| Rice is the most important food crop in China,and its planting area accounts for about 30% of the country’s total grain planting area;the total output exceeds 200 million tons,accounting for about 40% of the total grain output.The precise leveling of paddy fields can save more than 10% of irrigation water,improve the utilization of herbicides,fertilizers and pesticides,and reduce environmental pollution in farmland.It is one of the important ways to promote sustainable agricultural development.With the continuous development of the precision agriculture technology system,the laser-controlled farmland leveling machine has become an important farmland precise leveling equipment because of its reliability and cost advantages.Since the beginning of research on laser leveling machines in the 1990 s in China,a series of dry land and paddy field laser field systems and models with independent intellectual property rights suitable for China’s farmland environment have been formed,but there are still some problems in practice and application: the contradiction between the resolution accuracy and range of the laser receiver and its manufacturing complexity and cost;the height position of the flat shovel measured by the receiver cannot continuously reflect the change in the height of the flat shovel;the measurement frequency of the receiver is subject to the difficulty of increasing the speed of the transmitter,resulting in under-sampling of the height measurement during high-speed operation;the controller does not control the shovel height changes caused by the pitch angle in a timely manner when the bottom layer of the paddy field is in poor condition;the on-off valve-controlled electro-hydraulic actuator system is difficult to achieve continuous position adjustment;and the control algorithm has poor adaptability to paddy field differences and operating speed changes.In view of the above problems,on the basis of reviewing the literature and previous work,this article has carried out the following research concentrating on the electromechanical hydraulic system with a proportional valve controlled cylinder as the leveler height actuator based on the ricetransplanter powered paddy field laser leveler.Elevation motion states of the leveler was estimated.Based on the Kalman filter,the acceleration of the leveler elevation motion and the position of the leveler indicated by the laser receiver are fused to obtain a real-time estimate of the two-dimensional state vector composed of the position and speed of the leveler,this is called Kalman filter based method for fusing laser receiver and accelerometer(KFLA).The first-order low-pass filtering and the least squares method are used to reduce the noise and de-zero the free acceleration data collected under static conditions.The optimal filter coefficients(?=0.50)of the accelerometer used are obtained through traversal simulation.The calculation of variance obtains the covariance matrix parameters.Manually push and pull the slider with the accelerometer and laser receiver fixed to reciprocate between ± 77 mm and ± 11 mm along the parallel rail.The measured value of the displacement transducer was used as the reference of the true position of the receiver.The maximum absolute error(MAE)of the position estimate was 14.59 mm and 5.65 mm,and the root mean square error(RMSE)was 5.60 mm and 1.93 mm,respectively,and the position estimation accuracy is higher than that of using laser receiver only.Besides,the observation value of individual instantaneous jump has little effect on the KFLA estimation result.On the other hand,the first-order difference of the measured value of the displacement transducer was used as the slider speed reference.The results showed that the KFLA estimate tracked the reference change trend well and the process curve was smoother.The high-frequency signal increases the estimated frequency of the elevation motion state from 10 Hz to 50 Hz.Model for forecasting tractor pitch angle was built based on time series theory.By analyzing the autocorrelation function(ACF)and partial autocorrelation function(PACF)of pitch angles collected during field operations on three different leveling position,it is determined that the AR(15)model is used to describe the pitch angle motion of the leveler.The white noise test of the model fitting residuals further illustrated the applicability of AR(15).When N =400 was used as the parameter identification sample set capacity of leveler pitch angle online forecast AR(15)model,the recursive least squares(RLS)online recursion has been used to establish the AR(15)model parameters and use this model to forecast the pitch angle in next 5 steps(0.1 s),the mean absolute percentage error(MAPE)between the forecasted value and the real observation value was the smallest,which was 6.81%,and each forecast execution took 0.0015 s,which met the real-time requirement that sequence forecast must be completed between two adjacent pitch angle samples.Motion characteristics for electro-hydraulic actuating system of leveler were tested and