ERROR Pressure fell below zero SimHydraulic

Hello,

I'm modeling an hydraulic system with variable displacement pump, 3 way valves and a double actung hydraulic cylinder. I was trying to simulate the pipe line effects adding pipes between the pump and the valve, and between valve and cylinder.

But when I start the simulation or during the simulation an error is triggered and the simulation stops running:

• At time 15.160000, an assertion is triggered. Pressure fell below absolute zero. The assertion comes from:Block path: Modello_Pompa_A15VSO_280_R14/Hydraulic Pipeline7Assert location: between line: 96, column: 12 and line: 96, column: 41 in file: C:/Program Files/MATLAB/R2013a/toolbox/physmod/simscape/library/m/+foundation/+hydraulic/+elements/constant_chamber.ssc *

Even if I change the initial pressure of cylinder chamber or pipe line initial pressure I get the same messagge. And even if I put only a pipe between the pump and the valve the problem is the same.

How can I solve this issue?

Thanks Antonello

[Anonymous User]

Hi Antonello,

check following things

i) check tolerance setting set it to order of 1e-9 and above

ii) reduce the area of your direction control valve

iii) increase pressure setting of your VDPCP Pump(variable displacement pressure compensated pump) then try to simulate

[Lalitesh Kumar]

@ Antonello................Hello Antonello,

I am encountering the same problem for a different fuel cell system. Did you have overcome with the error you have mentioned ? If yes, then please help me out in mitigating this issue. I am trying from more than three weeks and still not able to solve this issue. My issue is as given below:

• An error occurred while running the simulation and the simulation was terminated Caused by:

At time 10.000200, an assertion is triggered. Pressure before the Orifice SHOULD NOT be Negative.

Thanks

Lalitesh Kumar

Answer 1

[Yifeng Tang]

Pressure below zero error occurs most likely when the liquid is pulled through an orifice/resistance/pipe/valve or forced to expand in a volume (e.g. in an actuator). A few occasions I remember seeing:

1. turning on an ideal flow rate source too fast.
2. hydraulic cylinder incorrectly sized for the load
3. oscillation at the hard stop at the end of a hydraulic actuator (small, quick expansion of volume)

And something to try:

1. think (really hard) about what may be a physical explanation of the low pressure, and see if any change of the model may help
2. add a bit entrained air in the liquid setting block. This reduces the modulus at low pressure significantly and help avoid low pressure scenario. It's physically reasonable, too. The default IL property settings in R2022a now has a bit of air
3. if error occurs at end of actuator travel, check hard stop setting. https://www.mathworks.com/matlabcentral/answers/114892-how-can-i-get-the-parameters-for-contact-stiffness-and-contact-damping?s_tid=srchtitle
4. Don't turn on ideal source blocks suddenly. Add a first order transfer function or filter in the Simulink-PS converter.