Quick Loop Tuning

Purpose

Tune SISO or MIMO feedback loops using a loop-shaping approach in Control System Tuner.

Description

Quick Loop Tuning lets you tune your system to meet open-loop gain crossover and stability margin requirements without explicitly creating tuning goals that capture these requirements. You specify the feedback loop whose open-loop gain you want to shape by designating the actuator signals (controls) and sensor signals (measurements) that form the loop. Actuator signals are the signals that drive the plant. The sensor signals are the plant outputs that feed back into the controller.

You enter the target loop bandwidth and desired gain and phase margins. You can also specify constraints on pole locations of the tuned system, to eliminate fast dynamics. Control System Tuner automatically creates Tuning Goals that capture your specifications and ensure integral action at frequencies below the target loop bandwidth.

Creation

In the Tuning tab of Control System Tuner, select New Goal > Quick Loop Tuning to specify loop-shaping requirements.

Command-Line Equivalent

When tuning control systems at the command line, use looptune (for slTuner) (Simulink Control Design) or looptune for tuning feedback loops using a loop-shaping approach.

Feedback Loop Selection

Use this section of the dialog box to specify input, output, and loop-opening locations for evaluating the tuning goal.

Specify actuator signals (controls)
Designate one or more signals in your model as actuator signals. These are the input signals that drive the plant. To tune a SISO feedback loop, select a single-valued input signal. To tune MIMO loop, select multiple signals or a vector-valued signal.
Specify sensor signals (measurements)
Designate one or more signals in your model as sensor signals. These are the plant outputs that provide feedback into the controller. To tune a SISO feedback loop, select a single-valued input signal. To tune MIMO loop, select multiple signals or a vector-valued signal.
Compute the response with the following loops open
Designate additional locations at which to open feedback loops for the purpose of tuning the loop defined by the control and measurement signals.
Quick Loop Tuning tunes the open-loop response of the loop defined by the control and measurement signals. If you want your specifications for that loop to apply with other feedback loops in the system opened, specify loop-opening locations in this section of the dialog box. For example, if you are tuning a cascaded-loop control system with an inner loop and an outer loop, you might want to tune the inner loop with the outer loop open.

Tip

To highlight any selected signal in the Simulink^® model, click . To remove a signal from the input or output list, click . When you have selected multiple signals, you can reorder them using and . For more information on how to specify signal locations for a tuning goal, see Specify Goals for Interactive Tuning.

Desired Goals

Use this section of the dialog box to specify desired characteristics of the tuned system. Control System Tuner converts these into Loop Shape, Margin, and Poles goals.

Target gain crossover region
Specify a frequency range in which the open-loop gain should cross 0 dB. Specify the frequency range as a row vector of the form [min,max], expressed in frequency units of your model. Alternatively, if you specify a single target frequency, wc, the target range is taken as [wc/10^0.1,wc*10^0.1], or wc ± 0.1 decade.
Gain margin (db)
Specify the desired gain margin in decibels. For MIMO feedback loops, this requirement guarantees stability for gain variations across all feedback channels. The gain can change in all feedback channels simultaneously, and by a different amount in each channel. For information about disk margins, see Stability Analysis Using Disk Margins (Robust Control Toolbox).
Phase margin (degrees)
Specify the desired phase margin in degrees. For MIMO feedback loops, this requirement guarantees stability for phase variations across all feedback channels. The phase can change in all feedback channels simultaneously, and by a different amount in each channel. For information about disk margins, see Stability Analysis Using Disk Margins (Robust Control Toolbox).
Keep poles inside the following region
Specify minimum decay rate and maximum natural frequency for the closed-loop poles of the tuned system. While the other Quick Loop Tuning options specify characteristics of the open-loop response, these specifications apply to the closed-loop dynamics.
The minimum decay rate you enter constrains the closed-loop pole locations to:
- Re(s) < -mindecay, for continuous-time systems.
- log(|z|) < -mindecay*Ts, for discrete-time systems with sample time Ts.
The maximum frequency you enter constrains the closed-loop poles to satisfy |s| < maxfreq for continuous time, or |log(z)| < maxfreq*Ts for discrete-time systems with sample time Ts. This constraint prevents fast dynamics in the closed-loop system.

Options

Use this section of the dialog box to specify additional characteristics.

Apply goal to
Use this option when tuning multiple models at once, such as an array of models obtained by linearizing a Simulink model at different operating points or block-parameter values. By default, active tuning goals are enforced for all models. To enforce a tuning requirement for a subset of models in an array, select Only Models. Then, enter the array indices of the models for which the goal is enforced. For example, suppose you want to apply the tuning goal to the second, third, and fourth models in a model array. To restrict enforcement of the requirement, enter 2:4 in the Only Models text box.
For more information about tuning for multiple models, see Robust Tuning Approaches (Robust Control Toolbox).

Algorithms

Control System Tuner uses looptuneSetup (for slTuner) (Simulink Control Design) or looptuneSetup to convert Quick Loop Tuning specifications into tuning goals.