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How to model actuator ports in Simscape fluids?

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HiWave on 6 Jun 2023
Commented: HiWave on 8 Jun 2023
A hydraulic actuator is outfitted with ports, whose size is different than the actuator areas A/B. I've reviewed a few examples given by Matlab and it's not clear to me how to properly account for the actuator port sizes. Above I've shown one possible example, which includes an "area change" block to represent the port size reduction from the cylinder diameters. This seems most intuitive to me. Other examples I've seen include an "orifice" block and others just use a "pipe". The "orifice" and "pipe" blocks don't seem right, because the pressure losses should be dependent upon the flow direction. In a double acting cylinder, flow goes both in and out. In the "area change" block, we can define an expansion and contraction coefficient, which seems to be the proper way to account for flow direction, however, I've never seen this used in an example. Can someone please tell me how to account for the port sizes in a hydraulic actuator? I don't fully understand why this isn't a characteristic of the actuator block....I've never seen an actuator whose port sizes are the same size as the cylinder!?!?!

Answers (1)

Yifeng Tang
Yifeng Tang on 8 Jun 2023
I'd encourage you to think more about the "effect" of a port in the actual system, instead of just having the actually values of area into the model. Usually it causes a pressure drop, and you can represent that using an orifice, or flow resistance, or pipe (which has pressure drop model). An area change block may also work, as long as you parametrize it so it provides the right amount of pressure drop for a given flowrate, just like how you would parametrize when using other blocks.
The Doc page for this double-acting actuator shows the diagram under the hood and the isothermal liquid ports are connected to translational mechanical converters. One assumption you see on the Doc page of the converter is that "the flow resistance between the inlet and the interior of the converter is negligible". You can of course add resistance, as you are already trying to do.
HiWave on 8 Jun 2023
Edited: HiWave on 8 Jun 2023
@Yifeng Tang...thanks for engaging, but this doesn't really help answer my question. I know the port size plays a signficant role in my overall problem (not a question...resistance is needed). I also highlighted in my question the different approaches I've come across. My question is about which approach is technically more accurate, so I was hoping someone with some experience could help. I think the main diff in either approach is an account for the flow direction. I'm not going to repeat my question, but I think it still stands.
HiWave on 8 Jun 2023
Tomorrow I will edit my question and include the case study I'm working on. I think that will help put my question in context.

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