A Mobile Robot with Tilting Quad-tracks for High Terrainability and Maneuverability

Abstract

This paper is about the development of PATRo (Passively Articulated Tracked Mobile Robot) which is a new mobile robot mechanism for rough terrain driving. PATRo mechanism consists of four drive tracks, two rocker links, and a main body. The rockers are connected to the main body via a differential joint, and each drive track is connected to the rocker via 2 degrees-of-freedom (DOF) passive joints. Thus, with 9 DOFs in total, the four drive tracks can be independently positioned and oriented to maximize the contact area with the ground on a random ground shape. There are two main advantages of the proposed PATRo mechanism. First, the loss of traction force due to imperfect contact is minimized by securing the contact area between the track and the ground. Second, it is possible to know the ground contact angle of each four drive tracks on the ground without prior information, and it is possible to observe the terrain around the robot. And the surrounding terrain information can be used in the rough terrain driving control to improve the reference trajectory tracking performance.<br/> Therefore, PATRo is a new mobile robot platform with improved maneuverability which is the disadvantage of the conventional track-typed mobile robots, and improved terrainability which is the disadvantage of the conventional wheeled mobile robots. In this study, rough terrain driving performance of PATRo was confirmed by both simulation and experiment. First, through 3D dynamics simulation, driving performance of PATRo is compared to those of seven existing mobile robot mechanisms. The same control algorithm was applied to verify only the influence of the mechanical characteristics of each platform purely, and the comparison test was repeated in the three different driving environments to prevent the biased result by selecting the driving environment. As a result, the PATRo platform showed the most superior performances in both terrainability and maneuverability among the total of 8 platforms. Compared to seven other platforms, PATRo showed 25.5% improved terrainability and 44.7% improved maneuverability, in average.<br/> Also, the design parameter optimization of the PATRo platform was performed. In order to improve the terrainability and maneuverability at the same time, the objective function of optimization was selected as the maximum value of the required friction coefficient. The length of the rocker and the length of the drive track were selected as the design variables through the sensitivity analysis using the orthogonal array and level average analysis. The optimal torque distribution based on kinetostatic analysis was used to calculate the required friction coefficient during the optimization process. Taguchi methodology is used for the optimization. Based on the initial and optimal values of the design variables, the rockers and drive tracks of the PATRo prototype were each produced in two versions.<br/> There are mainly four issues to be validated through rough terrain driving experiment of PATRo prototype. The first is the relative superiority compared to the existing mobile robots, which were confirmed through simulation. In the experiment, this can be confirmed by comparing the driving performance of PATRo when the nine passive joints are fixed, and when they are not. Second is to verify the effect of the proposed PATRo kinematic model. The third is the verification of the deign parameter optimization results. Experimental results show that the terrainability and maneuverability are improved by 10.1% and 26.8%, respectively. Finally, rough terrain driving experiment in the real circumstance is carried out and the result proved the effectiveness of the mechanism.