| Electrically controlled hydraulic system has fast response, large output power and high control precision, suitable for operation demand of large-scale variable fertilizer and seed drill, widely used in variable fertilization control system. Because the hydraulic servo system is uncertainty caused by time-varying parameters and external disturbance, some parameters are difficult to determine with theoretical method, and with the changes of main parameters such as pressure and flow conditions, the value of these parameters will change. The premise and key of control system is to establish an accurate mathematical model to describe the dynamic characteristics of electro-hydraulic system, pose the system identification modeling method, this method based on the input-output data of the system, fusing the parameter estimation and error criterion to establish the mathematical model of electro hydraulic variables system, which lays the foundation for the research of control algorithm. According to the law of conventional PID control which using the linear combination scheme, is difficult to coordinate the contradiction between rapidity and stability, and it’s robust performance is poor, the variable structure parameter perturbation and external disturbance with adaptive control strategy is given to overcome the nonlinearity and parameter uncertainty impact on system performance. Aiming at the defects of the chattering variable structure control is proposed to optimize the reaching process fuzzy control, suppress chattering and improve the system dynamic performance and steady-state precision. This study based on the nonlinear, time-varying parameter identification of hydraulic system perspective, and explore the way of system identification of the electro-hydraulic proportional variable fertilization system of mathematical model, and the variable structure robust control with intelligent control, to improve the system robustness and tracking features, and then to improve the variable fertilization seeder fertilization precision.Research contents:1. To establish the mathematical model of the electro-hydraulic proportional variable fertilization system based on system identification method. Through mechanism analysis, to determine the maximum working frequency of system clock cycle and the length of the sequence, M sequence. Based on the signal generator of the input signal for identification of M sequence and data acquisition instrument to obtain hydraulic motor speed, choosing a model class and using the least square method for identification of model parameters on the system, then from the final prediction error criterion (FPE), further to determine the optimal control system of electro-hydraulic variable fertilization model. In this study, according to the actual working condition, respectively choose autoregressive model ARX with controlled auto regressive moving average amount of ARMAX with control volume, identified model parameters on the system by least square method through MATLAB programming and the final prediction error (FPE) criterion for model structure identification, and a comparative study of the two model fitting degree, determined that ARMAX4431is the mathematical model of the electro-hydraulic variable fertilization system.2. Research on electro hydraulic variable fertilization system sliding mode variable structure control strategy. Using the ARMAX4431identification model of control system of variable rate fertilizer applicator, transformed into state space model, the model is converted into the control standard, and through the coordinate transformation method and optimal two quadratic coefficient method to determine the selection of proportion switching function, exponential rate, determine the sliding mode control strategy using the generalized sliding mode and system model, the sliding mode of electro-hydraulic variable fertilization system with variable structure control. Research on PID control and sliding mode variable structure control using MATLAB programming contrast (SMC) step tracking and sinusoidal tracking characteristic; control using MATLAB programming of PID, SMC control step tracking and sinusoidal tracking response characteristics.3. Discrete sliding mode variable structure control strategy of the electro-hydraulic variable fertilization system. Electro hydraulic variable fertilization system on computer is involved in the control of a discrete time system, the state variable is the time series of the sampling period, the formation of quasi sliding mode. By using MATLAB simulation study on the equivalent discrete variable structure control and discrete exponential reaching law of variable structure control performance, the system state switching in a certain width of the band is the formation of quasi sliding mode motion, to solve this problem, the variable exponent reaching law chattering amplitude of quasi sliding mode is mentioned to improve the motion performance of quasi sliding mode state stage. Considering the exponential reaching law in the state system, the system state in the reciprocating motion in band switching, can not be stable at zero; variable exponent reaching law asymptotically stable at the origin, but in early switching may produce buffeting larger amplitude, integrated two advantages of combined reaching control law, to optimize the control index approach.4. Fuzzy sliding mode of the electro-hydraulic variable fertilization system variable structure control strategy. Fuzzy sliding mode control, which use the two-dimensional fuzzy sliding mode controller, has input from the fuzzy sliding mode switching function5and its derivative values s(k), by the fuzzy logic reasoning, according to the position of the switching surface distance adjusting switch gain to eliminate the chattering, adaptive fuzzy sliding mode control, two input single output fuzzy controller, the switching function s(k) and its change rate ds(k) as input, the output fs(k) fuzzy reasoning, the sign function gain, eliminate chattering.5. Experimental study on electro hydraulic system of variable rate fertilizer PID, sliding mode variable structure and fuzzy sliding mode variable structure tracking performance and anti-jamming performance, the initial state of system operation, acquisition system sampling frequency of1000Hz, record the hydraulic motor speed and the controller output time domain data files, test data for processing, respectively, of three kinds of control strategy for step tracking performance and sinusoidal tracking performance, and compare the system steady-state error and anti disturbance performance.Results and conclusion:1. Through mathematical model parameter identification model of electro-hydraulic control system of variable rate fertilizer applicator, parameters, using least squares identification system model to determine the final prediction error criterion system structure, model fitting degree of the system for ARMAX4431.2. Through the contrast research of tracking performance and disturbance performance of PID control and SMC control, the result showed that PID control had better steady-state characteristics but long dynamically adjust time and poor disturbance performance; which SMC control had less dynamically adjust time and better robustness but system steady-state characteristics was impacted by chattering.3. Through the research of the equivalent control, exponential reaching law, variable exponent reaching law and the combination control of discrete sliding mode variable structure performance, results show that the equivalent control dynamically adjust quickly, the system response is oscillatory, and the presence of buffeting; discrete exponential reaching law is quasi sliding mode, system chattering, cannot be asymptotically stable zero; variable exponent reaching rate adjustment switch with shape, the state of the system is asymptotically stable at zero, but the initial stage of the switchover in large amplitude jitter; combined control integrated two advantages, realization of reaching sliding cutting surface finite step, and asymptotic stability of zero, chattering.4. The switching gain and index reaching study on fuzzy sliding mode variable structure control performance, the results show that the fuzzy adaptive sliding mode reaching law control system better tracking performance, control more stable output, buffeting smaller system.5. Based on the experimental study of PID,SMC,FSMC control, dynamic characteristics and steady-state characteristics of the anti interference characteristics, the results show that the fuzzy SMC control is better than that of the steady state characteristics and anti-interference characteristics before two, but also suppress chattering.Therefore, the use of system model parameter identification method can obtain more accurate mathematical model of variable structure control system to adjust quickly and without overshoot, but there are chattering, discrete combination control and fuzzy index reaching rate control can weaken chattering, but fuzzy control index reaching rate due to the combination control in tracking performance, and restrain the chattering effect is better. |
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