| The power plant for greenhouse operation is an urgent subject to be researched and developed,which is of great significance to the technological progress of facility agriculture.Based on the fact that the torque of each wheel of the in-wheel motor driving chassis can be controlled independently,a flexible Chassis was proposed,in which the motorized wheels was deviated from the steering axis,and relied on the torque of longitudinal force of the wheel relative to the steering axis to push the offset motorized wheel to steer.The Flexible Chassis has neither transmissions,differentials,half shafts and other transmission mechanisms,nor steering mechanisms such as steering trapezoids and steering assist motors,presenting a very simple chassis form.At the same time,the Flexible Chassis can realize special motion forms such as lateral driving and insitu rotation,and can also realize steering methods such as two-wheel steering,four-wheel steering,and crab steering.It is highly flexible.It can provide power chassis for greenhouse operations,storage and transportation of agricultural products and other operations in confined and narrow spaces,and is a new type of chassis structure with research value.The Flexible Chassis realizes special movements with zero steering radius such as spinning and lateral moving through in-situ mode conversion,which improves the flexibility of the chassis in greenhouse operations.Front wheel steering is the most basic steering method.Crab steering ensures that the chassis body and the crop row are kept parallel during the operations.Four-wheel symmetrical steering minimizes turning radius without stopping.However,the regular steering characteristics of the offset motorized wheels in the above process have not yet been proved.The purpose of this paper was to investigate dynamic characteristics of offset motorized wheel steering during mode conversion,front wheel steering,crab steering,and four-wheel symmetrical steering.It provides theoretical basis for the control strategies in various motions and steering modes of the Flexible Chassis,and develop electronic control systems and simulation models for practical application.The main research contents and conclusions are as follows:1.Improved the electronic control system of the flexible chassis.An embedded electronic control system was developed with a microcontroller as the core to detect the steering angle signal of the bias wheel,calculate the steering control algorithm,and output the corresponding duty cycle of PWM signal to softly control the motorized wheel.The research showed that: the standard deviation of steering angle detection of each offset motorized wheel is less than 0.33°,and the maximum absolute error is less than ±1.3°.System could accurately and stably identify the PPM signal output by the remote control system,and realized the remote control of Flexible Chassis.When the frequency of the motorized wheel velocity regulation PWM signal was 72 k Hz and the resolution was 1/1000,the standard deviation of velocity fluctuation was 0.85r/min,and the minimum adjustment value of no-load velocity was 2.9 r/min.When no load,the PWM value is proportional to the wheel speed;when the load is constant,the PWM value controls the velocity of motorized wheel;when the speed is constant,the PWM value controls the torque of motorized wheel.The straight-line driving test showed that the electronic control system controlled the Flexible Chassis precisely and stably.2.Clarified the tire model and dynamic characteristics of offset motorized wheel in-situ steering of Flexible Chassis.An offset motorized wheel in-situ steering tire model based on the Lu Gre friction model was established,the corresponding test bench was designed,and the lateral and longitudinal friction characteristics of the tire related to the offset distance and load were obtained through the test and model.A control method for offset motorized wheels in-situ steering of Flexible Chassis is formulated and applied in the mode conversion of spinning and crab moving.The results showed that: offset motorized wheel in-situ steering can be described by the tire model based on Lu Gre friction.When offset motorized wheel in-situ steering,the tire is subjected to the longitudinal friction force that hinders the rolling of the tire and the friction force increases rapidly with the decrease of the offset distance.The longitudinal friction force of Linglong 155R13 tire is proportional to the 1.82 power of the load and inversely proportional to the 1.61 power of the offset distance.When the offset motorized wheel is steering in situ,the tire is subjected to a relatively large lateral friction force directed outward.With the increase of the offset distance,the lateral force first increases and then decreases,but the change is relatively gentle.As the load increases,the lateral friction force increases.The offset motorized wheel in-situ steering control method based on fuzzy control