Integrating (Simulated) Acceleration to Position and Experiencing Positional Drift/Ramp

Hi,

I'm making a basic monotube shock absorber model in simulink using hydraulic continuity equations for the cylinder volumes, ideal gas law for the gas chamber, and equations of motion for the gas/oil separating free piston. Everything about the math seems correct, however I have positional drift of the free piston.

The displacement is integrated twice from the acceleration. Filtering it out after the simulation isn't an option as it actively effects internal pressures and flow rates during the simulation. I don't have any clue how to correct this, could anyone shed some light on it?

I attached the simulink model to view.

Thank you

[Daniel M]

I notice there is a large spike and ripple at the beginning of the Free Piston Velocity signal. Perhaps try using the integration + saturation block.

[Jordan Truitt]

The ripple/oscilations (from my understanding) in the beginning is from the sudden shock to the system of the pressurized gas chamber interacting with the rest of the unpressurized chambers and compressible oil. I implemented saturation and like cleaner simulation start, sadly it had no impact on positional drift.