modelled.The controlled kinematic characteristics of the electro-hydraulic actuating system of leveler elevation were studied,and the open loop transfer function model of leveler elevation was established.Through the input-output response test,the controlled kinematic process of the proportional valve controlled cylinder actuating system is decoupled into the following links: the system dead zone is [-0.5V,1V],the system saturation zone is[-3.5 V,-10 V] [8.5 V,10 V],the leveler falling gain zone is(1V,8.5 V),and the rising gain zone is(-3.5V,-0.5V).According to the non-linear measured results of flow,a variable gain process function was designed to compensate for it,so that the actual execution control quantity of the solenoid valve can cross the hydraulic dead zone and avoid the execution saturation.Using step identification method and analytical method,the mathematical models of valve control cylinder subsystem and mechanical transmission subsystem were established and realized in Matlab / Simulink.The validity of Simulink openloop transfer function with control signal as input and leveler motion height as output was verified with a two-dimensional light detection and ranging(Li DAR).The external disturbance adaptive compound controller was developed.First,a low-pass filter with a cut-off frequency of 8 Hz was used to extract the output response of the disturbance from the total output of the leveler height.Secondly,the main frequency of the disturbance signal was identified by the zero-crossing detection method,which was used as the characteristic parameter of coupling disturbances including the field difference and the operating speed.The fuzzy logic rules that take the main frequency of external interference as input and the differential parameters as output were designed to realize the self-adjustment of the differential parameters of the PD controller and the adaptation to external interference.The closed-loop response of the leveler under DAFC and conventional PD control were compared in Simulink with chirp signal with an amplitude of 50 mm.The results showed that,the closed-loop response of the conventional PD control system diverged with the increase of the disturbance frequency,and the maximum absolute motion deviation reached50 mm;while the DAFC achieved effective control of tracking and suppression of external disturbance and the height of the leveler,the maximum absolute motion deviation is 17 mm.The amplitude ratio of the closed-loop response to disturbance is 0.34.On above basis,the leveler elevation feedforward compensation device based on the AR(15)model was introduced,and a leveler elevation compound control system was constructed to enhance the controller’s adjustment speed for large height deviations caused by pitch angle.Based on dSPACEMicroAutoBox,a rapid control prototyping(RCP)for leveler height control system was developed and field experiments were conducted: 1)the KFLA results for tracking the contouring motion of the leveller machine in the field showed that the KFLA curve passed through the band represented by the rough height and corresponding deviation.2)the field ridge was selected to simulate the extreme situation of uneven bottom layer of the paddy field.When the leveller elevation movement exceeded the effective sensing range of the receiver for the laser beam due to field changes,the controller could adjust the height of the leveller to target height.Steps were selected to generate the pitch angle required for the work of the feed forward channel of the leveller elevation controller.The feed forward compensation device designed based on the AR(15)model outputs a linear signal within-2V when it forecasted that the pitch angle would change beyond the set threshold,which reduces the impact of pitch angle on the leveller height.3)Driving the leveller machine to perform levelling operations in paddy fields at medium and high speeds respectively,regard using a laser receiver as elevation sensor for leveller machine and levelling operation under PD control as a control group.A statistical comparison of a period of 360 s data from two groups of experiment showed that: at the same speed level,the statistical results of the maximum deviation,minimum deviation,average deviation and mean square deviation of the experimental group are better than the control group;during high-speed operation,the root mean square deviation of the controlled height of the leveller in the experimental group and the control group was 8.0 mm and 12.9 mm respectively,both of which meet the standard requirements of ± 25 mm,but the control deviation of the experimental group is smaller;the probability distribution histogram of the controlled height deviation data showed that the skewness and kurtosis of the experimental group are better than the control group at each operating speed,indicating that the method adopted by the experimental group has improved the physical asymmetry during the elevation motion and is more resistant to disturbance. |
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