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Create a Custom Block

How to Design a Custom Block

In general, use the following process to design a custom block:

  1. Defining Custom Block Behavior

  2. Deciding on a Custom Block Type

  3. Placing Custom Blocks in a Library

  4. Adding a User Interface to a Custom Block

Suppose you want to create a customized saturation block that limits the upper and lower bounds of a signal based on either a block parameter or the value of an input signal. In a second version of the block, you want the option to plot the saturation limits after the simulation is finished. The following tutorial steps you through designing these blocks. The library customsat_libcustomsat_lib contains the two versions of the customized saturation block.

The example model sldemo_customsatsldemo_customsat uses the basic version of the block.

Defining Custom Block Behavior

Begin by defining the features and limitations of your custom block. In this example, the block supports the following features:

  • Turning on and off the upper or lower saturation limit.

  • Setting the upper and/or lower limits via a block parameters.

  • Setting the upper and/or lower limits using an input signal.

It also has the following restrictions:

  • The input signal under saturation must be a scalar.

  • The input signal and saturation limits must all have a data type of double.

  • Code generation is not required.

Deciding on a Custom Block Type

Based on the custom block features, the implementation needs to support the following:

  • Multiple input ports

  • A relatively simple algorithm

  • No continuous or discrete system states

Therefore, this tutorial implements the custom block using a Level-2 MATLAB® S-function. MATLAB S-functions support multiple inputs and, because the algorithm is simple, do not have significant overhead when updating the diagram or simulating the model. See Comparison of Custom Block Functionality for a description of the different functionality provided by MATLAB S-functions as compared to other types of custom blocks.

Parameterizing the MATLAB S-Function

Begin by defining the S-function parameters. This example requires four parameters:

  • The first parameter indicates how the upper saturation limit is set. The limit can be off, set via a block parameter, or set via an input signal.

  • The second parameter is the value of the upper saturation limit. This value is used only if the upper saturation limit is set via a block parameter. In the event this parameter is used, you should be able to change the parameter value during the simulation, i.e., the parameter is tunable.

  • The third parameter indicates how the lower saturation limit is set. The limit can be off, set via a block parameter, or set via an input signal.

  • The fourth parameter is the value of the lower saturation limit. This value is used only if the lower saturation limit is set via a block parameter. As with the upper saturation limit, this parameter is tunable when in use.

The first and third S-function parameters represent modes that must be translated into values the S-function can recognize. Therefore, define the following values for the upper and lower saturation limit modes:

  • 1 indicates that the saturation limit is off.

  • 2 indicates that the saturation limit is set via a block parameter.

  • 3 indicates that the saturation limit is set via an input signal.

Writing the MATLAB S-Function

After you define the S-function parameters and functionality, write the S-function. The template msfuntmpl.mmsfuntmpl.m provides a starting point for writing a Level-2 MATLAB S-function. You can find a completed version of the custom saturation block in the file custom_sat.mcustom_sat.m. Save this file to your working folder before continuing with this tutorial.

This S-function modifies the S-function template as follows:

  • The setup function initializes the number of input ports based on the values entered for the upper and lower saturation limit modes. If the limits are set via input signals, the method adds input ports to the block. The setup method then indicates there are four S-function parameters and sets the parameter tunability. Finally, the method registers the S-function methods used during simulation.

    function setup(block)
    
    % The Simulink engine passes an instance of the Simulink.MSFcnRunTimeBlock
    % class to the setup method in the input argument "block". This is known as
    % the S-function block's run-time object.
    
    % Register original number of input ports based on the S-function
    % parameter values
    
    try % Wrap in a try/catch, in case no S-function parameters are entered
        lowMode    = block.DialogPrm(1).Data;
        upMode     = block.DialogPrm(3).Data;
        numInPorts = 1 + isequal(lowMode,3) + isequal(upMode,3);
    catch
        numInPorts=1;
    end % try/catch
    block.NumInputPorts = numInPorts;
    block.NumOutputPorts = 1;
    
    % Setup port properties to be inherited or dynamic
    block.SetPreCompInpPortInfoToDynamic;
    block.SetPreCompOutPortInfoToDynamic;
    
    % Override input port properties
    block.InputPort(1).DatatypeID  = 0;  % double
    block.InputPort(1).Complexity  = 'Real';
    
    % Override output port properties
    block.OutputPort(1).DatatypeID  = 0; % double
    block.OutputPort(1).Complexity  = 'Real';
    
