Pneumo-Hydraulic Actuator mechanical ports

I just discovered the Pneumo-Hydraulic Actuator which seems great for my application as I was previously using a gas accumulator connected to one hydraulic port of a double acting cylinder to model this. With the double acting cylinder there is 2 mechanical ports (R and C) and I'm using them to put for example a spring in parallel with the cylinder. On this new block there is only one mechanical port (L) so I'm confused. Shall I just assume the body of the cylinder is connected to the mechanical reference?

Also on both new block "Pneumo-Hydraulic Actuator" and "Double-Acting Hydraulic Cylinder (Simple)" the initial pressure in the fluid chamber is not an input parameter anymore as opposed as in "Double-Acting Hydraulic Cylinder". Is the initial pressure just taken from the block the cylinder is connected to for those new blocks?

Lastly "Double-Acting Hydraulic Cylinder (Simple)" help seems to suggest that friction is negligible for it as opposed to "Double-Acting Hydraulic Cylinder". However "Double-Acting Hydraulic Cylinder" does not include any friction. It seems to me that the "Cylinder Friction" block can be use with both blocks to simulate friction. am I missing something?

Thanks Alex

Answer 1

[Sebastian Castro]

Hi Alex,

1. You are right. The L port is equivalent to the R port in a traditional Mechanical Simscape block. If you want access to the C port for whichever reason, you may have to build your own "custom" Pneumo-Hydraulic actuator. In other words, combine a hydro-mechanical converter, mass, and pneumatic piston chamber from the Simscape Foundation Library.

2. I'm not sure about this one. My guess would be that the simplified equations don't require the component to keep the pressure stored internally in its equations, so you don't need initial conditions. The equations simply look at (Pa - Pb) at a particular time to figure out the force transmitted.

Again, don't quote me on this one. You could log the pressure across this cylinder block (with varying initial conditions) to test it out for yourself.

3. I think you're right here too. While the Simple block lists assumptions about friction, the documentation for the regular (non-simplified) block also lists this:

Basic Assumptions and Limitations

• No leakage, internal or external, is taken into account.
• No loading on piston rod, such as inertia, friction, spring, and so on, is taken into account. If necessary, you can easily add them by connecting an appropriate building block to cylinder port R.

- Sebastian