后轮驱动大功率拖拉机牵引力-滑转率联合自动控制方法

针对后轮驱动大功率拖拉机犁耕作业工况,提出了大功率拖拉机牵引力-滑转率联合自动控制方法。基于Freescale MC9S12XS128型微处理器开发了联合控制器硬件系统;采用PID(proportion integral derivative)控制算法,制定了牵引力PID控制和滑转率开关控制的联合控制策略,基于模块化设计开发了联合控制软件系统;进行了牵引力控制、牵引力与滑转率联合控制的田间实车对比试验,分析了2种不同控制方法下滑转率、牵引力和耕深的控制效果,验证了联合控制系统的性能。试验结果表明:当设定滑转率阀值区间0.1~0.2,牵引力阀值6 000 N时,联合控制下的实际耕深平均误差为1.45 cm,均方根误差为2.79 cm;实际牵引力平均误差为270.73 N,均方根误差为366.23 N;滑转率采样时间50 s,阀值区间以内43.78 s,有效控制时间范围为88%。与单独的牵引力控制相比,实际耕深、牵引力的平均误差和均方根误差均明显减小,实际滑转率的控制效率有较大提高。结果表明,该文提出的大功率拖拉机牵引力-滑转率联合自动控制方法,可以实现牵引力和滑转率的双目标联合控制,能够满足实际生产的农艺要求。

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

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 6000 N. With the slip ratio between 0.1 and 0.2, traction of 6000 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.