    % Register parameters. In order:
    % -- If the upper bound is off (1) or on and set via a block parameter (2)
    %    or input signal (3)
    % -- The upper limit value. Should be empty if the upper limit is off or
    %    set via an input signal
    % -- If the lower bound is off (1) or on and set via a block parameter (2)
    %    or input signal (3)
    % -- The lower limit value. Should be empty if the lower limit is off or
    %    set via an input signal
    block.NumDialogPrms     = 4;
    block.DialogPrmsTunable = {'Nontunable','Tunable','Nontunable', ...
        'Tunable'};
    
    % Register continuous sample times [0 offset]
    block.SampleTimes = [0 0];
    
    %% -----------------------------------------------------------------
    %% Options
    %% -----------------------------------------------------------------
    % Specify if Accelerator should use TLC or call back into
    % MATLAB script
    block.SetAccelRunOnTLC(false);
    
    %% -----------------------------------------------------------------
    %% Register methods called during update diagram/compilation
    %% -----------------------------------------------------------------
    
    block.RegBlockMethod('CheckParameters',      @CheckPrms);
    block.RegBlockMethod('ProcessParameters',    @ProcessPrms);
    block.RegBlockMethod('PostPropagationSetup', @DoPostPropSetup);
    block.RegBlockMethod('Outputs',              @Outputs);
    block.RegBlockMethod('Terminate',            @Terminate);
    %end setup function
    
  • The CheckParameters method verifies the values entered into the Level-2 MATLAB S-Function block.

    function CheckPrms(block)
    
    lowMode = block.DialogPrm(1).Data;
    lowVal  = block.DialogPrm(2).Data;
    upMode  = block.DialogPrm(3).Data;
    upVal   = block.DialogPrm(4).Data;
    
    % The first and third dialog parameters must have values of 1-3
    if ~any(upMode == [1 2 3]);
        error('The first dialog parameter must be a value of 1, 2, or 3');
    end
    
    if ~any(lowMode == [1 2 3]);
        error('The first dialog parameter must be a value of 1, 2, or 3');
    end
    
    % If the upper or lower bound is specified via a dialog, make sure there
    % is a specified bound. Also, check that the value is of type double
    if isequal(upMode,2),
        if isempty(upVal),
            error('Enter a value for the upper saturation limit.');
        end
        if ~strcmp(class(upVal), 'double')
            error('The upper saturation limit must be of type double.');
        end
    end
    
    if isequal(lowMode,2),
        if isempty(lowVal),
            error('Enter a value for the lower saturation limit.');
        end
        if ~strcmp(class(lowVal), 'double')
            error('The lower saturation limit must be of type double.');
        end
    end
    
    % If a lower and upper limit are specified, make sure the specified
    % limits are compatible.
    if isequal(upMode,2) && isequal(lowMode,2),
        if lowVal >= upVal,
            error('The lower bound must be less than the upper bound.');
        end
    end
    
    %end CheckPrms function
    
  • The ProcessParameters and PostPropagationSetup methods handle the S-function parameter tuning.

    function ProcessPrms(block)
    
    %% Update run time parameters
    block.AutoUpdateRuntimePrms;
    
    %end ProcessPrms function
    
    
    function DoPostPropSetup(block)
    
    %% Register all tunable parameters as runtime parameters.
    block.AutoRegRuntimePrms;
    
    %end DoPostPropSetup function
  • The Outputs method calculates the block's output based on the S-function parameter settings and any input signals.

    function Outputs(block)
    
    lowMode    = block.DialogPrm(1).Data;
    upMode     = block.DialogPrm(3).Data;
    sigVal     = block.InputPort(1).Data;
    lowPortNum = 2; % Initialize potential input number for lower saturation limit
    
    % Check upper saturation limit
    if isequal(upMode,2), % Set via a block parameter
        upVal = block.RuntimePrm(2).Data;
    elseif isequal(upMode,3), % Set via an input port
        upVal = block.InputPort(2).Data;
        lowPortNum = 3; % Move lower boundary down one port number
    else
        upVal = inf;
    end
    
    % Check lower saturation limit
    if isequal(lowMode,2), % Set via a block parameter
        lowVal = block.RuntimePrm(1).Data;
    elseif isequal(lowMode,3), % Set via an input port
        lowVal = block.InputPort(lowPortNum).Data;
    else
        lowVal = -inf;
    end
    
    % Assign new value to signal
    if sigVal > upVal,
        sigVal = upVal;
    elseif sigVal < lowVal,
        sigVal=lowVal;
    end
    
    block.OutputPort(1).Data = sigVal;
    
    %end Outputs function

Placing Custom Blocks in a Library

Libraries allow you to share your custom blocks with other users, easily update the functionality of copies of the custom block, and collect blocks for a particular project into a single location. This example places the custom saturation block into a library.

