The Challenge of Designing a Semiactive Damper for Heavy Truck Seat Suspensions

The close proximity of seat suspensions to human body presents several challenges in terms of the perception of the suspension forces by the vehicle operator. This is particularly true of the suspensions with time-varying forces, such as semiactive seat suspensions. The major challenge in such suspensions is changing the suspension force from one state to under, without causing excessive amounts of dynamic jerk. This paper looks into the cause of dynamic jerk in semiactive suspensions with skyhook control, and presents two alternative implementations of skyhook control, called “no-jerk skyhook,” and “skyhook function,” for the purpose of this study. An analysis of the relationship between absolute velocity of the sprung mass and the relative velocity across the suspension is used to show the damping force discontinuities that result from skyhook control. This analysis shows that at relative velocity zero crossings, skyhook control introduces a sharp increase (jump) in damping force, which, in turn, causes a jump in sprung mass acceleration. This acceleration jump, or jerk, causes a significant reduction in isolation benefits that is offered by skyhook suspensions. The alternative implementations of skyhook control considered in this study offer modifications to the formulation of conventional skyhook control such that the damping force jumps are eliminated. The alternative policies are compared with skyhook control, using a laboratory implementation of a heavy truck seat suspension with base excitation in vertical direction. An evaluation of the damping force, seat acceleration, and the electrical current to the magneto-rheological (MR) seat damper shows that the alternative implementations of skyhook control can entirely eliminate the damping force discontinuities and the resulting dynamic jerks caused by skyhook control.