Integrated control method of traction & slip ratio for rear-driving high-power tractors

Abstract: Ploughing with rear-driving tractors plays an important role in agricultural production during which the electronic-hydraulic hitch system is regularly adjusted for desired performance. In operation, severe driving wheel slip easily occurs because of the complex terrestrial environment and the frequent changing of operating load. Studies show that tractors have a higher traction efficiency when slip ratio is between 0.1 and 0.15; however, when slip ratio is larger than 0.2, the efficiency drops dramatically while the soil texture is destroyed, the tires are worn faster and the energy consumption is increasing. Therefore the effective control of the 2WD tractors' driving slip is highly desired. In this paper, both the structure of the electro-hydraulic hitch and the working principle of 2WD tractor's driving slip system are firstly analyzed. Then the integrated control of traction and slip-rate based on PID (proportional integral differential) is proposed. The integrated control method that contains a control factor with 2 detailed control objectives is presented. The automatic control system mainly performs the traction control while constantly monitoring the changes of slip ratio. When the slip ratio is in the desired range mentioned above, the traction control continues; when the slip ratio is out of range, the controller switches to slip rate control, which outputs a voltage signal to the electromagnetic proportional control valve (if the slip ratio is smaller than 0.1, proportional descent control valve works with an input voltage of 0.5 V; if the slip ratio is greater than 0.2, proportional lift control valve works with an input voltage of 4.5 V); and it returns to traction control once the slip is addressed. According to the characteristics of heavy tractor electro-hydraulic hitch，taking account of the complex operating condition of ploughing, an integrated automatic controller is developed for the electro-hydraulic hitch system based on microprocessor MC9S12XS128 of Freescale. At the same time, the software is designed using CodeWarrior. Taking the heavy tractor with electro-hydraulic hitch system as the test platform, after the successful selection and installation of the sensors, in order to verify the effectiveness of the joint controller, contrast experiment was carried out after wheat harvest in the farm of Xinli Machinery Company in Shunyi District, Beijing. First of all, we measured the actual traction with cultivated land operation in homogeneous soil conditions. Actual tractor traction was measured by force sensor, in which ploughing depth was set to 20 cm, tractor was geared to B2, and speed of tractor was set to 7.5 km/h. In the end, we identified the target of traction was 6 000 N. With the slip ratio between 0.1 and 0.2, traction of 6 000 N and sampling time of 50 s, the maximum of slip ratio changed from 0.2766 to 0.2112, the minimum changed from 0.0419 to 0.0786, the range reduced by 41.71% and the effective control of time range increased by 20.55%; ploughing depth's average error changed from 1.69 to 1.15 cm, and root-mean-square error changed from 4.07 to 2.09 cm; traction's average error changed from 426.38 to 270.73 N, and root-mean-square error changed from 503.18 to 366.23 N. The results demonstrate that the joint controller has not only the effectiveness, but also the efficiency, precision and stability. The proposed integrated controller raises the traction efficiency, improves traction performance and can serve as a guidance to field production.