COMPUTER AIDED DYNAMIC ANALYSIS AND OPTIMAL DESIGN OF SUSPENSION SYSTEMS FOR OFF-ROAD TRACTORS

Ride improvement of agricultural tractors is investigated through optimal suspension systems at the seat and at the cab. The investigation has been carried out in four phases, namely: passive seat suspension incorporating bounce, longitudinal, lateral, roll, and pitch modes of vibration of a rigidly mounted cab; cab suspension with a rigidly mounted seat; seat suspension with a suspended cab; bounce seat suspension employing semi-active "on-off" damping.

Passive seat suspension configuration for longitudinal and lateral modes and a gimbal supported arrangement for rotational modes is proposed. Mathematical models are developed for the passive seat, cab, and cab-seat suspensions. The terrain induced ride vibrations for simulation of the suspension models are taken from the artificial track constructed at the National Institute of Agricultural Engineers, Silsoe, England.

A frequency dependent linearization technique based on the local value of dissipated energy is developed to represent the non-linear seat suspension models by their linear equivalents. The sensitivity of response characteristics of suspension models to variations in suspension parameters is investigated to establish the base for formulating an optimization problem.

Objective functions are formulated for each suspension model, such that the ride performance is maintained within the ISO recommended fatigue decreased proficiency limits corresponding to at least 4 hours exposure time. The relative displacement response is constrained to certain maximum permissible value. The objective functions are minimized using non-linear programming techniques. The ride performance of optimal suspensions in terms of acceleration PSD and ISO weighted rms acceleration at the driver's location are presented.

Finally, the concept of semi-active "on-off" damping is developed. The "on-off" damper utilizes a simple 2-position on-off valve, which operates with the command signal manipulated from the directly measurable variables. The response characteristics of bounce seat suspension using "on-off" damping are evaluated and compared to that of a passive suspension.