Modeling and Validation of Hitched Loading Effects on Tractor Yaw Dynamics

Abstract

This thesis develops a yaw dynamic model for a farm tractor with a hitched implement, which can be used to understand the effect of tractor handling characteristics for design applications as well as for new automated steering control systems. A model is found in which hitched implement conditions can be accounted for, and an improvement in yaw rate tracking prediction in both steady state and dynamic conditions is seen vs. traditional models. This model is termed the “3-wheeled” Front and Hitch Relaxation Length (“3-wheeled” FHRL) Model. Experimental data from a hitch force dynamometer are used to validate the way the hitched implement forces are derived in the “3-wheeled” FHRL Model and to determine if differential hitch forces can be ignored. Steady state and dynamic chirp data taken for a variety of implements at varying depths and speeds are used to quantify the variation in the hitch parameter and to find the front and hitch relaxation length values. Finally, a model which accounts for four-wheel drive forces is derived, and experiments are taken which provide a preliminary look into the effect of four-wheel drive traction forces on the yaw dynamics of the tractor. In comparisons with other traditional models, the “3-wheeled” FHRL Model is shown to be superior in its steady state yaw rate tracking ability with an RMS error of .245 deg/s vs. 1.96-2.07 deg/s for other models at a certain depth and also superior in its dynamic tracking ability with an RMS error of .675 deg/s vs. .748-1.37 deg/s for the other models. The experimental results from the hitch force dynamometer show that the implement performs according to the linear tire model and that the moment caused by differential forces at the hitch can be ignored. The hitch parameter, Cah , ranges from 452- 3385 N/deg for various implements and depths tested in this thesis. The front tire relaxation length is found to be .37 m and the hitch relaxation length is found to be .4 m. The four-wheel drive experiments show that using four-wheel drive provided an increase in yaw rate from 9-21%, depending on the implement depth and speed.