2nd Order Differential Problems

Question

Hey I'm creating a second order differential solver and I'm using the differential solver VI's that come with the package. However, the solver won't do 2nd Order ODE's that are critically damped. Does anyone know what this problem is and how I can remedy it? I put a copy of my program here to show you the problem. For an example of a critically damped equation, input 4 in both "dy/dt" and "y". Thanks.
Attachments:
2nd_Order_Solving2.vi ‏170 KB

Answer 1

Hey Jay,
The modification I made is not that complex, so I'll explain what I did step by step. Before I go through the process, I should offer a disclaimer.
When modifying ODE Linear nth Order Numeric VI, you may want to save a local copy for yourself. This will ensure that the shipping version of the VI is not altered. If you modify the shipping version, then leave it that way, you may run into errors when you try to use the VI in the future.
Also remember to verify the results of the critically damped case. Since the VI is not designed for ODE's with repeated eigenvalues, it's possible that you could be getting incorrect results after the modification. With that said, here's the procedure I used to modify the VI so that it runs even with repea
ted eigenvalues.
1. The modification I made was to the ODE Linear nth Order Numeric.vi. You can access the VI's code by simply double clicking on it from your VI.
2. The change I made is located in the red circle on the bitmap I sent you.
3. Look for the "Multiple Roots.vi", the gray one with a sigma in the upper left corner and some arrows and dots on it. The output of this VI is a boolean, and it is connected to a not gate.
4. Delete the wire from the output of the "Multiple Roots.vi" to the not gate.
5. Right click the input port of the not gate and create a constant.
6. Make sure that the boolean constant connected to the not gate is false.
This is what I did to ensure that the VI will run regardless of the eigenvalues. Please be aware of the issues I addressed above. Otherwise, good luck!
Marcus G.
National Instruments