  1. In the Simulink® Library Browser, select File > New > Library.

  2. From the User-Defined Functions library, drag a Level-2 MATLAB S-Function block into your new library.

  3. Save your library with the filename saturation_lib.

  4. Double-click the block to open its Function Block Parameters dialog box.

  5. In the S-function name field, enter the name of the S-function. For example, enter custom_sat. In the Parameters field enter 2,-1,2,1.

  6. Click OK.

    You have created a custom saturation block that you can share with other users.

You can make the block easier to use by adding a customized user interface.

Adding a User Interface to a Custom Block

You can create a block dialog box for a custom block using the masking features of Simulink. Masking the block also allows you to add port labels to indicate which ports corresponds to the input signal and the saturation limits.

  1. Open the library saturation_lib that contains the custom block you created,

  2. Right-click the Level-2 MATLAB S-Function block and select Mask > Create Mask.

  3. On the Icon & Ports pane in the Icons drawing commands box, enter port_label('input',1,'uSig'), and then click Apply.

    This command labels the default port as the input signal under saturation.

  4. In the Parameters & Dialog pane, add four parameters corresponding to the four S-Function parameters. For each new parameter, drag a popup or edit control to the Dialog box section, as shown in the table. Drag each parameter into the Parameters group.

    TypePromptNameEvaluateTunablePopup optionsCallback
    popupUpper boundary:upMode 

    No limit

    Enter limit as parameter

    Limit using input signal

    customsat_callback('upperbound_callback', gcb)
    editUpper limit:upValN/Acustomsat_callback('upperparam_callback', gcb)

    TypePromptNameEvaluateTunablePopup optionsCallback
    popupLower boundary:lowMode 

    No limit

    Enter limit as parameter

    Limit using input signal

    customsat_callback('lowerbound_callback', gcb)
    editLower limit:lowValN/Acustomsat_callback('lowerparam_callback', gcb)

    The MATLAB S-Function script custom_sat_final.mcustom_sat_final.m contains the mask parameter callbacks.Save custom_sat_final.m to your working folder to define the callbacks in this example. This MATLAB script has two input arguments. The first input argument is a string indicating which mask parameter invoked the callback. The second input argument is the handle to the associated Level-2 MATLAB S-Function block.

    The figure shows the completed Parameters & Dialog pane in the Mask Editor.

  5. In the Initialization pane, select the Allow library block to modify its contents check box. This setting allows the S-function to change the number of ports on the block.

  6. In the Documentation pane:

    • In the Mask type field, enter

      Customized Saturation
    • In the Mask description field, enter

      Limit the input signal to an upper and lower saturation value
      set either through a block parameter or input signal.
  7. Click OK.

  8. To map the S-function parameters to the mask parameters, right-click the Level-2 MATLAB S-Function block and select Mask > Look Under Mask.

  9. Change the S-function name field to custom_sat_final and the Parameters field to lowMode,lowVal,upMode,upVal.

    The figure shows the Function Block Parameters dialog box after the changes.

  10. Click OK. Save and close the library to exit the edit mode.

  11. Reopen the library and double-click the customized saturation block to open the masked parameter dialog box.

To create a more complicated user interface, place a Handle Graphics user interface on top of the masked block. The block OpenFcn invokes the Handle Graphics user interface, which uses calls to set_param to modify the S-function block parameters based on settings in the user interface.

Writing the Mask Callback

The function customsat_callback.mcustomsat_callback.m contains the mask callback code for the custom saturation block mask parameter dialog box. This function invokes local functions corresponding to each mask parameter through a call to feval.

The following local function controls the visibility of the upper saturation limit's field based on the selection for the upper saturation limit's mode. The callback begins by obtaining values for all mask parameters using a call to get_param with the property name MaskValues. If the callback needed the value of only one mask parameter, it could call get_param with the specific mask parameter name, for example, get_param(block,'upMode'). Because this example needs two of the mask parameter values, it uses the MaskValues property to reduce the calls to get_param.

The callback then obtains the visibilities of the mask parameters using a call to get_param with the property name MaskVisbilities. This call returns a cell array of strings indicating the visibility of each mask parameter. The callback alters the values for the mask visibilities based on the selection for the upper saturation limit's mode and then updates the port label string.

The callback finally uses the set_param command to update the block's MaskDisplay property to label the block's input ports.

function customsat_callback(action,block)
% CUSTOMSAT_CALLBACK contains callbacks for custom saturation block

%   Copyright 2003-2007 The MathWorks, Inc.

