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Why do I get negative values though the connector?
Why do I get negative values though the connector?
Hi!
This is my model. It's a pump, hydraulic cylinder and two tanks.
The problem is that if the pump pumps out 9 l/min, then the cylinder is going to observe those flow values as -9 l/min. How can I "by pass" this problem?
The code for the pump has those equations
Code:
C1.Q = D * n * ve / 1000;
C2.Q = C1.Q;
The connector C2(input) and C1(output) has two variables: P(pressure) and Q(flow).
The cylinder has those equations
Code:
// Velocity equation of piston
v = C2.Q * ve / (A2 * 6);
// Compute with area A2 and flow C2.Q
// Input flow equation
C1.Q = C2.Q * A1 / A2;
// Saturation - Piston position equation
if v * time > L then
s = L;
elseif v * time < 0 then
s = 0;
else
s = v * time;
end if;
// set position at piston
piston.s = s;
// ODE equation of the cylinder
M * der(v) = 10 * A1 * C1.P * me - piston.f - 10 * A2 * C2.P * me;
Where C2 is output and C1 is input.
I have attached my library.
1510783475_EasyHydraulics.mo
Thank you
Re: Why do I get negative values though the connector?
I think you're a bit confused about how Modelica connections work. If your pump is sending 9 l/min from pump.C.Q, then from the cylinder's perspective, cylinder.C1.Q must be -9 l/min (it is receiving 9 l/min from some other component); because connections are zero-sum. If you check for example a simple electrical circuit in the standard library, and connect a constant source of 1V to a 1Ohm resistor, then one port of the resistor will be -1A and the other port will be 1A; but the component will also calculate an internal parameter i which is 1A and is the actual flow through the resistor. You could naturally calculate such a flow for the component itself if you'd like and think that's more natural than "+ is outgoing, - is incoming".
- sjoelund.se
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Re: Why do I get negative values though the connector?
sjoelund.se wrote:
I think you're a bit confused about how Modelica connections work. If your pump is sending 9 l/min from pump.C.Q, then from the cylinder's perspective, cylinder.C1.Q must be -9 l/min (it is receiving 9 l/min from some other component); because connections are zero-sum. If you check for example a simple electrical circuit in the standard library, and connect a constant source of 1V to a 1Ohm resistor, then one port of the resistor will be -1A and the other port will be 1A; but the component will also calculate an internal parameter i which is 1A and is the actual flow through the resistor. You could naturally calculate such a flow for the component itself if you'd like and think that's more natural than "+ is outgoing, - is incoming".
Thank you. That was a good answer on my question. Kirchhoff strömlag.
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