MATLAB: Does algebraic loop not use values from previous time step

algebraicdelayerrorloopoutputsimulinkunitwrong

I am expecting the output of a block in an algebraic loop at time step (n-1), to be the input to the next block in the loop at time step n. But it does not use the previous value, and uses a different value. Why is that?

Best Answer

If you want the output of a block in an algebraic loop at time step (n-1), to be the input to the next block in the loop at time step n, then you would have to introduce a memory/state element in the loop.

This can be achieved by introducing a 'Unit Delay' anywhere in the loop, preferably where you know the exact initial conditions.

Related Solutions

MATLAB: What is the cause of “In numerictype(s) s must be a scalar constant or a fi” error

The error is caused by the fact that built-in integers, like the int16 constant in the example model you kindly provided, are not treated as fixed point object.

There is two possible workarounds in this case.

1. You can set the treatment of inherited Simulink signal types to 'Fixed-point & integer' in the Ports and Data Manager, that is available from the toolbar when editing the MATLAB Fucntion Block. This is shown in the image below

2. You can ensure the casting of the input data yourself, so that integers will always be converted to fi objects. The following code segment achieves that and could be used in the MATLAB Function Block:

>> if isinteger(u_in) 
>>     u = fi(u_in); 
>> else 
>>     u = u_in;
>> end

Please note that the input argument has been changed to 'u_in' in order to allow for proper casting.

MATLAB: How to create a Unit Delay block that accepts variable-size signals and does not need to be placed inside a conditionally executed subsystem in Simulink 7.7 (R2011a)

As mentioned in the error message above, the Unit delay block propagates sizes only during initialization of conditionally executed subsystems. This behavior is explained here:

http://www.mathworks.com/help/releases/R2011a/toolbox/simulink/ug/br65qom-1.html

In order to work around this situation, it is possible to implement a different Unit Delay using an S-function.

Attached to this solution is both a Level 2 MATLAB file S-Function and a C S-function which implement a Unit Delay that does need to be placed in a conditionally executed subsystem.

Note that the attached examples illustrate one possible way to implement a Unit Delay. Depending on your specific application you may want to alter what exactly happens when the size of the variable-size signal changes to obtain the behavior you are looking for.

Best Answer

Related Solutions

MATLAB: What is the cause of “In numerictype(s) s must be a scalar constant or a fi” error

MATLAB: How to create a Unit Delay block that accepts variable-size signals and does not need to be placed inside a conditionally executed subsystem in Simulink 7.7 (R2011a)

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