| Facility agriculture is an essential mode of agricultural production.It plays a vital role in providing fresh agricultural products,adjusting agricultural production structure,ensuring food supply,and social stability.As the primary form of facility agriculture in China,greenhouse is still a labor-intensive industry.Manual handling with high labor intensity is typical in greenhouse work.There is an urgent demand for greenhouse transfer machinery that is suitable for narrow roads and a closed environment.The wire-controlled electric chassis was first developed in automotive field.It is widely favored on the ground of its outstanding advantages,such as environmental protection,high efficiency,flexibility,and so on.Industrially speaking,there are already many mature products.Moreover,the application of wire-controlled electric chassis in agriculture is also a future development trend.Agricultural flexible chassis(FC)is a new type of wire-controlled electric chassis driven by in-wheel motor(IWM).It has a unique off-center steering mechanism that amalgamates the steering and drive systems into one.Furthermore,the structure of the FC is simple at a low cost.It can achieve a variety of unique motion modes that can adapt to narrow road transportation in the greenhouse.It has strong adaptability in a closed environment.However,at present,the steer-by-wire control strategy of the FC off-center steering system(OSS)has not been proved,and it cannot be widely applied for engineering practice in agriculture.Therefore,to solve the aforementioned problems,we conducted a study on the steering control strategy and control parameters optimization of the FC OSS,expecting to provide a reference for the steering motion control of the FC.The main contents and conclusions are as follows:First,tests on collaborative control characteristics of driving and steering were conducted for the FC.A methodology using Pulse Width Modulation(PWM)technology was proposed for the motion control of the FC.The electromagnetic friction lock(EFL)was controlled by PWM signal to realize the collaborative motion of the driving and steering of the off-center steering mechanism.A PWM control test system was built to test the torque transmission characteristics and steering characteristics of the off-center steering mechanism.The results of two-factor experiment showed that the PWM frequency,duty cycle and their interaction had highly significant influences on the tightening torque of the EFL(P<0.05).When the frequency was 4?24 Hz and duty cycle was 20%?80%,the tightening torque of the EFL varied from 6.82 to 40.05 N·m.The average steering angular velocity was remarkably influenced by the PWM frequency,duty cycle and their interaction as well as the initial rotation speed of IWM(P<0.05).The effect of duty cycle on average steering angular velocity was the most obvious.With the increment of the duty cycle and the initial speed of IWM,the average steering angular velocity decreased rapidly but slowly increased with the increasing of frequency.When the frequency was 4?24 Hz,the duty cycle was 20%?80%,and the initial speed of IWM was 30?120 r/min,the average steering angular velocity varied from 0 to 0.514 rad/s.The results can provide a reference for the motion control of the FC.Second,a PWM parameter dynamic control method was developed and verified for the FC OSS.The influence of PWM duty cycle on the off-center steering mechanism under different working conditions was firstly tested by using the off-center steering shaft test bench.Then,a fuzzy dynamic controller using quantization factor and scale factor self-tuning was designed for the dynamic control of PWM duty cycle as well as stable and fast response of the steering motion.The verification test results showed the control performance with quantization factor and scale factor self-tuning was better than that of non-self-tuning fuzzy control and fixed PWM duty cycle control.The adaptability of the OSS to working condition changes was effectively improved by the proposed control methodology.This work can lay a foundation for the steering motion control of the FC.Third,a coupling control strategy was put forward for the steering control of the FC OSS.Steer-by-wire models of the FC were established based on the Ackermann steering geometry.A fuzzy PID controller was then designed to reduce the contour error of two wheel steering and angular velocity coupling error of four wheel steering mode switching.Simulation of the designed control strategy was then conducted based on MATLAB/Simulink.The results showed that the FC responded fast during step steering,snake steering and random steering.The steering angles of the left and right front wheel demonstrated good tracking performances for their respective target angles.The EFL cooperated well with the steering motion of the IWM.The linkage control effect of the two off-center steering mechanisms under the coupling control was better than that of the distribution control.In the coupling control simulation,the four wheel steering mode switching time was 4.2 s and the average steering angle error was 0.6°.The maximum and average angular velocity coupling error values were 0.003 and 0.0009 rad/s,respectively.The maximum longitudinal and lateral accelerations were 0.028 and 0.004 m/s~2,respectively.The above results under the coupling control were better than those of the distribution control,and all errors were within acceptable range.The proposed control strategy was effective.Fourth,tests and optimization on working parameters were conducted for the FC OSS.The effects of IWM speed,chassis load,locking voltage,and stepper motor speed of steering bridge circuit on the motion performance of the OSS were tested based on the FC test bench.The results showed that the speed of IWM,the load of the FC and their interaction had significant effects on the steering comprehensive evaluation index(P<0.05).The suitable range of the stepper motor speed and the locking voltage during steering were 150?180r/min and 18?24 V,respectively.The inner locking voltage,the outer locking voltage,the inner stepper motor speed and the outer stepper motor speed had a significant effect on the the steering comprehensive evaluation index(P<0.05).In the case of no load,the optimal inner and outer locking voltages were 22 and 20 V,and the optimal stepper motor speeds were 180 and 170 r/min,respectively.In the case of rated load,the optimal inner and outer locking voltages were 24 and 22 V,respectively.The optimal stepper motor speeds were still180 and 170 r/min,respectively.During the four wheel steering mode switching,the optimal combination of the locking voltage and the stepper motor speed were 4.35 V and 72 r/min.Finally,systematic hardened pavement tests were conducted to obtain the comprehensive motion characteristics of the FC.A integrated motion control system was built for the FC.A software was also developed for motion monitoring and management of the FC.Then the system control performances under the proposed methodologies were obtained through hardened pavement tests.The results showed that the FC steered smoothly and steadily as expected in front wheel steering tests.The maximum tracking errors of the two off-center steering mechanisms were 1.5°and 2.1°,respectively.The maximum and average contour errors of the linkage motion were 1.2°and 0.6°for step steering,1.1°and0.6°for snake steering,and 1.0°and 0.5°for random steering,respectively.During the four wheel steering mode switching process,the maximum angular error of off-center steering mechanism were 1.6°,and the maximum and average angular velocity coupling errors were0.013 rad/s and 0.006 rad/s,respectively.The steering performance under coupling control was better than that under the distribution control.The effectiveness of the proposed control strategy was verified by these stable control performances.This study can provide references for the steering control and engineering application of the FC. |
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