Change the minimum displacement of a variable-displacement pressured-compensated pump

Hello,

I am using a variable-displacement pressured-compensated pump in my hydraulic system. The MathWorks Help says: The displacement can be changed from its maximum value down to zero. Is there a way to change the minimum displacement from zero to another value smaller than its maximum displacement? For expample: The datasheet provides a p-Q-Diagram of a variable-displacement pressured-compensated pump with a maximium displacement of 35 L/min at minimum pressure and a minimum displacement of 5 L/min at maximum pressure.

Thank you very much for your help

Best regards,

Sebastian

Answer 1

[Jocie Kluger]

Hi Sebastian,

This is a limitation of the Variable-Displacement Pressure-Compensated Pump in the Hydrualic domain. The Pressure-Compensated Pump (IL) in the Isothermal Liquid library has an option to set a nonzero Minimum displacement. You can interface the Isothermal Liquid block with the Hydraulic domain network using an Interface (H-IL) block, or upgrade the entire model to the new Isothermal Liquid domain using the hydraulicToIsothermalLiquid function.

Regards,

Jocie

[Sebastian Stumpf]

Hello Joice,

thank you very much for your help. The Pressure-Compensated Pump (IL) in the Isothermal Liquid library looks promising. I get an error: Pressure at port A must be greater than or equal to Minimum valid pressure. The assertion comes from: Block path: /Hydraulic Subsystem/Pressure-Compensated Pump (IL). This is a screenshot of my system with the new Pressure-Compensated Pump (IL).

Port A comes from a tank, Port B goes to a 4/3 control valve. Port R is a diesel engine with positive rotation relative to C. With the previous variable-displacement pressured-compensated pump everything worked like a charm. The values I know are:

• Maximum displacement 30 at minimum pressure of 0 MPa
• Minimum displacement 2 at maxmimum pressure of 24.5MPa

Did I connect X and Y correctly and are my values for Set pressure differential and Pressure regulation range according to the given values correct?

I am struggling right know to set the right values. I am very happy about your help. I already invested a lot of time in troubleshooting with the help of the web and the MathWorks Help.

Thank you very much and kind regards,

Sebastian

[Jocie Kluger]

Hi Sebastian,

The connections look correct. To match the known values listed in the bullet points, you should set Set pressure differential to a value slightly above 0 MPa, like 1e-5 MPa (this is a numeric requirement), and set Pressure regulation range to 24.5 MPa. If the error persists, please feel free to attach the model or a simplified verison of the model that produces the error, and I'm happy to take a look.

Regards,

Jocie

[Sebastian Stumpf]

Hello Jocie,

thanks again for your help. I tried a lot, again. But the error persists. You can find my simplified verison of the model that produces the error with the necessary CAD-parts attached. I included the variable-displacement pressured-compensated pump, which work absolutly fine and the new Pressure-Compensated Pump (IL) in the Isothermal Liquid library which produces the error.

Thank you very much for your help.

Kind regards,

Sebastian

[Jocie Kluger]

Hi Sebastian,

Thanks for sharing the model. Something I do notice is that in the model with the original Hydraulic-domain variable-displacement pressure-compensated pump, the pressure dips slightly below absolute 0 at the pump T port. The Hydraulic domain permits this to occur, while the IL domain always throws an error, since this is not a strictly physical result.

Since this is not a strictly physically viable result, I believe the next thing to focus on is what parameters can be adjusted to keep the pressure above absolute 0 at the pump T port. I suspect this generally involves decreasing flow resistance in the system. It looks like the original model has the pressure going below 0 at around 6.76 seconds. At this time, the 4-way valve shifts to the positive position (Orifices P-A and B-T open), and fluid flows out the Hydraulic Reference, through the Fixed Orifice Empirical1, through Orifice B-T of the valve and into the cylinder. When the fluid flows through Fixed Orifice Empirical1 out of the reservoir, the orifice resistance at that flow-rate causes the pressure to drop relative to the Hydraulic Reference.

One other detail- I'm sorry I didn't mention it before: when using the Interface blocks in this way, there needs to be an Isothermal Liquid Properties block connected to the Isothermal Liquid network, to use the custom fluid rather than the default fluid in the Isothermal Liquid domain (which is water). The attached model includes the IL Properties block that you can use, with properties set to match ISO VG 46 at 60 degC.

Let me know how this sounds,

Jocie

[Sebastian Stumpf]

Hello Joice,

thanks again for your detailed answer. I solved the issue with some kind of superpositioning. Instead of one pump with a variable displacement from 5L/min till 30L/min I used two variable displacement pumps. The first one from 0L/min till 25L/min and the second one from 0L/min till 5L/min. But the second one is a Variable-displacement pump with a constant control member offset. Thanks to this I achieve a variable displacement from 5L/min till 30L/min. Validation with real world data proves this procdure correctly.

Thank you very much for your support. You helped me a lot.

Kind regards,

Sebastian

[Jocie Kluger]

I'm glad to hear you resolved the issue and the results match real-world data. It's a neat workaround trick.

Regards,

Jocie