COMBINED CONTROL OF SEMI-ACTIVE AXLE AND CAB SUSPENSION

摘要

Farm tractor operators are exposed to a high level of whole body vibrations. In the past, a number of systems have been developed to reduce these vibrations using both active and semi-active suspension of the axle, the cab or the seat. With an axle suspension it is also possible to reduce wheel load fluctuations. This is important for driving safety, handling and - especially in the case of farm tractors - for tractive efficiency. The common control strategies of skyhook and groundhook control either focus on comfort or roadholding. The conflict of objectives between a comfortable passive suspension design on the one hand and a safe passive suspension design on the other hand does also exist for semi-actively controlled suspensions. A gain in comfort results in increased wheel load fluctuation and vice versa. Therefore, a combined control strategy using a semi-active two stage suspension system consisting of an axle suspension and a cab suspension is proposed. Simulations of a quarter car model show the conflict of objectives for passive and single stage semi-active suspension systems. Possible control strategies for a two stage semi-active system are discussed and a combination of groundhook and skyhook control is proposed to solve the conflict. A modified tractor equipped with a semi-active hydropneumatic front and rear axle suspension and a semi-active cab suspension system is used to test different combinations of passive and semi-active suspension settings on a hydraulic four-post-test-rig. Results show that using a two stage suspension system can reduce dynamic tyre forces while at the same time improving the driver's comfort.