Abstract:
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.