%% Use function handle to call appropriate callback
feval(action,block)

%% Upper bound callback
function upperbound_callback(block)

vals = get_param(block,'MaskValues');
vis = get_param(block,'MaskVisibilities');
portStr = {'port_label(''input'',1,''uSig'')'};
switch vals{1}
    case 'No limit'
        set_param(block,'MaskVisibilities',[vis(1);{'off'};vis(3:4)]);
    case 'Enter limit as parameter'
        set_param(block,'MaskVisibilities',[vis(1);{'on'};vis(3:4)]);
    case 'Limit using input signal'
        set_param(block,'MaskVisibilities',[vis(1);{'off'};vis(3:4)]);
        portStr = [portStr;{'port_label(''input'',2,''up'')'}];
end
if strcmp(vals{3},'Limit using input signal'),
    portStr = [portStr;{['port_label(''input'',',num2str(length(portStr)+1), ...
        ',''low'')']}];
end
set_param(block,'MaskDisplay',char(portStr));

The final call to set_param invokes the setup function in the MATLAB S-function custom_sat.m. Therefore, the setup function can be modified to set the number of input ports based on the mask parameter values instead of on the S-function parameter values. This change to the setup function keeps the number of ports on the Level-2 MATLAB S-Function block consistent with the values shown in the mask parameter dialog box.

The modified MATLAB S-function custom_sat_final.mcustom_sat_final.m contains the following new setup function. If you are stepping through this tutorial, open the file and save it to your working folder.

%% Function: setup ===================================================
function setup(block)

% Register original number of ports based on settings in Mask Dialog
ud = getPortVisibility(block);
numInPorts = 1 + isequal(ud(1),3) + isequal(ud(2),3);

block.NumInputPorts = numInPorts;
block.NumOutputPorts = 1;

% Setup port properties to be inherited or dynamic
block.SetPreCompInpPortInfoToDynamic;
block.SetPreCompOutPortInfoToDynamic;

% Override input port properties
block.InputPort(1).DatatypeID  = 0;  % double
block.InputPort(1).Complexity  = 'Real';

% Override output port properties
block.OutputPort(1).DatatypeID  = 0; % double
block.OutputPort(1).Complexity  = 'Real';

% Register parameters. In order:
% -- If the upper bound is off (1) or on and set via a block parameter (2)
%    or input signal (3)
% -- The upper limit value. Should be empty if the upper limit is off or
%    set via an input signal
% -- If the lower bound is off (1) or on and set via a block parameter (2)
%    or input signal (3)
% -- The lower limit value. Should be empty if the lower limit is off or
%    set via an input signal
block.NumDialogPrms     = 4;
block.DialogPrmsTunable = {'Nontunable','Tunable','Nontunable','Tunable'};

% Register continuous sample times [0 offset]
block.SampleTimes = [0 0];

%% -----------------------------------------------------------------
%% Options
%% -----------------------------------------------------------------
% Specify if Accelerator should use TLC or call back into
% MATLAB script
block.SetAccelRunOnTLC(false);

%% -----------------------------------------------------------------
%% Register methods called during update diagram/compilation
%% -----------------------------------------------------------------

block.RegBlockMethod('CheckParameters',      @CheckPrms);
block.RegBlockMethod('ProcessParameters',    @ProcessPrms);
block.RegBlockMethod('PostPropagationSetup', @DoPostPropSetup);
block.RegBlockMethod('Outputs',              @Outputs);
block.RegBlockMethod('Terminate',            @Terminate);
%endfunction

The getPortVisibility local function in custom_sat_final.m uses the saturation limit modes to construct a flag that is passed back to the setup function. The setup function uses this flag to determine the necessary number of input ports.

%% Function: Get Port Visibilities =======================================
function ud = getPortVisibility(block)

ud = [0 0];

vals = get_param(block.BlockHandle,'MaskValues');
switch vals{1}
    case 'No limit'
        ud(2) = 1;
    case 'Enter limit as parameter'
        ud(2) = 2;
    case 'Limit using input signal'
        ud(2) = 3;
end

switch vals{3}
    case 'No limit'
        ud(1) = 1;
    case 'Enter limit as parameter'
        ud(1) = 2;
    case 'Limit using input signal'
        ud(1) = 3;
end

Adding Block Functionality Using Block Callbacks

The User-Defined Saturation with Plotting block in customsat_libcustomsat_lib uses block callbacks to add functionality to the original custom saturation block. This block provides an option to plot the saturation limits when the simulation ends. The following steps show how to modify the original custom saturation block to create this new block.

