ideas and technical references for the navigation system of unmanned four-wheel drive and four-rotation agricultural machinery in ridge tillage.

Electric tractors have presented promising potential in modern agriculture, due to their energy saving, high efficiency, green and clean. The efficiency and accuracy can be further improved by the distributed drive electric tractors with a simple structure and many control dimensions. Among them, the hub motor drive system of electric tractors also requires high performance of speed tracking and disturbance rejection, when operating on complex roads and in various conditions. Linear active disturbance rejection control (LADRC) can be used to estimate the external disturbance of the drive system using an extended state observer (ESO). The external disturbance can be suppressed using feedforward compensation. Furthermore, the LADRC can be expected to serve as the hub motor drive system, in order to effectively improve the operation performance of electric tractors. However, the high-gain observer, ESO can amplify the high-frequency interference noise, and then enhance the speed detection noise amplitude of the system, thus causing the electric tractor to deviate from the preset trajectory. Fortunately, the low-pass pass filter (LPF) can be used as the common tool to eliminate the high-frequency noise caused by speed disturbance. But the detected speed can be caused to lag behind the actual, thus reducing the dynamic performance of the hub motor drive system. To this end, the LPF can be adjusted to the observation error filtering of ESO, where the detected speed can be synchronized with the actual. But the intermediate-frequency disturbance of the system can also be amplified as well. In this study, a sigmoid function ESO (SFESO) was proposed to realize the active disturbance rejection control of the permanent magnet hub motor. The experimental platform was designed and constructed with the RTU-BOX204 as the core controller. Three control strategies were selected under the speed mutation and load mutation, namely LADRC with the traditional ESO, LADRC with the filtered observation error ESO (FOEESO), and LADRC with the SFESO. The experimental results show that the speed pulsation of FOEESO-based LADRC with the time constant of 2 ms decreased by 16% and 13.33%, respectively, compared with the ESO-based LADRC. While, the quadrature-axis current pulsation increased by 12.5% and 25%, respectively, and the speed drop and overshoot under the load mutation increased by 5.13% and 8.11%, respectively, where the speed recovery time increased by 1.16% and 5.81%, respectively. The variation amplitude of each data increased with the time constant of 10ms in the FOEESO-based LADRC. The speed pulsation decreased by 26% and 33.33%, respectively, whereas, the quadrature-axis current pulsation increased by 18.75% and 45.83%, respectively, and the speed drop and overshoot increased by 15.38% and 13.51%, where the speed recovery time increased by 17.44% and 21.29%, respectively. The speed pulsation of SFESO-based LADRC decreased by 32% and 41.67%, respectively, while the quadrature-axis current pulsation decreased by 6.25% and 4.17%, respectively, compared with the ESO-based LADRC. The speed drop and overshoot under the load mutation increased by 2.56% and 1.35%, respectively, whereas the speed recovery time increased by 1.74% and 1.90%, respectively. The SFESO-based LADRC effectively suppressed the system noise with less impact on disturbance rejection, compared with the ESO- and FOEESO-based LADRC. Therefore, the SFESO-based LADRC significantly improved the noise suppression of the hub motor drive system, while quickly and accurately tracking the given speed. The finding can also provide innovative ideas and technical references to realize the high-precision operation of electric tractors in complex environments.

Electric tractors have been widely used in agricultural production in recent years, Among them, the decoupling structure can be adopted as an electric power-take-off (PTO) system, due to its flexible control. A variable leakage flux permanent magnet (VLF-PM) Motor can be expected to serve as the driving motor for the PTO system, in order to improve the efficiency of electric tractors. Light load wide speed regulation and high torque for heavy loads can fully meet the needs of different working conditions. However, the complex disturbances have often limited the VLF-PM motor drive system under the PTO system of electric tractors. Particularly, motor parameters, load shocks, and system failures can seriously disturb the operational accuracy and efficiency of the PTO system. In this article, a compensation control strategy was proposed for the complex disturbance of the VLF-PM motor drive system under electric tractor PTO. Firstly, the control performance of the motor was varied significantly, according to the VLF-PM motor electromagnetic parameters and uncertain external disturbances, such as load shocks, soil conditions, and different crops. A sliding mode anti-disturbance control strategy was designed using a nonlinear disturbance observer (NDO). Specifically, a separate design was adopted in the sliding mode to greatly reduce the complexity of parameter adjustment for the dynamic and steady-state performance of the motor. In the control system of NDO, the strong anti-disturbance performance was required to resist the impact of the fault. The reason was that there was the a greater and more fatal impact of PTO motor failure on click work efficiency. The lumped disturbances after faults were compensated to suppress the impact of perturbing system faults for the robustness of the motor. A quasi-resonant feedforward nonlinear disturbance observer was designed for the stability of VLF-PM motor speed and torque output under faults. A series of experiments were carried out to validate the control strategy at different PTO standard speeds. A load test was firstly conducted on the VLF-PM motor, in order to verify its feasibility of PTO motor. The results demonstrated that the motor was maintained a typical standard speed output during loading, indicating the higher efficiency, compared with the traditional interior permanent magnet synchronous motors. Secondly, the performance of three control methods—PID, traditional ADRC, and SMC-NDO was compared under parameter and load disturbances. The results showed that the SMC-NDO was reduced the speed oscillation and adjustment time by 60.0% and 13.4%, respectively, indicating the most effective anti-interference performance. Therefore, the high operational accuracy was fully met the requirements of electric tractors; The frequency of the fault disturbance signal was consistent with theoretical analysis during current sensor faults and motor phase loss. The disturbance observation level was returned to the normal state. Meanwhile, the q-axis feedback current ripple was suppressed under improved observer control. Once the current sensor failed, the output torque ripple decreased by 33.6%, 69.0%, and 49.7%, respectively, under three operating conditions. The output torque ripple decreased by 68.5%, 51.4%, and 52.8%, respectively, under phase loss fault. At the same time, there was almost no pulsation in the speed, when the motor failed. This control strategy was effectively improved the anti-disturbance and system robustness of the PTO motor. The findings can provide new ideas and references on the anti- disturbance and efficient operation of electric tractors.

Nowadays, with increasing concerns about environmental pollution and energy crisis electric vehicles are rapidly developing owing to their significant environmental-friendly benefits. To meet the diverse driving requirements of electric vehicles, the PM motors with high-performance rare-earth PMs have been widely employed in electric vehicle powertrains, which shows the performance merits of high power density and high efficiency. Yet, rare-earth PMs, as non-renewable strategic resources, usually suffer from unstable supply and fluctuation prices, which increases the potential risks of further large-scale application of rare-earth PM motors. And this also poses a negative factor for the long-term sustainable development of electric vehicles or other applications that rely heavily on rare-earth PM materials. Under this background, a type of less-rare-earth PM motors, which aims to effectively alleviate the dependence of high-performance PM motors on rare-earth PMs, has recently drawn increasing attention from experts and scholars. It implies that the investigation and development of less-rare-earth PM motors without compromising performances is becoming a new and hot research direction in the motor field. This paper reviews the existing main alternatives for less-rare-earth PM motors. Based on the dominated torque component, the less-rare-earth PM motors are divided into two types, which are the less-rare-earth PM-dominated motor and less-rare-earth PM-assisted motor. The operation principle, design considerations and restrictions of each type of less-rare-earth PM motor is sequentially discussed. Finally, combined with the potential electric vehicle application, the key problems of less-rare-earth PM motor are summarized and the corresponding technological means are prospected.