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UBC Theses and Dissertations

A low level motion controller for path tracking control of a tracked mobile robot Ardron, Kevin

Abstract

The main goal of this thesis is to improve the path-tracking performance of a tracked mobile robot (an excavator) at the low level. The three contributions of this research are: the implementation of a cross-coupling motion controller on top of the existing fuzzy logic track controllers, the integration of a vision motion estimation system to be used in conjunction with hydraulic flow meters to detect track slippage, and the implementation of a basic track slippage controller. The low level system, consisting of the cross-coupling motion controller and the track controllers, was modeled and simulated. The stability of the designed controller was determined by the use of a Bode plot stability analysis. The vision motion estimation system, utilizing a trinocular video camera (Triclops™), was integrated with the hydraulic flow meters of the excavator. The mounting location of the camera on the excavator, the maximum allowable motion of the camera, and the time between image grabs were all determined. A method for detecting and quantifying individual track slippage was also developed. Finally, a basic slippage controller was developed. The slippage controller used the track slippage values obtained from the vision motion estimation system and the hydraulic flow meters. A minimum slippage threshold was determined for the slippage controller as well as a means to reduce and eliminate the track slippage. The results of the thesis work show that the cross-coupling motion controller does improve the path-tracking accuracy of the excavator. For the experiments completed for this thesis, the lateral path error incurred during a 30 second (6 meters long) commanded straight line path was reduced from 16 centimeters to almost zero by the use of the cross-coupling controller. The slippage results show that the use of the slippage controller can be advantageous in the area of outdoor mobile robots, even though the existing system has speed and accuracy limitations. The experiments conducted show that track slippage magnitudes of over 20% could be detected and the machine slowed down to eliminate these track slippages.

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