Set Pressure and Flow Rate in Simulink (SimHydraulics)

Hi!

I would like to ask if it is possible to set a user-defined pressure and flow rate in a model with Simulink.

I have tried using a "Hydraulic Flow Rate Source" together with a "Hydraulic Pressure Source", however by using the sensors I have noticed that if I check the flow rate after the pressure source, the values that I have set initially have changed.

I have uploaded a screenshot showing this issue.

Does anyone of you know if it is possible to set both values?

Thank you in advance!

Best Regards,

David

Answer 1

[Sebastian Castro]

I don't believe that is possible, because the Ideal Flow Rate Source will provide whatever pressure is necessary to maintain a given flow rate. If you add more pressure drops in the system, the flow rate source will simply compensate for those.

Suppose you need 10 MPa to cause a desired flow rate. If you put a Pressure Source of 3 MPa in series, then all that will happen is that the Flow Rate Source will supply 7 MPa instead, because it's being "helped" by the other pressure source.

Another reason why your flows are different is that the flow in the left sensor has two possible pathways. Some of the fluid flows through the Pressure Source into the second sensor, but most of it ends up going down into that Hydraulic Reference block. So, it makes sense for those flow rates to not be the same.

- Sebastian

[David Yu]

I understand, then there is no way that I could adjust myself these two parameters?

The reason why I put the "Hydraulic Reference block" was to avoid a "initial conditions" error from the compiler. If i remove it, Simulink won't be able to solve it due to the pressure conflict that you mentioned.

Thank you for your reply!

[Sebastian Castro]

It will work only if these conditions are non-conflicting. You typically specify either a flow rate or a pressure, and see what the other value is based on all your hydraulic elements.

What exactly are you trying to model? Maybe there's another way we can resolve this.

[David Yu]

I am just modelling a "Hydraulic Constant Pressure Source" into a custom valve system (valve1) that then goes into a throttle or goes to another custom valve (so the fluid splits into the throttle or another custom valve, lets call it valve2).

The pressure set by the "Constant Pressure Source" is given, so with that I calculate the flow rate after the valve1 (here is the problem, by using the "Hydraulic flow rate source" I set the value to the desired one but the pressure suddenly drops to 0 if I check with the "Hydraulic pressure sensor"). I have to also set the pressure after the valve1 with one that I calculated.

Later at the throttle I will also have to use a "Hydraulic flow rate source" to set a different flow rate. And similarly in the other path, I will have to set a flow rate after the valve2.

Sorry for the long message, but does that help you imagine the system?

[Sebastian Castro]

I can think of two tips that might help with that.

1. You mentioned that after connecting a Pressure Sensor, your pressures drop to zero. This is likely because you're connecting the Pressure Sensor in series, thus causing an infinite resistance in the system. Try connecting the sensor in parallel (the P port at the node you want to measure, and the T port connected to a Hydraulic Reference).
2. Typically, you have a constant pressure source and you control the flow rates by modifying the opening of the valves. I think you should keep the Pressure Sources there, and then measure flow rates into a closed-loop controller that modifies the openings of Valve1 and Valve2. This will let you control the flow rates at both systems.

For the valves, what are you using? Variable Orifice blocks, or something more elaborate?

[David Yu]

Thank you very much for your help!

I have been trying your tips today, here is what I found out.

1. You are correct, the "Pressure sensor" should be connected in parallel as you said, I have uploaded a picture showing that it is indeed possible to have a pressure and flow rate that one desires. However, you can see on the Scope graphs that the values of both flows are similar but there is a small sudden fluctuation in front of them. Do you know the cause of this?

2. The valves that I am using are simply Simulink Subsystems that uses signals and produces an output of flow rate, so it is basically formulas instead of an actual valve block from the library. Therefore I cannot play with the valve openings.

If I find a way to implement using signals the openings of the valve I will let you know.