Methods for the Reduction of Odometry Errors in Over-Constrained

Abstract. This paper presents an analysis of odometry errors in over-constrained mobile robots, that is, vehicles that have more independent motors than degrees of freedom. Examples of over-constrained vehicles are the various 6-wheeled Mars Rovers like Rocky-7, Rocky-8, or Fido. Based on our analysis we developed two novel measures aimed at reducing odometry errors. We also developed a novel method that serves as a framework for the implementation of the two new measures, as well as for other, conventional error reducing measures. One of the two new measures, called “Fewest Pulses Measure, ” makes use of the observation that most terrain ir-regularities, as well as wheel slip, result in an erroneous overcount of encoder pulses. The second new measure, called “Cross-coupled Control Measure, ” optimizes the motor control algorithm of the robot to reduce synchronization errors that would otherwise result in wheel slip with conventional controllers. The novel method that serves as a framework for other measures is based on so-called “Expert Rules. ” In this paper we formulate three expert rules aimed at reducing dead-reckoning errors. Two of these expert rules are related to the foregoing discussion on error reducing measures. The third expert rule adds a gyroscope to the system and we re-examine the effectiveness of the odometry error-reducing measures in the context of this addition. In the work described in this paper we modified a Pioneer AT skid-steer platform by providing it with four independent drive motors and encoders. We implemented our error-reducing measures and the expert rule method on this over-constrained platform and present experimental results