I created a model of a system of ODE's. The system contains 3 functions, say x,y,z I'd like to investigate.
The problem is that the time variable in the system is not
t (time)
but
omega*t
where omega has a fixed value. My question is how can I take the new time variable into considerations in my system? thanks.
Related
I am building model in Dymola. I have defined the mass of this model as a parameter, because it would be transfered into other moduls and called in them. But the mass should be changing during the simulation in different time intervals. For example, during the first 100 seconds the mass should remain 500kg, and during 100 to 200 sec, a passenger is going to get in, so that a new mass should be calculated including the mass of the passenger. But it has been showed, that "The problem is structurally singular", because to the parameter values have been twice assigned. Could someone give some tips to solve this problem? Thanks a lot.
If you define the mass of your component as an input rather than a parameter then you can change it during simulation by assigning e.g. the output from a TimeTable to it. For example
model Component
input Modelica.SIunits.Mass mass "Passenger dependent mass";
equation
...
end Component;
model systemModel
TimeTable timeTable;
Component component(mass=timeTable.y);
OtherComponent otherComponent(mass=component.mass);
equation
...
end systemModel;
Note that the other components using the mass must also have their internal mass 'parameters' defined as input to allow higher variability than parameters.
Best regards
Rene Just Nielsen
Modelica parameters are defined by the fact, that they don't change over time. Therefore you would need to stop the simulation, change the parameter and restart the simulation (see another question). Given you description I would rather not use this possibility, as it seems your variable is designed to change over time.
A better alternative seems to be defining the mass as a variable. If this is done, you can:
Transfer this variable from one model to the others using interfaces. This could be a bit tedious depending on the amount of classes using the variable.
Use inner/outer (basically global variables) is a feasible concept for this use-case. This concept is used in the MultiBody libraries world model.
With both solutions you will have to modify the original mass model, as m would then have to be a variable instead of a mass.
I make a structure using Comsol then I want to make this structure subjected to a temperature variation ( T(begain)=25C then a temperature ramp (100 C/min) till T=250C and it lasts for 30 min then another temperature ramp (-100 C/min) till T=25C ).How could I make these temperature sweep?
You can define a function (e.g foo) that follows exactly your desired temperature with time profile. Then in the place where you specify your temperature (whether it is a boundary condition or domain condition) you insert foo(t), t being COMSOL's exclusive variable name for time.
You can do that for other variables too, space for instance. The easiest way to define foo is through the 1D interpolation option. Unfortunately, I do not currently have a COMSOL license to check it but I think you can simply enter the time and temperature values in the 1D interpolation table, choose a name and the interpolant style and just use it in the later part of the program.
I'am simulating magnetic fields in time domain with moving coils. Time dependent solver is needed for the movement and for temperature ramps as well. I think that you can use something like this, T=T_start+rate_of_change*t. The t variable is available with the time dependent solver and you can simply write the equation I mentioned. However, I think that you need to use time dependent solver three times, one for ramp up second for the constant temperature and third for the ramp down. Set the times for time dependent solvers so that you can made the desired temperatures.
First t=0s->(225/100*60)135s
second t=135s->(135+30*60)1935s
and last one t=1935s->(1935+135)2070s
You might also need to use compile solutions steps as well to add these three solutions together. I can try to do this tomorrow and check it.
Hope that this helped a bit
What am I trying to do is to save in a variable (global or constant) the system time. I am using a S-function in Simulink. The problem is that when I store the value of the system time in a variable it is continuously incrementing so when I do the difference between the current system time and the time stored in my variable is always 0. What do you think is the solution storing the system time in a variable and what type of variable should I use a global one or a constant. If you have any answer please give me an example because I am new in Matlab.
P.S I am using C language for the S-function.
It sounds as if you are trying to store the system time at the start of the simulation, then during the simulation compare the system time to that stored value. If so, then you should be using anR-Work vector to store the initial system time.
So in mdlInitializeSizes you want
ssSetNumRWork(S, 1);
Then in mdlStart you want
real_T *P_Tinit=ssGetRWork(S);
P_Tinit[0]=((real_T) clock())/CLOCKS_PER_SEC;
Then when you want to use the value use,
real_T itime;
itime=ssGetRWorkValue(S,0);
(The above assumes that you know how to actually get the system time, i.e. include the correct libraries, which from your question it sounds as if you do.)
How can I determine value from previous time step during simulation in Modelica?
I have equation Q=m*c*(Ts2-Ts1-Tr) I need to extract value of Ts2 and Ts1 from it.
Ts2 - is the value from time step 2
Ts1 - is value from previous time step
Ts is input signal and it has variations during the time. Each step
it has different value. In my case time step is 1s. Other values are
fixed.
Can I set in equation variable time?
For example:
Ts2 (start=time);
Ts (start=time-1);
Or it should be input inside this model?
regards Tymofii
This was addressed in a similar question already.
The key point is that equations describing physical behavior cannot refer to time steps. This is because there is no "timestep" in nature or the laws of physics and so the response of a system cannot depend on it.
You don't really explain why you need to do what you are doing. Are you trying to extract simulation results? Are you trying to correlate to experimental data? Or, are you just trying to solve a differential equation?
It isn't clear what you want to do. Please elaborate and we can probably give you some guidance on how to proceed in Modelica.
Update
Using values from a "previous interval" is fine. For example, if you wanted to sample your solution at regular intervals, express a "z transform" or implement a Kalman Filter in Modelica, you could do each of those very easily (for example, see the 'sample' keyword here). In other words, it is possible to store as many previous values as you would like.
What you cannot do is use the timestep of the continuous solver in expressing how your system behaves. The intervals you reference must be independent from any intervals that the solver is using.
Is it possible to use the previous value of the time varying variable
for eg:
Suppose I have pipe whose inlet temperature is 298K with a specified uniform mass flow(m_flow), now suppose i am heating the pipe using a heater of 100 watts.
The outlet temperature will be attain a higher temperature of suppose 302K, now if i have to use this outlet temperature as my inlet temperature (in the sense i am recircuilating the water), how would i be doing it?
is it possible to update the value of the inlet temperature based on the outlet temperature at the previous timestep? so that for the next iteration the inlet temperature will be the same as the oulet temperature in the previous iteration (in other words the fluid would be recirculating).
Thanks
You cannot access the value in the previous time step. The closest you can get in Modelica is using delay(exp,T) to get the value T units of time ago.
The timestep does not enter into it at all. A model that uses information about timestep is just wrong. Nature doesn't know or care about integration time steps, the model should reflect that.
It seems to me what you want to capture is transport delay. Transport delay is the delay introduced by the time it takes for molecules, electrons, etc. through the system. So presumably what you wish to model is the time it takes the inlet fluid to reach the exit. Again, this has nothing to do with the integration timestep but rather the velocity of the fluid and the distance it must travel. Once you know how long that takes (by either a priori knowledge of the system of by looking at the simulation results themselves), you can follow Marco's suggestion of using the delay operator.
In order to setup a proper model for the system you described I suggest you to look at the example :
Modelica.Thermal.FluidHeatFlow.Examples.IndirectCooling
of the modelica standard library ver. 3.2. Instead of one pipe you can put an ambient or control volume component to better suit you needs. Moreover using continous and differentiable equations (the delay function is not) you will benefit from some of the advantages of the Modelica code, e.g. you will be able to reuse your models in a much wider range of cases, solve inverse problems, solve initial value problems, ...
I hope this helps,
Marco