Research On Kinematical Features And Control Strategy Of Agricultural Flexible Chassis

To achieve mechanical cultivation under the small and relatively closed environment,the agricultural flexible chassis(FC)is a new type of electric chassis system developed in Northwest A&F Universtiy.This chassis consists of four off-centered steering with wheel(OCSW)modules and a chassis,and its drive and steerin was done through the control of four in-wheel motors(IWM),which combines the steering and driving systems of a conventional chassis into one.FC can complete many movement forms,which the existing chassis systems can not,such as oblique movement,runing left-right and in situ rotation movement.In this dissertation,supported by the National Natural Science Foundation of China "The Work Mechanism and Control Strategy of the Flexible Chassis for Greenhouse"(Grant NO.51375401),the kinematical analysis and control strategies of FC are deeply studied.Research contents include: design and construction of test bench for OCSW module,experiment on steering torque and response characteristics of OCSW module was conducted on this test bench;and optimization of controller for automatic tracking steering(ATS)system for OCSW module based on the bridge circuit and exploration of its steering response characteristics;and optimization of test bench for FC and its measurement and control system;and the dynamic characteristics of FC under different motion conditions are analyzed.The motion analysis of the FC was carried out and the method of half-mode controlling the posture of the FC was put forward.What's more,the motion model of IWM and OCSW module,dynamic simulation model of FC and its motion model were built based on the Matlab/Simulink.The comparison and analysis of the test and simulation results under the same conditions were performed.A theoretical and technical reference is provided for the further development of FC and its controller.The main contents and achievements of the paper are as follows:1.Research on rotation speed response characteristics of IWM and steering ability of OCSW module was conducted on its test-bench.According to structural features of OCSW module,a test bed with horizontal turntable for this module was designed and built,and data acquisition system was developed.The motion model of IWM was developed based on Matlab/Simulink.What's more,the test of rotation speed response characteristics of IWM and steering ability of OCSW module at different vertical loadings and different speed of IWM were carried out.The experiment results showed that rotation speed response time of IWM was independent of the vertical loadings and only had a positive correlation with the set speed.With the 500.00 N loading,the steering torque of OCSW module with a 500 W IWM was 19.54±0.91 N·m,and was linearly related to the loading.2.Research on ATS response characteristics of OCS axis was completed on its test-bench.The ATS controller based on bridge circuit was designed and the dynamic model of the ATS system of OCSW module was established by Matlab/Simulink.The experiment on characteristics of the ATS under different vertical loadings,different rotation speed of IWM and different steering target angles of OCS axis were carried out.The experiment results showed that the ATS system could realize the automatic tracking and steering of different steering target angles.When the target angle was set in the range of-90.00°~90.00°,the automatic tracking response angle range of the OCS axis were-88.63±1.54°~+88.17±1.02°,and the maximum average absolute error was 1.77°;the vertical loadings had no significant effect on response angle accuracy and steering response time of ATS system.The steering response time was only positively related to the steering target angle when the OCS axis was rotated in the forward direction,while the steering response time was negatively correlated with steering target angle and rotation speed of IWM,when the OCS axis was rotated in the backward direction;the use of locking torque of electromagnetic lock(EL)could improve the stability of the steering of ATS system.3.Research on the motion and dynamic characteristics of FC with four 500 W IWMs under different motion postures was finished using are tested on its test bench with 4 horizontal turntables.The dynamic model of FC based on its test bench was established by using Matlab/Simulink.The experiment on the motion and dynamic characteristics of four motion forms,such as car up-down(CUD),parallel moving(PM)at 45 °,situ steering(SS)and car left-right(CLR),were conducted under different rotation speed of IWMs.The test results show that: the overall rotation speed response time of the four IWMs changing from 0 to the set speed was about 2 ~ 3 s after FC is started.And the traction of FC with four 500 W IWMs is 727.47±17.72 N,760.14±13.22 N and 729.29±19.46 N,respectively;what's more,rotation speed of IWMs had no significant relationship with the dynamic characteristics of FC.4.This paper proposed the half-mode control method of FC changing its postures,and the motion simulation of this control method was completed.Based on the kinematic analysis of FC,the activity,the operability and the maneuverability of the FC were obtained.According to the principle that the instantaneous center(ITS)'s position of FC decided its running posture,the half-mode control method of FC changing its postures was proposed.The simulation control interface based on the polar coordinate system was designed to select ITC of FC.The simulation model of FC motion was established by using Matlab/Simulink and motion simulation of FC changing it running postures from the straight posture to the CUD posture,PM posture,CLR posture and SS posture were carried out.The simulation results show that: the motion model could reflect the dynamic response characteristics of FC and the half-mode control changing posture method was effective and feasible.5.The half-mode controller of FC changing its postures was designed and the hardening road tests were performed.Aiming at the working principle of the half-mode controlling FC's postures changing,a half mode controller of FC was designed and manufactured.Based on the CAN bus and sensor technology,the data acquisition function of OCS axis of FC is realized by using the modular design method.Based on the low-speed Ackerman four-wheel steering model and closed-loop controller of IWM's rotation speed,effectiveness of the half-mode controller and its control algori thm were proved.The road experiment was conducted in which the FC changed changing it running postures from the straight posture to the CUD posture,PM posture,CLR posture and SS posture.The experiment results show that the controller could realize the control function of posture changing of FC.When the FC is stationary,the OCS axis could achieve free rotation in the range of-90.00° ~ +90.00°,and the maximum average absolute error of the steering angle control was 0.31°.And it took 2 s for the FC to complete its posture change,after the half-mode controller set ITC of FC.