Main Content

Tune Custom Masked Subsystems

This example shows how to enable custom masked subsystems in Control System Designer. Once configured, you can tune a custom masked subsystem in the same way as any supported blocks in Simulink Control Design. For more information, see What Blocks Are Tunable?.

Lead-Lag Library Block

For this example, tune the Lead-Lag Controller block in the scdexblks library.


This block implements a compensator with a single zero, a single pole, and a gain. To open the Block Parameters dialog box, add the Lead-Lag Controller block to your model, and double-click the block.

The block uses the specified Gain, K, Zero Frequency, wz, and Pole Frequency, wp, to implement the compensator transfer function:

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

Configure the Subsystem for Control System Designer

To configure a masked subsystem for tuning with Control System Designer, you specify a configuration function. In this example, use the configuration function in scdleadexample.m. To open this file, at the MATLAB® command line, type edit scdleadexample. This configuration functions specifies that:

  • There is only one pole allowed (MaxPoles constraint)

  • There is only one zero allowed (MaxZeros constraint)

  • The gain is tunable (isStaticGainTunable constraint)

Register the configuration in the subsystem using the SCDConfigFcn block callback function. Right-click the Lead-Lag Controller block and select Properties. In the Block Properties dialog box, on the Callbacks tab, set SCDConfigFcn.

Alternatively, you can set SCDConfigFcn using the command set_param.

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


The scdspeedctrlleadlag model uses the Lead-Lag Controller block to tune the feedback loop in Single Loop Feedback/Prefilter Compensator Design. In this model, the SCDConfigFcn property is already set.


Step 1 To open Control System Designer, in the Simulink model window, on the Apps tab, click Control System Designer.

Step 2 In the Edit Architecture dialog box, on the Blocks tab, click Add Blocks. In the Select Blocks to Tune dialog box, click Feedback Controller, and select Lead-Lag Controller.

Step 3 On the Signals tab, the analysis points defined in the Simulink model are automatically added as Locations.

  • Input: scdspeedctrlleadlag/Speed Reference output port 1

  • Output scdspeedctrlleadlag/Plant Model output port 1

Step 4 On the Linearization Options tab, in the Operating Point drop-down list, select Model Initial Condition.

Step 5 Create new plots to view the step responses while tuning the controllers.

  • In the Control System Designer, click New Plot, and select New Step. In the Select Response to Plot drop-down menu, select New Input-Output Transfer Response. Configure the response as follows:

To view the response, click Plot.

Tune Compensators

The Control System Designer app contains four methods to tune a control system:

  • Graphically tune the compensator poles, zeros, and gains using open/closed-loop Bode, root locus, or Nichols editor plots. Click Tuning Methods, and select an editor under Graphical Tuning.

  • Compute initial compensator parameters using automated tuning based on parameters such as closed-loop time constants. Click Tuning Methods, and select either PID tuning, IMC tuning, Loop shaping (requires Robust Control Toolbox™ software), or LQG synthesis.

Complete Design

The design requirements for the reference step response in Single Loop Feedback/Prefilter Compensator Design can be met with the following Lead-Lag Controller block 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:

Update Simulink Model

To write the compensator parameters back to the Simulink model, click Update Blocks. You can then test your design on the nonlinear model.


See Also