Hello,

So I worked some more on a spring mass damper model.  I put together a new model based on the example, but with no initial stretch of the spring, but instead have a step input of force at 0.5s which is a more interesting response and kind of along the lines of adding an actuator control the mass at some point.  I attached the model if anyone is interested.

One question I have is when I connect a force a window comes up called "Connect Array" (picture attached for clarity) and says

"Connect step1.y with force.force[index]
Enter index value for force.force"

What am I supposed to enter in this box?  I tried 1,2,3... etc and anything I put gets the following error when I try to run the model

"Error: Type mismatch in binding height = {0, 10, 0}, expected subtype of Real, got type Integer[3]."


I manually edited the modelica code to get the model to run.  I made these changes:

Removed [1] in "force connect(step1.y, force.force[1])" to get "connect(step1.y, force.force)"

Added [3] after step1 to get "Modelica.Blocks.Sources.Step step1[3](height = {0, 10, 0}, startTime = 0.5)"

This got the model to solve, but now I get this warning:

"Warning: Non-array modification '0.5' for array component, possibly due to missing 'each'."

What does the warning mean?

If you think the attached model is helpful I would be happy to to keep posting some models as I am learning how to use the software.

Thanks,
Mark

I didn't quite get the attachments the first time.  Hopefully this edit gets them in the post



spring-mass-stepinput-mf.mo

Hello,

I've started learning how to use open modelica and I'm looking through the examples and trying to build them by myself to try to understand them and I have a few questions concerning using the multibody library.  I've got the example "SpringWithMass" to compile and run, but when I try to re-create it in OMEdit it didn't  quite run the first time.

I looked at the error from why it didn't solve and looked properties of the body in my simulation and I have:

Modelica.Mechanics.MultiBody.Parts.Body body1(m = 1, angles_fixed = true, r_CM = {0, 0, 0}, w_0_fixed = true, animation = false) annotation(Placement(visible = true, transformation(origin = {20, 40}, extent = {{-10, -10}, {10, 10}}, rotation = 90)));

and the example has the following:

Modelica.Mechanics.MultiBody.Parts.Body body(r_0(start = {0, -0.3, 0}, each fixed = true), v_0(each fixed = true), angles_fixed = true, w_0_fixed = true, r_CM = {0, 0, 0}, m = 1) annotation(Placement(transformation(origin = {10, -10}, extent = {{-10, -10}, {10, 10}}, rotation = 270)));

If I change my model to have "r_0(start = {0, -0.3, 0}, each fixed = true)" then it solves.  Which I think gives it an initial guess for r_0 to fix the solver error about the distance between the origin of frame_a and the origin of frame_b

Is there a way to set these variables in the body dialog box?  If I try to enter "r_0(start = {0, -0.3, 0}, each fixed = true)" into the modifiers tab of body I get errors that it isn't formatted right.  I tried some formatting combinations to try and get it to accept it but I couldn't figure out the correct format.

I got the model to solve by manually changing the model in the Text View.  Is this the only way to do it?  Is there another way I can make this change?

Thanks,
Mark