# Issue with simHydraulic and double-acting hydraulic actuator

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Thomas Fournier on 17 Aug 2020
Answered: Yifeng Tang on 24 Aug 2020
Hi,
I'm trying to make a closed loop hydraulic actuator but i cannot manage to make it work.
I'm using a bidirectionnal pump, the input signal has this shape:
If i plot the flow rate on the line B of the cylinder i have,almost as excpected, something like that:
So if there is a flow rate, the cylinder should move back and forth right? But if i plot the displacement of the cylinder i get this figure: The cylinder move to the end of its stroke and stay stuck there. I really don't get why this is happening.
I'm conscious that there should be some kind of tank on the hydraulic line, this being a closed loop system i was planning to use accumulators, but when adding them, the initial conditions fails to converge.
I have used variable step ode 23 solver and fixed step solver, and played with the step size to avoid the error " pressure drop below absolute zero".
Does anyone have an idea on how to make it work?

Yifeng Tang on 17 Aug 2020
Hi Thomas,
Looks like you are using a much earlier version of MATLAB. I wonder whether it would help to see if where is any shipping demo using that variable displacement pump. In R2020a shipping demo, there is one example with that pump and a reservior, which, if available in your version, can be a good reference.
I also believe that "displacement" quantity refers to the ratio between flowrate and angular velocity, instead of the instaneous location of the pistons, so you won't see any reciprocal motion.
Thomas Fournier on 24 Aug 2020
Thanks for your answer, i couldn't adapt an exemple to my need but i added a reservoir with ckeck valves at the entry of my pump and it worked (even though this don't represent a closed loop circuit anymore).
After doing some research, i think the better solution tu resolve the solver problem " pressure fell bellow absolute zero " would be to upgrade my version of matlab, as newer version can continue the simulation after this problem, but not the 2013 version.

Yifeng Tang on 24 Aug 2020
Sounds like the question is resolved. Will paste the comments here:
Yifeng Tang
Hi Thomas,
Looks like you are using a much earlier version of MATLAB. I wonder whether it would help to see if where is any shipping demo using that variable displacement pump. In R2020a shipping demo, there is one example with that pump and a reservior, which, if available in your version, can be a good reference.
I also believe that "displacement" quantity refers to the ratio between flowrate and angular velocity, instead of the instaneous location of the pistons, so you won't see any reciprocal motion.
Thanks for your answer, i couldn't adapt an exemple to my need but i added a reservoir with ckeck valves at the entry of my pump and it worked (even though this don't represent a closed loop circuit anymore).
After doing some research, i think the better solution tu resolve the solver problem " pressure fell bellow absolute zero " would be to upgrade my version of matlab, as newer version can continue the simulation after this problem, but not the 2013 version.

R2013b

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