Online Frequency Response Estimation Basics
The Frequency Response Estimator block in Simulink Control Design lets you measure
the frequency response of your system in operation. The block performs an experiment
that injects signals into the plant and measures the plant output. If you have a code-
generation product such as Simulink Coder™, you can generate code that implements the
estimation algorithm on hardware. Deploying the algorithm to hardware lets you measure
the frequency response of a physical plant in real time.Embedded frequency-response estimation is a useful option when you have a physical
plant and a test bed or control environment to operate in. In this case, you can deploy the
Frequency Response Estimator block to your hardware. You trigger the tuning process via
an input to the block, so you can tune your controller at any time. For details, see “Deploy
Frequency Response Estimation Algorithm for Real-Time Use” on page 6-11.If you have a plant model in Simulink, you can use the Frequency Response Estimator
block to preview plant response and adjust the experiment settings before performing
estimation in real time. Doing so helps ensure that real-time estimation does not drive
your system out of the desirable operating range. For details, see “Online Estimation
Using Plant Modeled in Simulink” on page 6-6. You can also use the block to obtain the
frequency response of a plant that cannot be linearized in Simulink, as an alternative to
offline frequency response estimation with Linear Analysis Tool or frestimate.When Not to Use Online Frequency-Response Estimation
You can use online frequency response estimation with any stable SISO plant. For an
unstable plant, online estimation works in a closed-loop configuration, provided that the
closed loop is internally stable. A closed-loop system is internally stable if and only if the
roots of the nominal closed-loop characteristic equation all lie in the open left half-plane.
For a plant with transfer function G = NG/DG and controller C = NC/DC, the characteristic
equation is:
DGDC + NGNC = 0.In practice, this condition means that no unstable poles in G are stabilized by pole-zero
cancellation in GC. Do not use online estimation with an unstable plant that does not meet
this condition.Online frequency response estimation does not work well when there are large
disturbances in the plant during the estimation experiment. Disturbances distort the plant
response to the perturbation signals, yielding poor estimation results.6 Online Frequency Response Estimation