Research On Coordinated Movement Control Of A Four-wheel Drive Agricultural Mobile Platform

In recent years,with the acceleration of urbanization process and the aging of the population,less and less people are engaged in agricultural production.A lot of monotonous and heavy filed work needs to be done by mechanized,automated and intelligent machines.The development of agricultural information technology and agricultural intelligent equipment technology has provided an opportunity for the transformation of agricultural production modes from traditional agriculture to modern agriculture.Agricultural robots,as an important part of agricultural intelligent equipment technology,are drawing more and more attentions from researchers.Based on an intensive literature review of agricultural robots,this thesis pointed out that there are still some open problems for agricultural robots,such as large mechanical structure,weak environmental adaptability,poor obstacle-crossing capability and short endurance.To address these problems,an agricultural mobile platform--AgriRover01 was developed,which is four-wheel drive and has two(front and rear)simultaneously steering wheel pairs.According to the characteristics of the unique configuration and driving mode of the platform,the related coordinated movement control was researched.Firstly,this thesis described the overall framework of the four wheel drive agricultural mobile platform: we gave an analysis on the configuration design and the key component selection;then,we conducted the hardware system building and the software system integration according to the special requirements of wheeled agricultural robots.The hardware system building includes mechanism/wiring design and the selection for related hardware,such as motors,drivers,an industrial personal computer,and a series of sensor connections(encoders,RTK-GPS receiver,etc.)The software system integration mainly contributes to a host computer software,which enables the closed-loop control for the servo system with the help of sensor information and communication protocols.Secondly,in view of the unique configuration and driving characteristics of the agricultural mobile platform,this thesis presented a new coordinated movement control strategy--Extended Ackermann Steering Strategy.The control strategy can reduce the slippage through reasonably adjusting four wheel linear velocities.The new control strategy mainly consists of three steps: I)we used a steering-motor-turning-angle based model to estimate the steering angles of the front/rear wheels;II)we selected the optimal position of the virtual turning center to meet the application requirements of Ackermann Steering Principle(ASP),by minimizing the deviations between the inside and outside wheel steering-toe-in angles;III)we generated the linear velocities of inside and outside wheels following ASP.Finally,we carried out field trials to verify the coordinated movement control performances of the proposed strategy.We selected three different experimental sites(cement floor,bare farmland and wheat field)to judge whether this strategy can carry out coordinated movement control on various types of terrain.These types of terrain have different features: cement floor has even and hard surface;bare farmland has uneven and hard soil terrain;wheat field has uneven terrain,humid and soft soil surface with small soil blocks.In the experiments,we collected data of in-wheel motor current and repetitive error of trajectory tracking to help evaluate the level of slippage.We conducted several groups of contrast experiments.Through the analysis and comparison of experimental data,we drew the conclusion that the Extended Ackermann Steering Strategy could meet the application requirements of coordinated movement control.In summary,the platform has advantages of compact structure,strong driven power and small turning radius,etc.These advantages endow it with high application feasibility on open farmland.