  1. Add a check box to the mask parameter dialog box to toggle the plotting option on and off.

    1. Right-click the Level-2 MATLAB S-Function block in saturation_lib and select Mask + Create Mask.

    2. On the Mask Editor Parameters pane, add a fifth mask parameter with the following properties.

      PromptNameTypeTunableType optionsCallback
      Plot saturation limitsplotcheckcheckboxNoNAcustomsat_callback('plotsaturation',gcb)

    3. Click OK.

  2. Write a callback for the new check box. The callback initializes a structure to store the saturation limit values during simulation in the Level-2 MATLAB S-Function block UserData. The MATLAB script customsat_plotcallback.mcustomsat_plotcallback.m contains this new callback, as well as modified versions of the previous callbacks to handle the new mask parameter. If you are following through this example, open customsat_plotcallback.mcustomsat_plotcallback.m and copy its local functions over the previous local functions in customsat_callback.m.

    %% Plotting checkbox callback
    function plotsaturation(block)
    
    % Reinitialize the block's userdata
    vals = get_param(block,'MaskValues');
    ud = struct('time',[],'upBound',[],'upVal',[],'lowBound',[],'lowVal',[]);
    
    if strcmp(vals{1},'No limit'),
        ud.upBound = 'off';
    else
        ud.upBound = 'on';
    end
    
    if strcmp(vals{3},'No limit'),
        ud.lowBound = 'off';
    else
        ud.lowBound = 'on';
    end
    
    set_param(gcb,'UserData',ud);
  3. Update the MATLAB S-function Outputs method to store the saturation limits, if applicable, as done in the new MATLAB S-function custom_sat_plot.mcustom_sat_plot.m. If you are following through this example, copy the Outputs method in custom_sat_plot.m over the original Outputs method in custom_sat_final.m

    %% Function: Outputs ===================================================
    function Outputs(block)
    
    lowMode    = block.DialogPrm(1).Data;
    upMode     = block.DialogPrm(3).Data;
    sigVal     = block.InputPort(1).Data;
    vals = get_param(block.BlockHandle,'MaskValues');
    plotFlag = vals{5};
    lowPortNum = 2;
    
    % Check upper saturation limit
    if isequal(upMode,2)
        upVal = block.RuntimePrm(2).Data;
    elseif isequal(upMode,3)
        upVal = block.InputPort(2).Data;
        lowPortNum = 3; % Move lower boundary down one port number
    else
        upVal = inf;
    end
    
    % Check lower saturation limit
    if isequal(lowMode,2),
        lowVal = block.RuntimePrm(1).Data;
    elseif isequal(lowMode,3)
        lowVal = block.InputPort(lowPortNum).Data;
    else
        lowVal = -inf;
    end
    
    % Use userdata to store limits, if plotFlag is on
    if strcmp(plotFlag,'on');    
        ud = get_param(block.BlockHandle,'UserData');
        ud.lowVal = [ud.lowVal;lowVal];
        ud.upVal = [ud.upVal;upVal];
        ud.time = [ud.time;block.CurrentTime];
        set_param(block.BlockHandle,'UserData',ud)
    end
    
    % Assign new value to signal
    if sigVal > upVal,
        sigVal = upVal;
    elseif sigVal < lowVal,
        sigVal=lowVal;
    end
    
    block.OutputPort(1).Data = sigVal;
    
    %endfunction
  4. Write the function plotsat.mplotsat.m to plot the saturation limits. This function takes the handle to the Level-2 MATLAB S-Function block and uses this handle to retrieve the block's UserData. If you are following through this tutorial, save plotsat.m to your working folder.

    function plotSat(block)
    
    % PLOTSAT contains the plotting routine for custom_sat_plot
    %   This routine is called by the S-function block's StopFcn.
    
    ud = get_param(block,'UserData');
    fig=[];
    if ~isempty(ud.time)
        if strcmp(ud.upBound,'on')
            fig = figure;
            plot(ud.time,ud.upVal,'r');
            hold on
        end
        if strcmp(ud.lowBound,'on')
            if isempty(fig),
                fig = figure;
            end
            plot(ud.time,ud.lowVal,'b');
        end
        if ~isempty(fig)
            title('Upper bound in red. Lower bound in blue.')
        end
        
        % Reinitialize userdata
        ud.upVal=[];
        ud.lowVal=[];
        ud.time = [];
        set_param(block,'UserData',ud);
    end
  5. Right-click the Level-2 MATLAB S-Function block and select Properties. The Block Properties dialog box opens. On the Callbacks pane, modify the StopFcn to call the plotting callback as shown in the following figure, then click OK.

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