Robust H∞ control of a 7-DOF active suspension system for cargo-to-passenger vehicle conversions with variable payload and center of gravity height

Abstract Active Suspension system of vehicles plays a crucial role in ensuring the ride comfort and stability of vehicles, particularly for cargo vehicles converted into passenger buses, where variation in payload and center of gravity height significantly affect dynamic performance. This study proposes a robust H∞ controller for a seven degree of freedom (7 DOF) full vehicle active suspension system to simultaneously improve the ride comfort, road holding and rollover stability under parameter uncertainties. The controller is developed using an augmented generalized plant with appropriately designed frequency dependent weighting functions. The vehicle model considers four independent road disturbance inputs and lateral acceleration during cornering. The proposed controller is evaluated under single bump, triple bump and ISO 8608 class D random road excitations and is compared with passive suspension and linear quadratic regulator (LQR) control strategies. Under single bump excitation, the proposed controller reduces the weighted RMS heave acceleration, roll acceleration, suspension deflection and tire deflection by 77.1%, 40.2%, 94.6% and 89.4% respectively, compared with the passive suspension system. Similar improvements are achieved under triple and random road conditions. Robustness analysis under 30% variations in sprung mass and center of gravity height demonstrate that the proposed controller maintains superior performance with significantly smaller performance degradation than both passive and LQR controlled suspension systems. Frequency domain analyses, including power spectral density and singular value analyses, further confirm that the proposed H∞ controller excellent disturbance rejection capability and robust performance. These results demonstrate that the proposed robust H∞ controller effectively balances the conflicting requirement of ride comfort, suspension travel, road holding and rollover stability, making it a best solution for cargo to passenger vehicle conversions operating under varying operating conditions.