Legacy C code function o/p mismatch in simulink

Hi,

I am facing a unique problem here, must be some mistake of mine but I'm not able to see it.

I am using a legacy c function which is a mathematical model of an engine providing thrust as output. Now here is the problem:

When I am running the function as a c project independently, with some constant i/p's for one sample time, it is giving me correct results. In Simulink, after reading the documentation, I've made a s-function using the s-function builder as well as the legacy code tool. I am running the model with the same set of constant i/p's for one sample time(solver:fixed step), it is giving some completely different results, whereas the same function when run in a c compiler is giving correct results. I'm trying all the mentioned methods as mentioned to create the s-functions, but it is ending with the same result.I've tried with all the fixed step solvers just to check, but its the same.

I have also tried creating a mex function which I am calling from an embedded matlab function, this one is giving me correct results! Since I don't want to include an embedded matlab function in my model, s-function is my preference.

My objective is to include this legacy function in an aerospace non-real time simulation model which I have already made which uses the fixed step ODE4(R-K method) as the solver.

Will be really glad if some one sheds any light on this.

Regards, Devjit

Answer 1

[Kaustubha Govind]

It's hard to tell just from your description, but it looks like you might not be setting up your S-function correctly. What is your function's prototype? Have you ensured that the S-function's inputs and outputs have been configured with the right type/size? Have you examined the S-function mdlOutputs function to ensure that your function is getting called correctly?

I would also recommend trying to debug your S-function with a C/C++ debugging tool.

[Devjit Naha]

@Kaustubha: Thanks for your reply. I have checked the ip/op types, i am using a set of constants as i/p just for a start before connecting it to the bus. Also, if I use the legacy code tool to create the S-function, the i/p & o/p ports don't have to be written manually I guess? When I am using the S-function builder or writing it manually then for a constant i/p and subsequent o/p, I am keeping the port width as 1.

I'll keep on trying though :-). Another question: Suppose I have created a mex function out of my c program. How do you I include that in my simulink model? Is it mandatory to use S-functions?

[Kaustubha Govind]

A MEX-file can be treated like any other MATLAB function. You can either call it directly using an Interpreted MATLAB Function (http://www.mathworks.com/help/toolbox/simulink/slref/interpretedmatlabfunction.html) or from a C S-function using mexCallMATLAB (http://www.mathworks.com/help/techdoc/apiref/mexcallmatlab.html).

[Kaustubha Govind]

With the legacy code tool, the input/output types/sizes are parsed from the OutputFcnSpec.

[Devjit Naha]

Thanks for your reply! It has helped me indeed :-)