Tuning Custom Masked Subsystems

This example illustrates how to enable custom masked subsystems in a Simulink Compensator Design Task. Once configured a block can be used in the same way as any supported blocks in Simulink Control Design.

Lead-Lag Library Block

The example configured lead-lag block is in the library scdexblks


The lead-lag block implements a compensator element with a single zero and pole. There are three parameters K, wz, and wp which are configured in the block dialog:

The parameters are then used inside the mask create an equivalent transfer function:

$$G(s) = K {{{s \over {wz}} + 1} \over {{s \over wp} + 1}}$$

Configuring the Subsystem for a Simulink Compensator Design Task

The configuration of a masked subsystem for Simulink Control Design is specified in a configuration function. In this example the configuration function has been written in the M-Function scdleadexample.m. This configuration function specifies that:

  • There is only a maximum of 1 pole allowed (MaxPoles Constraint)

  • There is only a maximum of 1 zero allowed (MaxZeros Constraint)

  • The gain is tunable (isStaticGainTunable Constraint)

The configuration is registered in the subsystem using the SCDConfigFcn parameter for the block. This function can be set using the command set_param or through the Block Properties dialog which is accessed by right clicking on any subsystem.

After setting the SCDConfigFcn the block is now ready to be used in a Simulink Compensator Design Task.


The lead-lag block can be used to tune the feedback loop in example entitled "Single Loop Feedback/Prefilter Design". To begin the compensator design process:

Step 1 Start a new Simulink Compensator Design Task for the model scdspeedctrlleadlag by choosing Analysis -> Control Design -> Control System Designer from the Simulink model.

Step 2 Select the following block to tune by clicking the Select Blocks... button on the Tunable Blocks panel:

  • scdspeedctrlleadlag/Feedback Controller/Lead-Lag Controller

open_system('scdspeedctrlleadlag/Feedback Controller')

The lead-lag block will show up inside the subsystem Feedback Controller.

Step 3 Select the closed loop signals by right clicking and using the Linearization Points menu:

  • Input: scdspeedctrlleadlag/Speed Reference output port 1

  • Output scdspeedctrlleadlag/Speed Output output port 1

Step 4 In the Operating Points panel select Default Operating Point.

Step 5 Click on the Tune Blocks... button to launch the Design Configuration Wizard.

To tune the Open Loop at the outport 1 of scdspeedctrlleadlag/Feedback Controller/Lead-Lag Controller, select Open Loop 1 for Plot 1 and select Open-Loop Bode as its Plot Type.

In the wizard select Closed Loop 1 for Plot 2 and select Closed-Loop Bode as its Plot Type.

In step 2 of the wizard select Step responses for Plot 1 and Plot 2. Then in the Contents of Plots table

  • Select Plot 1 for the Closed Loop from /Speed Reference to scdspeedctrl/Speed Output|

After completing the wizard a SISO Design Task node is created. Use this task node to complete the design. Go to the Compensator Editor tab to tune the lead-lag compensator. The block parameters for the lead-lag block can now be tuned.

Completed Design

The design requirements for the reference step response in the example "Single Loop Feedback/Prefilter Design" can be met with the following controller parameters

  • scdspeedctrlleadlag/Feedback Controller/Lead-Lag Controller has parameters:

         Gain = 0.0075426
         Zero Frequency (rad/s) = 2
         Pole Frequency (rad/s) = 103.59

The responses of the closed loop system are shown below:

Writing the Design to Simulink

You can then test the design on the nonlinear model by clicking the Update Simulink Block Parameters button. This writes the parameters back to the Simulink model.

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