MATLAB: Changing Simevent attribute using Matlab Function Block

matlab function block delaySimEvents

Hello,

Lets suppose I want to select an Output Switch's port based on the value of an entity's attribute. If Attr == 0 then port 1 is selected, else port 2. There are several ways to do it, but I chose to implement it as shown in the image bellow, using a Get Attribute block a Matlab Function block with a Server in parallel in order to add a delay until Matlab Function's output is calculated.

The Matlab function is the following:

function PortSel = fcn(Attr)
PortSel = (Attr > 0)+1;

My question is why does the Server need a delay of 1 instead of zero in order the system to work as expected?

What I am trying to understand is when the Matlab Function block's code is calculated in order to use it in other situations. For example the same question arises if instead of an output switch I used a Set Attribute Block as stated by Devdatt Lad in this thread.

Best Answer

The Server does not need a delay of 1 for this to work - it should work with a delay of 0. The reason for the extra Server is to hold the entity temporarily while we perform a computation that affects the entity's path (output switch). So a time delay is not required.

If you are not seeing the expected behavior then it could be an issue of event priorities. Here are the sequence of events that will happen above:

Entity arrives at Server, which will schedule a departure event for this entity for timeNow+0. This event will have an event priority as specified in the Server block.
The signal computations will then be executed. This includes the MATLAB Function block and hence a new value will be produced at the "p" port of the Output Switch.
The Output Switch will sense this change and schedule an update event for tNow to switch its selected port. This update event will be at the priority specified in the Output Switch block, if specified.
The system will then schedule the above two events (both for tNow) in the order of ascending event priorities.

So if you are seeing the entity leave from the wrong port, then it is likely that the Departure event has a higher priority than the PortSelection event. Try making the event priority parameter on the Output Switch to be a lower number (higher priority) than the event priority value on the Server. Or alternatively, turn off the checkbox "Resolve simultaneous signal updates using event priority".

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MATLAB: Simevents: change the server capacity during simulation

Hi Kar,

You are correct, Entity Server does not allow for changing the capacity at run-time. Resource Pool block allows changing the resource amount at run-time through a signal port, but it only allows incrementing the available resources in the pool. And not the other way around.

For the variable capacity server, you can try using a MATLAB discrete-event system block and try including a queue of infinite capacity. You can then control whether to allow or disallow an entity from entering the queue by implementing the testEntry method. That way you can control the capacity at run-time. For example you can do something like the following:

function isAllowed = testEntry(obj, storage, ~, ~)
    % Specifies whether entities are allowed into the storage
    isAllowed = obj.mCurrentOccupancy <= obj.mCurrentCapacity;
end
function events = entry(obj, storage, ~, ~)
    % Increment the current occupancy

    obj.mCurrentOccupancy = obj.mCurrentOccupancy + 1;
    
    % Post a forward event after service-time

    events = obj.eventForward('output', 1, obj.mServiceTime);
end
function events = exit(obj, ~, ~, ~)
    % Decrement the current occupancy
    obj.mCurrentOccupancy = obj.mCurrentOccupancy - 1;
    
    if(obj.mCurrentOccupancy < obj.mCurrentCapacity)
        % Since there is room for entities, check if any entity
        % wishes to enter the block.
        events = [events obj.eventTestEntry(1)];
    end
end

In the above example, you can define mCurrentCapacity as a tunable parameter (not recommended). If the capacity decreases, then the new capacity would kick in only after the delta amount of entities would have left the block. You can have a more complex policy based on your particular use-case.

One caveat with the above piece of code is that, when the capacity of the server changes (increases), we need to test for entry (by posting a testEntry event), as there might be entities that were previously blocked out. But there is no way to do that since, we cannot associate an event when a tunable parameter changes. The best way would be to increment the server capacity by using another entity port. This other port would accept an entity that specifies the new capacity. This would enable us to return a testEntry event from the entry method.

function events = entry(obj, storage, ~, ~)
    % Increment the current occupancy
    if storage == 1
        % Book-keeping
        obj.mCurrentOccupancy = obj.mCurrentOccupancy + 1;
        
        % Post a forward event after service-time
        events = obj.eventForward('output', 1, obj.mServiceTime);
    elseif storage == 2
        obj.mCurrentCapacity = entity.data; % Assuming anonymous entity
        % Omitting checks for brevity
        events = [events obj.eventTestEntry(1)];
    end
end

An alternative to using DES block would be to use an infinite capacity server and use an entity gate to control the number of entities entering the server. The gate could be operated by adding some logic which would be driven by the statistics ports of the Entity Server.

Hope this helps!

- Krishna

MATLAB: Access current simulation time in SimEvents blocks

You can create a Simulink Function t = getTime() which contains a Digital Clock block and then call this function from any event action from a SimEvents block to get the current time.

Best Answer

Related Solutions

MATLAB: Simevents: change the server capacity during simulation

MATLAB: Access current simulation time in SimEvents blocks

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