Hydraulic Fluid Temperature -Imposed Nonlinearities in Automotive Active Hydraulic Suspension Systems

Abstract

In hydraulic active suspension systems, an often overlooked source of nonlinearity is the performance deviation of the working hydraulic oil through the wide band of operating temperatures. In this paper, a quarter-car, Simulink® Simscape nonlinear hydraulic active suspension model has been constructed, taking ISO VG 22 oil as the working oil[29]. A PID controller has been tuned for the active suspension system. Targeted response parameters were tire deflection and suspension travel, as measures of vehicle handling, and sprung-mass acceleration as a measure of riding comfort. At a typical moderate vehicle speed of 40 km/hr, system response was investigated for both a single sinusoidal bump and a continuous wavy sinusoidal road inputs. Simulation was performed at two distant, extreme, operating temperatures, namely -30°C and 60°C. Results showed that at higher temperatures, the decrease in oil viscosity of the hydraulic actuator results in higher tire-rebound amplitudes and longer settling times, and a higher sprung mass acceleration amplitude, worsening both tire-road holding and ride comfort compared to those at -30°C. The most significant conclusion is that an active suspension’s hydraulic actuator actually behaves as a nonlinear damper, besides being a force generator.

Keywords

• iso vg 22 fluid viscosity
• hydraulic actuator
• active vehicle suspension
• nonlinear actuator dynamics
• pid controller