Extended Ackerman Steering Principle for the coordinated movement control of a four wheel drive agricultural mobile robot

This paper presented a new extended Ackerman Steering Principle (ASP) for the coordinated movement control of an agricultural mobile platform, which is four-wheel drive and has front/rear steering gears. The mobile platform was particularly developed for agricultural applications, such as soil/crop data collection and spraying/fertilizing. To fit the complex farmland operation environment, it has 4 driving wheels equipped with in-wheel motors and two steering motors connected to front/rear steering gears. When the platform is turning, its steering mechanism will lead to linear velocity deviations between inside and outside wheels. As a result, coordinated movement control will be employed to reduce the slippage. The new control strategy mainly consists of three parts: firstly, we use a steering-motor-turning-angle based model to estimate the steering angles of the front/rear wheels; secondly, we select the optimal position of the virtual turning center, by minimizing the deviations between the inside and outside wheel steering-toe-in angles; thirdly, we generate the linear velocities of inside and outside wheels following the ASP. We conducted 9 sets of comparison experimental trials on 3 types of terrain (cement floor, bare farmland, and wheat field), to verify the feasibility of the new strategy. Experimental data analysis shows: first, the proposed strategy can decrease the total motor energy consumption (the sum of inside and outside wheels' average motor current values) in all trial sets; second, the new coordinated control strategy can help the mobile platform to track a circle curve with much less slippage in all trial sets.