MATLAB: Does NextTimeHit always return 0 in Matlab S-Functions

s-functionsimulink

I need to compute the step size (i.e. dt) in the discrete update method of a Matlab S-Function whose Sample Time is set to be "Inherited" (i.e. [-1 0]). However, regardless of the Sample Time specification (e.g. Inherited, Continuous, Discrete, Variable), the "block.NextTimeHit" always return "0" even though it is decleared as R/W in its Matlab Documentation.

Why does NextTimeHit always return 0 in Matlab S-Functions?
How can I infer the actual step size in update method of (Discrete) Matlab S-Functions for inherited and variable sample times?

Best Answer

the NextTimeHit is the API used specifically for variable discrete rates. You can read more about it in the page:

https://www.mathworks.com/help/simulink/ug/types-of-sample-time.html#brrdmmw-11

To get the actual step size used in the update method, you will have to use Dwork vectors. The link below demonstrates an example for using Dwork vectors:

https://www.mathworks.com/help/simulink/sfg/writing-level-2-matlab-s-functions.html#brgtux6

Related Solutions

MATLAB: How to use Dwork vectors in the S-Function Builder in Simulink 6.0 (R14)

Dwork vectors are not supported in the S-Function Builder.

Execute 'sfundemos' as a starting point to manually write S-Functions.

MATLAB: Does the Simulink Accelerator 6.0 (R14) produce incorrect results when using a C MEX S-function with variable step timing in a Simulink model

This is expected behavior when the sample time is set to inherited: S-functions will only get a Variable Sample time if the sample time is set explicitly and never if the sample time is set to be inherited. When the S-function's sample time is inherited, the mdlGetTimeOfNextVarHit is not called and therefore, the S-function produces unexpected results.

Here are a few suggestions for writing variable step S-functions:

1. Set the sample time of the S-function to variable, i.e.,

ssSetSampleTime(S, 0, VARIABLE_SAMPLE_TIME);

2. Instead of using the ssGetSimMode check, check for a variable step solver using the ssIsVariableStepSolver macro, e.g.,

static void mdlInitializeSampleTimes(SimStruct *S)
{
      if(ssIsVariableStepSolver(S))
     {
         ssSetSampleTime(S, 0, VARIABLE_SAMPLE_TIME);
     }
     else
     {
        ssSetSampleTime(S, 0, INHERITED_SAMPLE_TIME);  
     }
     ssSetOffsetTime(S, 0, 0.0);
}

The code generation that works with variable step solvers (Simulink Accelerator and RSim Target) will work with variable sample time S-functions if the S-function sets the sample time explicitly. Therefore, be sure that the sample time of your S-function is not based solely on the results of the ssGetSimMode macro, which does not query the solver being used for simulation.

Best Answer

Related Solutions

MATLAB: How to use Dwork vectors in the S-Function Builder in Simulink 6.0 (R14)

MATLAB: Does the Simulink Accelerator 6.0 (R14) produce incorrect results when using a C MEX S-function with variable step timing in a Simulink model

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