has high control stability during t the mode conversion of spinning and crab moving,and the fixed mode motion is smooth.The max error of each offset motorized wheel is less than 1.8° for spinning mode conversion,and the max error for crab moving mode conversion is less than 1.6°.3.Determined the dynamic characteristics of the front wheel steering of the flexible chassis.A 7-DOF vehicle dynamics model of a Flexible Chassis was established.According to the model,a Flexible Chassis dynamics simulation system that could be interactively controlled was established through Matlab/Simulink software.According to the simulation system,the dynamic characteristics of the front wheel steering was analyzed.Finally,verified the steering characteristics through the real vehicle test.The research showed:The real vehicle test shows that the 7-DOF dynamic simulation model has high accuracy and can be used as a virtual simulation platform for the follow-up control strategy research.During the steering process of the front wheel of the Flexible Chassis,there is a strong coupling effect between the offset motorized wheels.The key factor of the coupling is the tire aligning torque caused by the steering angle of the two front offset motorized wheels deviates from the Ackermann steering geometry,which prevents the deviation.At the same time,the greater the degree of deviation,the greater the aligning torque,and vice versa.The front wheel steering system of the flexible chassis has strong nonlinearity.The main factor causing the nonlinearity is the tire alignment torque caused by the tire lateral force that maintains the steering.When the steering is stable,the alignment torque is approximately proportional to the mass of the vehicle,and the square of the vehicle velocity and the inverse of the steering radius.So as the vehicle velocity increases,when the steering angle is the same,the driving torque required for steering is proportional to the square of the velocity.The real vehicle test confirmed that the higher the vehicle velocity,the greater the wheel driving torque required during the steering process.4.Determined the dynamic characteristics of four-wheel wheel steering of the flexible chassis.Using the Flexible Chassis dynamics simulation system,through the simulation of crab steering and four-wheel symmetrical steering,the dynamic characteristics of the two steering processes were analyzed,and finally verified by the real vehicle steering.The results showed that: in order to balance the sum of the yaw moment of the lateral force of the rear wheel with the sum of the yaw moment of lateral force of the front wheel and the the longitudinal force of the 4 wheels,and it is ensured that the vehicle body does not turn during the oblique steering.It can be controlled by constant driving torque,specifically,the longitudinal force of the two front wheels is equal,and the longitudinal force of the two rear wheels is also equal,but the longitudinal force of the rear wheel is 10% larger than that of the front wheel.The above-mentioned constant driving torque control was performed for crab steering.Since the steering radius is infinite in the crab steering process,the steering angle of the offset motorized wheel increases but the return torque remains unchanged.When the steering is stable,the steering velocity of the offset motorized wheel does not change.Because under the same lateral force,the higher the vehicle velocity is,the slower the body slip angle increases,resulting in the higher the vehicle velocity and the smaller the steering velocity of the offset motorized wheel.Quantitative analysis showed that the offset motorized wheel steering velocity is proportional to the difference between the average wheel longitudinal force minus 4.2 N,and inversely proportional to the vehicle velocity,and has nothing to do with the offset motorized wheel steering angle.This shows that the flexible chassis has the characteristics of a linear system when it is turned obliquely.The real vehicle test of crab steering verifies that the offset motorized wheel steers at a constant velocity and the steering velocity is inversely proportional to the vehicle velocity.When the four-wheel is symmetrical steering,the yaw velocity of the car body lead to the acceleration of the rear offset motorized wheel steering,and the acceleration of the rear offset motorized wheel in turn lead to an increase in the yaw velocity of the chasis body.The two promote each other,making the four-wheel symmetrical steering system an unstable system.In addition,the yaw rate of the chasis body can cause the front offset motorized wheel to steer in the opposite direction.The real vehicle test of four-wheel symmetrical steering verifies that the yaw velocity of the chasis body lead to the acceleration of the steering of the rear offset motorized wheel and the reverse steering of the front offset motorized wheel. |
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