The front suspension of agricultural tractors

Apparently, in the last few years the manufacturers of agricultural tractors feel an increasing interest in the development of suspension systems of the front axle, ranging from the passive schemes to the active schemes through the intermediate semi-active schemes. The expected advantages are a combination of comfort and handling targets, mostly related with the high speed travel on roads and partly related with the low speed travel (and work) on unprepared surfaces. Generally, the suspension schemes are implemented by hydraulic circuits, i.e. more or less complex networks of valves and accumulators coupled with one or more actuators acting on the front axle through a proper mechanical linkage. The semi-active and active schemes also include an electronic circuit to perform a control strategy, to be evaluated on the basis of some figures of merit tested or computed in the operational environment of the machine. This paper offers an introduction to the above problems by means of a simulation model of the tractor (a simplified multi-body description) and the suspension circuit (a reference architecture useful for both passive and semi-active schemes). The system performance is studied in presence of discrete and continuous perturbations active on the front and rear wheels, and the relevant figures of merit are used to estimate the improvement offered by the passive suspension and investigate some aspects of the conversion to a semi-active one.